Influence of traces of oxidized polymer on the performances of bulk heterojunction solar cells

A key challenge in the field of organic photovoltaics (OPVs) is making them efficient and stable devices despite their being composed of organic materials, which are susceptible to becoming photodegraded in the presence of atmospheric oxygen. It is therefore essential to determine to what extent the donor material used in the active layer can be oxidized before the oxidation results in a loss of solar cell performance. Here we mainly focused on thieno[3,4-b]thiophene-alt-benzodithiophene polymer (PTB7), and compared it to the well-known poly(3-hexylthiophene) (P3HT). The complexity of the PTB7 chemical structure, based on an alternation of benzodithiophene (BDT) and thienothiophene (TT) and flanked with alkoxy and alkyl side chains, necessitated a re-investigation of the first step of the photooxidative process. Neither the intrinsic photochemical process nor the presence of an alkoxy side chain was found to be critical for the photostability. The high initial sensitivity of PTB7 in photooxidative conditions was instead related to attack of singlet oxygen on the conjugated backbone, with this attack shown to give rise to the formation of carbonylated species. In addition, traces of PTB7 oxidation, resulting from processing or very short durations of irradiation under ambient air, were found to result in a significant drop in solar cell performance. Also in this work, PTB7 was found to be more susceptible to photooxidation than was P3HT, in line with the higher instability of PTB7-based solar cells. The novel bottom-up approach implemented in this work revealed the importance of the formation of traces of polymer oxidation products in altering solar cell efficiency. The use of unstable materials is suspected to play a key role in the poor initial performances and/or reduced lifetimes of organic solar cells.