Most PV researchers I've met have a physics or materials science background. Maybe some come from chemistry backrounds as well, and I'm fairly certain there are EEs around.
Electric vehicles may require EEs. There are definitely chemical engineers working on fuel cells and probably batteries.
MEs might work in concentrating solar power, but I highly doubt you'd find any in PV. MEs would work on a number of other renewables as well, such as wind power.
When you mention PV and electric vehicles (separately), are you talking about solar-powered vehicles? I know an EE who has worked on a solar-powered car as (I think) a hobby--either that or for a school competition of some sort. I don't know that there's actually much research or development going on in that area in general, but you'd probably need a team of engineers with different backgrounds, and you wouldn't really need someone with experience in PV (you'd probably just buy the best solar panels possible!)