Enhanced Open-Circuit Voltage of Eco-Friendly Silver Bismuth Iodide Thin-Film Photovoltaics with PTB7 Polymer-Based Hole Transport Layer

Next-generation and solution-processed thin-film solar cells have been attracted considerable attention because of their low cost, light weight, flexibility, and aesthetics. However, most of solution-processed thin-film solar cells are now focused on the use of photovoltaic absorbers containing the toxic element of Pb. In this study, eco-friendly silver-bismuth-iodide (Ag-Bi-I) thin-film photovoltaic devices with high open-circuit voltages (V_OC) are developed by utilizing polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) as the hole transport layer (HTL). The solution-processed AgBi₂I₇ semiconductor, which is an Ag-Bi-I ternary compound, exhibit features suitable for photovoltaic layers in thin-film solar cells, including a three-dimensional (3D) crystal structure, good surface morphology, and low optical bandgaps of 1.87 eV. Meanwhile, the solution-processed AgBi₂I₇ thin-film solar cell based on the PTB7 HTL exhibit a power conversion efficiency of 0.94% with an improved V_OC value of 0.71 V owing to the deeper highest occupied molecular orbital (HOMO) energy level compared to that of poly(3-hexylthiophene-2,5-diyl) (P3HT). In other words, the V_OC of the PTB7 HTL-based device is 20% higher than that of the P3HT HTL-based control device. Our results provide a new approach for increasing the V_OC of eco-friendly Ag-Bi-I thin-film photovoltaics and indicate that further HTL engineering is necessary to simultaneously improve the V_OC and performance of the devices.

Graphical Abstract