The electrical properties of a series of Au/Langmuir-Blodgett film/n-GaP metal-insulatorsemiconductor diodes are reported. The fabrication of diodes incorporating varying numbers of monolayers on the same GaP substrate has enabled a direct comparison of device properties with insulator thickness to be made. The devices have been operated both as photovoltaic solar cells and as electroluminescent diodes. The Langmuir-Blodgett film thickness required to optimise the electroluminescence efficiency is found to be 21 nm. This corresponds to the insulator thickness at which the short-circuit photocurrent of the solar cell begins to decrease. Results are discussed in terms of a simple energy band diagram for the metal-insulator-semiconductor structure.