Chemically Engineered Avenues: Opportunities for Attaining Desired Carrier Cooling in Perovskites

Abstract

Hot carrier extraction‐based devices are presently being persuaded as the most revolutionary means of surpassing the theoretical thermodynamic conversion efficiency limit (∼67 % for a model hot carrier solar cell). However, for practical realisation, there stand various hurdles that need to be surmounted, a major among all being the rapid hot carrier cooling rate. Though, the perovskite family has already demonstrated itself to exhibit slower cooling in contrast to the prototypical semiconductors. Decelerating this entire process of cooling further can prove to be a crucial stride in this regard. Quite contrarily, for the optoelectronic applications the situation is entirely conflicting where quick rate of cooling is a chief prerequisite. In the recent times, there have been various key developments that have targeted altering this cooling rate by various chemically engineered strategies. This review highlights such blueprints that can be utilized towards the advantageous alteration of the carrier cooling in accordance with the device requirements.

Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

“…Also, the size and mass of the polaron would increase with temperature since the lattice is more capable of supporting large polarons at elevated temperatures. 23,46,75 This work highlighted the tremendous potential of facet engineering in modulating the HC cooling dynamics in perovskite NCs. Adding new facets on the NC surface decelerates the cooling rate, allowing easy extraction of hot carriers by a carefully chosen composite material.…”

“…46 We further rule out the effect of Coulomb screening in our case, which requires at least a modest fluence. 15,75 Also, the sizes of all the NCs (r-/d-/c-CsPbBr 3 ) are nearly the same (∼17 nm) and are greater than the Bohr diameter (∼7 nm). 23 Therefore, the variation in the cooling time scale cannot be a result of the hot phonon bottleneck, which is generally liable for decelerated cooling in tightly confined systems.…”

“…Several factors, including rapid polaron formation, Coulomb screening effect, surface passivation, hot phonon bottleneck, acoustic-optical phonon upconversion, Auger heating, etc., can be responsible for the slow carrier cooling in our NCs. ,,,− Auger effects and acoustic-optical phonon upconversion influence the cooling rate only at high fluence; however, we used a low fluence, producing only ∼0.08 exciton per pulse per NC, where most of the excitation-power-induced processes, including Auger, play an insignificant role . We further rule out the effect of Coulomb screening in our case, which requires at least a modest fluence. , Also, the sizes of all the NCs (r-/d-/c-CsPbBr 3 ) are nearly the same (∼17 nm) and are greater than the Bohr diameter (∼7 nm) . Therefore, the variation in the cooling time scale cannot be a result of the hot phonon bottleneck, which is generally liable for decelerated cooling in tightly confined systems.…”

“…The extent of this coupling is highly sensitive to lattice temperature. A polaron is basically the carrier dressed with LO phonon clouds all around and characterized by a higher effective mass than the bare charge carrier. ,, Polaron formation causes an overall slower cooling of the carriers owing to their heavier masses and bigger sizes, where Fröhlich interactions with the crystal lattice control their relaxation. Very recently, Ghosh and colleagues observed not only a faster polaron formation (∼0.25 ps) in d-CsPbBr 3 but also a greater size of the polaron (∼39.6 A) compared to polarons (∼0.55 ps and ∼34.7 A) in its cubic counterpart (c-CsPbBr 3 ) .…”

“…However, at an elevated temperature (∼50 °C), the rise time tends to slow to ∼1.8 ps, implying decelerated cooling stemming from fast polaron formation due to the stronger exciton–LO phonon coupling (Figure B, red curve). Also, the size and mass of the polaron would increase with temperature since the lattice is more capable of supporting large polarons at elevated temperatures. ,, …”