Cost-Effective Cobalt(II) Acetate as an Efficient and Stable Hole Transport Layer in Inverted Organic Photodetectors

To address the upcoming large-scale production demands of organic photodetectors (OPDs) and image sensors, there is a growing need to develop interfacial materials that balance cost-effectiveness, performance, and stability. In this work, we systematically investigated the characteristics of a solution-processed cobalt(II) acetate (Co(OAc)₂) hole transport layer (HTL) in the OPDs. The work included a detailed analysis of the suitability in both conventional and inverted device architectures, the choice of the solvent system for the precursor solution, reaction conditions, and thickness optimization. We also evaluated the performance of a Co(OAc)₂ HTL in a top-illuminated device architecture to assess potential applications in image sensors. The results indicated that the device composed of a Co(OAc)₂ HTL exhibited lower noise current compared to the device with vacuum-based MoO₃ HTLs, and the device also demonstrated excellent stability in an unencapsulated condition. Consequently, in a top-illuminated architecture composed of a short-wave infrared (SWIR) photoactive layer, the device using Co(OAc)₂ as the HTL achieved a dark current density of 1.44 × 10^–5 A/cm² and a detectivity of 1.25 × 10⁸ Jones in the SWIR region at 1260 nm, outperforming the MoO₃-based device, which exhibited a dark current density of 2.47 × 10^–5 A/cm² and a detectivity of 4.73 × 10⁷ Jones. This solution-processed HTL meets the industrial demands for performance, stability, and cost efficiency. The Co(OAc)₂ HTL has the potential to become a crucial interfacial technology in OPD development, contributing to advancements in the organic image sensor industry.