The electrical and optical properties of tandem organic light-emitting devices (OLEDs) fabricated utilizing an organic bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) and 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) charge generation layer (CGL) were investigated to enhance their efficiency. While the operating voltage of the tandem OLEDs with a BEDT-TTF and HAT-CN CGL at 50 mA/cm2 was 11.2 V lower than that of the tandem OLEDs without a CGL, the current efficiency of the tandem OLEDs with a BEDT-TTF and a HAT-CN CGL at 50 mA/cm2 was 0.8 cd/A higher than that of the tandem OLEDs without a CGL. An increase in the current efficiency and a decrease in the operating voltage of the tandem OLEDs with a BEDT-TTF and an HAT-CN CGL were attributed to the enhancement of the electron injection due to its existence in the highest occupied molecular orbital level of the BEDT-TTF between the HAT-CN and the tris-(8-hydroxyquinoline)aluminum layer.