Timothy Coutts - Academia.edu (original) (raw)

Papers by Timothy Coutts

Applied Physics Letters, 1985

Solar cells have been fabaricated by RF sputter depositing indium tin oxide onto single crystal p... more Solar cells have been fabaricated by RF sputter depositing indium tin oxide onto single crystal p-type indium phosphide. Four different substrate doping densities have been used but in all cases the dopant was zinc and the wafers were (100) oriented. The optimum doping density from the range studied was 3 x 10 to the 16th per cu cm and devices based on such substrates have yielded total area efficiencies up to 16.2 percent using the air mass 1.5 spectrum normalized to 100 mW/sq cm, which correspond to active area efficiencies of 19.1 percent. A doping density less than the optimum yielded devices with excessive series resistance. Higher doping densities led to a marked loss of red response.

Chalcopyrite ZnGeAs2 lattice-matched to GaAs(001) is a promising 1.1 eV band gap semiconductor fo... more Chalcopyrite ZnGeAs2 lattice-matched to GaAs(001) is a promising 1.1 eV band gap semiconductor for applications in nonlinear photonic devices and multijunction solar cells. Knowledge of the optical functions of a material over a wide photon energy range is of importance to optimize photonic and photovoltaic device structures. We present room-temperature optical properties of a ZnGeAs2 thin film grown epitaxially on a GaAs(001) substrate by metalorganic vapor phase epitaxy. Spectroscopic ellipsometry was employed to measure the pseudodielectric function of the ZnGeAs2 thin film, and was compared with a theoretical calculation within the quasiparticle self-consistent GW approximation. The interband-transition critical-point energies were obtained from a standard lineshape analysis of the measured spectrum. We will also present a comparison of the optical properties of ZnGeAs2 with those of other II-IV-V2 chalcopyrite compounds as well as their corresponding III-V zincblende compounds. This abstract is subject to government rights.

IEEE Transactions on Electron Devices, 1999

We first discuss the similarities between generation of electricity using thermophotovoltaic (TPV... more We first discuss the similarities between generation of electricity using thermophotovoltaic (TPV) and high-opticalconcentration solar photovoltaic (PV) devices. Following this, we consider power losses due to above-and below-bandgap photons, and we estimate the ideal bandgap by minimizing the sum of these, for a 6000 K black-body spectrum. The ideal bandgap, based on this approach, is less than that previously predicted, which could have a significant influence on the performance of devices and systems. To reduce the losses, we show that the low-energy photons may be removed from both types of cells and consider the specific case of a back-surface reflector. This approach to the management of waste heat may offer a useful additional tool with which to facilitate the design of high-photonflux solar cells. In the case of the high-energy photons and the associated problem of thermalization of hot electrons, however, the heat must be removed by other means, and we consider the applicability of microchannel cooling systems. These appear to have the potential to handle thermal loads at least several times those generated by 1000 times concentrators, or by black-body TPV radiators at a temperature of far greater than 1500 K. We go on to consider the management of the very high currents generated in both concentrator TPV and PV systems and discuss the concept of the monolithically integrated minimodule.

We report results from in situ real-time spectroscopic ellipsometry (RT-SE) investigations into t... more We report results from in situ real-time spectroscopic ellipsometry (RT-SE) investigations into the sputter deposition and subsequent annealing behavior of permittivity-engineered transparent conductive oxide (TCO) films. RT-SE reveals information about the growth dynamics and evolution of the films' electro-optical properties during deposition. We investigate the correlations of the ex situ optical and electrical measurements with structural analysis for select films.

Journal of Non-crystalline Solids, 2008

ZnO-based transparent conducting oxide (TCO) thin films have received increased attention recentl... more ZnO-based transparent conducting oxide (TCO) thin films have received increased attention recently because of their potential to reduce production costs compared to those of the prevalent TCO indium tin oxide (ITO). Undoped ZnO and ZnO:Al (0.1, 0.2, 0.5, 1, and 2 wt% Al 2 O 3 ) polycrystalline films were deposited by RF magnetron sputtering. Controlled incorporation of H 2 and O 2 in the Ar sputtering ambient was investigated. Though optimal substrate temperature was found to be 200°C for films grown in 100% Ar, the addition of H 2 permits improved electrical performance for room-temperature depositions. Temperature-dependent Hall data suggest that ionized impurity and acoustic phonon scattering dominate at high and intermediate carrier concentration levels, respectively, with evidence of temperature-activated transport at the lowest levels. Lightly doped ZnO:Al demonstrates reduced infrared absorption compared to the standard 2 wt%-doped ZnO:Al, which may be beneficial to device performance.

Semiconductor Science and Technology, 2003

In this paper, we discuss some of the highlights of the Fifth Conference on Thermophotovoltaic Ge... more In this paper, we discuss some of the highlights of the Fifth Conference on Thermophotovoltaic Generation of Electricity. The paper is organized into three principal sections, which deal with systems, infrared radiation emitters, and photovoltaic cells. Significant areas of progress and trends are identified in each of these areas. Progress is occurring at the fundamental level of materials' science, radiation physics, and systems. In the third of these topics, the quaternary alloy GaInAsSb appears to have become increasingly favoured. In the second topic, interesting developments in surface structured radiators and close-proximity radiators is discussed. Finally, we note that increased work on systems is still required, particularly for non-military applications.

