Symposium Organizers
Timothy Gessert National Renewable Energy Laboratory
Sylvain Marsillac University of Toledo
Takahiro Wada Ryukoku University
Ken Durose Durham University
Clemens Heske University of Nevada-Las Vegas
Y1: Growth and Performance of Compound Thin Film Solar Cells
Session Chairs
William Shafarman
Takahiro Wada
Tuesday PM, April 10, 2007
Room 2024 (Moscone West)
9:30 AM - **Y1.1
Challenges in Understanding Chalcopyrite Semiconductors for Photovoltaics.
Hans-Werner Schock 1
1 Solar Energy Technology, Hahn-Meitner-Institut, Berlin Germany
Show Abstract10:00 AM - **Y1.2
Progress in CIGS Solar Cell Processes and Characterization.
Shigeru Niki 1 , Syogo Ishizuka 1 , Kei-ichiro Sakura 1 , Koji Matsubara 1 , Akimasa Yamada 1 , Hitoshi Tampo 1 , Hisayuki Nakanishi 2 , Norio Terada 3
1 RCPV, AIST, Tsukuba, Ibaraki, Japan, 2 , Tokyo University of Science, Noda, Chiba, Japan, 3 , Kagoshima University, Kagoshima, Kagoshima, Japan
Show Abstract10:30 AM - Y1.3
A Novel Laser-Assisted Deposition Technique for CIGS Thin Films and Solar Cells.
Tokio Nakada 1 , Keita Nozawa 1 , Yasuyuki Ishii 1
1 Electrical Engineering and Electronics, Aoyama Gakuin University, Sagamihara Japan
Show AbstractHigh-quality Cu(In1-xGax)Se2 (CIGS) thin films are required for achieving high efficiency thin-film solar cells. This contribution presents a novel laser-assisted-deposition (LAD) technique for improving the crystallographic quality of CIGS thin films and cell performance. In LAD technique a pulse excimer laser (KrF : 248nm, 100Hz) was irradiated onto the substrate surface during CIGS deposition by three-stage process. The crystallographic properties of CIGS thin films grown by three-stage process and LAD were characterized by SEM, TEM and XRD analysis. Time-resolved photoluminescence (TRPL) measurement was also carried out in order to predict a photovoltaic potential of CIGS thin films grown by LAD. The grain size of CIGS thin films became large, and the (112) orientation was enhanced by LAD process. The CIGS deposition was stopped at each step of three-stage process to check the crystallographic properties. As a result, the (InGa)2Se3(110)orientation which leads to the (112) orientation of CIGS was enhanced by the laser irradiation at the 1st step. In contrast, almost no effect was observed when the excimer laser was irradiated at 2nd or 3rd steps. Therefore it becomes evident that the excimer laser irradiation at the first step was most effective to improve the crystalline quality of CIS thin films. TRPL measurement revealed that the PL life time increased from 33 to 45 ns by the laser irradiation during CIGS deposition. This suggests that high efficiency solar cell would be achieved by using LAD process. The solar cells with a ZnO:Al/ZnO/CIGS/Mo/SLG structure were fabricated using two types of CIGS thin films prepared by three-stage process and LAD technique. The average Ga/(In+Ga) determined by ICP (Inductively Coupled Plasma Spectroscopy) was around 0.4. It was found that the open circuit voltage (Voc), fill factor (FF) and the efficiency of the solar cells increased significantly by LAD technique. For instance, a 16.3% efficiency (with Voc=0.659 V, Jsc=34.8 mA/cm2, FF=0.709) was achieved for LAD. It contrasts well with a 14.4 % efficiency (with Voc=0.624 V, Jsc=34.7 mA/cm2, FF=0.662) for three-stage process with the same deposition conditions. This result agrees well with the TRPL data mentioned above. Further experiment was carried out for CIGS thin films with different Ga contents. It was also found that the crystalline quality of CIGS thin films and cell performance were improved by LAD process for all range of Ga content. In conclusion, we found that the LAD process is effective to improve not only crystalline quality of CIGS thin films but also the cell performance.This work was supported in part by NEDO as “R & D for Future technology of PV system”.
10:45 AM - Y1.4
A Liquid Precursor Approach to CuInSe2 Based Photovoltaics.
Jennifer Nekuda 1 2 , Maikel van Hest 2 , Calvin Curtis 2 , Alex Miedaner 2 , Ryan O'Hayre 1 , David Ginley 2
1 Metallurgy and Materials Engineering, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractTuesday, April 10Transferred Poster Y3.38 to Y1.4 @ 9:45 amA Liquid Precursor Approach to CuInSe2 Based Photovoltaics. Jennifer Ann Nekuda
11:30 AM - **Y1.5
Development of Bi-facial CdTe Solar Cells on Glass and Polymer Substrates.
Genadiy Khripunov 2 , Alessandro Romeo 3 , Hans Zogg 2 , Ayodhya Tiwari 1 2
2 Thin Film Physics Group, ETH Zurich, Zurich Switzerland, 3 Faculty of Science, University of Verona, 37134 Verona, Italy Italy, 1 Electronic and Electrical Engineering, Loughborough University, Leicestershire United Kingdom
Show AbstractTueday, April 10New Presentation Time and Paper NumberUpgraded to Invited StatusY1.4 @ 9:45 am to *Y1.5 @ 10:30 amDevelopment of Bi-facial CdTe Solar Cells on Glass and Polymer Substrates. Ayodya Nath Tiwari
12:00 PM - Y1.6
Preparation of Cu(In1-xGax)Se2 Thin Films and Solar Cells Using a Se-radical Beam Source.
Shogo Ishizuka 1 , Akimasa Yamada 1 , Hajime Shibata 1 , Keiichiro Sakurai 1 , Paul Fons 2 , Koji Matsubara 1 , Shigeru Niki 1
1 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki Japan, 2 Center for Applied Near-Field Optics Research, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan
Show Abstract12:15 PM - Y1.7
Temperature-dependent Degradation Modes in CdS/CdTe Devices.
David Albin 1 , Samuel Demtsu 2 , Anna Duda 1 , Wyatt Metzger 1
1 PV Devices and Electronic Materials, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , SoloPower Inc, Milipitas, California, United States
Show AbstractA set of 24 identically made CdTe devices were subjected to accelerated lifetime testing (ALT) using open-circuit bias, 1 sun illumination, and heating under room ambient (dry) conditions at 60, 80, 100, and 120 *C. A total of 6 devices were tested at each temperature. Data was periodically collected for up to 2000+ hours. A systematic pattern of degradation became apparent with increased stress temperature. At 60-80 *C, degradation mechanisms associated with the CdTe/backcontact dominate. In this temperature regime, degradation modes include increased series resistance and increased backcontact barrier height resulting in increased 1st quadrant rollover. As temperature was increased above 80 *C, we observed a significant increase in voltage-dependent collection, which directly affected FF but not Voc. With higher ALT stress temperatures of 100-120 *C, recombination and loss in Voc were the dominant degradation modes. Increased voltage-dependent collection and recombination are believed to be associated with the forward-bias enhanced diffusion of Cu+ from the backcontact to the heterojunction during these ALT studies. This latter understanding was demonstrated by a parallel study using devices in which Cu introduced at the backcontact in a systematically controlled fashion. In these devices, Cu layers of thickness varying from 0 to 100 nm were evaporated onto CdTe devices that had undergone prior nitric-phosphoric etches. This etch produces a Te-rich layer on the surface of the CdTe within which evaporated Cu can react to form CuxTe. Time resolved photoluminescence measurements show a systematic decrease in bulk lifetime near the CdS/CdTe junction with increased evaporated Cu layer thickness. This decrease has been correlated with a decrease in device Voc. Quantum efficiency measurements have also confirmed a decrease in red response with increasing Cu that helps explain the voltage-dependent collection observed in devices stressed at higher ALT temperatures.Further validation of our belief that Cu migration is responsible for degradation modes observed at higher ALT stress temperatures was demonstrated by extracting activation energies from measured degradation rates as a function of time and temperature. Best-fit degradation (D) models using a power law type relationship were used to determine the effective life (L) of devices assuming a 10% drop in efficiency as the "definition" of useable life. Arrhenius-derived activation energies clearly associate an activation energy of 2.8 eV with the lower temperature degradation. The higher temperature activation energy was determined to be 0.63 eV, which agrees well with the literature value of 0.67 eV for Cu diffusion in CdTe. Thus, Cu diffusion is identified as the rate-dominating mechanism for degradation in our CdTe devices stressed at higher ALT temperatures.This abstract is subject to government rights.
12:30 PM - Y1.8
Investigation of ZnO:Al Doping Level and Deposition Temperature Effects on CIGS Solar Cell Performance.
Joel Duenow 1 2 , Timothy Gessert 2 , David Wood 1 , Brian Egaas 1 2 , Rommel Noufi 2 , Timothy Coutts 2
1 Dept. of Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract12:45 PM - Y1.9
InGaAs Solar Cells Grown on Wafer-Bonded InP/Si Epitaxial Templates.
James Zahler 2 , Katsuaki Tanabe 1 , Corinne Ladous 2 , Thomas Pinnington 2 , Frederick Newman 3 , Harry Atwater 1 2
2 , Aonex Technologies, Pasadena, California, United States, 1 Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California, United States, 3 , Emcore Photovoltaics, Albuquerque, New Mexico, United States
Show AbstractSolar cells fabricated on InP/Si epitaxial templates formed by wafer bonding and layer transfer have potential for significant cost and weight reduction relative to cells fabricated on bulk InP substrates. InP/Si substrates could be used as epitaxial templates for InGaAsP/InGaAs dual-junction solar cells lattice-matched to InP as well as conventional InP single-junction cells. Ultimately, InP/Si epitaxial templates are extendable to fabrication of ultrahigh efficiency four-junction (4J) AlInGaP/GaAs/InGaAsP/InGaAs cells via a direct bond interconnect between subcell structures of InGaAsP/InGaAs grown on InP/Si and AlInGaP/GaAs grown on GaAs to form the overall 4J cell structure. In this work we report on the first known demonstration of InGaAs solar cell fabrication on wafer-bonded InP/Si epitaxial templates.InP films ~900 nm thick were transferred onto thermally-oxidized Si substrates via wafer bonding. Transfer of the InP thin film was induced by helium-implantation of the InP and annealing of the bonded InP/Si structure to exfoliate the film. The damaged surface region of the as-transferred InP film was removed using an etching process to leave a film of ~400 nm with a roughness of ~10 nm-rms, as measured by contact mode atomic force microscopy. InGaAs solar cells were grown on the InP/Si epitaxial templates by metalorganic chemical vapor deposition. The solar cells had an n-type InGaAs emitter and a p-type InGaAs base with bandgap energies of 0.74 eV and nominally lattice-matched to (001)InP. Photovoltaic current-voltage (I-V) characteristics of the 1 mm2 InGaAs cells were measured under AM1.5 Global solar spectrum with 1-sun total intensity (100 mW cm-2). For comparison, photovoltaic I-V characteristics of InGaAs cells grown on bulk epi-ready (001)InP substrates were also measured. The device parameters for the InGaAs cell grown on the wafer-bonded InP/Si epitaxial template were Jsc = 35.8 mA cm-2, Voc = 0.29 V, FF = 0.56, and η = 5.8%, where Jsc, Voc, FF and η are short-circuit current, open-circuit voltage, fill factor and energy conversion efficiency, respectively. This performance was comparable to that of the InGaAs cells grown on bulk (001)InP substrates, Jsc = 26.9 mA cm-2, Voc = 0.31 V, FF = 0.63, and η = 5.2%.
Y2: Novel Materials and Processes
Session Chairs
Ken Durose
Hans-Werner Schock
Tuesday PM, April 10, 2007
Room 2024 (Moscone West)
2:30 PM - **Y2.1
Issues for Composition Control in the Deposition of CuInSe2-based Alloys.
William Shafarman 1 , Robert Birkmire 1 , Shiro Nishiwaki 1 , Greg Hanket 1
1 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractThe composition of Cu(InGa)Se2 thin films grown by elemental evaporation is controlled in a predictable manner by the delivery of the elemental fluxes. Under typical deposition conditions, the Cu and Se are distributed uniformly through the bulk of the film while the Ga and In distributions are determined by their flux–time profiles during growth. However, with different alloys and growth processes the film composition can be controlled by other factors which will be described in this work. The formation of stable intermetallic phases plays a critical role in the growth of CuAlSe2 by elemental evaporation and in the composition control of Cu(InGa)Se2 or Cu(InGa)(SeS)2 by the reaction of metal precursors. In each case, procedures to overcome the formation of the intermetallic phases and form single phase chalcopyrite materials with controlled composition will be described. In the elemental evaporation of CuIn(SeS)2 or Cu(InGa)(SeS)2 the relative incorporation of the chalcogen species is controlled by the presence of excess Cu and the effects of film composition versus relative flux variation will be described.
3:00 PM - Y2.2
Fabrication and Characterization of Cd1-xMgxTe Thin Films and Their Application in Solar Cells
Ramesh Dhere 1 , Kannan Ramanathan 2 , John Scharf 1 , Bobby To 1 , Anna Duda 1 , Helio Moutinho 1 , Rommel Noufi 1
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 , MiaSole, Santa Clara, California, United States
Show AbstractTwo-junction tandem solar cells require a top cell with a bandgap in the range of 1.6–1.8 eV. Ternary alloys of CdTe are of great interest for this application because of the success of CdTe solar cells. Considerable work has been conducted on Cd1-xZnxTe (CZT) polycrystalline thin films, but the development of CZT solar cells is hampered by the high affinity of Zn to oxygen and chlorine, which are routinely used in fabricating high-efficiency CdTe solar cells. Cd1-xMgxTe (CMT) offers several unique advantages, such as a large change in bandgap for the same composition as compared to CZT. The lattice mismatch between CdTe and CMT is minimal in the entire composition range, and the alloy retains the sphalerite structure of CdTe in a wide composition range. In this paper, we present our work on developing CMT polycrystalline thin films. CMT thin films were fabricated by co-evaporation of CdTe and Mg at a substrate temperature of 400ο C. The spatial separation of the two sources resulted in a compositional gradient, which allowed the deposition of a wide range of compositions from fewer runs. Stable films for compositions up to x=0.73 were fabricated, and the structural properties of the samples were studied by X-ray diffraction. The data show that the films have a sphalerite structure and are preferentially oriented in the <111> direction, and the lattice constant varies linearly with composition. Optical analysis of the samples shows that the optical bandgap varies linearly with composition and can be described byEg(x) = 1.6 x + 1.46 eV.The optical absorption coefficient of the alloy films for the entire composition range (up to x=0.73) is over 6x104/cm for energies above the bandgap, indicating a direct-gap semiconductor. We have fabricated solar cells using these films with the following structure: glass / SnO2 / CdS / CMT / back contact. Using the same processing conditions used for CdTe solar cells, the device efficiency was less than 2%. Post-deposition chloride heat-treatment in oxygen ambient resulted in partial dissociation of the CMT alloy and loss of Mg. With modified contact processing, we were able to improve efficiencies to over 5%. With modified chloride processing, we were able to minimize alloy decomposition and further improve the efficiencies. We have obtained devices with open-circuit voltages of up to 845 mV and efficiencies of 8% for CMT devices with a bandgap in the range of 1.6–1.62 eV. We will present a detailed analysis of the device properties and their correlation to materials properties of CMT. This abstract is subject to government rights.
3:15 PM - Y2.3
Nanostructured ZnS:In2S3 Buffer Layers. Can Voltage and Efficiency be Improved Through Interface Inhomogeneities on a Scale Below the Minority Carrier Diffusion Length?
Nicholas Allsop 1 , Andreas Haensel 1 , Christian Camus 1 , Sophie Gledhill 1 , Iver Lauermann 1 , Christian Fischer 1 , Martha Lux-Steiner 1
1 Solar Energy - Heterogeneous Material Systems, Hahn-Meitner-Institute, Berlin Germany
Show Abstract3:30 PM - Y2.4
Experimental and Theoretical Study of Fluorine Doping to Tin Oxide.
