Photovoltaic devices which convert solar energy into electricity are .... Efficiency gap between best laboratory cells, submodules/modules, and mass produced ...
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 o o •
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
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
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.
.
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
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 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
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
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
