Design of Low Crystallinity Spiro-Typed Hole Transporting Material for Planar Perovskite Solar Cells to Achieve 21.76% Efficiency

Hole transporting materials (HTMs) play a crucial role in achieving highly efficient and stable perovskite solar cells (PSCs). Spiro-typed materials being the most widely used HTMs are commonly utilized with dopants, such as Li-TFSI, to improve their carrier mobility significantly. However, dopants could affect the morphology of hole transporting layer negatively by forming defects and pinholes which restrict the performance of devices. Here, we adopt the extended π-conjugated structures N-ethylcarbazole and dibenzothiophene to substitute the donor group 4-methoxyphenyl of spiro-OMeTAD, devising two novel HTMs, SC and ST, respectively. Notably, SC possesses low crystallinity and good solubility due to the existence of ethyl in side groups, leading to decent miscibility with Li-TFSI to prevent unfavorable phase-separation. The SC-based device delivers the best power conversion efficiency (PCE) of 21.76% which is higher than that of spiro-OMeTAD (20.73%), attributed to the formation of smooth and pinhole-free morphology. Moreover, it exhibits long-term stability and retains over 90% of initial PCE value for more than 30 days without encapsulation in ambient air. In contrast, the ST-based device suffers from dense pinholes induced by its relatively high crystallinity and poor solubility, resulting in a low PCE of 18.18% and inferior stability. Thus, it is effective to modify the side groups in spiro-typed HTMs with specific structures to obtain predictable properties, fabricating PSCs with high efficiency and stability facilely.