Xiaonan Li 1 , Sally Asher 1 , Joel Pankow 1 , Lei Kerr 2 , Mailasu Bai 3 , Timothy Gessert 1
1 5200, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Miami University of Ohio, Oxford, Ohio, United States, 3 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractConductive tin-oxide (SnO2) films on glass substrates are used extensively for transparent electrodes in thin-film photovoltaic solar cells, electrochromic devices, flat-panel displays, and gas sensors. SnO2 with a tetragonal structure is naturally an n-type semiconductor because of a deviation from stoichiometry. The best n-type dopants for SnO2 is fluorine. With it, the electron concentration of 5-10^20/cm^3, and μ of 40 cm^2/V-s has achieved. However, the commonly used fluorine dopant for research used SnO2 film, bromotrifluoromethane (CBrF3), is a green–house gas. Thus, the alternative fluorine source is needed to replace the CBrF3. In this study, we first estimated the doping efficiency for different dopant precursors (CF4, CBrF3, and CIF3). We find that CIF3 is a most reactive precursor in the three precursors that studied. The reactivity of CF4 is much lower than the other two precursors. This prediction is consistent with the experimental observation. Then we did experiments to studied CIF3 source as replacement. The electric and optical properties of the SnO2 film that doped by CBrF3 and CIF3 source are compared. The effects of deposition temperature, different dopant and oxygen gas flow rates are studied. We found that due to the high volatility of CBrF3 precursor, the fluorine doping efficiency is strongly dependent on the substrate temperature and reaction chamber pressure. As a comparison, to achieve the similar carrier concentration, far less CIF3 dopant is required. However, for similar film thickness, with the CIF3 doping, the SnO2 film has lower mobility (<30 cm^2/Vs) and higher optical absorption compared with the films doped with CBrF3.This abstract is subject to government rights.
3:45 PM - Y2.5
Development of a Wide Band Gap Cd1-xMgxTe Film for Applications in Tandem Devices.
Xavier Mathew 1 2 , Anthony Vasko 1 , Jennifer Drayton 1 , Alvin Compaan 1
1 Physics and Astronomy, Univ. of Toledo, Toledo, Ohio, United States, 2 CIE, UNAM, Temixco, Morelos, Mexico
Show Abstract4:30 PM - **Y2.6
Zn-based Buffer Layer and High-quality CIGS Films Grown by a Novel Method.
Akira Yamada 1 , Fanying Meng 1 , Yoshiyuki Chiba 2 , Makoto Konagai 2
1 Quantum Nanoelectronics Research Center, Tokyo Institute of Technology, Tokyo Japan, 2 Department of Physical Electronics, Tokyo Institute of Technology, Tokyo Japan
Show Abstract5:00 PM - Y2.7
Growth and Characterization of Chalcopyrite Nanocrystals: Beyond Conventional Thin Films.
David Fuertes Marron 1 , Sebastian Lehmann 1 , Sascha Sadewasser 1 , Martha Ch. Lux-Steiner 1
1 Department of Solar Energy, Hahn-Meitner Institut, Berlin Germany
Show Abstract5:15 PM - Y2.8
Transparent Back Contacts in CdTe/CdS: Evaluation for Tandem Cells.
Viral Parikh 1 , Anthony Vasko 1 , Alvin Compaan 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show AbstractBy using a transparent back contact (TBC), a CdTe/CdS cell may be transparent to photons with energy less than the 1.5 eV bandgap of CdTe. This allows CdTe cells to be used as a top cell in a tandem structure, of either the two- or four-terminal kind. We have fabricated CdTe cells with transparent back contacts yielding efficiencies of up to 9%. We have evaluated the back contacts in terms of cell efficiency and transparency using combinations of p-type ZnTe:N and ZnTe:Cu, with n-type ZnO:Al and ITO (for total combinations of ZnTe:N/ZnO:Al, ZnTe:N/ITO, ZnTe:Cu/ZnO:Al, and ZnTe:Cu/ITO). CdTe thicknesses from 0.6 to 2.5 micrometers have been used. CdTe of lesser thickness may transmit some above bandgap photons, allowing for current matching in two terminal devices. We have performed bifacial IV and QE characterization under varying degrees of white light bias. Modeling has been employed to extract information on depletion widths and drift and diffusion lengths, and to understand the impact of TBC materials on the carrier collection from the primary junction. Acknowledgements – This work is supported by the NREL high performance PV program.
5:30 PM - Y2.9
An Indium Free Transparent Conductive Oxide: doped TiO2.
M. van Hest 1 , M. Gillispie 2 , M. Dabney 1 , J. Perkins 1 , D. Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Iowa State University, Ames, Iowa, United States
Show AbstractMost of the commonly used transparent conductive oxides (TCO) contain indium. Due to its cost, there is a clear need for an indium free alternative. Doped anatase TiO2 thin films grown on SrTiO3 (STO) using pulsed laser deposition (PLD) have been shown to be viable In-free transparent conductors with conductivities of 3500-5000 S/cm for 40 nm thick films. Little is known about this material. Here, we will show a detailed study of this material deposited by PLD and sputtering. This study demonstrates that the conductivity in this material is a bulk effect. It is also demonstrated that the TiO2 needs to be biaxially textured (004) anatase in order to be highly conductive. Since PLD is most likely not a viable way of deposition for large area, alternative methods for deposition of this material are studied. It is demonstrated that doped anatase TiO2 films can be deposited by RF magnetron sputtering. Thin films of Ti0.85Nb0.15O2 were deposited on STO and glass substrates using a ceramic oxide target at varying substrate temperatures in a pure Ar atmosphere. For films grown on STO, conductivity as high as 1500 S/cm has been obtained. X-ray diffraction (XRD) analysis showed that the most conductive films were (004)-textured anatase TiO2. Films grown on glass showed a maximum conductivity of ~100 S/cm, 15x smaller than films on STO. This decrease in conductivity is largely due to a decreased mobility. XRD analysis for the most conductive films on glass showed randomly oriented anatase TiO2.
5:45 PM - Y2.10
Towards Lower Deposition Temperatures of Spray Deposited ZnO Films.
Sophie Gledhill 1 , Pablo Thier 1 , Nicholas Allsop 1 , Martha Lux-Steiner 1 , Christian Herbert Fischer 1
1 Solar Energy 2, Hahn-Meitner Institute, Berlin Germany
Show AbstractY3: Posters
Session Chairs
Wednesday AM, April 11, 2007
Salon Level (Marriott)
9:00 PM - Y3.1
In-situ Characterization of As-grown Surface of CIGS Films.
Hirotake Kashiwabara 1 , Shimpei Teshima 1 , Kazuya Kikunaga 1 , Kazunori Takeshita 1 , Tetsuji Okuda 1 , Kozo Obara 1 , Keiichiro Sakurai 2 , Shogo Ishizuka 2 , Akimasa Yamada 2 , Koji Matsubara 2 , Shigeru Niki 2 , Norio Terada 1 2
1 Nano-Structure and Advanced Materials, Kaghosima University, Kagoshima Japan, 2 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan
Show AbstractControl of hetero interfaces in Cu(In1-xGax)Se2 [CIGS] based solar cells is one of the keys for pursuing higher conversion efficiency. For the interface between CdS buffer grown by chemical bath deposition (CBD) and In-rich CIGS absorber, it has been repeatedly observed that the CIGS region adjacent to the interface has Cu-deficient composition and much wider band gap energy about 1.3 eV, with respect to those of the corresponding bulk. It has been recognized that the presence of this wide gap region would suppress the carrier recombination and cause a notable improvement of cell-performances. Understanding of formation mechanism of the wide gap surface-region of CIGS is, therefore, important for the precise control of the interface. There are, however, few experimental reports about it. Theoretical calculations have indicated the structural instability of stoichiometric surface of CIGS, and stabilization of it by introduction of Cu-vacancies. It suggests the self-formation of Cu-deficient surface during growth process of CIGS. On the other hand, serious modifications of the CIGS surfaces in CBD process is also pointed out, since Cu ions in CIGS could be selectively dissolved in the bath.In order to determine in which sequence the wide gap region is formed, chemical formula and electronic structure of the as-grown CIGS layer by in-situ XPS, UPS and IPES.Specimen films with a nominal composition of Cu0.93(In0.65Ga0.35)Se2 were grown by conventional three stage co-evaporation. They were transported to the analysis system by using a vacuum vessel. During the transportation, pressure of the vessel was maintained at mid 10-9 Torr or better. This method yielded a clean feature of the as-received surfaces; carbon and oxygen were below detection limit of XPS.The as-grown surface exhibited seriously Cu and Ga depleted composition: Cu/(Cu+In+Ga+Se) = 0.08 ~ 0.10, Cu/(In+Ga) = 0.29 ~ 0.31 and Ga/(In+Ga) = 0.15. Though slow increases of these ratios were observed in the depth profile, the region 10 ~ 20 nm under the original surface had Cu/(Cu+In+Ga+Se) = 0.14, Cu/(In+Ga) = 0.40 and Ga/(In+Ga) = 0.38, which are between so-called 1-3-5 and 1-5-8 Cu-deficient phases. These results revealed that the Cu and Ga deficient phase is already formed in the final stage of the growth of CIGS.UPS/IPES measurements of the as-grown surface revealed on expansion of band gap energy up to 1.4 eV and n-type character. In the depth profile of the electronic structure, examined by using a low energy ion etching, a gradual rise of Fermi level was observed.These results mean that the as-grown surface of CIGS by the three stage method already has the wide band gap, n-type electronic structure and Cu-, Ga-deficient composition. They also suggested that the termination method of the CIGS growth should be one of the important factors of the nature of CdS/CIGS interface.
9:00 PM - Y3.10
Compositional Variance of Conductivity in Sputter-Deposited Amorphous Indium Zinc Oxide Transparent Films
Andrew Leenheer 1 2 , John Perkins 2 , Andrew Cavendor 1 2 , Matthew Taylor 2 , Maikel van Hest 2 , Ryan O'Hayre 1 , David Ginley 2
1 Materials Science, Colorado School of Mines, Golden, Colorado, United States, 2 National Center for Photovoltaics, National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractDue to its high electrical conductivity, high visible-spectrum transparency, smoothness and ease of deposition, amorphous indium zinc oxide (IZO) is a n-type transparent conducting oxide (TCO) of increasing interest for photovoltaic and other optoelectronic applications. IZO thin films deposited by magnetron sputtering are amorphous over the metals-only composition range ~55 to 85 atomic %. In this work, five different single-composition targets spanning the amorphous range were used to sputter thin films on glass and fused silica with varying oxygen content. The resistivity, carrier concentration, and hall mobility were measured for the films, as well as optical transmission and reflection spectra for λ=300-900 nm light. For these IZO films, the conductivity can be controllably varied over six orders of magnitude from ~ 2.5 x 103 S/cm down to ~ 10-3 S/cm depending on the composition and deposition conditions. Adding oxygen to the argon sputtering gas generally lowers the carrier concentration. In addition, the electron mobility increases with lower indium content, while the carrier concentration drops with lower indium content.
9:00 PM - Y3.11
Device Behavior of Cu(InGa)(SeS)2 Solar Cells with >1.5 eV Bandgap.
Shiro Nishiwaki 1 , William Shafarman 1
1 , Institute of Energy Conversion, Newark, Delaware, United States
Show Abstract9:00 PM - Y3.12
Characterization of ZnO Thin Film Prepared by Solid Source Chemical Vapor Deposition.
Hau-Ting Hu 1 , Yang-Chun Fang 1 , Shih-Hsuan Yang 1 , Liang Chao 1
1 Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan
Show Abstract9:00 PM - Y3.13
In situ Investigation of Reaction Pathways of Cu-Se, In-Se and Ga-Se Mixed and Bilayer Precursors.
Woo Kyoung Kim 1 , E. Andrew Payzant 2 , Tim Anderson 1
1 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 2 Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show Abstract9:00 PM - Y3.14
First-principles Calculations on Zn1-xMgxO Window Layer for CIS Thin Film Solar Cells.
Tsuyoshi Maeda 1 , Akio Shigemi 1 , Takahiro Wada 1
1 Materials Chemistry, Ryukoku University, Otsu, Shiga, Japan
Show Abstract9:00 PM - Y3.15
Fabrication of (Cu,Ag)InSe2 Thin films by a Combination of Mechanochemical and Screen Printing/Sintering Processes
Syuusuke Nomura 1 , Yoshihiro Matsuo 1 , Takahiro Wada 1
1 Materials Chemistry, Ryukoku University, Otsu, Shiga, Japan
Show AbstractCuInSe2 (CIS) and its solid solution such as Cu(In,Ga)Se2 (CIGS) are excellent thin-film photovoltaic materials. The CIGS films are usually deposited by physical vapour deposition (PVD) or sputtering and “selenization”. However, vacuum-based deposition processes, such as PVD and sputtering are complex and expensive. Therefore, some research groups have proposed non-vacuum deposition techniques for CIGS thin films. We have fabricated CIGS thin films by our original non-vacuum process, a combination of mechanochemical and screen printing/sintering processes [1]. Our preliminary CIGS solar cells showed an efficiency of 2.7%. Recently, wide-band-gap Ag(In,Ga)Se2 thin film solar cells with an efficiency of nearly 10% was demonstrated [2]. AgInSe2 has a bang gap (Eg) of 1.24 eV, which is a little wider than Eg of CuInSe2, 1.04 eV. In this study, we fabricated (Cu,Ag)InSe2 (CAIS) thin films by a combination of mechanochemical and screen printing/sintering processes and characterized their properties.
Elemental powders such as Cu, Ag, In, Se were weighted to give a molar ratio of (Cu1-xAgx): In:Se =1:1:2. CAIS powder was synthesized from elemental powders by a planetary ball milling in N2 gas atmosphere. The synthesized powder was determined to be chalcopyrite compound by X-ray powder diffraction using Cu-Kα radiation. Particulate precursors ink was prepared by a mixing of the obtained CAIS powder with an organic solvent. The precursor layer was deposited on a Mo coated soda lime glass substrate by the screen-printing. The organic solvent was removed from the screen-printed CIGS film by the heating in N2 gas atmosphere. Porous precursor layer was sintered into dense polycrystalline films by heat-treatment under an atmospheric-pressure N2 gas atmosphere. Crystal structure of the film was analyzed by x-ray diffraction and the microstructure was observed in a scanning electron microscope (SEM).
The x-ray diffraction patterns of CAIS powders showed characteristic peaks of the chalcopyrite structure such as 101 peak at 2θ=17° and 211 peak at 2θ=35°. The diffraction peaks was shifted low angle with increasing Ag amount. Therefore, we understood that the lattice parameters of CAIS solid solution increased with Ag contents. The particle size distribution measurements showed that the CAIS powders have average particle size of about 1μm. The surface and cross-sectional microstructure of the CAIS film were observed in an SEM. The thickness of the typical film is about 7-10μm, which is a little thicker than the CIGS films fabricated by the PVD or “sputtering and selenization” techniques.
This work was supported by the Incorporated Administrative Agency New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).
[1] T. Wada et al., phys. stat. sol. (a) 203, 2593-2597 (2006).
[2] K. Yamada et al., Science and Technology of Advanced Materials 7, 42-45 (2006).
9:00 PM - Y3.16
Validation of the p-type Behavior of an Ag-doped ZnSe Film Grown Heteroepitaxially on GaAs(100) Substrate.
Takashi Narushima 1 , Hiroaki Yanagita 1 , Masahiro Orita 1
1 R&D Center, HOYA Corporation, Tokyo Japan
Show Abstract9:00 PM - Y3.17
ZnO Thin Films Prepared by Thermal Oxidation of Metallic Zinc Films.
Chon-Chi Liau 1 , Liang Chao 1
1 Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan
Show Abstract9:00 PM - Y3.18
Synthesis of Size-controlled ZnO Seed for Nanorod Growth and its Application to Solar Cell.
Ki Seok Kim 1 , Gun-Young Jung 1
1 Material Science & Engineering, Gwnagju Institute of Science & Engineering, Gwangju Korea (the Republic of)
Show Abstract9:00 PM - Y3.19
Study of Band Alignment at CBD-CdS/Cu(In1-xGax)Se2 (x = 0.2 ~ 1.0) Interfaces by Photoemission and Inverse Photoemission Spectroscopy.
