Photovoltaics: Present Status and Future Prospects

Vasilis Fthenakis

Center for Life Cycle Analysis, Columbia University, New York, NY 10027

Summary

This report provides an overview of recent developments in photovoltaics (PV) efficiencies,

production costs, and deployment and discusses how these advances created the potential for

photovoltaics to become a major contributor in energy markets throughout the world.

Specifically the report deals with the following topics: a) PV market evolution; b) photovoltaic

materials and devices; c) PV manufacturing present status and prospects; d) efficiency

advancements for current and evolving technologies; e) sustainability and life cycle

assessments, and f) the intermittency challenge.

The technologies described in this report are crystalline silicon (c-Si), amorphous silicon (a-Si),

cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), copper zinc tin sulfide

(CZTS), dye-sensitized solar cells, and organic photovoltaics (OPV).

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Table of Contents

1. Introduction

2. Photovoltaic Materials and Devices

3. PV Materials, Cells and Modules: Manufacturing Status

a. Solar Grade Silicon Production

i. Silane and chlorosilane production

ii. Siemens process

iii. Fluidized bed reactor processes

iv. Vapor to liquid deposition

v. Upgraded metallurgical silicon

b. Alternative Si-cell technologies

i. Kerfless wafers

ii. Thin-film silicon

c. Amorphous and Nanocrystalline Silicon (a-Si)

d. Cadmium Telluride (CdTe)

e. Copper Indium Gallium Selenide (CIGS)

i. Co-evaporation

ii. Reactive Sputtering/selenization

iii. Solution-based processes

f. Copper Zinc Tin Sulfide (CZTS)

g. Dye-sensitized solar cells

h. Organic photovoltaics (OPV)\

4. Future Outlook

5. Sustainability-Life Cycle Analysis

6. The Intermittency Challenge

7. Conclusions

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1. Introduction

Global power consumption currently stands at approximately 15 TW (1 TW = 1012 W), the vast

majority of which is generated by the combustion of fossil fuels. The associated release of CO2

from these anthropogenic sources has dramatically altered the composition of the atmosphere

and may detrimentally impact global temperature, sea levels, and weather patterns.

Furthermore, the realization that fossil fuels are not inexhaustible and that enhancing recovery

of coal, oil and natural gas presents additional risks, drive energy policy scenarios that are

based on renewable forms of energy. In most of the proposed scenarios, solar energy is the

primary constituent as it is the major energy source over large regions. Solar energy in addition

to maintaining life on the planet, is used on demand in three basic forms based on

anthropogenic processes: Electricity from the direct conversion of solar energy using

semiconductor materials (solar photovoltaics, PV); electricity from captured thermal energy

(concentrated solar power, CSP), and heat from the sun (solar thermal). The later, in the form

of water solar collectors is cost-competitive with electric heaters in high solar irradiation

regions all over the world. Among the solar electric technologies, traditionally CSP were

considered as the most cost effective for central power generation in high insolation areas,

while PV were considered primarily for small dispersed (mainly residential) applications.

However, since about 2005, a large utility market has been created for PV and the costs of large

PV “solar

farms” has

been reduced

to below that of

comparable size

CSP plants.

Fig. 1. Recent growth of the PV Global Market; annual sales in GW modules(source:

http://solarcellcentral.com/markets_page.html)

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Overall, the market for photovoltaics has been growing at an average rate of 45% per year over

the past decade. The five year growth rate from 2007 to 2011 was approximately 70% per year,

but this was slowed down to 15% in 2012 as the incentives in several European countries were

reduced (Fig. 1). While the growth numbers are very impressive, the 27 giga-watts installed in

2011 is just a fraction of one percent of the total amount of electricity that was being generated

by all sources, indicating that there is plenty of room for further growth. 1,2

A synopsis of the market historical evolution is pertinent. Japan was the first country to enact

economic incentives for PV and led the global market in the 90s till the early 2000s in both

production and installations. Then in 2003, Germany enacted the Renewable Energy Standard

(RPS) and created an uncapped PV market that became a magnet for PV producers from all

over the world. The large market created in Germany is credited with enabling the economics

of scale in PV production and the manufacturing efficiencies that reduced the cost of producing

PV modules by a factor of three. In the period 2005-2009 cadmium telluride (CdTe) thin film

technology emerged rapidly as the low-cost PV production leader. A single company First Solar

become the world’s higher volume PV manufacturer in 2009 capturing 13% of the global

supply, from a negligible position in 2003. However, the rapid growth of CdTe PV was eclipsed

by an increase of crystalline PV production from China, which in 2010 became the world’s

leading manufacturer. This growth was spurred by strategic government investment, access to

cheap capital, low Chinese labor costs, and probable distortion of economics by

unconventional accounting and a low exchange rate. Protests by the European and US

manufacturers, resulted to the US Commerce Chamber issuing a preliminary determination for

a 31% tariff to be imposed on Chinese imports into the US market to counter-balance the

subsidies and cost distortions of the Chinese imports, whereas proposed legislation in the US

Senate calls for a “US-made” requirement to projects eligible for the business Investment Tax

Credit (ITC) eligibility. Thus, projects using a large fraction (e.g., >30%) of PV modules from

China would be ineligible for the 30% ITC in the United States. Although the market growth and

distribution of market share is fluid, we believe that the market is poised for further expansion

in the next decade as PV has reached cost parity with peak rates in south California, and South

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Italy and is expected to reach cost parity throughout the U.S. and many other regions by 2030

(Fig. 2).

Fig. 2 Wholesale Levelized Cost of Electricity in the U.S.: Central (utility) status and

projections (Source: Lushetsky J., Solar Technologies Program, US-DOE, 25th EUPV, Valencia,

Spain, Sept. 2010.

This report provides an overview of PV technologies and innovations which have the potential

to significantly contribute to an expansion of the PV markets in developed and developing

countries alike. The descriptions of the status and prospects of PV manufacturing are mainly

based on a National Science Foundation workshop held at the Colorado School of Mines.3

2. Photovoltaic Materials and Devices

Photovoltaics use semiconductor materials to generate electricity from solar energy. A

semiconductor is a solid, mostly crystalline, material such as silicon, selenium or germanium.

Semiconductors have electrical conductivities greater than insulators but lower than metals

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which are good conductors. At low temperatures they are insulators but at high temperatures

and/or when excited by sunlight, they conduct electrons. The most commonly used

semiconductor element is silicon belonging to column IV of the periodic table which has four

valence electrons and bond to four other atoms of the same element to have a fully satisfied

valence of eight electrons. A number of compounds can be semiconductors having a valence of

eight electrons. Galium arsenide (GaAs), and cadmium telluride (CdTe), are examples of III–V,

and II–VI compounds, respectively.

Semiconductors do not have ‘free’ electrons’ to exhibit electrical conduction at low

temperatures; however at room temperature, they show a modest electrical conductivity,

which increases as temperature increases. The conductivity behavior of semiconductors may

be explained with a simple model illustrating silicon’s atomic structure. Each silicon atom has

four nearest neighbors. It has 14 electrons in its atomic structure and four of them are in the

outermost orbit which is weekly-bound to the nucleus. These four electrons are called ‘valence

electrons’ which form covalent bonds with the four nearest-neighbor atoms (i.e., each valence

electron of a silicon atom is shared by one of its four nearest neighbors).

Fig.3 (a) Three-dimensional Silicon Lattice

Fig 3(b) Silicon lattice with one Si atom displaced by a pentavalent atom (e.g., As, P);

(c) Silicon Lattice with one Si atom displaced by a trivalent atom (e.g., Ga, B)

Source: Markvart T., Solar Electricity, UN Press4

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The valence electrons thus help to bind one atom to the nearest neighbor, resulting in these

electrons being tightly bound to the nucleus. In ‘chemically pure’ or ‘intrinsic’ silicon which

contains no foreign atoms, this ideal situation exists at 0 degrees Kelvin, and the crystal

behaves as an electrical insulator since no free electrons are available. When light is absorbed

by the material, the ‘bond’ gets broken and the valence electron becomes a free electron

leaving behind a ‘hole’. The hole is an incomplete

covalent bond. It takes energy equal to ‘band energy’

Eg, a characteristic parameter of a semiconductor, to

separate an electron from the bonding and make it a

‘free’ electron, also referred to as ‘conduction’ electron

(Fig. 4).

Figure 4. Schematic of energy gap in semiconductor materials

For silicon, Eg is equal to 1.1 eV at room temperature. When a hole exists in a bond, it is

relatively easy for a valence electron in a neighboring atom to leave its covalent bond to fill the

hole, leaving a hole in its original position. The ‘hole’ therefore effectively moves opposite to

that of the electron. The movement of holes, behaving like positive charges, constitute an

electrical current. Small amounts of dopants, few parts per million, introduced into the

semiconductor structure can significantly increase the number of electrons available for

breaking away from their atoms.

When, a silicon atom is replaced by a trivalent atom like boron (B) or gallium (Ga) which has

three valence electrons, then bonding with only three nearest neighbors is complete. This atom

can capture an electron from a nearby bond, establishing the fourth bond. The captured

electron is not available for conduction, but a hole created in the original position of the

electron enables conduction of electrons. This is called p-type doping or ‘acceptor’ type doping.

The valence electrons are confined to the valence band; but when light energy equal to or

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greater than Eg is incident, they move into the conduction band, creating an equal number of

‘holes’ in the valence band. In Fig. 4, Ec represents the lowest energy that conduction electrons

may have and Ev represents the lowest energy that ‘holes’ may have. Eg is the separation of the

conduction band edge from the valence band edge.

If a semiconductor of band gap energy Eg absorbs light energy (E=hν), greater than Eg, each

absorbed photon raises one electron from valence band to conduction band, creating one

electron and one hole. Here ‘h’ is Planck’s constant and ν is the frequency of the light wave. The

carrier generation rate per unit area of the semiconducting surface is therefore a function of Eg;

however, the reflection of incident light at the surface and incomplete collection of charge

carriers affect the generation rate.

The p-n junction

P-n junctions are formed by joining n-type and p-type semiconductor materials, as shown

below. Since the n-type region has excess electrons and the p-type has excess holes, electrons

diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type

side to the n-type side. If the electrons and holes were not charged, this diffusion process

would continue until the concentration of electrons and holes on the two sides were the same,

as happens if two gasses come into contact with each other. However, in a p-n junction, when

the electrons and holes move to the other side of the junction, they leave behind exposed

charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On

the n-type side, positive ion cores are

exposed. On the p-type side, negative ion

cores are exposed. In summary, electrons

move from the n-type to the p-type and

holes move from p-type to n-type and

this movement creates is a region where

the negative electrons have become positive and positive holes have become negative. This

changing of charges creates a barrier to further movement. An electric field Ê forms between

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the positive ion cores in the n-type material and negative ion cores in the p-type material; this

electric field pulls electrons and holes in opposite directions. This region is called the

"depletion region" since the electric field quickly sweeps free carriers out, hence the region is

depleted of free carriers (Fig. 5). The width, W, of the depletion zone in p-n silicon devices is in

the order of 100 nm. Due to Ê, a "built in" potential is formed at the junction.

Principle of Operation of a P-N Junction Solar Cell

Solar cell is an electronic device which converts solar energy directly into electrical energy

through the photovoltaic effect. It is a typical semiconductor p-n junction device. The principle

of operation of a p-n- junction solar cell are illustrated in Figures 6 and 7. When the light falls on

the device, the light photons of certain wavelengths are absorbed by the semiconducting

material (excites electrons which move from the valence to the conduction band) and electrical

charge carriers, electrons and holes, are generated. These carriers diffuse to the junction where

a strong electric field exists, the electrons and holes are separated by this field and produce an

electric current in the external circuit, this current, called photo-current, depends on the

incident photon intensity and the nature of semiconductors that constitute the junction device.

Since the extra energy of photons is lost into heat, only part of solar spectrum can be converted

(Fig. 6).

Fig. 6 and 7. Photovoltaic Effect in a Solar Cell

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Under illumination, a photo-generated current, Iph, flows as a reverse diode current, which is

linearly dependent on the intensity of incident light. Fig. 8 shows the current-voltage (I-V)

curves, without and with incident light.

Fig. 8. I-V characteristics of a solar cell with external voltage and with illumination

The resulting current is: I = -Iph + Io [ exp (qV/γkT) -1 ] and is represented by the lower curve in

Fig. 8. Typically the I-V curves of solar cells are presented only by the fourth quadrant (flipped

up to show positive current) showing the effect of both illumination and the diode, as shown in

Fig. 9.

