I second Guest Sooz's suggestion to focus either on contemporary song/singers/writers, which doesn't have a whole lot to do with traditional music, or focus upon traditional singers who write traditional songs in the traditional context.

I find the need to put Jez Lowe under "traditional" (just as an example) to be a BIG mistake in an academic context. Too much danger of your dissertation meandering meaninglessly across time, place, style, etc etc

Better off to use the bleeding obvious examples, and go from there, as Sooz suggests, for the dissertation anyway.