Influence of minimum position in [Ga]/([Ga]+[In]) profile of Cu(In,Ga)Se2 on flexible stainless steel substrate on its photovoltaic performances

Cu(In, Ga)Se2 (CIGS) solar cells on stainless steel (SUS) substrates are fabricated with various [Ga]/([Ga]+[In]), GGI, profiles in their absorbers. From GGI profiles, near-surface GGI is defined as average GGI within 200 nm from CIGS surface, mini GGI value is the lowest GGI, and mini GGI position is a distance between the lowest GGI position and CIGS surface. These are used as representatives of the GGI profile. Their impacts on cell performances are quantitatively examined. It is revealed that near-surface GGI is optimized in a range of approximately 0.40-0.45 (or band-gap energy of 1.28-1.30 eV), resulting in improvement of conduction band offset at CdS/CIGS interface. From simulation, optimized mini GGI position is in a range of 250-350 nm, well consistent with space charge region width. According to experimental results, fill factor and open-circuit voltage are increased when mini GGI position is decreased from 1.1 to 0.4 µm, while short-circuit current density is increased with decreasing mini GGI value. Ultimately, 17.3%-efficient CIGS solar cell on flexible SUS substrate with thin CIGS thickness of 1.5 µm is achieved, where near-surface GGI, mini GGI position, and mini GGI value are 0.42, 0.5 µm, and 0.06, respectively.