Nikolas J. Podraza

University of Toledo, Toledo, Ohio, United States

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Publications (102)153.33 Total impact

  • K. Ghimire · H.F. Haneef · R.W. Collins · N.J. Podraza ·
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    ABSTRACT: Optical properties of commercially available gadolinium gallium garnet (Gd3Ga5O12) single crystals have been studied over a spectral range from 0.034 to 5.887eV using spectroscopic ellipsometry and transmittance measurements via a near infrared to ultraviolet spectral range multichannel ellipsometer and a Fourier transform infrared (FTIR) ellipsometer. The complex dielectric function (ε{lunate}=ε{lunate}1+iε{lunate}2) spectra from 0.034 to 5.887eV have been determined. Transmission measurements have been performed to more accurately extract the absorption coefficient near the band edge and identify sub band gap features. Analysis of ε{lunate} for Gd3Ga5O12 indicates a direct band gap at 5.66±0.01eV, sub band gap absorption features from 3.95 to 5.06eV, and nine transverse optical phonon modes at 288, 313, 329, 361, 370, 467, 577, 609, and 670cm-1 and longitudinal optical phonon modes at 288, 327, 338, 370, 461, 490, 592, 663, and 707cm-1.
  • Yao O. Jin · David Saint John · Nikolas J. Podraza · Thomas N. Jackson · Mark W. Horn ·
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    ABSTRACT: Molybdenum oxide (MoOx ) and nickel oxide (NiOx ) thin films were deposited by reactive biased target ion beam deposition. MoOx thin film resistivity varied from 3 to 2000 ω· cm with a temperature coefficient of resistance (TCR) from ?1.7% to ?3.2%?K, and NiOx thin film resistivity varied from 1 to 300ω· cm with a TCR from ?2.2% to ?3.3%?K, both easily controlled by varying the oxygen partial pressure. Biased target ion beam deposited high TCR MoOx and NiOx thin films are polycrystalline semiconductors and have good stability in air. Compared with commonly used vanadium oxide thin films, MoOx or NiOx thin films offer improved process control for resistive temperature sensors. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).
    Optical Engineering 03/2015; 54(3). DOI:10.1117/1.OE.54.3.037101 · 0.95 Impact Factor
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    ABSTRACT: We present the optical function spectra of Cu 2SnSe3 determined from 0.30 to 6.45 eV by spectroscopic ellipsometry (SE) at room temperature. We analyze the SE data using the Tauc-Lorentz model and obtain the direct-bandgap energy of 0.49 ± 0.02 eV, which is much smaller than the previously known value of 0.84 eV for the monoclinic-phase Cu 2SnSe3. We also perform density-functional theory calculations to obtain the complex dielectric function data, and the results show good agreement with the experimental spectrum. Finally, we discuss the electronic origin of the main optical structures.
    Applied Physics Letters 01/2015; 106(4):043902. DOI:10.1063/1.4907202 · 3.30 Impact Factor
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    ABSTRACT: Assessment of the performance of single-junction hydrogenated amorphous silicon (a-Si:H) p-i-n configuration solar cells has been developed with a combination of real-time spectroscopic ellipsometry (RTSE) and current-voltage (I-V) measurements. For each layer, RTSE measurements enabled the determination of thickness and optical properties in the form of the complex dielectric function (ε = ε1 + iε2) spectra. RTSE tracked changes in a as a function of depth and was used to extract profiles in the i-layer bandgap and crystallite fraction in the n-layer. Through mapping I-V characteristic measurements, spatial variations in device performance were determined. By comparing individual devices at the location of the RTSE beam spot, the influence of a and thickness for each layer on device performance was identified through simulations of quantum efficiency yielding the shortcircuit current. This study compares two devices prepared with different superstrate preheating processes and finds that the combination of RTSE and I-V measurements along with quantum efficiency simulations were able to identify plasma damage to the transparent conducting oxide as the likely cause for variation in device performance. This comparison serves as one example of how the optically obtained information, such as thickness and a for each layer, can be used to understand the final device performance.
