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Poly(2-Methoxy, 5-(2'-Ethyl-Hexyloxy)-P-Phenylene

Vinylene), Meh-Ppv Solar Cell

(MEH-PPV) donor type polymer mixed with C[60] fullerene in a bulk hetro-junction

Spencer McAtee Colin Hicks

Quincy Garner

Abstract—Organic Solar Cells are becoming more efficient. This paper discusses (MEH-PPV) donor

type polymer mixed with C[60] fullerene in a bulk hetro-junction, aswell as discusses the differences

between organic and inorganic solar cells. Lastly we delve into the future of organic solar cell

technology.

I. INTRODUCTION

Organic solar cells such as Poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene] have become a

promising alternative to inorganic cells due to their low cost as well as cheaper scale production. Also the

chemical flexibility provides for easy modifications on organic materials using chemical synthesis. Blends

of conjugated polymers and fullerenes have been studied as a promising material for the active layer of

organic solar cells. The solar energy conversion has has had a reported efficiency between 4 and 6 % for the

best polymer/fullerene material solar cells.

A. History

In 1883, American scientist Charles Fritts coated the semiconductor material Selenium with a very

thin layer of gold to form a special metal-semiconductor junction, effectively creating the world’s first

photovoltaic cell and unbegnonsent to him sparking an alternative energy revolution. In 1946, fellow

American Russell Ohl patented the modern junction semiconductor solar cell [13] and finally in 1954 Bell

Laboratories developed the first practical modern photovoltaic cell using a diffused silicon p-n junction [14].

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Figure 1. MEH-PPV (chem.cmu.edu)

These original solar cells were quite inefficient and costly when compared to fossil fuel energies at this time

with the cost of producing 1 watt of energy being almost one hundred times more expensive in solar cells

compared to the cost of energy produced in a coal plant. Over the next twenty years, little progress was seen

in the efficiency and cost of solar cells as their main consumer the space industry which was willing to pay

the exorbitant fee to have a reliable and renewable energy source that could be used easily in space.

Between 1969 and 1973, an Exxon lab by the name of Solar Power Corporation began to drastically

decrease the cost production of solar cells by using rejected silicon from existing manufacturers and

reworking the production process by moving away from standard semiconductor fabrication methods [15].

Figure 2. C60 Fullerene (nano.gtri.gatech.edu)

From 1973 onward, solar cell technology has continued to improve with the adoption of the cheaper

polycrystalline silicon as the main fabrication material and the widespread use of flat screen televisions

leading to the availability of quality glass used in solar cell panels.

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Figure 3. Polycrystalline photovoltaic cells

When most people think about the main material used in photovoltaic cells they generally think

about Silicon. While they would be correct that Silicon is the main material used in 99% of the world’s

solar cells, they would be surprised to find that other semiconductor materials make excellent solar cells,

and even more surprised to find that Carbon based organic materials can be used as well.

In 1906, the organic compound anthracene (C14H10) was discovered to have photoconductive

properties by Pochettino[16]. Between the time of this discovery and the year 2000, little attention was paid

to organic solar cells due to its inefficiency compared to Silicon based solar cells. During the past ten years,

organic solar cells have gained a renewed interest due to two recent developments that have shown promise

for this technology. First, it was shown that the quantum efficiency of the electron transfer from an excited

polymer to C60 is quite high and the transfer of these electrons is very fast [17]. The second development is

based upon the creation of organic displays created from organic light emitting devices (OLEDs). These

displays have been shown to be just as efficient as standard LED displays and are significantly less costly to

produce. This technology lends credit to the idea that organic electrical components are a viable alternative

to non-organic components.

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B. Research topic

In this paper, we will be focusing on the physics, device characteristics, current technology, and

future prospects of organic solar cells. While there are many different types of organic solar cells, our paper

will be mainly focusing on poly [2-methoxy,5-(2’-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) onto

C60 (fullerene) heterojunction. This specific type of organic solar cell has shown to work well in real life

situations and has great potential to grab a significant share of the solar cell industry.

