Thermal Gradient During Vacuum-Deposition Dramatically Enhances Charge Transport in Organic Semiconductors: Toward High-Performance N-Type Organic Field-Effect Transistors

A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (PDI-CN2), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature one-dimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the PDI-CN2 film. The morphological and crystalline structures of the PDI-CN2 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the PDI-CN2-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm² V^-1 s^-1, more than a factor of 2 higher than the mobility of 0.25 cm² V^-1 s^-1 submitted to conventional thermal annealing and the mobility of 0.29 cm² V^-1 s^-1 from the horizontally applied temperature gradient.

Engineering Thin Films of a Tetrabenzoporphyrin toward Efficient Charge-Carrier Transport: Selective Formation of a Brickwork Motif

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 cm² V^-1 s^-1, which is approximately 14 times higher than the record value for pristine free-base BP (0.070 cm² V^-1 s^-1). 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.