solar photovoltaic technologies - department of energy ...chetan/lecture/lecture-34 spt-dec.pdf ·...

Solar Photovoltaic

Technologies

Prof. C.S. Solanki

Energy Systems Engineering

IIT Bombay

Lecture-34

8/1/2008 © IIT Bombay, C.S. Solanki Solar Photovoltaic Technologies 2

Contents

Brief summary of the previous lecture

Various Thin film solar cell technologies

Low temperature deposition

High temperature deposition

Solar cell structures


Classification of different approaches

• A large number of different technologies are under parallel

development

• A classification can be made based on different criteria:

• According to Tmax during layer formation

• According to grain size

• According to cell structure

•The R&D on the high-temperature routes is mainly driven

by considerations from classical bulk Si cells

Proven high efficiency and stability

• The R&D on the low-temperature routes is mainly driven

by considerations from a-Si:H solar cells

low thermal budget processing


Crystalline Si films: deposition

temperature200 400 600 800 1000 1200 1400

Kaneka

PECVD

Sanyo

SPC

Canon

VHF-

PECVD

ECN

LPE

IMEC,

CNRS-PHASE

CVD

ISE,

MITSUBISHI

ZMR + CVD

Neuchâtel,

Jülich

VHF-

PECVD

•

(oC)

• Low temperature deposition :

micro-crystalline Si ( g <1000 nm)

• Medium temperature deposition:

poly–Si ( g <100 m)

• High temperature deposition :

multi-crystalline Si, mono-Si

Grain size vs Voc,with SRV

107 cm/s

103cm/s


Crystalline Si materials

Type of Silicon AbbreviationCrystal Size

Range

Deposition

Method

Single-crystal

silicon

sc-Si >10cm Czochralski,

float zone

Multicrystalline

silicon

mc-Si 1mm-10cm Cast, sheet,

ribbon

Polycrystalline

silicon

pc-Si 0.1mm-1mm Chemical-vapor

deposition

Microcrystalline

silicon

mc-Si <0.1mm Ex: Plasma

deposition

• Thin film solar cell technologies requires a suitable substrate, and ……..


Basic components of crystalline Si solar

cells

Substrate

Active layer, 5 to

50 m

EmitterARC

Diffusion

barrier

•Base contact, if substrate is

conductive

• Substrate can be non-conductive, in that case both

the contact is taken from the front side



p-type

n-type

1. Homo-junction solar cell

for instance Mono-crystalline and multi-cystalline Si solar cells

p-type

n-type

2. Hetero-junction solar cell•p-type and n-type are different material

• more material choices some material can either be p-type or n-type

• used for material (thin-films) that absorbs light better than Si

• low series resistance window layer can be heavily doped

• CdTe and CIS are the examples

• in CdTe cell, CdS is used as window layer



i-layer

n-type

p-type

3. p-i-n / n-i-p solar cell

• Based on drift rather than diffusion

• Absorption take place in thicker intrinsic layer

• p-type a-Si / int a-Si / n-type a-Si

4. Multijunction solar cell

•Also called Tandem cells

•Can acieve high efficiency by capturing larger part of the spectrum

• individual cells with different bandgaps are stacked on top of one another

• Mechanically stacked and Monolithic

Eg1 > Eg2 > Eg3

Eg1

Eg2

Eg3


Diffusion vs drift in thin films

i

bidrift

L

VEL

q

kTDLdiff

High quality Crystalline-Si uses p-n junction

Carrier are transported by diffusion to the junction large

diffusion length

junction is very thin

diffdrift LL *10

Low-quality material should use p-i-n structure

Diffusion length are small

Drift length is about 10 times greater than diffusion length

intrinsic layer is thicker


Light trapping

• "light trapping" in which the optical path length is several times the

actual device thickness

• Light trapping is usually achieved by changing the angle at which light

travels in the solar cell

• texturing reduces reflection and increases optical path length

Following schemes are used for light trapping

2211 sinsin nnSnell’s law

substrate substrate substrate


Deposition techniques

• Physical vapor deposition

–Vacuum evaporation

–Sputtering

• Chemical deposition

– Chemical vapor deposition

(CVD)

– Hot wire CVD

– Plasma enhanced CVD

– Electro-deposition

– Spray pyrolysis

• Liquid phase deposition

– Liquid phase epitaxy


Contents

• Motivation

• Different thin-film solar cell technologies

• Why crystalline Si films?