II. INTRODUCTION TO OPV AND IPV CELLS

The conversion of solar energy into electrical or fuel energy is becoming more important as the

costs of our current energy sources become more apparent. Solar cells made from inorganic semiconductors

have been studied since the early 1950’s and have been used as a green power sources in applications

ranging from satellites (which need to not have bulky energy stores) to residential roof-tops (Which allows

the lowing of monthly electoral bills). More recently, several types of solar cells based on organic materials

have appeared, raising the intriguing possibility of inexpensive solar cells that can be made on flexible

substrates.[1,9] There are some differences between the photoconversion mechanisms in inorganic

photovoltaic or “IPV cells” and organic photovoltaic or “OPV” cells. Light absorption in OPV cells leads to

the production of exciton pair which are pairs of electrons and holes that are bound together and are neutral.

[5,6]. while in IPV cells it Instead leads directly to the creation of free electron-hole pairs. This difference

has fundamental consequences for the theory of the photoconversion process and any efforts to optimize the

performance of OPV cells.

Fig 4. Binding energy between a photogenerated hole at the origin and an electron at the indicated distance from the hole.

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Dye-sensitized solar cells or “DSSCs”, 2 planar organic semiconductor cells [22],and bulk-

heterojunction cells [23] are all valid for both IPV and OPV cells. This also is valid for quantum dot solar

cells[23]. OPV cells can be based on small semiconductor molecules, organic dyes, and even semiconducting

polymers. In some cases these even can be combined. In all cases light absorption by OPV cells result in the

production of exciton pairs instead of free electron-hole pairs. To have a photovoltaic effect, the electrically

neutral exciton pairs must either be created at DSSCs, or diffuse to, an interface where they will separate

into an electron and a hole and, so the charge carriers are already separated upon photogeneration.[24]

Exciton pair dissociation also can occur at bulk trap sites, leading to one trapped carrier and one potentially

free carrier, but this has little effect on the PV effect caused by exciton pair dissociation.

Internal electric fields are not strong enough to cause exciton pair dissociation: Given the

following values; exciton radius = 1 nm, binding energy = 0.25 eV. As such an E field of .106 V/cm would

be required to cause exciton pairs to dissociate.

III. BASICS OF PHOTOVOLTAIC CELLS

B. Excitations

When exposed to light some semiconductors will produce a small current do to the photons

energy being transferred to electrons in their lattice, this effect can be used to generate power known as

Solar Energy. Standard semiconductor photovoltaic cells use absorbed photons to create Electron Hole Pairs

(EHP). This is done by exciting an electron from the valance band to the conduction band, creating excess

charge carriers. When this affect occurs in a p-n junction the internal voltage (Vo) will cause movement of

the charged particles inducing a current in the material (Fig. 5). By sandwiching two photoactive materials

between metallic electrodes one of which is transparent to collect photo generated charges. The charges

move through the electrodes and produce current in the load by movement of holes and electrons.

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Fig.5 Energy band diagram of photovoltaic cell

The junction depth must be small, less than the diffusion length of the holes in the n-type

material, to allow holes generated at the surface to diffuse to the junction and recombine. Similarly the

thickness of the p-type layer must be thin enough to allow electrons to easily diffuse through to the junction

before recombination occurs. This requires a proper match between the electron diffusion length and the

thickness of the p-type region and the optical penetration depth of (1/a) [9].

((Eq. 4-2) in [9]) �(�) = �� ∗ ��

A. Design decisions

A large Vo is desirable to obtain a large photovoltage. Also heavy doping and long lifetimes are

desirable. This presents a problem as doping reduces lifetimes, leaving designers with a balance of

properties that they can achieve and properties they desire. The resistance of the device is also important.

We want to keep resistance low so power lost due to heat is reduced. In the bulk material this is not hard,

within the thin top layer. If this region is contacted at the edge, the current must flow along the thin region to

the contact, resulting in a large series resistance. To reduce this effect, contacts can be distributed over the

top surface by having small contact leads all over the lighted surface. The distributed contacts serve to

reduce the resistance without overly interfering with the incoming light.

