Hot carriers perspective on the nature of traps in perovskites

Abstract

Amongst the many spectacular properties of hybrid lead halide perovskites, their defect tolerance is regarded as the key enabler for a spectrum of high-performance optoelectronic devices that propel perovskites to prominence. However, the plateauing efficiency enhancement of perovskite devices calls into question the extent of this defect tolerance in perovskite systems; an opportunity for perovskite nanocrystals to fill. Through optical spectroscopy and phenomenological modeling based on the Marcus theory of charge transfer, we uncover the detrimental effect of hot carriers trapping in methylammonium lead iodide and bromide nanocrystals. Higher excess energies induce faster carrier trapping rates, ascribed to interactions with shallow traps and ligands, turning these into potent defects. Passivating these traps with the introduction of phosphine oxide ligands can help mitigate hot carrier trapping. Importantly, our findings extend beyond photovoltaics and are relevant for low threshold lasers, light-emitting devices and multi-exciton generation devices.

Fig. 1. Steady-state signatures of hot carrier losses.

Excitation energy-dependent photoluminescence quantum yield spectra for (a) MAPbI₃ and (b) MAPbBr₃ NCs, reported as a function of excess excitation energy (δ_E) with respect to the bandgap obtained from the fitting the absorption spectra with the Elliott formula (Table 1). Also known as ‘photo-action spectra’. Red squares and green circles represent MAPbI₃ and MAPbBr₃ NCs PLQY values, respectively. Lines are guides for the eye. Error bars represent the standard deviation, estimated through repeated measurements. Excitation energy ranges for potential applications of MAPbI₃ NCs are included below Figure a. c Absorption (solid line) and photoluminescence (dashed line) of MAPbBr₃ and MAPbI₃ NCs in anhydrous toluene solutions. d Fitting of the MAPbBr₃ NCs absorption spectrum with the Elliott formula, deconvolving the contributions due to free carrier absorption continuum (beige region), Rayleigh scattering (green dashes), and excitonic absorption (bright green region).

Fig. 2. Time-Resolved Signatures of Hot Carrier-induced Trapping.

a Transient Absorption spectrum of MAPbI₃ NCs in anhydrous toluene solutions under intense (<N > = 3.6 e–h) high-energy excitation of 3.10 eV (around 1.4 eV in excess of the bandgap). Under this intense excitation, biexponential cooling dynamics is observed and reveals the coexistence of Auger and phonon-mediated relaxation pathways. Inset: Corresponding hot carrier temperatures (black squares), estimated using the Boltzmann model. Multi-exponential fit is represented by the brown line. b Pump-push-probe differential spectrum of MAPbI₃ NCs in anhydrous toluene solutions, pumped at 2.07 eV (10 µJ cm⁻², <N> = 0.88), probed over the 1.60–1.90 eV interval, and pushed at 1.03 eV (1 mJ cm⁻²). The black dashed line represents the push time-zero, the positive ΔΔT signal (blue) implies a decrease in the PB signal, and a negative ΔΔT signal (red) an increase in the PB signal. c Pump-probe (PP) and pump-push-probe (PPP) kinetics pumped at 1.91 eV (10 µJ cm⁻², <N> = 0.8) and probed at 1.70 eV, with push energy of 1.03 eV (1 mJ cm⁻²). Thin lines represent the experimental data, and thick lines are exponential model fits to the data.

Fig. 3. Phenomenological model of hot carrier-induced trapping.

a Model schematic describing the interaction between carriers and shallow traps in PNCs. Free and trapped carriers are involved in a thermal equilibrium, whose barrier is dictated by the combination of reorganization energy, λ, and trapping free energy –ΔG₀. b The high-excess energy (δ_E) for the carriers increases the trapping rate and hence results in a higher number of trapped carriers, i.e., lower PLQY. c Vice versa, lower δ_E results in lower trapping rate and thus results in a lower number of trapped carriers, i.e., higher PLQY. HC and CC indicate the hot carriers and cold carriers, respectively. Model fitting of the photo-action spectra, for (d) MAPbI₃ NCs (e) and MAPbBr₃ NCs. The light-colored dots represent TOPO ligand-exchanged PNCs, while dark dots represent pristine PNCs. Error bars represent the standard deviation, estimated through repeated measurements. The corresponding colored lines represent the model fitting.