Yeah, it's not like it's made out of cloth, it's just a material that has elastic properties, so for a backpack it'd be useless, you'd be better off with a small fixed rectangular rigid panel mounted to the top of the pack which is the only part that will get any large degree of sun.
They'll form to cylindrical surfaces but not round, you could make a solar panel out of polymer round, but you couldn't bend a flat sheet of it, and I fail to see how a domed solar panel would be useful, you'd be better off with a fixed panel and a simple tracking system to orient the panel to the sun, little bit more expensive for the aiming system, but the sun doesn't move very fast so it would use very little power, the same square footage of solar panel would provide more power.
I honestly don't see polymer solar cells being useful in general situations, unless they can be made cloth like and absolutely dirt cheap.