Influence of π-bridge conjugation on the electrochemical properties within hole transporting materials for perovskite solar cells

Hole transporting materials (HTMs) play an important role in most efficient perovskite solar cells (PSCs). In particular, donor-pi-bridge-donor type oligomers (D-pi-D) have been explored extensively as alternative and economical HTMs. In the present work, a series of triphenylamine-based derivatives as alternatives to the expensive Spiro-OMeTAD were explored by using first-principles calculations combined with the Marcus theory. The electronic structures, optical properties and hole mobilities of all the molecules were investigated to reveal the relationship between their charge-transport properties and the pi-bridge conjugation. The HOMO levels decrease with the extension of the pi-bridge conjugation length, which may lead to higher open-circuit voltages. Moreover, we employed a quantum mechanical (QM) methodology to estimate the carrier mobility for organic crystals. Specifically, an orientation function mu(Phi) (V, lambda, r, theta, gamma; Phi) is first applied to quantitatively evaluate the overall carrier mobility of HTMs in PSCs. The theoretically calculated results validate that this model predicts the hole mobility of HTMs correctly. More importantly, it is revealed that enhancing the pi-bridge conjugation in HTMs can improve the hole mobility, which will definitely improve the performance of PSCs. We hope that our theoretical investigation will offer a reliable calculation method to estimate the charge-transport properties of novel HTMs applied in perovskite solar cells.