Don't let me discourage you, but there is a reason why there is little research on freemium business models: it is theoretically not very different from premium business models (price discrimination), except for the fact that the lowest tier is free instead of paid and that the company benefits from network effects from the free users. So the big issues are in price discrimination research and research on network effects.

Not that it isn't an important new idea in practice, since the psychological difference between 0.01 cents and 0.00 cents is huge.

If you end up doing the thesis with on freemium, you'll probably either end up doing a quantitative analysis (check out the quant research on shareware), or qualitative analysis around price discrimination, network effects and business models (check out this MA thesis https://jyx.jyu.fi/dspace/handle/123456789/23051).