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

Pradyumna Muralidharan, Mehdi Ashling Leilaeioun, William Weigand, Zachary C. Holman, Stephen M. Goodnick, Dragica Vasileska

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

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 Ωcm2, 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.

Original languageEnglish (US)
Article number8936950
Pages (from-to)363-371
Number of pages9
JournalIEEE Journal of Photovoltaics
Volume10
Issue number2
DOIs
StatePublished - Mar 2020

Keywords

  • Amorphous semiconductors
  • Contact resistance
  • Heterojunctions
  • Silicon
  • Simulation

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Electrical and Electronic Engineering

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