A GaAs/AlGaAs structure coated with molecules was investigated utilizing microwave modulated photoluminescence (MMPL) spectroscopy. This method enabled to resolve the photoluminescence (PL) band into localized and de-localized excitons. The results indicated a limited in-plan mobility of the excitons, due to electrostatic fluctuations or band bending induced by the adsorbed molecules. The reduction of exciton mobility is correlated with the reduction in the photocurrent observed in former studies. The present study provides an insight into the mechanism which couples molecules with a semiconductor structure and explains its operation as a molecular controlled semiconductor resistor (MOCSER), found to be a sensitive actuator in sensor applications.