Solar Cell

BariumCopperChFluorine (Ch= Sulfur, Selenium, Tellurium) p-type transparent conductors

by Andriy Zakutayev

Thesis (Ph.D.)--Oregon State University, 2010.;ISBN: 9781124216355

BaCuChF (Ch = S, Se, Te) materials are chalcogen-based transparent conductors with wide optical band gaps (2.9 – 3.5... more BaCuChF (Ch = S, Se, Te) materials are chalcogen-based transparent conductors with wide optical band gaps (2.9 – 3.5 eV) and a high concentration of free holes (10¹⁸ – 10²⁰ cm⁻³) caused by the presence of copper vacancies. Chalcogen vacancies compensate copper vacancies in these materials, setting the Fermi level close to the valence band maximum. BaCuChF thin film solid solutions prepared by pulsed laser deposition (PLD) have tunable properties, such as lattice constants, conductivity and optical band gaps. BaCuSF and BaCuSeF materials also feature room-temperature stable 3D excitons with spin-orbit-split levels. BaCuTeF has forbidden lowest-energy optical transitions which extends its transparency range. BaCuChF surfaces oxidize when exposed to air, but can be protected using Ch capping layers. Polycrystalline BaCuSeF thin films have a 4.85 eV work function, a 0.11 eV hole injection barrier into ZnPc, and 0.00 eV valence band offset with ZnTe. BaCuSeF should have s similar band offset and similar interfacial properties with CdTe and Cu(InGa)Se₂, and BaCuSF should have no valence band offset with Cu₂ZnSnS₄, according to the transitivity rule. Therefore, BaCuSeF is suitable for applications as a p-layer in organic light-emitting diodes, p-i-n double-heterojunction and tandem chalcogenide solar cells.

Band-structure, optical properties, and defect physics of the photovoltaic semiconductor SnS

by Andriy Zakutayev

Appl. Phys. Lett. 100, 032104 (2012

Direct Insight into Grain Boundary Reconstruction in Polycrystalline Cu(In,Ga)Se2 with Atomic Resolution

by Bernhard Schaffer

Phys. Rev. Lett., 108/7 (2012), pp. 075502

This work presents results from high-resolution scanning transmission electron microscopy and electron energy-loss... more This work presents results from high-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy on twin boundaries (TBs) and nontwin grain boundaries (GBs) in Cu(In,Ga)Se2 thin films. It is shown that the atomic reconstruction is different for different symmetries of the grain boundaries. We are able to confirm the model proposed by Persson and Zunger Phys. Rev. Lett. 91 266401 (2003) for Se-Se-terminated Σ3 112 TBs, showing Cu depletion and In enrichment in the two atomic planes closest to the TB. On the contrary, Cu depletion without In enrichment is detected for a cation-Se-terminated TB. At nontwin GBs, always a strong anticorrelation of Cu and In signals is detected suggesting that the formation of InCu or CuIn antisites within a very confined region of smaller than 1 nm is an essential element in the reconstruction of these GBs.

Correlation of Light-Induced Changes in a-Si:H Films with Characteristics of Corresponding Solar Cells

by Joshua Pearce

J. M. Pearce, R. J. Koval, R.W. Collins, C. R. Wronski, M.M. Al-Jassim, and K.M. Jones "Correlation of Light-Induced Changes in a-Si:H Films with Characteristics of Corresponding Solar Cells" 29th IEEE Photovoltaic Specialists Conf. Proc., (IEEE, 2002) pp. 1098-1101, 2002.

