Characterization of small molecule-based organic photodetectors for non-fullerene bulk heterojunction with strong response in the visible light region

• Fine-tuning the bulk heterojunction (BHJ) in the active layer is investigated in this study. • The blends of BCzPh:Cl 6 -SubPc demonstrate the degradation of optical (electronic) properties as the ratio tunes from 1:1 to 1:4, which implies an enhancement in photon harvesting. • Trap state density and bulk thermal generation increase in tune with alterations to the blend ratio from 1:1 to 1:4. • In spite of high bulk thermal generation, under a bias of -4 V, the blend ratio of 1:4 slightly provides higher specific detectivity by referring to the white noise current density. The study of organic photodetectors (OPDs) undergoes expansion as non-fullerene acceptors (NFAs) are introduced, allowing for tuning of the optical (electronic) properties with their donors. A novel active layer is promoted herein—an NFA of chlorinated subphthalocyanine (Cl 6 -SubPc) is composited with 9,9′-diphenyl-9H,9H′-3,3′-bicarbazole (BCzPh) in a bulk heterojunction (BHJ) configuration by vacuum deposition. Four blend ratios of BHJ (1:1, 1:2, 1:3, and 1:4)—particularly their intrinsic characteristics—are evaluated to understand their influence on specific detectivity, which is a pivotal performance metric in OPDs. Experimental studies reveal that a BHJ of 1:4 offers the highest responsivity as a consequence of its lower optical bandgap (2.010 eV), reduced Urbach energy (367 meV), and high trap state density (1.838 × 10 11 cm -2 ), while it also contributes to a high thermal noise current density (3.453 × 10 -13 A Hz -1/2 ). In contrast, a BHJ of 1:1 achieves the lowest responsivity, which is attributed to a high feature profile at the optical bandgap (2.018 eV) and Urbach energy (407 meV), as well as a low trap state density (1.215 × 10 11 cm -2 ); yet this also contributes to significantly low thermal noise current density (2.264 × 10 -13 A Hz -1/2 ). Notwithstanding the increase in bulk thermal generation as the blend ratio is fine-tuned, under a bias of -4 V and referring to the white noise current density, a BHJ of 1:4 is able to provide slightly higher specific detectivity. In brief, this comparative study highlights the bulk thermal generation that intrinsically limits the performance of OPDs.