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Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

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Abstract

Solar cells based on the organic–inorganic tri-halide perovskite family of materials have shown significant progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room-temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance as being due to spontaneous free-carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.

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Figure 1: Magnetic field dependence of the optical density for the perovskite CH3NH3PbI3.
Figure 2: Energy ‘fan’ diagrams.
Figure 3: Single-turn-coil results.
Figure 4: Transmission in high-temperature tetragonal phase.

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Acknowledgements

The authors thank: Meso-superstructured Hybrid Solar Cells—MESO NMP-2013-SMALL7-604032 project. H.J.S. thanks for financial support the Engineering and Physical Sciences Research Council (EPSRC), and the European Research Council (ERC-StG 2011 HYPER Project no. 279881). S. Stranks thanks Worcester College, Oxford, for additional financial support. P.P. thanks ANR JCJC project milliPICS, Region Midi-Pyrenee contract MESR 13053031. A. Miyata thanks the JSPS postdoctoral fellowships for research abroad for support. This work was supported by EuroMagNETII under the EU contract No. 228043.

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A.Miyata, A.Mitioglu, P.P., O.P. and R.J.N. collected and analysed the data. J.T-W.W. and S.D.S. prepared the samples. All authors contributed to the interpretation and the manuscript preparation. R.J.N. supervised and initiated the project.

Corresponding author

Correspondence to Robin J. Nicholas.

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The authors declare no competing financial interests.

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Miyata, A., Mitioglu, A., Plochocka, P. et al. Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites. Nature Phys 11, 582–587 (2015). https://doi.org/10.1038/nphys3357

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