Laminated OLEDs Avoid Vacuum Processing
Investigators at GE Global Research in Niskayuna, N.Y., may have found a way to make OLEDs practical for general lighting applications. They have demonstrated a simple roll-to-roll lamination process that could be used to fabricate an OLED-based diffuse light source at a cost well below what is currently possible. Ultimately, when combined with improvements in efficiency and lifetime at high brightness, the technique could enable OLEDs to surpass the performance of fluorescent lighting.
Key to the cost savings of the method is the removal of vacuum processing steps that have been used in high-performance OLED manufacturing to form electrodes and to boost performance. Previous fabrication techniques coated an organic layer and a top electrode onto a conducting substrate. The new process, in contrast, coats a transparent conductive substrate with an organic layer and a conductive substrate with another. The layers are then laminated together to form a final device. The method works best when the conducting layers are flexible.
In their demonstration, the researchers used a 175-μm-thick plastic film coated with a 120-nm-thick layer of indium tin oxide for an anode. For a cathode, they used a 100-nm-thick layer of aluminum coated on a plastic film. They spin-coated the hole-injection layer on the anode and the light-emitting polymer on the cathode, then laminated the two using a standard roll laminator. They compared the performance of the resulting device with one fabricated using vacuum processing.
They found that the laminated device had superior performance, operating at higher brightness and power efficiency than the vacuum-processed device. This indicated good electrical contact between the organic semiconductors, despite the fact that it was possible to pull the two apart by hand.
High-performance OLEDs either have a low work function cathode or an N-doped organic semiconductor electron-injection layer adjacent to the device cathode to enhance electron injection. Such techniques have required vacuum deposition. The new device demonstrates that it is possible to achieve similar performance by producing a metal-reduced conjugated polymer compatible with the lamination process.
The researchers note that further work must be done, particularly in the area of improving adhesion at the lamination interface, but that the process could be what is needed to produce high-performance OLEDs at low cost.
Applied Physics Letters, May 29, 2006, 223509.