Thickness dependent emission color of organic white light-emitting devices

White organic light emitting devices (WOLEDs) with an RBG stacked multilayer structure were demonstrated. In RGB stacked OLEDs, blue emitting, 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN) doped with p-bis(p-N,N-diphenyl-amono-styryl)benzene (DSA-Ph), green emitting, tris-(8-hydroxyquinoline)aluminum (Alq) doped with C545, and red emitting, tris-[8-hydroxyquinoline]aluminum (Alq) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)- 4H-pyran (DCJTB), were used. By adjusting the order and thickness of emitting layer in RBG structure, we got a white OLED with current efficiency of 5.60 cd/A and Commission Internationale De L’Eclairage (CIE) coordinates of (0.34, 0.34) at 200 mA/cm². Its maximum luminance was 20,700 cd/m² at current density of 400 mA/cm². The results have been explained on the basis of the theory of excitons generation and diffusion. According to the theory of excitons generation and diffusion, an equation has been set up which relates EL spectra to the thickness of every layer and to the exciton diffusion length.

This article provides a brief overview of the recent progress in white organic light emitting diodes (WOLEDs) that are considered to be the low-cost alternatives for solid-state illumination purposes. The article concentrates on various device designs and the underlying physical processes that govern the white-light creation. The operating principles and the problems associated with each device design are discussed. The emphasis is on identifying various approaches to achieve WOLEDs and their review based on publications appearing in the last two years.