Person:
Park, Helen

Profile Picture

Email Address

AA Acceptance Date

Birth Date

Research Projects

Organizational Units

Job Title

Last Name

Park

First Name

Helen

Name

Park, Helen
Author's Publications: search.filters.isAuthorOfPublication.391cd45e-b455-466d-8fd9-734b233ff58a

Search Results

Now showing 1 - 9 of 9
  • Thumbnail Image
    Publication
    Band alignment of SnS/Zn(O,S) heterojunctions in SnS thin film solar cells
    (American Institute of Physics, 2013) Sun, Leizhi; Haight, Richard; Sinsermsuksakul, Prasert; Bok Kim, Sang; Park, Helen; Gordon, Roy
    Band alignment is critical to the performance of heterojunction thin film solar cells. In this letter, we report band alignment studies of SnS/Zn(O,S) heterojunctions with various compositions of Zn(O,S). Valence band offsets (VBOs) are measured by femtosecond laser pump/probe ultraviolet photoelectron spectroscopy (fs-UPS) from which conduction band offsets (CBOs) are calculated by combining with band gaps obtained by optical transmission/reflection measurements. The SnS/Zn(O,S) heterojunctions with S/Zn ratios of 0.37 and 0.50 have desirable small positive CBOs, while a ratio of 0.64 produces an undesirable large positive CBO. The results are consistent with the device performance of SnS/Zn(O,S) solar cells.
  • Thumbnail Image
    Publication
    Co-optimization of SnS absorber and Zn(O,S) buffer materials for improved solar cells
    (Wiley-Blackwell, 2014) Park, Helen; Heasley, Rachel Lenox; Sun, Leizhi; Steinmann, Vera; Hartman, Katy; Chakraborty, Rupak; Sinsermsuksakul, Prasert; Chua, Danny; Buonassisi, Tonio; Gordon, Roy
    Thin-film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. The effects of deposition temperature and annealing conditions of the SnS absorber layer were studied for solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO. Solar cells were further optimized by varying the stoichiometry of Zn(O,S) and the annealing conditions of SnS. Post-deposition annealing in pure hydrogen sulfide improved crystallinity and increased the carrier mobility by one order of magnitude, and a power conversion efficiency up to 2.9% was achieved.
  • Thumbnail Image
    Publication
    Atomic layer deposition of Al-incorporated Zn(O,S) thin films with tunable electrical properties
    (AIP Publishing, 2014) Park, Helen; Jayaraman, Ashwin; Heasley, Rachel Lenox; Yang, Chuanxi; Hartle, Lauren; Mankad, Ravin; Haight, Richard; Mitzi, David B.; Gunawan, Oki; Gordon, Roy
    Zinc oxysulfide, Zn(O,S), films grown by atomic layer deposition were incorporated with aluminum to adjust the carrier concentration. The electron carrier concentration increased up to one order of magnitude from 1019 to 1020 cm−3 with aluminum incorporation and sulfur content in the range of 0 ≤ S/(Zn+Al) ≤ 0.16. However, the carrier concentration decreased by five orders of magnitude from 1019 to 1014 cm−3 for S/(Zn+Al) = 0.34 and decreased even further when S/(Zn+Al) > 0.34. Such tunable electrical properties are potentially useful for graded buffer layers in thin-film photovoltaic applications.
  • Thumbnail Image
    Publication
    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.
  • Thumbnail Image
    Publication
    Efficient Readout of a Single Spin State in Diamond via Spin-to-Charge Conversion
    (American Physical Society (APS), 2015) Shields, Brendan John; Unterreithmeier, Quirin; de Leon, Nathalie Pulmones; Park, Helen; Lukin, Mikhail
    Efficient readout of individual electronic spins associated with atomlike impurities in the solid state is essential for applications in quantum information processing and quantum metrology. We demonstrate a new method for efficient spin readout of nitrogen-vacancy (NV) centers in diamond. The method is based on conversion of the electronic spin state of the NV to a charge-state distribution, followed by single-shot readout of the charge state. Conversion is achieved through a spin-dependent photoionization process in diamond at room temperature. Using NVs in nanofabricated diamond beams, we demonstrate that the resulting spin readout noise is within a factor of 3 of the spin projection noise level. Applications of this technique for nanoscale magnetic sensing are discussed.
  • Thumbnail Image
    Publication
    Trapping and Manipulation of Isolated Atoms Using Nanoscale Plasmonic Structures
    (American Physical Society (APS), 2009) Chang, D. E.; Thompson, J. D.; Park, Helen; Vuletic´, V.; Zibrov, Alexander; Zoller, P.; Lukin, Mikhail
