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Soluble small molecule organic semiconductors combine the high-performance of small molecule organic semiconductors with the versatile processability of polymeric materials, but the control of device performance and uniformity is challenged by the complex film microstructure formed in these materials, and its strong dependence on processing conditions. These films crystallize via a nucleation and growth mechanism that can be difficult to control. In this study we used highly fluorinated self-assembled monolayers (SAMs) to modify the surface of the source and drain contacts and improve the performance of organic thin-film transistors (OTFTs) through controlling film microstructure and lowering the contact resistance. We reached charge carrier mobilities as high as 5.7 cm2/V in 2,8-Difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), one order of magnitude greater than what we obtained in devices on untreated substrates, and on par with the value reported for single crystal devices. Kelvin probe measurements distinguished an increase in the work function between 0.28 eV and 0.5 eV, depending on the molecular structure of the SAM. Selected area electron diffraction (SAED) confirmed the preferential “edge-on” molecular orientation of the semiconductor. We discuss the device performance in relation to the film morphology and contact resistance.

The hybrid halide perovskites combine the low-cost processing characteristics of organic materials with the performance factors of inorganic compounds. Recently the power conversion efficiencies of perovskite photovoltaic solar cells have reached a respective value of ∼20%. The charge transport properties were indirectly approximated in these compounds because of lack of available field-effect transistors (FETs). Here we report the fabrication and room-temperature operation of FETs based on the hybrid perovskites. We obtained balanced electron and hole transport with mobilities of ∼1 cm2/Vs. We also found that the yield, as well as the operational and environmental stability of the fabricated transistors is limited.

The electrical properties of organic field-effect transistors are governed by the quality of the constituting layers, and the resulting interfaces. We compare the properties of the same organic semiconductor film, 2,8-difluoro- 5,11-bis (triethylsilylethynyl) anthradithiophene, with bottom SiO2 dielectric and top Cytop dielectric and find a 10× increase in charge carrier mobility, from 0.17 ± 0.19 cm2 V−1 s−1 to 1.5 ± 0.70 cm2 V−1 s−1, when the polymer dielectric is used. This results from a significant reduction of the trap density of states in the semiconductor band-gap, and a decrease in the contact resistance.

Organic semiconductors are viable candidates for flexible electronic applications given their facile processing and malleability to any substrate shape, size and type. But the performance of the emerging devices still remains insufficient for many technologies. Recent work however has suggested that this is not an intrinsic property of the organic semiconductor layer, but it originates from extrinsic effects, such as the presence of impurities, thin-film microstructure, traps at the organic semiconductor-dielectric interface, contact effects, etc. These effects not only diminish the electrical performance, but can also prevent the access to the intrinsic properties of these materials. For example, while a band-like transport is expected for high quality single crystal devices, such behavior was observed in a very limited number of field-effect transistor (FET) measurements, and only in devices with vacuum-gap or organic dielectrics [1]. The formation of Fröhlich polarons or broadening of the density of states due to static dipolar disorder at the interface between the organic semiconductor and the dielectric can localize the charges, yielding a temperature activated transport. In this study we show that the microstress induced in the semiconductor layer as a result of the mismatch in the thermal expansions of the organic and dielectric layers also increases the trap density, lowers mobility and can even induce a crossover from a band-like to an activated transport. We fabricate organic thin-film transistors (OTFTs) with a novel small molecule organic semiconductor, 2,8-difluoro-5,11-bis(triethylgermylethynyl) anthradithiophene, diF-TEG ADT, a material that exhibits charge carrier mobilities greater than 5 cm2/Vs and is compatible with large-area spray deposition [2]. We evaluate the temperature effects on “vacuum-gap” and SiO2 dielectric and by tuning film texture and structure via processing we access ratios between the coefficients of linear thermal expansions (CTE) of the organic semiconductor and the dielectric varying between 1 and ~ 200 ppm/K. We observe band-like transport in the case of no CTE mismatch (vacuum dielectric) and find a performance degradation in SiO2 OTFTs. This is due to generation of trap states as a result of different thermal expansion between the dielectric and organic semiconductor layers. In the latter case, a thermal expansion coefficient of 90 ppm/K and a room temperature mobility of 3.7 cm2/Vs is measured for the films consisting of large grains, and 177 ppm/K and 5*10-4 cm2/Vs for the case of small grains. We show that the relative differences in thermal expansion between the dielectric and the organic film produce stress-induced defects that act as trapping sites for the injected charges. The trap density at the semiconductor/SiO2dielectric (thermal expansion coefficient 4.1 ppm/K, [3]) interface increases by 22% between room-temperature and 150K for the large grain devices, and by 5 times for the small grain devices, where the difference in thermal behavior is significantly larger. We compare these results with the case of other organic semiconductors, characterized by different thermal expansion coefficients (pentacene, 78 ppm/K and rubrene, 28 ppm/K). References [1] I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iossad, G. Rastelli, S. Ciuchi, A. F. Morpurgo, Nat. Mater., 5, 982 – 986, 2006. [2] Y. Mei, M. A. Loth, M. Payne, W. Zhang, J. Smith, C. S. Day, S. R. Parkin, M. Heeney, I. McCulloch, T. D. Anthopoulos, J. E. Anthony, O. D. Jurchescu, "High Mobility Field-Effect Transistors with Versatile Processing from a Small-Molecule Organic Semiconductor," Adv. Mater., 25, 4352-4357, 2013. [3] G. W. McLellan and E. B. Shand, Glass Engineering Handbook, 3rd ed. McGraw-Hill, New York, 214–215, 1984.

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