A cleaning–healing–cleaning method can effectively eliminate ionic defects at the surface of perovskite films, resulting in reliable and high-performance perovskite transistors.
Metal halide perovskite semiconductors, such as methylammonium lead iodide (CH3NH3PbI3), have been intensively investigated for use in optoelectronic devices such as solar cells and light-emitting diodes1. Research into their use in field-effect transistors (FETs) has though received significantly less attention, despite the fact that perovskite semiconductors can potentially offer high charge carrier mobilities. The earliest perovskite transistors, which were reported two decades ago, used two-dimensional layered organic–inorganic perovskite semiconductors, with alternating organic phenethylamine and inorganic tin iodide sheets2. The bulky organic species of such structures can improve the ambient stability of the devices by supressing ion movement, but the organic part is also an electrical insulator and hinders efficient charge transport in the vertical direction that runs through the polycrystalline organic–inorganic layers of the films.
Three-dimensional (3D) perovskites are more likely to show high mobility as the metal halide forms an octahedral scaffold that extends continuously in the vertical and lateral directions. However, one of the main obstacles to achieving reliable transistor operation is ion migration, as unbound mobile ions in the channel can screen the applied gate field, resulting in poor current modulation and significant current–voltage hysteresis3,4. Writing in Nature Electronics, Henning Sirringhaus and colleagues now report a solvent-based surface cleaning and passivation approach that can create high-performance 3D perovskite transistors5.
Inspired by the observation that surface degradation caused by an electron beam in a scanning electron microscope can be repaired by simply washing in isopropylalcohol (IPA), the researchers developed a cleaning–healing–cleaning process to ‘wash away’ the ionic defects in methylammonium lead iodide perovskite films and suppress the problem of ion migration (Fig. 1). The process follows three steps: a cleaning step to remove weakly bonded defective species from the surface; a healing step to remove surface organic–halogen defects/vacancies left after (or, potentially, introduced during) the first step; and a further cleaning step to remove residual ionic species left after the healing step and to ensure a clean surface prior to gate dielectric deposition.
The technique relies on a carefully chosen co-solvent system that combines a polar solvent with a non-polar solvent in order to avoid creating significant changes in film morphology or the introduction of undesirable impurities. And by also optimizing the cleaning process temperature, perovskite transistors could be created with field-effect mobilities for electrons and holes of up to 4.2 cm2 V−1 s−1 and 2.1 cm2 V−1 s−1, respectively, at room temperature. Notably, these mobilities are high enough that researchers should be able to conduct in-depth studies on the charge transport physics and structure–property relationships of perovskite semiconductors, something that was previously obscured by ion migration effects.
Though the approach of Sirringhaus and colleagues — who are based at the University of Cambridge, Imperial College London, the Australian National University and the University of Oxford — provides reliable perovskite transistors, there are still several challenges to address before practical devices are a reality. In particular, for various applications of perovskite transistors, including switching and driving transistors for organic light-emitting diode displays, charge carrier mobilities need to be improved to the level of polycrystalline silicon. Despite perovskites having high intrinsic charge carrier mobilities, actual mobilities are frequently degraded by strong polaronic scattering and various extrinsic factors6. One approach to reduce the polaronic scattering is to use a lighter metal cation than lead, while another is to create highly crystalline films with reduced grain boundaries and defects through a better understanding of film formation dynamics.
Another issue is the use of lead in such devices, which is harmful to the environment. A promising alternative metal cation is tin, especially for high-performance p-channel transistors7, but tin-based perovskites also have worse stabilities than lead-based perovskites. However, tin-based perovskites can potentially borrow technologies from lead-based perovskites in order to address stability issues. For instance, thermal stability can be enhanced by tuning the composition of lead-based perovskites by incorporating multi-cations and stabilizing additives8. Air stability can also be improved through strict encapsulation, as long as the structure phase itself is stable. Nevertheless, it remains unclear whether an ideal lead-free material, which combines optical and electrical properties with excellent stability, can be found. And potential candidates should not be limited to the family of conventional lead or tin-based halide perovskites: novel materials such as double perovskites with stable cations, and ‘perovskite-like’ materials with edge-sharing or face-sharing octahedral structures, are also worth consideration.
Research on perovskite-based transistors is still at a relatively early stage, but the work of Sirringhaus and colleagues provides a valuable surface cleaning approach to create reliable perovskite transistors, and it will be intriguing to observe how close perovskite FETs may reach to their intrinsic performance limits. Furthermore, their cleaning approach should be applicable to a range of other perovskite devices with various compositions, as ion migration also deteriorates the efficiency and stability of photovoltaics and light-emitting diodes9.
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The authors declare no competing interests.
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Zhu, H., Liu, A. & Noh, YY. Perovskite transistors clean up their act. Nat Electron 3, 662–663 (2020). https://doi.org/10.1038/s41928-020-00470-z