Sunflower needs another compatible variety nearby to set fruit. Pawpaws usually need pollen from a different pawpaw cultivar nearby, and hand pollination can improve set. It flowers in mid bloom, so a partner has to be flowering at the same time to be any use. Usually grown in USDA zones 5-9.
No single variety stands out as the obvious partner, but that is not a problem here: Allegheny, Mango, Shenandoah, NC-1, and PA Golden all bloom close enough to work. There are 1 more in the list below. Pick whichever your nursery actually carries. Plant pawpaws close enough for beetles, flies, and other blossom visitors to move easily between them. In home orchards, closer is better.
One thing to keep in mind. Often marketed as unusually easy, but it is still safer to plan on a second pawpaw cultivar.
To put that into practice: at a minimum, plant at least two different pawpaw cultivars. Better still, plant two to three different pawpaw cultivars close together for better overlap and more dependable fruit set. If you already planted one pawpaw, add a second named cultivar nearby rather than waiting to see if the single tree surprises you.