Solar energy used to halve in price every 7 years. in the last 7 it more than halved. Battery performance also has a nice exponential improvement curve.
Various forms of solar are probably one of our better bets, though I'm not convinced that large chunks of the recent gains don't come from massive effective subsidy from China and eventually the cost of the materials themselves could become insignificant compared to complexity and maintenance and end-of-life-recycling cost which are not likely to decrease much. Though battery performance... I haven't seen anything about it that even looks vaguely exponential.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.