Observation of 3D Biexcitons in Pristine‐Quality CH₃NH₃PbBr₃ Single Crystals

Abstract

Regarding the latter, for example, the basic issue such as "the identity of optical excitation at room temperature" is still controversial, because there are contradicting evidences for two candidates, free carriers and excitons. [9,10] The key question is the stability of the exciton against thermal ionization of the constituents. The exciton binding energy ϕ X in 3D HPs such as MAPbX 3 (MA = CH 3 NH 3 , X = I, Br, Cl) ranges from 15 meV to 70 meV, [9,11,12] which is indeed comparable to room temperature (≈26 meV), and therefore, an exciton may or may not be stable. However, optical excitation is certainly excitonic at low temperatures, for example, 10 K (≈0.9 meV).Like a hydrogen molecule formed by binding of two hydrogen atoms, two excitons can bind to form an excitonic molecule, typically known as a biexciton. The numerical value for the biexciton binding energy ϕ XX can be roughly estimated by a theoretical model, which predicts its universal dependence on ϕ X and the effective mass ratio σ between the electron and the hole. [13][14][15] According to variational methods, [16,17] the predicted ϕ XX for MAPbBr 3 (ϕ X = 38 meV [9] and σ ≈ 1 [18] ) can range from 1.0 meV to 4.8 meV, which is sufficiently large to ensure the stability of the biexciton at nominal cryogenic temperatures. However, there is no single experimental evidence for their existence in the 3D HPs, which is quite surprising because biexcitons are well established in conventional semiconductors like Si and Ge with similar and even much smaller values of ϕ XX = 1.5 meV [19] and 0.3 meV, [20] respectively.Here, we demonstrate that biexcitons do exist in 3D MAPbBr 3 single crystals at low temperatures (T < 30 K). However, biexcitons are unstable at the crystal exposed to ambient, explaining their elusive nature when probing as-grown crystals with typical photoluminescence (PL) spectroscopy. Therefore, the freshcut pristine quality of the crystal should be established for the stable biexciton formation. The evidence of the biexcitonic phase is very clear from the rapidly growing PL peak having an inverted Boltzmann shape, reflecting the kinetic energy distribution of the biexcitons. [21,22] We show that a cold biexciton can be directly generated using giant resonant two-photon absorption (2PA) with polarization control aided by the nanoscale surface flatness of the crystal with fine-scale photoluminescence Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH 3 NH 3 PbBr 3 (MAPbBr 3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon ex...

Impact of the Hexagonal Phase on Multiphoton‐Absorption Properties of Mixed‐Cation Halide Perovskite: FA0.8MA0.2PbI3

“…[49] The suggested concept of double resonance is similar to the giant two-photon generation of biexcitons via simultaneous resonance at the exciton and biexciton levels. [50,51] Our main thesis is further confirmed by the same MPA behavior observed from mixed perovskites prepared with different MA doping ratios, FA 1-x MA x PbI 3 (x = 0 and 0.3). Our results suggest that the MPA mechanism of the mixed perovskite is drastically altered by the presence of the hexagonal phase, which can be tuned via selective control of temperature and excitation wavelength for future NLO applications.…”

“…A similar concept of double resonance actually occurs for the direct generation of biexcitons, where the corresponding 2PA coefficient is known to be several orders of magnitude larger than typical 2PA coefficients for band-to-band transition. [50,51]…”

Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements

“…The synthesized crystals have a rhombic dodecahedral shape (Figure S1, Supporting Information). The surfaces of as-grown crystals were carefully wiped to remove residual solvents . For all measurements, we used samples synthesized on the same day to minimize the potential degradation of the sample quality caused by atmospheric reactions or any other external stimuli.…”

Discrete Donor–Acceptor Pair Transitions in CH3NH3PbI3 Perovskite Single Crystals

“…We prepare pristine surfaces by cleaving the crystals. [41,42] Applying pressure with a tweezer, we break the crystals into fragments. In comparison with the surface of as-grown crystals, which can be rough and inhomogeneous, cleaved fragments present smooth, mirrorlike surfaces (Figure S1, Supporting Information).…”