n-Type Thin-Film Transistors Based on Diketopyrrolopyrrole Derivatives: Role of Siloxane Side Chains and Electron-Withdrawing Substituents

The physical properties, packing, morphology, and semiconducting performance of a planar π-conjugated system can be effectively modified by introducing side chains and substituent groups, both of which can be complementary to the π framework in changing the intermolecular association, frontier molecular orbital energy, optical band gap, and others. We herein show that installation of end-capped electron-withdrawing groups (EWGs), such as dicyanovinyl (−DCV), 3-ethylrhodanine (−RD), and 2-(3-oxo-indan-1-ylidene)-malononitrile (−INCN), together with siloxane side chains to the backbones of dithienyldiketopyrrolopyrrole (DPPT), such as DPPT-Si-DCV, DPPT-Si-RD, and DPPT-Si-INCN, can greatly improve its solubility, air stability, and film morphology to serve as an n-channel in thin-film transistor fabrication. The EWGs attached to the DPPT core narrowed the optical band gap (Egopt) and changed the highest occupied molecular orbital and the lowest unoccupied molecular orbital energies (EHOMO and ELUMO), making them suitable for n-channel field-effect transistor (FET) applications. The benefits of introducing siloxane side chains to the DPPT core include enhanced solubility, low crystallization barrier, enantiotropic phase behavior, and much improved FET performance. The DPPT-Si-INCN film displayed low-lying HOMO (−5.82 eV) and LUMO (−4.60 eV) energy levels and an optical band gap as low as 1.22 eV, all of which suggest that this derivative can be quite resistant toward aerial oxidation. Thin films of these derivatives were prepared by the solution-shear method. A comparison of the solution-sheared films indicated that the molecular packing motif of DPPT-Si-INCN film was somehow different from that of DPPT-Si-DCV and DPPT-Si-RD, in which the π–π stacking tended to align orthogonally to the shearing direction. This specific π–π stacking alignment could have an impact on the electron mobility (μe) values in transistors based on the solution-sheared films.