If you're talking about investment in the usual sense, can't you just buy stocks of solar/clean energy companies?
Otherwise, if you just mean financially supporting solar technology, one good way is donating to political action/lobbying groups like Greenlight America or CATF (though that supports clean energy as a whole, not just solar)
Otherwise, if you just mean financially supporting solar technology, one good way is donating to political action/lobbying groups like Greenlight America or CATF (though that supports clean energy as a whole, not just solar)