The electrical and electroluminescent properties of Au/phthalocyanine Langmuir-Blodgett film/GaP diodes are reported. The electroluminescence conversion efficiency is shown to depend on the number of Langmuir-Blodgett layers and is a maximum for a 5.6 nm-thick film. This optimum can be explained in terms of simple tunnel injection theory. A preliminary investigation reveals that the devices are relatively stable and that the maximum power conversion efficiency approaches that of an unencapsulated p-n junction diode.