Red Shift of Bleaching Signals in Femtosecond Transient Absorption Spectra of CsPbX3 (X = Cl/Br, Br, Br/I) Nanocrystals Induced by the Biexciton Effect

Metal halide perovskites are affected by the quantum effect, their size is equal to the Bohr radius of the material, and they exhibit excellent emission performance and charge-transfer performance. We have synthesized CsPbX3 (X = Cl/Br, Br, Br/I) monodisperse nanocrystals (NCs) by a simple heat injection method. The band gap energy and emission wavelength can be changed by adjusting the composition of nanocrystals. CsPbX3 NCs have an emission linewidth of 18–29 nm, a photoluminescence lifetime of 5.7–20.8 ns, and a fast carrier-transfer rate. In this paper, the transient absorption (TA) spectra of CsPbX3 NCs were studied by a femtosecond laser. The carrier dynamics is studied by changing the excitation power and delay time. The experimental results show that the bleaching signal of the TA spectrum has a red shift of 5–12 nm initially and then remains unchanged. With an increase of excitation power, the bleaching signal also shows obvious fast decay components, indicating the emergence of biexciton states. In the TA spectrum, the corresponding wavelengths are obtained from the bleaching signals of single and biexciton states, and then, the binding energy of biexciton in the range of 21.7–61.2 meV is calculated according to the optical quantum energy formula.