Fig. 9. Solar Cell I-V Curve

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Two important parameters are the short-circuit current (Isc), and the open-circuit voltage (Voc). If

the terminals of the junction are shorted, no current flows in the junction. When the junction is

illuminated, a current called ‘short-circuit current’ flows in the junction from p- to n- side or

otherwise depending on which side of the junction, the light is incident. This photocurrent is

due to three contributions: holes generated in the n-region, electrons in the p-region and

electrons and holes in the depletion region before they recombine.

Isc is a function of cell design and is proportional to photon flux. If the load is an open circuit, the

corresponding Vout is called ‘open-circuit voltage’, Voc. Isc and Voc are related to the output power

at the ‘optimum operating point’, Pmax as

Pmax = Voc x Isc x FF

where FF, the fill factor.

FF= Vm x Im/Voc x Isc

Vm and Im are the voltage and current at the optimum operating point. Typically Vm is 75% to

90% of Voc; and Im is 85 % to 99 % of Isc for silicon.

The efficiency of the solar cell is typically measured as:

η = Voc x Isc x FF/ Incident Solar Power

The efficiency will be high is Isc and Voc are high and so also FF. it means the dark current must

be low. A sharp corner in the I-V characteristic is an indicator of high FF.

In practice, the conversion efficiency of a solar cell is measured under Standard Test Conditions

(STC), which include (i) solar irradiance intensity of 1000 watts/m2, (ii) AM 1.5 solar reference

spectrum, and (iii) cell temperature during measurement, of 250C.

The Voc is directly proportional to the band gap energy whereas the generation rate of the

current decreases with increasing energy gap, thus that the cell efficiency reaches a maximum

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at certain range of values of Eg that corresponds to a maximum voltage-current product. More

specifically, a change in energy gap influences the energy conversion of a solar cell in two ways.

First, a larger Eg reduces the reverse saturation current and increases Voc which in turn tends to

increase the efficiency. Secondly, a larger Eg means that fewer photons can be absorbed

because only those photons greater than or equal to Eg are absorbed, which in turn decreases

the efficiency. The net result of these two opposing effects is shown in Fig.10. For single-

junction devices, the calculated efficiency has a maximum of about 30% at Eg equal to about 1.5

eV and falls off on either side of this value.

The semiconducting compounds, cadmium telluride(CdTe), gallium arsenide (GaAs) and indium

phosphide(InP) have Eg around this value. In practice, several semiconductors with energy band

gaps in the range 1.0 to 1.7 eV are

used for maximum efficiency

devices/solar cells. These also

include monocrystalline and

multicrystalline silicon (Eg = 1.12

eV), copper indium diselenide(Eg =

1.05 eV), amorphous silicon (Eg =

1.7 eV), cadmium telluride (Eg =

1.45 eV), gallium arsenide (Eg = 1.25

eV).

Fig. 10. Estimated Efficiency versus Energy Gap of the Cell Material

Figure 11 displays the champion overall power conversion efficiencies for laboratory solar

cells.13, 14, 15 It is shown that the PV technologies are at different points in their respective

learning curves when compared to the Shockley–Queisser (S-Q) limit of ~31% solar energy

conversion efficiency for single junction devices.17 As shown in Figure 10, the band gap energy

dependence of the Shockley–Queisser limit is flat around the maximum, thus the theoretical

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efficiency limits for these technologies are only marginally different and are not a deciding

factor in their competitiveness with each other. Nevertheless, crystalline silicon, which is the

most mature technology, having been benefitted by investments in IC applications, has reached

record efficiencies of ~24%, whereas the less researched CdTe and CIGS thin-film technologies

have reached record cell efficiencies of 17.3% and 20.4% correspondingly. The module

efficiencies lag those of the laboratory cells; the highest commercial production module

efficiencies are 20.1%, 12.5% and 11.5% correspondingly for mono-crystalline Si, CdTe and CIGS

modules.

Fig. 11. Record efficiencies for different types of solar cells in the laboratory (Source: NREL)

In most cases, the record cells have seen little or no improvement over the past decade while

economies of scale and advances in manufacturing science and technology have fueled the

growth of the PV market through cost reduction and module performance improvements16.

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Solar Cell Design Principles

Solar cell design involves specifying the parameters of a solar cell structure to maximize

efficiency. The theoretical efficiency for photovoltaic conversion is in excess of 86.8%.6

However, the 86.8% figure uses detailed balance calculations and does not describe device

implementation. For silicon solar cells, a more realistic efficiency under one sun operation is

about 29%.7 The maximum efficiency measured for a silicon solar cell is currently 24.7% under

AM1.5G. The difference between the high theoretical efficiencies and the efficiencies measured

from terrestrial solar cells is due mainly to two factors. The first is that the theoretical

maximum efficiency predictions assume that energy from each photon is optimally used, that

there are no unabsorbed photons and that each photon is absorbed in a material which has a

band gap equal to the photon energy. This is achieved in theory by modeling an infinite stack of

solar cells of different band gap materials, each absorbing only the photons which correspond

exactly to its band gap. The second factor is that the high theoretical efficiency predictions

assume a high concentration ratio. Assuming that temperature and resistive effects do not

dominate in a concentrator solar cell, increasing the light intensity proportionally increases the

short-circuit current. Since the open-circuit voltage (Voc) also depends on the short-circuit

current, Voc increases logarithmically with the level of the incident light. Furthermore, since the

maximum fill factor (FF) increases with Voc, the maximum possible FF also increases with light

concentration, allowing concentrators to achieve higher efficiencies.

In designing single junction solar cells, the principles for maximizing cell efficiency involve the

following: a) increasing the amount of light collected by the cell that is turned into carriers; b)

increasing the collection of light-generated carriers by the p-n junction; c) minimizing the

forward bias dark current; d) extracting the current from the cell without resistive losses.

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3. PV Materials and Modules: Manufacturing Status

A. Crystalline Silicon

Silicon is the most developed and best understood semiconductor, benefiting from decades of

development by the integrated circuit (IC) industry. Multi-crystalline silicon (mc-Si)

photovoltaics have the greatest market share followed by mono-crystalline silicon, followed by

CdTe thin-film photovoltaics. Although CdTe thin-film has the lowest module production cost,

mc-Si has higher efficiency, reducing, therefore, the cost requirement for the mounting

structure and installation, since those are proportional to the area required for the installation.

The mc-Si industry is looking forward to lower polysilicon prices, and improvements in wire

cutting technology that reduced wafer thicknesses that could keep c-Si silicon competitive..

Several companies have started producing solar grade silicon at less energy and at a lower cost

than the current production via the“modified Siemens” process, and R&D is taking place for the

production of kerfless wafers, and the development of solar cells from depositing a thin-film

silicon on substrates as amorphous silicon is deposited today.

A.1 Production of Solar-grade Silicon

The solar grade silicon production is known to be the most energy intensive stage in the life

cycle of silicon PVs when the typical Siemens process is used. This process was originally

designed to produce the semiconductor grade silicon, which is purer than the solar grade

silicon. In order to reduce the high electric energy requirement, modified version of Siemens

process which operates under a relaxed condition, have widely been used. In addition, novel

process schemes and equipment have begun emerging such as fluidized bed reactor (FBR) and

upgraded metal-grade (UMG) processes. Figure 12 presents the currently available routes to

produce solar grade silicon; these are reviewed below.

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Most SGS production is involving purification through gasification of MG silicon to silane gases

and deposition of Si by reducing the silane feedstock gases. A new process that upgrades the

purity of MG silicon (UMG-Si) has also become available to commercial production.

A1.1 Silane and Chlorosilane Production

Most solar grade silicon (SGS) currently is obtained through silicon deposition in reactors from

mono- or trichloro- silane gas; the vast majority of the SGS is produced in Siemens reactors with

a small fraction of the production based on the fluidized bed reactors (FBR). Silane gases are

produced by gasifying metal-grade silicon (MG-Si, >99%) that is acquired by reacting silica

(SiO2) with coke (C) in electric arc furnace. Trichloro-silane (SiHCl3, TCS) gas is synthesized

reacting MG-Si with HCl in a Fluidized Bed Reactor (FBR) at 200-500 ° C: Si + 3HCl → SiHCl3 +

H2. In this process, a side reaction becomes dominant as the temperature increases, i.e., Si +

HCl SiCl4+ 2H2, producing SiCl4, the byproduct of this process. Therefore, removing the

reaction heat is critical to improve the efficiency of TCS synthesis.7 In mono-silane (MS, SiH4)

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gas production process, first, SiHCl3 is synthesized from MG-Si, SiCl4, and H2 (Si + 3SiCl4 + 2H2

→ 4SiHCl3), then converted to SiH4. Mono-silane is also commercially produced from SiF4

through the following process: SiF4 + NaAlH4 → SiH4 + NaAlF4.7

(a) Siemens Reactor (b) Fluidized Bed Reactor

Figure 13. CVD Reactors to Produce Polysilicon8

A1.2 Siemens Process

The typical Siemens process based on TCS deposits silicon on rods at 850 - 1100 °C.8 The bell

jar type inner wall of the Siemens reactor should be cooled by water to prevent Si precipitation

on the wall of the reactor, causing significant heat losses (Figure 13 (a)). The electricity

consumption ranges 60-150 kWh per kg of purified silicon.7 This process generates byproducts

of H2, HCl, SiH2Cl2 and SiCl4. The latter is recycled to SiHCl3 by reacting with H2 in a FBR from

the following reactions: SiCl4 + H2 = SiHCl3 + HCl, 3SiCl4+2H2+Si = 4SiHCl3. MS-based Siemens

can lower the deposition temperature to 650-800 °C. 7 But, formation of unwanted fine silicon

powder causes product quality and process control issues. 7,9

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Once the silicon rods reach 10-15 cm and no further room to grow inside the Siemens reactor,

the deposition operation should stop and the completed rods be removed. This type of batch

process, therefore, requires frequent assembly/disassembly of the reactor, which considered

labor intensive and time consuming.7

A1.3 Fluidized Bed Reactor (FBR) Processes

Fluidized Bed Reactor method produces particle silicon compared with rod shape silicon ingot

produced in the Siemens process. Silane or trichlorosilane gas is continuously supplied released

into a superheated FBR chamber that contains seed grains of silicon for continuous production

(Figure 13 (b)). The electric energy demand of producing SGS through FBR is reported 10-30%

of what is required for the Siemens process.7,9,10 The FBR route is more energy efficient than

the Siemens process for several reasons: It does not waste energy as in the Siemens process,

and produces more silicon per cubic meter of reactor space. 11

This process is easily contaminated due to the large surface area of Si particles than the bar Si

obtained in the Siemens process, and to the fact that those particles frequently contact with

the reactor wall. The particle size of Si should be constant for continuous production in FBR

that seed particles may be supplied to the reactor. The inner wall of the reactor is coated with

SiC as a barrier against contamination. But, in a high temperature beyond 1000°C, carbon can

be released from the coating to contaminate silicon particles.8 To avoid precipitation of silicon

on the wall and contamination, radiant heating such as high frequency, micro waves, and

infrared rays and so on is often used in an attempt to heat only Si particles. 9 The large surface

area of the Si particles also causes trapping of hydrogen gases inside and their burst triggers

generation of unwanted fine silicon powders. An additional dehydrogenation process may be

required at 1150 °C. 9

The MEMC Inc. has been commercially producing SGS in a FBR process since 1987 from MS

(SiH4) synthesized through reduction of SiF4. In addition, a new FBR SGS process in REC uses

SiH4 produced from disproportionation of SiHCl3 (4SiHCl3 → 3SiCl4 + SiH4). It is known that

this method reduces electricity consumption to 20% of the Siemens process and the cost to less

than 70%. 9 MS-FBR has a lower deposition temperature compared with TCS-FBR as MS

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deposits at 750°C compared with TCS at 1050°C.12 However, MS-FBR tends to produce a large

amount of unwanted fine silicon dust, which largely forms at the empty space above the

fluidized bed or inside gas bubbles.7

Hydrogen reduction in FBR using TCS as a raw material also produces purified particle silicon,

which is used by Wacker. However, the details of Wacker FBR process are not well known.12 In

TCS-FBR, residual Cl could be the major impurity, formed from dissociation ofSiCl2 [2, 3]. The

density of the latter is known to be lesser with a lower temperature and high pressure reactor

condition. 7

A1.4 Vapor to Liquid Deposition (VLD)

This is the process used by Tokuyama, Japan. It is similar to the Siemens process but produces

liquid silicon instead of solid silicon bar. The precipitation rate is 10 times faster than the

Siemens process but it is hard to remove impurities. Thus, this process is suitable for SGS rather

than electronic grade silicon which requires a higher purity. 9

A1.5 Upgraded Metallurgical Grade Silicon

Solar-grade silicon based on metallurgical refining processes, often called upgraded

metallurgical-grade silicon (UMG-Si), is expected to play an important role in reducing the

energy and cost of silicon production for photovoltaics. The leading company in this area is

Norway’s Elkem which reports the use of UMG-Si in solar cells with comparable efficiencies to

polysilicon (poly-Si) from the traditional Siemens process. Elkem’s process starts from the

production of metallurgical silicon in a reduction furnace with advanced process controls,

followed by purification stages involving slag treatment, leaching and solidification, and

subsequent post-treatment. The purification stages are designed to remove impurities like

boron and phosphorous which can adversely affect the operation of the p-n junction in solar

cells. Elkem reports having reduced the concentration of B and P to 0.22 ppm and 0.62 ppm

correspondingly, making their product comparable to solar grade silicon from the modified

Siemens process.

