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:PC₆₁BM 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 PC₆₁BM was only detected for the N-H-containing PZ:PC₆₁BM blends, corresponding very well to the stabilization effect of the PZs, which indicates that the PET process between PZ and PC₆₁BM stabilizes 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 calculations confirmed the formation of N-H···O-C and N-H···πbonds 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 (E_A) and excitation energy (E_Ex) of PC₆₁BM with/without piperazine confirmed that piperazine doping is able to promote the electron transfer (which 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.