The Importance of Entanglements in Optimizing the Mechanical and Electrical Performance of All-Polymer Solar Cells

Abstract

Organic solar cells that have all-polymer active layers may have several advantages compared with polymer−small molecule systems including improved mechanical and thermodynamic stability; however, an all-polymer active layer does not guarantee robust mechanical behavior. Here, we consider key parameters that may influence the mechanical behavior and power conversion efficiency of all-polymer solar cells (all-PSCs). Considerations include the thermal transition temperature of the polymers, the molecular weight (MW) of the polymers, and film morphology. The impact these features have on mechanical behavior is probed by measuring the cohesive fracture energy (G c ), crack onset strain, and elastic modulus. We find that the selection of ductile polymers with high MW enhances interchain interactions that improve the mechanical resilience of the films. High-MW polymers are also found to maximize the power conversion efficiency (PCE). Using this strategy, BHJ films with the best reported combination of G c (7.96 J m −2 ) and PCE (6.94%) are demonstrated. Finally, it is found that increasing the film thickness increases the fracture energy of the films but at the cost of PCE. These findings provide a fundamental perspective on the design strategy to achieve high performance and mechanically robust organic solar cells.

Thermodynamic Properties and Molecular Packing Explain Performance and Processing Procedures of Three D18:NFA Organic Solar Cells

“…This corresponds to a distance of 22.4 Å that agrees with structural expectations for extended chains (see Figure S2, Supporting Information for illustration). Such a signature that includes higher-order (00l) peaks is very rare and extremely unusual for a high-performance donor polymer used in OSCs, [33][34][35] and cannot be clearly observed in FTAZ/J71, [36,37] PTB7-Th, [38] and PBDB-T/PM6. [10,39] These (00l) peaks suggest strong chain extension of D18 and excellent ordering along the backbone direction in the solid film.…”

The Role of Demixing and Crystallization Kinetics on the Stability of Non‐Fullerene Organic Solar Cells

“…FTAZ was deliberately chosen because it is a very ductile, viscoelastic material that generally exhibits poor shelf-life stability with low T g acceptors. [19,53] It thus allows us to investigate the intrinsic contributions of the small molecule to limitations in stability. Absolute efficiencies achieved are not important in our fundamental study, and the relative PCE is simply used as a proxy for morphology changes.…”

Efficient, Thermally Stable, and Mechanically Robust All‐Polymer Solar Cells Consisting of the Same Benzodithiophene Unit‐Based Polymer Acceptor and Donor with High Molecular Compatibility

“…The long P A chains can interact with each other and bridge adjacent crystalline domains by forming tie molecules to dissipate external stress, unlike the NFSMA as previously reported. [ 4b,12 ] Furthermore, the formation of well intermixed domains in the PBDB‐T:P(BDT2BOY5‐X) blends should also dissipate stress throughout the films. In contrast, we expect that the stress in the PBDB‐T:Y5‐2BO blend is highly concentrated into the fragile region around hard agglomerates driven by high crystallinity of NFSMA and its incompatibility with P D .…”

“…[ 7r ] It is well known that the entropy of P D – P A mixing is inversely proportional to the degree of polymerization of the polymers. [ 7r,11 ] Thus, at the high molecular weights, required to form chain entanglements that realize mechanical and morphological stabilities of all‐PSCs, [ 4b,7p,12 ] the entropic contribution is very small and the miscibility of the system is mainly dependent on the enthalpy of mixing. As a result, the mechanical properties and the morphological stabilities of the all‐PSCs with NFSMA‐based P A s are not high.…”