In a new paper – Spin-dependent charge transfer state design rules in organic photovoltaics – published in Nature Communications, we probe intermolecular charge transfer states with the aid of their luminescent thermally activated delayed fluorescence (TADF) channel. In addition to probing this TADF with time-resolved photoluminescence alone, we examine the dynamics of the system under a variation of the electron and hole spacing of the CT states, controlled through external pressure. The comparison of two different acceptor molecules with either low or high local triplet energies directly shows the significance of the careful energetic design of charge separating interface in organic photovoltaics. Here, a low lying triplet level on the acceptor significantly quenches the CT triplet states, increasing the overall recombination losses.
Singlet exciton fission – a process found in special organic semiconductors – splits a high energy singlet state, which is created upon photon absorption, into two, low-energy triplet states of equal energy. Incorporating such organic materials in a photovoltaic cell paves the way to more carriers produced per incident photon flux.
Based on the singlet fission archetype material pentacene, in an exciton confining architecture around a heterojunction with the fullerence acceptor C60, we (Congreve et al., Science 340, 334 (2013)) have demonstrated an external quantum efficiency of an organic solar cell exceeding 100%, breaking the barrier of one electron per photon in the visible spectrum.