The femtosecond energy diffusion sensor is presented as a non-contact tool for the use in the characterization of thin film hydrogenated amorphous silicon (a-Si:H) photovoltaic cells. The sensor is based on the pump-probe technique and when used with the appropriate models, this non-contact, non-destructive tool is shown to be capable of measuring important material characteristics of each layer of a p-i-n junction including bandgap and density of states. When fully developed, it is believed that the sensor could be used in a factory environment to detect and solve problems rapidly and to maintain control of the entire manufacturing process.