Towards first-principles prediction of open-circuit voltage and short-circuit current density in small-molecule BHJ solar cells

Abstract

We present a theoretical framework to predict the open-circuit voltage and short-circuit current density of small-molecule bulk heterojunction solar cells. The method is based on the reciprocity relation and on a quantum mechanical description of the elementary processes. Radiative and non-radiative decay rates are evaluated through Fermi's golden rule. The Franck-Condon weighted density of states, computed via the generating function formalism, is employed as a genuine predictive tool, in that it enters the expression for both radiative and non-radiative decay rates and determines the absorption profile governing photogeneration. The method requires only a limited set of experimentally accessible inputs, namely the redox potentials and absorption spectra of the donor and acceptor materials, and involves a small number of adjustable parameters with negligible impact on the results. This makes the framework suitable for studying different donor-acceptor combinations in small-molecule organic solar cells based on either fullerene or non-fullerene acceptors. The model provides a direct interpretation of open-circuit voltage by linking voltage losses and photocurrent generation to specific processes, such as non-radiative recombination, subgap effects, and absorption-edge broadening. It also captures the temperature dependence of open-circuit voltage and short-circuit current density, which is essential for evaluating device performance under realistic operating conditions. Overall, the proposed approach provides a unified and computationally efficient framework for predicting open-circuit voltage and short-circuit current density and for relating them to the underlying physical processes in small-molecule organic solar cells.