Device performance and polymer morphology in polymer light emitting diodes: Morphology dependent emission spectra

Polymer thin films for the light-emitting devices (LEDs) are usually obtain ed by spin-casting from slightly viscous solutions. The performance of poly mer light-emitting diodes is strongly influenced by the thin film morpholog y. It is known that the polymer thin film morphology (or aggregation) is de termined by the solvents used for obtaining polymer solutions. In this manu script, we demonstrate that the polymer aggregation can also be controlled by the concentration of the polymer solution and the rotational speed used during spin-casting of the films. The correlation between the film morpholo gy and emission spectra for polymer light-emitting diodes fabricated using poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) has been studied. Our findings indicate that there is a strong correlation betw een the "vibronic structure" observed in the emission spectra of the MEH-PP V devices and the aggregation of the polymer chains.