Recycled Photons Traveling Several Millimeters in Waveguides Based on CsPbBr₃ Perovskite Nanocrystals

Abstract

Such multiple (re)absorption/(re)emission cycles result in a certain population of "recycled photons" concentrated inside the semiconductor. Consequently, an efficient PR effect provides another degree of freedom to control the photon and carrier densities in a semiconductor [2] and hence a way to tailor-made its optoelectronic properties. [3,4] Demonstrated applications include solar cells, light-emitting diodes, or optical modulators. [2] In this context, PR has been recently claimed in halide perovskites (HP), as an effect that could contribute to a certain extent to the excellent conversion efficiencies and emission rates reported for this family of semiconductors. [5,6] Indeed, the strong absorption coefficient above the bandgap and sharp (excitonic) band edge, [7,8] the small Stokes shift (SS) between photoluminescence (PL) and absorption, [9,10] and the high PL quantum yield (PLQY) [11,12] are outstanding characteristics of HPs and needed ingredients for leading to important PR effect. In this way, over the last 3-4 years, different experimental and theoretical reports have analyzed the efficiency of PR in HPs and the potential benefits in solar cells [13][14][15] or light-emitting diodes, [15,16] among other devices. 15 In particular, experimental studies carried out in CH 3 NH 3 PbX 3 (X = Cl, Br, I) polycrystalline thin films, [17,18] CH 3 NH 3 PbX 3 single crystals, [10,19,20] CsPbBr 3 nano/microwires [21][22][23] or CsPbBr 3 nanocrystals [9] always show that PL spectra experience an important redshift and an elongation of the decay time after traversing some microns of the HP material. Although there has been a controversy about the impact of PR in the total PL spectra, [19,24] or if PR dominates or not over carrier diffusion on the effective decay time, [23,25] recent studies on perovskite single crystals [6] and MAPI polycrystalline thin films [18] confirm that PR is the dominant transport mechanism for propagation lengths longer than the diffusion of carriers. Besides, the theoretical analysis predicts that multiple absorption and emission processes produce a certain "diffusive regime of traveling photons" that increases the effective lifetime of photons outcoupled from the sample (thin film, microwire, single crystal …). [26] Indeed, recent theoretical and experimental works demonstrated an enhancement of the open-circuit voltage in solar cells [14,18,27,28] or radiation efficiency in light-emitting diodes [29] when PR is optimized. The standard optical configuration that has been chosen to demonstrate the PR effect in most works consists of a semiconductor thin film deposited on a specific Reabsorption and reemission of photons, or photon recycling (PR) effect, represents an outstanding mechanism to enhance the carrier and photon densities in semiconductor thin films. This work demonstrates the propagation of recycled photons over several mm by integrating a thin film of CsPbBr 3 nanocrystals into a planar waveguide. An experimental set-up based on a frequency modulation spectroscopy allows to ch...

Photon Recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals

“…Evidently, to be able to measure any photons from indirectly excited NCs, these NCs must re-emit as well; therefore, the occurrence of a rise time in the TRPL traces is another indication of the photon recycling process. Indeed, measurements on waveguides of low-dimensional perovskites show a qualitatively similar increase of photoluminescence rise times with increasing excitation-detection distance, conceptually consistent with the dilution-dependent rise times presented here. − …”

“…In all cases, there is a clear discrepancy between experiment and Beer–Lambert prediction, with the experiment revealing a considerably stronger PL red shift than the Beer–Lambert simulated one. Similar spectral changes were also observed in drop-casted MAPbBr 3 nanocrystal films, CsPbBr 3 microwires, and CsPbBr 3 nanocrystal film waveguides, 24 , 26 , 30 showing their dominance in these perovskite nanocrystal materials.…”

“…Indeed, measurements on waveguides of low-dimensional perovskites show a qualitatively similar increase of photoluminescence rise times with increasing excitation-detection distance, conceptually consistent with the dilution-dependent rise times presented here. 24 − 26 …”

“… 23 Additional work performed on photon recycling in CsPbBr 3 -based or 2D perovskite waveguides showed repeated photoluminescence spectral and lifetime changes with increasing distance between excitation and detection. 24 − 26 Colloidal suspensions of perovskite NCs offer to further resolve the ongoing dispute as carrier diffusion is totally inhibited. Moreover, the absorption cross section near the NC band gap is typically much larger than that of the bulk, which provides yet additional advantage.…”

“…However, while the variation of PL intensity with distance from the surface suggested the presence of PR, , other work argued that the PR-related component constituted only approximately 0.5% of the total external emission, and the observed PL intensity gradient arises due to the long intrinsic carrier diffusion lengths . Additional work performed on photon recycling in CsPbBr 3 -based or 2D perovskite waveguides showed repeated photoluminescence spectral and lifetime changes with increasing distance between excitation and detection. − Colloidal suspensions of perovskite NCs offer to further resolve the ongoing dispute as carrier diffusion is totally inhibited. Moreover, the absorption cross section near the NC band gap is typically much larger than that of the bulk, which provides yet additional advantage .…”