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B. Alternative Si-cell Technologies

B.1 Kerfless Wafers

Improvements in wire-saw technology have enabled the reduction of wafer thicknesses from

~400 µm a decade ago to 140 µm at present. However, over than 50% of the silicon is lost as

silicon kerf and recycling of this kerf is difficult as it is contaminated with solvents and abrasives.

Kerfless wafer could potentially reduce Si use by 50% or more . Techniques for the direct

production of wafers from the melt were invented in the 1970s,20, 21 and after decades of

development they have now reached the market. The two closely related techniques are edge-

defined, film-fed growth (EFG) and string ribbon silicon technologies. In the EFG process the Si

wafers are pulled out from the melt through a graphite die using capillary action. This process

was developed by ASE Americas22 and is now employed by Schott Solar. Ribbon silicon is

produced by pulling a pair of high-temperature strings through a crucible of molten Si, and this

technology has been promoted and implemented by Evergreen Solar. These two growth

techniques produce vertical sheets of mc-Si approximately 300 m thick and up to a 100 mm

wide.

The quality of the material produced by these techniques is somewhat inferior to standard

block-cast mc-Si, but is continuously improving with champion laboratory cells now reaching

power conversion efficiencies over 18% and 14.5% efficient modules already on the market.14, 15

Passivation of surface and bulk defects is critical to achieving high efficiency, and this is usually

achieved through the deposition of a hydrogen-rich silicon nitride layer, which also serves as an

anti-reflection (AR) coating.23 These technologies are expected to become more cost-

competitive as energy costs continue to rise, but improvements in manufacturing are needed to

compete with conventional c-Si technology (Fig. 3). In particular, further reductions in wafer

thickness coupled to improvements in throughput are viewed as the most important tasks for

scaling these technologies to TW levels. Increasing the grain size and crystal quality will also be

important for further improvements in efficiency.3

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B2. Thin-film- Silicon

Silicon has an indirect band gap, and, therefore, relatively thick layers are needed to allow for

momentum extraction and energy absorption. It is assumed that silicon must be thicker than

100 μm to effectively absorb light,24 however theoretical modeling has shown that ~40 μm may

be ideal for obtaining maximum performance,25, 26 and that efficient cells could be obtained

with only 1 μm of single crystal silicon with state of the art surface passivation.27 If such thin

layers could be produced using a kerfless process, and therefore avoid the large silicon loss due

to cutting, this would result in an order of magnitude reduction in materials cost and energy

with respect to today’s state-of-the-art wafers. Thin silicon is also amenable to use of bifacial

architectures,28, 29 which harvest light from both directions.

There are two general approaches to the fabrication of thin-film-Si; these are a) heteroepitaxial

growth followed by lift off or removal of a sacrificial substrate30, and b) “peeling” ultrathin

silicon layers off of silicon ingots using techniques such as stress-induced liftoff.31 The startup

company Silicon Genesis has recently introduced a process where this is achieved through a

combination of ion implantation and thermal treatment, producing kerf-free wafers as thin as

25 microns.32 Substantial challenges remain once thin-film-Si is produced. Achieving high

efficiency will require the use of the most advanced technologies with respect to both surface

passivation33, 34 and light trapping.35, 36 However, results to date are encouraging. The University

of Stuttgart has fabricated a 16.7% solar cell from 45 μm thick Si produced by lift off, while 8.2%

modules have been fabricated using 2 μm polycrystalline Si.13 Commercialization of such efforts

will need to address the nontrivial challenge of mechanically handling these very thin wafers

while maintaining high throughput and low cost.

C. Amorphous and Nanocrystalline Silicon (a/nc-Si)

Solar cells based on hydrogenated amorphous silicon (a-Si:H or a-Si) were invented by Carlson

and Wronski at RCA Labs in 1976.61 Manufacturing is achieved by plasma-enhanced chemical

21

vapor deposition (PECVD) using mixtures of H2 and SiH4. Hot-wire chemical vapor deposition has

been offered as an alternative,62 but has yet to be implemented in large scale manufacturing.

The capability to dope a-Si in a controllable fashion is relatively poor and attempts to do so

leads to the creation of additional recombination centers. Because of these effects p-i-n device

structures are almost always used.63 Benefiting from synergies with the IC industry a-Si was

rapidly commercialized and the first PV products appeared in the early 1980s. Early devices

rapidly surpassed the 10% conversion plateau, but it was quickly recognized that these devices

suffered from light-induced degradation through the now well-known Staebler-Wronski

effect64; light exposure leads to a reduction of the solar cell efficiency over months which

eventually stabilizes at efficiencies around 6-7%. Nevertheless, for decades a-Si was by far the

most successful thin film technology, achieving market shares in excess of 10% before the

phenomenal growth of CdTe PV production.

Leading manufacturing companies include Sharp and Oerlikon. One of the most attractive

features of a-Si is that devices can be deposited at low temperature (< 200 °C), enabling the

fabrication of lightweight, flexible laminates on temperature sensitive substrates. This is a

unique trait that provides a competitive advantage in markets such as consumer products and

building integrated photovoltaics (BIPV). Though discovered much earlier,65 another major

change that has occurred over the past decade is the integration of micro (μc-Si) or

nanocrystalline (nc-Si) into device structures. The quality of PECVD deposited material is

strongly influenced by the level of silane dilution in hydrogen, and high H2 dilution levels (>90%)

lead to the formation of crystalline domains within the material. The primary advantage of nc-Si

is that it is much less susceptible to Staebler-Wronski degradation.64 Another important feature

is that a/nc-Si is amenable to the formation of multi-junction devices; most commercial devices

are based on either tandem cells or even triple junction cells. A common configuration is the

“micromorph” tandem, which pairs an a-Si top cell with a nc-Si bottom cell.66 Solar cells with

record efficiencies are based on triple junctions that employ germanium alloys to further

improve absorption in the red region of the solar spectrum.67 A related success story has been

the introduction of the a-Si/c-Si heterojunction with intrinsic thin layer or HIT cell, which boasts

22

21% conversion efficiency. 68 In addition to boosting current the intrinsic a-Si:H layers appear to

be important for passivation of the underlying c-Si material.

D. Cadmium Telluride (CdTe)

The first reports of CdTe-related PV devices appeared in the 1960s40 but efficiencies were very

low till the early 90s.41 CdTe has a number of intrinsic advantages as a light absorber. First, its

band gap of 1.45 eV is ideally positioned to harness solar radiation. Its high optical absorption

coefficient allows light to be fully captured using only two microns of material. CdTe sublimes

homogeneously and the compound’s thermodynamic stability makes it is nearly impossible to

produce anything other than stoichiometric CdTe.42 Thus, simple evaporation processes may be

used for film deposition. Close-space sublimation employs diffusion as the transport

mechanism,41 while very high rates (>20 m/min) may be obtained using convective vapor

transport deposition.43

Standard CdTe-based devices employ a superstrate configuration: production begins with a

glass substrate followed by the successive deposition of the transparent conducting oxide (TCO,

SnO2:F), the n-type window layer (CdS), the p-type CdTe absorber, and finally the back contact

(ZnTe/Cu/C). CdTe PV manufacturing is uniquely equipped to be integrated with the production

of float line glass44. Glass exits a float line at ~600 °C, which happens to be an optimal

temperature for vapor-phase deposition of the SnO2:F (FTO), CdS, and CdTe. Part of First Solar’s

success has been due to their ability to integrate these various process steps into an in-line

manufacturing process reducing the processing time from glass to a finished module down to

2.5 hours.

With low manufacturing costs established, the biggest opportunities for CdTe lie in the

improvement of device efficiency. Champion cells (Fig. 4) convert just over 50% of their S-Q

potential, while commercial modules are at ~11% power conversion efficiency. Improving

efficiency will require enhancements in both current and voltage. The former is perhaps the

most straightforward route, as much of the blue region of the solar spectrum is absorbed in the

TCO and CdS layers that make up the front contact. Top laboratory cells have replaced the FTO

23

with advanced TCOs such as cadmium stannate45 and ITO.46 Likewise, the CdS window layer (2.6

eV) absorbs a significant fraction of the blue light. Integration of advanced front contacts into

manufacturing appears to be the near term strategy. This will not be trivial because ITO is

expensive and cadmium stannate is a complex material.47 Furthermore it is not clear what

might be used to substitute for CdS though sulfides of zinc and indium have attracted significant

interest.48

The more daunting challenge is improving the voltage. The open-circuit voltage (Voc) of

champion CdTe cells is well below that of similar band-gap PV materials. For example, the best

Voc obtained in CdTe is 230 mV short of GaAs devices, which has a similar band gap. Short

carrier lifetimes are at the root of this limitation. The combined effect of defects and grain

boundaries limits minority carrier lifetimes in polycrystalline CdTe to a few nS, even the best

devices. These lifetimes are very short compared to almost 1 s for epitaxial CdTe49 or

hundreds of ns for CIGS.50 Sites and Pan51 showed through simulation that increasing the carrier

lifetime or the use of a p-i-n device structures may be two viable routes to increase the

efficiency to above 20%. The short term goal of commercial manufacturers is to raise module

efficiencies from current levels to >15% by 2014 through a combination of process integration,

research, and development. In the longer term they are targeting 18% as an achievable goal for

module efficiency.52

A number of fundamental questions remain unanswered for CdTe PV. At present, the issue of

extending carrier lifetime is partially addressed by chemical passivation. Examples include the

introduction of O2 during CdTe growth,53 post-deposition CdCl2 treatments,54 and controlled

diffusion of Cu from the back contact.55 The empirical recipes associated with these processes

constitute the “black art” of CdTe manufacturing. Clearly a preferable route would be to

understand the nature of the defects states so one could prevent their formation in the first

place or develop alternative and perhaps better passivation strategies. Fundamental research

in understanding these defects and how to passivate them would be transformative leading to

improvements in one of the most promising solar cell technologies. Another fundamental

question concerns the role of grain boundaries in these devices.56 CdTe is an interesting and

unusual material in that solar cells based on polycrystalline CdTe outperform devices made

24

using single crystal CdTe. It is thought that grain boundaries can have both positive and

detrimental impacts on charge transport, but the current level of understanding is not sufficient

to suggest how one might engineer a desired morphology. The use of p-i-n structures to create

high-efficiency devices requires deliberate control of the sample free carrier density, which is

not yet fully understood or achieved. A final area that deserves attention is the back contact. It

is difficult to contact CdTe because it has low conductivity. Moreover, the back contact has

been implicated as a potential contributor to degradation.57, 58 The issues discussed above are

non-trivial and will require substantial investment and fundamental research to resolve.

Another issue to be mentioned with respect to large-scale CdTe manufacturing is perceptions

with respect to both cadmium toxicity and tellurium availability. The toxicity issue is one of

public perception which has been amplified by competitive business interests. Cadmium is

indeed a toxic element, but the risk of exposure once incorporated into PV modules is minimal.

Extensive testing of broken and heated panels has shown that emissions due to fire59 or

leaching60 are negligible. To their credit, all CdTe manufacturers are committed to 100%

ownership of recycling, which in part is related to the issue of Te availability discussed later in

this paper. One also notes that Cd will continue to be produced as a natural byproduct of Zn

mining. Perhaps the best argument for CdTe PV is that it serves as a means to sequester this

element in an environmentally beneficial manner. While scientific arguments can be made that

the toxicity of Cd is not a significant issue, governmental policy in individual countries may

dictate whether this issue hampers the deployment of CdTe solar cell technology.

E. Copper Indium Gallium Selenide (CIGS)

Copper indium diselendide (CuInSe2 or CIS) CIS has a band gap of 1.05 eV slightly off the

maximum efficiency energy gap (fig 10). However the band gap may be continuously

engineered over a very broad range (1.05 – 2.5 eV) by substituting either Ga for In or S for Se. 74

CIGS is currently the efficiency leader among thin film technologies cells with 20.3% efficiencies

currently reported in the laboratory75, 76. Commercial production of CIGS began in 2007, with a

few companies operating facilities with 10-30 MW/year capacities but the foretold large

25

production has not happened yet as reproducibility and production yield have proven to be

challenging. However, Solar Frontier in Japan recently expanded production to 900 MW.