• Classification based on grain size

• Thin-film solar cell structures

• Deposition techniques

• low temperature

• High temperature approaches

Mono-crystalline Si thin films

• Other concepts


Low-temperature approaches

Property

Deposition temperature 200 – 550oC

Deposition technologies Plasma-enhanced (PECVD, VHF-PECVD,

microwave, ECR)

Hot-wire CVD

Solid Phase crystallisation of a-Si:H

Si-precursor SiH4

Dilution with H2 is necessary for PECVD

microcrystalline Si

Deposition rate 0.1 –1 nm/s

1 nm/s (Kaneka), mostly below 0.5 nm/s

Cell structure Mostly p-i-n

Dual junction: Micromorph


Low-temperature processes

Technology Main R&D-players Features / results

PECVD

VHF-PECVD

IPV-Juelich

Neuchatel

(VHF-Technologies)

Kaneka Solartech

Pacific Solar

Systems (13.56, 27.12, 40.28 MHz, 4-

chamber, 6-chamber system, 30x30 cm2)

Micromorph cell: > 13%

Micromorph cell: 12%

Module: 9%

Micromorph cell: 14.5%

Micromorph module: 10%

Module (30x30 cm2): 7%

!p-n polycrystalline Si solar cell!

Hot-wire CVD University Utrecht

IPV-Juelich

Micromorph cell on stainless steel: 8%

Solid Phase

crystallisation

Sanyo Staring from n+-a-Si:H/a-Si:H-layer

With p+-a-Si:H HIT-emitter: 9.2%


Low-temperature approaches: Strength/Weakness

Pro’s

Substrate Compatible with glass

Plastic

Efficiency Micromorph cell concept compatible

with 15%

Upscalability Upscalability up to 1 m2 modules

seems feasible with cost < 1$/Wp

Con’s

Deposition rate Best efficiencies are obtained with

rates below 1 m/h

Stability Topcell degradation?


Low temperature deposition:

c-Si films

Features

•Grain size ~ 100nm

• Temperature < 600 C

• Small Minority carrier diffusion length

< I micron

• P-I-N structure,

• ~ 10 %

Deposition techniques

• Solid phase crystallization

• Plasma enhanced CVD

• Hot wire CVD

• Sputtering +

Metal induced crystallization

Substrates

• glass

• SnO2/ZnO coated glass

• metal : stainless steel

Back contact

c-Si i layer

front contactTCO

Glass/metal

n layer

p layer


High-temperature approaches

Property

Deposition temperature 900 – 1300oC

Deposition technologies CVD

Solution Growth

Electrodeposition

Chemical Vapor Transport (CVT)

IMEC, PHASE

ECN, Stuttgart

NREL

NREL

Si-precursor SiH4, SiH2Cl2, SiHCl3

Deposition rate 1 – 10 m/min

Cell structure Always p-n Epitaxial cells

Interdigitated cells on

non-conductive

substrate


Technology Main R&D-players Features / results

CVD

RTCVD

Continuous CVD

IMEC, ISE,

Stuttgart

PHASE

ISE

Monocrystalline epitaxial cells: 17.8%

Multicrystalline epitaxial cells: 14%

Polycrystalline Si solar cells: 6%

Chemical Vapor

Transport

NREL Based on iodine as transporting agent

Efficiency < 2%

Solution Growth /

Liquid Phase

Epitaxy

MPI-Stuttgart

ECN

UNSW

ANU

Monocrystalline epitaxial cells: 17.4%

Multicrystalline epitaxial cells: 15%

Electrodeposition NREL Made from molten salts of Si




Pro’s

Substrate Compatible with Si (perfect match of TEC)

Efficiency High efficiency proven with epitaxial concept

Homogeneity/

reproducibility

Large expertise from microelectronics available

Doping and thickness homogeneity < 5%

Con’s

Substrate Only high-temperature resistant substrates: Si, ceramics,

glass-ceramics

Strong requirements on TEC-match and purity

Blocking layers

Efficiency Non-recrystallised polycrystalline Si-layers on ceramic suffer

from low efficiency

Upscalability High-throughput concepts are available but not proven


Pro’s

Efficiency High efficiency proven

10.5% on Si:SiC, 8-9% on mullite, SiN,

Homogeneity/

reproducibility

Because of the extreme conditions, small deviations during

recrystallisation (thickness of ceramic, change in thermal

properties) can lead to unstable solidification and increased

defect densities

Con’s

Substrate Only very high-temperature resistant substrates: Si, SiN, mullite,

Al2O3

Very strong requirements on TEC-match and purity

Thick blocking layers

Process Rather complex process ( 4 additional steps to realise active

layer on ceramic)

