Charge recombination in methylammonium lead triiodide at low temperatures
Arvydas Ruseckas a, Oskar Blaszczyk a, Jonathan R. Harwell a, Lethy Krishnan Jagadamma a, Ifor D. W. Samuel a
a Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, U.K.
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Benidorm, Spain, 2018 May 28th - 31st
Organizers: Emilio Palomares and Rene Janssen
Oral, Arvydas Ruseckas, presentation 154
DOI: https://doi.org/10.29363/nanoge.hopv.2018.154
Publication date: 21st February 2018

Trap-mediated charge recombination is the major factor limiting the efficiency of hybrid metal halide perovskite solar cells. Previous studies have shown coexistance of two crystal phases in methylammonium lead triiodide (MAPI) films at low temperatures and some evidence of charge transfer from the predominant  orthorhombic phase to small inclusions of the tetragonal phase. These inclusions act as efficient radiative recombination centers and recently a continuous wave lasing under optical pumping was demonstrated in this material. Trap-mediated recombination in such small inclusions may also be different from the bulk material but has not been studied in detail yet.  

In this work we studied charge recombination in MAPI at low temperatures using a wide range of pulsed excitation densities, including very low ones. We combined time-resolved photoluminescence with transient absorption spectroscopy to distinguish  radiative and non-radiative recombination. We observe a higher rate of radiative bimolecular recombination at early time after photoexcitation when all carriers are mobile which allows us to determine the density of mobile and trapped carriers in each crystal phase. We find that the density of trapped charges is lower in small inclusions of the tetragonal phase as compared to the dominant orthorhombic phase which leads to suppression of trap-mediated recombination at low temperature. Our results contribute to better understanding of recombination processes.

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