Effects of thermal annealing on the performance of polymer light emitting diodes

Thermal annealing plays an important role in controlling morphologies of polymer thin films and consequently the device performance, such as emission spectra, turn-on voltages, quantum efficiency of photoluminescence (PL) and electroluminescence (EL). In thermal annealing there is a tradeoff between hole-injection efficiency and PL efficiency. Annealing at a temperature higher than the glass transition temperature can improve the efficiency of hole injection at the expense of the PL efficiency, and vice versa. Optimizing the annealing conditions can improve the overall EL efficiency. The high efficiency of poly(2-methoxy-5-(2^′-ethyl-hexyloxy)-1,4-phenylene vinylene based polymer light-emitting diodes is demonstrated: 2.7 cd/A at a luminescence of $\text{1000\hspace{0.5em}}{\mathrm{cd/m}}^2.$