Thin Film Solar Cells (A Status Review)

Prof K L Chopra Former Director , IIT Kharagpur

Founder, Thin Film Laboratory, IIT Delhi & Microscience Laboratory, IIT Kharagpur

OUTLINE

Requirements for an ideal solar cell

Thin film materials for viable solar cells

Strengths and Weaknesses of various thin film cells

Comparative production status of various cells

New concepts to enhance cell conversion efficiency

Concluding Remarks

SOLAR Cell:PHOTOVOLTAICS

• Direct Conversion of light into electrical energy is called PHOTOVOLTAICS

(PV)

• Photovoltaic devices which convert solar energy into electricity are

called SOLAR CELLS

• Two electronically dissimilar materials (with different free electron

densities) brought together to form a junction with a barrier form a PV

device. Typical examples are :

metal1-oxide-metal2

metal-semiconductor (Schottky)

p-type semiconductor-n-type semiconductor (Homojunction)

n+-n semiconductor

p-type semiconductor(1)-n-type semiconductor(2) (Heterojunction)

p- (Insulator)-n

(p-i-n)1-(p-i-n)2- p-i-n)3 ………. (Multijunction)

Jct 1/Jct 2 /Jct 3 ………(Tandem)

SOLAR CELL

• Solar Cell operations depend on :

o Absorption of light to create electron-hole pairs (carriers)

o Diffusion of carriers

o Separation of electrons and holes

o Collection of carriers

• A Solar cell is a light driven battery with an open current voltage (Voc), short circuit current (Isc), maximum power point current and voltage (In, Vm), and a series and a parallel resistance (Rs, Rsh).

• Solar Cell Efficiency η – output = Im Vm = I siVIL FT input Σ nhv Σ nhv depends on quantum efficiency of creation of carriers, effectiveness of separation of carriers before recombination and collection of the separated carriers.

• Highest Theoretical Efficiency of known Jct Materials Homojunction ~ 30%

Heterojunction ~ 42%

36 Tandem Multigap Jctns 76%

What is required for an ideal Solar Cell ?

1.Cheap,Simple and Abundant Material

2.Integrated Large Scale Manufacturabilty

3.Cost (< 1$/watt)and Long Life HIGH ABSORPTION COEFFICIENT > 105 cm-1 with direct band gap ~1.5 eV

JUNCTION FORMATION ABILITY

HIGH QUANTUM EFFICIENCY

LONG DIFFUSION LENGTH

LOW RECOMBINATION VELOCITY

ABUNDANT,CHEAP & ECO-FRIENDLY MATERIAL

· CONVENIENCE OF SHAPES AND SIZES

· SIMPLE AND INEXPENSIVE INTEGRATED PROCESSING/MANUFACTURABILITY

· MINIMUM MATERIAL / WATT

· MINIMUM ENERGY INPUT/ WATT

· ENERGY PAY BACK PERIOD < 2 YEARS

· HIGH STABILTY and LONG LIFE (> 20 Years)

· COST (< 1$/Watt)

POSSIBLE Solar Cell Materials

Single Elements:

Si ( epi, mc, nc, mixed)

Carbon (nanotubes, DLC)

Binary alloys / Compounds:

Cu2S, Cu2O Cu-C, CdTe, CdSe,

GaP, GaAs, InP,ZnP , a-Si : H, Dye coated TiO2

Ternary (+) Alloys / Compounds:

Cu-In-S, Cu-In-Se,Cu-Zn-S, CdZnSe , CdMnTe, Bi-Sb-S,

Cu-Bi-S, Cu-Al-Te, Cu-Ga-Se, Ag-In-S, Pb-Ca-S,

Ag-Ga-S, Ga-In-P, Ga-In-Sb ,and so on.

Organic Materials:

Semiconducting Organics / Polymers and Dyes

Solar Cell Technologies • Crystalline Silicon solar cells - Single, Multi, Ribbon • Thin Film solar cells - Silicon, Cu2 S , a-Si, m-Si,n-Si, CdTe, CIGS,CNTS • Concentrating solar cells - Si, GaAs • Dye, Organic ,Hybrid & other emerging solar cells • New Ideas

Laboratory for Thin Films and Photovoltaics: Courtesy :Ayodhya Tiwari Swiss Federal Laboratories for Material Testing and Research 9

Spectral response of solar cells

Source: unknown

Crystalline Silicon :Present Scenario

Efficiency of single crystal Si cells (Laboratory) has been rising steadily to ~ 25% as a

result of better understanding of the junction properties and innovations in cell design

and fabrication technologies.

