Careful molecular engineering has enabled solution processing of well-performing bulk-heterojunction photovoltaic layers comprising insoluble materials.

Organic heterojunctions are widely used in organic electronics and they are composed of semiconductors interfaced together. Good ordering in the molecular packing inside the heterojunctions is highly desired but it is still challenging to interface organic single crystals to form single-crystalline heterojunctions. Here, we describe how organic heterojunctions are formed by interfacing two single crystals from a droplet of a mixed solution containing two semiconductors. Based on crystallization of six organic semiconductors from a droplet on a substrate, two distinct crystallization mechanisms have been recognized in the sense that crystals form at either the top interface between the air and solution or the bottom interface between the substrate and solution. The preference for one interface rather than the other depends on the semiconductor–substrate pair and, for a given semiconductor, it can be switched by changing the substrate, suggesting that the preference is associated with the semiconductor–substrate molecular interaction. Furthermore, simultaneous crystallization of two semiconductors at two different interfaces to reduce their mutual disturbance results in the formation of bilayer single crystals interfaced together for organic heterojunctions. These single-crystalline heterojunctions exhibit ambipolar charge transport in field-effect transistors, with the highest electron mobility of 1.90 cm2 V–1 s–1 and the highest hole mobility of 1.02 cm2 V–1 s–1. Hence, by elucidating the interfacial crystallization events, this work should greatly harvest the solution-grown organic single-crystalline heterojunctions.

Benzene-fused bis-(borondipyrromethene)s (bis-BODIPYs) were synthesized by retro-Diels-Alder reaction of the corresponding bicyclo[2.2.2]octadiene-fused (BCOD-fused) bis-BODIPYs, which were, in turn, prepared from 4,8-ethano-4,8-dihydropyrrolo[3,4-f]isoindole derivatives. The π-fused bis-BODIPY chromophores were designed to show intensive absorption and strong fluorescence in the near-infrared region and not to have any strong absorption in the visible region. A 6,10-dibora-5a,6a,9a,10a-tetraaza-s-indaceno[2,3-b:6,5-b']difluorene derivative (syn-bis-benzoBODIPY) obtained by a thermal retro-Diels-Alder reaction of the corresponding BCOD-fused BODIPY dimer has strong absorption and emission bands at 775 and 781 nm, respectively. The absolute quantum yield is 0.36. The absorption is more than 5.0 times stronger than other absorptions observed in the visible region. In the case of 6,15-dibora-5a,6a,14a,15a-tetraaza-s-indaceno[2,3-b:6,7-b']difluorene derivatives (anti-bis-benzoBODIPY), the absorption and emission maxima exceed 840 nm.

Various pure organic semiconducting molecules exhibit extraordinarily large Seebeck coefficient which cannot be elucidated by conventional physical models of thermoelectricity.

Tetrabenzoporphyrin (BP) is a p-type organic semiconductor characterized by the large, rigid π-framework, excellent stability, and good photoabsorption capability. These characteristics make BP and its derivatives prominent active-layer components in organic electronic and optoelectronic devices. However, the control of the solid-state arrangement of BP frameworks, especially in solution-processed thin films, has not been intensively explored, and charge-carrier mobilities observed in BP-based materials have stayed relatively low as compared to those in the best organic molecular semiconductors. This work concentrates on engineering the solid-state packing of a BP derivative, 5,15-bis(triisopropylsilyl)ethynyltetrabenzoporphyrin (TIPS-BP), toward achieving efficient charge-carrier transport in its solution-processed thin films. The effort leads to the selective formation of a brickwork packing that has two dimensionally extended π-staking. The maximum field-effect hole mobility in the resulting films reaches 1.1 cm V s, which is approximately 14 times higher than the record value for pristine free-base BP (0.070 cm V s). This achievement is enabled mainly through the optimization of three factors; namely, deposition process, cast solvent, and self-assembled monolayer that constitutes the dielectric surface. On the other hand, polarized-light microscopy and grazing-incident wide-angle X-ray diffraction analyses show that there remains some room for improvement in the in-plane homogeneity of molecular alignment, suggesting even higher charge-carrier mobilities can be obtained upon further optimization. These results will provide a useful basis for the polymorph engineering and morphology optimization in solution-processed organic molecular semiconductors.