The effects of valence band maximum (VBM) position control on the Cu(In,Ga)Se2 (CIGS) photoelectrode surface during the photoelectrochemical (PEC) water splitting using CIGS thin films were studied. First, a hole-blocking layer was obtained by replacing Se with S atoms to achieve VBM position control. The corresponding increase in photocurrent density suppressed the recombination at the photoelectrode/electrolyte interface. Subsequently, the CdS layer with lower VBM position than the CIGS layer was deposited on the CIGS photoelectrode. The results showed that the onset potential and photocurrent density increased due to the VBM position of the CdS layer. This study shows that water splitting could be effectively improved by controlling the VBM position of the photoelectrode surface.

Metastable behavior of cesium fluoride (CsF)‐treated Cu(In1–x,Gax)Se2 (CIGS) solar cells is investigated under heat‐light soaking (HLS) and heat‐soaking (HS) treatments. HLS increases open‐circuit voltage, fill factor, efficiency, and net carrier concentration and decreases short‐circuit current density, whereas heat‐soaking treatment acts oppositely. The performance of a CsF postdeposition treatment to CIGS thin film in selenium vapor, and closer to stoichiometry copper content, did not mitigate the open‐circuit voltage improvement after HLS. These results argue the traditional concept of the VSe–VCu divacancy complex for the total beneficial effect of HLS in alkali‐treated CIGS solar cells. The metastable behavior observed in the CsF‐treated devices due to the HLS and HS treatments is explained by the specific behavior of alkali‐containing new compounds at the surface and/or the migration of alkali metals at the surface and bulk regions.

For multilayered thin film solar cells, the degradation mechanism of high energy particle damage is usually evaluated by photovoltaic parameters. However, for each interface of the complex system still lacks direct detection techniques to distinguish the radiation damage layer-by-layer. Herein, we propose to apply electrochemical impedance spectroscopy for diagnosing the degradation properties of the irradiated thin film solar cells. Our result shows that the irradiation-induced new capacitance with resistance from high energy proton is not only at the interface of n-CdS/p-CIGS pn-junction, but also the interfaces of n-ZnO/n-CdS and p-CIGS/Mo, which can be further distinguished by the impedance spectroscopy with bias flow.

Recently, we demonstrated the positive effects of heat-light soaking (HLS) and subsequent heat-soaking (HS) on cesium fluoride (CsF) treated Cu(In 1x, Ga x)Se 2 (CIGS) solar cells. However, the role of defects formation and its influence on the electronic properties have not been analyzed. With this motivation, here, we analyzed the electronic properties of CsF-free and CsF-treated CIGS solar cells before and after HLS and subsequent HS treatments using temperature-dependent current-voltage (J-V-T), admittance and low-temperature capacitance-voltage (C-V) measurements. We noticed that CsF-treated CIGS solar cells form a minority carrier trap level after HLS. The subsequent HS treatment was found to be beneficial to compensate this defect level. The admittance measurement showed a shift of the shallow energy position to a higher value after HLS and subsequent HS treatments, irrespective of Cs incorporation. This is expected to be due to the formation of a secondary diode toward the CIGS/molybdenum contact. The positive and negative effects of HLS and subsequent HS treatments on CsF-treated CIGS solar cell are discussed using low-temperature C-V measurements. By optimizing the HLS and HS processes, CsF-treated CIGS solar cells yielded total efficiencies of over 20%.

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