An unsymmetrical bifluorenylidene–fluorene based hole-transporting material for perovskite solar cells†

Hole-transporting materials (HTMs) play a crucial role in perovskite solar cells (PSCs). Herein, we have designed and synthesized a new hole-transporting molecule, denoted as sp-35, from low-cost, commercially available reagents via a simple two-step synthesis route. The molecular architecture of sp-35 consists of a bifluorenylidene core moiety covalently linked with phenylfluorenamine units at the end. The suitable energy levels, ideal surface morphologies, high hole mobility of 2.388 × 10⁻³ cm² V⁻¹ s⁻¹, and stable chemical structure of sp-35 make it an effective HTM. As a result, PSCs constructed with sp-35 exhibit a high power conversion efficiency (PCE) of 21.59%, while spiro-OMeTAD shows a PCE of 20.42%. Promisingly, the device with sp-35 exhibits significantly better long-term and thermal stabilities than spiro-OMeTAD. This work presents a new molecular design and an in-depth understanding of the HTL strategy and its potential for the development of highly efficient cell performances.