There are already multiple solar roof cars. This should make it easier to integrate solar into these designs.
This is the worlds cheapest SOLAR CAR: https://www.youtube.com/watch?v=Hk6aTKV-des https://www.solarkycar.com/
Alin Chinese Electric Car with Solar Panel CE Approved Electric City Cars, US$1,500.00-2,000.00: https://alinmachinery.en.made-in-china.com/product/zZHTdphPH...
Chinese EV company tests flexible solar sunroof for 50 miles of additional range per day: https://interestingengineering.com/transportation/hongqi-per...
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.
Here's a similar article from 2013, talking about a paper from 2005:
https://www.fastcompany.com/1681384/you-may-soon-just-spray-...
"Some day, homeowners eager to produce their own energy might not line their roofs with solar panels. Instead, they’ll spray-paint them with a mist full of electricity-generating solar cells."
With some states only requiring UL listed inverters for small plug-in systems, it could become a more viable product.