Issue 48, 2022

Low dark current density in organic photodiodes achieved by reduced trap states of conjugated polymers bearing a fluorinated moiety

Abstract

A low dark current density (Jd) is critical for enhancing the performance of organic photodiodes (OPDs), and is evaluated using device parameters such as the signal-to-noise ratio, specific detectivity (D*), and linear dynamic range. However, the design rules for conjugated polymers to reduce the Jd in OPDs are rarely accessible. In this study, we present a strategy to obtain low Jd values in OPDs from a material point of view, by investigating the optoelectric properties of OPDs based on donor–acceptor (D–A) type conjugated copolymers. To this end, two D–A type copolymers were prepared with thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) as a common electron-withdrawing unit and 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT) or fluorinated BDT (FBDT) as an electron-donating unit. We then examined OPDs with an active layer formed by blending the fullerene derivative [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and the newly synthesized D–A type copolymers (PBDT-biTPD and PFBDT-biTPD). Under a reverse bias of −2 V, a low Jd of 6.09 × 10−10 A cm−2 was observed for the PFBDT-biTPD-based OPD, which showed a maximum responsivity of R = 0.30 A W−1 and D* = 2.13 × 1013 Jones at 500–650 nm, while the PBDT-biTPD-based OPD possessed a relatively higher Jd (1.43 × 10−9 A cm−2). Systematic analyses based on structural characterization of these active thin-film layers and the sub-gap density of states (DOS) measured using the OPDs and organic field-effect transistors revealed that fluorination of the electron donor unit in a D–A type conjugated polymer is an effective way to reduce the Jd of OPDs by suppressing the trap DOS in the blended active layer.

Graphical abstract: Low dark current density in organic photodiodes achieved by reduced trap states of conjugated polymers bearing a fluorinated moiety

Supplementary files

Article information

Article type
Paper
Submitted
29 sept. 2022
Accepted
14 nov. 2022
First published
18 nov. 2022

J. Mater. Chem. C, 2022,10, 18127-18136

Low dark current density in organic photodiodes achieved by reduced trap states of conjugated polymers bearing a fluorinated moiety

Y. Bai, S. Seo, J. Ha, M. Yoon, N. Yang, H. J. Park, S. C. Yoon, C. Lee, D. Hwang and J. Lee, J. Mater. Chem. C, 2022, 10, 18127 DOI: 10.1039/D2TC04116B

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