Efficiency gap between best laboratory cells, submodules/modules, and mass

produced modules varies with the maturity of technology and can be at least 10% lower

at every step so that the manufactured cell may be as low as 50% of the efficiency of

the best laboratory cell.

The world PV production of ~ 7900 MW in FY 2009 is primarily (~ 93%) based on single,

crystal and polycrystalline silicon.

With increasing production of Si-PV from 200 kW in 1976 to 6900 MW in 2008, the cost

of solar cells has decreased from $100 to about $3/Wp

With the existing technology and the material cost, the cost of Si cells can not be

decreased significantly unless major innovations in the production of appropriate

quality silicon I thin sheets take place.

Present day technology uses 8”or larger pseudo square of ~ 200µ m thickness, with an

efficiency of ~ 15-16%. The energy (16-5 kWH/Wp) pay back period of such cells is ~3-4

years.The module life is about 25 years

Specially designed silicon solar cells with efficiency ~ 18-20% are being manufactured

on a limited scale for special applications (e.g for concentration).

Polycrystalline silicon solar cells with efficiency ~ 12-14% are being produced on large

scale.

Specially designed thin(~ 20 m) films silicon solar cells with efficiency ~ 12% have been

fabricated on a lab scale . Production of hybrid thin film Si cells on MW scale is being

pursued

11

Solar module production for different technologies

EPIA expects thin film shares will grow: 20% in 2010 with about 4 GW 25% in 2013 with about 9 GW

CIGS is emerging with about 1% share CdTe is leading with over 6% share a-Si:H: About 5% share

Source: Paula Mint, Navigant Consulting

Laboratory for Thin Films and Photovoltaics Swiss Federal Laboratories for Material Science and Technology

C- Si dominates with ~ 90% share

------

WHY THIN FILM SOLAR CELLS ?

SMALL THICKNESS REQUIRED DUE TO HIGH ABSORPTION, SMALL DIFFUSION LENGTH &

HIGH RECOMBINATION VELOCITY

MATERIALS ECONOMY, VERY LOW WEIGHT GHT PER UNIT POWER

VARIOUS SIMPLE & SOPHISTICATED DEPOSITION TECHNIQUES

A VARIETY OF STRUCTURES AVAILABLE : AMORPHOUS, PLOYCRYSTALLINE, EPITAXIAL

TOPOGRAPHY RANGING FROM VERY ROUGH TO ATOMICALLY SMOOTH

DIFFERENT TYPES OF JUNCTIONS POSSIBLE –HOMO, HETERO, SCHOTTKY, PEC

TANDEM AND MULTI JUNCTION CELLS POSSIBLE

IN-SITU CELL INTEGRATION TO FORM MODULES

COMPATIBILITY WITH SOLAR THERMAL DEVICES

• TAILORABILITY OF VARIOUS OPTO-ELECTRONIC PROPERTIES ( e.g; Energy Gap ,Electron Affinity ,Work function ,Graded Gap ,etc)

Thin Film Cu2S –CdS Cell • One of the simplest solar cell to produce with

simple chemical conversion technique

• Highest efficiency obtained ~10 %

• Large scale production of modules with ~5% efficiency demonstrated during 70’s

• Stability of cells due to cuprous-cupric conversion remained an issue

• Due to the emergence of higher efficiency Si cells, this cell lost the battle of survival

• Revival of this cell with suitable modifications is a possibility

C-Si & Poly-Si

a-Si – amorphous Si

a-Si:H – amorphous hydrogenated Si

uc-Si:H – microcrystalline Si (hydrogenated)

Crystalline states of Si: Long range or short range order of atoms

Uncoordinated atoms and broken

bonds (called dangling bonds are

characteristics of a-Si

Hydrogen passivates the

dangling bonds in a-Si:H. Almost

any impurity can be added to

this open structure to obtain

asuitable semiconducting

behaviour

10 -1

10 0

10 1

10 2

10 3

10 4

10 5

0.5 1 1.5 2 2.5

Energy (eV)

a-Si:H

c-Si

µc-Si:H

Different Eg Different optical properties

Absorption coefficient of Si can change with the crystalline state

Ab

sorp

tio

n c

oef

fici

ent

(cm

-1)

Courtesy : Vikram Dalal

(small areaeff ~15%)

Laboratory for Thin Films and Photovoltaics Swiss Federal Laboratories for Material Testing and Research 19

Triple junction a-Si:H/SiGe:H/nc-Si:H solar cell Area: 0.25 cm2

Initial efficiency: 15.1%; Stable efficiency: 13.3%

• Amorphous Silicon (a-Si-H) : A Review

The glow discharge technology is well established production process.