Substrates include soda lime glass, metal foils, or high temperature polyimide (PI). The latter

has garnered substantial interest for applications such as BIPV and small portable applications.

In the case of deposition on flexible substrates it is critical to match the coefficient of thermal

expansion, with highest efficiencies obtained on titanium and stainless steel foils.

Fabrication of the CIGS device begins with the deposition of a Mo back contact followed by

the p-type CIGSS absorber (1-3 μm), a thin buffer layer (50-100 nm), with doped ZnO serving as

the transparent front contact. Many different approach exist for the formation of the CIGSS

absorber.77 At present, the performance of commercial modules is only about 11% although

the efficiency record of lab cells is 20.3% and much of this difference attributed to the quality

of the absorber layer.78 The approaches to CIGS fabrication may be classified into three basic

categories: co-evaporation, reactive sputtering/selenization of metal films, and non-vacuum

and solution printing techniques. Below is a description of the major advantages and issues

associated with each approach (Wolden et al, 2011).

Co-evaporation

Co-evaporation is practiced either in a single or three-stages. The former is employed by Q-cells

and Wurth Solar and has produced 12.7% efficient modules(Q-cells). The three-stage co-

evaporation is the process that has produced the world-record cells79 but so far it has produced

12.2% efficient modules (Global Solar, Yohkon). Co-evaporation alternates between copper-rich

and copper-poor conditions to produce the large grains and graded Ga/In profiles characteristic

of high efficiency material.80 There are a number of important practical challenges involved in

the manufacturing of CIGS solar cells. Evaporation sources typically have a cosine flux

distribution, and it is difficult to introduce sharp changes in composition or maintain uniformity

over large areas under the diffuse conditions of high vacuum. In addition, sources must be

mounted in a top-down configuration in order for large glass substrates to be supported and

heated to 600 °C. In situ diagnostics such as thermometry and laser light scattering, which are

26

critical for process control in the batch process, are being adapted for use in the manufacturing

environment.81 Likewise, atomic absorption spectroscopy and X-ray fluorescence are employed

for controlling element flux and in-line detection of film composition, respectively. Another

challenge with co-evaporation is that the relatively unreactive Se must always be supplied in

great excess, leading to practical concerns related to condensation and materials management.

Despite these challenges Q-Cells has announced 12-13% mass-produced modules and a 14.2%

champion module with this process. Through systematic optimization and accompanying

improvements in yield this may turn out to be a viable large-scale manufacturing strategy.

Reactive Sputtering/Selenization

Another method for synthesizing CIGS films is selenization of a stack or alloy of the constituent

metal films predeposited on a substrate in a predetermined stoichiometry. There are many

variations of this approach but the most common is a two-step process where the metals are

sputtered onto the substrate and then converted to CIGS through annealing in a chalcogen-

containing environment. Practitioners include Solar Frontier who have recently announced a

scaling up of their production to 900 Mw/yr, Avancis, Stion and TSMC Solar. The highest module

efficiency obtained by the two-step process is 12.6% (Solar Frontier). There are many pathways

and intermediates involved in transforming the metal into the chalcopyrite, requiring careful

optimization of the time-temperature-reactant profiles employed.78 This approach creates a

copper-indium-gallium-selenide-sulfide (CIGSS) film, with shifts the band gap towards the

maximum efficiency and also improves the interface with the window layer, which are also

sulfides (CdS, ZnS, In2S3).78

Non-vacuum and Solution-printing Processes

The third general approach to CIGS manufacturing has been to eliminate vacuum processing. In

general these are also two-step processes, application of a coating followed by a high

temperature step for annealing or sintering. Ostensible advantages include reduced capital

requirements, improved materials utilization, potentially lower energy requirements, and

compatibility with roll to roll (R2R) processing.82 A general challenge with the non-vacuum

27

based approaches is the potential of contamination introduced by either the compounds

themselves or the solvents employed. As such, it has been much harder to produce dense,

homogenous absorber layers. It is also more challenging to produce chemically-graded

structures with this technique. Record cell efficiencies trail co-evaporation and metal

selenization, but values up to 14% have been obtained by a number of techniques.

The non-vacuum strategies may be further divided into electrodeposition, particulate

deposition, and solution processes. Electrodeposition has been around for decades83 and

achieved cell efficiencies as high as 13.8%,84 but concerns about up scaling appear to have

limited commercial interest. The particulate route is currently the most actively pursued, with

variations employing particles composed of CIGS, metal, metal oxides, and/or metal selenides.

In all of these methods a coating of particles is first formed on the substrate surface and

reacted and/or sintered at high temperature to form the final film. It was found that CIGS

particles required excessive temperature for sintering.85 Likewise, problems with handling and

premature oxidation have limited the utility of metal particles. The best results have come

using slurries containing mixtures of metal oxide or selenide powders.86 This approach was

pioneered by Kapur and co-workers at ISET,87, 88 and more recently championed by Nanosolar.

The latter has reported 14% efficient cells,89 and has announced that 10-11% modules will be

available in the near future. Solution approaches have employed the use of soluble metal salts,

organometallics, and hydrazine-based compounds. Best results have been obtained with the

latter,90, 91 however the highly reactive and toxic nature of hydrazine poses additional

complications for manufacturing.

With five elements and numerous binary and ternary phases, the CIGSS system presents much

greater complexity than the PV technologies described previously3. Extensive theoretical work

has made great advances in understanding the electronic structure and role of defects in this

system.92 These studies have been aided by improvements in advanced characterization

techniques. Raman and time resolved photoluminescence are becoming useful for identifying

the presence of secondary phases and certain defects.81 It is well-known that sodium plays a

critical role in the morphology and electronic properties of CIGS. When soda lime glass

28

substrates are used, sodium diffuses into the CIGS layer from the glass. Once the importance of

Na was realized, more controlled and systematic approaches that employ sputtered layers of

Na-containing material have been developed to gain control over Na introduction into the CIGS.

There is also significant attention being paid to the window layers deposited on the CIGS

absorber. While CdS remains the leading choice, both indium and zinc sulfides are being

pursued and in some cases commercialized. Part of the interest is due to the desire to remove

Cd, but a second motivation is improving the blue response of these devices. There are strong

interactions between the buffer and the underlying absorber, and simultaneous optimization of

these layers is required for best performance. Some concerns remain about the use of ZnO as

the front TCO, and its potential impact on long-term device stability. Moisture exposure is

particularly detrimental, both to the TCO and the heterojunction itself. Encapsulation in glass

partially alleviates this effect, but further development of transparent ultra-barriers is required

to improve the long term stability of flexible CIGS solar cells.93

A longer-term concern is the availability and price of In. Recycling of indium will alleviate

constraints on CIGS long-term production, but research is needed to develop technologies for

efficient and low cost recycling of all the elements from the CIGS modules. The possibility of

substituting indium/gallium with earth abundant alternatives such as zinc/tin is discussed

below.

F. Copper Zinc tin Sulfide Selenide (CZTSS) Thin Films

There is interest in using “abundant” materials in thin-film photovoltaics, driven by concerns

about the availability of In, Te, Ga and Ge in the current 2nd generation thin-film PV. Such

materials include oxides and sulfides of base metals (e.g., Fe, Cu) that have band gaps in the

range of 1 – 2 eV. The most successful system to date has been copper-zinc-tin-sulfide

(selenide), or CZTS.125 Pioneered by Katagiri,125 in the past decade champion CZTS devices have

gone from less than 3% to approaching 10% efficiency (Fig. 5). These results have come with

only a handful of publications. CZTS shares great similarities with CIGS, including similar device

structures and fabrication techniques for the formation of the absorber layer. Initial studies

29

focused on sulfidization of metal layers,126 but more recently co-evaporation127 and non-vacuum

techniques128-130 have garnered significant attention. The current efficiency champion includes

Se and was derived from hydrazine precursors.130 The similarities to CIGS may have accelerated

CZTS solar cells’ initial success, but these same similarities may become limitations in the long

run.3

G. Dye-sensitized Solar Cells (DSC)

Dye-sensitized solar cells are based on the photo electrochemical effect discovered by Bequerel

in 1839. A relatively new concept, DSC was introduced by Grätzel and co-workers in 1991.99 A

comprehensive review on the complex chemistry and processes involved in this system was

recently published by Hagfelt and co-workers.100 This hybrid material is typically composed of

organometallic dye molecules adsorbed to a mesoporous titania nanoparticle film, with the

pore space filled by an electrolyte. In this structure light is absorbed by the dyes, which then

inject an electron into the conduction band of a wide band gap semiconductor like TiO2. The

electron is transported by hopping through the nanoparticle network to the front contact

where it exits and performs useful work before returning to the platinized back contact. Here

the electron reduces a redox couple, which in turn diffuses through the electrolyte and

regenerates a dye molecule to complete the cycle. Dye sensitization of oxides was well-known

at the time, and Grätzel’s key innovations were in creating a nanoparticle film with high surface

area to improve light harvesting and in choosing components with appropriate kinetics for fast

charge transfer and slow recombination. Grätzel’s group rapidly optimized the device to over

10% within a few years of its introduction.101 This brought the attention of industry and today a

number of small companies including G24i, Solarprint and Dyesol are engaged. Most current

products are directed at the consumer market; for example, DSC on flexible substrates that

replace rechargeable batteries for portable electronics. A unique feature of DSC is that their

performance improves under diffuse and low light conditions,102 enabling their use indoors and

without direct solar exposure. Devices can be fabricated in a number of colors and levels of

30

transparency, which is an attractive feature for architectural and BIPV applications.

Manufacturing can also be done at low temperature using flexible substrates.

It is noted that champion cell efficiency has been stagnant at ~11% for the past 15 years (Figure

5). The three main components in a DSC, the Ru-based dye, the photoanode, and the iodine-

based redox couple, have also remained unchanged. Further optimization of any one of these

components individually is not likely to yield significant improvements in efficiency. The recent

review by Hamann and co-workers103 provides an excellent overview of the complexity of the

issues involved. First, the leading dye does not capture much light past 750 nm, and harvesting

the red and near-infrared portion of the spectrum is needed to increase current densities.

Second, the I3-/I- redox couple is positioned with a 550 mV overpotential relative to dye

regeneration. An alternative redox couple could potentially allow the Voc to be improved by up

to 300 mV, but recombination rates are typically much faster with non-iodine redox couples. A

combination of these two changes could elevate device performance to > 16%. However as

cautioned by Hamann and colleagues,103 this will most likely require simultaneous optimization

of both dye and electrolyte and perhaps the development of new photoanodes with faster

charge transport as well. While there are photoanode designs based on wide band gap

semiconductor nanowires that attempt to improve efficiencies, six years after their first

introduction, the efficiencies remain low.

With respect to manufacturing numerous module fabrication strategies are being pursued,

which general can be divided into monolithic or sandwich constructions. The former offers

advantages with respect to materials cost, while the latter may be more amenable to R2R

processing. Substrates include glass, metal, and polymer foils, with best performance being

obtained on glass. Critical issues include stability and the production of large area modules. At

present mini-modules with areas < 100 cm2 are used, with resistance losses being a one of the

major challenges. The stability of a DSC module is strongly related to the device encapsulation.

Accelerated lifetime testing has indicated that carefully encapsulated glass-based DSC can last

for over 20 years, although current products using DSC on plastic substrates have lifetimes of

only a few years. For outdoor applications, the sealing material must, for example, be

31

mechanically and thermally stable, stable under UV exposure, and chemically inert to the

electrolyte. Moreover, it should prevent mass transport between adjacent cells. The issue so

important that Hagfelt et al. 100 suggested that the leading manufacturing approach for DSC may

be the one that provides the most functional encapsulation method. Replacing the liquid

electrolyte with a gel or solid would greatly improve encapsulation issues, but these changes

have resulted in decreased efficiency. Elimination of glass, implementation of R2R

manufacturing methods, and increased lifetimes will be critical to economics, particularly if

device efficiency remains below 12%.

H. Organic PV (OPV)

Spurred by the elemental abundance of carbon, extensive efforts to develop solar cells using

organic semiconductors are underway. Brabec and colleagues104 recently provided a

comprehensive review of the developments in OPV over the past decade and the challenges

that lie ahead. Figure 5 charts the progress of champion cell efficiencies for the past 15 years.

