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 having at least one organic group, 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 the composition at a temperature of about 250 C. or less for about 30 minutes or less, to produce a substantially crack-free and substantially void-free silicon polymer film, which silicon polymer has a weight ratio of organic groups to SiO groups of about 0.15:1 or more, and which silicon containing polymer film has a field breakdown voltage of about 2.5 MV/cm or more and a transparency to light in the range of about 400 nm to about 700 nm of about 95% or more.