Traps in the Spotlight: How Traps Affect the Charge Carriers Dynamics in Cs₂AgBiBr₆ Perovskite

Since their discovery as versatile semiconductors for photovoltaic applications, perovskite materials, with general formula ABX₃, have been intensively studied. Tuning of their opto-electronic properties make it possible to reach remarkable power conversion efficiencies, currently topping at 25.6%, in roughly a decade of development. However, a rising concern regarding the use of lead in the photoactive layer has pushed research into lead-free alternative candidates. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs₂AgBiBr₆. However, efficiencies of Cs₂AgBiBr₆-based photovoltaics are still modest. To elucidate the loss mechanisms, in this report we investigate charge dynamics in Cs₂AgBiBr₆ films in thin films by means of TRPL, TA measurements and double pulse excitation time-resolved microwave conductivity (DPE-TRMC). 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. In addition, we show that both carriers are trapped on sub-ns timescales, while their depopulation occurs over tens of microseconds. Furthermore, we observe a higher mobility for holes compared to electrons, which amount to 5 and 0.01 cm²/(Vs) respectively, in agreement with the imbalance in their effective masses. Localization of holes causes an effective loss in mobility for the holes, dropping to ca. 1.7 cm²/(Vs), while no effect can be discerned for the electrons. Knowing these kinetic parameters allows us to predict the charge carrier dynamics under AM1.5, explaining the solar cell performance. Our new developed DPE-TRMC methodology gives direct insight on the timescales involved with population and depopulation of the various trap states in Cs₂AgBiBr₆, essential for the design of more efficient devices.