Roll-to-roll fabrication of highly transparent Ca:Ag top-electrode towards flexible large-area OLED lighting application

Transparency and bendability are some of the most attractive unique selling points for organic light-emitting diode (OLED) lighting, enabling a variety of new application possibilities for industrial and private end-users. The roll-to-roll (R2R) OLED fabrication based on thermal evaporation offers a fast, continuous-running solution for producing flexible OLEDs at a lower cost. In this work, R2R fabricated large-area highly transparent thermal co-evaporated Ca:Ag (2:1 vol.%) top-electrode with mean transmittance of 64% over the visible range and a corresponding sheet resistance of as low as 21 omega sq(-1) was achieved. By increasing the film thickness to 24 nm, the sheet resistance can be further reduced to 12 omega sq(-1), albeit with a lower mean transmittance of 58%. The Ca:Ag electrode demonstrates a high mechanical durability through 1000 bending cycles, which is highly desirable as a characteristic for flexible electronics, as well as for the R2R fabrication. Finally, the stability of the Ca:Ag electrode in vacuum and ambient was studied as well.