Shimpei Teshima 1 , Hirotake Kashiwabara 1 , Keimei Masamoto 1 , Kazuya Kikunaga 1 , Kazunori Takeshita 1 , Tetsuji Okuda 1 , Keiichiro Sakurai 2 , Shogo Ishizuka 2 , Akimasa Yamada 2 , Koji Matsubara 2 , Shigeru Niki 2 , Norio Terada 1 2
1 Nano-Structure and Advanced Materials, Kagoshima University, Kagoshima Japan, 2 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan
Show AbstractIt is widely recognized that nature of the hetero-interfaces in Cu(In1-xGax)Se2 [CIGS] based solar cells should be one of the keys for pursuing higher conversion efficiency, especially the wide gap CIGS based cells. Direct characterization of changes of electronic structure around the interfaces buried in the cell structure with band gap energy of CIGS should be useful for clarifying of origins of serious deterioration of the cell performances, and for deducing guiding principles towards the improvement. In the present study, we have attempted a characterization of valence- and conduction band alignments at the interfaces between CdS buffer layer grown by chemical bath deposition and CIGS one with Ga substitution ratio x up to 1.0 grown by three stage co-evaporation by means of photoemission and inverse photoemission spectroscopy, and scanning Kelvin probe microscopy.Obtained results so far are summarized as follows.i) a low region of Ga substitution ratio x ~ 0.2; The CIGS region adjacent to the interface had a band gap around 1.3 ~ 1.4 eV, which was much wider than that of bulk-specimen with corresponding composition. VBO and CBO at these interfaces were finite, where the conduction band minimum of CdS located above that of the CIGS (positive CBO). VBO and CBO of the CdS/Cu(In0.80Ga0.20)Se2 specimen were 0.7 and 0.25 eV, respectively. ii) region of x < 0.4; an increase of VBO and a reduction of CBO with a rise of x were observed. CBO reached around 0 ~ -0.15 at the CdS/Cu(In0.55~0.60Ga0.40~0.45)Se2 interface. This result is consistent with the fact that the conversion efficiency of our cells took a maximum around x = 0.3. iii) region of x from 0.5 to 1.0, the CBO continuously decreased. Consequently, CBO was negative over the Ga-rich region including CGS. In this region, the energy spacing between conduction band minimum of CdS and valence band maximum of CIGS gave the minimum of band gap energy in the interface region, which was almost constant over the Ga-rich region. Additionally, in this region of x, oxygen impurities localized at the interface was apparently detected which also should promote decrease of Fermi level. Local enhancement of the negative CBO was observed just at the interface. iv) Depth profiles of work function were consistent with the band alignments at the interface and gradient of conduction band minimum in interior region of CIGS layer. These results are consistent with the saturation tendency of open circuit voltage and a deterioration of conversion efficiency in the wide gap CIGS based cells. The present study indicates usefulness of the direct characterization of the CdS/CIGS interfaces for clarification of the correlation between the interface electronic structure of the CIGS based cells over the wide range of the Ga concentration.
9:00 PM - Y3.2
Room Temperature Growth of Indium Oxide Films by Reactive Ion Beam Assisted Deposition.
Kai Wang 1 , Yuriy Vygranenko 1 , Arokia Nathan 2
1 Electrical & computer engineering, University of Waterloo, Waterloo, Ontario, Canada, 2 London Center for Nanotechnology, University College London, London United Kingdom
Show Abstract9:00 PM - Y3.20
Characteristics of ZnO thin film Deposited by Atomic Layer Deposition Method.
Jae Sung Hur 1 , Chang-Sik Son 2 , Dong-Gun Lee 1 , Samseok Jang 1 , Jung-Bin Song 1 , In-Hoon Choi 1 , Dongjin Byun 1 , Byoung-Hoon Lee 1
1 Material science and engineering, Korea Univ., Seoul Korea (the Republic of), 2 Photonics, Silla Univ., Pusan Korea (the Republic of)
Show Abstract9:00 PM - Y3.21
Copper-Indium-Boron-Diselenide Absorber Materials.
Ned Ianno 1 , Tobin Santero 1 , Rodney Soukup 1 , Chrisropher Exstrom 2 , Scott Darveau 2 , Jiri Olejnicek 2
1 Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Chemistry, University of Nebraska at Kearney, Kearney, Nebraska, United States
Show AbstractOne of the most useful thin film solar cell materials is CuInxGa1-xSe2 (CIGS). Although the band gap increases as x increases, the most efficient cells have been produced with x=0.24 with open circuit voltages slightly greater than 1V. Higher values of x result in lower voltage due to changes in the band structure. In order to increase the open circuit voltage we have substituted boron, rather than gallium, for indium and have fabricated novel CuBSe2 (CBS) and CuInxB1-xSe2 (CIBS) materials.. Based on preliminary calculations we have estimated the percentage of boron substitution required to yield a band gap of 1.5 eV in CIBS to be 24%.The precursor copper-indium-boron, CuInxB1-x, films are magnetron co-sputtered onto bare soda-lime glass and Mo coated soda-lime glass, where x has been varied from 1 to 0. The films are selenized in a separate step from a solid selenium source. The resulting films are analyzed by a variety of methods. These methods include characterization by x-ray diffractometry, photon transmission/absorption, Fourier Transform Infrared and Raman spectroscopies and spectroscopic ellipsometry. Spectroscopic results are compared to CIS films deposited in our laboratory and CIGS films reported in the literature. We will show the effect of boron substitution on the lattice parameters, optical band gap, and phonon frequency shift.
9:00 PM - Y3.22
Effect of Bias Light on Photovoltage Decay in CdTe/CdS Solar Cells.
Kent Price 1
1 , Morehead State University, Morehead, Kentucky, United States
Show AbstractWe have measured the open-circuit photovoltage decay of CdTe/CdS solar cells with and without a constant bias-light source. We used red probe light with and without white bias light, and white probe light with and without red bias light. In all cases the voltage decays exponentially with time from approximately 750 mV to zero after the turn-off of the probe light. The decay time depends on the sample and varies from tens of microseconds to milliseconds. With both red and white bias light, the decay is faster in the presence of bias light. In addition, the presence of white bias light results in a small (~3 mV) increase in voltage after the initial decay. The increase occurs over a time-scale of milliseconds. The relatively slow nature of the increase and the fact that it occurs only in the presence of white bias light suggests it is due to a re-arrangement of charge in deep states of the CdS layer. The work therefore adds evidence that photoconductivity in the CdS layer may be important in CdTe/CdS device performance. This work is supported in part by the Kentucky Science and Engineering Foundation.
9:00 PM - Y3.23
All-CSS CdTe Solar-Cell Fabrication Process with Thinner CdS Layers.
Alan Davies 1 , J. Sites 1 , R. Enzenroth 1
1 , Colorado State University, Fort Collins, Colorado, United States
Show AbstractCadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin-film solar cells have been fabricated by an in-line, Close-Space-Sublimation (CSS) process at Colorado State University. The goal of the current work is to reduce the CdS layer thickness to achieve high short-circuit current, while maintaining open-circuit voltage and fill factor at the levels typical with thicker CdS. Current-Voltage (J-V) and Quantum Efficiency (QE) measurements showed the expected increase in Jsc due to enhanced blue response as CdS is thinned over a range from 200 to 10 nm. However, thin CdS yielded reduced Voc (800mv for 200 nm CdS, 350mv for 10 nm) and FF (71% to 52%) due to areas of weak-junction formation where CdTe comes into close contact with the SnO2 TCO layer. As a result, gains in Jsc were offset and efficiencies reduced from 12.6% for the thick CdS layer to 4.5% for devices with the thinnest CdS. Localized weak junction formation in cells with thin CdS was characterized by high-resolution Laser-Beam-Induced Current (LBIC) 2-D mapping. Greater incidence of spatial non-uniformities in photocurrent response accompanied thinning of the CdS layer. At 638 nm, photocurrent varied spatially by 1.3% for thin CdS devices compared to 0.3% for thicker CdS. Non-uniformities of cells with thin CdS were highly sensitive to voltage biasing, consistent with the reduced voltages of these devices.
9:00 PM - Y3.24
Optimization Of Electro Deposited In2S3 Buffer Layers In CuInS2 Cells.
Teodor Todorov 1 , Juan Carda 1 , Eloisa Cordoncillo 1 , Purificacion Escribano 1 , Joachim Klaer 2 , Reiner Klenk 2
1 Inorganic and Organic Chemistry, Universidad Jaume I, Castellón de la Plana, Castellón, Spain, 2 , Hahn-Meitner-Institut , Berlin Germany
Show AbstractCurrently the best chalcopyrite photovoltaic cells employ CdS buffers deposited by chemical bath deposition. For large-scale inline production it is desirable to replace both the toxic material and its batch formation route related with low material utilization and necessity to manage large amounts of residual liquids. In the search for alternatives, during the last years significant progress has been made with different materials such as ZnS and In2S3-based materials deposited by atomic layer deposition (ALD), metal organic chemical vapour deposition (MOCVD), ion layer gas reaction (ILGAR), sputtering and thermal evaporation. Nevertheless, all of them either have inconveniencies for industrial application (ALD, ILGAR) or employ vacuum equipment which adds significantly to the production facilities cost. In an attempt to develop a process suitable for low-cost large-scale industrial manufacturing of CuInS2 PV modules, results of our recent work have demonstrated the feasibility of electro deposited In2S3-based buffer layers. Relatively good open-circuit voltage and short-circuit current were obtained, however, the fill factor as well as the deposition reproducibility and homogeneity were not entirely satisfactory. The present contribution reports the different approaches which were undertaken with the objective to improve these properties. The deposition parameters are discussed in relation with film characteristics and solar cell performance. Along with the influence of the starting composition and electro deposition regime, the fluid behavior of the solution was examined and several setup configurations were tested with or without the use of stirring methods. Film deposition proved very sensitive to the flow regime, which was a dominant factor in most cases. When stirring was absent, the convection-driven laminar flow at the substrate surface caused strong gradients in film thickness due to inhomogeneous mass transport and/or electrochemical modification of the boundary layer. Possible improvements of the process are discussed in terms of existent techniques for industrial electro deposition.
9:00 PM - Y3.25
Surface Morphologies and Optical Properties of Homoepitaxial ZnO by Close-Spaced Chemical Vapor Transport.
Koji Abe 1 , Tetsuya Tokuda 1 , Yuta Banno 1 , Osamu Eryu 1
1 Electrical and Electronic Engineering, Nagoya Institute of Technology, Nagoya Japan
Show AbstractEpitaxial ZnO films on sapphire substrates are formed by various techniques such as chemical vapor deposition, laser ablation, and plasma-enhanced molecular beam epitaxy. Sapphire substrates, however, introduce a large mismatch into interfaces. Homoepitaxial ZnO films are expected to lead to better optical and electrical properties. We have studied homoepitaxial ZnO growth by close-spaced chemical vapor transport (CSCVT). It is found that surface morphologies and growth rates significantly depend on the substrate temperature. We report growth conditions which lead to flat surfaces.ZnO layers were homoepitaxially grown on O-polar (000-1) surfaces by CSCVT at the pressure of 900 Torr (Ar:CO2=5:3). ZnO powder (99.999%) was used as Zn and O sources. Zn and O species are transported to a ZnO substrate by the chemical reaction between ZnO and CO (ZnO+CO↔Zn+CO2). ZnO substrates were cleaned with organic solvents. Subsequently, a ZnO substrate and ZnO powder were placed in a carbon crucible in a vertical quartz reactor. ZnO growth was performed in the substrate temperature range from 840°C to 940°C. The source temperature was kept at 950°C. The distance between the substrate and source was set at 1.5 mm with a carbon spacer. The decomposition of ZnO powder increased the distance because the substrate was fixed on the carbon crucible. Photoluminescence (PL) measurements were carried out at 6 K by exiting the films with a He-Cd laser (325 nm).The growth rate of ZnO films decreases with increasing substrate temperature because the decomposition of ZnO substrates becomes dominant. The growth rates at 840°C and 860°C were 9 μm/h and 4 μm/h, respectively. The etching of ZnO substrates was caused at a substrate temperature of 900°C. Observation of surface morphologies shows that there are macro steps along <1-100> crystal orientations on the film grown at 840°C. The films grown at 860°C and 880°C have smooth hexagonal islands. The surface roughness decreases with increasing substrate temperature. However, flat surfaces were not obtained.To perform ZnO growth at a higher temperature, we kept the distance between the substrate and source constant (1.5 mm). The temperature of the substrate continuously increased from 920°C to 940°C during the growth. AFM observations show that keeping the distance constant improves surface morphologies. The ω rocking curves of the (0002) peak were measured to determine the crystallinity of the ZnO film. The FWHM value of the rocking curve of the ZnO film is as small as that of as-received substrates. In the PL spectrum of the ZnO film, the D0X line at 3.36 eV was clearly observed. It is found that the intensity of the broad emission related to deep levels is decreased by keeping the distance constant.
9:00 PM - Y3.26
Effect of Stress Field on Component Diffusion in Multilayer Thin Films.
Phaniraj Madakashira 1 , Vikash Sinha 1 , Om Prakash 1
1 , GE India Technology Center, Bangalore India
Show Abstract9:00 PM - Y3.27
Solvothermal Synthesis of Copper Indium Diselenides using Alkyamines as Additives
Juyeon Chang 1 , Hye Jin Nam 1 , Geun-Tae Cho 1 , Duk-Young Jung 1
1 Chemistry, SungKyunKwan Univ., Suwon, Gyeonggi-do, Korea (the Republic of)
Show AbstractHigh quality polycrystalline CuInSe2 (CIS) samples were prepared by a solvothermal process of various solvent and temperature conditions. Toluene was used as a primary solvent, and akylamines were added as catalytic additives, such as diethylamine, ethylenediamine, diethylenetriamine and triethylenetetramine. Powder X-ray diffraction of the products and its stoichiometric composition by ICP-AES support the successful syntheses of CIS. X-ray diffraction peaks indicated that synthesis of CIS required only a small quantity of akylamine, which could be optimized to increase the purity of CIS and to reduce the reaction temperature below about 110°C. The relationship between concentration of akylamine and the morphologies of the prepared CIS was studied by scanning electron microscopy.
9:00 PM - Y3.28
Cu-doped ZnSe Film with Stoichiometric Composition Deposited at Room Temperature using Compound Sources.
Masahiro Orita 1 , Takashi Narushima 1 , Hiroaki Yanagita 1
1 R&D Center, HOYA Corporation, Akishima-shi, Tokyo, Japan
Show Abstract9:00 PM - Y3.29
The Formation of CuInSe2 Thin Film Solar Cell Absorbers from Electroplated Precursors.
Stefan Jost 1 , Frank Hergert 1 , Rainer Hock 1 , Torsten Voss 2 , Joerg Schulze 2 , Andreas Kirbs 2 , Michael Purwins 3 , Markus Schmid 3
1 Chair for Crystallography and Structural Physics, University of Erlangen-Nürnberg, Erlangen Germany, 2 , Atotech Deutschland GmbH, Berlin Germany, 3 Crystal Growth Laboratory, Department of Materials Science VI, University of Erlangen-Nürnberg, Erlangen Germany
Show Abstract9:00 PM - Y3.3
Study of CIGS Solar Cells Back Contact.
Sylvain Marsillac 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show Abstract9:00 PM - Y3.30
Characterization of CdS/CdTe Solar Cells Fabricated By Different Processes.