While most technologies have been relatively stagnant in their champion efficiency, organic PV

has made great strides in the past decade, with Heliatek and Konarka being the current

champions, each with devices certified at 8.3%.105, 106 Unfortunately Konarka run out of cash

and went out of business in June of 2012. Others leading the development of OPV for small

devices include Solamer and Plextronics who held the efficiency record in recent years. OPV

devices are comprised of a heterojunction between an electron donor molecule (e.g., P3HT,

poly(3-hexylthiophene) or CuPC, copper phthalocyanine) and an electron acceptor molecule

(e.g., C60 or its derivatives such as PCBM, phenyl-C61-butyric acid methyl ester).107 The

essentially limitless varieties of candidate organic semiconductor materials may be categorized

as either solution-processable (polymers, dendrimers, oligomers, or small molecules) or

vacuum deposited (small molecules or oligomers). Although superficially similar to inorganic p-n

junctions, the OPV junction is fundamentally different. Instead of directly creating an e-/h+ pair,

photon absorption produces an exciton, an uncharged excited state that must diffuse to a

donor/acceptor interface in order to dissociate into a free e-/h+ pair. In organic materials the

32

exciton can typically only diffuse 5-10 nm before decaying to the ground state, a problem that

limits performance and is typically referred to as the exciton bottleneck. There are two ways to

deal with this. One can make a multilayer device that uses very thin donor/acceptor layers such

that a majority of excitons can diffuse to a heterojunction interface.107 This approach is

commonly used in vacuum deposited devices. Or one can reduce the distance the exciton has

to diffuse before reaching the heterojunction by mixing the donor and acceptor materials on a

nanometer length scales to form a single-layer interpenetrating bicontinuous network called a

bulk heterojunction. This approach is commonly used in solution-processable materials.

In the OPV device structure, the heterojunction active layer(s) is(are) sandwiched between a set

of contact electrodes, with buffer layers likely to be present. In the bulk heterojunction

approach an asymmetry in the device must be imposed by either using electrodes of different

work function (typically a front TCO contact modified with a conducting polymer PEDOT-PSS,

poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate) and a back contact metal of Ca or Al)

or by inserting a buffer layer that blocks carriers from leaving one side of the device. An oxide

buffer layer is commonly inserted to block holes from leaving the device through the front TCO

contact, which inverts the direction of operation of the device and allows the use of a high

workfunction Ag back contact.108-110. In the vacuum deposited multilayer approach, co-doping of

buffer layers has been used to great effect to produce a true p-i-n structure that obviates the

need for a mismatch in the contact work-functions.111

There are several challenges to improve the efficiency of organic solar cells.104, 112 These are

being addressed through the development of novel donor and acceptor materials, new buffer

layer and electrode geometries, innovative processing, and through the use of tandem

architectures. A key issue is to significantly raise the short circuit currents (Jsc) to above 20

mA/cm2. Present values are typically 10-12 mA/cm2 with champion values approaching 17

mA/cm2.104 The main problem is that leading photoactive layers do not efficiently harness

photons in the red and infrared region of the solar spectrum. Significant efforts have been

directed at developing improved low band gap polymers.113, 114 Advanced photon management

33

strategies are also being pursued to increase optical density.115 A second challenge is to

increase the open circuit voltage. Key to this is achieving optimal band alignment of the device

structure and minimizing the band offset between donor and acceptor molecules while

retaining efficiency charge transfer. 113,116 It is predicted that the maximal Voc in a standard

donor-acceptor device is 0.6 V less than the bandgap energy/e. Thus the goal is for a Voc of 0.8

– 0.9 V for low band gap absorbers with band gaps ~1.4 – 1.5 eV. Voc’s above 1 V have been

achieved, but only with high band gap materials. Third, the fill factors (FF) have to be increased

beyond 0.7, which has been achieve in only a few champion devices.104 Organic solar cells

typically have poor FF relative to conventional p-n junctions. This is due to high series resistance

and/or carrier recombination as the carrier mobilities in organic thin films are lower than their

inorganic counterparts.

Simultaneous achievement of Jsc = 20 mA/cm2, Voc = 0.8 V, and FF = 0.7, leading to an efficiency

of 11% has not been achieved yet. Doing this in a single junction device will require

simultaneous optimization of all the materials and interfaces. A possibly faster route to this goal

will involve the use of tandem configurations.117 These have been demonstrated using the bulk

heterojunction approach and are being used to effectively boost efficiency in the evaporated

small molecule approach as implemented for instance by Heliatek. Passing the psychological

milestone of 10% efficiency could bring organic solar cells within striking distance of the existing

thin film technologies, particularly because manufacturing costs are expected to be low.104 Even

with existing materials and devices, the energy payback time for OPV has been estimated at 0.3

years.118 However, an efficiency of closer to 15% may be needed to achieve a true grid parity

LCOE of ~$0.07/kWh.119 Much work still needs to be done to demonstrate acceptable

performance in large area modules. 120 At present OPV submodule (200 cm2) efficiencies from

leading companies are approaching 4%.121 This value lags substantially behind the 9.2%

efficiency in comparable sized DSC modules.13 Also, published champion OPV devices are

fabricated on glass. To be economical, the substrate will likely need to be a low cost flexible

material that is suitable for R2R processing.

Another important issue that has to be resolved is the stability of organic solar cells. The

chemical, physical and mechanical degradation that are predominant in OPV materials and

34

devices have been well discussed.122 The list of failure mechanisms of OPV cells is long and

certainly as extensive as for any other photovoltaic technology. Major issues include

photodegradation and the sensitivity of OPV components to oxygen, requiring the use of

ultrabarriers for encapsulation. The current goal is to increase lifetimes from 3 to 10 years,

which is expected to be sufficient for consumer applications.104 Due to the flexibility of organic

synthesis, it can be estimated that there are on the order of 1013 different material

combinations that could be employed. Whether the right combination of properties (e.g., band

gap, charge mobility, exciton diffusion length, etc.) exists and how to identify them remain

open questions. Optimizing the photoactive organic layer may be best addressed using a

combinatorial approach. On the other hand, candidate structures and trends may be identified

using a rational method that combines computational methods with targeted synthesis.

4. Future Outlook

As discussed earlier, there is a large gap between the module and the cell efficiency for all

currently commercial PV technologies which can likely be narrowed with manufacturing R&D.

For crystalline silicon modules, efficiencies can be improved by maximizing the collection of

current, since little can be done to further improve the voltage. Related efforts include

improvements in front side texturing, integration of back side reflectors, and the use of

advanced AR coatings.37 Another strategy is to reduce the level of shadowing associated with

front contacts by either reducing the line-widths or by completely eliminating them using back

side contacting schemes such as those employed in Sunpower’s high efficiency modules.38 Also

the limitation of quantum efficiency in the blue region of the spectrum can be addressed with

selective emitter designs and improved passivation strategies.39 Through a combination of

these strategies it is expected that micro-crystalline-Si modules will exceed 20% within the

decade, and mono-crystalline -Si modules would approach 25%.3 Currently the best efficiencies

in nono-crystalline Si modules are slightly above 20% for modules manufactured by SunPower

and Sanyo.

35

The next few years will prove critical to amorphous silicon technology’s long-term viability.

While necessary to improve stability, the transition from a-Si to nc-Si has come at an expense.

Due to its relatively low absorption coefficient,69 nc-Si based devices need to be up to five times

thicker than a-Si to collect sufficient light. This issue is exacerbated by the fact that the

deposition rates for nc-Si are much lower than those for a-Si.70 Combined with the relatively low

efficiencies, this has made manufacturing of a-Si/nc-Si based solar cells prohibitively expensive

when compared to alternative technologies such as CdTe and recently with cheap imports of c-

Si modules from China. Efforts to improve deposition rates include: a) use of very high

frequency (VHF: 25 – 100 MHz) plasma sources; b) operation at higher pressures and c)

development of linear plasma sources to maintain large area uniformity with VHF

modulation.71-73 Barring a significant breakthrough, a/nc-Si may need to focus on market sectors

that benefit from its low temperature, low weight capability. Another strategy might be to

examine if a-Si could be used as a route to form ut-Si, perhaps by coupling with rapid thermal

processing.3

The improvement of the CdTe PV technology is expected to continue as first solar continues to

invest in R&D and others are following their lead (e.g., PrimeStar, Abound). Through

improvements in efficiency and further integration of manufacturing CdTe manufacturing costs

could drop to $0.5/Wp within this decade.

However, the c-Si industry has shown that they can also rapidly reduce costs, and it is important

to note that silicon PV module prices does not have to be reduced as much as CdTe, if their

efficiency is high. As module costs drop below $1/Wp they become increasingly less important,

as balance of system issues including inverters, racks, installation, and space become important

cost drivers. These costs drop with module efficiency, and thus silicon can afford to remain

competitive despite higher manufacturing costs than CdTe. It is too early to project what may

happen to CIGS, though it is plausible to expect that it might end up in a similar region, most

likely somewhere between CdTe and c-Si.3

36

There have been numerous scientific breakthroughs in the area of 3rd generation PV,132-135 but

practical devices are far from the point of commercialization.136, 137 In general 3rd generation PV

rely on the use of nanostructures such as quantum dots and nanowires to generate the desired

effects. A critical yet unresolved problem with devices that employ such structures is that they

will be dominated by interfaces, the anathema of PV technology.3 Interfaces typically serve as

either recombination centers or barriers to charge transport, and the demonstrated pathway to

high efficiency has been through their elimination. Record heteroepitaxial multi-junction cells

are produced by molecular beam epitaxy.138 The detrimental impact of interfaces is quite

plainly seen by comparing the performance within the silicon system (in terms of performance

c-Si > mc-Si > nc-Si > a-Si). Likewise, record CdTe and CIGS thin film devices are characterized by

their large grain size.139 Development of optimized manufacturing techniques requires

sophisticated modeling to understand how to maintain uniformity with respect to both area

and time. Accompanying this goal is the development of in-line diagnostics for real time process

control. Developments in intelligent and potentially self-correcting control of process flow

would help enable and accelerate throughput volume.

5. Sustainability –Life Cycle Analysis

Photovoltaics do not need any fuel to produce electricity but energy is needed for generating

their materials, cells, modules, and systems. As in all types of products and systems, a complete

evaluation of their environmental profile must be done under the framework of a life-cycle

analysis.140 The life-cycle of photovoltaics starts from the extraction of raw materials (“cradle”)

and ends with the disposal (“grave”) or the recycling and recovery (“cradle”) of the PV’s

components (Figure 14).

37

E

M, Q

M, Q: Material and energy inputs

EEEEE

M, QM, QM, QM, QM, Q

E

M, Q

Recycling

Operation

Treatment /Disposal

Decommis-sioningManufactur-

ingMaterial

Processing

Raw

Material

Acquisition

Figure 14: Flow of the life-cycle stages, energy, materials, and effluents for PV systems

The mining of the raw materials, for example, quartz sand for silicon PVs, copper-, zinc-, and

aluminum-ores for mounting structures and thin-film semiconductors, is followed by the

multiple stages of separation and purification. The silica in the quartz sand is reduced in an arc

furnace to metallurgical-grade silicon that must be purified further into solar-grade silicon (i.e.,

99.999% purity), requiring significant amounts of energy. Metallurgical-grade cadmium,

tellurium, indium, gallium, and selenium for CdTe and CIGS PV primarily are obtained as

byproducts of zinc-, copper-, and lead-smelting, and then further purified to solar grades. The

raw materials include those for encapsulations and balance-of-system components, for

example, silica for glass, copper ore for cables, and iron- and zinc-ores for mounting structures.

The production of all these materials requires large amounts of energy, as does the

manufacture of the solar cells, modules, electronics, and structures, their installation,

operation, and eventually their dismantling and recycling141 or disposal.

Thus, the energy payback time (EPBT) is defined as the period required for a renewable energy

system to generate the same amount of energy (in terms of primary-energy equivalent) that

was used to produce the system itself.

Energy Payback Time (EPBT) = (Emat+Emanuf+Etrans+Einst+EEOL) / (Eagen – Eaoper)

where,

Emat : Primary energy demand to produce materials comprising PV system

Emanuf : Primary energy demand to manufacture PV system

Etrans : Primary energy demand to transport materials used during the life cycle

Einst : Primary energy demand to install the system

EEOL : Primary energy demand for end-of-life management

Eagen : Annual electricity generation in primary energy terms

38

Eaoper : Annual energy demand for operation and maintenance in primary energy terms

An indicator more commonly used for comparing different types of energy-production

technologies is the Energy Return on Energy Investment (EROI) that quantifies the benefit that

the user gets out of exploiting an energy source. Usually, it is expressed as the dimensionless

ratio of the energy generated from a system over that energy, or its equivalent from some

other source, that is “invested” in extracting, growing, or processing a new unit of the energy in

question. Thus, EROI even can be used for fuel-based power production that never pays back

its energy requirement as it continuously needs energy in the form of depletable fuel to

operate. The traditional way of calculating the EROI of PV is EROI = lifetime/ EPBT; thus, an

EPBT of 1 year and a life expectancy of 30 years corresponds to EROI of 30:1. The EROI of

conventional thermal generation from fossil fuels has been viewed as much higher than those

of photovoltaics; this recently was shown to be a misconception fostered by using outdated

data and a lack of consistency among calculation methods. 142

Several published PV LCA studies give differing estimates of the EROI. Such divergence reflects

the varied assumptions about key parameters, like product design, solar irradiation,

performance ratio, and lifetime. These assessments also deviate because of the different types

of installation used, such as ground mounts, rooftops, and façades. Also, assessments still are

calculated from outdated information in the literature collected from antiquated PV systems.

