Bias-tuned reduction of self-absorption in polymer blend electroluminescence

We present a concentration- and bias-dependent electroluminescence study on MEH-PPV aggregation in a binary polymer blend with the blue-emitting Me-LPPP as host. In low-concentration blends the spectral features of MEH-PPV peak at 560 nm, identical to its photoluminescence spectrum in dilute solution, and therefore suggesting effective hindering of aggregation-induced self-absorption. At higher concentrations the electroluminescence spectra are dominated by MEH-PPV peaking at 600 nm and a dramatic shift of spectral weight to the 560 nm peak is observed with increasing bias. We attribute this novel effect to a reduction of self-absorption caused by either photo- or charge-induced bleaching.