First-principles calculations of the internal conversion and intersystem crossing rates of IDIC, Y6, and PC70BM

The efficiency of OSCs is controlled by the elementary processes occurring within the donor and acceptor domains and at their interface. Herein, we introduce a fully quantum-mechanical, first-principles framework for quantitatively predicting the rates of these elementary electronic transitions with high accuracy and at an affordable computational cost. The approach integrates Fermi's Golden Rule with density functional theory (DFT), enabling direct evaluation of both radiative and non-radiative transition rates from the knowledge of equilibrium nuclear configurations and vibrational frequencies and normal modes. It has been shown that the method yields excellent agreement with experiments and provides new insights into the role of electronic coupling and vibronic interactions in voltage losses. Here, we apply it to evaluate the internal conversion and intersystem crossing rates of IDIC, Y6, and PC₇₀BM.