Phonon-driven transient bandgap renormalization in perovskite single crystals

Abstract

Tailoring the electronic structure of a perovskite material on ultrafast timescales is expected to shed light on optimizing optoelectronic applications. However, the transient bandgap renormalization upon photoexcitation is commonly explained...

Band Gap Renormalization at Different Symmetry Points in Perovskites

“…We used the visible spectral region, which covers the VB1 → CB1 transition at the R point (fundamental BG transition), as previously studied. ,, Figure a shows the time-energy TA map in a 0–5 ps time window of the MAPbBr 3 perovskite excited at 3.10 eV, and Figure b shows the corresponding spectral traces at various delay times, revealing distinct negative and positive features from low to high energies along with a small positive shoulder on the lower-energy side at 0.2 and 0.4 ps. These figures display the characteristic bleach at the optical BG transition at ∼2.35 eV (see Figure b), mainly due to phase-space filling, accompanied by a broad, weak absorption signal on the higher-energy side, due to Coulomb screening. ,, These features are consistent with previous reports on similar perovskite systems. ,,, In addition, a small positive signal promptly emerges on the lower-energy side of the BG (indicated by the dashed red circle in Figure a), which we attribute to BGR caused by the presence of hot carriers on time scales of tens to hundreds of femtoseconds .…”

“…We used the visible spectral region, which covers the VB1 → CB1 transition at the R point (fundamental BG transition), as previously studied. 7 , 32 , 30 Figure 2 a shows the time-energy TA map in a 0–5 ps time window of the MAPbBr 3 perovskite excited at 3.10 eV, and Figure 2 b shows the corresponding spectral traces at various delay times, revealing distinct negative and positive features from low to high energies along with a small positive shoulder on the lower-energy side at 0.2 and 0.4 ps. These figures display the characteristic bleach at the optical BG transition at ∼2.35 eV (see Figure 1 b), mainly due to phase-space filling, 2 accompanied by a broad, weak absorption signal on the higher-energy side, due to Coulomb screening.…”

“…The choice of MAPbBr 3 is motivated by its potential applications in solar energy research, as detectors for high-energy radiation and for optoelectronic devices. , It is and has been richly investigated, especially at the fundamental BG in the visible spectral range, and BGR is also well documented. ,,− Its absorption spectrum shows modulations above the fundamental band gap (VB1 → CB1, ∼ 2.3 eV labeled 1), which correspond to at least three edges representing transitions from the upper VB at the M and X high-symmetry points to the lowest CB (labeled 3 and 4: VB1 → CB1, ∼3.8 eV and VB1 → CB1, ∼4.5 eV), or from a VB sub-band to the lowest CB at the R point (labeled 2: VB3 → CB1 at ∼3.4 eV), based on the BS diagram. , Here, we excite the material with a low pump fluence pulse at 3.1 eV, i.e., well above the bang gap at 2.3 eV, which implies that we generate free carriers. These will induce a BGR that survives as long as free carriers decay to form neutral excitons.…”

Small to Large Polaron Behavior Induced by Controlled Interactions in Perovskite Quantum Dot Solids

“…The asymmetric PB line shape arises from phase-space filling of hot polarons thermalized via carrier–carrier scattering (∼100 fs) . The low-energy PIA band has been attributed to bandgap renormalization (BGR). , By 1 ps delay, the hot polarons dissipate excess energy to the lattice and become thermally equilibrated with it. − During this cooling process, the PB band sharpens and gains intensity, while the BGR signature vanishes . Beyond 1 ps, an additional PIA is evident at probe energies above 1.75 eV.…”