Evaluating the influence of heteroatoms on the electronic properties of aryl[3,4-c]pyrroledione based copolymers

Abstract

A donor-acceptor-type conjugated copolymer (PBDT-PPD) composed of benzodithiophene (BDT) and pyrrolopyrroledione (PPD) was synthesized using the Stille cross-coupling reaction. Using both experimental and theoretical data, the optical, electrochemical, and photovoltaic properties of PBDT-PPD were compared with those of its sulfur analog, PBDT-TPD, which is composed of BDT and thienopyrroledione (TPD). The optical bandgaps of the polymers were determined to be 1.86 and 2.20 eV, respectively. While both materials possessed similar highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) level for PBDT-PPD was raised relative to that of PBDT-TPD. Devices incorporating PBDT-PPD had a higher open-circuit voltage and fill factor, yet drastically lower short-circuit current density (Jsc) than PBDT-TPD leading to a lower power conversion efficiency (PCE). The lack of significant intramolecular charge transfer (ICT) combined with the high LUMO of PBDT-PPD resulted in poor spectral overlap with the solar spectrum, lowering Jsc. Additionally, there was poor electron injection into PCBM, which also reduced the PCE.