Low-cost dopant-free fluoranthene-based branched hole transporting materials for efficient and stable n-i-p perovskite solar cells

It has been widely recognized that hole transporting materials (HTMs) play a key role in the rapid progress of perovskite solar cells (PVSCs). However, common organic HTMs such as spiro-OMeTAD not only suffer from high synthetic costs, but also usually require the additional chemical doping process to improve their hole transport ability, which unfortunately induces the terrible stability issue. Therefore, it is urgent to develop low-cost dopant-free HTMs for efficient and stable PVSCs. In this work, we have successfully developed a new class of efficient dopant-free fluoranthene-based HTMs (TPF1-5) with quite low lab synthetic costs by combining donor-acceptor and branched structure designs. The detailed structure-property study revealed that tuning the twisted arms at different substitution sites would regulate the intermolecular interactions and film-forming ability, thereby significantly affecting the performance of the HTMs. By applying these HTMs in conventional PVSCs, the dopant-free TPF1-based devices not only achieved the best efficiency of 21.76%, which is comparable to that of the doped spiro-OMeTAD control devices, but also showed much better operational stability, which maintained over 87% of the initial efficiency under maximum power point tracking after 1038 h.