Upscalability Quality of Si-layers, subjected to ZMR, decreases at

recrystallisation speeds above 10 cm/min

Zone Melting Recrystallisation


High temperature deposition:

Poly-Si films

Features

Grain size up to ~ several microns

• Temperature > 600 C

• diffusion length ~ 10s of microns

• P-N structure,

• ~ 11 %

Deposition techniques:

• Thermal CVD

• Ion assisted deposition

• Liquid phase epitaxy

(Zone melting recrystallization)

Substrate requirements

• Cost-effective

• Heat resistant

• Chemically inert

• Thermal expansion co-efficient matching

• Substrates: Alumina, mullite, graphite, low-cost Si

• Diffusion barrier: SiC, oxide/nitride

Back contact

Front contactARC

Epi-Si filmp+

p

n+

Ceramic substrate

Diffusion barrier

CVD si layer

Conducting substrate


Monocrystalline Si thin films

• Best possible thin-film solar cell performance

• Thin mc-Si films are obtained using “Layer transfer processes”

Starting substrate is a Si wafer

surface conditioning for forming separation layer

thin-film transfer to a foreign substrate

recycling of starting Si substrate

Device fabrication

How to form a separation layer?Example-2

Intermediate Oxide layer Example-1

Hydrogen implantation

Si

Si

Example-3

Porous Silicon layer

Si

Si


Porous Silicon Layer Transfer

(PSLT)

PSLT processes

• ELTRAN (Canon, Japan)

• SPS (Sony, Japan)

• PSI (ZAE, Germany)

• QMS (IPE, Germany)

• LAST (IMEC, Belgium)

• FMS (IMEC, Belgium)

• High monocrystalline Si layer can be deposited.

• Substrate can be re-used several times.

Porous silicon serves two purposes

Pores

• Anodization of Si in HF results in the formation ofporous silicon, columns of Si etched out (p+ Si).

• Layer porosity is a function of anodization parameters.

What is porous silicon ?


Integral Steps of PSLT

• Porous silicon formation

Silicon


SiliconSilicon



• Active layer deposition

- Annealing

- CVD epitaxial layer





• Device fabrication

Silicon





• Device fabrication

• Layer separation and transfer to

foreign substrate

Silicon

Substrate


Film Separation:

One-step anodization

Porous Silicon

Film

Silicon Substrate

Porous Silicon Film

20 m film

Features

• Homogeneous film thickness

• Film thickness from few microns to several tens of microns

• Film area is limited by experimental set up

• Film thickness is function anodization parameters

• Separation occurs for limited set of parameters

• US patent # 6649485


Silicon ingot

Electrolyte

Pt electrode

Continuous production of films

• HF conc. resumes at the surface after film separation


Silicon ingot

Pt electrode

Electrolyte





Silicon ingot

Pt electrode

Electrolyte

Porous silicon

films


• Patent pending


FMS cell process

Solar cell realization:

FMS (Freestanding Mono-Si) solar

cellsPS Film

Epitaxial layer

Epi layer after PS

removal

• Two-side contacted cell structure

PS Film

Epi layer

Emitter

0

0.2

0.4

0.6

0.8

1

400 600 800 1000 1200

Wavelength (nm)

IQE

FMS -1

FMS -2

Ref-20um

IQE analysis

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6

Voltage (Volts)

Curr

ent

(mA

/cm

2) Voc: 602.6 Volts

Isc: 33.12 mA/cm2

FF: 60.18

Eff.: 12.01%

Area: 0.65 cm2

Film thickness: 20 m

I-V curve

• Patent pending

Device is ready

• 9.6% FMS cell with HIT emitter


Other processes:

a-Si/c-Si hetero-junction

•This configuration has the following advantages:

• potential for high efficiency;

•low processing temperatures.

• low thermal budget for processing. Reduction of

energy pay back time;

• reduced cost of cell technology.

Epi layer, p-type

Epi layer, p+ type Al back contact

int. a-Si:H

n+, a-SiH

Front contactITO layer Combination of low

production cost of

amorphous cell

technology and high

efficiency of Mono-

crystalline Si cell

technology

Bandgaps: a-Si 1.7 to

1.8 eV, C-Si 1.12eV


Other processes:

Aluminum induced crystallization• for growing polycrystalline silicon (poly-Si) films on inexpensive glass

• Films is formed by aluminum-induced crystallization (AIC) of amorphous

silicon (a-Si)

• Annealing transforms an initial glass/Al/a-Si stack into a glass/poly-

Si/Al(Si) below the eutectic temperature of the Al/Si system (Teu=577 °C).

