Organic Amines as Targeting Stabilizer at the Polymer/Fullerene Interface for Polymer:PC₆₁BM Solar Cells

Herein, it is demonstrated that a small amount (0.05% in weight ratio) of polyethyleneimine (PEI) can effectively suppress the “burn-in” degradation of both PTB7-Th:PC₆₁BM and P3HT:PC₆₁BM cells, similar to the piperazine derivatives, suggesting that organic amines can serve as universal stabilizer in polymer:PC₆₁BM solar cells. Light-induced electron spin resonance (LESR) spectroscopy measurement shows a higher ESR signal intensity of PC₆₁BM anions in 0.2% PEI-doped film than in 1% piperazine-doped film. Moreover, no piperazine is detected in a 10% (w/w) piperazine-doped film by gas chromatography–mass spectrometry (GSMS). These results suggest that the residual piperazine in the blend film is very low, which can be understood by the high volatility of piperazine. Quantum calculations are performed on the intermolecular binding energy (E_B) between polymer (using model repeating units), PC₆₁BM, and piperazine molecules. Results reveal that piperazine prefers to localize at the polymer:fullerene interface by complexing with PC₆₁BM (in P3HT:PC₆₁BM) or PTB7-Th (in PTB7-Th:PC₆₁BM system), which indicates that the photo dimerization of PC₆₁BM which causes the “burn-in” degradation of polymer:fullerene solar cells mainly happens at the donor/acceptor interface, and the organic amine serves as the targeting stabilizer at the interface.