By introducing 1,8-diiodooctane (DIO) into solutions used to spin-cast poly (9,9-di-n-octylfluorene) films, the beta-phase content can be tailored between 0 and 45%. The absorption of films cast from o-xylene containing between 0 and 16% DIO by volume is shown at left. In turn, this morphological change can lead to a ca. 50% increase in the efficiency of polymer light-emitting diodes.