A theoretical exploration of charge transfer dynamics in PTB7Ir/PC71BM triplet-material-based organic photovoltaics

Abstract

In the field of organic photovoltaics (OPVs), triplet materials are expected to be good candidates for improving power conversion efficiency (PCE) due to their long-lifetime and energy-tunable triplet excitons. A comparative photophysical analysis (including absorption spectra, exciton diffusion lengths, singlet and triplet charge transfer dynamics and so on) of PTB7/PC71BM and PTB7Ir/PC71BM cell devices was investigated, by DFT and TD-DFT methods, to explore why PTB7Ir/PC71BM device has obvious higher PCE (8.71%) than PTB7/PC71BM device (6.64%) in the same condition. The results manifest that the enhanced PCE of the PTB7Ir/PC71BM device is mainly attributed to the improving exciton diffusion lengths owing to the longer lifetime and increasing diffusivity of triplet excitons. Besides, to reveal the structure-property relationships of the polymer donor materials, we calculated the values of driving force for charge recombination to the triplet (CRT) with the changes of torsional angles between electron donating and electron withdrawing units of PTB7 and PTB7Ir. It is found that suitably increasing the angle and number of torsional angles of polymer donors to limit conjugation extent will prevent the CRT process at donor/acceptor interface and thus improve photovoltaic performance. This work would provide theoretical guidelines for the molecular design of high-efficiency triplet-material-based OPVs.