to get 3v from 1.5v you need a lot of current. the joule thief takes advantage of the generous current yet low voltage remaining in a "dead" alkaline cell.
depending on the type, a solar cell offers a generous open circuit voltage but a very low short circuit current. there are however cells offering a low OC voltage but a very high SC current. Often these cells put out 0.5v OC and upwards of 2A SC. A few of these in series would have no trouble running a puny led, but they're going to cost as much as a small case of alkaline batteries.