Modeling and optimization of thin-film devices with Si1-xGex alloys

Alloys that have a lower bandgap than silicon can extend the infrared response of a silicon cell and hence increase the current generation. One group of materials that are compatible with silicon is Si_1-xGe_x alloys as silicon is completely miscible with germanium. One problem associated with this method is that, because the introduced material has a lower bandgap, it will therefore also cause the device to suffer a loss in voltage. Most research to date has focused on single-junction bulk devices and shows that the loss in voltage overrides the increase in current except for very low germanium content alloys. This work looks at incorporating these Si_1-xGe_x alloys into a thin-film multilayer structure where the flexibility offered through controlling the number and location of junctions facilitates the achievement of high collection probabilities even in thin regions of high germanium concentration where the diffusion lengths are extremely short. PC1D (a one-dimensional circuit simulation package) has been used to simulate the effect of incorporating a layer of Si_1-xGe_x alloy into the multilayer structure. Results show that considerable efficiency enhancement is achieved with this structure, especially for high germanium concentration alloys. The whole range of germanium concentrations is explored