Crossover from Band-like to Thermally Activated Charge Transport in Organic Thin-Film Transistors as a Result of Microstress

Abstract

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).