Journal of Crystal Growth, 2006

We studied the role of impurities in nitrogen-doped ZnO thin film to understand the difficulty of... more We studied the role of impurities in nitrogen-doped ZnO thin film to understand the difficulty of producing p-type ZnO via nitrogen doping. The ZnO:N films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD) using diethylzinc (DEZ) and nitric oxide (NO) precursors. Although very high levels of nitrogen incorporation were observed ($10 21 cm À3 ), acceptor concentrations were typically low (10 14 -10 17 cm À3 ). The investigation suggests that the low carrier concentrations are possibly due to compensation and passivation effects by hydrogen and carbon impurities unintentionally incorporated into the films from the metalorganic precursor. X-ray photoelectron spectroscopy (XPS) demonstrated that carbon was a bulk impurity in MOCVD-grown films. Secondary-ion mass spectrometry (SIMS) analysis confirmed the presence of carbon and indicated that hydrogen was also a bulk impurity. The concentration of carbon contaminant was found to increase with nitrogen doping. Both XPS and Fourier transform infrared spectroscopy (FTIR) data indicated that defect complexes (CH x , NH x , and NC x ) are likely present in MOCVD-grown ZnO films. First-principles calculations predict that the N O -H and (NC) O defect complexes are neutral and 1 + charge state; therefore, the existing carbon and hydrogen passivate the nitrogen acceptor species. Thus, we believe a low hole concentration in MOCVD-fabricated ZnO:N films are partially due to inadvertent passivation by hydrogen and carbon. r 2005 Elsevier B.V. All rights reserved. PACS: 71.55.Gs impurity and defect levels in II-VI semiconductors

Solar Energy Materials and Solar Cells, 2001

This paper provides an overview of the developments in thermophotovoltaic (TPV) generation of ele... more This paper provides an overview of the developments in thermophotovoltaic (TPV) generation of electricity that have occurred relatively recently-from about 1994 to October 1998. The components considered are the semiconductor converter; the radiator; and the means of recirculating unusable, long-wavelength photons. A short account of the functions and performance of each of these components is given. Also discussed are operational systems and progress in modeling TPV systems.

We report two organic excitonic solar cell structures with the conventional ITO anode replaced by... more We report two organic excitonic solar cell structures with the conventional ITO anode replaced by a thin single-walled carbon nanotube (SWCNT) layer. The first structure, SWCNT/PEDOT/P3HT:PCBM/Al, gave a non-optimized device efficiency of 2.61%, which is competitive with ITO-based devices. The second, SWCNT/P3HT:PCBM/Al, is unique, as it did not use PEDOT as a hole selective contact. This had a promising efficiency of 1.43%. The open structure of the SWCNT network, and its inherent compatibility with the chlorobenzene solvent used to deposit the P3HT:PCBM, allow for intimate intermixing with the active layers of the device and the elimination of PEDOT. The strong rectifying behavior of the device shows that nanotubes are selective for holes and are not recombination sites in the active layer of the device. This excitonic solar cell is the first measurably efficient organic device produced without ITO and PEDOT

Transparent electrical contacts (TCs) play an important role in thin film photovoltaics (PV), inc... more Transparent electrical contacts (TCs) play an important role in thin film photovoltaics (PV), including in organic devices (OPV). OPV devices have different TC requirements than traditional thin film devices, due to the need to make them flexible, cost and manufacturing constraints, and the potential benefits that may be realized from using a hole conducting TC. Additionally, the active layer in an OPV device is far thinner than in an inorganic “thin-film” device, causing TC morphology to have a significant effect on device performance. Here, we present data on optimizing carbon nanotube networks for use as TCs in OPV. We look at network deposition techniques and required post-treatment methods along with electrical, optical, and morphological data to devise a fabrication process for a high quality network that is compatible with OPV device production.

Applied Physics Letters, 2006

We report two viable organic excitonic solar cell structures where the conventional In2O3:Sn (ITO... more We report two viable organic excitonic solar cell structures where the conventional In2O3:Sn (ITO) hole-collecting electrode was replaced by a thin single-walled carbon nanotube layer. The first structure includes poly(3,4-ethylenedioxythiophene) (PEDOT) and gave a nonoptimized device efficiency of 1.5%. The second did not use PEDOT as a hole selective contact and had an efficiency of 0.47%. The strong rectifying behavior

Applied Physics Letters, 2005

The role of hydrogen in nitrogen-doped ZnO thin films was studied by Fourier transform infrared ͑... more The role of hydrogen in nitrogen-doped ZnO thin films was studied by Fourier transform infrared ͑FTIR͒ absorption and modeled by first-principles calculations to understand the difficulty of doping ZnO p-type with nitrogen. Nitrogen-doped ZnO films were fabricated by low-pressure metal-organic chemical vapor deposition ͑MOCVD͒. High levels of nitrogen incorporation were observed, but the acceptor concentrations remained low. Theoretical analysis suggests there is a high probability that N O − and H + charged defects combine to form the neutral defect complexes, thereby compensating the nitrogen-related acceptors. Calculated values of the vibrational frequencies of the related infrared modes agree well with the measured spectra. Thus, we believe the difficulty of achieving p-type doping in MOCVD-grown ZnO films is due, at least partially, to inadvertent passivation by hydrogen.

Applied Surface Science, 2004

SnO2 and CdO films with high electrical and optical quality have been produced by using low-press... more SnO2 and CdO films with high electrical and optical quality have been produced by using low-pressure metal-organic chemical vapor deposition. Based on the knowledge gained from binary oxide formation, linear combinatorial synthesis is used to study the ternary oxide (Cd–Sn–O) compound. Compositional, structural, electrical, and optical properties of the films are found to vary along the reactant gas flow direction. The crystal structure indicates that the CdO-like films are formed near the location of gas injection and SnO2-like films near the end of the deposition zone. As composition and structure change, the carrier concentration varies around 1020cm−3, and the mobility increases from less than 1 to ∼60cm2V−1s−1. The optical bandgap of the films also increases from ∼2.75 to 3.65eV as composition moves from CdO-like films to SnO2-like films.