Optical and photoelectrical applications of Langmuir Blodgett films
The Langmuir-Blodgett (LB) technique is well known for the deposition of thin, uniform films of precisely defined thickness. These films can he used in electronic devices as an active layer, or in a passive role as insulators. Results are presented of investigations using films of both types. Novel LB films of a merocyanine dye have been developed for use as a spectral sensitiser, and films of phthalocyanine used as insulators for the enhancement of the electroluminescent (EL) properties of a metal-semiconductor structure. The merocyanine dye used has none of the clear amphiphilic molecular substitutions associated with classical LB film materials, but behaves well as a Langmuir film. Incorporated into a matrix of cadmium arachidate the dye can be deposited onto various substrates using the LB technique. The optical properties of such deposited films are characterised and coipared with those of a merocyanine dye featuring anphiphilic substitutions. For both dyes these properties indicate that good quality LB films have been deposited, and that some dimeric structure is present in these films. The feasibility of using these films for the spectral sensitisation of semiconductors is demonstrated by their modifying effect on the photoconductive response of polycrystalline zinc sulphide. The incorporation of LB films of phthalocyanine into the gold/ZnSeS system enables low voltage forward bias d.c EL to be observed with a threshold ~1V. Measurement of the increase in intensity of emitted EL with bias suggests that the presence of the insulating layer increases the minority current available for radiative recombination by supporting realignment of energy bands in the metal and semiconductor. EL efficiency of devices is shown to increase with insulator thickness up to ten deposited layers of phthalocyanine a thickness of ~23nm. A simple energy band model for the system is proposed which takes into account series resistance effects.