Jeffrey R. S. Brownson

Ellipsometric characterization and density-functional theory analysis of anisotropic optical properties of single-crystal α-SnS

We report on the anisotropic optical properties of single-crystal tin monosulfide (SnS). The components ea, eb, and ec of the pseudodielectric-function tensor ⟨e⟩ = ⟨e1⟩ + i⟨e2⟩ spectra are taken from 0.73 to 6.45 eV by spectroscopic ellipsometry. The measured ⟨e⟩ spectra are in a good agreement with the results of the calculated dielectric response from hybrid density functional theory. The ⟨e⟩ spectra show the direct band-gap onset and a total of eight above-band-gap optical structures that are associated with the interband-transition critical points (CPs). We obtain accurate CP energies by fitting analytic CP expressions to second-energy-derivatives of the ⟨e⟩ data. Their probable electronic origins and implications for photovoltaic applications are discussed.

Optical Properties of Sputtered SnS Thin Films for Photovoltaic Absorbers

Tin monusulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high-efficiency solar cells. The optical properties of SnS thin films are investigated to assess their compatibility with the solar spectrum. SnS thin films were RF magnetron sputter-deposited at target powers of 105-155 W and total pressures of 5 to 60 mtorr in argon at room temperature. X-ray diffraction patterns confirmed a dominant tin monosulfide herzenbergite phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 105 -106 cm-1. The direct gap, indirect gap, and forbidden direct gap for the films were found to be in the range of 1.2-1.6 eV, indicating a strong match with the solar irradiance spectrum.

Chemistry of Tin Monosulfide ( δ-SnS ) Electrodeposition Effects of pH and Temperature with Tartaric Acid

The conditions of formation of a tin sulfide 8-SnS phase by galvanostatic deposition with tartaric acid were investigated, changing parameters such as pH (1.5 and 2.5) and temperature (50, 70, and 90°C). Uniform films were deposited at both pHs. Cyclic voltammetry data suggested that tin-tartrate complexes were crucial to film electrodeposition and also indicated reduction of sulfur species rather than tin. X-ray diffraction showed the 8-SnS structure to be a pure phase over the range of deposition temperatures (50-90°C) at pH 2.5. This observation was limited to 70°C for films deposited at pH 1.5. Impurities of Sn and α-SnS were found in films deposited at pH 1.5 and temperatures of 50 and 90°C, respectively, indicating that the α-SnS phase is more likely to be formed at lower pHs. All the films annealed at 350°C displayed the α-SnS phase. Scanning electron microscopy showed deposited particles increased in size and became more platelike with increasing temperatures of electrodeposition, but did not change morphology with annealing. UV/vis spectrometry confirmed increased disorder in films deposited at 50°C and a direct allowed optical bandgap of lower energy for the 8-SnS phase compared to that of α-SnS.

Efficiency, Economic and Environmental Assessment of Ground Source Heat Pumps in Central Pennsylvania

The energy use of a ground-source heat pump (GSP) for heating, cooling and hot water in a Central Pennsylvania residence (namely, the authors house) is analyzed, compared to a simulation of electricity and a heating-oil furnace (with electric cooling) for these same energy uses. Energy demands for space conditioning in the house are simulated by building a model of the house using the Transient Energy System Simulation (TNRSYS) tool. Overall, the efficiency gain for the ground-source heat pump compared to electricity is 43% for cooling and 81% for heating. For home heating and hot water, the ground-source heat pump has a 42% efficiency gain over a fuel-oil furnace. The system modeled in this paper has a payback period of between four and five years compared to an all-electric system. The payback period compared to a hybrid system of fuel-oil heat and electric cooling is between two and three years.

Phase identification of RF-sputtered SnS thin films using rietveld analysis of X-ray diffraction patterns

Tin monosulfide (SnS) is a promising material for a photovoltaic absorber layer. Significant strides have been taken to better understand its material properties. The X-ray diffraction patterns of radio-frequency sputtered SnS thin films are investigated. Samples were deposited under varying total pressure, target power, substrate-to-target distance, and substrate temperature. Rietveld refinement of samples deposited under varying conditions yielded evidence of multiple phases present in SnS thin films. Refinements were completed with one or more tin sulfide phases, showing a dominant herzenbergite SnS phase (Pbnm). Possible secondary phases include orthrhombic (Cmcm) and cubic (Fm3m) crystal structures. Lattice parameters, cell volume, and unit cell density were investigated as a function of deposition conditions. Results indicate that growth mode is related to deposition rate. Early studies of heated stage depositions showed that SnS thin films have added mobility at the substrate.

Matching Photovoltaic Orientation to Energy Loads

Rather than orienting rooftop installation of photovoltaics (PV) to maximize power for the individual customer-generator, we analyze design and performance of integrated PV for two alternate objectives: 1.) maximizing the volume of grid sellbacks, and 2.) maximizing customer-generator revenue through net metering. These alternative orientation strategies attempt to maximize power output during times of peak demand on the grid, or when market prices are highest. Power output and PV system configurations were simulated using weather/radiation data for Pennsylvania. Given a system with no centralized storage capacity, we use relevant hourly system loads and Locational Marginal Prices (LMP) from the PJM Interconnection to determine the specific orientations required to integrate intervals of energy gains from the PV system with periods of high demand for electricity in a building. We also determined optimal orientation of PV systems for a time-of-use purchasing scenario, matching orientations and electrical gains with periods in the day when it is economically beneficial to sell to the grid. Given a net-metered PV system with no storage capacity, altering the PV panel orientations allows one to design peak solar energy production times to match any set of energy demands.

Investigation of RF-sputtered tin sulfide thin films with in situ heating for photovoltaic applications

Tin (II) Monosulfide (SnS) is of increasing interest to researchers due to its near-optimal optoelectronic properties for photovoltaic devices. In this work, we take a new approach using a Tin (IV) Disulfide target to sputter SnS thin films. Sulfur-rich SnS thin films are produced via in situ heating of the substrate. Experimentation with substrate heating has yielded two unique crystal structures, depending on constant or “pulsed” heating. Standardless Electron Dispersive Spectroscopy measurements indicate that the films are nearly 1:1 Sn:S ratio. Films with low resistivity (<;100 Ω-cm) were produced via this method. These results are promising for improving SnS-based photovoltaic device performance.

Skill and Skill Prediction of Cloud-Track Advection-Only Forecasting under a Cumulus-Dominated Regime

AbstractThe intermittency of solar power production is dependent on the evolution and advection of the nearby cloud field. A key problem related to solar energy integration is the improvement of 1-h-ahead forecasts to reduce the impact of intermittency on power systems operations. Many solar forecasts explicitly or implicitly assume Taylor’s hypothesis. While such advection-only forecasts can be presumed to be valid across sufficiently short time scales, it is not clear how rapidly the skill of such a forecast decays with increased lead time. As the goal is to improve the quality of 1-h-ahead forecasts, this work focuses on quantifying the skill of cloud-track wind-based cumulus-dominated cloud field forecasts with respect to lead time. No explicit connection is drawn to the quality of solar forecasts because of the importance of separating two potential sources of error: cloud field forecasting and radiative transfer estimation. It is found that the cumulus field forecast skill begins to asymptotically a...

Investigation of the absorption properties of sputtered tin sulfide thin films for photovoltaic applications

Tin sulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high efficiency solar cells. Sputtered SnS thin films were deposited at target powers 105-155 W and total pressures of 5 to 60 mTorr in argon. X-ray diffraction patterns confirmed a dominant tin monosulfide phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 105-106 cm-1.