ADVANCED MATERIALSCommunications tage. EL originates from the compensated region where the built-in electric field is a maximum. Note, however, that when the external bias is applied under steady state conditions, the external field is opposite to the built-in field so that near threshold, the net internal field is small during operation. The observed decrease in the PL within the junction is particularly interesting since external quantum efficiencies of LECs are comparable with those of polymer LEDs fabricated from the same semiconducting polymer.[lA1 This suggests that the quantum efficiency of the EL is a larger fraction of the PL than in conventional polymer light-emitting diodes.We emphasize once again that the results presented here were obtained with the LEC in the surface cell configuration with relatively widely spaced electrodes. For sandwich cells, the width of the junction must be less than the sub-micron thickness of the polymer layer, implying that the boundary conditions play a more critical role. ExperimentalSurface cell LECs were prepared with pairs of gold strips (5 mm long, 2 mm wide) as the contact electrodes, separated by a (approximately) 20 pm gap. Five pairs were fabricated onto an 1.5 x 1.5 cm2 glass substrate by vapor deposition through a shadow mask. A layer of chromium was