Polymer light-emitting displays with printed cathodes

High-resolution passive-matrix polymer light-emitting displays with printed cathodes are achieved by combining delicate cathode deposition and multifunctional buffer layer fabrication. Unlike other printing methods and printed materials, ink-jetting conducting nano-particles as cathode can provide high-resolution cathode patterns and excellent continuity along the fine cathode lines without any mechanical pressure on the organic layers. The buffer layer between the printed cathode and the organic active layers, was fabricated by mixing the water/alcohol-soluble polymer poly[9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene-alt-2,7-(9,9-dioctylfluorene)] (PFNR2) and a curable epoxy adhesive. It offers the functions of solvent-proof, electron-injection, and proper affinity with the cathode ink. While improving the performance of devices, especially blocking the leakage current greatly, the cross-linked buffer layer also induces a novel phenomenon of ‘linear η-J plot’, which can be derived to interesting and realistic results. Red, green, and blue monochrome and full-color polymer light-emitting displays with a content format of 96 × 3 × 64 show neither dead pixels nor dead lines. Under optimized steps of curing, the nano silver ink forms continuous, defect-free, and low-resistance cathode rows without any distortion. The red, green, and blue displays exhibit the current efficiencies of 0.62, 4.38 and 0.93 cd/A, and CIE color coordinates of (0.63, 0.37), (0.39, 0.57) and (0.18, 0.16), respectively. The cathode printing technique removes the need of high vacuum for thermal evaporation of the cathode metal, which could lead to the industrial roll-to-roll process to manufacture the flat panel displays.