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IV. ORGANIC PHOTOVOLTAIC CELLS

A. Organic excitation pairs

When a photon hits a photoelectric polymer a photoexcitations are created [5]. These excitons are

different than the electron hole pairs created in semiconductors. In a semiconductor when the electrons and

holes are separated they move and act independently of each other. In photo-polymers the exciton electrons

and holes are bound together into pairs. They are not fully separated from each other as they are in a

inorganic medium, and instead are excited into a higher atomic orbital. These “exciton pairs” are an

important intermediate step in the solar energy conversion process. Usually strong electric fields are

required to dissociate these electron hole pairs into free charge carriers, which are the desired final products

for photovoltaic conversion.

Fig. 6. Carrier differences between conventional and organic solar cells from [8]

The exciton pair must travel through the generating material without recombining to reach an

interface. This interface can be either the interface of the electron donor and receivers or the interface of the

electrodes. Once at the interface the exciton pair is broken into an electrons and a hole (Fig. 6). After the

exciton pair breaks into electron and hole pair the cell acts much like a normal photovoltaic cell, with

diffusion of the charge carrier’s through the two materials, with the donor material acting as p-type and the

acceptor acting as n-type.

Organic semi-conductors normally have a band gap of 2eV and are primarily hole conductors [5].

The larger band gap limits the absorption of photons by requiring the energy of the incident photon to be

higher. A major difference between inorganic and organic solar cells is that inorganic cells are minority

carrier materials; the diffusion in the built-in electric potential creates the photovoltaic current.

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Fig. 7 Excitions generated (1) move through material(2) at interface break into EHP(3) move through material as electron and hole (4) Flow into electrodes producing current(5)

While organic cells are majority carriers, because electrons and holes exist primarily in different

phases with their movements directly related to current flow (Fig. 7). The diffusion and distribution of

carriers operate under different mechanics. Conventional cells generate holes and electrons together.

Compared to Silicon’s band gap of 1.12eV, this means that the energy absorbed by organic solar cells is

higher per photon but fewer photons can be absorbed. This along with the fact that the average wavelength

of light on Earth (Fig. 8) with the relation of energy in a photon being E=hv [9]. This relates the energy in a

photon to its frequency, and its frequency is related to its wave length as λ=c/v. Using this relationship we

find that a 2eV band gap requires a photon with wavelength of 621nanometers. While Silicon’s band gap of

1.12eV can absorb photons with wavelength of 1.1µm.

Fig. 8 Solar spectrum on Earth.

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V. DEVICE DESIGN

For this paper are looking at poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] or MEH-

PPV donor type polymer mixed with C[60] fullerene in a bulk hetro-junction[4] with a wt.% of C[60] in

MEH-PPV of 50%, a 1:1 ratio. The device is fabricated by taking the MEH-PPV and mixing it with the

proper weight ratio of C[60] in toluene then ether drop casting or spin casting on to alumina or glass

substrates [4].

The bulk hetro-junction is a mixture of the donor type material and acceptor type material in a volume

rather than in a layer. The mix allows the materials to have a large surface area for the interface allowing for

shorter total excition movement in the cell. With a greater surface area more excitions can break into

electron hole pairs for better efficiency.

The ideal production type for this device is roll-to-roll production. The economies of roll-to-roll

methods can make plastic cells inexpensive enough to be practical in a number of uses where low price is

more important than high efficiency. This is causing the field of polymeric and organic solar cells to gain

much interest from experts, because of the easy up-scaling, fast processing speed, low production cost, and

simplicity [11].

Today there exist a great variety of forming techniques for fabrication of organic solar cells. Coating

and printing techniques compatible with roll-to-roll processing are the most promising methods for organic

solar cells these techniques have the potential to boost production throughputs by a factor of 10 to 100

compared to other thin-film technologies.

