The Role of the Hydrogen Bond between Piperazine and Fullerene Molecules in Stabilizing Polymer:Fullerene Solar Cell Performance.

Piperazine has been recently reported as a 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:PCBM cells was investigated. Results confirmed that only piperazine derivatives (PZs) with N-H bonds showed the stabilization effect, whereas the bis-alkyl-substituted piperazine compounds cannot improve the stability. An efficient photon-induced electron transfer (PET) process between PZ and PCBM was only detected for the N-H-containing PZ:PCBM blends, corresponding very well to the stabilization effect of the PZs, which indicates that the PET process between PZ and PCBM stabilizes the cell performance, and the N-H bond plays a critical role ensuring the PET process and the consequent stabilization effect. Both H-NMR spectroscopy and theoretical calculations confirmed the formation of N-H···O-C and N-H···π bonds for the PCBM: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 () and excitation energy () of PCBM with/without piperazine confirmed that piperazine doping is able to promote the electron transfer (which leads to the formation of PCBM anions) than the energy transfer (leads to the formation of PCBM excitons) between P3HT and PCBM, which is beneficial for the performance and stability improvement.