For the first time direct correlations are obtained between the light induced changes under 1 sun illumination in the... more For the first time direct correlations are obtained between the light induced changes under 1 sun illumination in the properties of a-Si:H and those in the characteristics of p-i-n cells incorporating identically-prepared i-layers. These correlations were obtained after account was taken of the effects that the location of the electron and hole quasi-Fermi levels have on the carrier recombination that occurs through the different gap states. The changes in midgap state density, as measured on the films and reflected in the subgap absorption at 1.2 eV, are directly correlated with changes in the dark I-V characteristics under low forward bias. In this case small quasi-Fermi level splitting is present so the recombination of the injected carriers is determined by the midgap states in the bulk of the i-layer. In addition, the changes in the electron mobility-lifetime products as measured on the films are correlated with changes in the fill factor measured on cells under the same conditions as long as large quasi-Fermi level splitting is present and recombination occurs through states spanning a wide region of the gap, such as occurs under 1 sun illumination. The results explain (i) the failure of numerous attempts to correlate the degradation of solar cells reliably with the creation of dangling bond defects and (ii) the inadequacy of the large number of modeling results that assume such a correlation.

Microstructurally Engineered p-layers for Obtaining High Open-Circuit Voltages in a-Si:H n-i-p Solar Cells

by Joshua Pearce

R. J. Koval, J. M. Pearce, Chi Chen, G. M. Ferreira, A. S. Ferlauto, R. W. Collins, and C.R. Wronski, "Microstructurally Engineered p-layers for Obtaining High Open-Circuit Voltages in a-Si:H n-i-p Solar Cells", 29th IEEE Photovoltaic Specialists Conf. Proc., (IEEE, 2002) pp. 1090-1093, 2002.

A study was carded out with the goal of obtaining high open circuit voltages (Voc) in a-Si:H n-i-p solar cells, taking... more A study was carded out with the goal of obtaining high open circuit voltages (Voc) in a-Si:H n-i-p solar cells, taking into account the evolutionary nature of the microstructure of the p-layers during growth. It is found that cells with players in the protocrystalline Si:H growth regime give the highest values of Voc not those with microcrystalline Si:H p-layers. Evidence for this conclusion is presented whereby Voc is related directly to the microstructure of the p-layers, as characterized using spectroscopic ellipsometry, atomic force microscopy, and electrical measurements. The results clarify the origins of (i) inconsistencies associated with attributing high Voc in n-i-p cells to the microcrystallinity of the p-layers, as well as (ii) the inability to obtain similarly high values in p-i-n superstrate cells. Because the microstructure of p-type protocrystalline Si:H depends on that of the underlying i-layer, it is not possible to optimize the cell parameters based on an understanding of the process unless detailed characterization of the p-layer in the actual cell configuration is performed.

The Role of Phase Transitions in Protocrystalline Si:H on the Performance their of Solar Cells

by Joshua Pearce

R. J. Koval, J. M. Pearce, A.S. Ferlauto, P.I. Rovira, R.W. Collins, and C.R. Wronski, "The Role of Phase Transitions in Protocrystalline Si:H on the Performance their of Solar Cells", 28th IEEE Photovoltaic Specialists Conf. Proc., (IEEE, 2000) pp. 750-753, 2000.

A systematic study has been carried out to quantify the effect of microcrystallite nucleation in the intrinsic layer... more A systematic study has been carried out to quantify the effect of microcrystallite nucleation in the intrinsic layer of protocrystalline Si:H p-i-n solar cells prepared by rf plasma enhanced chemical vapor deposition (PECVD). Real-time spectroscopic ellipsometry (RTSE) results that have previously identified the transitions from amorphous to microcrystalline phase were confirmed with atomic force microscopy (AFM) images. The effects of the phase transitions in the bulk intrinsic layer, as well as near the p/i interface of p-i-n cells, have been evaluated as a function of film thickness and H2-dilution ratio (R=[H2]/[SiH4]). Illuminated and dark J-V characteristics are correlated with the microstructural evolution of the Si:H films yielding insight into the “sharpness” of the transition. Evidence of abrupt changes in the mobility gap is obtained from the selfconsistent numerical modeling of cell characteristics.

