Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

Abstract

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH3NH3PbBr3 perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm2 V−1 s−1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications.

Photon Recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals

“…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.…”

“… 27 Evidence of radiative energy transfer was indeed obtained in mixtures of CsPbCl 3 and CsPbI 3 NCs, 28 where photons emitted by the larger-band-gap CsPbCl 3 NCs were re-absorbed and re-emitted by the smaller-band-gap CsPbI 3 NCs. Within a single NC ensemble, however, the occurrence of PR is unclear: it was inferred from the apparent Stokes shift in NC dispersions, 29 and carrier diffusion lengths in NC films, 30 but despite its importance for PV applications, a clear proof and quantitative determination of PR is missing. NC dispersions would provide the most unambiguous proof of photon recycling, as other forms of energy transport, notably carrier diffusion, are minimized, and NC concentrations can be varied easily, allowing for systematic exploration of the PR effect.…”

Are solution processed CsPbI3 perovskite nanocrystalline films similar to traditional crystalline semiconductors?

“…Since the hopping transport mechanism is well documented by the carrier trapping model in perovskite bulk films (6,27,28) and nanocrystals (29,30), here, we focus on pre-trapping dynamics to gain insights into the carrier-phonon interactions. The strength of the carrier-phonon interaction is related to the temperature-dependent carrier mobility (µ= µp + µe) as: ~ − , (4) As shown in Fig.…”

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

“…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 …).…”