The highest efficiency obtained in the lab cells is ~ 15%.

Single junction cells degrade down to ~ 5-7% efficiency over a period

dependent on how these are used.

Numerous innovations such as cell integration, graded gap, multi-junctions,

light trapping have contributed to the improvements in the cell performance.

Stability has been improved with double and triple layer cells. Large MW

plants forsingle and multiple junction cells have been set up . The best

stabilized (claimed !) module efficiency is ~ 8%.

The present day cost/watt of a-Si:H cells and modules is comparable (about

$3) to that of single crystal silicon.

Because of the lower throughput, complex and expensive deposition

technology for triple junction cells, and material cost, the cost can be

brought down only with much larger (>100MW_ scale production, or with

breakthroughs which help stabilize simpler single junction cells.

Major applications of a-Si-H cells are for small scale, small power,flexible

power packs ,value -added electronics.

a-Si-H PV technology has lost ground from ~ 39% world PV share in 1988 down

to ~ 10% in 1997 and less than 4% in Y 2010.

.

Thin Film Si Cells

• Thick Films (Etched, EFG, melt spun /drawn) : 20% efficiency for 50 micron films demonstrated

• Micro / Nano – Crystalline and Mesoporous Thin Films ( Vacuum Evaporated, CVD ) : 10% efficiency for 2 micron films demonstrated

• Hybrid and tandem amorphous and microcystalline films/ junctions : 12 % efficiency demonstrated

• Large Scale ( upto 50 MW) production established PROBLEMS : Thin Film deposition throughput limited to 2-3 microns / min which is not

cost effective Higher throughput with good quality opto-electronic properties required Photon trapping structures , Passivation and Cheap Substrate required

for lowering the cost

Highest: 20.3%

Cell area: ~0.5 cm2

Typical range:

Cells: 12% - 20%

Module: 8% - 13.5%

Highest: 15% - 16%

TCO contact

CdS or ZnS window

Substrate

Metal contact

CuInGaSe2 absorber

CdS window

Substrate

CdTe absorber

TCO contact

Metal contact

Thin Film CIGS, CdTe, a-Si Solar Cells

mc-Si:H

a-Si:H

Substrate

TCO contact

p

I absorber

n p

I absorber

n

Metal back contact

TCO

Highest: 16.5%

Cell area: ~1 cm2

Typical range:

Cell: 10% - 16.5%

Module: 9% - 11%

Highest: 11.5%

Highest: 13.3%

Cell area: ~0.25 cm2

Typical range:

Cell: 8% - 13.3%

Module: 4% - 9%

Highest: 10.3%

Lower efficiency of large area solar modules

Laboratory for Thin Films and Photovoltaics Swiss Federal Laboratories for Material Science and Technology

NANOSOLAR

Thin Film CdTe/CdS Cell

Theoretical Efficiency : ~ 30%

Deposition Techniques :

CdTe by Evaporation/sublimation/Chemical Solution/Screen

Printing

CdS by Evaporation/Sublimation/Chemical Solution

Lab Cell Efficiency Achieved : ~ 16%

Module Efficiency : ~ 10%

Nature of Junction : Controversial

Formation of Good Junction : Empirical requiring Suitable

Heat, Chemical and CdCl2

Treatment required

Estimated Production Cost ~ 1$/Wp for 100 MW plant

Pay back Time : 1.6 months for 10MW plant

Stability : Good

Problems : Cd Toxicity and Te Availability

Production Technology : Empirical & Temperamental

Thin Film Cu-In (Ga)-Se(S) Based Cell

Theoretical Efficiency : ~ 28-30%

Deposition Techniques :

Co-evaporation and homogenization

Layered vacuum deposition followed by selenization with Se or H2Se

Sputter deposition followed by selenization

Spray deposition

Screen printing followed by selenization

Electroplating

Lab Cell Efficiency Achieved : ~ 20.3%

Module Efficiency ~ up to 15.7% on flexible substrate

Estimated Cost : ~ 1$/Wp at > 50 MW Production

Pay Back Time : ~ 4 months for 100 MW plant

Stability : Good

Problems :

Multiple Binary Phases; Polymorphism;Structural and Electronic

Disorder

Availability of In and Ga

Sensitive Structure ;Role of Na ?

Sophisticated Controls required

Upscaling Problematic

PROBLEMS with CIGS Technology 1. Incompatibility of deposition processes for CIS and CdS (Evp /

Sputt/ED-followed by selenization for CIS and Evp/CS for CDS).