Tamara Potlog 1 , Khrypunov Gennadiy 2 , Kaelin Marc 3 , Zogg Hans 3 , Tiwari Ayodhya N. 2
1 Physics, Moldova State University, Chisinau Moldova (the Republic of), 2 Materials for Electronic and Solar Cells, National Technical University “Kharkov Polytechnic Institute”, Kharkov Ukraine, 3 Thin Film Physics Group, Laboratory for Solid State Physics, ETH Zurich ETH Building-Technopark, Technoparkstr.1 , CH-8005 Zurich Switzerland
Show Abstract9:00 PM - Y3.31
Photocurrent Spectral Distribution and Relaxation in CdS/CdTe Heterojunctions
Sergiu Vatavu 1 , Iuliana Caraman 2 , Petru Gasin 1
1 Physics, Moldova State University, Chisinau Moldova (the Republic of), 2 Engineering, University of Bacau, Bacau Romania
Show Abstract Thin film CdS/CdTe solar cells have proved their importance in use as solar energy converters achieving 16.5 % efficiency [1]. Thin film CdS/CdTe heterojunctions have been deposited by close spaced sublimation method (CSS), onto SnO2 covered glass plates (2×2 cm2). The thickness of CdS and CdTe thin films was 0.3-1.6 μm and 2.3-6.6 μm respectively. For to enhance CdS/CdTe heterojunctions photoelectrical parameters and photosensitivity, the deposition procedure was followed by annealing in presence of CdCl2 at 420°C for 15-60 min. Ni has been used as back contact to CdTe. After annealing, the photoelectrical parameters at 1 kW/m2 are: Uoc=0.73 V, Isc=21.7 mA/cm2, ff=0.43.¶ The analysis of the photosensitivity for samples with different thicknesses of the component layers in the 78-293K temperature range showed, that the increase of the photoconductivity for photon energies hν>1.9 eV is determined by the generation-recombination processes in CdSxTe1-x interface layer, and in the 1.42 eV-1.85 eV region, by light absorption mechanism in CdTe.¶ At direct biases of the CdS/CdTe heterojunctions (2Uoc), the photocurrent spectral distribution is determined by the generation-recombination processes in CdS layer. The annealing of the heterojunction in presence of CdCl2 results in photocurrent increase more than one order of magnitude.¶ CdS/CdTe at reverse biases, having a constant sensitivity in visible and near infrared spectral region, can be used in photometry as radiation detector.¶ The analysis of the absorption spectra and the photocurrent spectral distribution at direct biases resulted in determination of the holes mean free path in CdTe layer, being equal to 1.05 μm.¶ The ratio of the ambipolar diffusion coefficient and surface recombination velocity for CdS/CdTe interface is 0.3 μm and the diffusion length is 0.6 μm at 78K. The same parameters for Ni/CdTe interface are: 0.025 μm and 0.05 μm respectively.¶ The phococurrent relaxation curves have been studied for illumination throught both sides of the heterojunctions at different biases, temperatures and wavelength, determining different penetration depths and materials excitation. Nonequilibrium charge carriers lifetime has been determined. The thermal annealing of the CdS/CdTe heterojunctions in presence of CdCl2 determine the nonequilibrium charghe carrier lifetime increase by more than one order of magnitude from 22 μs up to 280 μs.¶[1] X. Wu et. al. 16.5% Efficient CdS/CdTe polycrystalline thin film solar cell. 17th European Photovoltaic Solar Energy Conference, Munich, Germany, 2001, Proceedings, p.995-999.
9:00 PM - Y3.32
Alternative Route To Obtain Photovoltaic Thin Film Of CdTe.
Claudio Carvalho 1 , Rodolfo Fernandes 1 , Victor Reynoso 1
1 Physics and Chemistry, Universidade Estadual Paulista - UNESP, Ilha Solteira, SP, Brazil
Show AbstractThe demand of electrical energy is growing up quickly as result of the industrial and technological expansion. However, this expansion requires efficient, cheap and environmentally safe converting methods. Photovoltaic (PV) technologies fit perfectly in this situation. Following this motivation, a PV material synthesis research is made in the present work. We obtained CdTe thin film using polymeric precursor solution deposited on glass substrate by dip coating technique. The films were submitted to thermal treatment around 450oC during 1 hour in resistive furnace with controlled argon-carbon atmosphere. A quick study using optical microscopy showed a smooth and uniform surface before and after thermal treatment. FTIR and UV-VIS spectroscopy have shown that some compounds were eliminated after thermal treatment and the band absorption is around 650 nm, still far from the expected CdTe band, around 889 nm. The film thickness is around 3μm. Chemical composition analysis using EDX has also shown a excellent stoichiometric control. Measurement of XRD, AFM and efficiency measurements are being carried out.
9:00 PM - Y3.33
Formation of Zn-In-Se Compounds on CIGS Film by the Thermal Annealing in Dimethylzinc Stream.
Mutsumi Sugiyama 1 , Atsushi Miyama 1 , Akihisa Umezawa 1 , Toshihiro Yasuniwa 1 , Atsuki Kinoshita 1 , Hisayuki Nakanishi 1 , Shigefusa Chichibu 2
1 Dept. of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba Japan, 2 Inst. of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba Japan
Show Abstract9:00 PM - Y3.34
On the Origins of Impurities in CdTe-based Thin Film Solar Cells.
Mahieddine Emziane 1 , Douglas Halliday 1 , Ken Durose 1
1 Department of Physics, Durham University, Durham United Kingdom
Show Abstract9:00 PM - Y3.35
Formation of Cu2ZnSnS4 and Cu2ZnSnS4-CuInS2 Thin Films Investigated by In-situ Energy Dispersive X-ray Diffraction.
Alfons Weber 1 , Immo Koetschau 1 , Susan Schorr 1 , Hans-Werner Schock 1
1 Solar Energy Division, Hahn-Meitner-Institut, Berlin Germany
Show Abstract9:00 PM - Y3.37
Electronic Structure of CuInSe2 Thin Films.
Christian Pettenkofer 1 , Carsten Lehmann 1 , Volker Eyert 2
1 SE6, HMI, Berlin Germany, 2 , Universität Augsburg, Augsburg Germany
Show Abstract9:00 PM - Y3.4
Photosensitivity of Thin Variable-gap p-n Structures with a Triangular Profile of Energy Band Diagram.
Bogdan Sokolovskii 1
1 Department of Electronics, Ivan Franko University of Lviv, Lviv Ukraine
Show Abstract9:00 PM - Y3.5
Structure and Microstructure of Zn2x(CuBIII)1-xX2 Semiconductors (BIII=Ga,In;X=S,Se,Te)
Susan Schorr 1 , Gerald Wagner 2 , Michael Tovar 3 , Denis Sheptyakov 4
1 Solar Energy Research (SE3), Hahn-Meitner-Institute Berlin, Berlin Germany, 2 Institute of Mineralogy, Crystallography and Materials Science, University Leipzig, Leipzig Germany, 3 Structural Research (SF1), Hahn-Meitner-Institute Berlin, Berlin Germany, 4 Laboratory for neutron Scattering, ETH Zuerich&PSI Villigen, Villigen Switzerland
Show AbstractSolid solutions of DIIXVI and AIBIIIXVI2 compounds (D=Zn; A=Cu; B=Ga,In and X=S,SeTe) are of interest due to possible photovoltaic applications. An advantage would be a variation of the band gap from the large value of the binary wide band gap semiconductor ZnX towards the band gap of the ternary chalcopyrite type end member CuBX2. Both end members belong to the adamantine compound family, their structures are closely related but non-isotype. The tetragonal chalcopyrite structure of ternary can be derived form the cubic sphalerite type structure of the isoelectronic binaries by doubling the cubic unit cell in c-direction. The two principal noncubic features are a tetragonal deformation (1-c/2a) and displacement of the anions (u-¼ ) (u is the anion x coordinate).In this work we present the systematic investigation of structure and microstructure in 2(ZnX)x(CuBX2)1-x series by neutron and X-ray diffraction as well as transmission electron microscopy (TEM) in a compact way.Powder samples were synthesized by solid state reaction from the elements in sealed evacuated silica tubes at elevated temperatures. Neutron diffraction experiments were performed at BENSC (HMI Berlin, Germany) and the Spallation Source SINQ (PSI, Switzerland). CuKα1,2 radiation was used in X-ray powder diffraction. TEM examinations have been carried out in a Philips CM200 STEM equipped with a super twin objective lens and an EDX analysis system.The results showed the occurrence of a structural phase transition in dependence of the composition, which can be foreseen as a combination of order-disorder (cation substructure) and displacement (anion substructure) process.The 2(ZnX)-CuBIIIX2 systems form solid solution series with a miscibility gap, within two phases coexist as tetragonal domains and cubic matrix. This region was named the 2-phase field. Hence the composition dependent structural phase transition goes along with a phase separation by chemical disorder. It was revealed that within a homologeous series the 2-phase field depends on the anion, for the same anion it depends on the B(III) cation. For cubic and tetragonal mixed crystals the lattice constant depend linear on chemical composition. Moreover the anion displacement and tetragonal deformation for tetragonal mixed crystals is decreasing linearly with increasing ZnX content in CuB(III)X2. This behaviour indicates the displacement process within the anion sublattice. As an effect of the microstructure within the 2-phase field the both phases try to match in the a-b-plane, which gives rise to an increase of the tetragonal lattice constant c of the domains and an increasing tetragonal deformation. Moreover a non-random Zn distribution on the both cation sites of the chalcopyrite type structure could be deduced for tetragonal mixed crystals. With increasing ZnX content in CuB(III)X a trend to a more statistic distribution of the cations was observed, indicating the tendency of disorder in the cation substructure.
9:00 PM - Y3.6
Growth and Characterization of Erbium Doped ZnO Prepared by Vapor Phase Transport Process.
Shih-Hsuan Yang 1 , Liang Chao 1
1 Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan
Show Abstract9:00 PM - Y3.7
Electronic Transport in CuIn(S,Se)2 Electrodeposited Based Solar cells, Limiting Causes for 10% Efficiencies.
Arouna Darga 1 , Veronica Bermudez 2 , Denis Mencaraglia 1 , Olivier Kerrec 2
1 , LGEP- CNRS, Gif Sur Yvette France, 2 , IRDEP (CNRS-EDF), Chatou France
Show Abstract9:00 PM - Y3.8
Electroluminescence from Cu(In,Ga)Se2 Thin-film Solar Cells.
Thomas Kirchartz 1 , Uwe Rau 1 , Julian Mattheis 1
1 Institute of Physical Electronics, University of Stuttgart, Stuttgart Germany
Show Abstract9:00 PM - Y3.9
Growth and Characterization of Electrodeposited CIS Absorbers.
Shalini Menezes 1 , Yan Li 1 , Subba Kodigala 1 , Sharmila Menezes 1 , Joe Rose 1 , Anura Samantilleke 2
1 , InterPhases Research, Westlake Village, California, United States, 2 Department of Chemistry, University of Bath, BATH BA2 7AY United Kingdom
Show Abstract
Symposium Organizers
Timothy Gessert National Renewable Energy Laboratory
Sylvain Marsillac University of Toledo
Takahiro Wada Ryukoku University
Ken Durose Durham University
Clemens Heske University of Nevada-Las Vegas
Y6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
9:00 AM - **L7.1
Local Electrostatic Modulation of Superconductivity in Nb-doped SrTiO3 Films.
Kei Takahashi 1 , Nicolas Reyren 1 , Andrea Caviglia 1 , Marc Gabay 2 , Keisuke Shibuya 3 , Tsuyoshi Ohnishi 3 , Mikk Lippmaa 3 , Didier Jaccard 1 , Jean-Marc Triscone 1
1 Département de Physique de la Matière Condensée, University of Geneva, Genève Switzerland, 2 Laboratoire de Physique des Solides, Université de Paris-Sud, Orsay France, 3 Institute for solid state physics, University of Tokyo, Chiba Japan
Show AbstractIn this talk, the focus will be on recent experiments performed on superconducting doped SrTiO3 films using the ferroelectric field effect. Using thin single crystal films of Nb-doped SrTiO3, we have used the polarization field of ferroelectric Pb(Zr,Ti)O3 to electrostatically tune the electronic properties of Nb-doped SrTiO3. Atomic force microscopy was used to locally reverse the ferroelectric polarization, inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature [1]. By poling sub-micron scale domains, the carrier density and the electronic properties of the superconducting channel were locally modified. Transport measurements reveal a steplike behavior in the current-voltage and resistance-magnetic field characteristics. This behavior, not observed in homogeneously poled samples, is ascribed to phase slip lines which are more easily generated in inhomogeneous superconductors. More generally, the technique developed should allow Josephson junctions, SQUIDS and superconducting circuits to be “directly written” in a Nb-doped SrTiO3 film using atomic force microscopy, an idea proposed in 1997 by C.H. Ahn et al. [2].[1] K. Takahashi, D. Jaccard, M. Gabay, K. Shibuya, T. Ohnishi, M. Lippmaa, J.-M. Triscone, Nature 441,195 (2006).[2] C.H. Ahn, T. Tybell, L. Antognazza, K. Char, R.H. Hammond, M.R. Beasley, Ø. Fischer, and J.-M. Triscone. Science 276, 1100 (1997).
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
9:30 AM - **Y4.1
Metastable Defect Distributions in CIGS Solar Cells and Their Impact on Device Efficiency.
Malgorzata Igalson 1
1 Faculty of Physics, Warsaw University of Technology, Warszawa Poland
Show AbstractY6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
9:30 AM - L7.2
Dynamic Studies of Ferroelectric Domain Switching for Thin Films with Interfaces of Varying Orientation using High Speed Piezo Force Microscopy.
Ramesh Nath 1 , Ying-Hao Chu 2 , Ramamoorthy Ramesh 2 , Bryan Huey 1
1 Chemical, Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show Abstract9:45 AM - L7.3
Internal Structure of Ferroelectric BaTiO3/ SrTiO3 Superlattices.
Dillon Fong 1 , A. Soukiassian 2 , S. Nakhmanson 1 , J. Eastman 1 , P. Fuoss 1 , S. Streiffer 3 , Y. Li 2 4 , L. Chen 2 , K. Rabe 5 , D. Schlom 2 , X. Xi 6
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Department of Materials Science & Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 3 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 4 Materials Physics & Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 5 Department of Physics & Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States, 6 Department of Physics, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractComplex oxide superlattices have proven to be excellent systems for the exploration of novel phenomena such as electronic reconstructions (1) and ferroelectric coupling (2-4). In the case of BaTiO3/SrTiO3 superlattices on SrTiO3(001) substrates, theoretical results have shown that the ferroelectric BaTiO3 layers can induce polarization and tetragonality in the adjacent, normally paraelectric SrTiO3 layers, resulting in a strain-enhanced polarization compared with bulk BaTiO3 and a rich and complex atomic displacement profile (2,3). While enhanced ferroelectric properties have indeed been found (3,4), experimental confirmation of this displacement profile remains a challenge, requiring a nondestructive method of probing buried structure with sub-Ångstrom resolution. We employ synchrotron x-ray scattering at the Advanced Photon Source to study the internal structure of three BaTiO3/SrTiO3 superlattices grown by reactive molecular-beam epitaxy on SrTiO3(001) substrates. Studies were carried out over a wide range of temperatures in 0.1 Torr of oxygen. The three superlattices, all less than 10 nm thick, are described by (BaTiO3)n/(SrTiO3)n where n = 3, 4, and 5 unit cells. In addition to identifying the transition temperature for each sample, multiple crystal truncation rods were measured in both the paraelectric and ferroelectric phases, and direct methods were used to determine the three-dimensional electron density profile throughout the superlattice (5). All samples exhibit ferroelectricity, with transition temperatures in good agreement with those for thicker films (4). For each bilayer thickness, the temperature dependent atomic displacement profiles will be described and correlated with first-principles theory. Special emphasis will be given to the atomic structure at the surface and the buried interfaces. The ferroelectric domain structure will also be discussed. 1.A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).2.J. B. Neaton, K. M. Rabe, Appl. Phys. Lett. 82, 1586 (2003).3.W. Tian et al., Appl. Phys. Lett. 89, 092905 (2006).4.D. A. Tenne et al., Science 313, 1614 (2006).5.D. D. Fong et al., Phys. Rev. B 71, 144112 (2005).
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
10:00 AM - Y4.2
Understanding Metastable Defect Creation in CIGS by Detailed Device Modeling and Measurements on Bifacial Solar Cells.
