We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid stateelectrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs sufferfrom a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that thisefficiency drop is not only due to device decomposition upon high voltage operation and we also reveal theintrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-typedoped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminancequenching effects.