I do not understand why their not putting solar panels on top of those vans?
The extra cost of the panels would offset the cost of fuel over the long run.
Solar cell technology is just not ready for prime time yet. The amount of energy gathered is just too small to make financial sense.
Let's say the roof of the car can fit about 5 m^2 worth of solar panels. Solar panels can generate about 200 watt/meter^2 in full sunlight. So if you have 10 hours of full sun (not likely in NYC with smog, snow), that's 2 kwh / day. In comparison, each gallon of gasoline has 36.6kwh worth of energy. Assume 20% internal combustion engine efficiency, 10 hours of sun converts to about 1/4 gallon worth of gasoline per day or about $360 worth of gasoline/year at $4/gal prices.
From looking at amazon prices of solar panel, 5 m^2 of solar panel will run you about $4000. It'll take a long time to recoup that cost assuming the solar panel works perfectly and did not need repairs.
Also (i think the main reason), is that electricity is actually very difficult to use if the car is not directly driven by electric motors. You'll need to either add tons of complexity/weight/cost to incorporate a hybrid drive-train, or limit solar power use to running the radio, computer, lights etc. The alternator already does a pretty good job of converting engine power to electrical power so I don't know how much fuel you can actually save with solar cells.
I wish it's not the case, but solar cell is just not ready. It's debatable when installed on houses at the right locations, but it just doesn't make sense in cars right now.