An example of such misrepresentation of the current reality was apparent in a recent PE

Magazine article wherein the author stated that “…photovoltaic electricity generation cannot

be an energy source for the future because photovoltaics require more energy than they

produce during their lifetime”. Statements to this effect were not uncommon in the 1970s

based on some early PV prototypes. However, today’s PVs return far more energy than that

embodied in the life cycle of a solar system; Fig.15 illustrates this historical trend to betterment.

To resolve these inconsistencies in the estimates, the International Energy Agency (IEA) PVPS

Task 12 published the “Methodology Guidelines on Life Cycle Assessment of Photovoltaic

Electricity” for conducting balanced, transparent, and accurate LCAs . 143 The results, presented

39

in Fig. 16, were obtained following these Guidelines and the associated Life Cycle Inventory

(LCI) data144; they represent consensus between LCA experts from the ten Task 12 member

countries.

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 20150.5

5

50

mono-c-Simulti-c-SiCdTemono-c-Simulti-c-SiCdTe

Fig. 15. EPBTs of various PV systems were reduced from about 40 yrs to 0.5 yrs from 1970 to

2010

Figure 16 plots the Energy payback times (EPBTs) of three major types of commercial PV

modules , i.e., mono-Si, multi-Si, and cadmium telluride (CdTe).

mono-Si PV 14% multi-Si 13.2% CdTe PV 10.9% 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

BOSFrameModule

Figure 16: Energy payback times (EPBTs) of rooftop mounted PV systems for U.S- and European-

production and installation under average U.S. irradiation of 1800 kWh/m2/yr (4.9

40

kWh/m2/day), a performance ratio of 0.75, and the module efficiencies shown above. Data

adapted from de Wild Scholten (2009) and Fthenakis et al. (2009); note that module efficiencies

have increased since 2009, and, correspondingly, EPBTs have decreased.

These results are based on detailed process data obtained through collaborations with thirteen

European- and US-PV manufacturers. 145,146 The EPBT for the same type of systems installed

in the US-SW were decreased in proportion to the solar irradiation ratio (1800/2380) between

the US average- and SW-solar conditions. Thus, for SW irradiation, the EPBTs for the three PV

technologies shown in Fig. 16 are 1.3-, 1.3- and 0.5-years and their corresponding EROIs are

23:1, 23:1 and 60:1.

Fig.17. Energy PaybackTimes for different insolation levels in the United States. This Solar

Resource map was produced by B. Roberts, National Renewable Energy Laboratory for the U.S.

Department of Energy; the colors show annual averages of daily insolation for the south facing

latitude-tilt plane. The EPBTs were estimated by V. Fthenakis, Brookhaven National Laboratory.

41

6. The Intermittency Challenge

The short-term variability of solar power recently garnered much attention because the

installed capacity is increasing very rapidly, and the technology is on its way to become a

significant part of the generator portfolio (power capacity) of several countries, notably

Germany (12%, 2010), and Spain (4.3%, 2010) (German Energy Ministry, 2011; Red Electrica De

Espana, 2010). German Energy Ministry. (2011). Erneuerbare Energien 2010. Vierteljahrshefte

zur Wirtschaftsforschung (Vol. 76). doi:10.3790/vjh.76.1.35

Red Electrica De Espana. (2010). Red Electrica De Espana. Retrieved from http://www.ree.es/

Variability can be smoothened by aggregating solar generation over large geographical regions

and by combining solar and wind generation, since in many places one compliments the other

(e.g., more wind at night than during the day and more in the winter than in the summer). This

has been illustrated for New York State 147 and the ERCOT grid system.148

7. Conclusions

Although recent market uncertainties have slowed down PV production in 2012, the long-term

potential of the technology is clearly upward, and it would accelerate when electricity

production by PV reaches parity with conventional electric power generation technologies in

large regions of the world. It is expected that crystalline silicon will continue dominating the

market for the foreseeable future. However, significant opportunities remain for thin-film

photovoltaics, mainly through continued increase in module efficiencies. If investments in CdTe

and CIGS thin-film PV technologies continue, it is not unreasonable to expect that the module

efficiencies in commercial production will reach those of the high efficiency crystalline-silicon

PV modules. Pending further improvements in efficiency, CdTe’s market share could evolve to

anywhere between 10 – 25%. After decades of being the leading thin film technology, the

prospects of amorphous silicon making a major contribution to the utility sector appear

severely constrained by high manufacturing costs and low efficiency, with the exit of UniSolar

42

from the market being the evidence of the technology lacking competitive merit. However, the

a-Si/nc-Si (micromorph) architecture is still holding up. CIGS has gained a foothold in PV

manufacturing at ~100 MW/year and higher (Solar Frontier) and the demonstrated potential in

efficiency provides reason for guarded optimism. The next five years will be critical for CIGS to

demonstrate robust manufacturing to allow it to break out and join CdTe as a major player in

the global PV market. Although encouraging industrial progress has been made, DSC and OPV

are still limited at present by low efficiency and stability. Konarka, the US front-runner on OPV

recently exited the market as their production costs were relatively high.

Concerning the future, the future of solar PV still looks bright. The industry has consistently

been able to lower the cost of solar panels. This author believes that this trend can be

maintained, and if subsidies are continued for the next 10 years, there is a real prospect for

solar to become cost competitive without any subsidies. What is needed more than anything

else at this critical conjuncture is the maintenance of federal tax incentives and state

“renewable portfolio standard” incentives or the enactment of feed-in-tariffs following

Germany’s example. With solar replacing the current fossil-fuel and nuclear based power

infrastructure, the environmental benefits, which are not accounted in the current cost

structures, will be enormous.

References

[1] Zweibel K., Mason J. and Fthenakis V. Solar Grand Plan: Solar as a Solution, Sun&Wind

Energy, 4 (2008) 112-117.

[2] Fthenakis V. Mason J. and Zweibel K., The Technical, Geographical and Economic

Feasibility for Solar Energy to Supply the Energy Needs of the United States, Energy

Policy, 37, 387-399, 2009Fthenakis

[3] Wolden C, Kurtin J., Baxter J., Repins I., Shaheen S., Torvik J., Rockett A., Fthenakis V.,

Aydil E. , Photovoltaic Manufacturing: Present Status and Future Prospects, J. Vac. Sci.

Technolo. A 29(3), 030801-16, 2011.

[4] Markvart T., Solar Electricity, 2nd edition, Wiley, 2000

43

[5] Honsberg, C. B., R. Corkish, and S. P. Bremner, "A New Generalized Detailed Balance

Formulation to Calculate Solar Cell Efficiency Limits", 17th European Photovoltaic Solar

Energy Conference, pp. 22-26, 2001.

[6] Swanson, R., "Approaching the 29% limit efficiency of silicon solar cells", Thirty-First IEEE

Photovoltaic Specialists Conference, Lake buena Vista, FL, USA, 01/2005, pp. 889-94,

2005.

[7] Kim, H.Y., Preparation of Polysilicon for Solar Cells. Korean Chemical Engineering Research,

2008. 46(1): p. 37-49.

[8] Filtvedt, W.O., et al., Development of fluidized bed reactors for silicon production. Solar

Energy Materials & Solar Cells, 2010. 94: p. 1980-1995.

[9] Solar&Energy. Polysilicon production method. 2011 [cited 2011 November 8]; Available

from: http://www.solarnenergy.com/eng/info/column.php.

[10] Alsema, E. and M.J. de Wild-Scholten, Reduction of the Environmental Impacts in

Crystalline Silicon Module Manufacturing, in 22nd European Solar Energy Conference.

2007: Milan, Italy.

[11] REC. REC's Lluidized Bed Reactor (FBR) Process. 2011 [cited 2011 November 8];

Available from: http://www.recgroup.com/en/tech/FBR/.

[12] Frost & Sullivan, Assessment of Polysilicon Global Market and it's Fabrication

Technology Landscape. 2010. http://www.scribd.com/doc/56217734/Frost-Sullivan-

Polysilicon-2010.

[13] M. A. Green, K. Emery, Y. Hishikawa, and W. Warta, "Solar cell efficiency tables (version

36)," Prog. Photovolt: Res. Appl. 18, 346-352, (2010).

[14] D. S. Kim, V. Yelundur, K. Nakayashiki, B. Rounsaville, V. Meemongkolkiat, A. M. Gabor,

and A. Rohatgi, "Ribbon Si solar cells with efficiencies over 18% by hydrogenation of

defects," Solar Energy Mater. Solar Cells 90, 1227-1240, (2006).

[15] S. Solar, "EFG-Solar Cell Product Specification Sheet," in May 1st, ed, 2010.

[16] M. A. Green, "Third generation photovoltaics: Ultra-high conversion efficiency at low

cost," Prog. Photovolt: Res. Appl. 9, 123-135, (2001).

[17] W. Shockley and H. J. Queisser, "Detailed balance limit of efficiency of p-n junction solar

cells," J. Appl. Phys. 32, 510-519, (1961).

44

[18] G. del Coso, I. Tobias, C. Canizo, and A. Luque, "Temperature homogeneity of polysilicon

rods in a Siemens reactor," J. Cryst. Growth 299, 165-170, (2007).

[19] S. Pizzini, "Towards solar grade silicon: Challenges and benefits for low cost

photovoltaics," Solar Energy Mater. Solar Cells 94, 1528-1533, (2010).

[20] T. F. Ciszek, "Edge-defined, film-fed growth (EFG) of silicon ribbons," Mater. Res. Bull. 7,

731-737, (1972).

[21] T. F. Ciszek, J. L. Hurd, and M. Schietzelt, "Filament materials for edge-supported pulling

of silicon sheet crystals," J. Electrochem. Soc. 129, 2838-2843, (1982).

[22] J. P. Kalejs, "Modeling contributions in commercialization of silicon ribbon growth from

the melt," J. Cryst. Growth 230, 10-21, (2001).

[23] A. G. Aberle, "Overview on SiN surface passivation of crystalline silicon solar cells," Solar

Energy Mater. Solar Cells 65, 239-248, (2001).

[24] G. P. Smestad, Optoelectronics of Solar Cells. Bellingham, WA: SPIE Press, 2002.

[25] M. Wolf, "Proceedings of the 14th IEEE PVSC," San Diego, 1980, p. 674.

[26] M. J. Kerr, J. Schmidt, and A. Cuevas, "Lifetime and efficiency limits of crystalline silicon

solar cells," in Proceedings of the 29th IEEE PVSC, New Orleans, 2002, p. 250.

[27] M. A. Green, "Limiting efficiency of bulk and thin-film silicon solar cells in the presence

of surface recombination," Progress Photovolt: Res. Appl. 7, 327-330, (1999).

[28] A. Luque, A. Cuevas, and J. M. Ruiz, "DOUBLE-SIDED N+-P-N+ SOLAR-CELL FOR BIFACIAL

CONCENTRATION," Solar Cells 2, 151-166, (1980).

[29] K. A. Munzer, K. T. Holdermann, R. E. Schlosser, and S. Sterk, "Thin monocrystalline

silicon solar cells," IEEE Trans. Electron Dev. 46, 2055-2061, (1999).

[30] V. Depauw, I. Gordon, G. Beaucarne, J. Poortmans, R. Mertens, and J. P. Celis, "Large-

area monocrystalline silicon thin films by annealing of macroporous arrays:

Understanding and tackling defects in the material," J. Appl. Phys. 106, 033516, (2009).

[31] F. Dross, J. Robbelein, B. Vandevelde, E. Van Kerschaver, I. Gordon, G. Beaucarne, and J.

Poortmans, "Stress-induced large-area lift-off of crystalline Si films," Appl. Phys. a-

Mater.Sci. Process. 89, 149-152, (2007).

[32] C. Podewils, "A proton axe for thin silicon," Photon International May, 116-120, (2009).

45

[33] M. J. Kerr and A. Cuevas, "Very low bulk and surface recombination in oxidized silicon

wafers," Semicond. Science Technol. 17, 35-38, (2002).

