Organic Photovoltaic (OPV) is facing with the key challenge of making efficient and stable devices with organic materials, which are susceptible to undergo photodegradation in the presence of atmospheric oxygen. It is therefore essential to determine to which extent a minimal oxidation for the donor material used in the active layer results in solar cell's performance loss. Here we mainly focused on thieno[3,4-b]thiophene-alt-benzodithiophene polymer (PTB7) to be compared to well known poly(3-hexylthiophene) (P3HT). The complexity of the PTB7 chemical structure, based on benzodithiophene (BDT) and thienothiophene (TT) alternation and flanked with alkoxy and alkyl side-chains, necessitates the re-investigation of the first step of the photooxidative process. It is shown that neither intrinsic photochemical process nor the presence of an alkoxy side-chain are critical for the photostability. The high initial sensibility of PTB7 in photooxidative conditions is related to singlet oxygen attack on the conjugated backbone giving rise to the formation of carbonylated species. In a second part, it is shown that traces of PTB7 oxidation, resulting from processing or very short irradiation under ambient air, results in a significant drop in solar cells performance. We also show that the easiness of PTB7 photooxidation is higher than the one of P3HT in line with the higher instability of PTB7-based solar cells. The novel bottom-up approach implemented in this work reveals the importance of traces of polymer oxidation products in altering the solar cells efficiency. The use of unstable materials is susceptible to play a key role in the poor initial performances and/or reduced lifetimes of organic solar cells.