Solar power on the roof of an EV car seems like an obvious play. And yes some energy can be derived this way. But it's important to understand the limitations of physics.

When the sun is directly overhead we get about 1kw per square metre. Very good panel effeciency is about 25%. ) I'm rounding a bit to make math easier.) The average car footprint is maybe 6 square meters. Obviously a lot of that is curved away from the sun, or is glass. 4 square meters here is perhaps a reasonable starting point.

Which means if we plaster the car with solar cells, we might get 1kw of output in pretty optimal conditions. Perhaps 12 hours of generation depending on latitude, time of year, and so on.

Most EV cars have 30 to 80 kw batteries (some more). So 12kw of charge per day is not nothing, but also not enough. Given pesky things like shade (from tall builings or trees), undercover parking, clouds and so on, daily average is much lower.

Solar film in general is great. But current pricing makes it cheaper to just install regular solar panels in most places. Less portable maybe, but more (financially) efficient to generate/store/charge as we currently do.

On the other hand the aesthetic of a solar panel car roof is very cool, and maybe covers the aircon power requirements on a hot day.

I don't know if it can be DIY, their website says manufacturers can integrate into their workflows.

Looks like they are also working on transparent. In development.

https://solarwindow.com/liquidelectricity/