Electron–Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films

Abstract

Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. To realize efficient devices, it is imperative to understand how exciton 2 transport progresses in NPL thin films. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 °𝐶 and 50 °𝐶. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and consequently the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.

Energy Transfer in Stability-Optimized Perovskite Nanocrystals

“…1,2 Scientifically, their flexible compositions and precise size control allow for studies on exciton fine structure, level inversion, polaron formation, carrier dynamics, and phonon interaction. [3][4][5][6][7][8][9] With emission wavelengths tunable throughout the visible range, quantum yields approaching unity, cheap and facile syntheses, and abundant precursor materials, potential applications range from light emission (light-emitting diodes (LEDs), lasers, and displays) to solar cells, photodetectors, field-effect transistors, and even photocatalysis. [10][11][12][13][14] Despite sounding like the perfect material, halide perovskites also (currently) exhibit limitations that have been impeding their widespread commercialization.…”

“…Since their first emergence in 2014, halide perovskite nanocrystals (NCs) have been drawing growing interest due to their exciting fundamental properties and their immense commercial potential. , Scientifically, their flexible compositions and precise size control allow for studies on exciton fine structure, level inversion, polaron formation, carrier dynamics, and phonon interaction. − With emission wavelengths tunable throughout the visible range, quantum yields approaching unity, cheap and facile syntheses, and abundant precursor materials, potential applications range from light emission (light-emitting diodes (LEDs), lasers, and displays) to solar cells, photodetectors, field-effect transistors, and even photocatalysis. − Despite sounding like the perfect material, halide perovskites also (currently) exhibit limitations that have been impeding their widespread commercialization. Perhaps the most critical property in this regard is stability, as incorporated NCs need to survive for the intended lifespan of the device.…”

Dark‐Bright Exciton Splitting Dominates Low‐Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

“…At room temperature, the values coincide nicely with previously determined values in comparable systems. [ 12,57,62,64 ] However, at lower temperatures, the diffusion lengths increase drastically, reaching values of 750 nm for the 14 nm NCs (at 100 K) and nearly 1.2 µm for the 8 nm SLs at 120 K. To our knowledge, these are the highest reported values in nanocrystalline systems, and show the high degree of homogeneity and order in the SL systems.…”

“…For higher temperatures, the 5 nm SLs exhibit the highest values (values for the 8 nm SLs could not be determined due to sample degradation in this intermediate regime), which are comparable to those in our previous report on PNCs at temperatures around RT. [ 62 ] This continued decline coincides with a steep rise in the PL lifetimes (Figure 4b) and a significant drop‐off in the total PL count rate at those temperatures (see Figure S17, Supporting Information). Notably, the temperature dependence of the PNC samples matches nicely with previous studies on CdSe quantum dots, which identified specific temperature‐dependent effects, strongly impacting the overall PL intensity.…”

Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

“…At 200 K all three diffusivities become comparable, and for higher temperatures, the 5 nm SLs exhibit the highest values, yet, below those in our previous report on PNCs at temperatures around RT. 44 This continued decline coincides with a steep rise in the PL lifetimes (Fig. 4b) and a signi cant drop-off in the total PL count rate at those temperatures (see Figure S11).…”