The invention relates to low temperature curable spin-on glass materials which are useful for electronic applications, such as optical devices. A substantially crack-free and substantially void-free silicon polymer film is produced by (a) preparing a composition comprising at least one silicon containing pre-polymer, a catalyst, and optionally water; (b) coating a substrate with the composition to form a film on the substrate, (c) crosslinking the composition by heating to produce a substantially crack-free and substantially void-free silicon polymer film, having a a transparency to light in the range of about 400 nm to about 800 nm of about 95% or more.