Solar Cells with Epitaxial or Gas Phase Diffused Emitters Above 21% Efficiency

Abstract

The formation of high efficiency epitaxial emitters by atmospheric pressure chemical vapour deposition (APCVD) for wafer based solar cells as well as thin film solar cells has several advantages compared to the standard diffusion by tube furnace. These advantages include a short process time of only 2-7 min, no need for wet chemical etching after the deposition and an adjustable emitter profile, i.e. doping and depthcan be adjusted independently. Simulations by PC1D show the potential of epitaxial emitters featuring low contact resistance in combination with high shunt resistance, a good blue response and low j0e values[1]. The emitter profiles can be designed in the range from 1x1017 cm-3 up to 1x1020 cm-3 for p-type and n-type emitters. In this work the fabrication process and the epitaxial emitter profiles have been adjusted to improve layer quality and to match the passivation and metallisation of high-efficiency concepts. These optimisations resulted in solar cells with an efficiency of 21.0% and an open-circuit voltage of 668 mV, a short circuit current of 39.2mA/cm2 and a fill factor of 80% for n-type solar cells with epitaxial boron emitter. Additionally, a selective deposition process has been developed which allows the structuring of the epitaxial emitter.Apart from the epitaxial approach an emitter formation process based on gas phase diffusion[2]using diborane as dopant gas in the APCVD reactor has been developed. This process includes most of the advantages of the epitaxial emitter, i.e. no silicate glass growth anda single side process.N-type PERL typesolar cells featuring this emitter result in an efficiency of 21.7% with an open-circuit voltage of 675 mV, a short-circuit current of 40.8 mA/cm2 and a fill factor of 78.7%. The realisation in industrial scaleis in principle possible by using the ProConCVD in-line reactor developed at Fraunhofer ISE [3]