Fig. 9 for section 7.1 from [4] (hw = 2.5eV)

Optical absorption spectra normalized for MEH-PPV unity at peak absorption

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VI. CHARACTERISTICS OF DEVICE

The optical absorption is a major factor in solar cell design; the spectrum of light that is absorbed

depends on the material. Ideally solar cells would absorb all wavelengths equally, allowing them to capture

all of the energy in white sunlight. However in reality this is not the case, and most materials have certain

wavelengths that excite more efficiently, and this is related to the band gap of the material. The ranges of the

energy of the photons absorbed by MEH-PPV/C[60] is given in Fig.9.

Here you can see the unmixed values for MEH-PPV and C[60] independently (a), and with different

wt.% values of C[60] in MEH-PPV. The two curves add with simple superposition of the two components

without any significant interaction between the two material in the ground state. The plots (a) and (b) are

normalized to unity at the peak of MEH-PPV absorption( hw=2.5eV planks const. *omega).

Another major concern in the design of this device is the resistance of the device. This is modeled in

Fig. 10. The conductance of the device is modeled with respect to the energy of photons entering the device.

Note that the MEH-PPV is given pure with 1wt.% 5wt.% and 50wt.%, the MEH-PPV curve shows a large

jump at 2eV and then slowing increasing with energy. When mixed with C[60] the curve starts at 1.6eV

instead and at lower concentrations has the large jump at 2eV, at 50% however the jump is minimal, but still

follows the shape of the pure MEH-PPV curve. This shows that the concentration of C[60] effects the

collection of photons at lower energy levels while having a minimal effect.

An important part of organic solar cells is the ease of processing that could accompany polymer mass

production soft lithography techniques could have use printing large sheets of solar cells rather then

assembling them from wafers of silicon. Right now the production of semi-conductor solar cells is limited

by the cost of silicon processing. If Organic cells reach the efficiencies of silicon then processing of large-

scale solar systems could become much cheaper.

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Fig. 10. from section7.1 from [4]

Along with the ease of production comes the ease of shaping the solar cells and the substrates. If the

substrate is fixable and durable it can be used in more situations. While current Silicon solar cells are more

efficient they require a fixed stable base that can support the weight of them. The low weight to power ratio

of organic cells is a large factor in placement of collectors, with small cells even being used in clothing [12]

this kind of implementation requires the flexibility and low weight that organic cell provide.

Fig. 11. Chart of band gaps of several materials Showing MEH-PPV

The band gap energy for MEH-PPV is 2.41 eV (Fig. 11), twice that of Silicon. This large band gap has

an effect on what type of photons get absorbed, the large band gap limits wavelengths that will cause

excitons in the solar cell. When choosing materials factors such as band gap, charge mobility, and diffusion

distances are important electrical factors to consider. While other factors like weight, durability, and

flexibility are mechanical properties that can also affect the usability of the product.

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VII. DISCUSSION OF SPECIFIC TECHNOLOGY

A. Efficiency

The interest in and technology related to organic solar cells has increased exponentially within

the past ten years. This renewed interest has led to an increase in the efficiency and decrease in the cost of

production of these photovoltaic cells. While much work has been done to increase the efficiency of organic

solar cells, their total efficiency pales in comparison to the more common non-organic solar cells. This is

mainly due to the low quantum efficiency of organic solar cells. Many organic solar cells are unable to

reach a quantum efficiency of anything higher than 3%, while single p-n junction crystalline silicon devices

can reach almost 38%. Non-organic multiple layer cells can have a quantum efficiency of almost 86% [18].

Organic solar cells are also more unstable against oxidation and reduction, while recrystallization and

variations in temperature can lead to device degradation and decreased efficiency over time. This

degradation is typically much faster than current non-organic solar cells. Finally, the large band gaps of

organic materials compared to non-organic materials contributes significantly to the inefficiency of the solar

process. The MEH-PPV onto C60 heterojunction solar cells have shown to have a higher quantum efficiency

than most organic single layer solar cells. C60 has a high electron affinity, which makes it a perfect electron

acceptor in a heterojunction based solar cell as seen in the figure below. Quantum efficiencies under

monochromatic light of 9% have been reported with power conversion efficiency of 1% and fill factor of

0.48[19].

