Mechanically robust and high-performance ternary solar cells combining the merits of all-polymer and fullerene blends

Abstract

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.