From my experience of entering YC as a single founder, you need to find a key and simple differentiator that separates you from all of the co-founded companies applying. It should be super clear and concise why you are going to succeed when other's aren't. Statistically, single founder companies fail a hell of a lot more than co-founded companies, you need to give them a reason to take a chance on you.

Also, don't place too much importance on the idea, they know you'll figure it out eventually.