Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression

Abstract

Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hotphonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2−3 times faster at 0.3 GPa than at ambient pressure. Our electron−phonon coupling calculations reveal ∼2-fold stronger electron−phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high chargecarrier densities.

Multi-Pulse Terahertz Spectroscopy: Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites

“…𝑑𝑛 ℎ𝑙 𝑑𝑡 population density from 400 to 650 fs, as was previously observed and attributed to the hot-phonon bottleneck effect. [21][22][23][25][26][27]43 The observed timescales of both cooling processes are in excellent agreement with Bretschneider et al 58 However, the fact that a ~450 fs local lattice cooling timescale kl is observed in our experiment where polaron formation precedes (and is therefore ruled out from) the observed dynamics may point toward an alternative interpretation of their results. bonds that strongly couple to the carriers.…”

“…[21][22][23] A growing number of works implicate polarons in this aspect of the hot carrier behavior too, owing to the enhanced spatial extent, strong coupling to phonons and sharing of phonon subpopulations inherent to overlapping polarons. [24][25][26][27] The interpretation of the transient optical signals that convey the aforementioned relaxation processes in MHP-based materials must be taken with care, especially when pumping far above the bandgap and at high fluences. These conditions can favor a myriad of processes that obfuscate the intraband relaxation, including, but by no means limited to: interband transitions between distinct electronic/spin states, [28][29][30] exciton formation/dissociation, 13,[31][32][33] energy/charge migration, [34][35][36] bandgap renormalization, 21,37 refractive index changes, 21,38 Stark shifting, 39,40 and many-body (Auger) processes.…”

“…The slowing of the carrier cooling dynamics at higher push fluences (hot carrier densities) can be understood through the framework of the hot-phonon bottleneck mechanism, [21][22][23] where the energy deposited into phonons through cooling is continually re-absorbed by overlapping polarons. [24][25][26][27] Generally, the low-energy THz probe ought to be sensitive to all carriers (cold and hot), regardless of their energy within the electronic band. The push is not capable of interband excitation and therefore does not change the overall concentration of the photoexcited carriers.…”