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

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-g - E-CT > 0.1 eV. Additionally, it is shown that subsequent charge recombination state to the lowest triplet excited state (E-T) of D or A (CT -> T-1) can occur when E-CT - E-T > 0.1 eV. From these relations, it is concluded that in D-A blends optimized for photovoltaic action: i) the maximum attainable V-oc is ultimately set by the optical band gap (eV(oc) = E-g - 0.6 eV) and ii) the singlet-triplet energy gap should be Delta E-ST < 0.2 eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area.