Design of two novel hole transport materials via replacing the core of spiro-OMeTAD with tetrathiafulvalene and tetraazafulvalene for application in perovskite solar cells

Two novel hole transport materials (HTMs) were designed by way of replacing the core of 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'spirobifluorene (spiro-OMeTAD) with tetrathiafulvalene (TTF) and tetraazafulvalene (TAF). The frontier orbitals, stability and solubility, reorganization energy, and hole mobility of the new materials were investigated via density functional theory (DFT) calculations in combination with the Marcus hopping model. Time-dependent density functional theory (TD-DFT) was also used for evaluating the optical properties including the absorption and emission spectra. It was found that the HOMO level of the two proposed HTMs (-5.25 and -5.20 eV) is lower than that of spiro-OMeTAD ( -5.17 eV). The results showed that the maximum absorption band of the new designed HTMs with the TTF and TAF cores is blue and red-shifted compared to spiro-OMeTAD. This may indicate that these materials have no competition with the perovskite in absorbing light. Additionally, the hole reorganization energy is less than the electron reorganization energy for our structures, revealing these materials can act as HTMs more efficiently than electron transport materials. The calculated hole mobility of TAF- and TTF- OMeTAD was obtained as 4.41 x 10(-4) and 1.15 x 10(-6) cm(2) v(-1) s(-1), respectively. This result indicates that TAF-OMeTAD shows a great potential for use as an alternative HTM for expensive spiro-OMeTAD.