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 ECT 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 ECT correlates linearly with the open-circuit voltage (V oc) of photovoltaic devices in D-A blends via eVoc=E CT0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1 CT) occurs when E g-ECT>0.1 eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT T1) can occur when ECT-E T>0.1 eV. From these relations, it is concluded that in D-A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc=Eg=.6 eV) and ii) the singlet-triplet energy gap should be dEST