Thienothiophene-Assisted Property Optimization for Dopant-Free pi-Conjugation Polymeric Hole Transport Material Achieving Over 23% Efficiency in Perovskite Solar Cells

Hole transport materials (HTMs) play essential roles in achieving high photovoltaic performance and long-term stability in the n-i-p structure of perovskite solar cell (PSC) devices. Recently, dopant-free polymeric materials as HTMs in PSCs have attracted considerable attention owing to high carrier mobility and excellent hydrophobicity. However, achieving similar efficiencies to those of doped small molecule HTMs such as Spiro-OMeTAD is a big challenge. Herein, a thienothiophene pi-bridge is selected as a stabilizer and energy level regulator incorporated into a donor-acceptor-type HTM to synthesize a new polymer, Nap-SiBTA. The incorporation of the thienothiophene group improves the thermal stability and favors the high planarity and face-on orientation, promoting high charge carrier mobility and tunable optical band gap. Finally, the dopant-free polymer Nap-SiBTA-based PSC achieves an excellent power conversion efficiency (PCE) of 23.07% with a high fill factor of 80.85%. To the best of the authors' knowledge, this is one of the best efficiencies in dopant-free HTM PSCs. Moreover, the unencapsulated device retains 93% of its initial PCE after 1000 h owing to the excellent hydrophobicity of Nap-SiBTA. This work provides a general and practical method to design dopant-free HTMs for the high efficiency and long-term stability of PSCs.