The Excel format is published by Microsoft as an open specification . You can get the spec here in a beautifully formatted, intimidating 1200-page PDF file.
I used to use Microsoft specification documents to figure out how to implement partial (incomplete) systems that just serve my specific use cases. In other words, if you can calculate the exact bits you need (i.e. just convert the 2d array to some excel strings), you can often ignore the rest of the specification as it is not applicable to you.
I also found that finding solutions written in other languages helped me - for example, here is a project that uses C #.
Depending on what is available in your backend system, you can create a child process that handles the conversion.
Or you can upload all this to a web service. The Google Docs API list allows you to store and process Google Docs files, and then the Google Spreadsheets API you can read and modify data in Google Spreadsheets .
I understand that none of this really affects your question about the availability of the library, but it can give you a new way to solve the problem.