[34] J. Schmidt, A. Merkle, R. Brendel, B. Hoex, M. C. M. van de Sanden, and W. M. M.

Kessels, "Surface passivation of high-efficiency silicon solar cells by atomic-layer-

deposited Al2O3," Prog. Photovolt: Res. Appl. 16, 461-466, (2008).

[35] D. Shir, J. Yoon, D. Chanda, J. H. Ryu, and J. A. Rogers, "Performance of ultrathin silicon

solar microcells with nanostructures of relief formed by soft Imprint lithography for

broad band absorption enhancement," Nano Letters 10, 3041-3046, (2010).

[36] S. Pillai and M. A. Green, "Plasmonics for photovoltaic applications," Solar Energy Mater.

Solar Cells 94, 1481-1486, (2010).

[37] P. C. Rowlette and C. A. Wolden, "Digital control of SiO2-TiO2 mixed-metal oxides by

pulsed PECVD," ACS Appl. Mater. Interfaces 1, 2586-2591, (2009).

[38] S. Eglash, "Competition improves silicon-based solar cells," Laser Focus World 45, 39-44,

(2009).

[39] P. Engelharet, S. Hermann, T. Neubere, H. Plagwitz, R. Grischke, R. Meyd, U. Klug, A.

Schoonderbeek, U. Stute, and R. Brendel, "Laser ablation of SiO2 for locally contacted si

solar cells with ultra-short pulses," Prog. Photovolt. 15, 521-527, (2007).

[40] D. A. Cusano, "CdTe solar cells and photovoltaic heterojunctions in II-VI compounds,"

Solid-State Electron. 6, 217-218, (1963).

[41] C. Ferekides and J. Britt, "Thin film CdS/CdTe solar cell with 15.8% efficiency," Appl. Phys.

Lett. 62, 2851-2852, (1993).

[42] R. F. Brebrick and A. J. Strauss, "Partial pressures and gibbs free energy of formation for

congruently subliming CdTe," J. Phys. Chem. Solid 25, 1441-1445, (1964).

[43] J. M. Kestner, S. McElvain, S. Kelly, L. M. Woods, T. R. Ohno, and C. A. Wolden, "An

experimental and modeling analysis of vapor transport deposition of cadmium

telluride," Solar Energy Mat. Solar Cells 83, 55-65 (2004).

[44] P. V. Meyers and S. P. Albright, "Technical and economic opportunities for CdTe PV at

the turn of the millennium," Progress Photovolt: Res. Appl. 8, 161-169, (2000).

46

[45] X. Wu, "High-efficiency polycrystalline CdTe thin-film solar cells," Solar Energy 77, 803-

814, (2004).

[46] A. D. Compaan, A. Gupta, S. Lee, S. Wang, and J. Drayton, "High efficiency, magnetron

sputtered CdS/CdTe solar cells," Solar Energy 77, 815-822, (2004).

[47] X. Wu, S. Asher, D. H. Levi, D. E. King, Y. Yan, T. A. Gessert, and P. Sheldon,

"Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline

thin-film solar cells," J. Appl. Phys. 89, 4564-4569, (2001).

[48] N. Naghavi, D. Abou-Ras, N. Allsop, N. Barreau, S. Bucheler, A. Ennaoui, C. H. Fischer, C.

Guillen, D. Hariskos, J. Herrero, R. Klenk, K. Kushiya, D. Lincot, R. Menner, T. Nakada, C.

Platzer-Bjorkman, S. Spiering, A. N. Tiwari, and T. Torndahl, "Buffer layers and

transparent conducting oxides for chalcopyrite Cu(In,Ga)(S,Se)(2) based thin film

photovoltaics: present status and current developments," Prog. Photovoltaics 18, 411-

433, (2010).

[49] M. Carmody, S. Mallick, J. Margetis, R. Kodama, T. Biegala, D. Xu, P. Bechmann, J. W.

Garland, and S. Sivananthan, "Single-crystal II-VI on Si single-junction and tandem solar

cells," Appl. Phys. Lett. 96, 153502-3, (2010).

[50] W. K. Metzger, I. L. Repins, and M. A. Contreras, "Long lifetimes in high-efficiency

Cu(In,Ga)Se2 solar cells," Appl. Phys. Lett. 93, 022110-3, (2008).

[51] J. Sites and J. Pan, "Strategies to increase CdTe solar-cell voltage," Thin Solid Films 515,

6099-6102, (2007).

[52] M. Beck, "Personal Communication," N. P. Workshop, Ed., ed, 2010.

[53] D. Cahen and R. Noufi, "SURFACE PASSIVATION OF POLYCRYSTALLINE, CHALCOGENIDE

BASED PHOTOVOLTAIC CELLS," Solar Cells 30, 53-59, (1991).

[54] H. R. Moutinho, M. M. Al-Jassim, D. H. Levi, P. C. Dippo, and L. L. Kazmerski, "Effects of

CdCl2 treatment on the recrystallization and electro-optical properties of CdTe thin

films," J. Vac. Sci. Technol. A 16, 1251-1257, (1998).

[55] D. Grecu and A. D. Compaan, "Photoluminescence study of Cu diffusion and

electromigration in CdTe," Appl. Phys. Lett. 75, 361-363, (1999).

47

[56] Y. Yan, D. Albin, and M. M. Al-Jassim, "Do grain boundaries assist S diffusion in

polycrystalline CdS/CdTe heterojunctions?," Appl. Phys. Lett. 78, 171-173, (2001).

[57] S. Erra, C. Shivakumar, H. Zhao, K. Barri, D. L. Morel, and C. S. Ferekides, "An effective

method of Cu incorporation in CdTe solar cells for improved stability," Thin Solid Films

515, 5833-5836, (2007).

[58] T. A. Gessert, S. Asher, S. Johnston, M. Young, P. Dippo, and C. Corwine, "Analysis of

CdS/CdTe devices incorporating a ZnTe:Cu/Ti Contact," Thin Solid Films 515, 6103-6106,

(2007).

[59] V. M. Fthenakis, M. Fuhrmann, J. Heiser, A. Lanzirotti, J. Fitts, and W. Wang, "Emissions

and encapsulation of cadmium in CdTe PV modules during fires," Progress Photovolt:

Res. Appl. 13, 713-723, (2005).

[60] H. Steinberger, "Health, safety and environmental risks from the operation of CdTe and

CIS thin-film modules," Progress Photovolt: Res. Appl. 6, 99-103, (1998).

[61] D. E. Carlson and C. R. Wronski, "Amorphous silicon solar cell," Appl. Phys. Lett. 28, 671-

673, (1976).

[62] A. H. Mahan, Y. Xu, E. Iwaniczko, D. L. Williamson, B. P. Nelson, and Q. Wang,

"Amorphous silicon films and solar cells deposited by HWCVD at ultra-high deposition

rates," J. Non-Crystal. Solids 299-302, 2-8, (2002).

[63] A. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C.

Droz, and J. Bailat, "Thin-film silicon solar cell technology," Prog. Photovolt: Res. Appl.

12, 113-142, (2004).

[64] D. L. Staebler and C. R. Wronski, "Reversible conductivity charge in discharge-produced

amorphous Si," Appl. Phys. Lett. 31, 292-294, (1977).

[65] S. Veprek and V. Marecek, "The preparation of thin layers of Ge and Si by chemical

hydrogen plasma transport," Solid-State Electronics 11, 683-684, (1968).

[66] A. Shah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsch, J. Guillet, and U. Graf,

"Microcrystalline silicon and micromorph tandem solar cells," Thin Solid Films 403-404,

179-187, (2002).

48

[67] J. Yang, A. Banerjee, and S. Guha, "Triple-junction amorphous silicon alloy solar cell with

14.6% initial and 13.0% stable conversion efficiencies," Appl. Phys. Lett. 70, 2975-2977,

(1997).

[68] M. Taguchi, H. Sakata, Y. Yoshimine, E. Maruyama, A. Terakawa, M. Tanaka, and S.

Kiyama, "An approach for the higher efficiency in the HIT cells," in Conference Record of

the 31st IEEE Photovoltaic Specialists Conference, 2005, pp. 866-871.

[69] A. Poruba, A. Fejfar, Z. Remes, J. Springer, M. Vanecek, J. Kocka, J. Meier, P. Torres, and

A. Shah, "Optical absorption and light scattering in microcrystalline silicon thin films and

solar cells," J. Appl. Phys. 88, 148-160, (2000).

[70] T. Roschek, T. Repmann, J. Muller, B. Rech, and H. Wagner, "Comprehensive study of

microcrystalline silicon solar cells deposited at high rate using 13.56 MHz plasma-

enhanced chemical vapor deposition," J. Vac. Sci. Technol. A 20, 492-498, (2002).

[71] M. Fukawa, S. Suzuki, L. H. Guo, M. Kondo, and A. Matsuda, "High rate growth of

microcrystalline silicon using a high-pressure depletion method with VHF plasma," Solar

Energy Mater. Solar Cells 66, 217-223, (2001).

[72] Y. Mai, S. Klein, R. Carius, J. Wolff, A. Lambertz, F. Finger, and X. Geng, "Microcrystalline

silicon solar cells deposited at high rates," J. Appl. Phys. 97, 2005).

[73] J. Rudiger, H. Brechtel, A. Kottwitz, J. Kuske, and U. Stephan, "VHF plasma processing for

in-line deposition systems," Thin Solid Films 427, 16-20, (2003).

[74] L. L. Kazmerski, F. R. White, and G. K. Morgan, "Thin-film CuInSe2/CdS heterojunction

solar cells," Appl. Phys. Lett. 29, 268-270, (1976).

[75] J. H. L. Stolt, J. Kessler, M. Ruckh, K.-O. Velthaus, ans H.-W. Schock, "ZnO/CdS/CuInSe2

thin-film solar cells with improved performance," Appl. Phys. Lett. 62, 597-599, (1993).

[76] A. M. Gabor, J. R. Tuttle, D. S. Albin, M. A. Contreras, R. Noufi, and A. M. Herman, "High-

efficiency CuInxGa1-xSe2 solar cells made from (InxGa1-x)2Se3 precursor films," Appl. Phys.

Lett. 65, 198-200, (1994).

[77] A. N. Tiwari, D. Lincot, and M. Contreras, "The time for CIGS," Progress Photovolt: Res.

Appl. 18, 389-389, (2010).

49

[78] S. Niki, M. Contreras, I. Repins, M. Powalla, K. Kushiya, S. Ishizuka, and K. Matsubara,

"CIGS absorbers and processes," Prog. Photovolt: Res. Appl. 18, 453-466, (2010).

[79] M. A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, F. Hasoon, and R. Noufi, "Progress

toward 20% Efficiency in Cu(In,Ga)Se2 polyscrystalline Thin film solar cell," Prog.

Photovolt: Res. Appl. 7, 311, (1999).

[80] K. Ramanathan, G. Teeter, J. C. Keane, and R. Noufi, "Properties of high-efficiency

CuInGaSe2 thin film solar cells," Thin Solid Films 480, 499-502, (2005).

[81] R. Scheer, A. Pérez-Rodríguez, and W. K. Metzger, "Advanced diagnostic and control

methods of processes and layers in CIGS solar cells and modules," Prog. Photovolt: Res.

Appl. 18, 467-480, (2010).

[82] C. J. Hibberd, E. Chassaing, W. Liu, D. B. Mitzi, D. Lincot, and A. N. Tiwari, "Non-vacuum

methods for formation of Cu(In, Ga)(Se, S)2 thin film photovoltaic absorbers," Prog.

Photovolt: Res. Appl. 18, 434-452, (2010).

[83] R. N. Bhattacharya, "SOLUTION GROWTH AND ELECTRODEPOSITED CULNSE2 THIN-

FILMS," J. Electrochem. Soc. 130, 2040-2042, (1983).

[84] D. Lincot, J. F. Guillemoles, S. Taunier, D. Guimard, J. Sicx-Kurdi, A. Chaumont, O.

Roussel, O. Ramdani, C. Hubert, J. P. Fauvarque, N. Bodereau, L. Parissi, P. Panheleux, P.

Fanouillere, N. Naghavi, P. P. Grand, M. Benfarah, P. Mogensen, and O. Kerrec,

"Chalcopyrite thin film solar cells by electrodeposition," Solar Energy 77, 725-737,

(2004).

[85] M. G. Panthani, V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P.

F. Barbara, and B. A. Korgel, "Synthesis of CuInS2, CuInSe2, and Cu(InxGa1-x)Se2 (CIGS)

Nanocrystal "Inks" for Printable Photovoltaics," J. Am. Chem. Soc. 130, 16770-16777,

(2008).