    IEEE Journal of Photovoltaics 01/2015; 5(1):307-312. DOI:10.1109/JPHOTOV.2014.2362294 · 3.17 Impact Factor

  • IEEE Journal of Photovoltaics 01/2015; DOI:10.1109/JPHOTOV.2015.2478056 · 3.17 Impact Factor
  • L. Karki Gautam · H. Haneef · M. M. Junda · D. B. Saint John · N. J. Podraza ·
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    ABSTRACT: Ellipsometric spectra have been analyzed using a procedure whereby the complex dielectric function (epsilon = epsilon(1) + i epsilon(2)) is parameterized using different, physically realistic models in isolated regions of a single spectrum while layer thicknesses are kept common. The isolated spectral ranges correspond to regions where the material of interest is either non-absorbing or heavily absorbing and physically realistic models describing epsilon are available. Common structural parameters (thicknesses) obtained by this divided spectral range approach are used to extract epsilon by numerical inversion over the full spectral range. The divided spectral range analysis has been applied to study amorphous hydrogenated silicon, nanocrystalline zinc oxide, and epitaxial bismuth stannate thin films, as well as single crystal bismuth germanate. Layer thicknesses obtained from the divided range analysis are compared to those using continuous, but not necessarily physically supported, parameterizations of epsilon over the full spectral range. The divided spectral range analysis yields inverted spectra in epsilon which are free of discontinuities beyond noise present in experimental data and whose determination does not require any assumption to be made about the line shape of epsilon in weakly absorbing regions.
    Thin Solid Films 11/2014; 571:548-553. DOI:10.1016/j.tsf.2014.03.020 · 1.76 Impact Factor
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    ABSTRACT: Real time spectroscopic ellipsometry (RTSE) from the near-infrared to ultraviolet has been applied for analysis of the deposition of polycrystalline thin films that form the basis of two key photovoltaic heterojunction configurations, superstrate SnO2/CdS/CdTe and substrate Mo/Cu(In1-xGax)Se-2/CdS. The focus of this work is to develop capabilities for monitoring and controlling the key steps in the fabrication of these device structures. Analysis of RTSE data collected during sputter deposition of CdS on a rough SnO2 transparent top contact provides the time evolution of the CdS effective thickness, or film volume per unit substrate area. This thickness includes interface, bulk, and surface roughness layer components and affects the CdS/CdTe heterojunction performance and the quantum efficiency of the solar cell in the blue region of the solar spectrum. Similarly, analysis of RTSE data collected during co-evaporation of Cu(In1-xGax)Se-2 (CIGS; x similar to 0.3) on a rough Mo back contact provides the evolution of a second phase of Cu2-xSe within the CIGS layer. During the last stage of CIGS deposition, the In, Ga, and Se co-evaporants convert this Cu2-xSe phase to CIGS, and RTSE identifies the endpoint, specifically the time at which complete conversion occurs and single-phase, large-grain CIGS is obtained in this key stage. Published by Elsevier B.V.
    Thin Solid Films 11/2014; 571:442-446. DOI:10.1016/j.tsf.2013.10.158 · 1.76 Impact Factor
  • Hamna F. Haneef · Nikolas J. Podraza ·
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    ABSTRACT: Optical properties in the form of the complex dielectric function spectra (epsilon = epsilon(1) + epsilon(2)) of commercially produced single crystal bismuth germanate (Bi4Ge3O12) have been studied using spectroscopic ellipsometry from 0.033 to 6.478 eV. Transmission measurements have also been performed in order to more accurately obtain low values of the absorption coefficient close to the band edge. Critical point parameters have been determined by analyzing the interband transitions. Infrared extended measurements yield vibrational modes corresponding to chemical bonding environments and the lattice structure. The overall analysis yields e from 0.033 to 6.478 eV and provides information about the crystal such as the energy of the band gap at 4.16 +/- 0.01 eV; interband transitions at 4.49, 4.75, 4.81, 5.08, 5.59, and 6.08 eV; transverse optical phonon modes at 283, 364, 395, 447, 702, 729, and 778 cm(-1) and longitudinal optical phonon modes at 292, 386, 445, 458, 710, 779, and 817 cm(-1). (C) 2014 AIP Publishing LLC.
