Influence of shape on the carrier relaxation dynamics of CsPbBr perovskite nanocrystals.

Lead halide perovskite nanocrystals (NCs) have recently emerged as a new class of functional materials for designing efficient light harvesting systems because of their unique photophysical properties. Here, we report the influence of different shapes on the relaxation dynamics of perovskite nanocrystals. The structural transformation of CsPbBr NCs from cubic shape to rod shape occurs by changing the solvent from toluene to dichloromethane (DCM). Rietveld analysis reveals that the crystallinity along with the preferred orientation (PO) of the orthorhombic phase plays a vital role for the unidirectional growth of rod shaped CsPbBr NCs in DCM. Time-resolved emission spectroscopy and ultrafast transient absorption spectroscopy are used to understand the photoinduced relaxation processes. Global and target analysis of femto-second transient absorption kinetics has been done to understand the individual excited-state species. The analysis reveals that trap states play an important role in the carrier relaxation dynamics of cubic and rod shaped NCs. The lifetime of the shallow trap (ST) changes from 25 ps to 45 ps and the lifetime of the deep trap (DT) state changes from 163 ps to 303 ps with changing the shape of the nanocrystals from cubic to rod. This work highlights the tuning of the crystal phase, shape and the exciton dynamics of CsPbBr NCs that would be beneficial for designing efficient photovoltaic devices.