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

Journal name:
Nature Physics
Volume:
11,
Pages:
582–587
Year published:
DOI:
doi:10.1038/nphys3357
Received
Accepted
Published online

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.

At a glance

Figures

  1. Magnetic field dependence of the optical density for the perovskite CH3NH3PbI3.
    Figure 1: Magnetic field dependence of the optical density for the perovskite CH3NH3PbI3.

    a, A sequence of optical density (Log(1/transmission)) spectra measured during a single pulse of the magnetic field. For easier comparision the spectra are offset. The arrows show the energies for the Landau level absorptions at 65 T. b,c, Sequences of ratios of the transmission in magnetic field T(B) to that measured at zero field T(0), which improves the resolution of smaller field-dependent features and where the resonant absorption features correspond to minima. b shows the 2s absorption visible at lower fields and c shows the Landau levels seen at higher fields. The dashed arrows show the magnetic field evolution of the excitonic (b) and Landau level (c) absorptions as plotted in Fig. 2. The feature highlighted at 2.03 eV in c is a band-edge absorption from a previously undetected higher-energy band edge and will be the subject of a future publication.

  2. Energy /`fan[rsquor] diagrams.
    Figure 2: Energy ‘fan diagrams.

    a, Full fan using data from long-pulse fixed-field spectra (black circles) and fixed-energy fast-field-sweep data (red stars). The calculated transition energies are shown for the free-electron and hole levels (solid lines) and the excitonic transitions (dashed lines). Inset to a, lower fields measured using fixed-field spectra. b, A schematic of the energy levels and transitions between the free-electron and hole levels (solid lines) and the excitonic transitions (dashed lines).

  3. Single-turn-coil results.
    Figure 3: Single-turn-coil results.

    Plots of the magneto-transmission measured using a single-turn fast-pulse magnetic field, with a schematic of the experimental system. For better comparison spectra are offset. a, The temperature dependence, with the arrows indicating the position of the N = 1 inter-Landau level transition. The inset shows a schematic of the single-turn experimental system showing the coil and its firing circuit, the sample and the optical fibre illumination and collection system. b, The transmission spectra in the tetragonal (intermediate temperature) phase with spectra measured for different wavelengths in the temperature range 155–190 K. The linked arrows show the positions of the N = 1 and N = 2 transitions.

  4. Transmission in high-temperature tetragonal phase.
    Figure 4: Transmission in high-temperature tetragonal phase.

    a, Optical density, with the dashed line showing the anomalous behaviour of the 1s exciton transition. The arrows show the positions of the excitonic and Landau level absorptions as plotted in b. b, Fan diagram using data from long-pulse fixed-field spectra (black circles) and fixed-energy fast-field-sweep data (red stars). The calculated transition energies are shown for the free-electron and hole levels (solid lines) and the excitonic transitions (dashed lines). c, Temperature-dependent bandgaps and resonance positions for the N = 1 inter-Landau level transition.

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Author information

  1. These authors contributed equally to this work.

    • Atsuhiko Miyata &
    • Anatolie Mitioglu

Affiliations

  1. Laboratoire National des Champs Magnetiques Intenses, CNRS-UJF-UPS-INSA, 143 Avenue de Rangueil 31400 Toulouse, France

    • Atsuhiko Miyata,
    • Anatolie Mitioglu,
    • Paulina Plochocka &
    • Oliver Portugall
  2. University of Oxford, Clarendon Laboratory, Parks Road Oxford OX1 3PU, UK

    • Jacob Tse-Wei Wang,
    • Samuel D. Stranks,
    • Henry J. Snaith &
    • Robin J. Nicholas

Contributions

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.

Competing financial interests

The authors declare no competing financial interests.

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