[86] M. Kaelin, D. Rudmann, F. Kurdesau, T. Meyer, H. Zogg, and A. N. Tiwari, "CIS and CIGS

layers from selenized nanoparticle precursors," Thin Solid Films 431-432, 58-62, (2003).

[87] V. K. Kapur, B. M. Basol, and E. S. Tseng, "LOW-COST METHODS FOR THE PRODUCTION

OF SEMICONDUCTOR-FILMS FOR CUINSE2/CDS SOLAR-CELLS," Solar Cells 21, 65-72,

(1987).

50

[88] V. K. Kapur, A. Bansal, P. Le, and O. I. Asensio, "Non-vacuum processing of CuIn1-xGaxSe2

solar cells on rigid and flexible substrates using nanoparticle precursor inks," Thin Solid

Films 431-432, 53-57, (2003).

[89] J. v. Duren, D. Jackrel, F. Jacob, C. Leidholm, A. Pudov, M. Robinson, and Y. Roussillon,

"The next generation in thinfilm photovoltaic process technology," in Conference Record

of the Seventeenth International Photovoltaic Science and Engineering Conference,

Fukuoka, Japan, 2007.

[90] D. B. Mitzi, "Solution Processing of Chalcogenide Semiconductors via Dimensional

Reduction," Adv. Mater. 21, 3141-3158, (2009).

[91] W. Liu, D. B. Mitzi, M. Yuan, A. J. Kellock, S. J. Chey, and O. Gunawan, "12% Efficiency

CuIn(Se,S)2 Photovoltaic Device Prepared Using a Hydrazine Solution Process," Chem.

Mater. 22, 1010-1014, (2010).

[92] S. Siebentritt, M. Igalson, C. Persson, and S. Lany, "The electronic structure of

chalcopyrites—bands, point defects and grain boundaries," Prog. Photovolt: Res. Appl.

18, 390-410, (2010).

[93] G. L. Graff, R. E. Williford, and P. E. Burrows, "Mechanisms of vapor permeation through

multilayer barrier films: Lag time versus equilibrium permeation," J. Appl. Phys. 96,

1840-1849, (2004).

[94] C. Wadia, A. P. Alivisatos, and D. M. Kammen, "Materials availability expands the

opportunity for large-scale photovoltaics deployment," Environ. Sci. Technol. 43, 2072-

2077, (2009).

[95] V. Fthenakis, "Sustainability of photovoltaics: The case for thin-film solar cells," Renew.

Sustain. Energy Rev. 13, 2746-2750, (2009).

[96] K. Zweibel, "The impact of tellurium supply on cadmium telluride photovoltaics," Science

328, 699-701, (2010).

[97] K. Zweibel, "Thin film PV manufacturing: Materials costs and their optimization," Solar

Energy Mater. Solar Cells 63, 375-386, (2000).

51

[98] M. Raugei, S. Bargigli, and S. Ulgiati, "Life cycle assessment and energy pay-back time of

advanced photovoltaic modules: CdTe and CIS compared to poly-Si," Energy 32, 1310-

1318, (2007).

[99] B. O'Regan and M. Gratzel, "A low-cost, high-efficiency solar cell based on dye-sensitized

colloidal TiO2 films," Nature 353, 737-740, (1991).

[100] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, "Dye-sensitized Solar Cells,"

Chem. Rev. 110, 6595-6663, (2010).

[101] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N.

Vlachopoulos, and M. Graetzel, "Conversion of light to electricity by cis-X2bis(2,2'-

bipyridyl-4,4'-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-,

and SCN-) on nanocrystalline titanium dioxide electrodes," J. Am. Chem. Soc. 115, 6382-

6390, (1993).

[102] G. E. Tulloch, "Light and energy--dye solar cells for the 21st century," J. Photochem.

Photobiol. A: Chem. 164, 209-219, (2004).

[103] T. W. Hamann, R. A. Jensen, A. B. F. Martinson, H. Van Ryswyk, and J. T. Hupp,

"Advancing beyond current generation dye-sensitized solar cells," Energy Environ. Sci. 1,

66-78, (2008).

[104] C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. J. Jia, and S. P. Williams, "Polymer-

Fullerene Bulk-Heterojunction Solar Cells," Adv. Mater. 22, 3839-3856, (2010).

[105] Heliatek. (12/1/10). Available: http://www.heliatek.com/news-19

[106] Konarka. (12/1/10). Available: http://www.konarka.com/index.php/newsroom/press-

release-list/

[107] C. W. Tang, "Two-layer organic photovoltaic cell," Appl. Phys. Lett. 48, 183-185, (1986).

[108] M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. Ginley, "Inverted bulk-

heterojunction organic photovoltaic device using a solution-derived ZnO underlayer,"

Appl. Phys. Lett. 89, 143517-3, (2006).

[109] M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, and T. J. Marks, "p-type

semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in

polymer bulk-heterojunction solar cells," Proc. Nat. Acad. Sci. 105, 2783-2787, (2008).

52

[110] C. Tao, S. Ruan, G. Xie, X. Kong, L. Shen, F. Meng, C. Liu, X. Zhang, W. Dong, and W.

Chen, "Role of tungsten oxide in inverted polymer solar cells," Appl. Phys. Lett. 94,

043311-3, (2009).

[111] M. Riede and et al., "Small-molecule solar cells—status and perspectives,"

Nanotechnology 19, 424001, (2008).

[112] H. Spanggaard and F. C. Krebs, "A brief history of the development of organic and

polymeric photovoltaics," Solar Energy Mater. Solar Cells 83, 125-146, (2004).

[113] E. Bundgaard and F. C. Krebs, "Low band gap polymers for organic photovoltaics," Solar

Energy Mater. Solar Cells 91, 954-985, (2007).

[114] R. Kroon, M. Lenes, J. C. Hummelen, P. W. M. Blom, and B. d. Boer, "Small Bandgap

Polymers for Organic Solar Cells (Polymer Material Development in the Last 5 Years,"

Polym. Rev. 48, 531-581 (2008).

[115] M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, and M. A. Baldo, "High-efficiency organic

solar concentrators for photovoltaics," Science 321, 226-228, (2008).

[116] D. Veldman, S. C. J. Meskers, and R. A. J. Janssen, "The Energy of Charge-Transfer States

in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar

Cells," Adv. Funct. Mater. 19, 1939-1948, (2009).

[117] T. Ameri, G. Dennler, C. Lungenschmied, and C. J. Brabec, "Organic tandem solar cells: A

review," Energy Environ. Sci. 2, 347-363, (2009).

[118] A. Anctil, C. Babbit, B. Landi, and R. P. Raffaelle, "Life-Cycle Assessment of Organic Solar

Cell Technology," in 35th IEEE Photovoltaic Specialist Conference, Honolulu, Hawaii,

2010.

[119] J. Kalowekamo and E. Baker, "Estimating the manufacturing cost of purely organic solar

cells," Solar Energy 83, 1224-1231, (2009).

[120] R. Tipnis, J. Bernkopf, S. J. Jia, J. Krieg, S. Li, M. Storch, and D. Laird, "Large-area organic

photovoltaic module-Fabrication and performance," Solar Energy Materials and Solar

Cells 93, 442-446, (2009).

[121] V. Shrotriya, "Organic photovoltaics: Polymer power," Nat. Photon. 3, 447-449, (2009).

53

[122] M. Jørgensen, K. Norrman, and F. C. Krebs, "Stability/degradation of polymer solar

cells," Solar Energy Mater. Solar Cells 92, 686-714, (2008).

[123] A. O. Musa, T. Akomolafe, and M. J. Carter, "Production of cuprous oxide, a solar cell

material, by thermal oxidation and a study of its physical and electrical properties,"

Solar Energy Mater. Solar Cells 51, 305-316, (1998).

[124] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonsovante, K. Buker, M. Bronold, C.

Hopfner, and H. Tributsch, Solar Energy Mater. Solar Cells 29, 289-370, (1993).

[125] H. Katagiri, "Cu2ZnSnS4 thin film solar cells," Thin Solid Films 480, 426-432, (2005).

[126] H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani, and S. Miyajima,

"Development of thin film solar cell based on Cu2ZnSnS4 thin films," Solar Energy Mater.

Solar Cells 65, 141-148, (2001).

[127] K. Oishi, G. Saito, K. Ebina, M. Nagahashi, K. Jimbo, W. S. Maw, H. Katagiri, M. Yamazaki,

H. Araki, and A. Takeuchi, "Growth of Cu2ZnSnS4 thin films on Si (100) substrates by

multisource evaporation," Thin Solid Films 517, 1449-1452, (2008).

[128] Q. J. Guo, H. W. Hillhouse, and R. Agrawal, "Synthesis of Cu2ZnSnS4 Nanocrystal Ink and

Its Use for Solar Cells," J. Am. Chem. Soc. 131, 11672-+, (2009).

[129] C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, and B. A. Korgel,

"Synthesis of Cu2ZnSnS4 Nanocrystals for Use in Low-Cost Photovoltaics," J. Am. Chem.

Soc. 131, 12554-+, (2009).

[130] T. K. Todorov, K. B. Reuter, and D. B. Mitzi, "High-efficiency solar cell with earth-

abundant liquid-processed absorber," Adv. Mater. 22, E156-E159, (2010).

[131] "Basic Research Needs for Solar Energy Utilization," Department of Energy Report2005.

[132] R. D. Schaller and V. I. Klimov, "High efficiency carrier multiplication in PbSe

nanocrystals: Implications for solar energy conversion," Phys.Rev. Lett. 92, 186601,

(2004).

[133] A. J. Nozik, "Multiple exciton generation in semiconductor quantum dots," Chem. Phys.

Lett. 457, 3-11, (2008).

[134] W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, and X.-Y. Zhu, "Hot-

electron transfer from semiconductor nanocrystals," Science 328, 1543-1547, (2010).

54

[135] J. B. Sambur, T. Novet, and B. A. Parkinson, "Multiple exciton collection in a sensitized

photovoltaic system," Science 330, 63-66, (2010).

[136] J. M. Luther, M. Law, M. C. Beard, Q. Song, M. O. Reese, R. J. Ellingson, and A. J. Nozik,

"Schottky solar cells based on colloidal nanocrystal films," Nano Letters 8, 3488-3492,

(2008).

[137] Y. Wu, C. Wadia, W. L. Ma, B. Sadtler, and A. P. Alivisatos, "Synthesis and photovoltaic

application of copper(I) sulfide nanocrystals," Nano Letters 8, 2551-2555, (2008).

[138] J. F. Geisz, S. Kurtz, M. W. Wanlass, J. S. Ward, A. Duda, D. J. Friedman, J. M. Olson, W. E.

McMahon, T. E. Moriarty, and J. T. Kiehl, "High-efficiency GaInP/GaAs/InGaAs triple-

junction solar cells grown inverted with a metamorphic bottom junction," Appl. Phys.

Lett. 91, 2007).

[139] J. D. Beach and B. E. McCandless, "Materials challenges for CdTe and CuInSe2

photovoltaics," MRS Bulletin 32, 225-229, (2007).

[140] Fthenakis V.M., Sustainability of photovoltaics: The case for thin-film solar cells,

Renewable and Sustainable Energy Reviews, 13, 2746-2750, 2009.

[141] Fthenakis V., End-of Life Management and Recycling of PV Modules, Energy Policy, 28,

1051-1058, 2000.

[142] Fthenakis V., Energy Return on Investment (ROI) tracks efficiency gains, Solar Today, 24-

26, June 2012.

[143] Fthenakis V.M. et al., Methodology Guidelines on Life Cycle Assessment of Photovoltaics

Electricity, PVPS Task 12, International Energy Agency. 2009.

144] Fthenakis V.M. et al., Life Cycle Inventories and Analyses of Photovoltaic Systems: 2011

Status, PVPS Task 12, 2nd Draft, International Energy Agency, July 2011

[145] Fthenakis V.M., Kim H.C. and Alsema E., Emissions from photovoltaic life cycles, Environ.

Sci. Technol., 42 (6), 2168–2174, 2008.[

[146] Fthenakis V., Raugei M., Held M., Kim H. C., Krones J. An update of Energy Payback Times

and Greenhouse Gas emissions in the Life Cycle of Photovoltaics, Proceedings 24th

European Photovoltaic Solar Energy Conference, Hamburg, Germany, 21-25 September

2009.

55

[147] Nikolakakis T. and Fthenakis V. The Optimum Mix of Electricity from Wind- and Solar-

Sources in Conventional Power Systems: Evaluating the Case for New York State, Energy

Policy, 39(11), 6972-6980, 2011.

[148] Denholm, P., Margolis, R., 2007. Evaluating the limits of solar photovoltaics (PV) in

traditional electric power systems. Energy Policy 35, 2852–2861.

56