Enhancement of optical micro-cavity effect coupled with surface plasmon in an organic light emitting device with nanosized multi-cathode structure

Organic light-emitting devices (OLEDs) are widely recognized as a high quality flat panel displays. The internal quantum efficiency of OLED has approaches to 100% by using phosphorescent materials. However, the external... [ view full abstract ]

Organic light-emitting devices (OLEDs) are widely recognized as a high quality flat panel displays. The internal quantum efficiency of OLED has approaches to 100% by using phosphorescent materials. However, the external quantum efficiency (EQE) remains 20-25% because of a poor light extraction efficiency due to surface plasmon (SP) losses in a metal cathode. We found that the emission efficiency becomes higher by using multi-cathode (MLC) structure. In addition, the color purity in blue, green and red emission has improved by the external micro-cavity effect coupled with SP resonance.

Fig.1 [A] shows the device structure used in this work. The normal structure (a) consists of ITO/PEDOT:PSS/NPB/CBP:color dopant/Bu-PBD/MgAg cathode. MLC structure (b) has a feature of thin film stacked cathode consisting of a semi-transparent MgAg, ITO optical buffer and high reflection Ag. Fig.1 [B] shows optical power density as a function of normalized horizontal wave-vector (k_h/h₀) calculated by near-field optics. In the normal cathode, a strong SP band appears in an evanescent region in k_h/h₀ >1.8. In contrast, it almost disappears and waveguide TM and substrate modes become dominant in the MLC structure. Fig.1 [C] shows a power density distribution obtained by FDTD analysis when a vertical dipole is excited. In the normal cathode, almost all the excitation power disappears in a short lifetime as SP loss in the cathode. However, we can see an intense forward emission as well as waveguide mode in the MLC structure. The optical efficiency has improved by a factor of 1.8. These phenomena can be explained by considering two kinds of SP resonance, long range SP and short range SP, which interact with each other on both sides of very thin MgAg layer.

Fig.2 shows electroluminescent spectra and three primary color coordinates in CIE diagram in the devices with and without MLC structure. Sharp emission band with narrower half-width is an advantage in the MLC structure because of the external micro-cavity effect enhanced by the increase of waveguide mode. Color coordinates of blue, green and red are shown in a table and color gamut has improved greatly.