Influence of thermal treatment on micro-Cu(In,Ga)Se₂ solar cells for micro-concentrator photovoltaic applications

The micro-concentrator photovoltaic concept consists in miniaturizing asolar cell to the micrometer scale and using an optical concentrator to collect and focus sunlight onto the micro solar cells. As a result, this approach reduces the use of critical raw materials, while potentially reducing module costs and enhancing power conversion efficiency (PCE). Thin-film Cu(In,Ga)Se₂-based solar cells currently hold a record power conversion efficiency of 23.6% and 23.3% under concentrated illumination.

In this study, micro-holes with diameters of 200 and 250 μm were patterned into a SiO_x insulating matrix on a SLG/SiON/Mo substrate using photolithography [3]. A 1 µm thick Cu-In-Ga precursor was deposited by sputtering; this was followed by selenization at 480 ºC in a tube furnace to form CIGS micro-absorbers. One of the precursors underwent a thermal treatment at a nominal temperature of 450 ºC prior to selenization. The micro solar cells were completed with a CdS buffer layer deposited by chemical bath deposition and an i-ZnO/ZnO:Al window layer deposited by RF sputtering.

The Cu-In-Ga precursor exhibited a rough surface characterized by island-like grains, with CGI and GGI ratios of 0.75±0.04 and 0.24±0.03, respectively. The thermal treatment resulted in small areas of exposed Mo; however, both thermally treated and untreated micro-absorbers exhibited smooth surfaces after selenization. The treated CIGS micro-absorber exhibited improved elemental composition with a CGI ratio of 0.81±0.06, compared to 0.76±0.13 for untreated micro-absorber.

The annealed CIGS micro-cells display better overall performance, with higher average open-circuit voltage (V_oc). The best annealed CIGS micro-cell achieved a maximum PCE of 1.44%, with a V_oc and J_sc of 249 mV and 16.5 mA/cm². In contrast, the untreated CIGS micro-cells achieved a PCE of 0.47%, with a V_oc of 198 mV and a J_sc of 9.2 mA/cm², respectively.