White Organic Light emitting diodes based On exciplex states by using a new carbazole derivative as single emitter Layer

In this work we report the fabrication of organic light-emitting diodes based on the carbazole derivative 1,4-bis(9H-carbazol-9-yl)phenyl)ethynyl)benzene, showing excellent CIE coordinates (0.31, 0.33). To achieve this, we developed an initial architecture where the emissive layer was sandwiched between ITO/PEDOT:PSS, that acted as anode/hole transport layer, and Ca/Ag as cathode. The electroluminescence for this initial architecture was similar to the photoluminescence of the carbazole derivative, showing blue emission with CIE coordinates (0.20, 0.22). Subsequently, when the thickness of the PEDOT:PSS layer was reduced from 60 nm to 30 nm, the electroluminescence changed from blue to greenish, with new CIE coordinates (0.26, 0.45). We attributed this effect to intermolecular interactions producing the observed emission from excitons within the emissive layer and exciplexes at the interface of the hole transport and the emissive layer. Based on these preliminary results, a second architecture using 15 nm of 4,7-diphenyl-1,10-phenanthroline as the electron transport layer (ETL) was developed. In this manner, the observed electroluminescence spectrum became wider and the devices showed white emission, with outstanding CIE coordinates (0.31, 0.33). For this architecture, new exciplexes generated at the interface between emissive and electron transport layer emitted at longer wavelengths giving the complementary emission for white light. The generation of exciplex states at the two interfaces of the emitting layer made possible the fabrication of white light organic diodes with the simplest and more commonly used OLED architecture. Interestingly, by using an ETL in the second architecture, and by changing the thickness of the hole transport layer from 60 to 30 nm, it was possible to further fine-tune the white light from cold to warm emission.