Development of large area transparent conducting oxides from a combinatorial lead for organic solid state lighting

Organic light emitting devices (OLEDs) are projected to provide a low-cost, long-lived, and efficient wide area lighting solution if challenges in reliability, cost, and efficiency can be overcome. Development of new transparent conducting oxides (TCOs) that do not contain indium for use as the anode in bottom-emitting OLEDs can lead to cost savings and provide longer device lifetimes. Indium-free TCOs need to meet or exceed performance targets including high conductivity and visible light transmission, acceptable stability and, for blue or white OLEDs, a high work function to match the deep HOMO of the hole transport material. In this work, we report results from our efforts to scale up sputter deposition on large area substrates (up to hundreds of cm2) of a Ga-doped ZnO TCO having a composition identified using combinatorial methods. We present the results of initial scale-up efforts and evaluate relevant properties for these films. Finally, we have incorporated these materials in the production of OLEDs, and show performance comparisons between devices fabricated on the scaled-up GZO and commercial indium tin oxide (ITO). The results demonstrate that we are able to generate substrates with the appropriate work function to reduce the operating voltage of blue phosphorescent OLEDs compared to commercial ITO. This work function-HOMO energy matching leads to more efficient charge injection into the device hole transport layer.