Abstract
The development of highly efficient hole transporting materials (HTMs) for perovskite solar cells (PSCs) is still one of the most thrilling research subjects in the development of this emerging photovoltaic technology. Inner ring engineering of the aromatic core of new HTMs - consisting of three fused rings endowed with four triarylamine units - reveals major performance effects over the fabricated devices. In particular, substitution of the central pyrrole ring in dithienopyrrole (DTP) by a benzene ring -benzodithiophene (BDT) - allows enhancing the power conversion efficiency from 15.6% to 18.1%, in devices employing mixed-perovskite (FAPbI(3))(0.85)(MAPbBr(3))(0.15) (MA: CH3NH3+, FA: NH=CHNH3+) under 1 sun illumination. In comparison, 2,2',7,7'-tetrakis(N, N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) yielded a power conversion efficiency of 17.7%. The novel HTM molecules show an efficient quenching of the perovskite photoluminescence, indicating an efficient charge transfer from the active layer to the HTM, along with a good conductivity (comparable to that of the spiro-OMeTAD reference). Density functional theory (DFT) calculations allowed rationalizing the electrochemical and optical properties and predicting a reorganization energy (lambda) for the best performing BDT-based HTM (0.101 eV) significantly lower than that computed for the benchmark spiro-OMeTAD (0.139 eV).