Impact of Argon, Nitrogen, and Oxygen Exposure on the Structural and Optoelectrical Properties of Mixed Tin-Lead Halide Perovskites

Abstract

Mixed tin-lead halide perovskites are considered promising materials for narrow-bandgap photovoltaic applications, particularly in tandem solar cells. However, their practical implementation is hindered by stability issues, especially due to tin oxidation and trap-state formation. In this study, we investigate the impact of argon, nitrogen, and oxygen storage environments on the structural, optical, and electronic properties of mixed tin-lead halide CsFAPb_0.5Sn_0.5I₃ perovskites. Optical absorption, transient photoluminescence (PL), transient photocurrent, and time-delayed collection field (TDCF) measurements reveal the significant role of environmental conditions on carrier dynamics. Carrier trapping over tens of nanoseconds is observed in samples prepared and stored in argon, with a trapping rate increasing several times after exposure to nitrogen (with less than 0.1 ppm of oxygen) and further increasing upon exposure to O₂. Photocurrent transients also show a fast photocurrent decay component occurring within tens of nanoseconds, independent of the oxygen-created traps. Based on the TDCF measurements, we attribute this fast photocurrent decay component to the spatial traps created by the perovskite boundaries, which reduce the carrier mobility to values below 0.05 cm²/V·s, as estimated from transient photocurrent measurements. Our findings highlight the importance of carefully controlling fabrication and storage conditions, often overlooked due to their initially minor impact on device performance, as these conditions critically affect material stability and charge carrier dynamics.

1

Absorption spectra of CsFAPb_0.5Sn_0.5I₃ perovskite films fabricated under different atmospheric conditions after varying storage times.

2

XRD patterns of perovskite films initially fabricated under an argon atmosphere and measured after 26 h of storage in different environments: argon, nitrogen, and oxygen. The right side of the figure shows a zoomed-in view of the (220) diffraction peak over time for each environment, highlighting a slight shift toward lower angles for oxygen-stored films.

3

Surface morphology SEM images of the CsFAPb_0.5Sn_0.5I₃ perovskite films fabricated in an argon atmosphere: (a) as-prepared and (b) after being kept in an oxygen atmosphere for 26 h. The scale bars correspond to 2 μm.

4

Photoluminescence decay kinetics of mixed lead–tin perovskite films measured under different environmental conditions. All measurements were performed immediately after fabrication, starting in an argon atmosphere.

5

Transient photocurrent decay kinetics of CsFAPb_0.5Sn_0.5I₃ perovskite films on IDEs measured under different gas environments. (a–c) show kinetics measured at varying excitation intensities with an applied voltage of 0.1 V, while (d–f) display kinetics measured at different applied voltages with a constant excitation intensity of 0.1 μJ/cm².

6

Normalized transient photocurrent kinetics at different applied voltages for the sample in an Ar atmosphere obtained after dividing measured kinetics by kinetics at 0.5 V. Arrows indicate time intervals when the photocurrent decreases twice. The inset shows an ideal time-of-flight kinetics in case of homogeneous along the carrier drift direction excitation.

7

Time-delayed collection field data for CsFAPb_0.5Sn_0.5I₃ perovskite films on interdigitated electrodes (IDE), measured in (a) argon, (b) nitrogen, and (c) oxygen environments. All measurements were performed under a 0 V prebias and a 1 V extraction voltage, with an excitation intensity of 0.1 μJ/cm². Inset plots show the normalized extracted charge Q(t)/Q(0) versus delay time, illustrating the decay of extractable charge carriers.