Hot Carrier Dynamics in Perovskite Nanocrystal Solids: Role of the Cold Carriers, Nanoconfinement, and the Surface

Abstract

Carrier cooling is of widespread interest in the field of semiconductor science. It is linked to carrier-carrier and carrier-phonon coupling, and has profound implications for the photovoltaic performance of materials. Recent transient optical studies have shown that a high carrier density in lead-halide perovskites (LHPs) can reduce the cooling rate through a "phonon bottleneck".However, the role of carrier-carrier interactions, and the material properties that control cooling in LHPs, are still disputed. To address these factors, we utilize ultrafast "pump-push-probe" spectroscopy on LHP nanocrystal (NC) films. We find that the addition of cold carriers to LHP NCs increases the cooling rate, competing with the phonon bottleneck. By comparing different NCs and bulk samples, we deduce that the cooling behavior is intrinsic to the LHP composition, and independent of the NC size or surface. This can be contrasted with other colloidal nanomaterials, where confinement and trapping considerably influence the cooling dynamics.

Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites

“…These dynamics agree with the literature [6][7][8][9][10][11][12] and our own previous findings which invoke the hot-phonon bottleneck effect. 16,19 Interestingly, we also find that t relax tends to decrease with increasing n exc , suggesting that the hot-phonon bottleneck plays a lesser role in intraband relaxation when there are many cold carriers in the system. From this, we can deduce that the carrier relaxation occurs via at least two competing pathways.…”

“…Here, we propose a practical kinetic model to describe the effects of the hot and cold carrier populations on the carrier relaxation kinetics in CsPbBr 3 bulk and nanocrystal (NC) films, and apply this model to explain the results of ''pump-push-probe'' measurements. 19 Our results convey two competing pathways for the dissipation of the excess hot carrier energy into either optical phonons or cold carriers. By applying simple approximations, we reduce this model into a single intuitive equation containing terms for the carrier-carrier and carrier-phonon interactions.…”

“…A more lengthy discussion of the material properties of the two samples is given elsewhere. 19 Taking together the weak confinement and ordered packing of the NCs, we anticipate that carriers can move within and between NCs. The high mobility of carriers in films of similarly sized CsPbBr 3 NCs was recently demonstrated by optical pump-terahertz probe experiments.…”

“…24 To investigate the intraband relaxation dynamics in the samples, we used a ''visible pump-IR push-IR probe'' pulse sequence, which has been outlined in previous works. 16,19,25,26 and recently adapted to study hot carrier extraction in LHPs. 27 As depicted in Fig.…”

“…At higher cold carrier density, relaxation via carrier-carrier interactions starts to dominate, resulting in the overall faster relaxation and the reduced impact of the hot-phonon bottleneck. 19…”