RESUMEN
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 CH3 NH3 PbBr3 (MAPbBr3 ) 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 excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.
RESUMEN
Two-dimensional (2D) semiconductors have emerged as an excellent platform for studying various excitonic matter under strong quantum and dielectric confinements. However, such effects can be seriously overestimated for Coulomb binding of two excitons to form a biexciton by a naive interpretation of the corresponding photoluminescence (PL) spectrum. By using 2D halide perovskite single crystals of [CH3(CH2)3NH3]2Pb1-xMnxBr4 (x = 0-0.09) as a model system, we investigated both population and relaxation kinetics of biexcitons as a function of excitation density, temperature, polarization, and Mn doping. We show that the biexciton is formed by binding of two dark excitons, which are partially bright, but they radiatively recombine to yield a bright exciton in the final state. This renders the spectral distance between the exciton peak and the biexciton peak as very different from the actual biexciton binding energy (Ï) because of large bright-dark splitting. We show that Mn doping introduces paramagnetism to our 2D system and improves the biexciton stability as evidenced by increase in Ï from 18.8 ± 0.7 to 20.0 ± 0.7 meV and the increase of the exciton-exciton capture coefficient C from 2.4 × 10-11 to 4.3 × 10-11cm2/ns within our doping range. The precisely determined Ï values are significantly smaller than the previously reported ones, but they are consistent with the instability of the biexciton against thermal dissociation at room temperature. Our results demonstrate that electron-hole exchange interaction must be considered for precisely locating the biexciton level; therefore, the Ï values should be reassessed for other 2D halide perovskites that even do not exhibit any dark exciton PL.
