Tandem organic light-emitting devices (OLEDs) with a mixed organic n-type 4,4',4"-methylidynetris (N,N-dimethylaniline) and organic p-type 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile layer embedded into a charge generation layer (CGL) were fabricated to enhance their current efficiency. The operating voltage of the tandem OLEDs with a mixed layer at 10 mA/cm2 was 1 V lower than that of the tandem OLEDs without a mixed layer due to the electrons generated from the mixed layer. The matrix-assisted laser desorption-ionization-time-of-flight mass spectrometer spectra clarified the charge generation behaviors of the mixed layer into the CGL. The current efficiency of the tandem OLEDs with a mixed layer at 10 mA/cm2 was 48.2 cd/A, which was much larger than that without a mixed layer. The increase in the current efficiency for the tandem OLEDs was attributed to the enhancement of the electron injection efficiency in a CGL resulting from the insertion of the mixed layer.