2. Complex deposition processes and controls.

3. CIS synthesis : • Narrow stoichiometry range, • polymorphism • Multiple

Binaries • Numerous Structural Defects • Nonunifority • Electronic

Disorder • Non Stoichiometry / defect dominated conductivity type-

depend on deposition parameters.

4. CdS Microstructure and Morphology very sensitive to deposition

process.

5. Mo/CIS Adhesion & Interficial strain.

6. TCO/CdS Interface (?)

7. Role of sodium ?

8. Cell-to-Cell mismatch.

9. Encapsulation

CZTS (Se) Cell • Band Gap :1.4-1.6 eV – Direct

• Deposition Techniques : PVD;Sputtering;Spray Pyrolysis; Electrodeposition; Screen Printing

• Theoretical Efficiency : ~30%

• Efficiency Obtained : up to about 9.6 %

• Abundant, cheap and green materials

Problems :

• Multiphasic ;Mixed Phases (monoclinic,orthorhombic,cubic,tetragonal,stannite)

• Multistructural;Structural and Electronic Inhomogeneities

• Difficult to control complicated synthesis process

• Time and temperature stability questionable

Thin Film GaAs Cell

Deposition techniques include MBE, MOCVD, CVD and LPE

Homo, Hetero, Stacked ,Multijunction, Tandem Junction and PEC

possible

Efficiencies : Homo (23.3%), AlGaAs/Si (26.9%), AlGaAs/GaSb

Tandem (32.6%), GaAs/InGaP (30%), Stacked InGaAs and InGap

(33%)

Junction Formation Straightforward

Various types of junctions possible

Suitable for stacked cell application

Stable Cell. Good for high temperature applications

Expensive materials and processing

Limited Laboratory batch size production for specialized

applications

New & Emerging Excitonic Cells

• Photoeletrochemical (PEC) Cell : Efficiency up to 12% Dyed TiO2/ Electrolyte/TCO (Gretzel Cell) & Variations with Polymeric Solid , Gel, and Hybrid Electrolytes • Organic ( Plastic ) Cells:Polymer / Polymer , Polymer/ Inorganic Semicon Jct :

Efficencies up to 9 % • Carbon Nanotube Cells- Hybrid , and Hetero Jct ( concept stage ) PROBLEMS

• Stability , • Empirical Processing and understanding • Low Efficiency, • Excitonic Transport and Charge Transfer Processes not well understood • Encapsulation problems

_

Organic Exitonic Solar Cell

2 - Generation of excitons

3 – Exciton

diffusion 4 – Exciton dissociation

by charge transfert at

interface

5 –Holes transport

1 – Photon

absorption

5 – Electron transport

hν

Cath

od

e

Tra

ns

pare

nt

an

od

e Donor

Principles of photovoltaic energy generation process :

D-A interface

Acceptor

Gretzel- Dye Sensitive Solar Cell

. Cathode

+-

ITO

Glass

ITO

Active layer

PEDOT

Anode

LiF

Al

h?

Cathode

+-

ITO

Glass

ITO

Active layer

PEDOT

Anode

LiF

Al

h?

Cathode

+-

ITO

Glass

ITO

Active layer

PEDOT

Anode

LiF

Al

h?

Organic Solar Cell

Organic Exitonic Solar Cells

• Development of tailored conducting and semiconducting polymers and co-polymers has made possible photonic junction devices with these materials. Solar cells of about 8% efficiency have been achieved. Impressively rapid progress is being made to understand the physics of the cells to improve the efficiency on large area cells

• Fabrication techniques are simple and manufacturable on a large scale

• Polymers used so far are rather expensive and thus cost of cells remains a question mark

• Poor stabilty of the cells is a major concern

• Sophisticated encalpsulation techniques need to be developed

http://en.wikipedia.org/wiki/Image:PVModuleLabEffic.jpg

Modeled losses from an ideal solar cell

Useful energy 29%

Thermalization 32%

Sub-bandgap losses 21%

Other losses 18%

Incident solar radiation 100%

The most noticeable loss mechanism in solar energy conversion relates to the fact that the basic electronic excitation process in Photovoltaics and also in photochemical processes & photobiological such as photosynthesis

Efficiency Enhancement by Fundamental Processes

• Multiple Junction and Tandem cells (feasible and useful)

• Graded bandgap cells (feasible but complicated)

• Quantum Well & Q-Dot structured cells ( feasible on

small area cells)

• Hot electron cells ( questionable)

• Multi-carrier generation cells (possible by using inverse

Auger Effect,impact ionization ,field emission if e-ph interaction can be controlled which is the main limitation today )

