Under the auspices of the CombOLED project, CEA-LETI (a French Research Laboratory) has developed an efficient white OLED with overall transparency, equivalent to 65 to 70% over the whole visible spectrum (400 - 750nm) depending on the semi-transparent cathode film thickness.
The diodes consist of an organic semiconductor thin film stack on 5 x 5 cm2 active emitting area, which is deposited using thermal evaporation of small molecules. These ultra thin layers encapsulate the diodes in order to protect the underneath OLED from moisture and oxygen.
The transparent OLED's white luminance, simultaneously reaches emissions of 1000Cd/m² at 4 and 4.6 volts from the bottom and top, respectively. The white OLED efficacy is 11Lm/W and 3Lm/W respectively at 1000Cd/m² for bottom and top emission, which leads to an overall efficiency of 14Lm/W.
Although the OLED structure is optimized when the colour coordinates are identical for both sides, the luminance efficiency for each side is slightly reduced to 5Lm/W. This leads to an overall efficiency of 10Lm/W for a 65% transparent white OLED.
CEA-LETI plan to improve the luminance efficiency and the permeation properties of the thin encapsulation layers.