Excitonic Feature in Hybrid Perovskite CH3NH3PbBr3 Single Crystals

Abstract

We have measured the temperature dependence of the reflection and photoluminescence spectra of a lead bromide hybrid material (CH 3 NH 3 PbBr 3 ) using high-quality macroscopic single crystals. Single crystals provide clear and sampleindependent reflection spectra that permit discussion of the properties of the photoexcited carriers. We have found that excitons still exist in CH 3 NH 3 PbBr 3 , even at room temperature. We have also determined the temperature dependence of the exciton resonance energy and the line width.In recent years, lead halide organicinorganic hybrid perovskite materials have attracted considerable attention for photovoltaic applications. 16 The reported power conversion efficiencies (PCE) of the hybrid perovskite-type solar cells with lead iodide materials (CH 3 NH 3 PbI 3¹x Cl x ) are more than 20%. Meanwhile, such materials have been studied from the viewpoint that they form ideal low-dimensional systems and have stable excitons. 812 Even in three-dimensional hybrid materials, the exciton binding energies were reported to be 3750 meV. 13This suggests that stable excitons might exist even at room temperature, and seems to contradict the fact that these materials function as excellent photovoltaic materials, for which dissociation of photoexcited electronhole pairs is necessary to extract the photocurrent.While most studies of the excitonic properties have been performed at low temperatures, solar cells work at room temperature. Therefore, in order to resolve the contradiction stated above, it is important to study the temperature dependence of photoexcited carrier properties in these materials. Recent studies of CH 3 NH 3 PbI 3 indicate that the static dielectric constant shows a rapid increase with increasing temperature, suggesting that this effectively decreases the exciton binding energy. 14,15 Consequently, these results support the view that the excitons dissociate at room temperature in CH 3 NH 3 PbI 3 . In order to clarify the intrinsic properties of these materials, however, such investigations should be performed with high-quality single crystals, which are less affected by defects or impurities. Nevertheless, there are few studies on hybrid perovskite single crystals. 1618In this work, we used high-quality macroscopic single crystals of lead bromide hybrid material (CH 3 NH 3 PbBr 3 ), which has similar properties to CH 3 NH 3 PbI 3 but has a larger band gap (ca. 2.35 eV at room temperature).13 Although CH 3 NH 3 PbBr 3 does not absorb near infrared light, a PCE of more than 10% was reported. 19 The reason why we chose CH 3 NH 3 PbBr 3 is that macroscopic single crystals are more easily synthesized than those of CH 3 NH 3 PbI 3 . Using the single crystals, we have measured the temperature dependence of the reflection and photoluminescence (PL) spectra to discuss the properties of the photoexcited carriers. Our results indicate that excitons still exist in CH 3 NH 3 PbBr 3 , even at room temperature. In addition, we have determined the temperature dependence of the exciton reso...

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

“…27), MAPbBr 3 (refs 52, 53), CsPbBr 3 (refs 54, 55) as 13 meV, 18 meV and 16 meV, respectively, by taking into account both the experimental and theoretical estimations performed before and also the band structure we presented in Fig. 3.…”

Strong Exciton–Photon Coupling in Hybrid Inorganic–Organic Perovskite Micro/Nanowires

“…The bulk parameters ε b = 4.7, [73] ℏγ = 5.9 × 10 −2 eV, and ℏω T = 2.303 eV [74] were taken from literature value. The bulk parameters ε b = 4.7, [73] ℏγ = 5.9 × 10 −2 eV, and ℏω T = 2.303 eV [74] were taken from literature value.…”

Exploring the cation dynamics in lead-bromide hybrid perovskites

“…Interestingly, a study on point defects in MAPbBr 3 [19] showed that the materials' performances are highly tolerant to defect formation, suggesting that stable defects may induce shallow levels in the band gap. Kunugita et al [20] recently investigated the excitonic properties of these compounds as a function of temperature. The study revealed that electron-hole pairs remain bound even at room temperature.…”

“…In this study the photoluminescence line broadening with temperature was attributed to exciton-phonon interaction. An evaluation of the coupling between the exciton and the longitudinal optical (LO) phonon [20] resulted in the frequency of the LO phonon being estimated to be ω LO = 16.8 ± 2.4 meV (135 ± 20 cm −1 ). Hence, it appears that the dynamic of the MA and FA molecules crucially impacts the optical and electronic properties of this class of materials.…”