Despite the meteoric rise in the development of a variety of perovskite electronic and optoelectronic devices, the phenomenon of ion migration remains a common and longstanding Achilles’ heel limiting their performance and operational stability. In particular, ionic screening of the applied gate potential especially near room temperature reduces the gate modulation of carriers in the semiconducting channel of lead (Pb) perovskite thin film transistors (PeTFTs), resulting in inferior carrier mobilities and non-idealities in device characteristics [1]. Similarly, ionic movements under light and/or bias have been shown to result in current-voltage hysteresis [2], open circuit voltage gain [3] and short circuit current losses [4] in operating Pb perovskite solar cells (PSCs). Moreover, such propensity of facile ionic motion in mixed halide perovskite compositions also contributes to the segregation of individual halide phases under photo and/or electrical excitation [5], resulting in the inferior device performance and stability of PSCs and colour instabilities in perovskite light emitting diodes (PeLEDs). Despite several efforts in the literature to mitigate such effects through compositional and additive engineering, there appears no clarity on the impact of tin (Sn) incorporation on the resulting ion dynamics and compositional instabilities of Pb halide perovskites.

In this talk, I will share our research on understanding the ionic transport in methylammonium (MA)-free mixed Pb-Sn perovskites using PeTFTs and PSCs as two different platforms. Firstly, we demonstrate that mixed Pb-Sn based PeTFTs do not suffer from ion migration effects as significantly as their pure-Pb counterparts, thereby reliably exhibiting hysteresis-free p-type transport with high mobility reaching 5.4 cm²/Vs, which is one of the highest reported TFT mobilities for hybrid perovskite thin films. The reduced ionic migration in mixed Pb-Sn PeTFTs is manifested in the relative invariance of the current-voltage hysteresis as well as ON-current for a range of scan rates and improved temporal stability of the ON-current with time. Moreover, the observation of an activated temperature dependence of the field-effect mobility with low activation energy (< 50 meV for T > 200 K) in the case of mixed Pb-Sn compositions (as opposed to the negative temperature coefficient resulting from ionic screening effects in the pure-Pb composition) is consistent with the presence of shallow defects present in these materials, thereby reinforcing the suppressed role of ionic transport in dictating the charge transport in these materials. Furthermore, by performing photoluminescence microscopy under bias on lateral two-terminal devices, we visualize the suppressed in-plane ionic migration in Sn-containing perovskite compositions compared to their pure-Pb counterparts [6].

Next, we performed scan-rate dependent current-voltage (and hysteresis) measurements and temperature dependent impedance spectroscopy measurements on optimized MA-free Pb and Pb-Sn perovskite solar cells, which demonstrate the suppressed motion of ions in Pb-Sn devices as compared to their Pb-only analogues, thereby generalizing our earlier observations from PeFETs. We also obtain mechanistic insights into these experimental observations by conducting first-principles calculations, which reveal the key role played by Sn vacancies (with low formation energy) in increasing the migration barrier for iodides due to severe local structural distortion in the lattice. Finally, we also show our preliminary results on the improved photostability of mixed-halide perovskite compositions with Pb-Sn alloying under a host of processing, atmospheric and excitation conditions, which will have significant implications for the development of future perovskite optoelectronic devices with extended stability.

All in all, our results emphasize the bright side of tin in arresting the undesirable ionic migration and compositional instability challenges in halide perovskite materials and devices.