Stability and degradation in CdTe / CdS and CIGS photovoltaic cells

1. Stability and degradation in CdTe/CdS and CIGS photovoltaic cells Gary Hodes and David Cahen Dept. of Materials and Interfaces Weizmann Institute of Science, Rehovot 76100, Israel

2. Degradation rates of different types of modules/systems <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000 Jordan & Kurtz,2011 (August), National Renewable Energy Laboratory (NREL) Photovoltaic degradation rates – An analytical review

3. CdTe/CdS CdTe CdS CdSxTe1-x CdS TCO front contact CdTe Cu/Cu2-xTe/CdTe:Cu back contact Te-rich CdTe surface

4. Actual CdTe/CdS module stabilities From: Photovoltaics International Aug. 2012. N. Strevel, L. Trippel& M. Gloeckler, First Solar, Perrysburg, Ohio, USA 8.4% drop over 16.6 yr 0.5%/yr Beginning 2011 From: Francesco Biccari Master’s thesis 2012 0.6%/yr 480 kW thin film CdTe solar field installed in 2003 Tucson Electric/First Solar First Solar’s long-term degradation modellingrecommendation: -0.5%/yrfor moderate climates and –0.7%/yrfor hot climates (-0.5% with ZnTe)

5. Diffusion of Cu from the back contact through the cell From: Dobson et al., Sol. En. Mater. Sol. Cells, 62, 295 (2000)

6. Diffusion of Cu from back contact to CdS Cu 200ºC/N2/15 h Cu From: Visoly-Fisher et al., Adv. Funct. Mater. 13, 289 (2003) Accumulation of Cu in CdS due to stronger Cu-S bond (than Cu-Te) and relatively Larger grain surface (smaller grains) of CdS.

7. How impurities move in the cell from different back contacts Au Au/Cu Mo/Sb Mo/Sb2Te3 (n.b. Cu contacts gave somewhat more efficient cells) Au Au Bätzner et al., Thin Solid Films, 451-2, 536 (2004) Back contact

8. What is the effect of Cu on the cell performance? In the short term – positive In the long term – if too much, causes degradation Why?

9. Sensitivity of CdTe (CdCl2-treated) resistivity to Cu CuCd Cui From: Perrenoud et al., J. Appl. Phys. 114, 174505 (2013) 2.1018/cc There is always some Cu (and other impurities) in CdTe

10. Some of the causes of temporal changes in cell Cu into CdTeand via grain boundaries to CdS Cu deep acceptor in CdS Cui donor in CdS CuCdand [VCd-ClTe] acceptors in CdTe Cui donor in CdTe CdTe CdS A little Cu acceptor compensates. A lot forms recombination centres and Increases RS. Also forms shunt path via grain boundaries between CdTe and CdS Optimum doping – space charge layer width Cu+to CdS enhanced under illumination since barrier is decreased CuCdhas a charge of -1 (relative to lattice) and therefore will drift in opposite direction to Cu+ Recovery of slightly/medium degraded cells by Storageor anneal in dark could be due to dissociation of acceptors (CuCd) or VCd-Cuito VCd + Cui and back diffusion of Cu+ in field

11. Predicted radiation damage to different cell types by protons or electrons Bätzner et al., Thin Solid Films, 451-2, 536 (2004) Decrease in efficiency for CdTe at high DS Such high Ds would not normally occur in space for many years. Performance recovery occurs over days/weeks in absence of radiation.

12. Not to forget – mechanical stability Module thermal cycling (TC) is a standard test exposing modules in a controlled environment between -40 ºC to + 85 ºC to test mechanical stability of the modules (connections and differences in thermal expansion). First Solar modules are not affected by these (or even more cycles) tests. Long-term degradation– probable cause: Drop in fill factor due to increase in RS believed due to changes in back contact

13. CIGS/CdS <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000 ??? Fundamental diff between the 2 cells due to thermodynamic Preference of Cu for CdS in CdTe cell and for CIGS in CIGS cell?

14. ZSW S. Niki et al., Prog. Photovolt: Res. Appl., 18, 453, (2010)

15. Thermodynamic reaction possibilities for CIGS cells ∆G KJ/mol Mo CIGS CdSZnO If CIS not Cu-rich (no Cu2Se ) No reaction Cu2Se + CdS Cu2S+CdSe -5.6 In2Se3 + 3CdS 3CdSe + In2S3 -26 CdS + In2Se3 CdIn2(Se3)S ca. -25 Ga2Se3 + 3CdS 3CdSe + Ga2S3 -78 Increasing Ga, increased Possibility of reaction

16. High temperature stability of cells (Kijima and Nakada, Appl. Phys. Xpress, 1 075002 (2008)) not heated 280ºC 360ºC 400ºC At higher temperature, Cd diffuses to CIGS Space charge layer moves towards Mo. Stable up to 320 ºC (30 min in vacuum) Shows high chemical stability

17. Even if chemically stable, there could be other electrical degradation mechanisms Formation of defects, e.g. through Cu+ diffusion/migration e.g., low energy (of formation) defects: [Vcu][2VCuInCu]complex neutral defect or changes in ZnO stoichiometry Large variation in degradation rates from module to module, but main effect is drop in fill factor (increase in RS)

18. Bias-induced Cu migration in CIGS CIGS Cu+ Cu+ CIGS CdS CdS Operating conditions (PMAX) Field at the interface of CIS/CdS pulls the Cu+ into the CISmakes the CIS ‘surface’ Cu poor Reverse bias may occur for single cells within a series-connected module By partial shading of the module. Thus, the cell should be stable against: Forward bias, found under normal operation Reverse bias

19. We thus conclude that the electrical stability of CI(G)S is not static but is rather dynamic. In other words, stability is not obtained because the material is “strong”, but because it is flexible. Guillemoles et al. J. Phys. Chem. B, 104, 4849 (2000) Maybe also true to some extent for CdTe/CdS?

20. Overall conclusions There are many possible degradation pathways for CdTe and CIGS cells Results show module/system stabilities have improved (along with efficiencies) over the years Present mean panel/system efficiencies are 0.5% for CdTe and 1% for CIGS, but can expect this to fall considerably for CIGS <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000 <2000 >2000

21. Thanks to Kevin Dobson (Brian McCandles and Steve Hegedus) (U. Delaware) for helpful input