Cu(In,Ga)Se₂-based solar cells for space applications: proton irradiation and annealing recovery

In this work, we present an experimental study of a Cu(In,Ga)Se-2 (CIGS)-based solar cell (SC), irradiated with protons of energy 80 and 180 keV and with fluences of 10(12) , 10(13) , and 10(14) cm(-2), as well as a strategy to recover the induced damage. The possible modifications of the structural, electrical, and optical properties, induced by the proton irradiation, were investigated. Although the irradiation did not promote any major modification in the crystalline structure, it did induce the creation of defects responsible for changes in the electronic structure which caused a partial PL quenching and significant changes in the PL spectral shape, as well as a reduction of the power conversion efficiency and open-circuit voltage of up to 30% as revealed by J-V measurements. The photoluminescence results showed a broadening, redshift and decrease in the signal-to-noise ratio. The recovery of damage induced by irradiation in several SCs was tested through annealing steps performed at different temperatures and time intervals. It was found that the best recovery strategy for the investigated irradiation parameters was carrying out several isothermal annealing at 200 degrees C for 30 min. This strategy is compatible with the intermittent variation of the temperature in space and allowed to recover a power conversion efficiency comparable to that of the as grown cell. In particular, it must be highlighted that keeping the SC at room temperature in ambient atmosphere and in the dark, did not promote significant recovery in contradiction with some previous reports. This recovery methodology was applied in parallel for non-irradiated SCs and no increase in power conversion efficiency was found, but rather a slight decrease. The dominant radiative recombination channel was, apparently, unchanged with the irradiation and the subsequent recovery process. Nonetheless, changes in the concentration of defects of different types cannot be excluded, which is in line with a significant influence of fluctuating potentials in both as grown and after recovery stages of the solar cell. This work constitutes a first systematic study that simultaneously encompasses the influence of proton irradiation on the optical and electrical properties of CIGS SCs and a damage recovery methodology with a high potential to be explored in space applications. Additionally, it contributes to reinforcing the high potential of CIGS technology in the context of creating constellations of small satellites that are being developed by different entities, particularly private ones.