Tantalum-Doped Tin Oxide Films with Post-Annealing in H2 Strategy Enabling Improved Performance of Indium-Free Perovskite Solar Cells

Abstract

Indium tin oxide (ITO) films are widely used as transparent electrodes in perovskite solar cells (PSCs). However, the limited supply and high cost of rare indium pose challenges to the sustainable development of PSCs. In this study, sputtered tantalum-doped tin oxide (TTO) films with post-annealing in H₂ (TTO-H₂), are proposed as an attractive alternative to ITO electrodes in PSCs. It is demonstrated that TTO films post-annealed in H₂ atmosphere exhibited lower sheet resistance of 91.8 Ω sq⁻¹, with a higher carrier concentration of 6.51 × 10²⁰ cm⁻³, and a higher mobility of 6.96 cm² V⁻¹ s⁻¹, compared to TTO films post-annealed in O₂ and N₂ atmospheres. The mechanism behind the improvement in electrical characteristics is revealed. Indium-free inverted PSCs using TTO-H₂ bottom electrodes are constructed. High-quality TTO-H₂/MeO-2PACZ/perovskite interface is demonstrated with advantages of higher built-in electric field, lower defect state density, and improved carrier transport ability, compared to the reference ITO/MeO-2PACZ/perovskite interface. Thus, the prepared PSCs using TTO-H₂ electrodes exhibit improved photovoltaic performance compared to reference ITO-based PSCs: the open-circuit voltage (V_oc) is enhanced from 1.100 to 1.134 V; and the power conversion efficiency (PCE) is enhanced from 20.58% to 20.77% with 1% increase. This study demonstrates the indium-free PSCs with enhanced V_oc and PCE for the first time, and elucidates the underlying mechanisms, providing new insights for high-performance and low-cost PSCs.