Comparative Study on Manufacturability of Selective Emitter and Double Printing on Mono-Si PV Cells
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Comparative Study on Manufacturability of Selective Emitter and Double Printing on Mono-Si PV Cells
Y. Liu1, F.D. Jiang1, S. Wang1, Y. Zeng1, and W. Shan1,2 1 2
JA Solar Holdings Co. Ltd., Shanghai, 200436, China JA Solar USA, Inc. Milpitas, CA 95035, USA
ABSTRACT Selective emitter structure has long been regarded as a good and relatively simple approach to improve the energy conversion efficiency of Si wafer-based single-junction photovoltaic (PV) cells. Recently emerged double printing method, on the other hand, potentially has the capability of improving the efficiency with no requirement for device structure modification. The manufacturability of these two approaches has been studied on a mass-production platform at JA Solar recently with large scale sampling. The experimental results collected from over two hundred thousand cells demonstrated that both approaches are capable of achieving significant conversion-efficiency gain in a cost-effective way with high yield rate on the PV industry commonly used mass production platform currently adopted by the vast majority of cell manufacturers. INTRODUCTION There are several technological approaches to reach high conversion efficiency for wafer based silicon PV cells. Among them, selective emitter (SE) structure is the one that can be relatively easier to implement on the platform of currently well-accepted and widely used screenprinting cell production lines, as compared to the other sophisticated cell structures including implementation of rear-side point contacts [1], replacement of homojunctions by heterojunctions [2], as well as placement of both solar cell contacts at the rear side of a solar cell [3]. SE structure, in principle, only needs to add one more diffusion step with oxide mask to form a low contact resistance of heavy doping underneath metal grid and a lightly doped region between the grid to improve front-surface passivation and reduce recombination therefore a better spectral response at shorter wavelength region of the spectrum of solar irradiation [4]. However, the technical complexity involved in forming selective emitter using conventional twostep diffusion process and the difficulty in precisely screen printing metal grid onto heavily doped contact pattern, along with its associated high cost and low yield rate during manufacturing, make mass production of screen printed SE cells nearly inhibitive in the past. A few variety of SE structure manufacturing methods including emitter etch-back [5], laser doping followed by electroplating [6,7], and ion implantation followed by thermal annealing process [8], etc. have also been developed to make SE cells more cost-effectively in the last a couple of years. Among them, silicon ink assisted diffusion is a very different technical approach to make PV cells based on SE structure. This new approach radically simplifies the process of forming selective emitter and greatly eases the painful alignment problem during the process of screen printing contact fingers, therefore paves the way for mass production
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