Figure 4. Solar Cell Efficiency (National Renewable Energy Laboratory)

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B. Production

The one area where organic solar cells have a major advantage over non-organic solar cells is in

the ease and cost of fabrication. Organic materials have a high optical absorption coefficient when compared

to non-organics and thus fewer materials are needed to build the most efficient solar panels. This property

of organic solar cells has lead to the creation of flexible and pliable organic solar cells which have a myriad

of potential uses. It is estimated that purely organic solar cells could have a cost of around $50/m2 or about

49 cents per kilowatt hour if their efficiencies remain under 5% [20]. Organic films used to build

photovoltaic cells are typically deposited by either spin coating or vapor-phase deposition. Spin coating is a

technique in which an excess amount of solution, in this case organic material, is deposited in excess on a

substrate and spun at high rotational velocity to evenly spread the solution via centrifugal force. Spin

coating is advantageous in that it can coat large areas with speed, however this technique is more likely to

further degrade the polymer layer due to the use of solvents. Vapor-phase deposition is a technique that

transports molecules to a cold substrate by a hot inert carrier gas [21]. This technique is known to be more

reliable for producing higher quality organic solar cells, but is significantly slower and more costly than the

spin coating technique. A less frequently used technique, but worth noting is the vacuum thermal

evaporation process. In this technique, the organic material is heated in a vacuum while the substrate is

placed a few centimeters away from the source allowing for the evaporated organic material to be directly

deposited onto the substrate. The benefit of this technique is that it allows for the deposition of many layers

without chemical reactions occurring between two consecutive layers. The downside of this technique is that

the deposition is often not very uniform over large areas and is thus a less reliable technique than vapor-

phase deposition.

Figure 12. Bilayer organic photovoltaic cell

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While all of these techniques are commonly used to fabricate organic solar cells, many people believe that

the most efficient way of producing organic solar cells is by literally printing the organic material onto the

substrate. Much like an inkjet printer, organic material can be transferred from a writing head onto the

substrate without direct contact with the surface [21]. Printing organic materials is possible in part due the

low processing temperature required for the molecular and organic solutions. This style of production results

in high resolution patterning and rapid fabrication.

VIII. FUTURE PROSPECTS

The future of organic solar cells appears to be very bright. Doing a quick literature search through

multiple journal databases, one would find several entries published in November of this year solely

regarding Fullerene based organic solar cells. If the search is expanded to include all organic solar cell

publications, one would find scores of results from this past year. Clearly there is a major buzz around the

alternative energy industry regarding organic solar cells. While the focus in the past has been on non-

organic solar cells, this viewpoint has recently begun to shift toward the organic side due to the fact that

non-organic solar cells are beginning to approach their maximum efficiencies.

While organic solar cells have greatly advanced in the past decade, more work must be done to

improve efficiency, durability, lifetime, and cost of production of these photovoltaic cells. Many

researchers are currently looking at which organic materials are best suited for solar cell use. Pokhrel et al.

recently looked at the photovoltaic properties of Rhodamine B dye embedded in Poly(tolyl-1,1’-binaphthyl

carbamate) and poly(hexamethylene-1,1’-binaphthyl carbamate) matrices [22]. Ooyama et al. observed the

photophysical properties of 2,10-disubstituted benzofuro[2,3-e]naphthoxazole-type fluorescent dyes [23].

The possibilities are seemingly endless considering there are over 60 million known chemical substances

that need to be examined[13].

The study of organic solar cells has even begun to take hold of faculty members at Iowa State.

Assistant professor of Electrical and Computer Engineering Sumit Chaudhary was recently awarded a

National Science Foundation career award for the project “Utilizing Ferroelectrics for Multifaceted Device

Engineering of Polymer Solar Cells” [24]. In the Mechanical Engineering Department, assistant professor

Baskar Ganapathysubramanian has taken a more analytic approach by modeling the diffusion processes

involved in random heterogeneous media and applying these results to develop more efficient organic solar

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cells [15]. The future of organic solar cells appears to be very promising, with more and more progress being

made each month in the feasibility of this growing alternative energy source.