    Journal of Applied Physics 10/2014; 116(16). DOI:10.1063/1.4898762 · 2.18 Impact Factor
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    ABSTRACT: Four applications of real-time spectroscopic ellipsometry (RTSE) and ex-situ mapping spectroscopic ellipsometry (SE) in thin-film hydrogenated silicon (Si:H) photovoltaics (PV) technology are reviewed with the common theme being the development and application of SE-derived growth evolution diagrams. The goals of these applications are to understand and consequently further advance this technology. In the first application, fabrication of engineered thin films consisting of periodic arrays of silicon (Si) nanocrystallites in an amorphous Si:H (a-Si:H) host matrix has been guided by a growth evolution diagram developed by RTSE for radio-frequency plasma-enhanced chemical vapor deposition (PECVD) using SiH4+H-2 mixtures. Such precisely controlled microstructures are of interest as possible intrinsic-layer components of p-i-n and n-i-p thin-film PV devices, and RTSE is shown to be a key technique for guidance in fabrication and for structure verification. In the second application of growth evolution diagrams, very-high-frequency PECVD intrinsic a-Si:H, hydrogenated amorphous silicon-germanium alloys (a-Si(1-x)Gex:H), and hydrogenated nanocrystalline silicon (nc-Si:H) have been investigated for use as the top, middle, and bottom-cell i-layer components, respectively, of triple-junction n-i-p solar cells. The growth evolution diagram for the bottom-cell i-layer, starting from an underlying mixed-phase amorphous + nanocrystalline silicon [(a + nc)-Si:H] n-layer, reveals a bifurcation at a critical H-2-dilution flow ratio R (R=[H-2]/[Si2H6], in this application) between mixed-to-amorphous phase evolution [(a+nc)-> a] at low R and mixed-to-nanocrystalline phase evolution [(a+nc)-> nc] at high R. The highest performance single-step nc-Si:H solar cell is found at minimal R while remaining on the nanocrystalline side of the identified bifurcation where suitable grain boundary passivation can be assured. Because of the importance of the roll-to-roll flexible substrate configuration in such multi-junction Si:H-based PV technology, RTSE has been demonstrated in a third application for monitoring PECVD of a-Si:H n-i-p solar cell structures on back-reflector-coated flexible roll-to-roll polymer substrates. RTSE has been used for probing along the center line of the moving substrate during deposition, and ex-situ mapping SE has been used over the full substrate area after deposition. Detailed studies of the top-most p-layer of the n-i-p solar cell have been performed, with the goal being to develop spatially-dependent (in contrast to R-dependent) growth evolution diagrams in order to evaluate uniformity across the width of the substrate and thus to enable optimization of the resulting a-Si:H PV modules. In this study, efficiency optimization occurs at the p-layer transition region in which a-Si:H nucleates from the i-layer surface, but evolves to predominantly nc-Si:H for improved contact to the top-most In2O3:Sn layer. In the fourth and final application reviewed here, the mapping-SE-deduced properties of the Si:H i and p-layers have been spatially correlated with device performance parameters from an array of n-i-p a-Si:H-based dot cells over a 13 x 13 cm(2) substrate area. Analysis of the SE data acquired over the full area provides property maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n-i-p structure. The mapped values adjacent to the PV devices have been correlated with the device performance parameters. When sufficient non-uniformity exists, these correlations enable optimization based on specific ranges of values that characterize the fundamental properties of the material and the film structure.
    Solar Energy Materials and Solar Cells 10/2014; 129:32-56. DOI:10.1016/j.solmat.2014.01.028 · 5.34 Impact Factor
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    ABSTRACT: Vanadium oxide (VOx) and hydrogenated silicon germanium (SixGe1-x) are the two predominant thin film material systems used as the active layer in resistive infrared imaging. Thin films of VOx used in microbolometers have a resistivity typically between 0.1 and 1 Omega-cm with a temperature coefficient of resistance, TCR vertical bar between 1.4%/K to 2.4%/K, while SixGe1-x: H thin films have a resistivity between 200-4,000 Omega-cm with a vertical bar TCR vertical bar between 2.9%/K to 3.9%/K. Future devices may require higher TCR materials, however, higher TCR is loosely associated with higher resistivity and therefore also with high noise. This work compares 1/f noise of high resistivity VOx and Ge:H thin films having vertical bar TCR vertical bar > 3.6%/K. The high TCR thin films of VOx were found to be amorphous while, depending on the deposition conditions, the Ge:H thin films were either amorphous or mixed phase of amorphous + nanocrystalline. Evaluation of these VOx and Ge: H thin films indicates a prospects for a superior process-property relation of 1/f noise in Ge: H thin films in comparison with thin films of VOx
    SPIE Defense + Security; 06/2014
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    ABSTRACT: The origin of the deep subgap states in amorphous indium gallium zinc oxide (a-IGZO), whether intrinsic to the amorphous structure or not, has serious implications for the development of p-type transparent amorphous oxide semiconductors. We report that the deep subgap feature in a-IGZO originates from local variations in the oxygen coordination and not from oxygen vacancies. This is shown by the positive correlation between oxygen composition and subgap intensity as observed with X-ray photoelectron spectroscopy. We also demonstrate that the subgap feature is not intrinsic to the amorphous phase because the deep subgap feature can be removed by low-temperature annealing in a reducing environment. Atomistic calculations of a-IGZO reveal that the subgap state originates from certain oxygen environments associated with the disorder. Specifically, the subgap states originate from oxygen environments with a lower coordination number and/or a larger metal-oxygen separation.
