Development of flexible Cd-free Cu(In,Ga)Se2 solar cell on stainless steel substrate through multi-layer precursor method

Cu(In,Ga)Se2 (CIGS) absorbers on flexible stainless steel (SUS) substrates in thin-film solar cells are deposited by multi-layer precursor method with different [Ga]/([Ga]+[In]) grading profiles, thus yielding different band-gap energy (Eg) grading profiles. In Eg grading profiles, near-surface Eg is defined as average Eg within 200 nm from CIGS surface, and minimum Eg is the lowest Eg. It is disclosed that near-surface Eg and minimum Eg have impacts on open-circuit voltage and short-circuit current density, respectively. 17.6%-efficient CIGS solar cell on flexible SUS substrate using CdS buffer is obtained with the suitable Eg grading profile consisting of near-surface Eg and minimum Eg of 1.26 and 1.08 eV, respectively. Flexible Cd-free CIGS solar cell on SUS substrate using ZnS(O,OH) buffer is next fabricated with relative low conversion efficiency of 8.5%, attributed to a relatively low Zn diffusion into CIGS and/or the large conduction band offset (CBO) at ZnS(O,OH)/CIGS interface. However, all photovoltaic parameters of the Cd-free CIGS solar cell are improved after light-soaking process owing to improvement of the CBO at ZnS(O,OH)/CIGS interface. Ultimately, 14.8%-efficient Cd-free CIGS solar cell on flexible SUS substrate utilizing ZnS(O,OH) buffer is obtained, thereby demonstrating the ability for portable power source.