Last, the presence of electronegative N atoms in BQQDI allows alternating electronegative/electropositive positions, corresponding to O and N and C-H moieties, respectively, along the lateral direction of the molecule, which is expected to promote core-to-core interactions, improving intermolecular charge transport and increasing both chemical and physical robustness. However, the replacement of two bay C sites with N atoms should theoretically lower the LUMO level from −3.81 eV in the case of methyl-substituted PDI (Me−PDI) to −4.17 eV for Me−BQQDI, thus improving the chemical stability of n-type transistors made from this compound ( 25). In addition, BQQDI has a molecular structure and orbital configuration similar to those of perylene diimide (PDI), which has been widely studied. 1C), exhibits several beneficial features, including a rigid, planar structure based on the BQQ π-core that allows effective charge transport. The base compound used in this work, 3,4,9,10-benzoisoquinolinoquinolinetetracarboxylic diimide (BQQDI) ( Fig. The present report proposes a strategy for the molecular design of n-type OSCs, based on the concept of a π-core containing electronegative N atoms, where the N atoms play multiple roles for tuning and enhancing electronic and structural properties. This difficulty in achieving intermolecular interactions in the aggregated form of the material has impeded the development of high-performance n-type OSCs with limited molecular motions together with suitable robustness for use in high-end devices.
This limited overlap leads to poor charge transport in n-type OSCs. 1B) compared with that in high-mobility p-type OSCs having herringbone-type packing structures ( Fig. In particular, the overlap is reduced obviously in the lateral direction of the molecule ( Fig. Because almost all n-type OSCs reported, including PDI−FCN 2, form 2D brickwork-type packing structures, these compounds exhibit less effective orbital overlaps with a higher degree of anisotropy. However, simultaneously obtaining effective orbital overlap and suppression on molecular motions using this strategy is difficult. This compound also shows free-electron–like carrier behaviors, such as band-like transport ( 27). In particular, PDI−FCN 2 ( 30), in which the perylene π-core contains one cyano group on each side at the bay positions and linear fluoroalkyl substituents on the imide N, exhibits high electron mobilities exceeding 1 cm 2 V −1 s −1 in solution-grown single-crystalline (SC) transistors under ambient conditions ( 27, 31, 32). However, n-type OSCs showing the same level of performance have not yet been achieved because of the lack of molecular design strategies based on suppression of molecular motions.īecause of the unique electronic requirements for n-type OSCs ( E LUMO < −4.0 eV), a common molecular design strategy involves the introduction of strong electron-withdrawing groups such as imide, cyano, and halogen moieties ( 25). The n-type OSCs have been developed on the basis of electron-deficient π-cores ( 23– 25), such as arylene diimide derivatives ( 26), and hence, some high electron mobilities greater than 5 cm 2 V −1 s −1 have been reported ( 27– 29). These materials have been incorporated into solution-processed transistors and have exhibited hole mobilities exceeding 10 cm 2 V −1 s −1 and ambient stability and thermal durability features that are required for practical applications in future high-end printed electronics ( 15, 16, 20, 22).
Recent advances in the design of p-type, bent-shaped π-electron systems ( 15– 21) have resulted in two-dimensional (2D) charge-transport herringbone-type packing structures due to multiple intermolecular interactions as well as suppressed molecular motions ( Fig. However, reductions in dynamic disorder originating from thermally induced intermolecular vibrations (that is, molecular motion) ( 8– 14) also can be important factors for improving mobility.
For weakly bound OSCs, mobility is determined primarily by effective mass ( m*) that results from the overlap of molecular orbitals between two adjacent molecules ( 5– 7). Charge-carrier mobility (referred to as mobility) is one of the most important performance indicators for a semiconductor. The intrinsic high solubility of OSCs in organic solvents also enables low-cost mass production using solution-processing techniques ( 1– 4). These properties are in contrast to those of covalently bonded inorganic semiconductors such as silicon. The aggregation of π-electron systems in organic semiconductors (OSCs) via weak intermolecular interactions can produce soft, lightweight, and mechanically flexible materials.
0 kommentar(er)
