Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency

We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of pi electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at similar to 630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our finding suggests that the thin-film preparation, and thus the formation of polymer morphology, is a much more complicated process than previously assumed. (C) 2000 American Institute of Physics. [S0021-8979(00)02109-5].