ITO-on-top organic light-emitting devices: a correlated study of opto-electronic and structural characteristics

This paper reports on a complementary investigation of the structural and opto-electronic characteristics of top-emitting organic light-emitting diodes (OLEDs). The key point of such a procedure is the deposition of the indium tin oxide (ITO) transparent top electrode onto the organic films structure. This was achieved by RF sputtering under soft enough conditions so as to avoid any deterioration of the underlying organic layers (room temperature deposition, low plasma power density and minimal bombardment effect) as much as possible. The electrical and optical properties of ITO as a function of the deposition conditions are described first. Hall effect measurements show that ITO films can be grown at room temperature with a high mobility (40 cm(2) V-1 s(-1)) and a carrier density exceeding 5 x 10(20) cm(-3). The surface roughness as a function of the plasma conditions was determined by AFM and can be as low as 1.2 nm. Hole only silicon/Al/poly(N-vinylcarbazole) (PVK)/poly(3,4)ethylenedioxythiophene (PEDOT)/sputtered ITO diodes were processed and characterized both structurally (high resolution TEM in cross section) and electrically. An excellent agreement between the interfacial ITO/PEDOT structure and the hole injection performance was evidenced. Glass/ITO/PEDOT/PVK/Alq/BCP/ITO top-emitting OLED were processed and characterized.