Polymer film light-emitting diodes based on dialkylfluorene-thiophene copolymers with different thiophene content were made. The device structure was ITO/PEDOT:PSS/dialkylfluorene-thiophene copolymer layer/Ba/Al. Typical turn on voltage was about 5V and the highest external quantum efficiency reached 1.8% and the highest current efficiency reached 12cd/A. It was found that the electroluminescent spectra of the LED based on this series of copolyfluorenes could be adjusted effectively by changing the content of the thiophene comonomer. By comparing the spectral variation of the devices along with the increase of the current density or after thermal annealing at different temperatures

it was found that the emission spectra were rather stable when the thiophene content reached 5～10%. For device based on the polydialkylfluorene-co-thiophene with 10% thiophene comonomer

no apparent spectral change was found even if the current density was as high as 520mA/cm

2

or after the devices were annealed at 160℃ for 2 hours on hotplate. Mechanism of the spectral adjustment with thiophene content was preliminarily discussed and it was believed that the energy level structures corresponding to the two kinds of conjugated chain segments were hybridized to a high extent. As to the excellent spectral stability of devices with current increasing or thermal annealing temperature

the origin was believed to be the introduction of low bandgap comonomer which had destructed the coplanar structure of polyfluorene main chain and thus increased the energy barrier to form excimer.