We have observed optical second harmonic generation (SHG) from a space charge layer (SCL) in a stacked indium tin oxide (ITO)/molybdenum trioxide (MoO3) /N- N′ -diphenyl- N- N′ -bis(1-naphthly)- 1, 1′ -biphenyl- 4, 4′ -diamine (α -NPD) system. When the MoO3 thicknesses were increased, the SHG signals from this system decreased sharply at smaller MoO3 thicknesses, and were saturated at MoO3 thicknesses larger than 1 nm. These results prove the vital role of SCL in improvement of drive voltages of organic light-emitting diodes.
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