"Ultralow Dark Current in Near-infrared Organic Photodetector via Crosslinked Conjugated Polyelectrolyte Hole Transporting Layer"
S. Chaea, H. M. Luonga, A. Yia,b, J. Chatsirisupachaia,c, B. M. Kima, Y. Wana, V. Promarakc, Hyo Jung Kimb, and Thuc-Quyen Nguyena*
a Center for Polymers and Organic Solids (CPOS), Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA
b Department of Organic Material Science and Engineering, School of Chemical Engineering,
Pusan National University, Busan 46241, Republic of Korea
c Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong 21210, Thailand
In the realm of infrared photodetectors (IR-PDs), there is a burgeoning market for various applications, encompassing medical uses, imaging, and motion sensors for robotics and self-driving systems. However, current inorganic IR-PDs are prohibitively expensive due to their manufacturing processes and are limited in flexible applications due to material constraints. Consequently, active research aims to replace them with organic semiconductors featuring a low band gap capable of absorbing the infrared spectrum. Nonetheless, in the case of infrared organic photodetectors (IR-OPDs), device detectivity is hindered by a notably high dark current, presenting a challenge to achieving optimal functionality. This increased leakage current arises from inherent thermal generation within the low bandgap organic semiconductors. Additionally, the role of the interfacial layer, which governs the flow of this leakage current, is a critical factor influencing device performance. Consequently, the interfacial layer in conventional photodetection structures has predominantly utilized PEDOT:PSS and inorganic materials due to the necessity of introducing an organic solvent-resistant material during the manufacturing of the photoactive layer to prevent damage to the underlying interfacial layer, thereby limiting the application of an ideal interfacial layer. In this study, a water-soluble conjugated polymer electrolyte (CPE) was employed as the interfacial layer, capitalizing on its properties. Furthermore, an insulator crosslinking technique was introduced to regulate the conductivity of the interfacial layer. Initially, a conjugated electrolyte polymer with an optimal energy level molecular structure suitable for an infrared photodetector was incorporated, and the amount of cross-linking was finely adjusted. The result was a device with significantly reduced dark leakage current, achieving one of the highest performances among reported near-infrared organic photodetectors operated at practical applied voltages (over 2.0 V). Expanding on these advancements, we demonstrated its applicability as a flexible photoplethysmography device with a large area for heart rate monitoring.
