Exciton Binding Energy of Methylammonium Lead Tri-iodide and Tri-bromide Perovskites

Murad Tayebjee^a, Anita Ho-Baillie^a, Arman Mahboubi Soufiani^a, Rui Sheng^a, Martin A. Green^a, Fuzhi Huang^b, Yi-Bing Cheng^b, Peter Reece^c

^aUniversity of New South Wales, Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Engineering, Sydney 2052, Sydney, Australia

^bSchool of Materials Engineering, Monash University, Victoria 3800

^cSchool of Physics, The University of New South Wales, Sydney, NSW 2052

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Arman Mahboubi Soufiani, 143
Publication date: 5th February 2015

The last few years have seen the rapid emergence of organometal halide perovskites as a promising medium for light harvesting and carrier transport in solar cells. As for any novel material entering the photovoltaic field, appropriate knowledge of the basic physical properties is essential. Exciton binding energy is one of them, and its accurate calculation and/or measurement can help in determining the electronic band gap of the semiconductor, its transport properties and in understanding the origin of luminescence. The latter, in particular, is strong in methylammonium lead halides (iodide, bromide or mixed halides) of interest [1]. In this study, for the first time, we report the exciton binding energy of CH₃NH₃PbBr₃ to be 40±6meV at room temperature (RT) for its cubic crystallographic phase. The absorption band-edge of CH₃NH₃PbBr₃ shows a clearly resolved excitonic feature as a single peak from 7K up to RT (Fig. 1). However, the corresponding peak smears out at RT in CH₃NH₃PbI₃ due to its lower exciton binding energy value of 15.4±3.4meV compared to its corresponding thermal broadening parameter. The values measured here fall in between the two extremes of exciton binding energies calculated based on the largely different optical permittivity and static dielectric constant in these materials [2], as argued in the following. In these polar materials with ionic bonding character, which intensifies as smaller halogen ions are substituted on the octahedron corners, the interaction of polarons, the composite entity formed by the introduced charge carrier and the superposition of longitudinal optical phonons [3], with photo-excited quasi-particle species should be considered. As such, spatial-dependent screening of the electron-hole interaction should be included when estimating the exciton binding energy of methylammonium lead halides. Calculations along these lines are consistent with the abovementioned experimentally measured values, implying incomplete relaxation of the lattice. Our results further support the free carrier character of the photo-excited species in the most commonly studied organic lead halide perovskite, CH₃NH₃PbI₃ [4,5], while providing insights into an appropriate approach for determining the exciton binding energy of these materials.
Fig. 1 Temperature-dependent optical density. Optical density spectrum of CH3NH3PbBr3 measured from room temperature to 7-8K.
1. Deschler, F., et al., High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. The Journal of Physical Chemistry Letters, 2014. 5(8): p. 1421-1426. 2. Green, M.A., A. Ho-Baillie, and H.J. Snaith, The emergence of perovskite solar cells. Nat Photon, 2014. 8(7): p. 506-514. 3. Iadonisi, G., J. Ranninger, and G. De Filippis, Polarons in bulk materials and systems with reduced dimensionality. Vol. 161. 2006: IOS Press. 4. D’Innocenzo, V., et al., Excitons versus free charges in organo-lead tri-halide perovskites. Nat Commun, 2014. 5. 5. Lin, Q., et al., Electro-optics of perovskite solar cells. Nature Photonics, 2014.

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