Effects of Conjugation Spacers in Diketopyrrolopyrrole-Based Copolymers for All-Polymer-Based Photodiodes

Abstract

The selection of the π-conjugation spacers in semi-conducting polymer backbone is one of the important factors for determining the optoelectrical and morphological properties in organic photodiodes. To study the effects of π-conjugation spacers in donor–acceptor (D-A)-type alternating copolymers on their device performances in all-polymer-based photodiodes (all-PPDs), a series of diketopyrrolopyrrole (DPP)-based copolymers as polymer donors (PDs) were designed and synthesized. In detail, three different π-conjugation spacers, thiophene (T for P1), thienothiophene (TT for P2), and bithiophene (BT for P3), were incorporated into the DPP-based copolymer structures. Interestingly, all-PPDs based on the series of P1–P3 as PDs and N2200 as a polymer acceptor (PA) exhibited totally distinct device performances in terms of external quantum efficiency (EQE), dark current density (JD), and ideal detectivity (D*). The P1-based device showed suppressed JD (6.1 × 10−11 A/cm2 at −1 V) compared to those of the P2- and P3-based devices due to the lower lying of the highest occupied molecular orbital (HOMO) level of P1. However, the P3-based all-PPD showed higher EQE (16% at 630 nm wavelength and −1 V) compared to those of the P1- and P2-based devices. And, it mainly originated from the better molecular packing and final blend film morphology, as confirmed by morphological analyses.