Extending the defect tolerance of halide perovskite nanocrystals to hot carrier cooling dynamics

Abstract

Defect tolerance is a critical enabling factor for efficient lead-halide perovskite materials, but the current understanding is primarily on band-edge (cold) carriers, with significant debate over whether hot carriers can also exhibit defect tolerance. Here, this important gap in the field is addressed by investigating how intentionally-introduced traps affect hot carrier relaxation in CsPbX₃ nanocrystals (X = Br, I, or mixture). Using femtosecond interband and intraband spectroscopy, along with energy-dependent photoluminescence measurements and kinetic modelling, it is found that hot carriers are not universally defect tolerant in CsPbX₃, but are strongly correlated to the defect tolerance of cold carriers, requiring shallow traps to be present (as in CsPbI₃). It is found that hot carriers are directly captured by traps, instead of going through an intermediate cold carrier, and deeper traps cause faster hot carrier cooling, reducing the effects of the hot phonon bottleneck and Auger reheating. This work provides important insights into how defects influence hot carriers, which will be important for designing materials for hot carrier solar cells, multiexciton generation, and optical gain media.

Fig. 1. Effect of intentionally introduced defect states on the optoelectronic properties of CsPbX₃ nanocrystals (X = I, Br, or I/Br mixture).

a Change in the normalized photoluminescence quantum yield (PLQY) of CsPbI₃ (red), CsPbBr_xI_3−x (orange) and CsPbBr₃ (blue) nanocrystals after successive purification steps. All PLQY values were normalized to the PLQY of the pristine samples. Illustrations inset show the changes to the surface defect and ligand density of the nanocrystals before and after each purification step. Comparison of the photoluminescence (PL) and absorption spectra of pristine (low defect density), singly purified (moderate defect density), and doubly purified (high defect density) b CsPbBr₃, c CsPbBr_xI_3−x and d CsPbI₃ nanocrystals. Excitation-wavelength-dependent PLQY for e CsPbBr₃, f CsPbBr_xI_3−x, and g CsPbI₃ nanocrystals. All PLQY values were normalized to the PLQY measured with the excitation laser energy that is closest to the bandgap of the materials. In each case, a comparison is made between nanocrystals with low (red) and high (blue) defect densities. The model fit to the excitation-dependent measurements was obtained from ref. . The PLQY and PL lifetime measurements here, along with the XPS measurements (Supplementary Figs. 2 and 3), suggest that we have successfully prepared NCs with different defect densities.

Fig. 2. Evidence of carrier trapping from pump-probe transient absorption spectroscopy.

a, b Transient absorption spectra of singly-purified (low defect density) and doubly-purified CsPbBr₃ NCs (high defect density). Red and blue regions indicate the sub-bandgap regions (containing both trap bleach (TB) and photo-induced absorption (PIA); 535–545 nm) that were integrated to determine the kinetics. The kinetics of sub-bandgap regions for singly (low defect density)- and double-purified (high defect density) c CsPbBr₃ NCs (probed at 535–545 nm), d CsPbBr_xI_3-x NCs (probed at 635–645 nm), and e CsPbI₃ NCs (probed at 715–725 nm). The TA measurements were performed at 116.6 μJ cm⁻², and the spectra are shown in Supplementary Figs. 8–10. TB was extracted from the measured spectra through singular-variable decomposition (SVD), shown in Supplementary Fig. 7. f Schematic representation of the charge-carrier relaxation processes highlighting the carrier trapping events, with energy levels indicated here arbitrarily. The NC solution was pumped with photon energies significantly higher than the bandgap (ħω_pump = 3.1 eV).

Fig. 3. Hot carrier energy loss rate based on pump-probe transient absorption spectroscopy measurements.

TA maps for colloidal solutions of low, moderate and high defect density a–c, CsPbBr₃, d–f, CsPbBr_xI_3−x and g–i CsPbI₃ perovskite nanocrystals under 400 nm wavelength excitation. Energy loss rate for different defect concentrations in j CsPbBr₃, k CsPbBr_xI_3−x, and l CsPbI₃ NCs. A vertical dotted line is placed at 1000 K carrier temperature (T_c) to visually aid the direct comparison between pristine (low defect density, red), singly purified (moderate defect density, yellow) and doubly purified NCs (high defect density, blue). The energy loss rate is based on the relaxation lifetime at 194.3 μJ cm⁻² for CsPbBr₃ (carrier density of 12.5 × 10¹⁷ cm⁻³), 178.2 μJ cm⁻² for CsPbBr_xI_3−x (carrier density of 43.5×10¹⁷cm⁻³) and 193.4 μJ cm⁻² for CsPbI₃ (carrier density of 16.6×10¹⁷cm⁻³).

Fig. 4. Hot carrier cooling kinetics based on pump-probe transient absorption spectroscopy.

HC lifetime (τ_cool) obtained by fitting pump-probe transient absorption spectroscopy measurements of a CsPbBr₃, b CsPbBr_xI_3−x, and c CsPbI₃ NCs with low (red), yellow (moderate) and high (blue) defect densities. d Hot carrier lifetime against defect density of CsPbI₃ (red), CsPbBr_xI_3−x (yellow) and CsPbBr₃ (blue) perovskite NCs. The normalized τ_cool for CsPbBr₃ is based on the relaxation lifetime at 194.3 μJ cm⁻² fluence (carrier density of 12.5 × 10¹⁷ cm⁻³), for CsPbBr_xI_3−x at 178.2 μJ cm⁻² fluence (carrier density of 43.5 × 10¹⁷ cm⁻³), and for CsPbI₃ is at 193.4 μJ cm⁻² fluence (carrier density of 16.6 × 10¹⁷ cm⁻³). Error bars represent uncertainty in numerical fitting of the cooling lifetime at each carrier density.

Fig. 5. Hot carrier cooling kinetics based on pump-push-probe transient absorption spectroscopy.

a Schematic diagram of the pump-push-probe TA setup used. b Exemplar comparison of the pump-probe (PP, dark red) and pump-push-probe (PPP, blue) GSB decay kinetics with a pump-push delay of 10 ps for low defect density CsPbBr₃ NC solution. c Hot carrier density (or push-fluence) dependent representative GSB decay curves. HC lifetimes obtained from fitting the PPP TA measurements for d CsPbBr₃, e CsPbBr_xI_3−x, and f CsPbI₃ NCs, respectively, with low (red), moderate (yellow) and high (blue) defect densities. Dotted lines are results from the kinetic model described in final sub-section of Results. Error bars represent uncertainty in numerical fitting of the cooling lifetime at each carrier density.