INVESTIGATION INTO THE STABILITY OF SILICON HETEROJUNCTION SOLAR CELLS AND THE METASTABILITY OF CADMIUM TELLURIDE SOLAR CELLS

Abstract

Solar photovoltaic (PV) is currently the fastest growing source of energy in the US due to its cost-effective approach to harnessing the energy from the sun and its ability to decarbonize the energy sector, thereby helping to meet global climate goals. Remarkable progress has been made in the PV industry to improve efficiency at a reduced cost with different advanced solar cell architectures. However, as the design and the fabrication of the solar cells advanced further to improve the efficiency at a reduced cost, there is a challenge to ensure that the long-term reliability of the emerging solar cells is not compromised under outdoor field conditions. Therefore, it is necessary to investigate the long-term reliability issues in solar cells using accelerated indoor exposure conditions, and then come up with an approach to mitigate these reliability issues. This work involved the study of two very different types of solar cells, which are silicon (Si) and cadmium telluride (CdTe) solar cells, each with a history of commercial success and current manufacturing expansion. They were exposed to different types of accelerated lifetime testing (ALT) designed to stress their known weaknesses. Devices were characterized in their initial, degraded, and in some cases recovered states. Characterizations such as current density-voltage (J-V), temperature-dependent current density-voltage (J-V-T), quantum efficiency (QE) were carried on both types of solar cells, whereas Suns-Voc and electroluminescence (EL) were specifically conducted on the silicon solar cells, and capacitance-voltage (C-V) and drive level capacitance profiling (DLCP) were specifically conducted on the CdTe cells. In the first part of this work, silicon heterojunction (SHJ) solar cells were exposed to thermal ALT to investigate the effect of varying structures and the effect of varying ALT exposure conditions on their degradation mechanism. The cell structural variation (by eliminating the rear-side indium layer) resulted in much greater degradation in open-circuit voltage (Voc) compared to standard SHJ cells due to rear-side passivation loss (as revealed by characterizations such as spectral-dependent Suns-Voc and QE) when exposed to 1-sun illumination intensity at 90ºC in argon ambient for 1000 hours. The rear-side passivation loss is speculated to have occurred due to aluminum diffusion into the rear-side amorphous layer followed by recrystallization and counter-doping, thereby resulting in recombination and consequently Voc degradation under the ALT. Therefore, reducing the fabrication cost by eliminating the rear-side indium layer on SHJ solar cells will significantly compromise their long-term reliability. On the other hand, the standard SHJ cells with both-sides indium layer were stable at the same exposure condition at 90ºC under 1 sun in argon ambient for 1000 hours but degraded significantly in fill factor (FF) when the exposure temperature was raised from 90ºC to 120ºC. J-V measurements and single-diode model revealed that the FF degradation was caused by an increase in series resistance and an increase in collection barrier at the front side of the cell structure under the ALT. Therefore, standard SHJ modules installed in very hot climates may experience an increase in degradation over a long period. In the second part of this work, the effect of doping and metastable defects on the performance of cadmium selenide telluride/cadmium telluride (CdSeTe/CdTe) solar cells were investigated. The cells co-doped with both antimony (Sb) and copper (Cu) i.e. (Sb+Cu)-doped cells demonstrated higher Voc, FF and short-circuit current density (Jsc) than the Cu-doped cells. The doping concentration obtained, with C-V measurements, is slightly higher in (Sb+Cu)-doped cells than in Cu-doped cell. While the doping concentration in the Cu-doped cells is comparable to what has been previously reported, the doping concentration in (Sb+Cu)-doped cells is almost two orders of magnitude lower than what was previously achieved with the Sb doping technique. The lower doping concentration observed is likely due to compensating defects caused by competing doping between the Sb and chlorine on tellurium sites, and perhaps any other unknown defects in (Sb+Cu)-doped cells. The lower performance observed in the Cu-doped cells compared to the (Sb+Cu)-doped cells is mainly due to dominant recombination occurring at the interface, as confirmed by lower activation energy (EA) from the Voc(T) measurement, higher drop in QE, higher saturation current density (J0) and higher collection barrier. However, the exact interface where recombination is prominent is not known, and further study is needed to separate the interface recombination into the front and the rear contributions. Therefore, Sb doping did not significantly improve the doping concentration but rather reduces interface recombination. In addition to characterizing the influence of doping on device performance, the effect of metastable defects was investigated by exposing Cu-doped, (Sb+Cu)-doped and As-doped cells to light under 1 sun illumination intensity at about 25ºC for 50 hours. Characterizations, such as J-V-T or Voc(T), QE, CV vs DCLP, in conjunction with a single-diode model, revealed higher dominant recombination mechanism occurring at the interface in As-doped cells, which is caused by higher interface trap concentration. Upon light soaking, the As-doped cells showed a significant improvement both in Voc and the FF (∆Voc = +16%, ∆FF = +21%) due to passivation of interface traps by excess photogenerated carriers under light soaking, while the Cu-doped and (Sb+Cu)-doped cells displayed a negligible improvement in Voc with slight increase in FF (i.e. ∆Voc = +0.7%, ∆FF = +2.6% in Cu-doped cells and ∆Voc = +0.6%, ∆FF = +6.0% in (Sb+Cu)-doped cells). Thereafter, when all the cells were stored in the dark for 50 hours, their performance returned to the initial value before exposure, thereby confirming that the performance instability was due to metastable defects. Considering the instability observed, it is important to continue improving the standard measurements and pre-testing light soaking procedure for investigating the origin of metastable phenomena and come up with an approach to mitigate them, as the CdTe solar PV technology advances.