LAUSR

Piperazine was recently reported as stabilizer for polymer:fullerene solar cells that can minimize the "burn-in" degradation of the cell. In this paper, the influence of N-substituents on the stabilization effect of piperazine in P3HT:PC61BM cells was investigated. Results confirmed that only piperazine derivatives (PZs) with N-H bond showed the stabilization effect, whereas the bis-alkyl substituted piperazine compounds do not able to improve the stability. Efficient photon-induced electron transfer (PET) process between PZ and PC61BM was only detected for the N-H containing PZ:PC61BM blends, corresponding ver well the stabilization effect of the PZs, which indicates that PET process of PZ and PC61BM stabilize the cell performance and the N-H bond plays a critical role ensuring the PET process and the consequent stabilization effect. Both 1H-NMR spectroscopy and theoretical calculation confirmed the formation of N-H…O-C and N-H…π bonds for the PC61BM-piperazine adduct, which was considered as the driving force that promotes the PET process between these two components. In addition, comparison of the calculated electron affinity energy (EA) and excitation energy (EEx) of PC61BM with/without piperazine confirmed that piperazine doping is able to promote the electron transfer (leads to the formation of PC61BM anions) than the energy transfer (leads to the formation of PC61BM excitons) between P3HT and PC61BM, which is beneficial for the performance and stability improvement.