Trapping and Detrapping in Colloidal Perovskite Nanoplatelets: Elucidation and Prevention of Nonradiative Processes through Chemical Treatment

Abstract

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr 3 nanoplatelets, before and after chemical treatment with PbBr 2 that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr 2 . More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Direct Observation of Shallow Trap States in Thermal Equilibrium with Band‐Edge Excitons in Strongly Confined CsPbBr3 Perovskite Nanoplatelets

“…Figure S5, Supporting Information shows the dynamics on the ns range. As already observed by Vonk et al, [47] the PL decay is multiexponential (τ 1 = 5 ns and τ 2 = 13 ns) due to the NPl subpopulations in the ensemble. Moreover, the bi-exponential function cannot fit the decay curve on long time scales (>100 ns).…”

“…This may explain the extended PL lifetimes that have been reported previously in the literature. [ 39,40,47 ] Additionally, we have also demonstrated the importance of capping ligands to prevent the formation of deep trap states. Desorption of the ligand from the surface due to sample aging increases the number of deep trap states and directly influences the PLQY of the NPls.…”

“…This kinetic component has already been ascribed as a delayed PL induced by the trapping and detrapping of excitons from shallow trap states. [ 47 ]…”

“…This kinetic component has already been ascribed as a delayed PL induced by the trapping and detrapping of excitons from shallow trap states. [47] Focusing on NPlsBr, we monitored the effect of sample aging on the PL dynamics. Figure 3a,b shows that the linear optical properties remain unaffected after 80 days under light and room temperature.…”

Carrier diffusion coefficient is independent of defects in CH3NH3PbBr3 single crystals: Direct evidence

“…1c), indicating that the carrier trapping caused by surface defects in the as-prepared NWs and NPs is negligible. In previous studies, the trapping of carriers in perovskite nanoplates are demonstrated to occur in very fast (ps) time scales, the long radiative lifetimes are attributed to delayed photons from charge detrapping process [25]. In order to verify whether there exists fast trapping process, we performed PL up-conversion (PL-UC) measurement (Fig.…”