Transient Reflected Microwave Conductivity measurements for the assessment of the quality and stability of triple-cation, triple-anion Perovskite films

A very simple and compact measurement setup has been used, combining a Transient Reflected Microwave conductivity (TRMC) setup [1], operating in the Ka-band, realized using a Gunn-diode microwave source, a circulator and a fast diode as receiver, with a 520 nm pulsed semiconductor laser diode with variable optical pulse length between between 6 and 129 ns for optical excitation. The photo-generated charge carriers change the sample conductivity and consequently the reflected microwave power. From the TRMC transient charge information on carrier lifetime, recombination and trapping phenomena can be extracted. Due to the high charge carrier mobility in Perovskite, even at relatively low excitation pulse intensities, TRMC signals with a good signal-to-noise ratio can be obtained. First such experiments have been reported recently in the literature [2,3]. In this study the application of the TRMC-technique for the monitoring of the film properties of triple-cation, triple-anion Perovskite films is shown. These films are optimized for good long-term stability and films with 2 different bandgap values have been investigated. The beneficial effect of a PEAI passivation layer could been verified. These measurements on Perovskite films have been compared to the properties of solar cells, realized with these films as active layer. A very good correlation between TRMC lifetime data and the quantum efficiency spectra of the completed solar cells is found with an increase of the short wavelength photoresponse after passivation. While different runs of production of the low-bandgap films with a bandgap of 1.58 eV showed basically the same properties, in the case of the high-bandgap films (1.62 eV), initially much lower lifetimes and a smaller TRMC-signal amplitude have been measured. The latter indicates some fast decay processes, already active during the exciting pulse. Also this latter material could be optimized and TRMC measurements on later run showed decay times comparable to the ones of the low-bandgap material. These improvements have also been reflected in the corresponding device properties, for example in the solar cell performances under low-light illumination conditions. Finally tests of the long-term stability of the Perovskite film properties have been done. TRMC-lifetime remained basically unchanged over a period of some months, confirming also the good quality of the applied encapsulation technique.

(1) Kunst, M.; Beck, G. The study of charge carrier kinetics in semiconductors by microwave conductivity measurements: J. Appl. Phys. 1986, 63, 3558−3566.
(2) Li, Y.; Jia, Z.; Yang, Y.; Yao, F.; Liu, Y.; Lin, Q. Shallow traps-induced ultra-long lifetime of metal halide perovskites probed with light-biased time-resolved microwave conductivity, Appl. Phys. Rev. 2023, 10, 011406.
(3) Caselli, V.M.; Thieme, J.; Jobsis, H.J.; Phadke, S.A.; Zhao, J.; Hutter, E.M.; Savenije, T.J. Traps in the spotlight: How traps affect the charge carrier dynamics in Cs2AgBiBr6 perovskite. Cell Reports Physical Science 2022, 3, 101055.