Hereagain, I make no claims at being an expert on the subject, but it does seem a bit odd that cadmium is being (rightly) shunned in electronics, while caesium is even being considered as a component of perovskites, at all. There may be different flavors of 'caesium' for all I know, but the highly radioactive caesium-137 is the first thing that comes to this laymans mind. On the other hand, it's not like these initial stages of research will be on our doorstep any time soon & any toxicity concerns would likely have to go thru the same approval gauntlet as cadmium.