Ambient Air Stability of Hybrid Perovskite Thin‐Film Transistors by Ambient Air Processing

Abstract

Despite the widespread research on organic–inorganic hybrid perovskites, the ambient air instability and ion migration‐induced hysteresis in the current–voltage characteristics of their devices remain unsolved. Here, it is shown that stable ambient air operation of methylammonium lead iodide (MAPbI3) thin‐film transistors can be achieved by solution processing of the MAPbI3 film in ambient air via solvent engineering. N,N‐dimethylformamide (DMF), mixed with dimethyl sulfoxide (DMSO) and hydroiodic acid (HI), is found to be the most suitable solvent for one‐step deposition in ambient air. While the HI promotes homogeneous nucleation, the low boiling point of DMF and the high vapor pressure and strong Lewis base property of DMSO lead to fast crystallization and consequent large grain size. The oxygen in air passivates grain boundary defects, thereby improving lateral conduction and minimizing grain boundary defect‐mediated ion migration. This approach paves the way for the simple fabrication of hybrid perovskites by eliminating the need for well‐controlled inert environments and provides a solution for the problem of ambient air stability in perovskite devices.

Reduction of Hysteresis in Hybrid Perovskite Transistors by Solvent-Controlled Growth

“…Previous studies have shown the presence of oxygen during annealing to reduce defects without affecting the surface morphology [ 37 , 38 ]. Moreover, it is suggested that oxygen reduces grain boundary defects, yielding better grain-to-grain connection, which is essential for good lateral conduction [ 24 ]. The presence of oxygen in the films ( Figure 4 ) was consistent with defect passivation during ambient air processing [ 21 , 24 , 25 , 38 , 39 ] and evaporation of MA via oxygen bonding at temperatures greater than 120 °C ( Figure 3 ).…”

“…Experimentally, we found temperatures around 70 °C to be sufficient for the seed formation. Given the high boiling points of GBL and DMSO (204 and 189 °C, respectively) [ 24 ], high-temperature annealing is required after SCG to effectively complete crystallization. This temperature should be around 120 °C, according to Figure 3 .…”

“…Additionally, there is still room to optimize the thermal treatment employed herein and further improvement can be achieved by employing additional strategies to suppress the formation of vacancies and ion migration such as the incorporation of multi cation mixtures and Lewis base acid treatments [ 53 , 54 , 55 ]. Additionally, various surface treatments such as the insertion of SAMs [ 16 ] and the use of additives to passivate surface defects [ 24 , 25 , 26 , 27 , 28 , 29 ] can be applied to further mitigate the difference in the surface energy or roughness between the G.I. and the perovskite semiconductor to achieve smoother pinhole-free thin films.…”

“…Using the Laurell WS-400B spin coater (Laurell, North Wales, UK), we spin-coated several MAPbI 3 films on bare glass substrates at room temperature in ambient air (relative humidity of 50 ± 5% and temperature of 25 ± 2 °C). For process simplicity and for better ambient air stability, the MAPbI 3 films were deposited in ambient air as described previously in [ 24 ]. We obtained the precursor solution by dissolving powdered MAI (99.999%, Greatcell Solar, Queanbeyan, Australia) and PbI 2 (99%, Merck Korea Ltd./Sigma-Aldrich Korea Ltd., Incheon, Korea) (MAI:PbI 2 = 1:1) in a mixed solvent composed of gamma-butyrolactone (GBL) (99%, Merck Korea Ltd./Sigma-Aldrich Korea Ltd., Incheon, Korea) and dimethyl sulfoxide (DMSO) (99.9%, Merck Korea Ltd./Sigma-Aldrich Korea Ltd., Incheon, Korea), followed by stirring for 12 h at 100 °C in ambient air.…”

“…Many researchers have attempted to eliminate or counteract the hysteresis via solvent engineering [ 24 , 25 , 26 ], insertion of self-assembled monolayers (SAMs) [ 16 ], suppression of interfacial ferroelectricity [ 18 ], doping [ 19 ], defect passivation [ 20 ], additives [ 24 , 25 , 26 , 27 , 28 , 29 ], ambient air processing [ 26 , 27 , 28 ], and so forth [ 30 , 31 , 32 ]. However, despite all these efforts, the current situation is still far from the complete annihilation of the hysteresis problem.…”