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 mu m) 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 mu m 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 mu m thick absorber layers, the highest reported value for submicron-based solution processed, low bandgap CISSe solar cells.