Photochemical stability of conjugated polymers, electron acceptors and blends for polymer solar cells resolved in terms of film thickness and absorbance

Abstract

Photochemical degradation at 1 sun under AM1.5G illumination was performed on six conjugated polymers and five different electron acceptors. Additionally, the respective polymer:PC₆₀BM and P3HT:electron acceptor blends were studied, and all degradations were resolved in terms of film thickness and absorbance. A fully automated degradation setup allowed for inclusion of in excess of 1000 degradations in this study to enable a discussion of reliability of the technique. Degradation rates were found to increase exponentially with decreasing film absorbance for all materials. The relative stabilities within each material group were found to vary for both the pure polymers and the blends. The stability ranking between the materials of the pure polymers was found to be similar to the ranking for their respective blends, implying that the photochemical stability of a pure polymer is a good measure of its associated blend stability. Different electron acceptors were found to stabilize P3HT decreasingly with decreasing donor–acceptor LUMO–LUMO gap. Destabilization of P3HT was observed in the case of the electron acceptor ICBA. Additionally, the decreased stabilization of P3HT by high LUMO electron acceptors poses a challenge to solar cell encapsulation if these materials are to be of commercial interest. The presented method is generally applicable to all types of organic materials to assess photochemical stabilities. The presented results of conjugated polymers demonstrate that this is a powerful tool for conjugated polymer stability assessment if the results are interpreted correctly.