Impact of tailoring BTBT-based hole-transporting materials on perovskite photovoltaics under indoor illumination†

Small organic molecules have garnered significant attention as hole-transporting materials (HTMs) in perovskite photovoltaic (PPV) devices due to their enhanced stability, cost reduction, and improved performance. To achieve optimal performance in PPVs, 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD) currently is the leading HTM but its instability over a prolonged period is insufficient for ensuring reliable long-term device operation, and the current high market price poses a barrier to its uninterrupted use in large-scale manufacturing. In this work, we synthesized and characterized two novel small organic molecules based on the central [1]benzothieno[3,2-b][1]benzothiophene (BTBT) core, termed BTBT-1 and BTBT-2, and applied as HTMs in indoor PPVs (i-PPVs). The dopant-free BTBT-2 demonstrated a power conversion efficiency (PCE) of 31.73%, which is higher than that of a device using BTBT-1 (29.19%) and doped Spiro-OMeTAD (28.87%) under the illumination of a 1000 lux LED lamp. Conspicuously, the hydrophobic nature of the BTBT-2 based dopant-free HTM afforded excellent stability compared to Spiro-OMeTAD doped i-PPVs, which enables better moisture resistance and long-term stability under indoor conditions. These results suggest that BTBT-2 is a promising candidate for high-performance, stable indoor photovoltaic technology, offering a cost-effective and reliable alternative for large-scale applications.