    We propose and analyze a scheme to interface individual neutral atoms with nanoscale solid-state systems. The interface is enabled by optically trapping the atom via the strong near-field generated by a sharp metallic nanotip. We show that under realistic conditions, a neutral atom can be trapped with position uncertainties of just a few nanometers, and within tens of nanometers of other surfaces. Simultaneously, the guided surface plasmon modes of the nanotip allow the atom to be optically manipulated, or for fluorescence photons to be collected, with very high efficiency. Finally, we analyze the surface forces, heating and decoherence rates acting on the trapped atom.
  • No Thumbnail Available
    Publication
    Development of Earth-Abundant and Non-Toxic Thin-Film Solar Cells
    (2014-12-08) Park, Helen; Gordon, Roy G.; Mazur, Eric; Buonassisi, Tonio
    Although solar energy is the most abundant energy resource available, photovoltaic solar cells must consist of sufficiently abundant and environmentally friendly elements, for scalable low-cost production to provide a major amount of the world’s energy supply. However, scalability is limited in current thin-film solar cell technologies based on Cu(In,Ga)(S,Se)2 and CdTe due to scarce, expensive, and toxic elements. Thin-film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. Solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO were studied by varying the synthesis conditions of the SnS and Zn(O,S) layers. Annealing SnS in hydrogen sulfide increased the mobility by more than one order of magnitude, and improved the power conversion efficiency of the solar cell devices. Solar cell performance can be further optimized by adjusting the stoichiometry of Zn(O,S), and by tuning the electrical properties of Zn(O,S) through various in situ or post-annealing treatments. Zn(O,S) can be post-annealed in oxygen atmosphere or doped with nitrogen, by ammonium hydroxide or ammonia gas, during the ALD growth to reduce the carrier concentration, which can be critical for reducing interface recombination at the p-n junction. High carrier concentration buffer layers can be critical for reducing contact resistance with the ITO layer. Zn(O,S) can also be incorporated with aluminum by trimethylaluminum (TMA) doses to either increase or decrease the carrier concentration based on the stoichiometry of Zn(O,S).
  • Thumbnail Image
    Publication
    Enhancing the Efficiency of SnS Solar Cells bia Band-Offset Engineering with a Zinc Oxysulfide Buffer Layer
    (American Institute of Physics, 2013) Sinsermsuksakul, Prasert; Hartman, Katy; Kim, Sang Bok; Sun, Leizhi; Park, Helen; Chakraborty, Rupak; Buonassisi, Tonio; Gordon, Roy
    SnS is a promising earth-abundant material for photovoltaic applications. Heterojuction solar cells were made by vapor deposition of p-type tin(II) sulfide, SnS, and n-type zinc oxysulfide, Zn(O,S), using a device structure of soda-lime glass/Mo/SnS/Zn(O,S)/ZnO/ITO. A record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S). Increasing the sulfur content in Zn(O,S) raises the conduction band offset between Zn(O,S) and SnS to an optimum slightly positive value. A record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37 exhibits short circuit current density \((J_{sc})\), open circuit voltage \((V_{oc})\), and fill factor (FF) of \(19.4 mA/cm^{2}\), 0.244 V, and 42.97%, respectively, as well as an NREL-certified total-area power-conversion efficiency of 2.04% and an uncertified active-area efficiency of 2.46%.
  • Thumbnail Image
    Publication
    Atomic layer deposition of Zn(O,S) thin films with tunable electrical properties by oxygen annealing
    (American Institute of Physics, 2013) Park, Helen; Heasley, Rachel Lenox; Gordon, Roy
    Zinc oxysulfide, Zn(O,S), films grown by atomic layer deposition were annealed in oxygen to adjust the carrier concentration. The electron carrier concentration of Zn(O,S) can be reduced by several orders of magnitude from \(10^{19}\) to \(10^{15} cm^{−3}\) by post-deposition annealing in oxygen at temperatures from 200 °C to 290 °C. In the case of Zn(O,S) with S/Zn = 0.37, despite the considerable change in the electron carrier concentration, the bandgap energy decreased by only ∼0.1 eV, and the crystallinity did not change much after annealing. The oxygen/zinc ratio increased by 0.05 after annealing, but the stoichiometry remained uniform throughout the film.