Their efficiency (in terms of how much solar energy is converted to electrical) is not that important I don't think. Yeah, more efficient would be better. But ATM cost is the bigger factor. Bring that down that solar becomes more and more viable. Lower cost means we can buy more solar panels - we have lots of places to put them so energy per square foot is far less important that the energy per cost of a cell you can get.

We have a staggering amount of otherwise unused space to put them - on top of buildings, over car parks, and if we really need to over roadways. Hell, there were some interesting projects about putting them over rivers/lakes which has the added benifit of reducing evaporation and increasing the amount of water preserved under them which also goes to help places that are in need of water.

There is a lot of places we could be putting them. On moving trucks and under roads these trucks drive on just seems like the last places we should be considering. I can see a time when solar on cars is useful to extend their range without needing to plug in as much - but that is when solar is cheaper and when we already have enough static solar (and other renewables) as static infrastructure.