Inorganic photovoltaics: research and perspectives

Inorganic photovoltaics: research and perspectives Alessia Le Donne, M. Acciarri and S. Binetti MILANO-BICOCCA SOLAR ENERGY RESEARCH CENTER CNISM and Department of Materials Science University of Milano-Bicocca

Present PV scenario Bulk c-si based devices: well established technology, which presently covers almost 83% of the PV market Thin films based devices: promising technology able to improve the ratio between manufacturing cost and total delivered power (a-si-h, μc- Si-H, nc-si CdTe, Cu(In,Ga)Se 2 ) Organic cells (DSSCs): technology in the state of advanced development and pilot production (indoor and low power applications)

Bulk c-si based devices- Analysis of the cost High efficiency modules and new assembly concepts Module assembly 40% Feedstock 14% Si reduction consumption for Wp and use of lower cost silicon Ingot growth 8% Wafering 11% Cell manufactory 27% High productivity Research activities Dr. A. Le Donne Material cost reduction -thickness from 180 to 80 mm --UMG-Si Conversion efficiency increase -high efficiency concepts - light harvesting

Si metallurgic production (from quartz or carbon) Material cost reduction SiHCl 3 purification via distillation Polysilicon deposition Si reaction with HCl SiO 2 + C -> SiC+ SiO 2 -> Si+ SiO+ CO Worldwide production facility ~ 1,000,000 MT/y Electronic grade silicon Multicrystalline Si Monocrystalline Si Si ingot growth

Metallurgical Silicon or Upgrade Metallurgical Silicon (UMG-Si) Advantages: lower cost lower energy payback time Metallurgical silicon Drawbacks : Fe, B, P contents higher than those required for solar applications a deeper understanding of defect interactions and their effects on t and h is necessary Solar specifications R. Davis, et al., IEEE Trans El. Dev. ED-27, 677 (1980) A. Luque, S. Hegedus Handbook of Photovoltaic Science and Engineering John Wiley & Sons Ltd, 2003 England

Efficiency [%] UMG silicon: state-of-the-art 18 17 mc-si 16 15 14 textured untextured 13 UMG CZ ingot 12 0 200 400 600 800 1000 1200 Position from the top Hoffman et al. 23rd EUPVSEC Valencia 2008 J. Libal, S. Binetti et al., J.Appl.Phys. 104 (2008) 104507 UMG silicon devices have shown no significant performance degradation UMG silicon feedstock is currently used for solar cells production the efficiency could be further improved.

High efficiency concepts for Si solar cells Based on n-type Silicon: h=22% (commercial efficiency by Sunpower) HIT (Heterojunction with Intrinsic Thin Layer) structure: c-si with a double a- Si/c-Si heterojunction on n-type (Sanyo) h= 23% (R&D) h=20.7% (commercial efficiency) Pluto by UNSW (based on PERL cells) h=25.5% (R&D world efficiency record) h=19.9% (commercial efficiency by Suntech) Theoretical maximum efficiency: 31 % Dr. A. Le Donne

Increase of Si solar cells efficiency: light harvesting strategies Increase of the energy conversion efficiency of first generation solar cells obtained exploiting the solar spectrum regions not efficiently converted from silicon (conversion of photons with energy E < Egap and E >> Egap in radiation around the maximum quantum efficiency value of the PV device) theoretical solar cell efficiency over 40%* *T. Trupke, M. Green, P. Würfel, J. Appl. Phys. 92 (2002) 4117 and J. Appl. Phys. 92 (2002) 1668

Exploitation of the high energy region of the solar spectrum*: E>>E gap Multiple Exciton Generation (MEG) Space Separated Quantum Cutting (SSQC) Down-conversion (DC) or Quantum Cutting (QC) Down-shifting (DS) The enhanced PV conversion efficiency could exceed the Shockley Queisser limit ( 31%) The enhanced PV conversion efficiency could never exceed the Shockley Queisser limit ( 31%) * B.S. Richards, Sol. Energy Mater. Sol. Cells 90 (2006) 1189 Dr. A. Le Donne

