Understanding Charge Transport in Lead-Tin Perovskite Field Effect Transistors with Superior Performance

Metal halide perovskites have earned the distinction of one of the most exciting semiconductor technologies in the last decade, with the photovoltaic power conversion efficiencies demonstrating a significant surge from 3.8% in 2009 to 25.5% in 2020. In addition, many of the early studies showed that halide perovskites also demonstrate superior charge carrier mobilities, as high as 10 -100 cm²/Vs from THz measurements, which promises a bright prospect for employing in field effect transistors (FETs) that form the bedrock of modern electronics. However, due to the soft nature of halide perovskites, charge transport studies on the 3D perovskite compositions have largely remained under the cloud of undesirable ionic migration effects near room temperature. The mobile ions can screen the applied potential and reduce the gate modulation of carriers at the interface, thereby leading to lower mobility at room temperature along with pronounced hysteresis and non-idealities in the device characteristics. Moreover, most of the 3D perovskites FETs contain methylammonium cation (MA⁺, CH₃NH₃⁺) in their compositions, which is known to be inherently unstable and can introduce additional problem of dipolar disorder which further reduces the carrier mobility near room temperature. In this work, we address the above challenges using a simple strategy of alloying tin (Sn) with lead (Pb) in 3D mixed formamidinium-cesium (FA-Cs) perovskite compositions with bottom-gate bottom-contact FET architectures. Incorporation of Sn to partially replace Pb at the B-site of perovskite modifies the electronic structure to obtain a reduced hole effective mass and additional density of states at the VBM, thereby resulting in a pronounced p-type transport. Perovskite FETs fabricated from such mixed Pb-Sn perovskites exhibit remarkable transport properties with µ_FET reaching as high as 5.4 cm²/Vs, ON/OFF ratio exceeding 10⁶ and normalized channel conductance of 3S/m, which are among the highest in the field of perovskite FETs. We then utilize these reliable high-performance FETs to probe the charge transport physics in mixed Pb-Sn perovskites. Temperature dependent transport measurements indicate a transition from ionic migration dominated negative coefficient of mobility to a classical activated behaviour, signifying the inherent electronic traps that are rife in solution-processed perovskite semiconductors. Such trends were never observed heretofore in 3D hybrid perovskite FETs owing to undesired ionic migration effects. We believe that the background p-type doping originating from ubiquitous Sn vacancies in mixed Pb-Sn perovskites completely overwhelms the gate-induced ionic motion to the extent that it is possible to mitigate the ionic migration and observe the intrinsic charge transport in solution processed perovskites. Hence, various molecular and interfacial doping strategies may be employed even for Pb perovskite FETs in future to mitigate such ionic migration effects near room temperature. We also visualize the suppressed in-plane ionic migration and associated electrochemical processes occurring under bias for Sn-containing perovskites using photoluminescence imaging measurements. Finally, we present results on the operational and shelf-life stability of fabricated FETs with different A-site cations, which further reinforces the need to go MA-free in future. It is also important to note that this work represents the first comprehensive study of FETs based on low bandgap mixed Pb-Sn compositions, which have recently demonstrated significant progress in single junction and tandem solar cells.Our work therefore establishes FETs as a powerful and reliable platform for investigating the device physics of doping, defects and instabilities in perovskite semiconductors, which can provide numerous inputs of relevance for furthering the progress of perovskites in solar cells, light emitting diodes and other optoelectronic applications.