A comprehensive structural and electrical characterization of solution-processed blend films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconductor and poly(alpha-methylstyrene) (PalphaMS) insulator was performed to understand and optimize the blend semiconductor films, which are very attractive as the active layer in solution-processed organic thin-film transistors (OTFTs). Our study, based on careful measurements of specular neutron reflectivity and grazing-incidence X-ray diffraction, showed that the blends with a low molecular-mass PalphaMS exhibited a strong segregation of TIPS-pentacene only at the air interface, but surprisingly the blends with a high molecular-mass PalphaMS showed a strong segregation of TIPS-pentacene at both air and bottom substrate interfaces with high crystallinity and desired orientation. This finding led to the preparation of a TIPS-pentacene/PalphaMS blend active layer with superior performance characteristics (field-effect mobility, on/off ratio, and threshold voltage) over those of neat TIPS-pentacene, as well as the solution-processability of technologically attractive bottom-gate/bottom-contact OTFT devices.
We demonstrate tuning of hole injection barriers in bottom contact triisopropylsilylethynyl pentacene ͑TIPS-pentacene͒ organic thin film transistors ͑OTFTs͒ by forming the self-assembled monolayers ͑SAMs͒ of thiophenol, 4-fluorothiophenol, or pentafluorothiophenol on the pristine Ag electrode. The work functions of SAM-treated Ag electrodes are measured by Kelvin probe method. The TIPS-pentacene OTFT devices were fabricated by a drop-cast method with a micropipette like an inkjet printing. The OTFTs with pentafluorothiophenol-Ag electrodes as source and drain exhibit carrier mobility of 0.17 cm 2 / V s and on/off current ratio of 10 5 because of almost no hole injection barrier to TIPS pentacenes. The SAM-treated Ag electrodes are robust over repeated electrical scans of 100 cycles.
The use of processing additives has emerged as a powerful approach for the optimization of active layer performance in organic photovoltaic devices. However, definitive physical mechanisms explaining the impact of additives have not yet been determined. To elucidate the role of additives, we have studied the time evolution of structure in polymer‐fullerene films blade‐coated from additive containing solutions using in‐situ spectroscopic ellipsometry and UV–vis transmission. Additives that are poor solvents for poly(3‐hexylthiophene) (P3HT), such as 1,8‐octanedithiol, and additives that are good solvents for P3HT, such as 1‐chloronapthalene, both promote improved polymer order, phase segregation, and device performance. Regardless of the presence or type of additive, the polymer order develops under conditions of extreme supersaturation. Additives, regardless of whether they are solvents for P3HT, promote earlier polymer aggregation compared to additive ‐ free solutions presumably by degrading the solvent quality. We find evidence that the details of the final film morphology may be linked to the influence of the substrate and long‐time film plasticization in the cases of the non‐solvent and solvent respectively.
A comprehensive structure and performance study of thin blend films of the small‐molecule semiconductor, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TESADT), with various insulating binder polymers in organic thin‐film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X‐ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X‐ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α‐methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution‐processed bottom‐gate/bottom‐contact thin‐film transistors.
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