Contact resistivity is a crucial parameter in continuing to drive the improvement of solar cell device performance. This is especially true in tandem cells where such resistive losses can manifest themselves in both top and bottom cells. Currently, crystalline silicon has emerged as the primary bottom cell used in most tandem configurations due to its high performance, low cost, and reliability. An established method for measuring the resitivity associated with contacts to silicon is the transmission line measurement (TLM) technique. In this work, TLM structures were fabricated to ascertain the properties of two important contacts to silicon bottom cells – intrinsic hydgrodenated amorphous silicon (a-Si:H(i))/ p-type hydrogenated amorphous silicion (a-Si:H(p)/indium tin oxide (ITO)/silver (Ag) stack (a-Si:H(p)/ITO/Ag) and a-Si:H(i,p)/aluminum. a-Si:H(p)/ITO/Ag contacts are commonly used silicon heterojunction solar cells
For the a-Si:H(i,p)/ITO/Ag stack we vary the a-Si:H(i) layer thickness, a-Si:H(p) doping and thickness, and oxygen partial pressure during ITO sputtering. By increasing the a-Si:H(i) thickness from 4 to 16 nm there is a marked increase in contact resistivity from 0.48 to 1.91 Ωcm2. Similarly, we find that by increasing the oxygen partial pressure from 0.14 to 0.85 mTorr the contact resistivity increases from 0.1 to 2.75 Ωcm2. The doping of the a-Si:H(p) show a unique trend in that increasing the trimethylborane flow from 9 to 18 sccm during PECVD results in an initial decrease in contact resitivity from 0.26 to 0.24 Ωcm2 while further increases up to 100 sccm results in contact resistivity values up to 1.24 Ωcm2. Interestingly, the contact resistivity does not vary for a-Si:H(p) layer thicknesses greater than 3 nm. The contact resistivity remains constant at 0.26 Ωcm2. For a thinner layer of 3 nm the contact resistivity is 0.99 Ωcm2.
The doped amorphous silicon thickness was also varied in the aluminum contacts and annealed between 150-240 °C. It was found that contact resistivities as low as 1x10-3 and 4x10-3 Ωxcm2 could be obtained for n- and p-type samples respectively. However, it was also observed that annealing at high temperatures counter-dopes the n-type samples leading to the formation of a rectifying junction and prohibitively high resistivity values. The same high thermal loads in the p-type samples exhibited a large decrease in contact resistivity due to the aluminum spiking to the wafer and thus making direct contact at the expense of passivation, which is ultimately undesired for solar cells. Thus, these studies have shown the promise of such contacts for application in silicon bottom cells and provided insight into the window of process parameters for their fabrication. Such techniques will continue to be used for parameter/process optimization and extended to other contact structures.