JinWoo Lee 1 , David Berney Needleman 1 , William Shafarman 2 , J. David Cohen 1
1 Physics, Univ. of Oregon, Eugene, Oregon, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractPreviously, we have investigated the role of metastable defect state creation by admittance measurements and suggested that it is initiated by the capture of two electrons by a precursor site. This defect creation mechanism seems to be consistent with the vacancy complex (Vse-Vcu) model proposed by Lany and Zunger. We also measured I V curves at several different temperatures for a series of metastable light-soaking treatments. The observed metastable changes generally led to substantial reductions in short circuit current and fill factor but very little change in the open circuit voltage. The lack of change in open circuit voltage suggests that recombination remains constant over metastable states or that the quasi-Fermi level for electrons is pinned by interface states.We compared our experimental results with detailed numerical modeling using the SCAPS-1D simulation program to understand these metastable changes. The most successful mechanism we have found for explaining the observed effects is a metastable conversion of a donor defect into a bulk acceptor. Our SCAPS modeling further indicates that some of these effects, specifically observed decrease in fill factor, are due to changes in photoconductivity in the CdS buffer and window layers under red illumination.Our admittance measurements primarily allow us to determine the metastable changes taking place in the neutral bulk region of the absorber. However, until now, we have not had a good method to estimate the minority carrier concentration in this region to help us better understand the details of the mechanism involved. We were recently successful in fabricating thin film CIGS solar cells using transparent Mo back contacts. We found the device performance of such bifacial CIGS solar cells were almost the same as the conventional thick Mo counterpart, although the thin Mo samples exhibited slightly larger series resistance and lower fill factors. Light-soaking through the back contact of such bifacial CIGS solar cells with 780 nm monochromatic light has now allowed us to examine changes in the CIGS bulk absorber properties that exclude effects of charge trapping at the barrier interface and metastable changes in CdS layer. In this manner, we are able to conclusively correlate how the metastable effects in the CIGS bulk absorber properties impact the device performance.
Y6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
10:00 AM - **L7.4
Electronic Properties of All-oxide Metal-ferroelectric Interfaces.
Marin Alexe 1 , Lucian Pintilie 1 , Ionela Vrejoiu 1 , Dietrich Hesse 1 , Gwenael Le Rhun 1 , Burc Misirlioglu 1 , Ulrich Goesele 1
1 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractY4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
10:15 AM - Y4.3
Prediction of Materials Properties for Cu(In,Ga)Se2 Photovoltaic Absorbers: Fermi Level Pinning, Light- and Bias Induced Metastability, and Electron Traps.
Stephan Lany 1 , Alex Zunger 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract10:30 AM - Y4.4
Energetics of Both Minority and Majority Carrier Transitions through Deep Defects in Wide Bandgap Pentenary Cu(In,Ga)(Se,S)2 Thin Film Solar Cells.
Adam Halverson 1 , Shiro Nishiwaki 2 , William Shafarman 2 , J. David Cohen 1
1 Department of Physics, University of Oregon, Eugene, Oregon, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractWe have applied both transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopies to characterize a series five Cu(InxGa1-x)(SeyS1-y)2 thin-film solar cells of similar bandgap. These devices were fabricated with deposition parameters chosen so that each absorber has a bandgap near 1.5 eV. We observe a broad, Gaussian shaped defect band in the sub-bandgap spectra whose optical energy threshold shifts systematically relative to the valence band edge as the alloy composition is changed. We also observe relatively broad bandtails with Urbach energies in the range 27 meV to 34 meV. This is significantly larger than the 20±4meV Urbach energies typically found for the quaternary Cu(InxGa1-x)Se2 materials. Thus, these broader bandtails may be indicative of enhanced disorder within the pentenary alloy system.In addition, we have been able to observe the thermal emission of electrons from the dominant defect band in these alloys into the conduction band. Specifically, we are able to model the occupation of the defect in our TPC spectra by incorporating an optical filling rate from the valence band into the defect, plus a thermal emission rate out of the defect into the conduction band. Observing this latter transition process is important because that emission rate depends upon the capture cross section of electrons from the conduction band into the defect. This capture rate of minority carriers into the dominant defect band is potentially important for understanding cell performance. By monitoring the TPC signal from the defect as a function of temperature and rate window for the alloys of these pentenary samples, we are then able to deduce an approximate thermal energy depth of the defect relative to the conduction band. For the alloy with S/(S+Se) =0.12, we find a value near 500 meV relative to the conduction band. This value decreases as the sulfur fraction is increased, implying that the energy level of this defect is moving closer to the conduction band of the material. These deduced values plus the optical threshold of the defect energy relative to the valence band agree quite well with the known bandgaps of these absorbers.
Y6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
10:30 AM - L7.5
Monodomain To Polydomain Transition In Ferroelectric PbTiO3 Thin Films: The Role Of Electrical Boundary Conditions.
Celine Lichtensteiger 1 , Matthew Dawber 1 , Nicolas Stucki 1 , Jean-Marc Triscone 1 , Jason Hoffman 2 , Jeng-Bang Yau 2 , Charles H. Ahn 2
1 DPMC, University of Geneva, Geneva Switzerland, 2 Department of Applied Physics, Yale University, New Haven, Connecticut, United States
Show AbstractFerroelectric finite size effects have been probed in a series of epitaxial perovskite films of c-axis oriented PbTiO3 ranging from 500 down to 24 Å (6 unit cells). The samples were grown using off-axis RF magnetron sputtering on metallic (001) Nb-SrTiO3 substrates and on La0.67Sr0.33MnO3 epitaxial electrodes prepared on insulating SrTiO3 substrates. The thickness of the films, the c-axis lattice parameter, and the epitaxial growth were studied using high resolution x-ray diffraction measurements. For the PbTiO3 series on Nb-SrTiO3, the films are monodomain and it is shown that the c-axis parameter systematically decreases with decreasing film thickness below 200 Å, a decrease in tetragonality related, using a model Hamiltonian approach, to a reduction of the polarization. The analyses demonstrate that films well below 100 Å are ferroelectric and that the progressive polarization reduction is due to the depolarizing field resulting from imperfect screening[1].The behavior observed for the series of PbTiO3 films on La0.67Sr0.33MnO3 is strikingly different. A decrease in tetragonality is first observed as the film thickness is reduced but is followed by a recovery for the thinnest films. This behavior is accompanied by a change from a monodomain to a polydomain configuration. This transition was confirmed by Piezoresponse Atomic Force Microscopy measurements, also allowing a direct demonstration of the ferroelectric switching of the polarization in PbTiO3 films as thin as 28 Å.[1] Lichtensteiger et al., Phys. Rev. Lett. 94, 047603 (2005).
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
Y6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
10:45 AM - L7.6
Breaking of the Tetragonality-polarization Coupling in Ultra-thin Epitaxial Ferroelectrics.
Chun-Lin Jia 1 , Nagarajan Valanoor 2 , Jia-Qing He 1 , Lothar Houben 1 , Tong Zhao 3 , Ramamoorthy Ramesh 3 , Rainer Waser 4 , Knut Urban 1
1 Institute of Solid State Research and Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Juelich, Juelich, NRW, Germany, 2 Materials Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia, 3 Materials Science and Engineering, University of California, Berkeley, California, United States, 4 Institute for Solid State Research and Centre for Nanoelectronic Systems for Information Technology, Forschungszentrum Juelich, Juelich, NRW, Germany
Show Abstract11:00 AM - Y6:DiscI
BREAK
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
11:15 AM - **Y4.5
Relationships Between CdS/CdTe Materials Composition and Device Performance.
Sally Asher 1 , T. Gessert 1 , M. Young 1 , C. Perkins 1 , G. Teeter 1 , J. Pankow 1 , D. Albin 1 , X. Wu 1
1 , NREL, Golden, Colorado, United States
Show AbstractThe last 15 years have seen an intense effort to understand compositional variations in the materials that compose CdS/CdTe devices; and to link these variations with deposition parameters, specific device fabrication steps, or post-processing history. Additional effort has been directed at associating these variations to device performance, stability, or device changes that result from accelerated life testing (ALT). In many cases the differences are large enough to be easily observable and the interpretation is straightforward. However, in cases where the differences are subtle, the interpretation may be complicated by limitations or understanding of the data from the analytical method. Compositional analysis of thin-film materials generally requires analytical methods that have surface or layer-by-layer sensitivity, and these commonly include Auger and photoelectron spectroscopies, secondary ion mass spectrometry, and electron probe microanalysis. In this presentation we will focus on our work using surface analytical methods to derive compositional information from CdS/CdTe materials and devices and our success at using this information to explain device results. The strengths, weaknesses, and limitations for each of the major techniques will be discussed. This work was performed with the support of US Department of Energy Contract No. DE-AC36-99GO10337. This abstract is subject to government rights.
Y6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
11:30 AM - L7.7
Chemical Switching of Ferroelectricity in Monodomain PbTiO3 Films.
Paul Fuoss 1 , Ruey-Ven Wang 1 2 , Dillon Fong 1 , Fan Jiang 1 , Stephen Streiffer 2 1 , Jeffery Eastman 1 , G. Stephenson 1 , Carol Thompson 3 , Kujtim Latifi 3
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Physics, Northern Illinois University, DeKalb, Illinois, United States
Show AbstractFree charge at interfaces is required to compensate the depolarizing field and to stabilize polarization in monodomain ferroelectric thin films [1]. In earlier work [2], we observed monodomain polarization of coherently strained ultra-thin PbTiO3 films grown on conducting SrRuO3 layers on (001) SrTiO3. Ab initio calculations predicted that ionic adsorption on the surface could provide sufficient charge to stabilize this polarization. Here we present experiments demonstrating reversible switching of the film polarization direction by changing the chemical environment above the film. Using high-resolution synchrotron x-ray surface scattering, we study the ferroelectric behavior of 10 nm thick PbTiO3 films. By following in real time the evolution of the out-of-plane lattice parameter, and hence film polarization, we determine the “chemical butterfly loop” as we reversibly switch the film by changing the O2 partial pressure. A newly discovered (4×1) reconstruction is present under reducing conditions. We interpret these results in terms of the chemical nature of both the absorbed ions and the surface structures. Work supported by U.S. Dept. of Energy under contract W-31-109-ENG-38.[1] A. K. Tagantsev and G. Gerra, J. Appl. Phys., 100, 051607 (2006)[2] D. D. Fong et al., Phys. Rev. Lett., 96, 127601 (2006)
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
11:45 AM - Y4.6
Study of Recombination in Cu(In,Ga)Se2 Solar Cells by Time-Resolved Photoluminescence.
Sho Shirakata 1 , Tokio Nakada 2
1 Department of Electrical and Electronic Engineering, Ehime University, Matsuyama, Ehime, Japan, 2 Department of Electrical Engineering and Electronics, Aoyama Gakuin University, Sagamihara, Kanagawa, Japan
Show AbstractY6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
11:45 AM - **L7.8
Spintronics with Multiferroics.
Agnes Barthelemy 1 , Helene Bea 1 , Martin Gajek 1 , Manuel Bibes 2 , Karim Bouzehouane 1 , Stephane Fusil 1 , Benedicte Warot-Fonrose 4 , Selia Cherifi 5 , Gervasi Herranz 1 , Eric Jacquet 1 , Cyrille Deranlot 1 , Josep Fontcuberta 3
1 , Unité Mixte CNRS/Thales, Palaiseau France, 2 , IEF, Orsay France, 4 , CEMES-CNRS, Toulouse France, 5 , Laboratoire Louis Néel, Grenoble France, 3 , ICMAB, Barcelona Spain
Show AbstractY4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:00 PM - Y4.7
Stability and Electronic Structures of CuxTe
Juarez Da Silva 1 , Su-Huai Wei 1 , J. Zhou 1 , X. Wu 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractCuxTe is currently considered as one of the best contacting material for CdTe solar cell [1]. However, unlike convention compounds, CuxTe has one of the most complex phase diagram among the chalcogenides [2] and controlling of the Cu concentration in CuxTe is found to be critical for achieving high-efficiency CdTe based solar cells. High Cu/Te ratio usually gives high conductivity, but also high diffusion rate of Cu into the absorber/window layers, which can destabilize the device. Despite many experimental studies on this interesting materials, it is still not very clear how the physical properties of CuxTe vary as functions of x. In this work, using first-principles methods, we have investigated the structural and electronic properties of CuxTe systems. We calculated the stability of CuxTe in the range x = 1.0 - 2.0. We find that CuxTe is more stable in the Rickardite tetragonal structure, whichconsists weakly binded CuxTe layers [3]. In the Rickardite structure, 1/2 of the Cu sites have occupation 1, while the remaining Cu sites are randomly occupied by the rest of Cu atoms sites. For each Cu composition, different configurations in a supercell were considered, and the total energy of the system are obtained by fully relaxation of the cell size and atomic positions using quantum mechanical stress and force. We find that the formation energy ΔH(x)=E(CuxTe) - xE(Cu)-E(Te) has minimum at xmin = 1.25, where E(Cu) and E(Te) are the total energy of bulk Cu and Te. Furthermore, we find that, strain in the CuxTe layer can change the position of xmin. For example, when CuxTe is constrained to have the lattice constant of Cu1.5Te, xmin shifts to 1.375. We also find that the formation energy of Cu2Te is positive, indicating that it is unstable with respect to phase separation into Cu and CuxTe, thus the extra Cu can easily diffuse into the absorber or window layers. We will also discuss the electronic structures, band alignment and Cu diffusion barriers in CuxTe and their relationship to the solar cell performance.
[1] X. Wu et al., in 17th European PVSEC, (2001), p. 995; X. Wu et al. Thin Solid Films (in press)
[2] S. Miyatani, S. Mori, and M. Yanagihara, J. Phys. Soc. Jap. 47, 1152 (1979).
[3] S. A. Forman and M. A. Peacock, Am. Mineral., 34, 441 (1949).
12:15 PM - Y4.8
Controlling Stoichiometric Defects in CdTe Polycrystalline Thin Films.
Brian McCandless 1
1 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractY6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:15 PM - L7.9
Properties of BaFeO3-Based Thin Films and Superlattices
Charlee Callender 1 , Mat Ivill 1 , David Norton 1 , Ritesh Das 2 , Arthur Hebard 2 , John Budai 3
1 Dept. of Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 2 Dept. of Physics, University of Florida, Gainesville, Florida, United States, 3 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe coupling of material properties (e.g. magnetic and ferroelectric) via the formation of artificial superlattices is of significant interest. These artificially-layered multiferroics add a new degree of freedom for tuning material properties. In this talk, we will report on the synthesis and properties of BaFeO3 thin films and related superlattices involving (Ba,Sr)TiO3 and K(Ta,Nb)O3 ferroelectrics. In bulk, BaFeO3 is a hexagonal structured material with ferromagnetic transition at 160 K. However, BaFeO3 can be stabilized in a pseudo-cubic perovskite structure via epitaxial thin film. We will describe the synthesis, structure, magnetic, and electronic properties of BaFeO3/K(Ta,Nb)O3 and BaFeO3/(Ba,Sr)TiO3 superlattices grown by pulsed laser deposition. Effects of oxygen stoichiometry on magnetic and structure properties will be discussed. The research was sponsored by the Army Research Office under contract W911NF0510519. The ORNL research sponsored by the Division of Materials Science, US Department of Energy, under contract with UT-Battelle, LLC. Additional support was sponsored by the South East Alliance for Graduate Education and the Professoriate (SEAGEP) [NSF HRD-0450279].
Y4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:30 PM - Y4.9
Locations of Cu in CdTe Solar Cells.
Yanfa Yan 1 , Kim Jones 1 , Xuanzhi Wu 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractY6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:30 PM - L7.10
Understanding the Ferromagnetic-antiferromagnetic Interface in Co/BiFeO3 Multifunctional Heterostructures.
Kilho Lee 1 2 , Ying-Hao Chu 1 2 , Junwoo Choi 2 , Mikel Barry 2 , Andreas Scholl 3 , Lane Martin 1 , Peiling Yang 1 , Pu Yu 2 , Seung-Yeul Yang 1 2 , Qian Zhan 1 2 , Zi-Qiang Qiu 2 , Ramamoorthy Ramesh 1 2
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractY4: Defects and Impurities
Session Chairs
Sally Asher
Ayodhya Nath Tiwari
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:45 PM - Y4.10
Effect of O2 Concentration During the Post-deposition Anneal on the Efficiency of CdS/CdTe Cells Annealed with CdCl2 or TeCl4.
