Cation-Dependent Hot Carrier Cooling in Halide Perovskite Nanocrystals

Abstract

Lead halide perovskites (LHPs) nanocrystals (NCs), owing to their outstanding photophysical properties, have recently emerged as a promising material not only for solar cells but also for lighting and display applications. The photophysical properties of these materials can be further improved by chemical engineering such as cation exchange. Hot carrier (HC) cooling, as one of the key photophysical processes in LHPs, can strongly influence performance of LHPs NCs based devices. Here, we study HC relaxation dynamics in LHP NCs with cesium (Cs), methylammonium (MA, CH3NH3 +), and formamidinium (FA, CH­(NH2)2 +) cations by using femtosecond transient absorption spectroscopy. The LHP NCs show excitation intensity and excitation energy-dependent HC cooling. We investigate the details of HC cooling in CsPbBr3, MAPbBr3, and FAPbBr3 at three different excitation energies with low excitation intensity. It takes longer time for the HCs at high energy to relax (cool) to the band edge, compared to the HCs generated by low excitation energy. At the same excitation energy (350 nm, 3.54 eV), all the three LHP NCs show fast HC relaxation (<0.4 ps) with the cooling time and rate in the following order: CsPbBr3 (0.39 ps, 2.9 meV/fs) > MAPbBr3 (0.27 ps, 4.6 meV/fs) > FAPbBr3 (0.21 ps, 5.8 meV/fs). The cation dependence can be explained by stronger interaction between the organic cations with the Pb–Br frameworks compared to the Cs. The revealed cation-dependent HC relaxation process is important for providing cation engineering strategies for developing high performance LHP devices.

Metal halide perovskite nanocrystals and their applications in optoelectronic devices

“…The higher the excitation energy, the longer the lifetime of hot carriers. In lead halide perovskites with Cs + , FA + , and MA + , the hot carrier cooling dynamic of Cs‐based perovskite NCs is the slowest compared to others, which show more dependence on carrier density . The much longer hot carrier lifetime of perovskites makes their extraction relatively easy, which helps to prevent their loss as thermal energy and increase the efficiency of solar cells prepared from these perovskites.…”

“…In lead halide perovskites with Cs + , FA + , and MA + , the hot carrier cooling dynamic of Cs-based perovskite NCs is the slowest compared to others, which show more dependence on carrier density. [78][79][80] The much longer hot carrier lifetime of perovskites makes their extraction relatively easy, which helps to prevent their loss as thermal energy and increase the efficiency of solar cells prepared from these perovskites. In addition, the study of microscopic mechanisms such as exciton dynamics and carrier trapping has become more and more intensive, which has provided a good guide for us to better understand the behavior of microscopic particles of perovskite NCs.…”

Charge Carrier Relaxation in Colloidal FAPbI3 Nanostructures Using Global Analysis

“…In more strongly confined perovskite nanostructures, the conventional approach extracting T c is invalid. Alternatively, researchers use the buildup of the bandedge bleach over time to follow the cooling dynamics [ 16 , 17 ]. However, single wavelength trace analysis can be strongly complicated by excitonic effects such as Stark effects or coupled optical transition [ 18 , 19 ].…”

“…For weakly confined MAPbBr 3 NCs, slightly slower carrier cooling has been observed than in the bulk counterpart when excited at 400 nm while not much difference could be observed when comparing the energy-loss rate [ 13 ]. In a subsequent work, the cooling times at the same excitation wavelength and low carrier density (~10 17 cm −3 ) were found to be slightly dependent on the composition of lead bromide perovskites NCs with different cations, Cs, MA and FA: 310, 235 and 180 fs, respectively [ 16 ]. At high carrier densities (~10 19 cm −3 ), the decay of T c with t presents an additional component on a time range order of magnitude higher: 5 ps for CsPbBr 3 and 3 ps for MAPbBr 3 and FAPbBr 3 NCs [ 16 ] and even 10–30 ps in MAPbBr 3 NCs [ 13 ].…”

Ultrafast dynamics in QD based photoelectrochemical cells

“…[1][2][3] Recent years have shown the potential of QDs from the new perovskite solar energy material for optoelectronics applications. [4][5][6][7][8] Such applications rely on separation of photo-generated electron-hole pairs for efficient photon to electron conversion, which is typically carried out through charge transfer from the QDs to electronically coupled electron (or hole) acceptors. Charge transfer in QD-acceptor systems have been extensively investigated by time-resolved spectroscopies.…”