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

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 (Cl6-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 x 1011 cm-2), while it also contributes to a high thermal noise current density (3.453 x 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 x 1011 cm-2); yet this also contributes to significantly low thermal noise current density (2.264 x 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.