Recombination Zone by Mixed-Source Evaporation in Organic Light Emitting Diodes with Graded Mixed-Layer Structures

Thermal evaporation of a mixed-source of electron and hole-transport materials at appropriate heating rates, namely, tris-(8-hydroxyquinoline)aluminum(3)(Alq3) and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-diphenyl-4,4-diamine (TPD) can produce a graded, mixed-layer structure (GM). This enables GM-OLEDs [1,2] to be fabricated on ITO-coated glass with sheet resistivity of 20 Ω/sq and optical transmittance of 90%. Although I-V characteristic of GM-OLEDs may not be significantly modified by a single mixed layer, as compared to the conventional bi-layer heterojunction (HJ) OLED, the brightness from the GM-OLEDs can be increased by a factor of 3. The improvement in GM-OLEDs can be largely explained by the broadening of its recombination zone, in which the spatial distributions of TPD and Alq3 depend on the external heating rate, internal individual evaporation rate, and the mixing ratio. The spatial distributions of Alq3 and TPD in GM-layers are obtained by depth-profiling of X-ray Photoelectron Spectroscopy. The optimum layer structure for GM-OLEDs is discussed qualitatively in term of charge balance and electrode quenching.