Research ArticleMATERIALS SCIENCE

Deciphering the effect of traps on electronic charge transport properties of methylammonium lead tribromide perovskite

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Science Advances  11 Sep 2020:
Vol. 6, no. 37, eabb6393
DOI: 10.1126/sciadv.abb6393
  • Fig. 1 ToF spectroscopy and MC simulation.

    (A) A schematic of physical principle of ToF method. The light pulse generates free carriers (holes) near the electrode (anode). The photogenerated free holes further drift toward the cathode by an electric pulse bias. The drifting holes interact with traps, and their interaction affects the current transient. (B) ToF experimental setup. (C) Top: Simulation of the holes drifting from anode to cathode in the bulk of a semiconductor characterized by a deep trap. a.u., arbitrary units. The time t = 0 represents the initial hole distribution immediately after illumination. At t = tk, the evolving carrier cloud reaches in the vicinity of the collecting electrode at the right side. Trapped holes (green crossed circles) remain localized in the traps. Bottom: This graph quantitatively shows the spatial distribution of trapped holes and evolution of free holes cloud-drifting between the two electrodes. (D) Normalized CWFs induced by drifting holes. The single exponential decay of CWF assigns the presence of a deep trap in the material. The current was normalized by the corresponding applied bias to show the influence of deep traps on the CWF shape at different biases. (E) Top: The visualization of free holes drifting in the semiconductor with one shallow trap and a high number of trapping-detrapping events. Free carriers drift toward the collecting electrode. The shallow traps delay the fraction of free carriers demonstrated by cyan circles. Bottom: Spatial distribution of never trapped and the delayed free holes drifting between the two electrodes. (F) CWF induced by drifting holes. The tails at the beginning and the end of CWF are affected by the shallow trap.

  • Fig. 2 Effect of traps on the dynamics of free holes depicted by ToF and MC simulation.

    (A, C, and E) ToF CWF measured at 100 V. The red curve shows the best MC simulation fit with three traps at different times t1 = 6 μs, t2 = 14 μs, and t3 = 23 μs, respectively. (B, D, and F) Top: The time evolution of the free charge cloud (subdivided according to their trapping-detrapping history) during the drift process at t1, t2, and t3, respectively. Bottom: The respective normalized concentrations of never trapped, trapped, and delayed holes in the material. The attached video (movie S1) shows the evolution of the charge cloud computed by the MC simulation with a 0.1-μs time resolution, which allows the direct visualization of the free charge movement in a MAPbBr3 device. (G) The ToF CWF and MC simulation at 80-V bias. MC simulation represents four models fitted according to the least square regression analysis. The inset demonstrates the residuals of fitting curves shown in (G). (H) Charge transport model in MAPbBr3. Upward and downward arrows illustrate energy transitions (trapping and detrapping). Table 1 summarizes the parameters of these transitions estimated by ToF measurements and MC simulations.

  • Fig. 3 ToF and MC validation of the three-trap model.

    (A) Bias dependence of CWFs. The green curves represent the simulated MC fit. The inset shows electric field profiles between the two electrodes. (B) Normalized CWFs at different biases. (C) CWFs dependence on normalized time according to Eq. 2.

  • Fig. 4 Spatial and temporal distribution of free and trapped charges in MAPbBr3.

    The evolution of free holes and the occupation of trap states in a MAPbBr3 device at electric biases of 100 V (A and B), 20 V (C and D), and 1 V (E and F). (A), (C), and (E) represent the temporal evolution of free carriers and traps occupation. Flag indicators separate the regions (i) to (iv) with different relaxation dynamics. (B), (D), and (F) show the simulated visualization of the spatial distribution of traps (top), the distribution of occupied trap density (middle), and the profile of the charge cloud in MAPbBr3 at different biases (bottom). The attached videos (movies S2 to S4) show the simulated evolution of the charge cloud and trap occupation with 0.1-μs time resolution, allowing direct visualization of the interplay between free holes and a particular trap.

  • Fig. 5 Effect of traps on the effective mobility, drift mobility evaluation, and mobility-lifetime product.

    (A) The effective hole mobility as a function of the electric field, as determined by the MC simulation for various thicknesses (L) of the material. The effective hole mobilities measured by the ToF for L = 0.2 cm are shown by the stars. (B) The temperature dependence of the effective mobility obtained by theoretical modeling. The observed behavior suggests that traps affect μeff(T) dependence in MAPbBr3 single crystals especially at T < 250 K. Two magenta dots show the experimental drift and effective mobilities [measured at room temperature (RT) under steady-state conditions]. (C) Hole drift mobility as a function of applied electric field obtained by different approaches using data from Fig. 3A and MC simulations. The inset demonstrates the linear fit of the transit time versus electric field. (D) Mobility-lifetime product as a function of collection time. The inset in (D) shows the Hecht fit according to data from Fig. 3A.

  • Table 1 Charge transport parameters of single-crystal MAPbBr3 found from the combination of ToF and MC simulation.

    TrapTrapping time (μs)Detrapping time (μs)Trapping/detrapping ratioσh × nt product*
    10−3 cm−1
    E12337.71.31
    E224141.71.25
    E3901200.750.34

    *Hole capture cross section and trap density product.

    Supplementary Materials

    • Supplementary Materials

      Deciphering the effect of traps on electronic charge transport properties of methylammonium lead tribromide perovskite

      Artem Musiienko, Jindřich Pipek, Petr Praus, Mykola Brynza, Eduard Belas, Bogdan Dryzhakov, Mao-Hua Du, Mahshid Ahmadi, Roman Grill

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      The PDF file includes:

      • Sections S1 to S3
      • Table S1
      • Figs. S1 to S3

      Other Supplementary Material for this manuscript includes the following:

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