Dark Current Suppression based on Theorical and Experimental Investigation of Barrier Energy between Transparent Anode and Photosensitive Layer via Material Design in Organic Photodetector

Organic semiconductor-based photodetectors (OPDs) have attracted a variety of attention attraction as a substitute for photodetectors with conventional inorganic materials for image sensors with high sensitivity. The organic semiconductor has tunable optical, electrical and mechanical properties by designing their structure. OPDs are evaluated by both dark current and photocurrent. However, research to investigate the causes of dark currants has been insufficient.<br/>Herein, we introduce non-fullerene acceptors which have opened new avenues with regard to improve the figure of merits of OPDs including wide absorption and fast response. [1] To investigate the effect of electron acceptors on dark current, OPDs are fabricated according to the types of acceptors in photosensitive layers, because anodic charge injection to acceptors has a major role of dark current under an applied reverse bias, which influences the performance of organic photodetectors. [2] The origin of dark current generated in an organic photodetector is observed through the modeling between current and voltage. To elucidate the dominant mechanism, the barrier energies in anodic contact are set from 0.6 to 1.0 eV using photosensitive layers composed of different acceptors. The current density–barrier energy characteristics measured under reverse bias strongly suggests the main source of dark current. The linear relationship observed in the current density–voltage characteristics also confirm tunnelling effects rather than thermionic emission. Consequently, the non-fullerene acceptor-based OPDs show a high detectivity and a faster response time, because of the excellent dark current suppression of the carrier injection and the low trap density of non-fullerene acceptor. Eh-IDTBR (ethylhexyl-rhodanine-benzothiadiazole-coupled indacenodithiophene) has been employed in OPDs and the results have been compared with fullerene derivatives (PC₇₁BM), in combination with electron donor PBDTTT-EFT material. eh-IDTBR based OPDs has shown excellent detectivity in comparison to PC₇₁BM based OPDs due to suppressed injected carriers and trap density. Furthermore, morphological and electrical changes depending on thermal and electrical stress have been investigated in the two different acceptor systems.<br/>Carbon nanotubes (CNTs) have attracted huge attention owing to their outstanding flexibility, transparency, and conductivity, so that CNTs can replace an indium-tin oxide (ITO) for efficient curved and flexible photodetectors. Especially, CNTs-based electrode with high chemical resistance is the excellent candidate for organic photodetectors (OPDs), because the devices have been fabricated through solution process. In addition, The OPDs require dense nanotubes arrays to form top layers suitably to enhance the performance. The CNT electrode with high transmittance reduces dark current below pA level and exhibits the higher detectivity than that of ITO-based devices owing to its deep work function related with the high electron injection barrier energy. [3] The flexible OPDs fabricated utilizing the CNT electrode stably operate after 500-bending tests. By investigating the charge injection mechanism according to barrier energy between transparent anode and photosensitive layer in organic photodetector, This work can help devise an effective strategy to suppress dark current for effective organic photodetector device implementation with increase of process yield and reduction of unit prices.<br/>[1] Jang et al., Adv. Funct. Mater., 2020, 30, 2001402<br/>[2] Jang et al., Adv. Funct. Mater., 2023, 33, 2209615<br/>[3] Jang et al., Nano Today, 2021, 37, 101081