Effect of an ambient environment on light-induced degradation of organic solar cells based on a benzodithiophene-quinoxaline copolymer in air

Degradation of polymer molecules is one of the main factors that reduces the lifetime of thin film organic solar cells. The mechanisms of such degradations are not at all well-established because of the different nature of the polymer molecules. In this work, PBDTQ-4O, a donor (benzodithiophene)-acceptor(benzo-1,4-dioxane substituted quinoxaline-benzodithiophene)-based copolymer was successfully designed and synthesized using the Stille cross-coupling reaction to study the effect of light stress on organic solar cells (OSCs). PBDTQ-4O:PC70BM-based inverted OSCs exhibited a power conversion efficiency (PCE) of 4.8%. The stability of light stressed and unstressed OSC devices was investigated to highlight the contribution of an ambient environment to the light-induced degradation in the devices. The solar cells showed a fast loss in PCE of 17 and 9% in the first 2 h followed by 34 and 24% loss after 10 h of ageing in light stressed and unstressed devices, respectively. Furthermore, the formation of a coarse morphology with ageing confirms the development of large domain sizes that hinder exciton dissociation and reduce the measured photo-current. Ageing of the devices was also found to enhance the bi-molecular recombination due to the drastic decrease in electron mobility due to the reduction of the conductivity of the electron transport layer, ZnO, in both devices. In conclusion, our findings show that ambient environmental conditions play a significant role in light-induced degradation recorded in air.