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Cocrystals, as an excellent solid state to improve the physicochemical properties of active pharmaceutical ingredients, have always been discussed together with solvates since the definition of cocrystals was proposed. However, investigation and discussion of the boundary between cocrystals and solvates have not been common. In this work, aiming at betulin (BE), a natural product with multiple pharmacological activities, we designed its polymorphism and cocrystal screening experiments with different liquid organic solvents and solid cocrystal coformers (CCFs) that possess low and high melting points, respectively. Afterward, a hemihydrate, a new solvate with benzyl alcohol, and two new cocrystals of BE with phenol (PHE) and 3-hydroxybenzoic acid (3-HBA), respectively, including their single crystal structures, were designed, prepared, and characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and other solid-state analysis methods. A special thermal property for pharmaceutical cocrystals was found in cocrystal BE–PHE, that is, it possesses an exact CCF loss process, which is different from cocrystal BE–3-HBA but is similar to the solvate of BE. To explain this phenomenon, theoretical calculation studies were performed by molecular electrostatic potential surface analysis, electron density difference analysis, energy decomposition analysis, and energy framework analysis to realize an in-depth study of the sites, quantities, and energy of the intermolecular interactions between BE and different coformer molecules. Results indicated that the fewer interaction sites and the small interaction energy between BE and PHE molecules are possible reasons for the unusual thermal properties of cocrystal BE–PHE. This finding also proved that the structural features of cocrystals can greatly affect their physicochemical properties.

Silver nanowire (AgNW) transparent electrodes are considered as a promising candidate for applications in flexible optoelectronic devices. However, it remains a great challenge to obtain flexible AgNW electrodes with excellent optoelectrical properties and mechanical flexibility. Here, highly stable Ag nanoparticle (AgNP)-enhanced plasmonic AgNW electrodes are demonstrated via the controllable in situ growth of AgNPs at the AgNW junctions and introduction of an l-histidine (l-His) wrapping layer. The flexible transparent electrodes of AgNW-AgNP/l-His possess a low sheet resistance (Rsh) of ∼17.5 Ω sq–1, a high transmittance of ∼92.5% (550 nm), and a robust mechanical stability (100,000 bending cycles). Benefiting from plasmon-coupling effects, flexible polymer light-emitting devices (FPLEDs) with AgNW-AgNP/l-His electrodes present a current efficiency (CE) of ∼14.8 cd A–1and an external quantum efficiency (EQE) of ∼5.6%, constituting ∼80% and ∼75% increases compared to those of the reference devices with AgNW electrodes, respectively. Additionally, the laminated flexible transparent PLEDs (FT-PLEDs) are demonstrated by integrating polydimethylsiloxane/AgNW-AgNP anodes by a soft lamination process. The FT-PLEDs present a CE of ∼7.1 cd A–1(cathode side: ∼3.9 cd A–1; anode side: ∼3.2 cd A–1) and an EQE of ∼2.7% (cathode side: ∼1.5%; anode side: ∼1.2%). Furthermore, the FPLEDs and FT-PLEDs exhibit robust mechanical durability, maintaining ∼89% and ∼86% of their initial luminance after 1000 bending cycles at a bending radius of 2 mm, respectively. This work opens up a new avenue for the development of high performance and stable flexible optoelectronic devices.

Flexible transparent metal electrodes (FTMEs) have significant application potentials in the fields of flexible optoelectronic devices due to their outstanding optical transmittance and electrical conductivity. However, obtaining excellent optoelectrical properties and mechanical flexibility of FTMEs is challenging because ultrathin metal layers usually follow an island growth mode. In this paper, flexible transparent ultrathin Ag electrodes with high mechanical stability and good optoelectrical properties were exploited by tailoring the surface properties of plastic substrates with ultraviolet–ozone (UVO) treatment for regulating the nucleation and growth kinetics of Ag films. The composite transparent electrodes of Ag (9 nm)/MoO3(20 nm) fabricated on the UVO-treated polyethylene terephthalate (PET) substrates possess a low sheet resistance of ∼7.9 Ω/sq, a high optical transmittance of ∼87.2% at 550 nm, a long-period environmental stability of 30 days (∼65 °C, ∼80% humidity), and excellent mechanical flexibility of 100,000 bending cycles at a bending radius of 1.5 mm. These properties are derived from the surface treatment of PET substrates by UVO, which increases substrate surface energy and produces chemical nucleation sites of the phenolic hydroxyl groups. The phenolic hydroxyl groups generated on the PET surface not only provided efficient nucleation sites for subsequent Ag film growth but also formed C–O–Ag bonds between the substrate surface and the Ag layer, which act as “anchor chains” to fix firmly the Ag atoms on the substrate surface. As a universal applicability strategy, the composite electrodes on the UVO-treated polyethylene naphthalate (PEN) and norland optical adhesive 63 (NOA63) substrates also possess excellent optoelectrical properties and mechanical flexibility. Based on the ultrathin Ag composite electrodes, the flexible white organic light-emitting devices with PET, PEN, and NOA63 as substrates present the maximum current efficiencies of 53.0, 77.0, and 65.2 cd/A, respectively.