Noise characterization of light-emitting devices based on conjugated copolymers of fluorene and thiophene moieties

Abstract

Degradation induced changes in the structural and optical properties of polyfluorene-based light-emitting diodes are examined by using electroluminescence and low frequency noise (LFN) spectroscopic techniques. The materials studied are poly[2,7- (9, 9′ -dihexylfluorene)-alt-bithiophene] (P1) and poly[2,7- (9, 9′ -dihexylfluorene)-alt-thieno[3,2- b] thiophene] (P2). Improved emission spectra for a light-emitting device based on polymer P2 in terms of current efficiency, spectra stability, and lifetime are observed. A polymer P2-based device also presents long lifetime predicted by the smaller slope in the initial LFN spectra. Correlation of device LFN spectra with polymer structure change and lifetime is established. The increase in noise level predicts the undergoing degradation in bulk material and the increase in the noise slope predicts the fluctuation of carrier number and change in polymer structure. The redshift in emission spectrum for P2 after long-time driving is also picked up by the LFN spectrum. © 2007 American Institute of Physics.