Highly efficient solution-processed white organic light-emitting diodes based on novel copolymer single layer
We fabricate by solution processed methods organic light emitting diodes with single-layer structure (ITO/(PEDOT:PSS)/co-polymer/Ba/Al) and study the transport properties of the final devices. The copolymer is novel poly(fluorene-alt-phenylene) (PFP) derivatives containing co-monomers, involving red-emitting 1,8-naphthalimide units as pendant groups (0.0005, 0.005, 0.02 and 0.08 wt%) covalently attached. All the devices exhibited emission at very low driving currents in the A range (47¿73 A). White emission with luminous efficiency of 9.42 Cd/A at 50 A is obtained for the co-polymer with the smallest amount of chromophore. Commission Internationale de L¿Eclairage (CIE) coordinates evolve from almost pure white color (0.26, 0.30) for low currents to stable cool white (0.21, 0.23). Increasing naphthalimide contents leads to stable green and orange emission with 3.07, 19.5 and 6.7 Cd/A efficiencies. The current-voltage response of the devices is analyzed by means of a numerical model that includes an injection mechanism based in the microscopic hopping theory and a field-dependent carrier mobility for the bulk transport regime. The fitting results allow to estimate the dependence of carrier mobility on polymer composition in the diodes.
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