Recent Advances in Conjugated Polymers for Light Emitting Devices
The novel violet-blue-emitting electroluminescent polymer I was blended at three different weight ratios with the green-emitting polymer II, providing materials which have been studied in terms of their absorbance, photoluminescence, electroluminescence, and morphology. The absorption and PL spectra in dilute solution and in the solid state were compared. Substantial red shifts were observed in photoluminescence from the solid state, which were attributed to intermolecular interactions in the films. Only green emission was obtained from films of the polymer blends and from corresponding double-layer LEDs, indicating an almost complete Förster energy transfer from I to II. Morphological studies indicate that the immiscibility of the two polymers and their differences in CHCl3 solubility result in submicron phase separation during film preparation. In a blend with a high concentration of I, large domains of I were responsible for an incomplete energy transfer, especially noticeable in the solid-state photoluminescence. In double-layer LEDs, both PPV and polyethylene dioxythiophene/polystyrene sulfonate (PEDOT/ PSS) were used as hole-transport layers to increase device efficiency. At 8 V bias, bright green emission (2700 cd/m2) was observed in an ITO/PEDOT/II/Ca device with an external quantum efficiency of 0.69%. The effectiveness of the two hole-transport materials was compared.