No, direct sun does not help tomatoes to ripen and in fact may be harmful.
Sunlight is Not Needed For Ripening
Sunlight is needed for tomato plants to grow, of course, but it is not needed for the fruits to ripen. In fact they can ripen in complete darkness! Furthermore, too much direct sunlight can inhibit the change to red and may cause damage to the skin. Ripening is promoted by warmth and ethylene, a chemical produced by the fruit that triggers softening, color change, and conversion of starch to sugars. It is those sugars that give a ripe tomato its natural sweetness.
Direct sun on the fruit can overheat the fruit surface, which inhibits normal red pigment (lycopene) synthesis and causes green or yellow shoulders or sunscald.
What Does Work to Speed Ripening of Tomatoes?
The ethylene that is essential for ripening is actually a gas, so by concentrating it you can speed the ripening process. This is easily done by putting the tomatoes in a brown paper bag and sealing the top. The ethylene that is naturally produced by the tomatoes will be concentrated and speed up the ripening process. Include a ripe banana in the bag to speed things up even more. Bananas give off a lot of ethylene of their own and that's why they help.
These Myths Might Interest You!
Tomatoes are not the only thing that can ripen after picking.
Bananas!
