Exploring the Formation and Growth of Organic Semiconductors with mm-Scale Grains

While record mobilities have been reported for organic semiconductors in their single crystal form, the bulk nature of such crystals prohibit their practical application in devices. Here, we discuss our efforts to realize pinhole free films of organic semiconductors with grains of up to 1 mm in extent. One such material is rubrene, but we will also show our efforts on other materials applicable to organic light emitting diodes, such as the electron transport layer 2,2’,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). For rubrene, we will show our efforts to understand crystal formation, epitaxy, and transistors. Homoepitaxial studies uncover evidence of point and line defect formation in these films, indicating that homoepitaxy is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result strain built into the thin film template, indicating that, in general, organic crystalline thin films may not adopt the exact lattice of a bulk crystal. Transistors made out of these large-grained films of rubrene display charge carrier mobility of up to 3.5 cm² V^–1 s^–1, very close to single crystal values, highlighting their potential for practical application.