In organic light-emitting diodes (OLEDs), an efficient charge injection is one of the most important requirements to obtain the high device performance. In this regard, nucleoside cytidine can be used as an efficient electron injection layer in OLEDs because of its low work function and solution processability. In addition, cytidine is soluble in nontoxic water allowing environmental-friendly device fabrication and can be obtained from the nature without the complex synthesis process.
 In this study, we fabricated inverted OLEDs with a cytidine electron injection layer. The current density-voltage-luminance characteristics of OLEDs with various thicknesses of a cytidine layer were measured to find the optimum thickness. The morphology and electronic structure of a cytidine layer were investigated using atomic force microscopy and photoelectron spectroscopy. The origin of the enhancement in device performance was demonstrated based on the measured film properties.