1.  Field
One or more embodiments of the present invention relate to organic light-emitting devices. 
2.  Description of the Related Art
Organic light-emitting devices (OLEDs) are self-emitting devices that can provide multicolored images and have desired characteristics such as wide viewing angles, excellent contrast, quick response time, excellent brightness, low driving voltage, and excellent response speed. 
An OLED has a structure including a first electrode disposed on a substrate, and a hole transport region, an emission layer (EML), an electron transport region, and a second electrode sequentially formed on the first electrode.  Holes injected from the first electrode move to the EML via the hole transport region, and electrons injected from the second electrode move to the EML via the electron transport region.  Thus, excitons are generated when carriers, such as holes and electrons, recombine in the EML.  When the excitons drop from an excited state to a ground state, light is emitted.