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

Alloys that hale a lower bandgap than silicon can extend the infrared respo nse of a silicon cell and hence increase the current generation. One group of materials that are compatible with silicon is Si1-xGex alloys as silicon is completely miscible with germanium, One problem associated with this me thod 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 Si1-xGex alloys int o a thin-film multilayer structure where the flexibility offered through co ntrolling the number and location of junctions facilitates the achievement of high collection probabilities even in thin regions of high germanium con centration where the diffusion lengths are extremely short. PC1D (a one-dim ensional circuit simulation package) has been used to simulate the effect o f incorporating a layer of Si1-xGex alloy into the multilayer structure. Re sults show that considerable efficiency enhancement is achieved with this s tructure, especially for high germanium concentration alloys, The whole ran ge of germanium concentrations is explored.