Development of high-efficiency large-area screen-printed solar cells on direct kerfless epitaxially grown monocrystalline Si wafer and structure

This paper demonstrates the potential of epitaxially grown Si wafers with doped layers for high-efficiency solar cells. Boron-doped 239 cm2 180–200 µm thick 2 Ω-cm wafers were grown with and without 15 µm thick p+ layer, with a doping in the range of 1017~1018 cm−3. A layer transfer process involving porous Si layer to lift off epi-Si wafers from the reusable substrate was used. The pp+ wafers were converted into n+pp+ passivated emitter rear totally diffused (PERT) cells by forming an oxide-passivated POCl3-diffused n+ emitter at the front, and oxide/nitride-passivated epitaxially grown p+ BSF at the entire back, with local screen-printed contacts. To demonstrate and quantify the benefit of the epi-grown p+ layer, standard passivated emitter and rear cells (PERCs) with local BSF and contacts were also fabricated on p-type epi-Si wafers as well on commercial-grade Cz wafers. Sentaurus 2D device model was used to assess the impact of the epi-grown p+ layer, which showed an efficiency gain of ~0.5% for this PERT structure over the traditional PERC. This was validated by the cell results, which showed an efficiency of ~20.1% for the PERC, and ~20.3% for the PERT cell using epi-Si wafers. Experimental data showed higher FF in PERT cells, largely because of the decrease in lateral resistance on the rear side. Efficiency gain, a result of higher FF, was greater than the recombination loss in the p+ layer because of the lightly doped thick p+ epi-grown region used in this study.