•The poly-Si forms a continuous layer which consists of large grains with a

preferential (1 0 0) orientation

Al

a-Si

Glass

Al

crystalline-Si

Glass

annealing


Conclusions

• Thin-film crystalline Si solar cells represent obvious way

to reduce costs PV

• A large number of techniques are under investigation

• There is a certain risk for subcritical R&D in this field

• Crucial issues are clear:

• Low-T techniques: increase of growth rate

• High-T techniques

• Availability of ceramic substrate

• Increase of recrystallisation speed for process

• Improvement of nucleation control for process

without ZMR

• On all of these questions there is a considerable R&D-activity


Concentrator PV

systems


Why solar concentrators?

• Replace expensive cell

material with cheaper material

• increased power output

per unit area

• Efficiency of solar cell increases under concentration

• Smaller area

solar cells are

easier to

manufacture

Solar cell

Concentration (suns)

Eff

icie

nc

y (

%)

Fil

l F

acto

r (%

)


Components of CPV systems

Solar cell

Heat sink

1 - Light collector

2 – Solar Cell

3 – Heat Sink

4 – Sun tracker


Light collectors

• Refraction and

reflection

• Concentration

ratio

•Line focus &

point focus

• Imaging & non-

imaging

concentrator


Solar cells for concentrators•Single-junction solar cells & Multi-junctionsolar cells

0.5 1 1.5 2 2.5

Wavelength (µm)

0.5

1.0

1.5

Su

nli

gh

t in

ten

sit

y

(kW

/m2/

m)

Single-junction solar cell

• Si is mainly used for

solar cell

• Lowering series

resistance is main

design parameter

• Laser grooved buried

contact solar cells

• Limited in solar

cell efficiency


Multi-junction solar cells• Bandgap engineering

• Materials are manipulated to adjust the bandgap according

to solar spectrum

• Double and triple-junction solar cells

• InGaP/GaAs/Ge on Ge substrate

0.5 1 1.5 2 2.5

Wavelength (µm)

0.5

1.0

1.5S

un

lig

ht in

ten

sit

y

(kW

/m2/

m)

Eg1 > Eg2 > Eg3

Eg1

Eg2

Eg3


Multi-junction solar cells

- 37.3 % (concentration

@175 Suns, 2004) World

record efficiency by

Spectrolab

- 13% with 6 junction

• Bandgap GaInP2 - 1.89eV

GaAs – 1.42 eV

Ge – 0.67 eV

• Design challenge is to match current from

each cell

• Higher number of junction can achieve higher

efficacies 40% is target by 2006

• Potentially 45% by 2010 (Spectrolab)


Comparison of technologies

Material t/ η Disadvantages Advantages, perspectives

Mono-Si

300

m,

15 -

18 %

Lengthy production

procedure, wafer

sawing necessary

Best researched solar cell

material in a next few years it

will dominate world market,

especially there, where

high power/area ratio is

required

Multi-c Si

300

m,

13 -

15 %

lengthier

production

procedure, wafer

sawing necessary

The most important

production procedure at

least for the next ten years

Polyc-Si

Transpare

nt

300

m,

10 %

Lower efficiency,

special procedures

to achieve optical

transparency

required

Attractive solar cells for different

BIPV applications. Possible

also production of double sided

cells



Material t / ηDisadvantage

sAdvantages, perspectives

EFG

250

m,

14 %

Limited use

of this

production

procedure

Very fast crystal growth, no

wafer sawing necessary,

significant decrease in production

costs possible in the future

Riboon-Si

300

m,

12 %

Limited use of

this production

procedure

No wafer sawing necessary,

significant decrease in production

costs possible in the future

a-Si

1 m,

5 - 8

%

Lower

efficiency,

shorter life

span.

No sawing necessary, possible

production in the flexible form. It

is a promising material in the

future if long-term stability

increases



Material t / ηDisadvanta

gesAdvantages, perspectives

CdTe

2-3 m ,

6 - 9 %

(mod.)

Poisonous

raw

materials

Significant decrease in

production costs possible in

the future

CIS

2-3 m,

7,5 -

9,5 %

(mod.)

Limited

Indium

supply in

nature

Significant decrease in

production costs possible in

the future

HIT 200 m,

18 %

Limited use

of this

production

procedure

Higher efficiency, better

temperature coefficient and

lower thickness.

solar photovoltaic technologies - department of energy ...chetan/lecture/lecture-34 spt-dec.pdf ·...

Documents