A thieno[3,4-f]isoindole-5,7-dione based copolymer for polymer solar cells

Abstract

A new polymer based on thieno[3,4-f]isoindole-5,7-dione (TID) and benzo[1,2-b:4,5-b]dithiophene was designed and synthesized. The copolymer was characterized by elemental analysis, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) absorption spectra as well as electrochemical cyclic voltammetry (CV) tests. Compared to PBDTTPD, introducing the quinoid unit could elevate the HOMO energy level and lower the LUMO energy level of the polymer, and consequently, the band gap can be reduced very effectively. The electronic structures of the two polymers were also studied by DFT calculations at the B3LYP/6-31G level. When a polymer solar cell (PSC) device was fabricated with an active layer of a blend of PBDTTID and PC₆₁BM using 3% (v/v) diiodooctane (DIO) as a solvent additive, a high V_oc of 0.80 V was obtained, with a PCE of 3.12% under AM 1.5G conditions. The PBDTTID-based PSC device shows a slightly lower V_oc but better J_sc and FF than the PBDTTPD-based PSC device, and the preliminary results indicate that TID will be a desirable building block for designing photovoltaic polymers with low-lying HOMO energy levels, as well as low band gaps.