Effect of terminal fluorine substitution of a nonfullerene small molecular acceptor on the thermal stability of organic solar cells

Abstract

The addition of fluorine atoms on the terminal group of nonfullerene small molecular acceptors (NFSMAs) is generally considered an effective method to improve device efficiency. However, it is still unknown how the thermal stability of the device changes after the introduction of fluorine atom. In this study, the influence of terminal fluorine substitution on the thermal stability of organic solar cells (OSCs) was investigated by choosing ITIC and ITIC-4F as acceptors. Although the efficiency of the ITIC-4F-based device was improved after the introduction of fluorine atoms, the compatibility between ITIC-4F and the polymer donor was worse compared with ITIC. The poor compatibility between ITIC-4F and the polymer donor gave rise to the precipitation of ITIC-4F from the polymer network during thermal annealing, leading to the aggregation and crystallization of ITIC-4F, which destroyed the optimal morphology of the active layer. Therefore, the efficiency of the ITIC-4F-based device decreased by 34.8% after 24 h of thermal annealing at 150 °C, while for the ITIC-based device, the efficiency decreased by only 5.4% after thermal annealing at 150 °C for 24 h. These results showed that although the efficiency of the device could be improved by adding fluorine atoms to the end groups of ITIC, the thermal stability of the device would also be poor when PBTIBDTT was used as the donor.