Joseph Beach 1 2 , F. Seymour 1 2 , V. Kaydanov 2 , T. Ohno 2 , S. Peabody 1 2
1 , PrimeStar Solar Inc., Longmont, Colorado, United States, 2 Physics Department, Colorado School of Mines, Golden, Colorado, United States
Show AbstractY6: Discussion Session I: What Else is Needed for a Healthy Thin-film PV Industry?
Session Chairs
Dieter Bonnet
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
12:45 PM - L7.11
Growth of BiFeO3 Thin Films on (001) SrTiO3 and (0001) GaN by Molecular Beam Epitaxy.
Jon Ihlefeld 1 2 , Wei Tian 1 , Ramamoorthy Ramesh 2 , Darrell Schlom 1
1 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States, 2 Materials Science and Engineering, University of California, Berkeley, California, United States
Show AbstractEpitaxial thin films of the multiferroic BiFeO3 have been grown by molecular beam epitaxy in an adsorption-controlled growth regime where substrate temperature and bismuth oxide over pressure establish phase and stoichiometry control. 30 nm thick BiFeO3 films have been deposited directly on (001) SrTiO3 and SrRuO3/(110) DyScO3, and on (0001) GaN containing a 1 nm thick TiO2 overlayer to enable the epitaxial transition between (0001) GaN and (0001) BiFeO3. Films grown on (001) SrTiO3 possess rocking curves identical to that of the underlying substrate, e.g., a full width at half maximum (FWHM) in ω of 25 arc sec in the best case. This is over 40 times narrower than the best published result for epitaxial BiFeO3 films deposited by any technique. On all of the above substrates, the BiFeO3 films exhibit rhombohedral symmetry. The in-plane epitaxial alignment observed for BiFeO3/TiO2/(0001) GaN differs by a 30° in-plane rotation from that observed in previous work for BiFeO3/SrTiO3/TiO2/(0001) GaN. Specifically, our (0001) BiFeO3/TiO2/(0001) GaN is oriented in-plane with [10-10]BiFeO3 ∥ [11-20]GaN, accompanied by a 180° in-plane rotational twin variant. The results of this work will be discussed in the context of the interfacial and crystallographic orientation dependence on the ferroelectric, antiferromagnetic, and dielectric properties of this multiferroic.
Y5: Industrial Perspectives
Session Chairs
Shigeru Niki
Bolko v. Roedern
Wednesday PM, April 11, 2007
Room 2024 (Moscone West)
2:30 PM - **Y5.1
On Industrial Production of Thin Film CdTe PV Modules
Dieter Bonnet 1
1 , SOLARPACT, D-61381 Friedrichsdorf Germany
Show AbstractAfter more than 30 years of development in industrial and academic laboratories, the CdTe thin film photovoltaic module has entered the production stage in two companies and first experience is gained in this process. This will lead to the next generation of plants at enhanced capacities. It is expected that the “learning curve” will be transgressed quite fast leading to a mature low-cost product.* CdTe is very well suited for use as active material in thin film solar cells due to four special properties:* CdTe has an energy gap of 1.45 eV.* The energy gap of CdTe is direct* CdTe has a strong tendency to grow as an essentially highly stoichiometric, but p-type semiconductor film and can form an p-n heterojunction with CdS. * Simple deposition techniques have been developed.Suitable films have been made by a number different processes, physical vapour deposition, electrodeposition, screen printing, chemical spraying, cathode sputtering metalorganic chemical vapour deposition. All have the common property to transport Cd + Te to the substrate where a reaction takes place to form stable, and stochiometric CdTe .In present factories modified PVD processes are used: Sublimation of CdTe from a source and condensation on the substrate to form a film. The p-n junction is created by depositing thin CdS films by similar processes. Most processes studied in the past allow very rapid deposition – a requirement for low-cost production. State of the art factories are are configured to use fully automated and integrated in-line processes, reducing manpower to an absolute minimum.Presently modules have efficiencies of 7 – 10%. By intense R&D at industrial and academic laboratories processes will be improved yielding efficiencies of 14% in the near future. Roadmaps to achieve this have been formulated by American and European research groups and funding agencies. Finally current densities of up to 27 mA/cm-2 and open-circuit voltages of 880 mV, leading efficiencies of 18% under AM1.5 illumination can be expected for CdTe modules made under a mature technology. To make cells and modules of such improved performance, the following basic properties of CdTe films are of importance: film quality (nucleation, morphology and grain boundary behaviour), doping and contacting, junction formation (activation), understanding and reduction of impurities. At the same time production technologies have to be improved by new reactor designs and prototyping of new equipment, aiming at still faster deposition and thinner films.Cost predictions show that drastic cost reductions can be expected by applying dimensions of scale. This positive outlook has recently led to the establishment of several new ventures.
3:00 PM - **Y5.2
Scalable, Manufacturable, TF-on-glass: from $1.59/W to $1/W.
David Eaglesham 1
1 , First Solar, Perrysburg, Ohio, United States
Show AbstractFirst Solar have the lowest reported cost-per-Watt in the PV industry for their TF-CdTe modules, and are in a phase of extremely rapid volume expansion. Over the last year, the company has grown to be the largest TF manufacturer in the world and the largest manufacturer of photovoltaics in the US. This presentation will outline some of the materials foundations for this technology, summarise progress in the TF-CdTe device and describe the outstanding challenges and the pathway to $1/W PV.
4:00 PM - **Y5.3
The Next Generation of Thin-film Photovoltaics.
Chris Eberspacher 1 , Craig Leidholm 1 , Alex Pudov 1 , Matthew Robinson 1 , Jeroen van Duren 1
1 , Nanosolar, Palo Alto, California, United States
Show AbstractSolar markets are growing rapidly, fueled in part by continuing advances in thin-film photovoltaics (PV) technology. Thin-film PV now dominates in the large-scale, ground-mounted systems market. Thin-film CuInGaSe2 (CIGS) PV technology in particular can provide a potent combination of high sunlight-to-electricity conversion efficiencies and low manufacturing costs -- provided that the processing technology is designed to achieve high throughput, high yield, and high materials utilization. This presentation provides an overview of Nanosolar’s CIGS processing technology with an emphasis on materials science issues related to nanoparticle-based materials processing, high-speed coating and film formation, and low-cost device construction. Cell efficiency drivers and accomplishments will be discussed.
Symposium Organizers
Timothy Gessert National Renewable Energy Laboratory
Sylvain Marsillac University of Toledo
Takahiro Wada Ryukoku University
Ken Durose Durham University
Clemens Heske University of Nevada-Las Vegas
Y7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday AM, April 12, 2007
Room 2024 (Moscone West)
9:30 AM - **Y7.1
The Role of Electronic Processes During Junction Formation in Cu(In,Ga)Se2 Thin Film Solar Cells.
Kannan Ramanathan 1
1 , Miasole, Santa Clara, California, United States
Show AbstractThe photovoltaic properties of Cu(In,Ga)Se2 thin film solar cells are influenced by the interactions that occur during the formation of the p-n junction. It is well known that solar cells made from common absorbers and the same emitter layer (n-type junction partner) can exhibit widely different properties. For example, CdS emitter layers can be deposited by chemical bath deposition, thermal evaporation or sputtering. In all cases, a CdS compound film is deposited. However, processing related impact on the electronic properties of the interface (junction region) is likely to be different because each process brings with it a unique set of conditions. In chemical bath deposition there are chemical driving forces present, whereas in sputtering one is concerned with ion bombardment during the growth of the emitter layer. A stronger effort is needed to adequately characterize the effect of such processes. To illustrate the importance of this, we will provide examples of junction studies in other solar cell materials (CdTe, InP). The paper will provide a historical account of the development of p-n junctions in CuInSe2 based solar cells, the current understanding of the CBD CdS interface, and the options that exist for simplifying the device structure. This review will be based on prior work performed at the National Renewable Energy Laboratory in support of the CIS National R&D Team.
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
9:30 AM - **Y13.1
Polarized Luminescence of Defects in CuGaSe2.
Susanne Siebentritt 1
1 , Hahn-Meitner-Institut, Berlin Germany
Show AbstractY7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
10:00 AM - Y7.2
Alternative Back Contacts for Cu(In,Ga)Se2 Solar Cells.
David Bremaud 1 , Adrian Chirila 1 , Chris Hibberd 2 , Ayodhya Tiwari 1 2
1 Thin Film Physics Group, ETH Zurich, Zurich Switzerland, 2 CREST (Centre for Renewable Energy Systems and Technology), Loughborough University, Leicestershire United Kingdom
Show AbstractY12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
10:00 AM - Y13.2
Optical Depth-Profiling of The CdTe Solar Cell Structure By Spectroscopic Ellipsometry.
Jie Chen 1 , Jian Li 1 , Robert Collins 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show AbstractY7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
10:15 AM - Y7.3
Variable Light Soaking Effect of Cu(In,Ga)Se2 Solar Cells with Conduction Band Offset Control of Window/Cu(In,Ga)Se2 Layers.
Takashi Minemoto 1 , Yasuhiro Hashimoto 2 , Takuya Satoh 2 , Takayuki Negami 2 , Hideyuki Takakura 1
1 Photonics, Ritsumeikan University, Kusatsu, Shiga, Japan, 2 Advanced Devices Development Center, Matsushita Electric Ind. Co., Ltd., Soraku-gun, Kyoto, Japan
Show AbstractY12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
10:15 AM - Y13.3
The Mechanism of J-V ``roll-over" in CdS/CdTe Devices.
Jie Zhou 1 , Xuanzhi Wu 1
1 , national renewable energy lab, Golden, Colorado, United States
Show AbstractThe “roll-over” phenomenon in current-voltage (J-V) curves of CdS/CdTe devices is recognized as a result of the formation of a higher back barrier. Knowing the reasons for forming the “roll-over” is important for CdS/CdTe device manufacturing. CdTe material has a high work function and is difficult to achieve high p+ doping. Back surface treatment to produce a p+ Te-rich layer and applying Cu are the general procedures to form an ohmic contact for CdS/CdTe devices. Therefore, without a Cu source, “roll-over” is apparent in the J-V curves. However, the mechanism was unclear for forming J-V “roll-over” in a CdTe cell with a back contact containing Cu. We studied the formation of different CuxTe phases on the back surface of CdTe and their effects on device performance. Cu/Te ratio and post-anneal temperatures are two parameters to control phase x in CuxTe film. J-V curves reveal the “roll-over” feature for devices using high Cu concentration. A SIMS depth profile shows that the cell has a large amount of Cu at the back of the CdTe film. We did extensive characterizations, including XRD, XPS, TEM, and EDS, and “re-contact” experiments to understand this issue. The results show that the “roll-over” comes from the formation of Cu-related oxides at the back side of the device during processing, rather than the over-diffusion of Cu to the front side of the device. Discussions related to the different possible oxide phases will also be presented.
Y7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
10:30 AM - Y7.4
Study of Band Alignment at the Interface between CBD-CdS and CIGS Grown by H2O-introduced co-evaporation.
Norio Terada 1 2 , Hirotake Kashiwabara 1 , Shinpei Teshima 1 , Keimei Masamoto 1 , Tetsuji Okuda 1 , Kozo Obara 2 , Shogo Ishizuka 2 , Keiichiro Sakurai 2 , Akimasa Yamada 2 , Koji Matsubara 2 , Shigeru Niki 2
1 Nano-Structure and Advanced Materials, Kagoshima University, Kagoshima Japan, 2 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan
Show AbstractA remarkable improvement of performances of rather wide-gap CIGS based solar cells due to the introduction of water vapor into the three-stage growth process of CIGS layer has been recently reported by Ishizuka et al. in AIST. Such cells, involving the CIGS layer with a Ga substitution ratio for In ~ 0.5 and band gap energy ~ 1.3 eV, showed an efficiency up to 18.1 % (without anti-reflection coating) and open circuit voltage Voc above 0.74 V. On the other hand, in the previous studies about the Ga substitution ratio dependence of band alignment at CdS/CIGS interfaces in the cells fabricated without the H2O introduction, non-positive conduction band offset was observed for the cells with the CIGS with the similar Ga content, which was accompanied with deteriorations of the efficiency and a saturation of Voc. These contrasts suggest that a certain change of crystallinity and/or electronic structure of the interface should be induced by the H2O introduction. In the present study, band alignments at the interface between CBD-CdS and CIGS fabricated with the H2O introduction [CIGS-H2O] have been studied by means of photoemission and inverse photoemission spectroscopy, and the effect of the H2O introduction on the interface nature has been investigated. Specimens, fabricated in AIST, had CBD-CdS/Cu(In~0.5Ga~0.5)Se2–H2O/Mo/soda lime glass structure. For growth conditions of the CIGS-H2O were the same as those for the high efficiency cells. Interior region of the samples were exposed by etching with Ar ion beam with a low kinetic energy below 350 eV. Band gap energy, a location of conduction band minimum (CBM) and valence band maximum (VBM) of the cleaned CdS layer over the CIGS-H2O were identical with those of the CdS over the conventionally grown CIGS. Within the CdS-region, VBM and CBM stayed constantly. In the present study of the CBD-CdS/CIGS-H2O (Ga~50 %) interfaces, CIGS related XPS core-signals started to be observable at the etching time of ~ 900 seconds. With the progress of etching from CdS to CIGS, apparent decrease of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was 0.2 ~ 0.3 eV, whereas the total rise of VBM was 0.9~1.0 eV. XPS core-level shifts over the interface showed downward band bending about 0.1 ~ 0.15 eV. Consequently, the conduction band offset (CBO) at the CBD-interface over the CIGS-H2O (Ga~50%) are slightly positive around 0 ~ +0.1 eV. The previous study about the Ga substitution ratio dependence of the band alignments at the interfaces between CBD-CdS and conventionally grown CIGS showed that a continuous decrease of CBO with the Ga content, where the CBO was already slightly negative at Ga ~ 40 %. These results indicate that the H2O introduction is effective to extend the upper limit of the Ga substitution ratio where the non-negative CBO is maintained. The observed band alignments are consistent with the rise of Voc and efficiency of the CIGS-H2O based cells.
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
10:30 AM - Y13.4
UPS Measurements of CdTe(111)-B with Surface-segregated Cu: Evidence of a Metallic Surface State and Implications for Ohmic-contact Formation.
Glenn Teeter 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractY7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
10:45 AM - Y13.5
Some are Different from others: High Temperature Structural Phase Transitions in Ternary Chalcopyrites
Susan Schorr 1 , Guillaume Geandier 2 4 , Boris Korzun 3
1 Solar Energy Research (SE3), Hahn-Meitner-Institute Berlin, Berlin Germany, 2 , European Synchrotron Radiation Facility (ESRF), Grenoble France, 4 Advanced Light Source, Lawrence berkeley National Laboratory, Berkeley, California, United States, 3 Institute of Physics of Solids and Semiconductors, National Academy of Science Belarus, Minsk Belarus
Show AbstractTernary AIBIIIX2VI chalcopyrites (A=Cu,Ag;B=Al,Ga;In, X=S,Se,Te) form a large group of semiconductors with diverse optical and electrical properties. These compounds are the isoelectronic analogs of DIIXVI binaries (D=Zn,Cd) and crystallize in the tetragonal chalcopyrite structur. The two principal noncubic features are a tetragonal deformation (1-c/2a) and a displacement of the anions (u-¼ ) (u is the anion x coordinate).For CuInX2 (X=S,Se,Te) and CuGaTe2 it is known that they undergo a temperature dependent structural phase transition from the chalcopyrite to the sphalerite structure. To our knowledge this behaviour is not reported in literature for CuGaS2, CuGaSe2 and CuAlX2. In this work we present a detailed study of the temperature dependent structural phase transition in CuBIIIX2VI chalcopyrites using in-situ high temperature synchrotron radiation diffraction.Experiments were performed at the beamline ID15B at the ESRF synchrotron radiation facility in Grenoble, France. The setup consisted of a ceramic oven with two small holes (incident and scattered beam apertures) and an on-line 2D detector, MAR 345 image plate. The samples were heated first with a rate of 300 K/h and later (200 K before TC) 38 K/h. The latter allows collecting diffraction patterns each 1K. An aluminum reference sample was used to calibrate the beam energy (λ=1.40185 nm) and the sample-detector distance. The lattice parameters, anion position and cation site occupancies were determined by Rietveld analysis of the data.The structural phase transition was obtained in CuInX2 and CuGaTe2 with transition temperatures according to literature. Structural parameters show a linear temperature dependence up to TC-10 K. Then they change their behaviour drastically, marking the critical region of the phase transition. Tthe anion parameter u remains nearly constant up to the critical region. This behaviour, du/dT=0, has been predicted theoretically, but never proved experimentally due to the absence of experimental data.The cation site occupancies remain constant with increasing temperature, within the critical region a Cu-In anti-site occupation can be observed, which was concluded as the driving force of the transition. The order parameter of the phase transition was introduced for the first time as the difference between the number of electrons at the cation site in disordered (cubic) and ordered phase (tetragonal). It vanishes approaching the phase transition with the critical behaviour IT-TCIβ. The value of the critical exponent β was determined and found to be in agreement with theoretical predictions for an order-disorder transition. The structural phase transition in CuAlTe2 and CuGaSe2, which was never reported in literature before, shows a complete different behaviour. Although the structure changes from [chalcopyrite] to [sphalerite], no Cu-BIII cation anti-site occupation takes place and no critical behaviour could be observed.
