In an example embodiment, the electron transport region may include at least one compound selected from 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (Bphen), Alq3, BAlq, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ), and NTAZ:
In an example embodiment, the electron transport region may include a phosphine oxide-containing compound (for example, TSPO I used in the following examples or the like). In an example embodiment, the phosphine oxide-containing compound may be used in a hole blocking layer in the electron transport region.