In the case of Moore's Law, a transistor can be any size. By contrast, solar cells absorbs too little energy if they are too thin.
TL;DR: There are real physical limitations to solar cells.
1. The grams of rare earths required per watt of solar have been dropping steadily. And Solar PV panels with no rare earths at all have been developed, though they are not yet common.
2. "Rare earths" are not so rare. Their prices have tumbled substantially as new mines have opened up and substitutes have been found.
Yes, but mining them is an environmental disaster.
Yeah, but the super-efficient solar cells are not the cheap ones that everybody is putting on their roof. They're the ones going on the international space station.
>Once a particular solar technology reaches grid parity
It already did in about half of the continental US, about two years ago.
I mean that seriously. Right now, pretty much all that matters is cost per watt. For normal terrestrial uses, a 20% efficient cell loses out to a 5% efficient cell that's 10x cheaper. (Numbers made up on the fly with no particular basis.) We're not currently hurting for space for these things, and sunlight is free.