Y7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
11:15 AM - Y7.5
Exploring Back Contact Technology to Increase CdS/CdTe Solar Cell Efficiency.
Alan Fahrenbruch 1
1 Dept. of Physics, Colorado State University, Fort Collins, Colorado, United States
Show Abstract11:30 AM - Y7.6
X-ray and Electron Spectroscopy Investigation of Interfaces and Surfaces in CdTe Thin Film Solar Cells.
Lothar Weinhardt 1 , XiangXin Liu 2 , Jie Zhou 3 , Marcus Baer 1 , Timo Hofmann 1 , Oliver Fuchs 4 , Alvin Compaan 2 , Xuanzhi Wu 3 , Clemens Heske 1
1 Chemistry Department, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 Center for Materials Science and Engineering, University of Toledo, Toledo, Ohio, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 4 Experimentelle Physik II, Universität Würzburg, Würzburg Germany
Show AbstractY12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
11:30 AM - **Y13.6
Preferred Orientation in Polycrystalline Cu(In,Ga)Se2 Thin-Films and Its Effect on Absorber Physical Properties
Miguel Contreras 1
1 NCPV, NREL, Golden, Colorado, United States
Show AbstractThere are several factors that influence the microstructure and preferred orientation (texture) of polycrystalline thin-films based on Cu(In,Ga)Se2, chief among them are; substrate type, selenium activity, substrate temperature during film growth, sodium content and the type of deposition process. In this work, we use high vacuum evaporation as the deposition process and a selected group of substrates (namely molybdenum coated soda lime glass, bare soda lime glass and bare sodium free glass) to obtain thin-film materials with varied types and degrees of preferred orientation. The microstructure and texture of as-deposited films have been characterized by electron microscopy; x-ray diffraction (2D diffraction patterns, pole figures) and electron diffraction (orientation definition mapping). Along with the above characterization, we attempt to correlate the effect of texture to some physical properties of these materials. For this purpose we consider films with random orientation, films with (112) preferred orientation and films with (220) preferred orientation. Specifically, and of relevance to photovoltaic devices, we study film crystallographic defects, defect density, electrical transport and optoelectronic properties as a function of preferred orientation.Perhaps the immediate consequence of texture is the resulting microstructure. We find that films characterized by a (220) preferred orientation in general consist of a microstructure with larger grain sizes (as compared to random or (112) oriented films) and a reduced density of crystallographic defects. These characteristics of the (220) oriented films also reflect upon electronic properties for which we find such films to be more homogeneous and present a lower density of optoelectronic defects (recombination centers). To elucidate the effect of such varied physical properties with preferred orientation we have fabricated solar cells and a synopsis of device performance and results will be presented.In summary, it is clear that preferred orientation influences certain physical properties of these materials that in turn (or consequently) affect the performance of solar cells made from them.
Y7: Contacts and Interfaces
Session Chairs
Dieter Bonnet
Miguel Contreras
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
11:45 AM - Y7.7
High Infra-red Transparency CdTe Cells with a CuxTe/SWCNT Back Contact.
Teresa Barnes 1 , Xuanzhi Wu 1 , Jie Zhou 1 , Anna Duda 1 , Garry Rumbles 1 , Timothy Coutts 1 , Chris Weeks 2 , Igor Levitsky 2 , David Britz 2 , Jorma Peltola 2 , Paul Glatkowski 2
1 , National Renewable Energy Lab, Golden, Colorado, United States, 2 , Eikos, Inc, Franklin, Massachusetts, United States
Show AbstractHigh efficiency polycrystalline thin-film tandem photovoltaic devices have the potential to demonstrate 25% efficiency at a reasonable cost. However, there are many challenges to producing a cost-efficient and stable 25% device. One of the most significant is the development of a highly efficient, wide bandgap, and infra-red transparent, top cell. CdTe is an excellent candidate material for the top cell because of its relatively wide band-gap and high efficiency. Wu et. al. have already demonstrated a highly efficient and transparent CdTe top cell using a CuxTe/In2O3:Sn (ITO) back contact.1In this work, we replace the ITO with a thin film of networked single-wall carbon nanotubes (SWCNT). The SWCNT networks are highly transparent in the visible and near-IR. Unlike ITO and other conventional TCOs, they are primarily hole conducting. Free carrier scattering does not occur in the visible or near-IR range in these materials, leading to significantly higher long-wavelength transmittance than ITO. We describe a 12.4% efficient device using the CuxTe/SWCNT back contact with 50% transmission. The transmission spectra for this device is significantly flatter than for a device with ITO. We also discuss ways to improve both the transmittance and efficiency of the devices. 1Xuanzhi Wu, J. Zhou, A. Duda et al., Prog. in Photovoltaics; 14, 471 (2005).
Y8: Grain Boundaries and Inhomogeneities I
Session Chairs
Malgorzata Igalson
Susanne Siebentritt
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
12:00 PM - Y8.1
Microscopic Investigation of the CdS Buffer Layer Growth on Cu(In, Ga)Se2 Absorbers
Sascha Sadewasser 1 , Wolfgang Bremsteller 1 , Thilo Plake 1 , Christian Kaufmann 1 , Christian Pettenkofer 1
1 , Hahn-Meitner Institut, Berlin Germany
Show AbstractIt was reported recently, that the growth of evaporated CdS onto clean Cu(In, Ga)Se
2 high efficiency absorbers results in an inhomogeneous growth. Work function variations around the absorber grain boundaries where interpreted to result from S diffusion into the grain boundaries resulting in passivation of defects [1]. In order to investigate the detailed processes in more depth, we performed photoemission electron microscopy (PEEM) on thin, evaporated CdS layers on clean Cu(In, Ga)Se
2 absorbers, prepared in vacuum by decapping a protective Se-layer. The experiments were performed at beamline U49/2 PGM at BESSY.For thin CdS layers (< 1 nm thickness) we observe a selective CdS growth on some grains, while other grains remain uncovered. For CdS layers of intermediate thickness (up to 20 nm) we observe cluster formation of metallic Cd at grain boundaries, while the S is homogeneously distributed on the surface. This indicates a loss of some S, possibly by diffusion into the grain boundaries, thus confirming the model of Ref. [1]. For thicker CdS layers, homogeneous Cd and S distributions are observed. When annealing such layers at 100°C for 30 min, an increase in the detailed In and Se structure is observed. As for such low temperature a re-evaporation of CdS is unlikely, we conclude towards an intermixing of CdS and Cu(In, Ga)Se
2.[1] M. Rusu, Th. Glatzel, A. Neisser, C.A. Kaufmann, S. Sadewasser, and M.Ch. Lux-Steiner, Appl. Phys. Lett. 88, 143510 (2006).* email:
[email protected], Tel. +49-30-8062 2164, Fax. +49-30-8062 3199
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
12:00 PM - Y13.7
Chemical and Electronic Properties of the Front and Back Surfaces of Chalcopyrite Thin Film Solar Cell Absorbers.
Marcus Bar 1 , Shiro Nishiwaki 2 , Sujitra Pookpanratana 1 , Lothar Weinhardt 1 , Timo Hofmann 1 , William Shafarman 2 , Clemens Heske 1
1 Chemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, , Delaware, United States
Show AbstractIn order to reduce the costs associated with In usage in Cu(In(1-X)GaX)(SYSe(1-Y))2 (“CIGSSe”) thin film solar cells, the possibility of utilizing thinner absorber layers (e.g., with thicknesses significantly below 1 μm) is currently being explored by various groups. However, corresponding devices show lower photocurrents compared to theoretical values predicted based on the absorption characteristics of the (thinner) absorbers1. It is currently unknown whether this is due to an increasing impact of absorber nonuniformities, back-contact recombination losses, or a different chemical (and thus electronic) situation at the absorber/back contact interface.The aim of our experiments is thus to shed light on the absorber/back contact interface in terms of its chemical and electronic properties. The investigations were based on two different types of samples, namely CIGSSe/Mo/glass and (sulfur-free) CIGSe/Mo/glass thin film stacks. As a starting point, we used absorber layers of standard thickness without intentional composition gradients. In order to make the interface between absorber and Mo accessible for characterization by X-ray photoelectron spectroscopy (XPS), we used a suitable lift-off (i.e. cleavage) technique, which allowed us to cleave the samples at the absorber/Mo interface. This is confirmed by the fact that no Mo can be found on the lifted-off absorber back surface and only minor amounts of the absorber components can be observed on the surface of the Mo layer. In a first step, we have compared the characteristics of the absorber front surface with the properties of the back surface of the absorber. For the CIGSSe absorber, the XPS analysis reveals a significant difference in the Ga/(Ga+In) ratio of its front and back surface. Assuming a stoichiometric absorber composition (e.g., no Cu deficiency towards the absorber surface), the determined compositions allow a direct (“theoretical”) estimate of the absorber band gap (Eg). For the CIGSSe front surface we determine Eg to 1.68 eV; for the CIGSSe back surface, Eg is 1.58 eV. Furthermore, our results clearly identify the formation of MoSe2 and Mo(S,Se)2 layers on the surface of the Mo back contact in the CIGSe and CIGSSe case, respectively.It is planned to extend the characterization of these samples to UV photoelectron spectroscopy and inverse photoemission, which will allow us to derive direct experimental information for the electronic “surface” band gaps and to compare them with “bulk” band gaps derived from the composition at the front and back surface. Furthermore, these experiments will allow us to gain insights into the electronic properties of the absorber/back contact interface. It is of special interest to understand the band alignment at this interface, and whether it changes significantly when going from CIGSe to CIGSSe and thus from a MoSe2 to a Mo(S,Se)2 interlayer between the absorber and the Mo back contact. 1O. Lundberg et al., Prog. Photovoltaics 11, 77 (2003).
Y8: Grain Boundaries and Inhomogeneities I
Session Chairs
Malgorzata Igalson
Susanne Siebentritt
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
12:15 PM - Y8.2
Impact of Nonuniformities on Thin Cu(In,Ga)Se2 Solar Cell Performance.
Ana Kanevce 1 , James Sites 1
1 Physics Department, Colorado State University, Fort Collins, Colorado, United States
Show AbstractY12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
12:15 PM - Y13.8
Tailoring the Work Function of Chalcopyrite Thin Films with Self-assembled Monolayers of Thiols.
Sebastian Lehmann 1 , David Fuertes Marrón 1 , Iver Lauermann 1 , Marcus Bär 2 , Harry Mönig 1 , Christian-Herbert Fischer 1 3 , Martha Ch. Lux-Steiner 1 3
1 solar energy research, Hahn-Meitner Institut Berlin, Berlin Germany, 2 Department of Chemistry, University of Nevada Las Vegas, Las Vegas, Nevada, United States, 3 , Freie Universität Berlin, Berlin Germany
Show AbstractY8: Grain Boundaries and Inhomogeneities I
Session Chairs
Malgorzata Igalson
Susanne Siebentritt
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
12:30 PM - Y8.3
Analysis of Band Gap Fluctuations in Cu(In,Ga)Se2 by Confocal Optical Transmission and Photoluminescence
Levent Gutay 1 , Gottfried Bauer 1
1 Institute of Physics, University of Oldenburg, Oldenburg Germany
Show AbstractIn this work we present newest insights into absorber inhomogeneities of Cu(In0.7,Ga0.3)Se2 by confocal optical transmission and spectrally resolved photoluminescence (PL) with lateral resolution in the one-micron regime. We carry out scans for both measurands at identical positions and observe explicit local variations of the PL- and transmission-spectra. The PL-yield shows fluctuations in the sub- and few-µm regime. As shown in our previous work we extract local variations for the splitting of quasi-fermi-levels of about 40-50meV, which contributes to the open circuit voltage of a cell, and of the optical threshold of the absorber of some tens of meV, that is related to the local optical band gap. Local transmission spectra similarly show lateral variations in the few-µm regime. From these spectra we calculate local absorption coefficients and absorbance functions which yield variations of the optical threshold of the absorber of 20-30meV, which extends to the same range as the results from photoluminescence. By applying Planck’s generalized law we are able to compute theoretical PL-spectra from the resulting absorbance functions. The discrepancies between measured and calculated hypothetical PL-spectra are discussed with respect to diffusion of excess charge carriers and fluctuations of the band gap in the sub-µm-regime below the spatial resolution of the experiment.
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
12:30 PM - Y13.9
Pressure Dependent Rapid Thermal Processing of CuInS2 Thin Films Investigated by in-situ Energy Dispersive X-ray Diffraction.
Immo Kotschau 1 , Humberto Rodriguez-Alvarez 1 , Alfons Weber 1 , Cornelia Streeck 1 , Manuela Klaus 2 , Ingwer Denks 2 , Jens Gibmeier 2 , Christoph Genzel 2 , Hans-Werner Schock 1
1 SE 3, Hahn-Meitner-Institute, Berlin Germany, 2 SF 2, Hahn-Meitner-Institute, Berlin Germany
Show AbstractY8: Grain Boundaries and Inhomogeneities I
Session Chairs
Malgorzata Igalson
Susanne Siebentritt
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
12:45 PM - Y8.4
Device Performance Mapping in Cu(In,Ga)Se2 Thin Film Solar Cells
Jehad AbuShama 1 , John Tuttle 1
1 , DayStar Technologies, Inc., HalfMoon, New York, United States
Show AbstractIn this paper, we discuss device performance losses in Cu(In,Ga)Se2 (CIGS) thin film solar cells. Photovoltaic (PV) performance losses in CIGS devices are observed upon transitioning the small area cells into larger-area ones. Fabricating CIGS large-area cells in mass-production involves, generically, the following steps: 1) transition optimum small-area device designs to larger area device designs; 2) transition CIGS deposition processes from low-volume, small-area methodologies to high-volume, large-area methodologies; and 3) transition balance-of-cell (BOC) processes to production-scale processes with high performance and yield. Upon transitioning to larger area CIGS devices, PV performance losses are observed. These losses are due to non-uniformities in a CIGS solar cell. For example, substrate roughness, elemental diffusion to the CIGS film, and compositional non-uniformities are possible sources for inhomogeneities in a CIGS device. Surface compositional non-uniformities in CIGS, specially, in the Cu- and the Ga-contents may be the cause for localized fluctuations in Voc. In this paper, we present a mapping study that sheds light on the large area device performance losses.
