Revealing the Complex Charge Carriers Dynamics in double metal Perovskites by Microwave Conductivity Techniques

Tom Savenije^a, Valetina Caselli^a, Jos Thieme^a, Huygen Jöbsis^b, Eline Hutter^b

^aDepartment of Chemical Engineering, Delft University of Technology Van der Maasweg, Van der Maasweg, 9, Delft, Netherlands

^bInorganic Chemistry and Catalysis, Department of Chemistry, Utrecht University, Daltonlaan, 200, Utrecht, Netherlands

Materials for Sustainable Development Conference (MATSUS)
Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)

#PbFreePero - The potential of lead-free perovskites: synthesis, properties, and applications

Barcelona, Spain, 2022 October 24th - 28th

Organizers: Loreta Muscarella, Eline Hutter and Hendrik Bolink

Invited Speaker, Tom Savenije, presentation 075
DOI: https://doi.org/10.29363/nanoge.nfm.2022.075
Publication date: 11th July 2022

A great challenge in today’s research on perovskites is finding stable, lead-free alternatives for photovoltaic applications. One interesting route is the replacement of divalent lead with a combination of monovalent and trivalent cations, as in Cs2AgBiBr6. Despite showing higher stability, the photovoltaic performance of Cs2AgBiBr6 based devices is inferior to lead perovskite based devices. The indirect bandgap, high exciton binding energies, and presence of trap states have been discussed as limiting factors of the power conversion efficiency. This work aims to provide new insight into the interplay of excitons and trap states on the charge carrier dynamics in Cs2AgBiBr6 thin films. To do so, we investigated Cs2AgBiBr6 thin films and bilayers of Cs2AgBiBr6 with a transport layer (TL) by means of time-resolved photoluminescence (TRPL), transient absorption (TA) measurements and time-resolved microwave conductivity (TRMC). In addition, an entirely novel technique i.e. double pulse excitation time-resolved microwave conductivity (DPE-TRMC) has been developed. Basically, in DPE-TRMC the sample gets illuminated by two laser pulses arriving with a short time delay. By comparing the photoconductance traces induced by the second pulse in presence and absence of the first pulse, we are able to examine the effect of the long-lived species on the charge carrier dynamics. For these experiments we used identical excitation wavelengths for both laser pulses but varying intensities and delay times. We modelled the results introducing a comprehensive model, which accounts for the free carrier generation yield, localization of free carriers, electron trapping by color centers, and shallow trap states for holes. The iterative analysis of the DPE-TRMC experiments with different intensities and delay times reveals the presence of a high concentration of both electron (1015 cm-3) and hole (1016 cm-3) trap states. Slow (partially) radiative decay of trapped electrons with their countercharges is in agreement with the TRPL measurements. Holes are rapidly trapped in shallow states, from where they get slowly released explaining the complementary TA measurements. These results are compared to results obtained on alloys of Cs2Ag(Bi1-xSbx)Br6. Finally, we also looked into the charge collection in double layers of Cs2AgBiBr6 and various transport layers including TiO2 and C60. In our opinion, the long-lived charge carriers in Cs2AgBiBr6 as observed in this work can offer suitable, lead-free alternatives for many other applications, including X-ray detectors or photocatalysis.

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