High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells

In the ALBA-II project, Q-Cells SE, Bitterfeld-Wolfen, Germany, and the Institute for Solar Energy Research Hamelin, Emmerthal, Germany, are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralski (Cz) method forms the basis of this high-efficiency solar cell development as it offers high bulk carrier lifetimes. The EWT device structure allows us to employ a simplified process sequence compared with interdigitated back-contact back-junction solar cells. High open-circuit voltages of our solar cells are achieved by different passivation layers for base and emitter surfaces and picosecond laser ablated contact openings. An optimization of the resistances along the current paths in base and emitter leads to an improvement in fill factor (FF) over former EWT solar cells. Together with the inherently high current densities of EWT solar cells, we achieve on our small-area (4-cm², designated area without busbars) cells a short-circuit current density J_SC of 40.4 mA/cm², an open-circuit voltage V_OC of 661 mV, FFs well above 80%, and, thus, cell efficiencies of up to 21.6%.