An Impedance Spectroscopy Investigation of nanocrystalline CsPbBr3 Films

Recently ternary compounds of CsPbBr3 produced by co-evaporating CsBr and PbBr2 purified powders are attracting much interest for their interesting optical characteristics and potential applications in luminescence based detector and devices. Indeed, nano-crystallites are produced in the thin film material as evidenced by optical spectroscopy and X-Ray diffraction. These ternary nano-aggregates are wide-gap semiconductors with direct band-to-band transition. Identification of the nano-crystallites in the film is based on a similarity of the observed spectroscopic properties with those of the bulk CsPbBr3 purified by the Bridgman technique. The materials object of this study have been deposited by Physical Vapour Deposition inside an UHV apparatus on DC7059 optical glass. We report on an impedance spectroscopy study in a wide frequency range and temperature aimed to identify the role of the surface located defects. These materials show an exciton absorption also at RT. The FWHM of the exciton peak is used to evaluate, together with X-ray analysis, the quality of the deposited material. The transport mechanisms have been analysed to achieve a better understanding of the current paths whereas spectral photoconductivity has been used to investigate the density of gap-states. Results show as the Fermi level is pinned by the surface states which induce an electron accumulation layer at the crystallite surface influencing the current transport at lower and intermediate temperature while, at the higher, an activate transport mechanism is observed. Hopping transport can be addressed both at high and low temperature with an average energy barrier of 0.95±0.02 eV. DC and AC measurements will be presented to illustrate the active transport mechanisms at the different temperatures.