Year 12s: what will be the first way you research your future options? >>

Struggling to decide on a PhD

So a bit of background fist I guess. I have a 1st in physics undergrad and a masters in sustainable energy. I'm currently taking a bit of time out and travelling/working as a mountain guide but know in the near future I'd like to do a PhD, most likely in physics, but I'm really struggling with which area.

My main interest is obviously energy, with solar energy being a favourite of mine, as well as energy storage solutions. For this reason I've been looking at various PhDs in hybrid or organic photovoltaics (what I did my undergrad dissertation in), specifically transparent ones. Energy storage though I have no idea what direction to even look in.

Although I know about this area and feel it's a worthwhile use of my time what inspires me the most is when I gaze up at the stars, especially where I am at the moment where there are more visible than you could hope to count. I feel if I'm going to be doing something for the next four years is really should be something that I'm passionate about/inspires me. I know it sounds like a silly childish dream but if my research/future career could ever contribute to people exploring those stars I would be a very happy man, and therefore I'm considering something along the lines of propulsion methods. Unfortunately this is a field that I know absolutely FA about. Any suggestions of possible PhDs/fields/likely universities?

Sorry that was a bit long winded, but I'm struggling and hope some of the combined wisdom of TSR can help.