The interfacial degradation mechanism of polymer:fullerene bis-adduct solar cells and their stability improvement

Abstract

Although fullerene bis-adducts have been widely used in polymer solar cells for their high LUMO energy level and good performance, the degradation behavior of this class of solar cells has not been well understood. In this paper, the performance and stability of the solar cells based on P3HT:fullerene bis-adducts, including bis-PC₆₁BM and ICBA, were systematically investigated. Different from the P3HT:PC₆₁BM cell, these bis-adduct based cells showed fast open circuit (V_OC) and fill factor (FF) decays. The partial recovery of V_OC and FF of the aged cells by renewing the MoO₃/Al electrode indicated that degradation at the photoactive layer and MoO₃ interface is the main reason for V_OC and FF decays. The X-ray photoelectronic spectroscopy analysis confirmed that under light illumination, Mo⁶⁺ of MoO₃ is partially reduced to Mo⁵⁺. By inserting a thin layer of C₆₀, both MoO₃ reduction and performance decays are slowed down, confirming that photoreduction of MoO₃ by P3HT is the degradation mechanism for P3HT:bis-PC₆₁BM cells. Finally, we found that doping a polymer:fullerene bis-adduct layer with piperazine increases the fullerene content on the surface of the photoactive layer, which consequently lowers the reduction of Mo⁶⁺ and improves the stability of the solar cells. This work gives a detailed understanding of the interfacial degradation of PSCs and provides effective solutions, which has important guiding significance for improving the stability of different types of polymer solar cells.