In this Letter, we had measured the spatial recombination-rate distribution of a mixed-emitter organic light-emitting device by inserting a red-dopant at different positions of the emitting layer as a probe. The approach of thin doped-layer to the maximum recombination-rate position generated a strong red emission. In the meantime, the electrical current increased by 8.12 times at 8 V due to the higher recombination coefficient of the dopant than the matrix. In our ME-EML, the maximum recombination-rate position was 10 nm to the hole-transport layer. Carrier piled-up near the EML and the electron-transport layer interface was concretely observed. (c) 2006 Elsevier B.V. All rights reserved.