Unusual photoluminescence of porous CdS (CdSe) crystals.

Abstract

Low temperature photoluminescence (PL) is used to study photoetched (PE) CdS (CdSe). This surface treatment produces a porous fractal-type morphology, with superior photovoltaic properties. While most of the usual features in the PL spectrum are suppressed after PE, a new broad spectral band is observed which is deeper than the original bound exciton (I2) line. In contrast with previously known centers, this band shows remarkably large shift as a function of light intensity, which can be described by scaling laws. A theoretical model is proposed, which considers the excitonic emission in porous media. According to this model, the coulombic energy of the exciton increases due to the reduced polarizability of the composite media. Good agreement is obtained between theory and experimental data, and self-consistency is established for the parameters of the theory.