Formation of High Efficiency Epitaxial Emitters by APCVD

Abstract

Improvements in emitter passivation as well as back contact passivation lead to several high efficiency solar cell concepts. PERC (Passivated Emitter Rear Contact) solar cells on monocrystalline wafers above 21% [1] have been achieved, but the diffusion processes with POCL3 and BBr3 have emerged as one of the limiting factors. The formation of high efficiency emitters by APCVD (Atmospheric Pressure Chemical Vapour Deposition) has several advantages compared to the standard diffusion. Epitaxial emitters can be optimised to match the passivation and metallisation of high efficiency concepts as well as industrial approaches like nickel plating. Simulations by PC1D show the potential of epitaxial emitters by featuring low contact resistance in combination with high shunt resistance, a good blue response and low emitter saturation current (J0e) [2,3]. The emitter profiles can be designed in depth and in doping in the range from 1x1017cm-3 up to 1x1020cm-3 for p-type and n-type emitters. Simple reference solar cells with J0e= 46 fA/cm2 and efficiencies of η=17.5% on float zone (FZ) and η=16.1% on multicrystalline (mc) wafers have been processed. Furthermore an epitaxial selective emitter has been developed. The deposition process itself has been enhanced to improve the emitter profiles as well as material quality.