Theoretical investigation on charge transport parameters of two novel heterotetracenes as ambipolar organic semiconductors

In the current work, the charge transport parameters of two novel dicyanovinyl heterotetracenes as potential ambipolar transport materials, 2-((10H-benzo[4,5]thieno[3,2-b]indol-2-yl)methylene)malononitrile (BTMN) and 2-((11H-benzo[a]carbazol-9-yl)methylene)malononitrile (BCMN), have been investigated at the molecular and crystal levels by means of the first-principles density functional theory (DFT) calculations and the incoherent charge-hopping model combining with the quantum-mechanical charge transfer approach. Based on the random-walk simulation of charge diffusion coefficient, the 3D-average mobilities of hole and electron at T = 300 K are predicted to be 6.387 × 10−2 and 1.936 × 10−2 cm2 V−1 s−1 for BTMN crystal, while they are as high as 2.404 × 10−1 and 1.418 × 10−1 cm2 V−1 s−1 for BCMN crystal. The predicted high and balanced carrier mobility for BCMN crystal suggests its potential application as ambipolar charge transport materials under favorable device conditions. However, this claim needs experimental verification. The temperature dependence of mobility shows that the carrier transport in both systems behaves in a “bandlike” manner over a wide range of temperatures when the nuclear tunneling effect is considered, as indicated by a decrease in mobility with the increasing temperature, in contradiction to the classical Marcus–Hush description. In addition, the simulation for the angle dependence of mobility shows that the hole transport is remarkable anisotropic in both crystals, and the maximum μh is 0.518 cm2 V−1 s−1 for BTMN and 0.368 cm2 V−1 s−1 for BCMN, which appears along the crystallographic a-axis direction due to the close face-to-face molecular stack and intermolecular π–π interaction that result to the large electronic coupling values.