Surfactant-assisted growth of CdS thin films for photovoltaic applications

A common nonionic surfactant, Triton X-100, was used to modify the chemical bath deposition of CdS buffer layers on Cu(In,Ga)Se-2 (CIGS) thin films. Addition of the surfactant to the CdS deposition bath allowed increased wetting of Cu(In,Ga)Se-2 substrates and an increase in the uniformity of films, especially on model hydrophobic substrates. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy data demonstrate that films produced with the surfactant have the same chemical and electronic properties as films grown without it. In CdS/Cu(In,Ga)Se-2 devices, it was found that Triton X-100 allowed the use of CdS layers that were three to four times thinner than those used normally in high efficiency CIGS-based devices and eliminated the large drops in open-circuit voltage that usually accompany very thin buffer layers. For these thin CdS layers and relative to devices made without the surfactant, average absolute cell efficiencies were increased from 10.5% to 14.8% or by a relative 41%. Visual inspection of the CdS depositions reveals one possible mechanism of the surfactant's effects: Bubbles that form and adhere to the CIGS surface during the chemical bath deposition are almost completely eliminated with the addition of the TX-100. Thus, junction nonuniformities, pinholes, and thin areas in the CdS layer caused by poor wetting of the substrate surface are sharply reduced, leading to large increases in the open-circuit voltage in devices produced with the surfactant. (c) 2006 American Vacuum Society.