• Up and Down wavelength conversion cells ( not much

to gain from poor efficiency)

• Plasmonic Effects for enhancing optical absorption(promising if reproducibleQ-dots can be printed)

Surface Plasmons

Scattering

Increase in EM field near particle (Near Field Effect)

Direct electron emission from metal nanoparticles

Increase in Photonic Mode Density near the particles

Scattering: The light hitting the solar cell excites a surface plasmon on the metal nanoparticle, which then re-radiates most of its energy into the silicon in such a way that the light is trapped inside the cell. Increase in EM field near particle (Near Field Effect): The strong interaction between light and metal nanoparticles also leads to increases in the electromagnetic field around the particles. The particles effectively concentrate the light into small regions. If a semiconductor is close to or surrounding the metal particles, this will increase the light absorbed by the semiconductor in that region.

Courtesy: Dr Vamsi

Enhancement of Optical Absorption

(antireflection,scattering , path length increase,plasmonics)

• Plasmonic Nanostructure as AR coating(size and shape dependent)

• Surface Plasma Polaritons

• Localised Surface Plasmons

• Nano-imprinted Back Reflector

• Textured Back Electrode

• Nano-dome ,Nano-moth eye graded index AR structures

• Integrated Diffraction &Light Coupled Grating

(Limited feasibility for small area applications)

Optical Absorption of Thin

Discontinous Silver Films

(Source : Thin Film Phenomena)

SPR position depends on material and

size and shape of Islands or Q-dots

(Possible Choice of Materials : Si ,Fe, Cu,Al ,C,Ca, Pb,Ba,Zn,S)

Concluding Remarks 1. Hybridized micro- and nano-crystalline and aa-Si:H silicon thin films technologies

with efficiencies ~ 10-12% have started competing with mulicrystalline silicon wafer technology .

2. a-Si:H PV technology will continue at a limited level and will cater to portable small/medium power and other photo-electronic application.

3. Both vapour deposited and screen printed , thin film solar cells on flexible and hard substrates, based on CIGS and CdTe films have reached MW scale production with claimed module efficiencies ~ 12-15% at a production cost of about $1/watt .

4.CdTe and CIGS based solar cells have only short range prospects. Only cells based on abundant,cheap and green materials such as Cu ans Fe will have a brighter future. Research on binary or at most tertiary Cu based cells hold the future key.Stabilized CuxS and CuxO thin films need a serious re-visit

5. Small area hybridised/ hybrized organic - inorganic thin film with efficiencies up to 9% and Dye –sensitized solid state electrochemical cells with efficiencies upto 12% are opening new vistas for Thin Film Solar Cells.

6.Economic viability and sustainabilitywill ultimately determine the successful thin film technologies. High efficiency at high cost , or low efficiency at low cost are two competing options depending on applications

NEW CONCEPTS :coOLING of HOT CARRIERS

NEW CONCEPTS:

Multiphoton Generation

Multiple electron-hole pair generation Schaller et al., Nano Letters 6, 424 (2006)

The challenge for photovoltaic application (a) Separating electron-hole pairs (b) Collecting them efficiently

Roll-to-roll CIGS solar module production concept

CIGS absorber layer Mo sputter deposition for back contact

P1 scribe

pressure reduction

pressure adjustment

Chemical bath deposition for buffer layer

P2 scribe

pressure reduction

ZnO/ZnO:Al sputter deposition for front contact & anti reflection layer

P3 scribe

pressure adjustment

electrical contacts lamination & protection

Challenge:

Transfer of static deposition processes to dynamic deposition on moving foils

Thermal mismatch induced stress

Laboratory for Thin Films and Photovoltaics (Courtesy : Ayodhya Tiwari)

Swiss Federal Laboratories for Material Science and Technology

Consequences of Nucleation & Growth of Films

• Grain Structure : Nano to Micro Size; Dense; Porous ;

Columnar ; Granular

• Morphology : Particles ; Quantum Dots; Nano-wires,- rods, -tubes,-sponges ;Films ;Multilayers (Superlattices, Q-Wells…)

• Microstructure :Amorphous ; Nano to Micro-Crystalline ; Oriented ; Epitaxial

• Topography :Atomically smooth to micron scale rough

• Crystal Structure :Normal ; Polymorphic ; Metastable

• Chemical Structure : Normal ;Variable and Extended Solubility ; Non-equilibrium structures

Opto-electronic Properties of Micro & Nano-structured Films depend very strongly on nucleation and growth processes and hence on numerous deposition paramaters