Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals

Abstract

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH3NH3PbBr3), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH3NH3PbBr3 perovskite single crystals is 1015 cm−3 , only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.

The Dominant Energy Transport Pathway in Halide Perovskites: Photon Recycling or Carrier Diffusion?

“…A similar emission redshift as a function of depth can be observed (Figure 6a,b). Since the photon recycling related parameters of MAPbBr 3 and (BA) 2 PbI 4 crystals are very similar, such as bandgap (2.3-2.4 eV), absorption efficiency (10 4 -10 5 cm −1 ), [42] internal PL QY (60-80%), [30,50] refractive index (≈1.7-2), [42] and intrinsic Stokes shift (90-100 meV). The relationships between peak position (E(l)) and depth are fitted by exponential decay curves, E(l) = E min + E 0 exp (−l/L 0 ), where E min + E 0 and E min are the photon energies of the initial emission without redshift and the emission through a distance l → ∞, respectively (see details in Note S1, Supporting Information).…”

“…A comparison of the TPL peak positions between 2D perovskite and 3D MAPbBr 3 crystal is presented in Figure 6c. [42] It is logical to accept the photon recycling terms, η PR , L α of MAPbBr 3 and (BA) 2 PbI 4 crystals are very close, so they exhibit a similar L 0 value without contribution from carrier diffusion. L 0 = 10.02 µm is acquired for 2D perovskites, approximately Adv.…”

“…[39][40][41] This debate was frequently investigated by the PL variation as a function of separation of excitation and emission position. [19,42,43] In these experiments, the excitation spot and emission collection position are different, thereby allowing photogenerated carriers or emission photons to travel internally before measurements. For example, when the excitation beam is irradiated on the front of a perovskite crystal, the PL emission spectra collected from the front and from the rear of the crystal are different, which is explained by the pure photon reabsorption process.…”

“…[19,42,43] In these experiments, the excitation spot and emission collection position are different, thereby allowing photogenerated carriers or emission photons to travel internally before measurements. [42] Tian et al measured individual single-crystal MAPbI 3 (MA = CH 3 NH 3 ) and MAPbBr 3 nanowires (NWs) and nanoplates (NPs) using PL-scanned imaging microscopy with a galvano-mirror. [42] Tian et al measured individual single-crystal MAPbI 3 (MA = CH 3 NH 3 ) and MAPbBr 3 nanowires (NWs) and nanoplates (NPs) using PL-scanned imaging microscopy with a galvano-mirror.…”

“…[19,30,[32][33][34] These reports claim that energy transport is not limited by diffusive charge transport but can occur over long distances through multiple reabsorption-diffusion emission events implying the repeated recycling between photons and electron-hole pairs. [18,19,[30][31][32][33][34][35][36][37][38][39][40][41][42][43] This controversy commonly exists in various perovskites and device studies. [30][31][32][33] Very similar redshifts of the PL maximum were consistently observed.…”