Multiple Excition Generation: one photon with energy Ephot negap yields up to n e h pairs with energy equal to Eg. Estimated efficiency enhancements of single junction solar cells up to 44%, under the assumption that an e h pair is created for every increase by Eg in the photon energy hindered by competing de-excitation processes, recombination of excitons without carrier separation and high impact ionization threshold. Examples: Quantum dots based systems (CdSe, PbSe, and PbS ) Space Separated Quantum Cutting: one high energy photon divides into two or more e h pairs through the interaction of two spatially separated neighbouring nanocrystals. SSQC efficiency can be optimized by changing the separation between individual nanocrystals a deeper knowledge on this topic is required to establish the possible efficiency of charge carrier generation. Examples: Quantum dots based systems (Si nanocrystals)

Down-conversion/Quantum Cutting: two low-energy photons are generated from absorption of one highenergy photon with energy exceeding 2Eg. Dr. A. Le Donne Examples of lanthanide based DCs (C. Strumpel et al., Sol.En.Mat. & Sol.Cells 91 (2007) 238) QC on single rare earth ions QC on rare earth ions pairs

Down-shifting: one high energy photon is converted into a single low energy photon. Down-shifters are molecular systems with: wide absorption band in the region where the EQE of the cells is low no absorption in other spectral regions narrow emission band around the maximum EQE of the cell large Stoke shift Examples*: organic dyes, rare earth organic complexes, QDs in high transmittance, low scattering and photo/thermal-stable matrixes (PMMA, PVA, EVA, Al2O3, SiO2) *K. R. McIntosh et al., Prog. Photovolt: Res. Appl. 17 (2009) 191 *E. Klampaftis et al., Solar Energy Materials & Solar Cells 93 (2009) 1182 *C. Strumpel et al., Solar Energy Materials & Solar Cells 91 (2007) 238

@ MIB-SOLAR: development of a DS procedure compatible with the industrial process for the PV module production Dr. A. Le Donne *Eu(dbm) 3 phen *Eu(tfc) 3 H 3 C CH 3 CF 3 N Eu O O * N O 3 H 3 C O 3 Eu CH 2 CH 3 H 3 C CH 3 NCH 2 CH 3 CF 3 O H 3 C O Eu O 3 DI sc up to +2.2% # DP max up to +2.9% # *Eu(tfc) 3 /4,4'-bis(diethylamino)benzophenone 1:1 molar ratio NCH 2 CH 3 CH 2 CH 3 # A.Le Donne et al., Prog.Photovolt:Res.Appl. 17 (2009) 51 # A.Le Donne et al., Optical Materials 33 (2011) 1012

Exploitation of the low energy region of the solar spectrum: E<E gap Dr. A. Le Donne Up-conversion*: one or two low energy photons are converted into one high energy photon, which can be absorbed from Si Examples*: lanthanide based materials, usually involving Er 3+ *C. Strumpel et al., Solar Energy Materials & Solar Cells 91 (2007) 238

Third generation solar cells based on silicon: high efficiency solar cells based on abundant, non-toxic materials and processes for large scale production (target 0.20 $/Wp) Tandem solar cells (stacks of individual cells) Limiting efficiency: 74% Si QDs

Thin films based devices Thin films based solar cells allow a manufacturing cost reduction, since the amount of material required for their realization is at least two order of magnitude lower than in the case of first generation ones thin films based PV devices match with the requirements for BIPV, which have lower overall costs than conventional PV.

BIPV examples Ljsselstein Row Houses, The Netherlands System Electrical Output: 1150 kwh/year/house PV Cell Type: Amorphous silicon Interconnection: Utility-Grid-Connected Maine Solar House (W. Lord) System Electrical Output: 4.573 MWh in 2009 PV Cell Type: Bulk crystalline silicon Interconnection: Off-Grid System Building-integrated DSC demonstrator from Dyesol (Photo by Thomas Bloch). Copyright Dr. A. Le Donne Dyesol Ltd. 2009 Solar façade with CIS thin film panels by Sulfurcell

Main thin films based devices 1. CdTe/CdS: maximum efficiency 16.5% 2. Thin film silicon: maximum efficiency 13% 3. CuGaInSe 2 /CdS: maximum efficiency 19.5% 2007 Cell Production by Technology (%)* 2010 Cell Production by Technology (MW-dc)* Total 23,889 MW *Data by PV News

CdTe Polycristalline material with direct bandgap (E gap = 1.45 ev, very close to the theoretical value for the best absorber material to be used in PV) stoichiometric growth for temperatures higher than 350 C very difficult doping (due to segregation at grain boundaries) for temperatures lower than 500 C intrinsic p-type doping for CdTe grown at temperatures higher than 500 C recovery of defects at grain boundaries for TT @ 400 C.

