Dipping pristine poly(9,9-di-n-octylfluorene) film into a mixed solvent/nonsolvent THF/MeOH transforms the conformation of minor amorphous-phase chains to beta-phase conformers (see figure) with a controllable content. The P phase acts as a dopant and causes electron trapping and promoted hole mobility. It allows efficient energy transfer and charge trapping and thus achieves pure-blue electroluminescence and enhanced device performance.