Hot carriers perspective on the nature of traps in perovskites

Hybrid lead halide perovskites (LHPs) achieve exceptional optoelectronic device performance despite high densities of defects, commonly interpreted as defect tolerance due to predominantly shallow traps. With device efficiencies plateauing, the authors re-examine the role of shallow traps, particularly under high-energy (non-resonant) excitation that creates hot carriers, conditions relevant to LEDs, lasers, and photovoltaics. Conflicting reports exist on whether hot carriers couple to traps and reduce photoluminescence quantum yield (PLQY). Perovskite nanocrystals (PNCs) offer a platform to probe these effects due to their high PLQY, surface-dominated defect landscape, and tunable passivation. The central research question is whether shallow traps, typically benign for cold carriers, become active and detrimental under hot-carrier excitation, thereby limiting conversion efficiencies, and whether surface passivation can mitigate such losses.

Prior studies have largely focused on deep traps (e.g., lead vacancies) as non-radiative centers, proposing mitigation via materials design and synthesis. Shallow traps have been considered less impactful due to high detrapping probability, underpinning the notion of defect tolerance. Recent literature shows conflicting observations on hot carrier–trap coupling and energy-dependent PLQY. In II–VI quantum dots (e.g., CdSe), shallow trap dynamics have been modeled using Marcus/Marcus–Jortner charge transfer frameworks, with trapping–detrapping equilibria limiting PLQY. Similar shallow-trap dynamics have been implicated in perovskite nanostructures for cold carriers. The authors position their work to resolve the role of shallow traps under hot-carrier conditions in PNCs and relate findings to established trap models.

• Materials and samples: Colloidal MAPbX3 (X = I, Br) nanocrystals synthesized via modified ligand-assisted reprecipitation (LARP). MAPbBr3: PbBr2 and MABr in DMF rapidly injected into toluene with oleylamine, n-octylamine, and oleic acid, followed by centrifugation steps. MAPbI3: PbI2 and MAI in acetonitrile with oleylamine and oleic acid, slowly added to toluene and purified. Suspensions dispersed in anhydrous toluene. Post-synthetic ligand exchange with trioctylphosphine oxide (TOPO) used to passivate surface traps.
• Structural and linear optical characterization: XRD confirmed cubic Pm3m phase. TEM assessed size distributions (MAPbBr3 radius 4.2 ± 1.3 nm; MAPbI3 radius 5.5 ± 1.5 nm). Absorption and PL spectra acquired; absorption deconvolved using Elliott formula to separate excitonic absorption, free-carrier continuum, and Rayleigh scattering. Extracted bandgaps and exciton binding energies: MAPbI3 Eg ≈ 1.73 eV, Eb ≈ 20 meV; MAPbBr3 Eg ≈ 2.41 eV, Eb ≈ 36 meV.
• Steady-state photophysics: Photoluminescence quantum yield (PLQY) measured with an integrating sphere under varied excitation energies to construct photo-action spectra (PLQY vs excess excitation energy δe above bandgap), at low excitation intensities to probe trapping losses.
• Ultrafast spectroscopy: Femtosecond transient absorption (pump–probe, PP) to monitor hot-carrier cooling dynamics and band-edge bleach kinetics; pump energies varied (including 3.10 eV, 400 nm; OPA-generated tunable pumps). Pump–push–probe (PPP) used to transiently reheat carriers after initial relaxation, with IR push (1.03 eV) or visible push (2.07 eV), to reveal hot-carrier-induced trapping. Low pump fluences were used in PPP to avoid Auger-induced complications; push–ground-state absorption contributions assessed and minimized for IR-push.
• Modeling: Phenomenological model built on Marcus charge transfer theory for trapping between free-carrier and shallow-trap manifolds. Population dynamics for free carriers (N) and shallow-trapped carriers (NT) include radiative recombination (bimolecular k2 for MAPbI3 or monomolecular k1 for more excitonic MAPbBr3), and energy-dependent trapping/detrapping rates kr(e) governed by free-energy driving force −(ΔG0 + δe) and reorganization energy λ, with hot-carrier temperature decay T(t) = 300 K + T0 exp(−t/τ). PLQY computed by integrating radiative emission over time. Model parameters include electronic coupling H and reorganization energy A (λ), fitted to reproduce photo-action spectra for pristine and TOPO-passivated samples.

