Heterojunction Silicon Wafer Solar Cells using Amorphous Silicon Suboxides for Interface Passivation

Abstract

Current industrial monocrystalline silicon wafer solar cells based on screen-printing technology for contact formation and selective emitter have an efficiency potential of around 20%. Heterojunction silicon wafer solar cells combine plasma-deposited amorphous silicon thin films and n-type crystalline silicon wafers to industrially viable high-efficiency solar cell devices. The excellent surface passivation provided by an intrinsic amorphous silicon interlayer leads to a very high open-circuit voltage, enabling industrial heterojunction cells with efficiencies of over 23%, as demonstrated by the Japanese company Sanyo. The key point of these structures is the removal of the highly recombination-active solar cell contacts from the crystalline surface, by insertion of a thin film with a wide bandgap. To reach the full device potential, the defect density at the hetero-interface must be minimised. Commonly used hydrogenated amorphous silicon films of only a few nanometer thickness are appealing candidates for this. Their bandgap (∼1.7eV) is much larger than that of c-Si and, when intrinsic, such films can reduce the c-Si surface state density by hydrogenation. Additional oxygen built into the amorphous network results in amorphous silicon suboxides, which in turn leads to an improved surface passivation. With this approach we are able to achieve implied voltages of 740mV and an optical bandgap of about 2eV, which improves the cell's blue response. In addition, microcrystalline silicon emitter and back-surface-field layers provide enhanced doping efficiency, enabling the fabrication of contacts with low saturation current density values. This new approach has already demonstrated promising cell efficiencies of about 21%. This paper summarises the recent progress in heterojunction silicon wafer solar cell research using amorphous silicon suboxides.