module? It's not very well fleshed out, but it will let you build a .XLS file with text and number fields (it doesn't do much else. No formulas, colors, multi-sheet workbooks, etc). You didn't mention what you're writing into the spreadsheet, so this may be insufficient for your needs.
I'm wishing that someone (besides me) would write a full BIFF5 compliant Excel module. There seems to be a reasonable demand for it, but the whole BIFF5 spec is rather convoluted.