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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% 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) . 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. [Copyright &y& Elsevier]

Abstract: Epitaxial emitters deposited by atmospheric pressure CVD have been studied using different characterization methods such as spreading resistance profiling (SRP) or electrochemical capacitance voltage profiling (ECV). Comparing simulations with measured data, very low minority carrier lifetimes of τeff = 20μs have been determined for FZ-material after a standard epitaxy process. LBIC measurements showed very low and inhomogeneous distributed diffusion lengths in the range of 100-250μm before process optimization. Therefore an emitter epitaxy process working at lower temperatures was developed [1] leading to diffusion lengths of 750μm. The effective minority carrier lifetimes and emitter saturation currents were determined using QSSPC measurements. For samples with an emitter, deposited at optimized temperature, a best value for the effective minority carrier lifetime of τeff = 212μs was measured and an emitter saturation current could be determined to J0e= 46 fA/cm2. In order to investigate the stress in the epitaxial layers μRaman and μPL measurements were performed showing no increased values at the interface. The doping density in boron doped emitters was determined with μPL and in good agreement with measured ECV-profiles. The Shockley-Read-Hall lifetime in the emitter bulk showed no increased values at the interface between substrate and epitaxial layer. [Copyright &y& Elsevier]

Abstract: Crystalline silicon thin film solar cells, based on the epitaxial wafer equivalent, require a reflecting interlayer between substrate and active layers to increase the generated current and reach similar efficiencies as wafer based solar cells. With the epitaxial lateral overgrowth technique, a reflecting dielectric layer can be implemented. In this paper results of solar cells with overgrown, patterned SiO2 films are shown. A beneficial optical effect due to the interlayer and also a reduced effective diffusion length within the epitaxially grown silicon layer are observed. Cells with reflecting interlayer and non-optimized absorber layer thicknesses therefore exhibit lower efficiencies than cells without SiO2. Slightly higher currents are observed with textured front sides. Significantly increased effective diffusion lengths and cell performances can be reached with non-merged active silicon layers. Reasons might be the avoidance of defects generated at merging points and of unpassivated surface areas in voids remaining where two growth fronts merge. [Copyright &y& Elsevier]