Effective Bandgap Lowering of CdS Deposited by Successive Ionic Layer Adsorption and Reaction

SILAR (successive ionic layer adsorption and reaction) is a solution process used to deposit semiconductors that has become very common in the past few years as a method to deposit light absorbers in nanoporous solar cells. Films of CdS (possibly the most commonly deposited semiconductor using this method) often show an anomalously low apparent bandgap (lowered by as much as 10%) for sufficiently thick films, although this effect has been ignored in most cases. Here, we study this bandgap lowering and show that it is due not so much to a lower bandgap but rather to a particularly long absorption tail that extends far into the red, and that is amplified by a large optical thickness in the high surface area nanoporous films. The tail is presumably due to as yet unidentified, but probably bulk defects in the CdS. Additionally, the absorption coefficient of the SILAR CdS was nearly twice as high as normal values.