Tuning the physical properties of pyrrolo[3,4-c]pyrrole-1,3-dione-based highly efficient large band gap polymers via the chemical modification on the polymer backbone for polymer solar cells

Abstract

A systematic modulation of the photo-physical properties of high energy converting large band gap (2.04 eV) alternating polymers (PBDTT–DPPD) containing electron rich 2D-conjugated benzodithiophene (BDTT) and weak electron accepting pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD) derivatives via the incorporation of a relatively strong electron accepting thieno[3,4-c]pyrrole-4,6-dione (TPD), thieno[3,4-b]thiophene (TT), or pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit on the polymer backbone was demonstrated. All three new random copolymers, RP1, RP2 and RP3, displayed broad absorption bands and lower optical band gaps compared to those of their parent alternating polymer, PBDTT–DPPD. The estimated band gaps of RP1, RP2 and RP3 decreased gradually from 2.04 eV for PBDTT–DPPD to 1.87 eV, 1.60 eV and 1.45 eV, respectively. The decrease in the band gaps of RP1, RP2 and RP3 was associated mainly with the alteration of their conduction bands. Interestingly, RP1 and RP2 showed slightly improved hole mobilities and RP3 exhibited one order lower hole mobility than that of PBDTT–DPPD. The estimated mobilities of RP1, RP2 and RP3 were 1.4 × 10⁻³ cm² V⁻¹ s⁻¹, 3.7 × 10⁻³ cm² V⁻¹ s⁻¹ and 4.9 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. The polymer solar cells (PSCs) prepared from RP1, RP2 or RP3 as donors and PC₇₀BM as an acceptor using a simple device configuration of ITO/PEDOT : PSS/polymer : PC₇₀BM + DIO/Al exhibited a maximum power conversion efficiency (PCE) of 5.35%, 5.05% and 2.41%, respectively.