Contact Resistivity of the p-Type Amorphous Silicon Hole Contact in Silicon Heterojunction Solar Cells

In silicon heterojunction solar cells made with high-lifetime wafers, resistive losses in the contacts dominate the total electrical power loss. Moreover, it is widely believed that the hole contact stack—a-Si:H(i)/a-Si:H(p)/ITO/Ag—is responsible for more of this power loss than the electron contact stack. In this article, we vary the a-Si:H(i) layer thickness, the a-Si:H(p) layer thickness and doping, and the indium tin oxide (ITO) doping, and determine the effect of each variation on the contact resistivity of the hole contact stack. In addition, we make complete solar cells with the same variations and correlate their series resistivity to the hole contact resistivity. We find that the contact resistivity is most sensitive to the thickness of the a-Si:H(i) layer and the oxygen partial pressure during ITO sputtering. Increasing the former from 4 to 16 nm results in a fourfold increase in contact resistivity, whereas increasing the latter from 0.14 to 0.85 mTorr raises the contact resistivity almost 30-fold. Optimized conditions produce a contact resistivity of 0.10 Ωcm2, while maintaining an implied open-circuit voltage of 720 mV measured on cell precursors, which is the lowest contact resistivity value reported in the literature for an a-Si:H hole contact.