Practical application of bulk-heterojunction polymer-fullerene solar cells requires the cells to be

stable. Polymer light-emitting diodes, like the organic solar cells must be protected from the air to prevent

degradation of the active layer and electrode materials by the effects of moisture and oxygen. Even with

these protections there are several degradation processes that need to be eliminated to ensure stability. The

materials must be photochemically stable and the uniformity of donors and acceptors in the active layer

should be kept the same.

New combinations of materials that are being developed focusing on improving the three parameters

that establish the energy conversion efficiency of a solar cell. These are; the open circuit voltage (Voc), the

short-circuit current (Jsc), and the fill factor (FF). For ohmic contacts the Voc of bulk-heterojunction

polymer photovoltaic cells is controlled by the energy levels of the highest occupied molecular orbital or

“HOMO” and the lowest unoccupied molecular orbital or “LUMO” of donor and acceptor[26]. In most

polymer/fullerene solar cells, the positioning of these band levels of donor and acceptor is such that up to

0.4 to 0.8 eV is lost in the electron-transfer reaction.[27] By more careful positioning of these levels, it is

possible to raise the Voc above 1 V. The substitute of increasing the donor-HOMO to acceptor-LUMO

energy is that eventually the photo-induced electron transfer will be hampered by a loss of energy

increase.[28]

One important parameter for escalating the photocurrent is the number of absorbed photons. We can

increase the number of absorbed photon by increasing the thickness of the layer and shifting the absorption

spectrum of the active layer to higher wavelengths. The major trade off of this is that when the mobility

becomes too low or the layer becomes too thick, This cause the transit time of photogenerated charges to

become longer than the lifetime. This results in charge recombination.

IX. CONCLUSIONS

In this paper we have seen that Organic Solar cells such as Poly(2-Methoxy, 5-(2'-Ethyl-

Hexyloxy)-P-Phenylene Vinylene), Meh-Ppv Solar Cell have become more and more efficient and as they

open up a wide range of possibilities of solar cell materials they can be seen as the future of solar cells. This

does not discount inorganic solar cells though as they still have many things that can be done with them and

they still need to be more fully explored and there are still many methods that can be found. We have gone

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very far in 60 years in the field of solar cells, and have moved from very low efficiency to solar cells

becoming the future of energy.

REFRENCES

[1] Maurizio Prato “[60] Fullerene chemistry for materials science applications”, Piazzale Europa 1,

34127 T rieste, Italy

[2] A. A. Damitha, T. Adikaari, D. M. Nanditha, M. Dissanayake, and S. Ravi P. Silva, “Organic–

Inorganic Solar Cells: Recent Developments and Outlook”, IEEE Journal of Selected Topics in

Quantum Electronics, VOL. 16, NO. 6, 2010

[3] By Christoph J. Brabec, N. Serdar Sariciftci, and Jan C. Hummelen “Plastic Solar Cells” Adv.

Funct. Mater. 2001, 11, No. 1, February

[4] C. H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N. S. Sariciftci, A. J. Heeger, and F. Wudl

“Sensitization of the photoconductivity of conducting polymers by C60: Photoinduced electron

transfer” Physical Review B VOLUME 48, NUMBER 20 15 November 1993-II

[5] Hoppe, H. and N.S. Sariciftci, “Organic solar cells: An overview.” Journal Of Materials Research,

2004. 19(7): p. 1924-1945.

[6] Tom J.Savenij “ORGANIC SOLAR CELLS.” Internet:

http://aerostudents.com/thirdyear/solarCells.php [Nov. 1, 2011].

[7] Joop Knol and Jan C. Hummelen, “FULLERENE DERIVATIVES AS COMPONENTS FOR

‘PLASTIC’ PHOTOVOLTAIC CELLS.” Electrochemical Society Proceedings Volume 98-8 P.