    Applied Physics Letters 06/2014; 104(23):232108-232108-4. DOI:10.1063/1.4883257 · 3.30 Impact Factor
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    ABSTRACT: In the scale-up of Cu(In1-xGax)Se2 (CIGS) solar cell processing for large-area photovoltaics technology, the challenge is to achieve optimum values of layer thicknesses, as well as CIGS Cu stoichiometry and alloy composition x within narrow ranges and simultaneously over large areas. As a result, contactless metrologies - those that provide such information in real-time or in-line process step by step, with the capabilities of large-area mapping - are of great interest in this technology. We have demonstrated high-speed multichannel spectroscopic ellipsometry (SE) in a number of modes for CIGS metrology, including 1) single-spot real-time SE monitoring of (In1-xGax)2Se3 as the first stage in multisource evaporation of three-stage CIGS; 2) control of Cu stoichiometry in the second and third stages of the process; 3) single-spot in situ SE analysis of alloy composition and grain size averaged through the thickness for the final CIGS film; 4) offline mapping of the CIGS thickness and composition over large areas, as well as mapping after each device fabrication step for correlation with local small area cell performance; 5) ex situ single-spot analysis of alloy composition profiles in CIGS and of completed solar cell stacks to extract thicknesses and properties of semiconductor and contact layers; and 6) predictive capability for quantum efficiency based on the results of SE multilayer analysis. With the future development of new instrumentation, the offline and ex situ capabilities in multilayer analysis and mapping will be possible in-line for both rigid and roll-to-roll flexible substrates.
    IEEE Journal of Photovoltaics 01/2014; 4(1):333-339. DOI:10.1109/JPHOTOV.2013.2282745 · 3.17 Impact Factor
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    ABSTRACT: An expanded-beam spectroscopic ellipsometer has been developed and applied toward in situ high-speed imaging/mapping analysis of large area spatial uniformity for multilayer coated substrates in roll-to-roll thin-film photovoltaics (PV). Slower speed instrumentation available in such analyses applies a 1-D detector array for spectroscopic mapping and involves width-wise translation of the ellipsometer optics over the moving coated substrate surface, measuring point-by-point in a time-consuming process. The expanded-beam instrument employs instead a 2-D detector array with no moving optics, exploiting one array index for spectroscopy and the second array index for line imaging across the width of a large area sample. Thus, the instrument enables imaging width-wise and mapping length-wise for uniformity evaluation at the high linear substrate speeds required for real-time, in situ, and online analysis in roll-to-roll thin-film PV. In this investigation, we employ the expanded beam technique to characterize the uniformity of the Ag, ZnO, and n-type hydrogenated amorphous silicon (a-Si:H) layers of an a-Si:H n-i-p structure deposited on a flexible polyimide substrate in the roll-to-roll configuration. Spectroscopic ellipsometry data across a line image were collected as the substrate was translated by a roll-to-roll mechanism. Coated areas as large as 12 cm × 45 cm were analyzed in this study for layer thickness and optical properties by applying the appropriate analytical models for the complex dielectric functions of the Ag, ZnO, and n-type a-Si:H layers.
    IEEE Journal of Photovoltaics 01/2014; 4(1):355-361. DOI:10.1109/JPHOTOV.2013.2284380 · 3.17 Impact Factor
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    ABSTRACT: Electrical properties for resistive microbolometer sensor materials including resistivity, temperature coefficient of resistance (TCR), and normalized Hooge parameter were explored in n-type a-Si:H and a-Si 1−xCx:H prepared by plasma enhanced chemical vapor deposition. The complex dielectric function spectra (ε = ε 1 + iε 2) and structure were measured by spectroscopic ellipsometry. Two-dimensional drift-diffusion simulations were used to understand the band-tail slope dependency of TCR and 1/f noise.