CdTe based devices CdTe solar cells are grown on soda lime glass in the sequence: front contact junction (CdS) CdTe absorber layer back contact. Back contact CdTe CdS Front contact Glass Example commercial CdTe PV modules by developer [J.Schmidtke, Opt.Expr. 18 (2010) A477]

Thin film silicon based devices Single-junction amorphous silicon (a-si), dual-junction a-si/a-si, tandemjunction mc-si-a-si and triple-junction germanium-doped a-si (a-si/a-sige/a- SiGe) are included the transparent conducting oxide typically is aluminum-doped zinc oxide, deposited by CVD or RF sputtering the active TF-Si absorber layers are commonly deposited by plasmaenhanced chemical vapor deposition the buffer layer usually is ZnO and the back contact is Al, Ag, or ZnO:Al. Example commercial TF-Si PV modules by developer [J.Schmidtke, Opt.Expr. 18 (2010) A477]

CIGS (CuInGaSe 2 ) Quaternary solid solution chalcopyrite lattice structure CuIn x Ga (1-x) Se 2 Intrinsic p-type doping direct bandgap: 1.1<E gap < 1.4 ev [E g (x Ga )=1.01 + 0.626(1-x) - 0.167x(1-x)] large variations in composition without appreciable differences in optoelectronic properties.

CIGS based devices The best CIGS solar cells are grown on soda lime glass in the sequence: back contact (thin Mo film deposited by magnetron sputtering, typically 500-1000 nm thick) CIGS absorber layer grown at temperature >500 C (to enhance grain growth and recrystallization) and in the presence of Na, either directly from the glass substrate or by evaporation of a Na compound (to enhance grain growth, passivation of grain boundaries and a decrease in the absorber layer resistivity ) junction (usually formed by chemical bath deposition of a thin (50-80 nm) CdS buffer layer).

CIGS absorber layer growth methods Substrate 500-550 C Evaporation: h = 19.5% Cu In Ga Se Vacuum sputtering of Cu, In and Ga followed by Se evaporation (preferred for large-scale depositions) spray-pyrolisis

CIGS @ MIB-SOLAR: hybrid sputtering* growth System compatible with roll-to-roll deposition (substrates up to 14x120 cm 2 ) up to now 11.5% record efficiency Cu In Ga precursors prepared by sequential DC-sputtering from In and Cu Ga alloy targets thermal evaporation of Se pellets to form the CIGS films Mo-coated soda lime glasses as substrates Na source Dr. A. Le Donne *Italian patent n TO2007A000684

Sunlight intensity (W/m 2 /nm) External Quantum Efficiency Average efficiency evolution in 2010 (%) MIB-SOLAR CIGS based PV devices 1.8 1.6 1.4 0.7 0.6 10 1.2 0.5 8 1.0 0.8 0.6 0.4 0.2 0.0 Dr. A. Le Donne AM 1.5G 0.1 cella solare CIGS 0.0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) 0.4 0.3 0.2 6 4 2 January March *M. Acciarri et al., Energy Procedia (2011) in press May July September December Month

Monolithic integration of CIGS cells in a module P1: Mo back contact removal (laser scribing) P2: active layers (CIGS, CdS) removal by laser scribing P3: insulation among the single cells after the front contact deposition (laser scribing)

CIGS large scale production* *J.Schmidtke, Opt.Expr. 18 (2010) A477

Conclusions New concepts for high efficiency c-si PV devices can increase the efficiency to cost ratio silicon QDs based solar cells with h > 40 % will no longer be a dream as demonstrated by the research community thin films based solar cells are expected to reach in few years a main role in the PV market thin films based devices can strongly reduce the Wp price. Dr. A. Le Donne

Thanks to: Voltasolar (M. Meschia, S. Marchionna, R. Moneta) Laserpoint and http://www.mibsolar.mater.unimib.it/