Systematic Optical Design of Constituting Layers to Realize High-Performance Red-Selective Thin-Film Organic Photodiodes

A high-performance red-selective thin-film organic photodiode (OPD) is designed. Dual-band absorbing poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT) is introduced as the photoactive donor layer of a planar heterojunction OPD in conjunction with a sol–gel synthesized ZnO acceptor layer. The active layer thickness is systematically controlled to suppress band II absorption (λ_max = 420 nm) of PPDT2FBT without sacrificing band I absorption (λ_max = 650 nm). The optimal PPDT2FBT thickness is 320 nm to realize red-selective absorption while maintaining the low dark current density of the OPD (predicted by optical simulation conducted using the transfer matrix method). In addition, the introduction of ZnO (with a strategically determined thickness) as an acceptor layer in front of PPDT2FBT in an illumination pathway enables further suppression of band II absorption because of the blue color filter effect. Consequently, the resulting OPD with a device architecture of indium tin oxide/ZnO/PPDT2FBT/MoO₃/Ag shows an outstanding red-selective photodiode performance with peak detectivity up to 3.04 × 10¹² Jones and a high linear dynamic range of 116 dB.