• Strong excitation-energy dependence of PLQY (photo-action spectra):
  • MAPbI3 NCs: PLQY decreases from ~25% at δe ≈ 0.35 eV to ~18% at δe ≈ 2.05 eV (~30% relative loss).
  • MAPbBr3 NCs: PLQY decreases from ~76% at δe ≈ 0.45 eV to ~53% at δe ≈ 1.25 eV.
• Hot-carrier dynamics (PP and PPP):
  • PP hot-carrier cooling in MAPbI3: carrier–phonon cooling τcooling ≈ 500 ± 100 fs (low fluence); at high fluence, Auger re-heating slows cooling (ps scale).
  • PPP with IR push (1.03 eV) on MAPbI3: immediate depletion of band-edge bleach followed by full recovery with thermalization τthermal ≈ 450 ± 100 fs. Post-push recombination accelerates with an added component τ ≈ 110 ± 10 ps, indicative of enhanced monomolecular trapping, not Auger (Auger contribution quantified to be minor for IR push).
  • Visible push (2.07 eV) can cause sudden carrier loss and overlaps with ground-state absorption, making disentanglement from Auger challenging; analysis shows Auger dominates post-push recombination under visible push.
  • Excitation-energy-resolved PP: increasing δe from 0.4 to 0.8 eV shortens bleach lifetime from 3.2 ± 0.6 ns to 1.9 ± 0.4 ns; further increase to 1.4 eV shows saturation of lifetime reduction.
  • MAPbBr3 PPP shows no additional fast recombination component; derivative-like differential PPP suggests excitonic nature limits direct observation, though photo-action spectra confirm hot-carrier-induced losses.
• Modeling outcomes:
  • Hot carriers experience higher trapping driving force −(ΔG0 + δe), increasing trapping rates, explaining PLQY decrease with excitation energy and lifetime shortening.
  • Model parameters (electronic coupling H and reorganization energy A) are consistent with literature for shallow traps and decrease upon TOPO passivation, indicating reduced carrier–trap coupling.
• Passivation effects:
  • TOPO ligand exchange improves PLQY overall and mitigates high-δe PLQY losses (notably in MAPbBr3 for δe > 0.2 eV; and in MAPbI3 across δe), evidencing the role of surface shallow traps/ligands in hot-carrier trapping.
  • PPP comparisons between pristine and TOPO-treated MAPbI3 show reduced carrier losses after push in passivated samples.
• Additional observations:
  • Cooling dynamics are unchanged within time resolution by passivation, suggesting electron–phonon coupling is not significantly altered by TOPO, whereas long-time recombination is affected.

The results directly address whether defect tolerance in perovskite nanocrystals extends to hot carriers. The pronounced decrease in PLQY with increasing excitation excess energy, lifetime shortening with higher δe, and the PPP-observed acceleration of recombination after transient reheating demonstrate that shallow traps become activated by hot carriers. This converts nominally benign shallow defects into effective non-radiative centers under high-energy excitation, thereby undermining defect tolerance in regimes relevant to LEDs, lasers, and solar energy conversion. The Marcus-based model captures this behavior by linking increased carrier energy to larger free-energy driving force for trapping, elevating trapping rates relative to relaxation to the band edge. Surface passivation with TOPO reduces the electronic coupling and reorganization energy associated with traps, thereby diminishing hot-carrier trapping and improving PLQY, particularly at higher δe. The contrast between MAPbI3 (free-carrier-dominated) and MAPbBr3 (more excitonic) PPP responses highlights how the nature of photoexcited species influences spectroscopic observables, yet both systems exhibit hot-carrier-induced efficiency losses in steady-state PLQY. Collectively, the findings refine the understanding of defect tolerance in LHPs by emphasizing the energy dependence of trap activity and highlight surface chemistry as a lever to sustain performance under high-energy excitation.

This study reveals that hot carriers in MAPbI3 and MAPbBr3 nanocrystals strongly couple to shallow surface traps, leading to excitation-energy-dependent PLQY losses and faster recombination. A phenomenological Marcus-type model explains the increased trapping rates with higher excess energy and quantifies how electronic coupling and reorganization energy govern hot-carrier trapping. Surface passivation via TOPO ligand exchange reduces carrier–trap coupling, mitigates PLQY losses at high δe, and improves overall emissive performance. These insights indicate that defect tolerance in perovskite nanocrystals does not automatically extend to hot carriers, with implications for high-energy-excitation devices (LEDs, lasers, photovoltaics, MEG). Future research should pursue optimized passivation strategies (alternative ligands, core–shell architectures) and deeper investigations of hot-carrier interactions with traps, including disentangling trapping from Auger processes under visible push and exploring device-level impacts on hot-carrier extraction and lasing thresholds.

• Spectroscopic disentanglement: Under visible push (2.07 eV), ground-state absorption contributes significantly and enhances Auger processes, complicating isolation of trapping signatures; analysis indicates Auger dominates post-push recombination in this regime.
• Technique sensitivity: PPP measurements in more excitonic MAPbBr3 show derivative-like signals and no additional fast recombination component, suggesting inherent limitations in detecting hot-carrier trapping directly, despite PLQY evidence.
• Energy range: Detailed analysis was focused on lower excess energies for PPP to minimize confounding effects; very high excess energies may involve additional mechanisms (e.g., photoionization) not fully explored.
• Temporal resolution: Cooling dynamics appeared unchanged by passivation within the setup’s time resolution, potentially masking subtle changes in electron–phonon interactions.
• Generalizability: While surface TOPO passivation is effective, broader ligand chemistries and core–shell structures require systematic study to generalize mitigation strategies across compositions and device architectures.