Understanding Transport in Hole Contacts of Silicon Heterojunction Solar Cells by Simulating TLM Structures

Abstract

Silicon heterojunction (SHJ) solar cell device structures use carrier-selective contacts that enable efficient collection of majority carriers while impeding the collection of minority carriers. However, these contacts can also be a source of resistive losses that degrade the performance of the solar cell. In this article, we evaluate the performance of the carrier-selective hole contact- hydrogenated amorphous silicon (a-Si:H)(i)/a-Si:H(p)/indium tin oxide (ITO)/Ag-by simulating transport in SHJ solar cell transfer length method structures. We study contact resistivity behavior by varying the a-Si:H(i) layer thickness, ITO(n±) and a-Si:H(p) layer doping, temperature, and interface defect density at the a-Si:H(i)/ crystalline silicon (c-Si) interface. In particular, we consider the effect of ITO/a-Si:H(p) and the a-Si:H(i)/c-Si heterointerfaces on contact resistivity as they play a crucial role in modulating transport through the hole contact structure. Transport models such as band-to-band tunneling, and thermionic emission models were added to describe transport across the heterointerfaces. Until now, most simulation studies have treated the ITO as a Schottky contact; in this article, we treat the ITO as an n-type semiconductor. Our simulations match well with corresponding experiments conducted to determine contact resistivity. As the a-Si:H(i) layer thickness is increased from 4 to 16 nm, the simulated contact resistivity increases from 0.50 to 2.1 Ωcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which deviates a maximum of 8% from the experimental measurements. It should be noted that we calculate the contact resistivity for the entire hole contact stack, which takes into account transport across the a-Si:H(p)/c-Si and ITO/a-Si:H(p) heterointerface. Corresponding experiments on cell structures showed a fill factor degradation from 77% to 70%. Our simulations indicate that a highly doped n-type ITO layer facilitates tunneling at the ITO/a-Si:H(p) heterointerface, which leads to low contact resistivities.