Influence of light-soaking treatment on the optoelectronic properties of polymorphous silicon thin films to be used in solar cells

Abstract

In this work we show the different trends in the optoelectronic properties of polymorphous silicon thin films under light-soaking for long exposure times. These thin films were grown by Plasma Enhanced Chemical Vapor Deposition using dichlorosilane as precursor gas and different hydrogen dilutions. When the samples were illuminated by white light at power density of 100 mW/cm2 (AM1.5 condition) during 250 h continuously, singular behaviors on the photoconductivity measurements were shown. The different trends are explained as a function of the crystalline fraction and chemical composition of these films. XPS shows that an oxidation process takes place throughout the film in some samples grown at high hydrogen dilutions, while other samples grown at low hydrogen dilution show only surface oxidation after ambient exposure. In the same way, XPS spectra show different silicon oxidation states and chlorine presence along of the films. On the other hand, FTIR spectra evidence the absence of mono-and dihydride Si-H bonds around 2000-2150 cm-1. These bonds are generally responsible for the degradation process in amorphous silicon thin films solar cells. With the control of the crystalline fractions and chemical composition, it is possible to avoid the degradation process in thin films silicon solar cells. The role of chlorine and hydrogen related bonds, which can be associated with the level of stability of the films, were inferred from XPS and FTIR analysis. Conductivity and photoconductivity changes on polymorphous silicon thin films were observed when the films are subjected to light-soaking for 15000 minutes. These changes are dependent on the chemical and structural properties of the films.