Impact of the photo-induced degradation of electron acceptors on the photophysics, charge transport and device performance of all-polymer and fullerene-polymer solar cells

Abstract

The photodegeneration of polymer solar cells (PSCs) based on polymer donors and fullerene acceptors limits their lifetime and reliability. Here, we demonstrate that replacing fullerene acceptors with naphthalenediimide (NDI)-based n-type polymers can significantly enhance the photo-stability of PSCs. As a model system, we compared the photo-stabilities of all-polymer solar cells (all-PSCs) and fullerenebased PSCs (fullerene-PSCs) using the same polymer donor (PBDTTTPD) exposed to one sunlight illumination. We observed a remarkable contrast in their photo-stabilities. After 4 days of irradiation, the performance of the fullerene-PSCs dropped by 50%, whereas the performance of the all-PSCs remained stable. The detailed analysis of charge dynamics measured by femtosecond transient absorption spectroscopy and space charge limited current measurements shows significant reduction in interfacial charge transfer and charge carrier mobility, which correlates with the loss in JSC and FF of the fullerene-PSCs under solar illumination. By contrast, for the all-PSCs, the ultrafast dynamics and charge mobility remained consistent after illumination. The NDI-based copolymer had high stability under light exposure, whereas the photo-induced dimerization of phenyl-C61-butyric acid methyl ester (PCBM) acceptors was responsible for the photo-degradation of fullerene-PSCs.