Broadband transient absorption study of photoexcitations in lead halide perovskites: Towards a multiband picture

Ultrafast transient pump-probe measurements of thin ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ perovskite films over a wide spectral range from 350 to 800 nm reveal a family of photoinduced bleach (PB) and absorption (PA) features unequivocally pointing to the fundamentally multiband character of the underlying electronic structure. Excitation pump-energy dependent kinetics of three long-lived PB peaks at 1.65, 2.55, and 3.15 eV along with a broad PA band shows the involvement of band-edge thermalized carriers in all transitions and at least four, possibly more, electronic bands. The evolution of the transient signatures is described in terms of the redistribution of the conserved oscillator strength of the whole system. The multiband perspective opens up different directions for understanding and controlling photoexcitations in hybrid perovskites.

Figure 1

Transient spectra measured in the 350–800 nm probe range for different time delays between pump and probe beams. Visible and near-UV parts of the spectrum here are taken with different excitation energies and matched to each other at the PB2 peak for $\ge 1$ ps time delays to account for differences in photoexcitation levels. The colored bands are used to indicate the positions of various features: bleaches PB1 (green), PB2 (purple), PB3 (orange), and the broad long-lived PA (white) and the short-lived PA signals (blue) adjacent to PB bands. Also shown is the steady-state UV-vis linear absorption spectrum (black curve referring to the right axis).

Figure 2

Normalized kinetics at the PB peaks measured following excitation with different pumps (a) at longer time scales and (b) at shorter time scales. (a) clearly illustrates the similarity of the decay of all three PB peaks independently of the pump, in agreement with the band-edge depopulation of photoexcited carriers.

Figure 3

Schematics of the alternative band diagrams: (a) The three valence bands (VB1, VB2, VB3) giving rise to optically allowed transitions into the conduction band (CB1) and (b) the two valence bands (VB1, VB2) giving rise to transitions into the two conduction bands (CB1, CB2). The common long-time dynamics of the three PB features reflects the population of the photoexcited carriers relaxed to the fundamental band edge(s). The displayed approximate transition energies correspond to the PB wavelengths monitored in Fig. 2. Also shown is the renormalization of the band edges (CBR and VBR) due to photoinduced changes in the exchange corrections to electronic energies [18]. See text for more discussion.

Figure 4

(a) Dynamics of the visible PA measured at different spectral positions. For a visual comparison, the PB1 dynamics (sign reversed) is also shown. The inset showing the early-time dynamics reveals a similar short-lived nature of PA near PB1 and PB3 peaks. (b) Normalized transient spectra obtained with the 400 nm pump for the initial $n_{\mathrm{eh}}\sim 1.7\times {10}^{18}{\mathrm{cm}}^{-3}$ displaying the behavior of the broad PA band between PB1 and PB2 features. The arrows indicate the directions of the evolution of the spectrum. The inset illustrates computational results that would follow from the state-filling model with an account of the photorefractive index change $\mathrm{\Delta }n$.