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Steinmann, Vera

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Steinmann

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Vera

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Steinmann, Vera
Author's Publications: search.filters.isAuthorOfPublication.e3c23588-b84a-4bb4-869f-de1448d8b280

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    Framework to predict optimal buffer layer pairing for thin film solar cell absorbers: A case study for tin sulfide/zinc oxysulfide
    (AIP Publishing, 2015) Mangan, Niall; Brandt, Riley E.; Steinmann, Vera; Jaramillo, Rafael; Yang, Chuanxi; Poindexter, Jeremy R.; Chakraborty, Rupak; Park, Helen; Zhao, Xizhu; Gordon, Roy; Buonassisi, Tonio
    An outstanding challenge in the development of novel functional materials for optoelectronic devices is identifying suitable charge-carrier contact layers. Herein, we simulate the photovoltaic device performance of various n-type contact material pairings with tin(II) sulfide (SnS), a p-type absorber. The performance of the contacting material, and resulting device efficiency, depend most strongly on two variables: conduction band offset between absorber and contact layer, and doping concentration within the contact layer. By generating a 2D contour plot of device efficiency as a function of these two variables, we create a performance-space plot for contacting layers on a given absorber material. For a simulated high-lifetime SnS absorber, this 2D performance-space illustrates two maxima, one local and one global. The local maximum occurs over a wide range of contact-layer doping concentrations (below 1016 cm−3), but only a narrow range of conduction band offsets (0 to −0.1 eV), and is highly sensitive to interface recombination. This first maximum is ideal for early-stage absorber research because it is more robust to low bulk-minority-carrier lifetime and pinholes (shunts), enabling device efficiencies approaching half the Shockley-Queisser limit, greater than 16%. The global maximum is achieved with contact-layer doping concentrations greater than 1018 cm−3, but for a wider range of band offsets (−0.1 to 0.2 eV), and is insensitive to interface recombination. This second maximum is ideal for high-quality films because it is more robust to interface recombination, enabling device efficiencies approaching the Shockley-Queisser limit, greater than 20%. Band offset measurements using X-ray photoelectron spectroscopy and carrier concentration approximated from resistivity measurements are used to characterize the zinc oxysulfide contacting layers in recent record-efficiency SnS devices. Simulations representative of these present-day devices suggest that record efficiency SnS devices are optimized for the second local maximum, due to low absorber lifetime and relatively well passivated interfaces. By employing contact layers with higher carrier concentrations and lower electron affinities, a higher efficiency ceiling can be enabled.
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    The impact of sodium contamination in tin sulfide thin-film solar cells
    (AIP Publishing, 2016) Steinmann, Vera; Brandt, Riley E.; Chakraborty, Rupak; Jaramillo, Rafael; Young, Matthew; Ofori-Okai, Benjamin K.; Yang, Chuanxi; Polizzotti, Alex; Nelson, Keith A.; Gordon, Roy; Buonassisi, Tonio
    Through empirical observations, sodium (Na) has been identified as a benign contaminant in some thin-film solar cells. Here, we intentionally contaminate thermally evaporated tin sulfide (SnS) thin-films with sodium and measure the SnS absorber properties and solar cell characteristics. The carrier concentration increases from 2× 10^16cm^−3 to 4.3×10^17cm^−3 in Na-doped SnS thin-films, when using a 13 nm NaCl seed layer, which is detrimental for SnS photovoltaic applications but could make Na-doped SnS an attractive candidate in thermoelectrics. The observed trend in carrier concentration is in good agreement with density functional theory calculations, which predict an acceptor-type NaSn defect with low formation energy
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    Effect of growth temperature on carrier collection in SnS-based solar cells
    (2017-04-14) Chakraborty, Rupak; Steinmann, Vera; Poindexter, Jeremy; Jaramillo, Rafael; Hartman, Katy; Polizzotti, Alex; Brandt, Riley; Mangan, Niall; Yang, Chuanxi; Gordon, Roy; Buonassisi, Tonio
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    Voc impact of orientation-dependent x in anisotropic PV absorbers
    (2015) Chakraborty, Rupak; Needleman, David; Doolittle, Kelsey; Mangan, Niall; Steinmann, Vera; Poindexter, Jeremy; Polizzotti, Alex; Yang, Chuanxi; Gordon, Roy; Buonassisi, Tonio
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    A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System
    (American Chemical Society (ACS), 2016) Steinmann, Vera; Chakraborty, Rupak; Rekemeyer, Paul H.; Hartman, Katy; Brandt, Riley E.; Polizzotti, Alex; Yang, Chuanxi; Moriarty, Tom; Gradecak, Silvija; Gordon, Roy; Buonassisi, Tonio
    As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing “false negative” results. Here, we demonstrate a device engineering solution towards a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400˚C) to stimulate grain growth, followed by a much thinner, low-temperature (200˚C) absorber deposition. At lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and demonstrate a more than 3.5x superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.