Contact resistivity of n-type amorphous silicon electron contacts in silicon heterojunction solar cells

Silicon heterojunction solar cells have historically suffered from high series resistivities. Yet, until recently, little had been done to understand the main factors behind this behavior. In this work, we present a systematic analysis in order to quantify and characterize the contribution from each layer of a-Si:H(i)/aSi:H(n)/ITO/Ag electron contacts. We attempt to address how the stack performs when its constituent layers are altered, using the transfer length method. Specifically, we demonstrate how the thickness of the a-Si:H layers and the doping of the ITO layers contribute to the overall series resistivity via changes in contact resistivity. From these results, we determine the optimum process conditions to minimize the resistivity of the electron contact, and thus its contribution to fill factor losses. We find that increasing the a-Si:H(i) thickness and the oxygen partial pressure during ITO sputtering leads to an increase in contact resistivity. Specifically, by increasing the a-Si:H(i) layer thickness from 0 to 15 nm the contact resistivity increases from 0.15 to 0.35 Ωcm2 for our standard ITO layer. By increasing the oxygen partial pressure during ITO sputtering from 0.14 to 0.85 mTorr the contact resistivity increases from 0.07 to 0.82 Ωcm2 for a standard a-Si:H(i) layer thickness. On the other hand, increasing the a-Si:H(n) layer thickness has little effect on the contact resistivity, with a constant value of 0.07 Ωcm2 for thicknesses greater than 3 nm.