Recovering power after severe space degradation: annealing time effects on the resilience of CIGS solar cells

In this study, we investigated the effect of annealing time at a fixed temperature (120 ºC or 180 ºC) on the recovery of Cu(In,Ga)Se2 (CIGS) thin solar cells irradiated with protons. The energy range, 80-180 keV, allowed the proton stopping region to encompass the depletion region. The photovoltaic performance and optical properties were investigated using photoluminescence, J-V measurements, and external quantum efficiency measurements along the irradiation and recovery stages. The irradiation caused a reduction in power conversion efficiency of ~92% and a quite strong modification of the radiative recombination channels, namely involving deep energy levels in the band gap. The increase in defect density resulted in an increase in Urbach energy, from 33 meV to ~38 meV, and a reduction in the estimated band gap energy from ~1.159 eV to ~1.146 eV. The annealing temperature used in the recovery of the solar cells has a decisive influence on the recovery. At 120 °C, the efficiency and the radiative recombination channels recover more rapidly during the first 18 h, but maximum recovery is only reached after 58 h. In contrast, at 180 ºC, these aspects recover predominantly within the first 30 min, although maximum recovery takes 8 h. Analysis of the photoluminescence’s temperature dependence after recovery highlighted the involvement of the VCu defect in radiative recombination channels, independently of the annealing temperature. The results provide evidence that some defects formed during irradiation persist even after the recovery process, leading to an increased total defect density relative to the pre-irradiation condition. The relationship between annealing time, temperature, and the self-healing capacity of the CIGS is quite important for accurately predicting the behavior of these solar cells to in-orbit temperature cycles. Our results support the potential of this technology for long-term missions in hostile space environments.