Organic Thin Film Transistors Based on Highly Dipolar Donor–Acceptor Polymethine Dyes

Abstract

Organic thin film transistors (OTFTs) of a series of twenty dipolar donor–acceptor‐substituted polymethine dyes (D–A dyes, dipole moments from 3–15 D) are investigated. The employed merocyanine dyes contain a dimethine bridge that is substituted with 1‐alkyl‐3,3‐dimethylindolin‐2‐ylidene (“Fischer base”), 3‐alkyl‐2,3‐dihydrobenzothiazol‐2‐ylidene or 1,3‐benzodithiole‐2‐ylidene, respectively, as electron‐donating unit and various acceptor heterocycles. These studies show that thin films formed by these D–A dyes upon deposition in high vacuum are all composed of antiparallel π‐stacked dimers. However, they are either amorphous, discontinuous or highly crystalline due to the interplay between molecule‐substrate and dimer–dimer interactions. With the help of single crystal X‐ray analysis, out‐of‐plane X‐ray studies (XRD), selected area electron diffraction (SAED), and atomic force microscopy (AFM), a correlation between the molecular structure, film ordering, and hole charge transport ability can be established. The mobility values are compared to Bässler's disorder charge transport theory and a film growth mechanism is proposed based on DFT calculations and single crystal structures. The results show that with carefully adjusted bulky substituents and high dipolarity an intimate centrosymmetric packing with a slipped, but tight π‐stacking arrangement could be realized. This provides two‐dimensional percolation pathways for holes and ultimately results in charge carrier mobilities up to 0.18 cm2 V−1 s−1.