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.

But even if some of the cost is subsidies and the real speed is only halving in price every 7 years that's still good enough.

I don't see why there shouldn't be any way to optimise end of life costs and maintenance.