Optimizing ionic strength of interfacial electric double layer for ultrahigh external quantum efficiency of photomultiplication-type organic photodetectors†

A synthetic approach for engineering an electric double layer (EDL)-photoactive layer interface in photomultiplication-type organic photodetectors (PM-OPDs), whereby the EDL is strategically embedded between a transparent cathode and the photoactive layer to enhance the photomultiplication mechanism, is demonstrated. To elucidate the effects of the EDL on the PM-OPD performances, a series of conjugated polyelectrolytes (CPE), which form EDLs in solid-state films, based on a poly(fluorene-co-phenylene) backbone, are synthesized by varying ionic densities of quaternary ammonium cations and bromide counterions (per polymer repeat unit). Together with inherent characteristics of the CPE EDL, including modifications in the work function of the transparent cathode suitable for Schottky junction formation and the development of a favorable morphological environment for on-coated polymer semiconductors to exhibit preferential orientations and form defectless films, we find an increase in the EDL ionic density improves the electron trapping ability, affording efficient gain generation. The optimized PM-OPD with the highest ionic density exhibits a record high external quantum efficiency of 4?440?000%, responsivity of 18?700 A W^?1, and gain-bandwidth product of 1.98 × 10⁷ Hz as well as an exceptionally large specific detectivity of 3.09 × 10¹⁴ Jones. This work contributes toward further improvements in PM-OPDs, particularly by adjusting the electrostatic environment.