Mechanical properties of conducting polymers are an essential consideration in the design of flexible and stretchable electronics, but the guidelines for the material design having both high mechanical and electrical properties remain limited. Here we provide an important guideline for the design of mechanically robust, electroactive polymer thin films in terms of the molecular weight of the polymers. These studies based on a highly efficient, representative n-type conjugated polymer (P­(NDI2OD-T2)) revealed a marked enhancement in mechanical properties across a narrow molecular weight range, highlighting the existence of a critical molecular weight that can be exploited to engineer films that balance processability and mechanical and electronic properties. We found the thin films formed from high molecular weight polymers (i.e., number-average molecular weight (M n) ∼ 163 kg mol–1) to exhibit superior mechanical compliance and robustness, with a 114-fold enhanced strain at fracture and a 2820-fold enhanced toughness, as compared to those of low molecular weight polymer films (M n = 15 kg mol–1). In particular, we observed a jump in the mechanical properties between the M n = 48 and 103 kg mol ‑1, yielding a 26-fold enhanced strain at fracture and a 160-fold enhanced toughness. The significant improvement of tensile properties indicates the presence of a critical molecular weight at which entangled polymer networks start to form, as supported by the analysis of the thermal and crystalline properties, specific viscosity, and microstructure. Our work provides useful guidelines for the design of conjugated polymers with recommendations for the best combinations of mechanical robustness and electrical performance for flexible and stretchable electronics.

Nonfullerene acceptors (NFAs), that are smallmolecule acceptors (SMA) and polymer acceptors (PAs), have been extensively explored, which has yielded significant enhancements in the photovoltaic performance of polymer solar cells (PSCs). The mechanical robustness of the PSCs is of vital and equal importance to ensure long-term stability and enable their use as power-generators in flexible and stretchable electronics. Here, we report a comparative study of the mechanical properties of SMA-based, PA-based, and fullerenebased PSCs. We chose ITIC (SMA), P(NDI2OD-T2) (PA), and PCBM (fullerene) as three representative acceptor materials and blended them with the same polymer donor PTB7-Th. To understand the difference between the mechanical properties of SMA-based and PA-based PSCs, we control the number-average molecular weight (M n ) of P(NDI2OD-T2) from 15 to 163 kg mol −1 in all-PSCs. The all-PSCs-based high-M n PAs exhibit significantly higher cohesion energy (4.03 J m −2 ) than SMA-PSCs (1.19 J m −2 ) and PCBM-PSCs (0.29 J m −2 ). Notably, the all-PSCs exhibit a high strain at fracture of 31.1%, which is 9-and 28-fold higher than those of SMA-PSCs and PCBM-PSCs, respectively. The superior mechanical robustness of all-PSCs is attributed to using a PA above the critical molecular weight (M c ), which produces tie molecules and polymer entanglements that dissipate substantial mechanical strain energy with large plastic deformation. This work provides useful design guidelines for photovoltaic active materials in terms of the mechanical properties and highlights the importance of incorporating high-M n PAs above the M c for producing PSCs with excellent mechanical robustness and device performance.

Perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising renewable light-harvesting technologies with high performance, but the utilization of hazardous dopants and high boiling additives is harmful to all forms of life and the environment. Herein, new multirole π-conjugated polymers (P1-P3) are developed via a rational design approach through theoretical hindsight, further successfully subjecting them into dopant-free PSCs as hole-transporting materials and additive-free OSCs as photoactive donors, respectively. Especially, P3-based PSCs and OSCs not only show high power conversion efficiencies of 17.28% and 8.26%, but also display an excellent ambient stability up to 30 d (for PSCs only), owing to their inherent superior optoelectronic properties in their pristine form. Overall, the rational approach promises to support the development of environmentally and economically sustainable PSCs and OSCs.

We delineate the important role of 2D conjugated alkylthiophene side chains of polymers in manipulating the molecular orientation and ordering at the polymer donor/polymer acceptor (P D /P A ) interface as well as the composition variations in the blend active layer of all-polymer solar cells (all-PSCs). To systematically investigate the impact of 2D conjugated side chains on the performance of all-PSCs, we synthesized a series of polypolymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDT-TPD (P1, 100% alkoxy side chain), PBDTT 0.29 -TPD (P2), PBDTT 0.59 -TPD (P3), PBDTT 0.76 -TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1−P5 polymer donors produced similar PCEs of ∼6% in fullerene-based PSCs. In contrast, for the all-PSC systems, the changes in the side chain composition of the polymers induced a strong increasing trend in the power conversion efficiencies (PCEs), from 2.82% (P1), to 3.16% (P2), to 4.41% (P3), to 5.32% (P4), and to 6.60% (P5). The significant increase in the PCEs of the all-PSCs was attributed mainly to improvements in the short-circuit current density (J SC ) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the P D /P A interfaces and improved domain purity, which led to enhanced exciton dissociation and charge transport characteristics of the all-PSCs. Our observations highlight the advantage of incorporating 2D conjugated side chains into polymer donors for producing high-performance all-PSC systems.

Side-chain fluorination of polymers is demonstrated as a highly effective strategy to improve the efficiency of all-polymer solar cells from 2.93% (nonfluorinated P1) to 7.13% (fluorinated P2). This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimizes the band alignment and the film morphology.