Origin and Energy of Intra-Gap States in Sensitive Near-Infrared Organic Photodiodes

Trap states in organic semiconductors are notoriously detrimental to the performance of organic electronics. However, the origin and energetics of trap states remain largely elusive and under debate, especially for bulk-heterojunction (BHJ) photodiodes consisting of electron donor and acceptor materials. Combining three sensitive techniques now enables locating the origin and energy of trap states in six state-of-the-art polymer - non-fullerene acceptor organic photodiodes (OPDs) with noise-based specific detectivities exceeding 10(13) Jones. Analyzing the temperature dependence of the reverse-bias dark current density (J(d)) identifies intra-gap states in the polymers, lying 0.3-0.4 eV above the energy of the highest occupied molecular orbital, as being responsible for J(d). Sub-bandgap external quantum efficiency spectra of donor-only and acceptor-only diodes confirm that intra-gap states are much more abundant in the polymers. Likewise, responsivity measurements at ultra-low light intensities (10(-7) mW cm(-2)) show trap-mediated charge recombination in BHJ and polymer-only diodes, but not in acceptor-only devices. The results imply that to further improve the specific detectivity of near-infrared OPDs, the intra-gap state energy, and density need to be reduced.