Swollen and orientated intermolecular packing toward suppressed dark current in organic photodetectors

Suppressing the dark current density in organic diodes is essential for developing practical consumer electronics such as in-door photovoltaics and photodetectors. While extensive research has explored dark current from thermodynamics and kinetics (of free and trapped charges) perspectives, a clear relationship between nanostructure and dark current remains elusive. In this contribution, the relationship between the structural/morphological order of non-fullerene acceptors (NFAs) with the dark current and shunts in their organic photodetectors (OPDs) is investigated. Starting with the state-of-the-art NFA C9-12 (BTP-eC9), we systematically tune the molecular structural symmetry and alkyl chain length. We find that unsymmetrizing the end groups and/or extending the alkyl chains of NFAs leads to swollen intermolecular packing in solid state, effectively suppressing microscopic shunting paths. Additionally, increasing alkyl chain length promotes more oriented intermolecular packing in the out-of-plane direction, reducing energetic disorder and consequently suppressing the thermal generation of dark charges. These combined benefits lead to a simultaneous suppression of both thermally activated diode current and shunt current in OPDs based on unsymmetrical non-fullerene acceptors with long alkyl chains.