High efficiency n-type emitter-wrap-through silicon solar cells

Abstract

In the ALBA-II project, Q-Cells SE and ISFH are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralsky method (Cz) forms the basis of this high-efficiency solar cell development as it offers high charge carrier lifetimes. The EWT solar cell concept nevertheless does not impose the same strict requirement onto the bulk material and front surface passivation quality than interdigitated back-contact back-junction (IBC-BJ) high-efficiency cell concepts. Thus, the relaxed front surface passivation requirement of our EWT cells allows us to employ a — compared to IBC-BJ solar cell concepts — relatively simple device structure and process sequence with only two dopant diffusion processes (phosphorus and boron). EWT solar cells are known for very high current collection efficiency. We achieve high open-circuit voltages of our cells by passivating the front and rear boron-diffused p-type emitter by a stack of aluminum oxide and silicon nitride (Al 2 O 3 -SiN). In order not to offset these advantages by current transport losses (such as the VIRE effect) we use relatively low Si wafer resistivities of 1.5 Ωcm and include a POCl 3 -diffusion process for the formation of a back-surface field (BSF). We passivate the BSF by a thermally grown oxide, which gets covered during the subsequent cell process by the Al 2 O 3 -SiN emitter passivation stack. We use pico-second laser ablation for the formation of contact openings through the passivation layers and employ nano-second laser ablation for all other structuring steps, including the aluminum rear contact structuring. With this approach we achieve on our small area (4 cm2) cells a short-circuit current density (J sc ) of 40.4 mA/cm2, an open-circuit voltage (V oc ) of 661 mV, fill factors (FF) well above 80% and thus cell efficiencies exceeding 21%.