| Before photovoltaics can compete with traditional sources of grid-connected power, the cost per Watt must come down. Toward this end, the Femtosecond Energy Diffusion (FED) sensor has been developed to improve the manufacturing process for thin film hydrogenated amorphous silicon (a-Si:H) solar cells. This may lead to both lower fabrication costs and increased cell efficiencies.; The FED sensor is a non-contact, non-destructive device based on the pump-probe technique that can measure many important properties of amorphous silicon. It is used in conjunction with two newly developed computer models to measure the properties of the individual layers of an a-Si:H solar cell. These properties include the bandgap, the doping level, the alloying level, optical absorption, and a simplified density of states.; In developing the sensor, a number of novel transient transmission experiments have been conducted. These experiments are among the first reported where the photon energies of both the pump and probe lasers fall within the bandtail energy region of a-Si:H. In addition to intrinsic a-Si:H, pump-probe experiments have been conducted on samples alloyed with germanium and carbon as well as samples doped with phosphorus and boron. Tests have been performed on samples with differing levels of hydrogenation created in two ways. Experiments performed on three samples comprising the build up of an a-Si:H p-i-n diode solar cell demonstrate the ability of the sensor to determine the properties of each new layer as it is deposited. This makes the FED sensor a potentially powerful tool for use in the manufacturing of a-Si:H solar cells.; Two new models have been developed in order to explain the experimental results. The absorption model calculates absorptivity by combining thermally-induced bandgap reduction due to carrier thermalization and recombination with carrier trapping in order to fit the data obtained by the FED sensor. A second model, the multi-layer model, combines experimental data with the output of the absorption model to find the properties of individual layers within multi-layer a-Si:H systems. The data provided by these two models is used in an existing device simulation program to predict the operating characteristics of complete solar cells based on the measured properties of the individual layers. |
