Effect of defect-rich epitaxy on crystalline silicon / amorphous silicon heterojunction solar cells and the use of low-mobility layers to improve performance
We present two-dimensional device physics simulations of amorphous silicon / crystalline silicon heterojunction solar cells to explain the effects of full and localized epitaxial layers, sometimes observed in the early stages of amorphous Si deposition, on cell performance. Minimizing the defect density, thickness, and wafer area fraction covered by the epitaxial region are shown to be important factors for maximizing cell open circuit voltage. We find that localized defect-rich epitaxial patches covering small percentages of the wafer surface (~5%) can cause significant reduction in open circuit voltage, which is explained by considering lateral carrier flow in the device. We also show that a thin layer of low-mobility material, such as microcrystalline silicon, included between the wafer and amorphous regions can impede lateral carrier flow and improve conversion efficiencies in cases where isolated defective pinholes limit device performance.