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Solar cells based on the organic-inorganic perovskite family of materials have made a dramatic impact on emerging photovoltatic (PV) research with efficiencies of around 20% [1], which offers a new route to low-cost solar energy devices with simple fabrication processes. However, the fundamental electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are poorly known. We have measured both properties for methyl ammonium lead tri-iodide (CH3NH3PbI3) using magneto absorption in very high magnetic fields up to 150 T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously assumed [2]. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously assumed [2], but in good agreement with recent calculations [3]. We also observe Landau levels in the room temperature phase (350 K > T > 140 K) in which PV devices actually perform, showing an evidence that the binding energy falls to a few milli-electron volts in the room temperature. This result indicates the performance of PV devices using this material is attributed to the spontaneous generation of free carriers following photo-absorption.

[1] NREL Best research cell efficiencies: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
[2] K. Tanaka et al., Solid State Commun, 127, 619 (2003).
[3] E. Menendez-Proupin et al., Phys. Rev. B, 90, 045207 (2014).

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