Realizing photomultiplication-type organic photodetectors based on C₆₀-doped bulk heterojunction structure at low bias

Photomultiplication (PM) structure has been widely employed to improve the optoelectronic performance of organic photodetectors (OPDs). However, most PM-type OPDs require a high negative operating voltage or complex fabrication. For obtaining high-efficiency OPDs with low voltage and easy process, here the bulk heterojunction (BHJ) structure of high exciton dissociation efficiency combined with the method of trap-assisted PM are applied to the OPDs. In this paper, we investigate the operating mechanism of OPDs based on poly(3-hexylthiophene) (P3HT):(phenyl-C61-butyric-acid-methyl-ester) (PC₆₁BM), and poly-{[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[3-fluore-2-(octyloxy)carbonyl-thieno[3,4-b]thiophene-4,6-diyl]} (PBDT-TT-F):PC₆₁BM doped with C₆₀ as active layer. Furthermore, the influence of C₆₀ concentration on the optoelectronic performances is also discussed. With 1.6 wt.% C₆₀ added, the P3HT:PC₆₁BM:C₆₀ OPD exhibits a 327.5% external quantum efficiency, a 1.21 A·W^-1 responsivity, and a 4.22×10¹² Jones normalized detectivity at -1 V under 460 nm (0.21 mW·cm^-2) illumination. The experimental results show that the effective electron traps can be formed by doping a small weight of C₆₀ into BHJ active layer. Thus the PM-type OPDs can be realized, which benefits from the cathode hole tunneling injection assisted by the trapped electrons in C₆₀ near the Al side. The efficiency of PM is related to the C₆₀ concentration. The present study provides theoretical basis and method for the design of highly sensitive OPDs with low operating voltage and facile fabrication.