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 fullerene-based 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 J_SC 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.

In this study, we demonstrate that the introduction of small amounts of phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) into all-polymer solar cells (all-PSCs) increases the photovoltaic performance without compromising mechanical properties. Ternary blend polymer solar cells (ternary-PSCs) consisting of a polymer donor (PTB7-Th) and an acceptor mixture with different weight ratios of a polymeric acceptor (P(NDI2HD-T2)) and PC₇₁BM demonstrate the effects of PC₇₁BM loading on the power conversion efficiency (PCE) and mechanical properties. A significant enhancement in the PCEs of ternary-PSCs, from 6.32% to 7.33%, is observed when PC₇₁BM is added into the active layer as up to 30 wt% of the acceptor mixture. Importantly, the excellent mechanical properties (i.e., crack onset strain = 11.6%, toughness = 2237 J m⁻³) of the blend films are well preserved at PC₇₁BM loadings at or below 30 wt%. In contrast, both the PCE and the mechanical performance of the ternary-PSCs significantly decrease at higher PC₇₁BM loadings (>50 wt%). Detailed morphological analysis via grazing incidence X-ray scattering measurements reveals that PC₇₁BM molecules are well-dispersed in the amorphous portion of the active layer at PC₇₁BM loadings up to 30 wt%. Therefore, both the mechanical and photovoltaic performances of the ternary-PSCs correlate closely with their morphological behavior, particularly in terms of the mixing behavior of PC₇₁BM with polymers. The well-dispersed PC₇₁BM molecules in the amorphous polymer domains facilitate efficient exciton dissociation, whereas the formation of PC₇₁BM aggregates above a critical concentration causes severe mechanical degradation of the ternary-PSCs due to the presence of weak interfaces between the brittle PC₇₁BM and polymer domains. Therefore, the ternary blends with optimal content of polymer/fullerene acceptors represent important candidates for flexible and wearable solar cells that require both high mechanical and photovoltaic performances.

We designed and synthesized a series of n-type conjugated polymers by introducing phenylnaphthalenediimide (PNDI) as a novel n-type building block, and investigated the effect of side-chain engineering in the polymer acceptor on the performance of all-polymer solar cells (all-PSCs).

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We have developed a simple yet versatile approach for enhancing the performance of all-polymer solar cells (all-PSCs) using a highly crystalline small-molecular additive, 6,6′-dithiopheneisoindigo (DTI).