1123-1126

[8] Travis L. Benanti & D. Venkataraman, “Organic solar cells: An overview focusing on active layer

morphology.” Photosynthesis Research (2006) 87: 73–81

[9] Ben G. Streetman and Sanjay Kumar Banerjee, “Optoelectronic Devices.” in Solid State Electronic

Devices, 6th ed. Upper Saddle River, New Jersey: Person Prentice Hall, 2006, ch. 8, p. 398-436.

[10] Sean E. Shaheen, Christoph J. Brabec, and N. Serdar Sariciftci et al “2.5% efficient organic plastic

solar cells.” Applied Physics Letters, Volume 78, Number 6, 5 February 2001.

[11] N. Espinosa, R. García-Valverde, M. S. García-Cascales, and A. Urbina, “Towards low-cost

manufacturing of organic solar cells: multi-criteria assessment of fabrication technologies.” in

17

International Conference of Renewable Energies and Power Quality, Granada, Spain. 23rd March,

2010.

[12] M. Schubert, J. Werner, “Flexible Solar Cells for Clothing.” Materials Today, Volume 9 Issue 6,

Pages 42-50.

[13] Russell S. Ohl, Little Silver, N.J. "Light sensitive device" U.S. Patent 2,402,662 Issue date: June

1946

[14] K. A. Tsokos, "Physics for the IB Diploma", Fifth edition, Cambridge University Press, Cambridge,

2008, ISBN 0521708206

[15] Perlin, John. “From Space to Earth: The Story of Solar Electricity.” Harvard University Press,

2002, pg. 53

[16] A. Pochettino. Acad. Lincei Rend. 15, 355, (1906).

[17] N. S. Sariciftci. “Photoinduced Electron Transfer from a Conducting Polymer to

Buckminsterfullerene.” Sciences 258, 1474, (1992).

[18] Li B. et al. Solar Energy Materials & Solar Cells 90, 549 - 537 (1982)

[19] Halls J.J.M. et al. "Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/c- 60

heterojunction photovoltaic cell"Appl. Phys. Lett. 68,3120-3122 (1996)

[20] Spanggaard, Holger, et al. “A Brief History of the Development of Organic and Polymeric

Photovoltaics.” Solar Energy Materials and Solar Cells. Vol 83. Issues 2-3. June 2004.

[21] J.C. Bernéde “ORGANIC PHOTOVOLTAIC CELLS: HISTORY, PRINCIPLE AND

TECHNIQUES.” J. Chil. Chem. Soc. Vol. 53, No. 3, pp.1549-1564, Sep. 2008.

[22] Brain A. Gregg and Mark C. Hanna, “Comparing organic to inorganic photovoltaic cells: Theory,

experiment, and simulation.” JOURNAL OF APPLIED PHYSICS VOL. 93, No 6, pp.3605-3614,

MARCH 2003

[23] A. L. Fahrenbruch and R. H. Bube, “Fundamentals of Solar Cells. Photo-voltaic Solar Energy

Conversion” (Academic, New York, 1983)

[24] Klaus Petritsch, “Organic Solar Cell Architectures.” PhD Thesis, Dipl.Ing. Technische Univ, Graz,

Austria. 2000.

[25] J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, and A.

B. Holmes, Nature �London� 376, 498 (1995)

[26] J. Simon and J.-J. Andre, “Molecular Semiconductors” Springer-Verlag, Berlin, 1985.

[27] P. Peumans, V. Bulovic, and S. R. Forrest, Appl. Phys. Lett. Vol. 76, pp. 2650, 2000.

[28] A. J. Nozik, Physica E ,Amsterdam 14, 115 2002.

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[29] B. A. Gregg, in “Molecules as Components in Electronic Devices,” edited by M. Lieberman,

American Chemical Society, Washington, D.C., 2002.

[30] Z. D. Popovic, A.-M. Hor, and R. O. Loutfy, Chem. Phys., vol. 127, pp. 451 – 457, 1988.