Understanding the charge transport mechanisms through ultrathin SiO_x layers in passivated contacts for high-efficiency silicon solar cells

Journal Article · 26 February 2019 · Applied Physics Letters

DOI: https://doi.org/10.1063/1.5081832 · OSTI ID:1502791

Kale, Abhijit S. ^[1]; Nemeth, William ^[2]; Guthrey, Harvey ^[2]; Kennedy, Ellis ^[3]; Norman, Andrew G. ^[2]; Page, Matthew ^[2]; Al-Jassim, Mowafak ^[2]; Young, David L. ^[2]; Agarwal, Sumit ^[4]; Stradins, Paul ^[2]

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1. Colorado School of Mines, Golden, CO (United States); National Renewable Energy Lab. (NREL), Golden, CO (United States)
2. National Renewable Energy Lab. (NREL), Golden, CO (United States)
3. Univ. of California, Berkeley, CA (United States)
4. Colorado School of Mines, Golden, CO (United States)

We report on the microscopic structure of the SiO_x layer and the transport mechanism in polycrystalline Si (poly-Si) passivated contacts, which enable high-efficiency crystalline Si (c-Si) solar cells. Using electron beam induced current (EBIC) measurements, we accurately map nanoscale conduction-enabling pinholes in 2.2 nm thick SiOx layers in a poly-Si/SiO_x/c-Si stack. These conduction enabling pinholes appear as bright spots in EBIC maps due to carrier transport and collection limitations introduced by the insulating 2.2 nm SiO_x layer. Performing high-resolution transmission electron microscopy at a bright spot identified with EBIC reveals that conduction pinholes in SiO_x can be regions of thin tunneling SiO_x rather than a geometric pinhole. Additionally, selectively etching the underlying poly-Si layer in contacts with 1.5 and 2.2 nm thick SiO_x layers using tetramethylammonium hydroxide results in pinhole-like etch features in both contacts. However, EBIC measurements for a contact with a thinner, 1.5 nm SiO_x layer do not reveal pinholes, which is consistent with uniform tunneling transport through the 1.5 nm SiO_x layer. Finally, we theoretically show that reducing the metal to the c-Si contact size from microns, like in the p-type passivated emitter rear contact, to tens of nanometers, like in poly-Si contacts, allows lowering of the unpassivated contact area by several orders of magnitude, thus resulting in excellent passivation, as has been demonstrated for these contacts.

• Research Organization: National Renewable Energy Laboratory (NREL), Golden, CO (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S)
• Grant/Contract Number: AC36-08GO28308
• OSTI ID: 1502791
• Report Number(s): NREL/JA--5900-72682
• Journal Information: Applied Physics Letters, Journal Name: Applied Physics Letters Journal Issue: 8 Vol. 114; ISSN 0003-6951
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English