Thickness Evolution of the Microstructural and Optical Properties of Si:H Films in the Amorphous-to-Microcrystalline Phase Transition Region

by Joshua Pearce

A. S. Ferlauto, G.M. Ferreira, R.J. Koval, J.M. Pearce, C.R. Wronski, R. W. Collins, M.M. Al-Jassim, and K.M. Jones, "Thickness Evolution of the Microstructural and Optical Properties of Si:H Films in the Amorphous-to-Microcrystalline Phase Transition Region" 29th IEEE Photovoltaic Specialists Conf. Proc., (IEEE, 2002) pp. 1076-1081, 2002.

The ability to characterize the phase of the intrinsic (i) layers incorporated into amorphous silicon [a-Si:H] and... more The ability to characterize the phase of the intrinsic (i) layers incorporated into amorphous silicon [a-Si:H] and microcrystalline silicon [μc-Si:H] thin film solar cells is critically important for cell optimization. In this research, a new method has been developed to extract the thickness evolution of the μc-Si:H volume fraction in mixed phase amorphous + microcrystalline silicon [(a+μc)-Si:H] i-layers. This method is based on real time spectroscopic ellipsometry measurements performed during plasma enhanced chemical vapor deposition of the films. In the analysis, the thickness at which crystallites first nucleate from the a-Si:H phase can be estimated, as well as the nucleation density and microcrystallite cone angle. The results show very good correlations with structural and electronic device measurements.

A New Approach to the Analysis of Forward Bias Dark Current Characteristics of a-Si:H Solar Cells

by Joshua Pearce

31st IEEE Photovoltaic Specialists Conf. Proc., pp. 1404-1407, 2005. http://dx.doi.org/10.1109/PVSC.2005.1488403

Generally the dark forward bias current voltage (JD-V) characteristics of a-Si:H solar cells are analyzed without... more Generally the dark forward bias current voltage (JD-V) characteristics of a-Si:H solar cells are analyzed without clearly separating their contributions due to carrier recombination in the bulk from that at the p/i interface regions nor those imposed by carrier injection from the p and n contacts. Furthermore their exponential regimes are interpreted and fitted with constant diode quality factor n with modeling which is based on many fitting parameters that have not been reliably established. A new approach has been developed for the analysis of JD-V characteristics for the entire voltage range relevant to the operation of solar cells, which has allowed the three contributions to be identified and characterized. It is based on the analysis of their bias dependent differential diode quality factors, n(V), from which important information on the energy distribution of the defect states in the i-layers has been obtained. Results are presented and discussed here for p-i-n a-Si:H solar cells having sufficiently low p/i interface recombination so that the limitations by the bulk recombination on open circuit voltage, Voc, can be identified. These results are then correlated with the defect state distributions previously obtained from the analysis on differential diode quality factor n(V) characteristics for hydrogen diluted and undiluted intrinsic layers both in the annealed state as well as after introducing light induced defects.

Evaluation of compositional depth profiles in mixed-phase (amorphous+crystalline) silicon films from real time spectroscopic ellipsometry

by Joshua Pearce

Thin Solid Films 455-456, pp. 665-669, 2004.

The ability to characterize the phase of the intrinsic (i) semiconductor layers incorporated into amorphous silicon... more The ability to characterize the phase of the intrinsic (i) semiconductor layers incorporated into amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin film solar cells is critically important for device optimization. In this study, a new method has been developed to extract the thickness evolution of the μc-Si:H volume fraction in mixed-phase amorphous+microcrystalline silicon [(a+μc)-Si:H] i-layers. This method applies real time spectroscopic ellipsometry (RTSE) performed using a rotating-compensator multichannel ellipsometer during r.f. plasma-enhanced chemical vapor deposition of the films, in conjunction with a two-layer virtual interface analysis of the RTSE data. In this analysis, the depth profile of the μc-Si:H volume fraction in the mixed-phase growth regime can be determined simultaneously with the evolution of the surface roughness layer thickness. From this information, the microcrystallite nucleation density and cone angle can be estimated, the latter describing the preferential growth of the silicon microcrystallites. The results from RTSE analysis correlate well with structural and p–i–n solar cell device measurements.

