The sun only provides X amount of energy per unit of area. And even if we had solar panels thats were 100% efficient in turning ALL available sunlight into electrical energy, the area of top of a car isnt nearly enough to drive indefinitely on. Not even factoring in sunlight angles and hours of sunlight per day.
I mean they cant HURT range, but dont get your hopes up on a chargeless solar car. I think they quote an extra 40 miles of range which I bet if you broke down the numbers, is very optimism and requiring perfect conditions.