I've been (successfully) using the
forumslader to that end. It generates enough power to actually recharge a depleted phone battery, while riding with the screen turned on. The forumslader is rated at 3A (vs typical 2A for a wall charger).
Interestingly, like many have advised above, I've moved away from this. I now prefer to use minimal navigation electronics (ex: garmin forerunner, if at all) and no longer ride with a dynamo hub. IMHO phone navigation is useful in large cities, not so much on open roads.