Proceedings Article | 3 October 2022
KEYWORDS: Perovskite, Crystals, Transistors, Ions, Thin films, Terahertz spectroscopy, Field effect transistors, X-rays, Thin film deposition, Terahertz radiation
The inorganic and hybrid (organic - inorganic) perovskites appear as an attractive, alternative solution for organic solar cells and light emitting diodes due to their large absorption coefficient, low exciton binding energy, and “long” carrier diffusion length in thin films. Thanks to their cost-effective and facile solution manufacturing perovskites have received enormous attention and showed great potential for industrialization and practical application. Despite the relatively high intrinsic - local charge carrier mobility determined by optical methods, the fabrication of reliable perovskites field-effect transistors with high performance at room temperature has remained as an enormous challenge. First of all, many strategies have been proposed to enhance the device performance, such as controlling the crystallization process, regulating the film microstructure, and adjusting the phase orientation. These approaches involve self-assembled monolayer, non-stoichiometry crystal engineering, mixed solvent, and additives. However, precise control of the grain size and crystallinity is still a challenge. First of all, the continuity and homogeneity of the films are the main limitations in an effective in-plane charge carrier transport. From one hand, strategy mainly used in photovoltaics, rapid nucleation supported by a slow crystal growth do not allow to obtain continuous and large area surface coverage without defects. Secondly, charge carrier transport in perovskite transistors, is obscured by serious ionic migration in both 3 dimensional (3D) and 2 dimensional (2D) structures. Ionic migration is induced by migration of both anions (I-) and cations (MA+, PB2+) in the presence of vacancies, supported by an external source – drain or source – gate field. This bring us to the fact, that all defects in perovskite structure created during not controlled nucleation or crystallization process are potential source of ions movement, which significantly reduce the in-plane charge carrier transport. From this reason, fundamental understanding of the perovskite formation process is the critical parameter which needs to be recognize and fully controlled, if high charge carrier transport, undisturbed by ions migration, is expected. In this report, the crystallization and growth kinetics of Sn(II)-based 2D perovskite, using 2-thiopheneethylammonium (TEA) as the organic cation spacer, was effectively regulated by the hot-casting method. Hot-casting is proposed as an effective approach to precisely control the crystallization and growth kinetics of (TEA)2SnI4 perovskite films enabling large grain sizes. With increasing crystalline grain size, the local charge carrier mobility, is found to increase moderately for ~ 25% from 13 cm2V–1s–1 to 16 cm2V–1s–1, as inferred by terahertz (THz) spectroscopy. In contrast, the FET operation parameters, including mobility, threshold voltage, hysteresis, and subthreshold swing improve substantially with increasing grain size, especially for large channel lengths. This behavior is mainly attributed to screening of the applied electric fields by ion migration that becomes severe for small grain sizes and high densities of grain boundaries. Electrical characterization at various temperatures (from 300K to 100K) presents an influence of the ion migration on the charge carrier transport in transistors architecture. Moreover, it is confirmed by grazing incident wide angle x-ray scattering and theoretical unit cell prediction that hot-casting method does not change the molecular organization in deposited thin films but only reduce the number of grain boundaries. These insights provide important guidance for the grain engineering for high-performance 2D perovskite FETs.