Charge-Transfer Properties of Dye-Sensitized Solar Cells via Long-Range-Corrected Density Functional Theory

by Bryan Wong

Materials Research Society Symposium Proceedings, 1120E, 1120-M01-03 (2008)

Coumarin Dyes for Dye-Sensitized Solar Cells: A Long-Range-Corrected Density Functional Study

by Bryan Wong

Journal of Chemical Physics, 129, 214703 (2008)

Dilute nitride GaInNAs and GaInNAsSb for solar cell applications

by Siew Li Tan

The dilute nitride GaInNAs(Sb) alloy system is challenging to grow and defects can cause short diffusion lengths and... more The dilute nitride GaInNAs(Sb) alloy system is challenging to grow and defects can cause short diffusion lengths and high background doping densities. Despite these difficulties, extremely high cell efficiencies have recently been achieved in multi-junction solar cells utilising 1 eV GaInNAs absorber layers. This study aims to highlight the trade-offs between the electrical and optical characteristics related to the performance of GaInNAs(Sb) diode structures grown by molecular beam epitaxy , with band gaps ranging from 0.90 to 1.04 eV. Post-growth annealing was necessary in some instances to reduce the background doping and dark current densities. The incorporation of Sb into GaInNAs has enabled the possibility of producing a dilute nitride cell with a band gap lower than 0.80 eV, although with an increased dark current.

Rectification at Graphene-Semiconductor Interfaces: Zero-Gap Semiconductor-Based Diodes

by Sefaattin Tongay

Effect of the TiO2 shell thickness on the dye-sensitized solar cells with ZnO–TiO2 core–shell nanorod electrodes

by Mohammad Mehdi Shahjamali

Well-aligned ZnO nanorod arrays with high aspect ratio have been grown on FTO substrate by hydrothermal process. FTO... more Well-aligned ZnO nanorod arrays with high aspect ratio have been grown on FTO substrate by hydrothermal process. FTO substrate was pre-modified with ZnO thin film as a seed layer by sol–gel coating. TiO2 thin shells with different thicknesses were grown on the ZnO nanorods by chemical vapor deposition method. Sample characterization was performed by X-ray diffraction (XRD), electron microscopy (FESEM, TEM, and SAD), X-ray photoelectron spectroscopy (XPS) and UV–vis Spectroscopy. XRD and TEM analyses indicated a wurtzite structured with high crystallinity and confirmed that each individual ZnO nanorod was a single crystal. The photoelectrochemical experiments were performed in a sandwich type two-electrode cell and I–V curves were obtained. Result of solar cell testing showed that addition of TiO2 shells to ZnO nanorod significantly increased the JSC, VOC, fill factor and efficiency relative to devices without TiO2 shells. Overall cell efficiency jumped from 0.45% for bare ZnO nanorod array to 0.92% for 14 nm thick TiO2 shells on ZnO which showed a twofold increase. The fill factor increased from 0.37 to 0.60, showing a 62% improvement. For a shell with thickness of 21 nm, VOC increased by about 150 mV to 0.68 V. The results showed that it is possible to fabricate core–shell cells of higher efficiency by using nanorod arrays and other morphologies with larger surface area.

ATOMIC TRANSPORT KINETICS ACROSS Al/Si INTERFACES

by Nicole Herbots

Stable hole doping of graphene for low electrical resistance and high optical transparency

