A low drive voltage, transparent, metal-free n-i-p electrophosphorescent light emitting diode

We demonstrate a transparent, inverted, electrophosphorescent n-i-p organic light emitting diode (OLED) exhibiting a luminance of 500 cd/m(2) at 3.1 V, and with a luminous power efficiency of 23 1m/W when light emitted from both top and bottom surfaces is summed. We find that 10% more light is emitted from the top surface; hence a power efficiency of 121m/W is obtained for a device viewed through the top, transparent contact. This device, with applications to head-up and displays employing n-type Si driver circuitry, has significantly higher power efficiency and lower drive voltage than undoped fluorescent inverted OLEDs. Efficient injection of both electrons and holes is made possible by controlled n- and p-doping of the transport layers with high doping levels. The light emitting region is protected from ITO sputtering damage by a 210 nm thick p-doped hole transport layer. The transparency of the device at the peak OLED emission wavelength of 510 nm is (80 +/- 5)%. (C) 2003 Elsevier Science B.V. All rights reserved.