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 :
Absorption of light to create electron-hole pairs (carriers)
Diffusion of carriers
Separation of electrons and holes
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%
•
o
o
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What is required for an ideal Solar Cell ?
1.Cheap,Simple and Abundant Material
2.Integrated Large Scale Manufacturabilty
3.Cost (< 1$/watt)and Long Life
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HIGH ABSORPTION COEFFICIENT > 105 cm-1 with direct band gap ~1.5 eV
JUNCTION FORMATION ABILITY
HIGH QUANTUM EFFICIENCY
LONG DIFFUSION LENGTH
LOW RECOMBINATION VELOCITY
ABUNDANT,CHEAP & ECO-FRIENDLY MATERIAL
CONVENIENCE OF SHAPES AND SIZES
SIMPLE AND INEXPENSIVE INTEGRATED PROCESSING/MANUFACTURABILITY
MINIMUM MATERIAL / WATT
MINIMUM ENERGY INPUT/ WATT
ENERGY PAY BACK PERIOD < 2 YEARS
HIGH STABILTY and LONG LIFE (> 20 Years)
COST (< 1$/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
Spectral response of solar cells
Source: unknown
Laboratory for Thin Films and Photovoltaics:
9 and Research
Swiss Federal Laboratories for Material Testing
Courtesy :Ayodhya Tiwari
Crystalline Silicon :Present Scenario
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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
Solar module production for different technologies
CIGS is emerging with about 1% share
CdTe is leading with over 6% share
a-Si:H: About 5% share
C- Si dominates with ~ 90% share
Source: Paula Mint, Navigant Consulting
EPIA expects thin film shares will grow:
20% in 2010 with about 4 GW
25% in 2013 with about 9 GW
Laboratory for Thin Films and Photovoltaics
Swiss Federal Laboratories for Material Science11and Technology
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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
Crystalline states of Si: Long range or short range order of atoms
C-Si & Poly-Si
a-Si – amorphous Si
a-Si:H – amorphous hydrogenated Si
Uncoordinated atoms and broken
bonds (called dangling bonds are
characteristics of a-Si
Hydrogen passivates the
dangling bonds in a-Si:H. Almost
uc-Si:H – microcrystalline Si (hydrogenated)
any impurity can be added to
this open structure to obtain
asuitable semiconducting
behaviour
Absorption coefficient of Si can change with the crystalline state
Different Eg
Different optical properties
105
Absorption coefficient (cm-1)
104
103
µc-Si:H
102
c-Si
a-Si:H
101
100
10-1
0.5
1
1.5
Energy (eV)
2
2.5
Courtesy : Vikram Dalal
(small areaeff ~15%
Triple junction a-Si:H/SiGe:H/nc-Si:H solar cell
Area: 0.25 cm2
Initial efficiency: 15.1%; Stable efficiency: 13.3%
Laboratory for Thin Films and Photovoltaics
Swiss Federal Laboratories for Material Testing and19
Research
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.
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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
Thin Film CIGS, CdTe, a-Si Solar Cells
Metal back contact
TCO contact
Metal contact
CdS or ZnS window
CdTe absorber
TCO
n
CdS window
I absorber
p
n
I absorber
Metal contact
TCO contact
p
TCO contact
Substrate
Substrate
Substrate
CuInGaSe2 absorber
Highest: 20.3%
Highest: 16.5%
Highest: 13.3%
Cell area: ~0.5 cm2
Cell area: ~1 cm2
Cell area: ~0.25 cm2
Typical range:
Typical range:
Typical range:
Cells: 12% - 20%
Cell: 10% - 16.5%
Cell: 8% - 13.3%
mc-Si:H
a-Si:H
Lower efficiency of large area solar modules
Module: 8% - 13.5%
Module: 9% - 11%
Module: 4% - 9%
Highest: 15% - 16%
Highest: 11.5%
Highest: 10.3%
Laboratory for Thin Films and Photovoltaics
Swiss Federal Laboratories for Material Science and Technology
NANOSOLAR
Thin Film CdTe/CdS Cell
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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
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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 (Evp /
Sputt/ED-followed by selenization for CIS and Evp/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 typedepend on deposition parameters.
4.
5.
CdS Microstructure and Morphology very sensitive to deposition
process.
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
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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
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Stability ,
Empirical Processing and understanding
Low Efficiency,
Excitonic Transport and Charge Transfer Processes not well understood
Encapsulation problems
Organic Exitonic Solar Cell
hν
Donor
2 - Generation of excitons
1 – Photon
absorption
3 – Exciton
diffusion
5 –Holes transport
5 – Electron transport
4 – Exciton dissociation
by charge transfert at
interface
Acceptor
D-A interface
Cathode
Transparent anode
Principles of photovoltaic energy generation process :
Gretzel- Dye Sensitive Solar Cell
Organic Solar Cell
Cathode
Al
LiF
.
Active layer
PEDOT
Anode
ITO
Glass
ITO
h?
+
-
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
Modeled losses from an ideal solar cell
Other losses
18%
Incident solar radiation
100%
Useful energy
29%
Thermalization
32%
Sub-bandgap
losses
21%
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
P1 scribe
Mo sputter deposition
for back contact
pressure
reduction
CIGS absorber layer
pressure
adjustment
Chemical bath deposition for
buffer layer
Challenge:
Transfer of static
deposition processes
to dynamic deposition
on moving foils
P2 scribe
pressure
reduction
ZnO/ZnO:Al sputter
deposition
for front contact
& anti reflection layer
Laboratory for Thin Films and Photovoltaics
(Courtesy : Ayodhya Tiwari)
P3 scribe
pressure
adjustment
electrical contacts lamination
& protection
Thermal mismatch
induced stress
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, QWells…)
• 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