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Hot carrier extraction in CH3NH3PbI3 unveiled by pump-push-probe spectroscopy

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Science Advances  15 Nov 2019:
Vol. 5, no. 11, eaax3620
DOI: 10.1126/sciadv.aax3620
  • Fig. 1 PPP spectroscopy on MAPI.

    (A) Representative pump-probe TA spectra of MAPI at different pump-probe delays using a 2.07-eV (600 nm) pump. Inset: A schematic of our three-pulse PPP experiment. a.u., arbitrary units. (B) The influence of push pulses of varying photon energies on the thermalized carriers. Injection of HCs to bphen occurs if the push could overcome the band offset and the interfacial barrier of bphen. IES, initial excited state; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital. (C) PPP ΔT transients of MAPI without bphen (neat film, black and orange solid symbols for push off and on, respectively) probed at PB1 [the 1.63-eV ground-state bleaching (GSB) and stimulated emission (SE)]. The 2.07 eV push pulse (15 μJ cm−2) arrives 3 ps after the 2.07-eV pump (5 μJ cm−2). The solid lines are multiexponential fits of the pump-probe and PPP kinetics. (D) The corresponding ΔΔT push-induced signal and fit transients are obtained by taking the difference of the ΔT transients [push off–push on in (C)]. The time window is cropped for clarity to focus on the effect of the push pulse.

  • Fig. 2 Push-induced transients in MAPI.

    PPP ΔΔT push-induced signals of MAPI without (orange solid symbols) and with bphen (blue open symbols) quenching layer probed at PB1 obtained by taking the push-on and push-off difference from the ΔT transients. The push pulse arrives 3 ps after the 2.07-eV pump (5 μJ cm−2), with varying energies to probe different excited states; (A) 2.07 eV (15 μJ cm−2) and (B) 1.03 eV (650 μJ cm−2). Solid lines are exponential fits of the pump-probe ΔT signals, and the raw data are depicted by symbols. For clarity, the transients of MAPI with bphen are scaled by a factor of 1.18 in (A) and 0.83 in (B).

  • Fig. 3 Determining the interface barrier in MAPI/bphen.

    (A) Push photon energy–dependent ΔΔT measurements at finer intervals, 2.07 eV (blue circles, 15 μJ cm−2), 1.98 eV (dark purple upright triangles, 10 μJ cm−2), 1.91 eV (purple inverted triangles, 10 μJ cm−2), 1.77 eV (maroon squares, 10 μJ cm−2), and 1.03 eV (pink right triangles, 650 μJ cm−2) reveal that, at minimum, a 1.98-eV push photon is required to overcome the barrier between MAPI and bphen. The transients are scaled according to the factors denoted in the graph and offset for clear comparison. (B) Transient transmission dynamics of MAPI pumped at 3.10 eV (400 nm, 5 μJ cm−2) and probed at 1.63 eV (760 nm) reveals no quenching of the lifetime. This indicates that the photoexcited carriers are not able to be extracted by bphen. Slight lengthening of the lifetime comes from surface trap passivation by the bphen layer.

  • Fig. 4 Schematic comparison of the carrier population under probe.

    In TA spectroscopy, at thermalized carrier temperatures of Tc ~ 1000 K, i.e., ~ 86 meV above the band edge, only a fraction (37%) exists. This contrasts with PPP spectroscopy, where a fraction of the excited carrier population is re-excited above the LUMO level of the HC acceptor, and the excited state lifetime is directly probed. Here, we assume a pump wavelength of 400 nm and an excited carrier density of 1018 cm−3.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaax3620/DC1

    Section S1. Interpretation of PPP signal

    Section S2. Charge extraction to bphen

    Section S3. PIA modeling based on refractive index change

    Fig. S1. Pump fluence–dependent measurements.

    Fig. S2. Push-induced photophysical processes.

    Fig. S3. Push-induced transient transmission dynamics of MAPI.

    Fig. S4. Push-induced transients in MAPI.

    Fig. S5. Comparison of fitting models.

    Fig. S6. Complex refractive index of MAPI.

    Table S1. F test parameter report.

    Table S2. F test analysis.

    Table S3. List of parameters.

    References (3235)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Interpretation of PPP signal
    • Section S2. Charge extraction to bphen
    • Section S3. PIA modeling based on refractive index change
    • Fig. S1. Pump fluence–dependent measurements.
    • Fig. S2. Push-induced photophysical processes.
    • Fig. S3. Push-induced transient transmission dynamics of MAPI.
    • Fig. S4. Push-induced transients in MAPI.
    • Fig. S5. Comparison of fitting models.
    • Fig. S6. Complex refractive index of MAPI.
    • Table S1. F test parameter report.
    • Table S2. F test analysis.
    • Table S3. List of parameters.
    • References (3235)

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