    Journal of Applied Physics 11/2013; 114(18):183705. DOI:10.1063/1.4829013 · 2.18 Impact Factor
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    ABSTRACT: Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.
    APL Materials 10/2013; 1(4):042112. DOI:10.1063/1.4824041 · 2.79 Impact Factor
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    ABSTRACT: Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.
    Journal of Applied Physics 07/2013; 114(3). DOI:10.1063/1.4813486 · 2.18 Impact Factor
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    ABSTRACT: Biaxial strain induces a phase transition from a pseudo-rhombohedral (R) to pseudo-tetragonal (T) phase in BiFeO3 (BFO) thin films. Using optical second harmonic generation, we measure the nonlinear optical dij coefficients at a fundamental wavelength of 1550 nm for R and T-BFO thin films. A large increase of the dij magnitudes is observed for T-BFO in comparison to R-BFO. The dij magnitudes for T-BFO were measured to be: |d33| = 18.1±2.4, |d31| = 60.8±8.1, and |d15| = 47.0±4.2, and for R-BFO: |d33| = 15.1±2.1, |d31| = 8.5±1.2, |d15| = 0.9±0.1, and |d22| = 18.7±2.6 (pm/V). The strain-enhanced nonlinear optical properties of T-BFO thin films make them potentially useful for optical applications.
    Applied Physics Letters 07/2013; 103(3). DOI:10.1063/1.4812978 · 3.30 Impact Factor
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    ABSTRACT: 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.
    IEEE Journal of Photovoltaics 07/2013; 3(3):1084-1089. DOI:10.1109/JPHOTOV.2013.2251758 · 3.17 Impact Factor
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    ABSTRACT: In this study, real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CuIn1-xGaxSe2; CIGS) thin film deposition by the three-stage co-evaporation process used for high efficiency photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1-xGax)2Se3 (IGS) at a temperature of 400°C on substrates consisting of soda lime glass coated with thin film molybdenum (Mo). This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result, precise composition and thickness control is desired in order to achieve the optimum open circuit voltage (Voc) and fill-factor (FF) of the resulting CIGS solar cell. In fact, RTSE has been applied broadly in previous studies for the characterization of complicated thin film deposition processes used in PV device fabrication - with the potential for process monitoring and control in many situations. In the case of the first-stage IGS deposition of this study, RTSE has been used to characterize the time evolution of (i) the Mo/IGS interface filling fraction, (ii) the IGS surface roughness layer thickness, and (iii) the IGS bulk layer thickness, as the depositing layer covers the rough Mo surface. In addition, one can extract the evolution of the bulk layer optical properties, expressed in the form of the complex dielectric function, which can serve as a fingerprint for IGS composition and also provide information on relative void vol.% and grain size (or defect density) in the IGS. Overall the structural and compositional information can assist in understanding the growth of three-stage CIGS absorbers for solar cells and in optimizing cell performance.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: Real time spectroscopy ellipsometry (RTSE) has been applied to study the evolution of thin film optical structure during sputter deposition of polycrystalline CdS/CdTe solar cell stacks on transparent conducting oxide (TCO) coated glass substrates optimized for high efficiency. RTSE provides information on (i) interface formation to the underlying high resistivity transparent (HRT) layer during initial CdS growth, (ii) bulk layer CdS growth and its surface roughness evolution, (iii) CdS/CdTe interface formation when the overlying CdTe layer is deposited on the CdS, and (iv) CdTe bulk layer growth and its roughness evolution. Structural/optical models developed in the analysis of RTSE data acquired at a single point are also applied in the analysis of ex situ mapping SE data obtained over the area of the completed solar cell stack. As a result, maps of the structural parameters can be extracted, which then can be correlated with maps of the small area device performance. When uncorrelated non-uniformities exist over the area, optimization by combinatorial methods is possible.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013

Publication Stats

847 Citations
153.33 Total Impact Points


  • 2006-2015
    • University of Toledo
      • • Department of Physics and Astronomy
      • • Center for Photovoltaics Innovation and Commercialization
      Toledo, Ohio, United States
  • 2011
    • William Penn University
      Worcester, Massachusetts, United States
  • 2004-2011
    • Pennsylvania State University
      • • Department of Electrical Engineering
      • • Department of Materials Science and Engineering
      • • Materials Research Institute
      • • Department of Physics
      University Park, MD, United States