Polymer Chemistry PAPER Cite this: Polym. Chem., 2017, 8, 3612 Received 23rd March 2017, Accepted 22nd May 2017 DOI: 10.1039/c7py00497d rsc.li/polymers Dark current reduction strategies using edge-on aligned donor polymers for high detectivity and responsivity organic photodetectors† Seung Hun Eom, a So Youn Nam, a Hee Jin Do, a Jaemin Lee, a,b Sangho Jeon, c Tae Joo Shin, c In Hwan Jung, * d Sung Cheol Yoon* a,b and Changjin Lee* a,b We synthesized the conjugated polymers PT2OBT, PVT2OBT, and PFBT2OBT for use in organic photo- detecting devices. An octyloxy benzothiadiazole (OBT) moiety was used as a weak electron-accepting building block in the polymer system to cause the dominant absorption in the green-light region via weakening the intramolecular charge transfer (ICT) interactions between adjacent electron-donating moieties. In particular, indirect X-ray detection using a scintillator requires a low leakage current; thus, we focused on designing a molecular structure that can enhance the detectivity. The difluorobenzene- incorporated PFBT2OBT polymer showed a strong edge-on orientation both in the pristine film and in the blend film; however, PT2OBT and PVT2OBT have no preferred molecular orientation in the blend film. In the edge-on structure, the alkyl side chains of PFBT2OBT align on the surface of the electrode, forming an insulating layer, which decreases the tunneling leakage current, whereas in the latter cases, the interface between the semiconducting polymer backbone and PC 70 BM can come into contact with the electrode, forming a pathway for the leakage current. Consequently, the PFBT2OBT:PC 70 BM devices showed promising detectivities of over 10 13 Jones over a wide range of reverse biases of up to -2 V, resulting from their low dark current density of less than 2.3 × 10 -9 A cm -2 . Introduction Printed electronic sensors have gained much attention as in- expensive and fully flexible devices because they can be fabri- cated using the cost-saving roll-to-roll printing technique and room-temperature processes on flexible plastic or paper substrates. 1–6 Currently, various types of applications, such as capacitive, 7,8 piezoelectric, 9–13 photoelectric, 14,15 temperature, 16,17 gas sensors, 18,19 etc., have been considered for printed sensors; among them, photoelectric sensors have been extensively studied in both academia and industry, 20,21 and much progress has been made for research into organic photodetectors including mole- cular engineering and interface engineering. 22–24 The photoelectric effect occurs when light is converted to an electric current, similar to the process in the eyes, and the most popular photoelectric devices are CMOS image sensors 25,26 and digital X-ray sensors. 27 In particular, conven- tional X-ray sensors using silicon-based photodiodes are usually large-area devices due to optical limitations in concen- trating numerous X-ray photons into a small area. Additionally, X-ray detection for medical diagnostics requires highly X-ray transparent devices for shadow-free imaging. 28 Therefore, it is essential to develop organic photodiodes with much higher absorption characteristics than silicon-based photodiodes. In addition, for flexible applications, silicon, the most common photoelectric material, must be replaced by solution-processible organic semiconductors 29,30 because silicon is fabricated by using a vacuum-deposition process and, in thin films, the photo-absorption of silicon is not as high as that of organic semiconductors owing to the indirect band gap of silicon. The absorption properties of photoactive materials are significantly affected by the “responsivity”, which is directly related to the current density upon light illumina- tion ( photo-induced current density, J ph ) in photoelectric devices, and thus excellent light absorption characteristics are required for developing organic photoelectric materials. † Electronic supplementary information (ESI) available: 1 H NMR of the syn- thesized monomers and polymers, J–V characteristics of hole-only and electron- only devices in the SCLC regime, and photodiode properties in detail. See DOI: 10.1039/c7py00497d a Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea. E-mail: yoonsch@krict.re.kr, cjlee@krict.re.kr b Department of Chemical Convergence Materials, University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c UNIST Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan, 689-798, Republic of Korea d Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Republic of Korea. E-mail: ihjung@kookmin.ac.kr 3612 | Polym. Chem. , 2017, 8, 3612–3621 This journal is © The Royal Society of Chemistry 2017 Published on 23 May 2017. Downloaded by Korea Research Institute of Chemical Technology (KRICT) on 6/29/2018 10:04:17 AM. View Article Online View Journal | View Issue