by Sefaattin Tongay

Published in Nanotechnology

We report on the p doping of graphene with the polymer TFSA ((CF3SO2)2NH). Modification of graphene with TFSA... more We report on the p doping of graphene with the polymer TFSA ((CF3SO2)2NH). Modification of graphene with TFSA decreases the graphene sheet resistance by 70%. Through such modification, we report sheet resistance values as low as 129 Ω, thus attaining values comparable to those of indium–tin oxide (ITO), while displaying superior environmental stability and preserving electrical properties over extended time scales. Electrical transport measurements reveal that, after doping, the carrier density of holes increases, consistent with the acceptor nature of TFSA, and the mobility decreases due to enhanced short-range scattering. The Drude formula predicts that competition between these two effects yields an overall increase in conductivity. We confirm changes in the carrier density and Fermi level of graphene through changes in the Raman G and 2D peak positions. Doped graphene samples display high transmittance in the visible and near-infrared spectrum, preserving graphene's optical properties without any significant reduction in transparency, and are therefore superior to ITO films in the near infrared. The presented results allow integration of doped graphene sheets into optoelectronics, solar cells, and thermoelectric solar cells as well as engineering of the electrical characteristics of various devices by tuning the Fermi level of graphene.

Measurement of residual stress in multicrystalline silicon ribbons by a self-calibrating infrared photoelastic method

by Miguel Centeno Brito

published in Review of Scientific Instruments, 2005

This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on... more This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on the infrared photoelastic technique. This self-calibrating method allows the in situ determination of the photoelastic coefficients and can thus be used for any crystal orientation. The method was validated by the experimental determination of the photoelastic coefficient of monocrystalline (100) silicon wafers and by comparison with strain measurements using asymmetrical x-ray diffraction. The distribution of residual stress in multicrystalline silicon ribbons was also measured. The results showed strong evidence for tensile stress in the central region and compressive stress near the edges of the ribbons. Both the measured residual stress and the photoelastic coefficient distributions are correlated to grain boundaries.

Unintended Consequences of Green Technologies

by Ozzie Zehner

Zehner, Ozzie. “Unintended Consequences.” In Green Technology, edited by Paul Robbins, Dustin Mulvaney and J. Geoffrey Golson, 427-32. London: Sage, 2011.

Green technologies (e.g. wind turbines, solar cells, and biofuels) and initiatives (e.g. efficiency, recycling, and... more Green technologies (e.g. wind turbines, solar cells, and biofuels) and initiatives (e.g. efficiency, recycling, and organics) yield distinct unanticipated consequences that can partially or fully offset intended environmental benefits.

Controlling the Directional Emission of Light by Periodic Arrays of Heterostructured Semiconductor Nanowires

by Olaf Janssen

S.L. Diedenhofen, O.T.A. Janssen, M. Hocevar, A. Pierret, E.P A.M. Bakkers, H.P. Urbach, and J. Gomez Rivas

We demonstrate experimentally the directional emission of light by InAsP segments embedded in InP nanowires. The... more We demonstrate experimentally the directional emission of light by InAsP segments embedded in InP nanowires. The nanowires are arranged in a periodic array, forming a 2D photonic crystal slab. The directionality of the emission is interpreted in terms of the preferential decay of the photoexcited nanowires and the InAsP segments into Bloch modes of the periodic structure. By simulating the emission of arrays of nanowires with the emitting segments located at different heights, we conclude that the position of this active region strongly influences the directionality and efficiency of the emission. Our results will help to improve the design of nanowire based LEDs and single photon sources.

BOOK: Anodically Fabricated Metal Oxide Nanotube Arrays: A Useful Structure for Efficient Solar Energy Conversion

by Nageh K. Allam

Nageh K. Allam
ANODICALLY FABRICATED METAL OXIDE NANOTUBE ARRAYS: A USEFUL STRUCTURE FOR EFFICIENT SOLAR ENERGY CONVERSION
VDM Verlag Dr. Müller, 2011
ISBN-13: 978-3-639-32597-3
ISBN-10: 3639325974
EAN: 9783639325973

The primary focus of this book is to present the protocols used to improve the properties of anodically fabricated... more The primary focus of this book is to present the protocols used to improve the properties of anodically fabricated metal oxide nanotube arrays; notably their band gap and crystallinity while retaining their tubular structure unaffected. Controlling the crystallinity and band gap of the materials, while retaining its tubular structure, results in an enormous enhancement in the photoconversion capability of the material upon use in photo-assisted water splitting and solar cell devices.