Anode modification of polyfluorene-based polymer light-emitting devices

A glycerol-modified poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonate) (PSS) layer was used as an anode buffer layer in polymer light-emitting devices using poly(9,9-dioctylfluorene) (F8) as the emitter. Devices with a configuration of indium tin oxide/PEDOT:PSS (with or without glycerol)/F8/CsF/Al were fabricated. It was found that the glycerol-modified device showed a much larger current density than the unmodified device. At an operating voltage of 6 V, the glycerol-modified device showed a luminance of $\text{1300\hspace{0.17em}}{\mathrm{Cd/m}}^2$ and a current efficiency of 1.7 Cd/A compared to the corresponding values of $\text{500\hspace{0.17em}}{\mathrm{Cd/m}}^2$ and 1.3 Cd/A in the unmodified device. Analysis by ultraviolet spectroscopy suggests that the two devices have the same energy level structure and the performance improvement should not be due to change in the PEDOT/polymer interface. It was further found that incorporating a suitable amount of glycerol into the PEDOT:PSS layer can increase its conductivity by six times. This leads to a better balance in the hole and electron currents and thus improved device efficiency.