Solar panels are giant photodiodes. Heavy metal doped silicon. a-Si something or SiGe or GaAs or InP or whatever pairs and trios of toxic metals. Generally more toxic more electronically open to trade therefore broader spectral response and better performance. You can't do, say, Al substrate PtFeCu semiconductor, that's not going to make sense.

They're not merely similar to a photodiode, but using giant photodiodes as batteries is literally the idea.

There are some versions based on toxic organic chemicals in place of toxic inorganic elements, few and far between, and I guess the technology will eventually move onto engineered nanoparticles later in this century after they've cracked fusion, but that hasn't happened yet.