The photostability of donor–acceptor (D–A) polymers remains a critical issue despite recent improvements in the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. We report the synthesis of three highly photostable polymers (PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2) and their suitability for use in high-performance OPV cells. Under 1 sunlight of illumination in air for 10 h, these polymer films demonstrated remarkably high photostability compared to that of PTB7, a representative polymer in the OPV field. While the PDTBDT-BZ, PDTBDT-BZF, and PDTBDT-BZF2 polymer films maintained 97, 90, and 96% photostability, respectively, a PTB7 film exhibited only 38% photostability under the same conditions. We ascribed the high photostability of the polymers to both the intrinsically photostable chemical moieties and the dense packing of alkyl side chains and planar backbone polymer chains, which prevents oxygen diffusion into the PDTBDT-BZ films. This work demonstrates the high photostability of planar PDTBDT-BZ series polymers composed of photostable DTBDT and BZ moieties and suggests a design rule to synthesize highly photostable photovoltaic materials.

We introduce a synthetic approach to enhance coalescence phenomenon during solidification of water-borne colloids so that thin, even, and continuous film morphology of polymer semiconductors can be realized. From the theoretical study of complex colloids, we show that small-sized and uniform colloid particles are essential to minimize depletion contact energy between colloid particles and thus to enhance coalescence. Therefore, the newly synthesized polymer semiconductor in this study is designed with the aim of better molecular affinity with surfactants, so that phase transfer of polymer semiconductors from organic phase to water phase can proceed more efficiently during mini-emulsion synthesis. This is achieved by substituting a Si atom to the branching C atom of the alkyl solubilizing group of a conventional donor−acceptor polymer semiconductor. Such a chemical modification increases the volumetric portion of hydrophobic alkyl chains and thus enables higher solubility as well as higher hydrophobicity, all of which are closely related with enhancing molecular affinity between polymer semiconductor and surfactant, as proved by surface energy, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy analyses. As a result, it is shown that the performance of organic field-effect transistors fabricated from water-borne colloids can be improved to a level similar to the case of organic solvents, 0.91 cm 2 V −1 s −1 . More importantly, we also show the reproducibility of transistor performance is greatly improved due to the uniform and small water-borne colloidal particles.

Here we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of the rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made with DTBDT-Rho:PCBM blends showed an extremely high power conversion efficiency (PCE; 7.10%); notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.

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The 5-(heptadecan-9-yl)dithieno[3,2-b:2 ,3-d]pyridine (NET) structure was designed as a new donor unit for copolymer synthesis. Benzothiadiazole (BT) was used as the electron acceptor unit. The new donor-acceptor copolymer was obtained through the Stille coupling reaction. The density functional theory analysis of the new copolymer showed that the highest occupied molecular orbital electron density was localized on the NET unit while the lowest unoccupied molecular orbital electron density was localized partially on the BT and partially on the pyridine group of the NET unit. The polymer presented good solubility and thermal stability, which are desirable properties for use in solution-processed devices. The new polymer is expected to show a potential for application in organic electronics, such as thin-film transistors.