LEDs Based on Polymer Blends

In addition to the opportunity to increase the overall device performance in LEDs based on polymer blends,126 127 another interesting aspect is that their emission color can be internally converted.128-130 Based on blue-light-emitting polymers,131 therefore, any other emission color can be realized for those also, which intrinsically are difficult to achieve with pure conjugated polymers, as is the case for pure white132,133 and red134,135 emission colors. 

An efficient internal color-conversion technique will only occur if the polymers in the blend are only weakly phase separated. Otherwise, the various polymers in the blend behave similar to independent parallel diodes and hence a strongly voltage-dependent emission color will be observed.131,137 

FIGURE 8.18. Absorption of m-LPPP photoluminescence of m-LPPP (—), absorption 

UP to rjp 41% for very small PPDB concentrations of 0.7% in the blend. For higher doping concentrations, r]F then slowly decreases (down to 11% for a concentration of 10% PPDB in the blend) and approaches the rjF-value of pure PPDB films, which only is 0.6% (see Fig. 8.19). 

The color conversion occurring in the EL cannot be solely ascribed to an excitation energy transfer (Forster-type), because the change in the emission color and the dependence of the EL efficiency on the doping concentration are slightly but distinct different from that of the PL (see Figs. 8.19 and 8.20). We therefore propose that additional to the Forster transfer, a charge transfer (polarons) occurs from m-LPPP to PPDB. 

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