Metal-Free All-Organic Polymer Light-Emitting Devices

Although they are classified as “organic electronic” devices, polymer organic light-emitting devices (OLEDs) have long required at least one, usually two, inorganic metal electrodes to operate. Processing these inorganic materials usually does not fit the low-cost, roll-to-roll printing-based manufacturing model proposed for OLEDs. Furthermore, OLEDs typically require the use of Indium-Tin-Oxide (ITO) as a transparent electrode, the Indium in which is predicted to become dramatically more expensive as demand for the material increases with the expanding use of electronic displays.

We have demonstrated a metal-free light-emitting device based on a light-emitting polymer (LEP) sandwiched between a transparent graphene cathode and a transparent poly(3,4,ethylenedioxythiophene) / poly(styrenesulfonate) (PEDOT/PSS) anode. The LEP layer contains poly(ethyleneoxide) and mobile ions, which allows electrochemical doping of the LEP and drastically changes the device's operation compared to a traditional polymer OLED. Graphene can be used as the cathode in this device, called a light-emitting electrochemical cell (LEC), because the mobile ions and the doped polymer make the device relatively insensitive to the work function of the electrodes. Furthermore, the device turns on at an applied potential only slightly larger than the LEPs, regardless of the thickness of the LEP layer, while maintaining power and current efficiencies comparable with other polymer OLEDs.

Others have demonstrated that graphene can be used to replace the ITO anode in traditional polymer OLEDs, and graphene can be used as the anode in an LEC as well. However, only LECs are shown to work without any inorganic metal electrode. We see the development of metal-free light-emitting devices like this one as a clear path for the development of efficient and easily-recycled electronics.

P. Matyba, H. Yamaguchi, G. Eda, M. Chhowalla, L. Edman and N. D. Robinson, Graphene and Mobile Ions: The Key to All-Plastic, Solution-Processed Light-Emitting Devices. ACS Nano 4, pp. 637-642 (2010)