Hi
I couldn't find anything very specific for leaf loss on paw paw trees, but this specimen looks pretty seriously ill, and perhaps past recovery. If it has only been newly purchased I'd suggest returning it to the nursery since it appears to still be in its pot. The sun shouldn't have been an issue, though it you happened not to have watered quite sufficiently too much sun can cause a plant to stress and lose leaves. This, however, seems somewhat beyond that. I'll attach a Univ of Kentucky fact sheet on general paw paw care and propagation; it may help you diagnose any concerns with care: http://www.pawpaw.kysu.edu/pawpaw/ppg.htm