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Couldn't we have a lead box lined with these radiation to electricity converters with a small amount of radioactive material in the center, and have an energy generating device that would last for thousands or even millions of years? Imagine putting the sun in a box lined with solar cells, but on a much smaller scale.
Is there a reason this wouldn't work?
Lot of comments about RTGs, but I don't think that's what OP is asking. RTGs convert heat to electricity, same as a conventional power plants
they just do it in a solid state way instead of steam. In RTGs it doesn't matter where the heat comes from; they are not really analogous to solar cells, as the title asks.
In fact, there are consumer products that use the same technology
you can buy a little electric fan that sits on top of a wood stove and, once up to temp, will start spinning. The electricity is generated by the thermal gradient using heat from the stove, essentially the same as an RTG.
You can buy Sterling engines for that too and skip the electric phase.
Take it up with Google voice dictation.
Yup, RTGs are still subject to the second law of thermodynamics.