Rear Surface Passivation for Ink-Based,Submicron CuIn(S,Se)2 Solar Cells

A N, N-dimethylformamide and thiourea-based route is developed to fabricate submicron (0.55 and 0.75 µm) thick CuIn(S,Se)₂ (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. However, reducing the absorber film thickness below 1 µm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly-recombinative, and mediocre-reflective Mo/CISSe rear interface. For the first time, to mitigate the rear recombination losses, a novel rear contacting structure involving a surface passivation layer and point contact openings is developed for solution processed CISSe films and demonstrated in tangible devices. An atomic layer deposited Al₂O₃ film is employed to passivate the Mo/CISSe rear surface while precipitates formed via chemical bath deposition of CdS are used to generate nanosized point openings. Consequently, Al₂O₃ passivated CISSe solar cells show an increase in the open-circuit voltage (V_OC) and short-circuit current density when compared to reference cells with equivalent absorber thicknesses. Notably, a V_OC increase of 59 mV contributes to active area efficiencies of 14.2% for rear passivated devices with 0.75 µm thick absorber layers, the highest reported value for submicron-based solution processed, low bandgap CISSe solar cells.