D-pi-A-Type Triazatruxene-Based Dopant-Free Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells

Three donor-pi-bridge-acceptor (D-pi-A)-type organic small molecules coded CI-B1, CI-B2, and CI-B3 are designed, synthesized, and used as dopant-free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron-donating triazatruxene central core (D), terthiophene conjugated arms (pi), and three different strong electron-accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end-capping, three different organizations of self-assembly with pi-pi stacking are observed in the solid thin films. Dopant-free CI-B1, CI-B2, CI-B3, and spiro-OMeTAD with dopants are used with triple cation perovskite composition Cs-0.1(MA(0.15)FA(0.85))(0.9)Pb(I0.85Br0.15)(3)(MA: CH3NH3+, FA: NH(sic)CHNH3+) in n-i-p architecture. The cells prepared with CI-B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state-of-art of spiro-OMeTAD with dopants (18.02%), but also demonstrates improved long-term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant-free nature, high durability, and edge-on molecular orientation of CI-B3 show its great promise as a HTM candidate for efficient and stable PSCs.