Influence of fullerene photodimerization on the PCBM crystallization in polymer: Fullerene bulk heterojunctions under thermal stress

ABSTRACT For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo‐stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light‐induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1209–1214 For an increased lifetime of polymer:fullerene bulk heterojunction solar cells, a thorough understanding of the factors that govern the morphological stability of the active layer blend under thermal stress is crucial. In this work, it is demonstrated that light exposure slows down fullerene crystallization and phase separation normally induced at elevated temperatures. The observed photostabilizing effect on active layer morphology is attributed to light‐induced dimerization of the fullerene.