Temperature-dependent optical spectra of single-crystal $\left({\mathrm{CH}}_3{\mathrm{NH}}_3\right){\mathrm{PbBr}}_3$ cleaved in ultrahigh vacuum

We measure temperature-dependent one-photon and two-photon induced photoluminescence from (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$ single crystals cleaved in ultrahigh vacuum. An approach is presented to extract absorption spectra from a comparison of both measurements. Cleaved crystals exhibit broad photoluminescence spectra. We identify the direct optical band gap of 2.31 eV. Below 200 K, the band gap increases with temperature, and it decreases at elevated temperature, as described by the Bose-Einstein model. An excitonic transition is found 22 meV below the band gap at temperatures $<200$ K. Defect emission occurs at photon energies $<2.16$ eV. In addition, we observe a transition at 2.25 eV (2.22 eV) in the orthorhombic (tetragonal and cubic) phase. Below 200 K, the associated exciton binding energy is also 22 meV, and the transition redshifts at higher temperature. The binding energy of the exciton related to the direct band gap, in contrast, decreases in the cubic phase. High-energy emission from free carriers is observed with higher intensity than reported in earlier studies. It disappears after exposing the crystals to air.

Figure 1

Conventional PL and TPI-PL from (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$. The different absorption lengths of 800 nm and 405 nm light provide a light source deep inside the crystal, and a reference spectrum from the surface-near region. Also indicated is the diffusion of photoexcited carriers.

Figure 2

PL spectra measured under different environmental conditions.

Figure 3

Intensity dependence of TPI-PL data. (a) shows spectra recorded on (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$ in its low-temperature orthorhombic phase, normalized to the second power of the incoming laser flux. Intensities in selected spectral regions, as well as the integral TPI-PL intensity at room temperature are given in (b). Red symbols give the intensity in the low-energy region. Green and blue symbols show the excitation density dependence of the main emission feature. Data for the room-temperature phase are given by black symbols.

Figure 4

(a) Photoluminescence spectra of single-crystal (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$ after excitation with 405-nm light (blue) and 800-nm light (red), as well as their ratio (green). (b) gives the extracted absorption coefficients and photoluminescence spectra corrected for reabsorption, together with fits to the absorption coefficients. Fit residua are given in (c). Emission and absorption features described in the text are labeled. Their positions are marked by ticks. For the absorption onset A0, the error is indicated in addition.

Figure 5

Diffusion-corrected PL spectra and fits to the data. The emission peaks $E_0$ and $E_1$ are indicated. The intensity in the high-energy part of the spectrum is scaled for reasons of clarity. The scaling factors increase significantly in the low-temperature phase.

Figure 6

Temperature dependence of the direct band gap (green symbols). Data for the lower-energy transition in the tetragonal and cubic phases are also shown (red), and shifted in energy by 95 meV for better visibility. The green line represents a fit using the Bose-Einstein model. Data for the low-temperature phase of (${\mathrm{CH}}_3{\mathrm{NH}}_3$)${\mathrm{PbBr}}_3$ [42], as well as of related ${\mathrm{CsPbBr}}_3$ [78], shifted by $-0.1$ eV, are given for comparison. [A]: Ref. [42], [B]: Ref. [78].

Figure 7

Results of fits to PL emission (blue: E0, E1, ${\mathrm{E}}_c$), TPI-PL data (red: T0, T1), and extracted absorption spectra (green: A0, A1, A2). (a) gives the extracted peak positions as discussed in the text and as labeled in Figs. 4 and 5. A schematic energy level diagram is given in (b). In (c), spacings between selected peaks are given by symbols. (c) also shows the carrier quasitemperature (open squares).