Phonon-assisted trapping of carriers in lead halide perovskites
Christopher Bailey a, Giacomo Piana a, Pavlos Lagoudakis a
a University of Southampton, Southampton, United Kingdom
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Christopher Bailey, 242
Publication date: 11th February 2019

Perovskite solar cells have attracted significant attention in the photovoltaics community, with power conversion efficiencies now greater than 23%. As a result, much work has been carried out to determine the fundamental properties and physical processes of perovskite materials. We develop a model describing the recombination dynamics in CH3NH3PbI3 thin-films that accounts for phonon-assisted exciton and free-carrier trapping. We obtained the longitudinal optical (LO) phonon energy, exciton binding energy and temperature-dependent bandgap by fitting data from absorption spectroscopy and steady-state photoluminescence measurements. Following these experiments, we observe significant coexistence of the tetragonal and orthorhombic structural phases at low temperatures. We determine the LO phonon energy to be 10 meV and the exciton binding energy to be 20.2 meV at room temperature and 23.3 meV at low temperatures. These parameters are used in the model to accurately reproduce the results of temperature- and fluence- dependent time-resolved photoluminescence measurements. This enables us to extract the recombination rates for different processes and also the trap depth, barrier height and trap-state density as a function of temperature. We find that the trap-state density increases at temperatures close to the phase transition at ~ 140 K, suggesting the formation of disorder-induced trap-states.

The authors would like to thank EPSRC and the centre for doctoral training in new and sustainable photovoltaics (CDT-PV) for funding the project.

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