Towards Lead-Free Perovskite: How Excitons and Charge Carriers Contribute to the Photovoltaic Performance in Cs2AgBiBr6 Solar Cells
Valentina M. Caselli a, Jos Thieme a, Job Hermans a, Sohan A. Phadke a, Huygen J. Jöbsis b, Eline M. Hutter b, Tom J. Savenije a
a Department of Chemical Engineering, Delft University of Technology (TU Delft), The Netherlands, Netherlands
b Utrecht University, The Netherlands, Princetonplein, 1, Utrecht, Netherlands
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Valentina M. Caselli, presentation 054
Publication date: 20th April 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 are all factors limiting the power conversion efficiency. Here, we investigated the interplay of excitons and trap states on the charge carrier dynamics in Cs2AgBiBr6 thin films by using double pulse excitation time-resolved microwave conductivity, DPE-TRMC. By exciting the sample with two laser pulses with identical excitation wavelength but with varying intensities and a delay time between 0 ns to 30 microseconds, we found that the excited charge carriers rapidly loose their mobility. In contrast, complete recombination to the ground state is extremely slow, extending over tens of microseconds. By modelling the time-resolved DPE-TRMC dynamics we revealed that both deep electron and hole trap states are present in high concentrations, in the orders of 1015 and 1016 cm-3, respectively. Furthermore, a low electron mobility of 0.01 cm2/Vs has been determined, in contrast to 5 cm2/Vs for the holes reducing within nanoseconds to 1.7 cm2/Vs. In addition, an exciton binding energy of approximately 120 meV has been estimated from the analysis of the photoconductance maxima of the DPE-TRMC traces. All these factors play a detrimental role in Cs2AgBiBr6 carrier dynamics and explain the poor photovoltaic performance. Lastly, from the analysis of TRMC traces recorded of Cs2AgBiBr6 in presence of TiO2 or Spiro-OMeTAD, which are commonly applied electron and hole selective transport layers in Cs2AgBiBr6-based devices, we reveal which collection of carriers is retarded. Our results offer fundamental insight of the complex dynamics in Cs2AgBiBr6 thin films.

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