Low cost and stable quinoxaline-based hole-transporting materials with a D–A–D molecular configuration for efficient perovskite solar cells
The use of expensive hole transporting materials (HTMs), such as spiro-OMeTAD, in perovskite solar cells (PSCs) is one of the critical bottlenecks to hinder their large-scale applications. Some low-cost alternatives have been developed by combining conjugated electron-rich cores with arylamine end-caps, usually in a donor–π spacer–donor (D–π–D) molecular configuration. However, incorporation of electron-rich cores can lead to undesirable up-shift in the HOMO energy level and low stability, and few of these new HTMs can outperform spiro-OMeTAD in terms of device efficiency. Given that electron-deficient units have shown many advantages in developing efficient and stable photovoltaic dyes and polymers, we herein present a couple of novel molecular quinoxaline-based HTMs (TQ1 and TQ2) with a donor–acceptor–donor (D–A–D) configuration, especially for rationally modulating the HOMO level, improving the stability and decreasing the cost. The TQ2-based PSCs exhibit a maximum efficiency of 19.62% (working area of 0.09 cm2), unprecedentedly outperforming that of spiro-OMeTAD (18.54%) under the same conditions. In comparison, TQ1 based devices only showed moderate efficiencies (14.27%). The differences in hole extraction and transportation between TQ1 and TQ2 are explored by photoluminescence quenching, mobility and conductivity tests, and single crystal analysis. The scaling-up of the TQ2 based device to 1.02 cm2 achieves a promising efficiency of 18.50%, indicative of high film uniformity and processing scalability. The significant cost advantage and excellent photovoltaic performance strongly indicate that the D–A–D featured TQ2 has great potential for future practical applications.