High glass transition temperature dopant-free hole transport material via D-A-π-A-D-strategy for perovskite solar cell

Despite the great leap forward perovskite solar cells (PSCs) have achieved in power conversion efficiency, the device instability remains one of the major problems plaguing its commercialization. Dopant-free hole transport material (HTM) has been widely studied as an important strategy to improve the stability of PSCs due to its avoidance of moisture-sensitive dopants and cumbersome doping process. In this work, a series of dopant-free HTMs L1F, L2F and L3F based on D-A-π-A-D configuration were synthesized through two steps of reaction. L3F presents a high glass transition temperature of 180 °C and thermal decomposition temperature of 448 °C. Notably, electron paramagnetic resonance signals of L1F, L2F and L3F powders indicate the open-shell quinoidal diradical resonance structure in their aggregation state due to aggregation-induced radical effect. All these HTMs present higher hole mobility than dopant-free Spiro-OMeTAD, and the dopant-free L3F-based PSC device achieves the highest power conversion efficiency of 17.6% among them. In addition, due to the high hydrophobic properties of L1F, L2F and L3F, the perovskite films spin-coated with these HTMs exhibit higher humidity stability than doped Spiro-OMeTAD. These results demonstrate a promising design strategy for high glass transition temperature dopant-free hole transport material. The open-shell quinoid-radical organic semiconductors are not rational candidates for dopant-free HTMs for PSC devices.