The Energy of Charge‐Transfer States in Electron Donor–Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells

Abstract Here, a general experimental method to determine the energy E CT of intermolecular charge‐transfer (CT) states in electron donor–acceptor (D–A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D–A blends to correct for a constant Coulombic term. It is shown that E CT correlates linearly with the open‐circuit voltage ( V oc ) of photovoltaic devices in D–A blends via eV oc = E CT − 0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S 1 with energy E g ) in the blend to the CT state (S 1 → CT) occurs when E…