INCREASED WORK FUNCTION DIFFERENCE BETWEEN METAL AND SILICON IN Al/SiO_x/p-Si MIS TUNNEL CONTACTS

摘要

A promising alternative to diffused emitters on silicon solar cells is the concept of metal-insulator-semiconductor (MIS) contacted inversion-layer (MIS-IL) solar cells. The limitations of that concept are mainly given by the elevated saturation current density (J_0) of the MIS-contact compared to pn-junction contacts. J_0 depends on the band bending (Ψ_s) of the MIS-contact, which can be increased by using low work function (Φ_m) contact materials. Since metals with low Φ_m like Mg react with the tunnel oxide and thus degrade the MIS-contact, less reactive materials must be used. A possible solution is to introduce CsCl between the tunnel-oxide and the Al-contact by immersion in a CsCl containing liquid before the deposition of Al. This already results in a remarkable reduction of J_0 compared to the standard MIS-contact. A further reduction of J_0 can be achieved by evaporation of a CsCl-layer with a thickness of ～ 5 nm as described in this work. The open circuit voltage of planar MIS-IL solar cells could be increased from 624 mV for conventionally contacted cells to 635 mV for cells immersed in CsCl-solution and to 648 mV for cells with additionally evaporated CsCl-layer.