All-Polymer Photodetectors with n-Type Polymers Having Nonconjugated Spacers for Dark Current Density Reduction

Abstract

Recently, all-polymer-based p-n bulk heterojunction organic photodiodes exhibited a high photocurrent density (Jph) through the state-of-the-art synthesis in n-type conjugated polymers. However, in the organic photodetector (OPD) application, the on-off ratio (Jph/dark current density (Jd)) is a performance-determining parameter; thus herein we focused on effectively increasing the on-off ratio by lowering the Jd of all-polymer OPDs. We developed four n-type polymers, with nonconjugated spacers, which effectively suppressed the Jd of the devices; the increased insulating properties and excellent hole-blocking properties under dark conditions aided Jd suppression. Moreover, breaking the π-conjugation of the n-type polymers, by adding short bulky nonconjugated spacers, effectively increased Jph and decreased Jd in relation with those upon the addition of long flexible alkyl spacers. Hence, the OPDs based on the n-type polymer with a nonconjugated spacer exhibited a high specific detectivity of >1012 Jones and a -3 dB cutoff frequency of 219 kHz. Notably, they also displayed ultrafast rising and falling response times of 2 and 9 μs, respectively, at -2 V.