Y12: Poster Session II
Session Chairs
Friday AM, April 13, 2007
Salon Level (Marriott)
12:45 PM - Y13.10
Hole Photocarrier Drift Mobility Measurements in CuInxGax-1 Se2 Solar Cells
Steluta Dinca 1 , Brian Egaas 2 , Rommel Noufi 2 , Eric Schiff 1 , William Shafarman 3 , David Young 2
1 Department of Physics, Syracuse University, Syracuse, New York, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Institute of Energy Conversion-University of Delaware, Newark, Delaware, United States
Show AbstractWe have measured temperature-dependent hole photocarrier drift-mobilities on copper-indium-gallium-diselenide (CIGS) solar cells using the photocarrier time-of-flight technique. We studied CIGS samples were prepared at the National Renewable Energy Laboratory (NREL) and at the Institute of Energy Conversion (IEC); samples had an alloy ratio [Ga]/([In]+[Ga]) in the range 0.25 – 0.3. The solar conversion efficiencies ranged from 11.7% to 16.6%.The values for the hole drift-mobility at 295 K ranged from 0.05 cm2/Vs to 0.3 cm2/Vs. These values are far below those obtained from Hall effect measurements on epitaxial, single-crystalline films (1). They are also significantly below most Hall effect estimates on thin films, which range from 10 – 100 cm2/Vs. Mobilities this low would have some direct effect on solar cell conversion efficiencies, but they are large enough that true mobility-limitation of the conversion efficiency seems unlikely (3).The temperature-dependence of the hole drift-mobility was measured over the range 100-300 K; there has been very little previous work on temperature-dependence of the carrier mobilities in thin films. For all samples studied, the drift-mobility has little temperature dependence (less than a factor 3 change over our range). The results suggest that the drift-mobility is not strongly affected by photocarrier trapping processes; we speculate that the mobility is determined by the microstructure of these fine-grained films.At 150 K our drift-mobility measurement on one IEC cell is 1 cm2/Vs; this value may be compared with the range 3-20 cm2/Vs previously reported on similar IEC cells based on high frequency admittance measurements. We speculate that this difference reflects a vertical inhomogeneity in the samples, for which there is some evidence in capacitance measurements; however, we cannot exclude instrumental differences between the two measurement techniques as the explanation for this difference.The NREL Thin Film Photovoltaics Partnership has supported this research.(1) D. J. Schroeder, J. L. Hernandez, G. D. Berry, A. A. Rockett, J. App. Phy. 83, 1519 (1998).(2) J.W. Lee, J. D. Cohen and W. D. Shafarman, Thin Solid Films 480-481, (2005) 336-340(3) J. Liang, E. A. Schiff, S. Guha, B. Yan, and J. Yang, Appl. Phys. Lett. 88 063512 (2006).
Y9: Grain Boundaries and Inhomogeneities II
Session Chairs
Malgorzata Igalson
Susanne Siebentritt
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
2:30 PM - Y9.1
Can Grain Boundaries Improve the Performance of Cu(In,Ga)Se2 Solar Cells?
Kurt Taretto 1 , Uwe Rau 2
1 Departamento de Electrotecnia, Universidad Nacional del Comahue, Neuquén Argentina, 2 Inst. Phys. Electron., University Stuttgart, Stuttgart Germany
Show Abstract2:45 PM - Y9.2
On Coincidence Site Lattice Modelling of Twins in the Sphalerite and Chalcopyrite Structures.
Ken Durose 1
1 Department of Physics, Durham University, Durham United Kingdom
Show Abstract3:00 PM - Y9.3
Preferred Orientation, Grain Sizes and Grain Boundaries of Chalcopyrite Thin Films.
Daniel Abou-Ras 1 , Melanie Nichterwitz 1 , Christian Kaufmann 1 , Susan Schorr 1 , Hans-Werner Schock 1
1 Solar Energy Division, Hahn-Meitner-Institut GmbH, Berlin Germany
Show Abstract3:15 PM - Y9.4
A Neutral Barrier at CGS Grain Boundaries – Compositional and Structural Dependencies.
Michael Hafemeister 1 , Susanne Siebentritt 1 , Sascha Sadewasser 1 , Martha Ch. Lux-Steiner 1
1 , Hahn-Meitner-Institut, Berlin Germany
Show AbstractY10: Structural, Optical, and Electronic Characterization I
Session Chairs
Robert Collins
Sylvain Marsillac
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
4:00 PM - Y10.1
Real-time Investigations on the Formation of Cu(In,Ga)Se2 While Annealing Precursors Produced with a Combination of Sputtering and Thermal Evaporation.
Stefan Jost 1 , Frank Hergert 1 , Rainer Hock 1 , Michael Purwins 2 , Markus Schmid 2
1 Chair for Crystallography and Structural Physics, University of Erlangen-Nürnberg, Erlangen Germany, 2 Crystal Growth Laboratory, Department of Materials Science VI, University of Erlangen-Nürnberg, Erlangen Germany
Show Abstract4:15 PM - Y10.2
Real Time Spectroscopic Ellipsometry of Magnetron Sputtered CdTe, CdS, and CdSxTe1-x Thin Films: Nucleation, Coalescence, and Optical Properties.
Jian Li 1 , Jie Chen 1 , Robert Collins 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show AbstractY11: Discussion Session II: Optimizing Thin Film Solar Cells: are We Really in Control?
Session Chairs
Brian McCandless
William Shafarman
Thursday PM, April 12, 2007
Room 2024 (Moscone West)
9:00 PM - Y12.1
The Effect of Ionizing Radiations on the Structural, Electrical and Optical Properties of AII-BVI Polycrystalline Thin Films Used as Solar Cell Materials.
Lucian Ion 1 , Vlad-Andrei Antohe 1 , Oana Ghenescu 1 , Marian Ghenescu 1 , Stefan Antohe 1
1 Faculty of Physics, University of Bucharest, Bucharest-Magurele Romania
Show Abstract9:00 PM - Y12.10
Influence of Metal Impurity Defects on the Electrical and Optical Properties of ITO Films on the PEN Substrates.
Hauk Han 1 , Terry Alford 1 2
1 Scool of Materials, Arizona State University, Tempe, Arizona, United States, 2 Flexible Display Center, Arizona State University, Tempe, Arizona, United States
Show AbstractIndium tin oxide (ITO) has drawn a great deal of attention due to its potential use as transparent electrodes in organic LED and photovoltaic applications. This work focuses on understanding the role of impurity defects on the electrical conduction and transmittance of ITO. Thin films of ITO with high carrier concentration have been deposited onto polyethylene napthalate (PEN) substrates by electron-beam deposition (e-beam) without introduction of oxygen into the chamber. The influence of annealing in air on the electrical and optical properties of ITO/PEN samples can be explained by changes in the free electron concentrations, which were evaluated in terms of the oxygen content. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis were used to determine the oxygen content in the film. Hall effect measurements were used to determine the dependence of electrical properties on oxygen content. The electrical properties of the ITO films such as carrier concentration, electrical mobility, and resistivity abruptly changed after annealing in the air atmospheres. The carrier concentration decreased from 2.04×1021/cm3 to 1.17×1020/cm3 and resistivity decreased from 1.5×10-1 Ωcm to 4.9×10-3 Ωcm after annealing because of improved hall mobility. In addition, optical transmittance improved from 7 to 71 % and optical band gap changed from 3.18 to 3.25 eV after heat treatment. The optical band gap narrowing behavior is observed in the as-deposited sample because of impurity band and heavy carrier concentration. Metal impurity cluster form in the films as a result of oxygen deficiency in the as deposited samples. These clusters also generate defects and/or impurity states in the band gap, which cause an optical band gap shift by merging of these impurity states and conduction band.
9:00 PM - Y12.12
Role of Sodium Fluoride in Cu-poor CIGS2 Thin Film Solar Cells.
Parag Vasekar 1 , Neelkanth Dhere 1
1 , Florida Solar Energy Center, Cocoa, Florida, United States
Show Abstract9:00 PM - Y12.13
The Effects of Sodium on the Properties of CIGSeS Thin Film Solar Cells.
Vinaykumar Hadagali 1 , Neelkanth Dhere 1
1 PV Materials Laboratory, Florida Solar Energy Center / University of Central Florida, Cocoa, Florida, United States
Show Abstract9:00 PM - Y12.14
Bandgap Engineering in In2S3 Thin Films by Tin Mixing.
Meril Mathew 1 , C. Sudha Kartha 1 , Kandathil Vijaykumar 1 , John Elgin 2 , Parameswar Hari 2
1 Department of Physics, Cochin University of Science and Technology, Kochi, Kerala, India, 2 Physics and Engineering Physics, University of Tulsa, Tulsa, Oklahoma, United States
Show Abstract9:00 PM - Y12.15
Characteristics of ZnO:Al Thin Films Prepared by RF Magnetron Sputtering Method.
Jihoon Kim 1 , Junje Seong 1 , Uiseok Jung 1 , A Ryoung Lee 1 , Hajong Bong 1 , Young Min Lee 1 , Deuk Young Kim 1 , Sejoon Lee 2
1 semiconductor science, Dongguk university, Seoul Korea (the Republic of), 2 Industrial Science, University of Tokyo , Tokyo Japan
Show Abstract9:00 PM - Y12.16
Kinetics of Charge Trapping and Emission in CIGS Solar Cells.
Aleksander Urbaniak 1 , Malgorzata Igalson 1 , Marika Edoff 2
1 Faculty of Physics, Warsaw University of Technology, Warsaw Poland, 2 Ångström Solar Center, Uppsala University, Uppsala Sweden
Show Abstract9:00 PM - Y12.18
CuGaSe2 Related Defect-chalcopyrites: Which Structure is the Right One and Do They Limit the Performance of the CuGaSe2 Based Devices?
Sebastian Lehmann 1 , David Fuertes Marrón 1 , Gerald Wagner 2 , Michael Tovar 3 , Yvonne Tomm 2 , Sebastian Fiechter 2 , Ernest Arushanov 4 , Thomas Schedel-Niedrig 1 , Martha Ch. Lux-Steiner 1 5
1 solar energy research, Hahn-Meitner Institut Berlin, Berlin Germany, 2 Department of Chemistry and Mineralogy, University of Leipzig, Leipzig Germany, 3 structural research, Hahn-Meitner Institut Berlin, Berlin Germany, 4 Institute of Applied Physics , Academy of Sciences of Moldova, Chisinau Moldova (the Republic of), 5 , Freie Universität Berlin, Berlin Germany
Show Abstract9:00 PM - Y12.19
Electron Spin Resonance and Ultra Violet (UV) Photoluminescence of Ge Implanted CuGaSe2 Thin Films Prepared by the CCSVT (Chemical Close-spaced Vapor Transport) Technique.
Serge Doka 1 , Jasmin Hofstetter 1 , Marin Rusu 1 , Ernest Arushanov 1 2 , Norbert Fabre 3 , Klaus Lips 4 , Thomas Schedel- Niedrig 1 , Martha Lux- Steiner 1
1 SE2, Hahn-Meitner-Institut, Glienicker Str. 100, D- 14109 Berlin Germany, 2 , Institute of Applied Physics, Academy of Sciences, Academiei 5, Chisinau 277028 Moldova (the Republic of), 3 , LAAS- CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse Cedex 4 France, 4 SE1, Hahn-Meitner-Institut, Kékuléstrasse 5, D- 12489 Berlin Germany
Show Abstract9:00 PM - Y12.2
Characterisation of Thin Films CuInxAl1-xSe2 Prepared by Selenisation of Magnetron Sputtered Metallic Precursors
Guillaume Zoppi 1 , Ian Forbes 1 , Paresh Nasikkar 1 , Robert Miles 1
1 Northumbria Photovoltaics Applications Centre, Northumbria University, Newcastle upon Tyne United Kingdom
Show AbstractThin film photovoltaic solar cells based on the use of CuInxGa1-xSe2 absorber layers are currently the most efficient thin film devices. Efficiencies in excess of 19% have been achieved for small area cells and modules have been produced with efficiencies greater than 12%. However, this material has an energy bandgap substantially lower than the 1.5 eV needed for optimum solar energy conversion. Wider energy bandgap material would results in higher efficiency devices by increasing the output voltage and reducing output current and collection losses. High efficiency CuInxGa1-xSe2 devices have been limited to the use of absorber layers with energy bandgaps of 1.3 eV as beyond this value there is substantial degradation of the physical properties of the layers produced. The most promising alternative for wider energy bandgap material is CuInxAl1-xSe2 and it has already been demonstrated that CuInxAl1-xSe2-based devices can be produced with efficiencies over 16%. However this material is not fully understood. This paper will present further details on the properties of this wide energy bandgap chalcopyrite semiconductor. Thin films of CuInxAl1-xSe2 were synthesised by selenisation of metallic Cu/Al/In multilayer precursor on Mo-coated soda-lime glass. The magnetron sputtered precursor films were Se-coated by thermal evaporation and then selenised in a tube furnace in an Ar or “forming gas” atmosphere at temperatures up to 550°C for times up to 60 min. A range of compositions were produced with Cu/(In+Al) ratios ranging from 0.6 to 1.4 and Al/(In+Al) ratios ranging from 0.15 to 0.65. The precursor layers (and the subsequently selenised thin films) were investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, optical spectroscopy and secondary ion mass spectroscopy. It is shown that the precursors have uniform composition with depth and that the composition can be controlled by the deposition process. The structural properties of the precursor films indicated a mixture of CuIn, Cu11In9, AlCu3 and AlCu4 phases with no ternary compound formed. Depending on the composition, selenised films showed a bilayer structure with a strong Al concentration towards the substrate-film interface. These films did not present the quaternary phase but instead segregation of the CuInSe2 and CuAlSe2 phases. For films with the highest Al content, near single phase CuInxAl1-xSe2 were synthesised. Data are presented for various annealing conditions and annealing environments.
9:00 PM - Y12.20
Sputtered Transparent Conducting Amorphous ZnO-SnO2 Thin Films.
Mariana Bertoni 1 2 , M. van Hest 2 , J. Perkins 2 , D. Ginley 2
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract9:00 PM - Y12.21
Mechanical Properties of Indium Tin Oxide on Polyethylene Napthalate Substrate.
S. Bhagat 1 2 , K. Lee 3 , S. Iyer 3 , J. Lewis 4 , S. Grego 4 , T. Alford 1 2
1 Flexible Display Center, Arizona State University, Tempe, Arizona, United States, 2 School of Materials, Arizona State University, Tempe, Arizona, United States, 3 Electrical and Computer Engineering, North Carolina A&T State University, Greensboro, North Carolina, United States, 4 , RTI International, Research Triangle Park, North Carolina, United States
Show Abstract9:00 PM - Y12.22
Influence of CSS Deposition Parameters on the Morphology of CdTe Films and Solar Cells.
Mathias Hadrich 1 , Sebastian Mack 1 , Heinrich Metzner 1 , Udo Reisloehner 1 , Thomas Hahn 1 , Wolfgang Witthuhn 1
1 Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Jena Germany
Show Abstract9:00 PM - Y12.23
All-chemically Deposited Solar Cells with Antimony Sulfide-Selenide/Lead Sulfide Thin Film Absorbers.
Sarah Messina 1 , M. T. Santhamma Nair 1 , P. Karunakaran Nair 1
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show AbstractPhotovoltaic structures: SnO2:F/CdS/Sb2(S/Se)3/PbS/silver print in which the semiconductor thin films have been deposited sequentially over commercial (TEC-8/15) transparent conductive oxide films have shown open circuit voltages (Voc) of up to 690 mV, short circuit current density (Jsc) of 5 mA/cm2, and fill factor of 0.34 with a conversion efficiency of > 1.5%, under tungsten halogen lamp illumination of 800 W/m2. We have used two different chemical baths: one using citrate complex [1], which produces hexagonal phase CdS film and the other using triethanolamine complex [2], which produces cubic phase CdS films, deposited at 80 oC for durations of 60 min and 90 min, respectively. Preliminary results show that using the cubic phase CdS thin films of a similar thickness (100 nm) in the structure results in a higher Jsc and a higher conversion efficiency of up to 2%, but Voc is relatively lower, 560 mV. The Sb2(S/Se)3 layers in the structures were produced by depositing first an Sb2S3 thin film of 500 nm on the CdS thin film, followed by heating at 250-300 oC in contact with a chemically deposited Se-thin film [3] for 30 min. A solid solution of antimony sulfide-selenide with an optical band gap of about 1.3 eV is formed through this process, as seen in a previous study [4]. Finally, PbS thin films of 200 nm have been deposited in 2 h at 40 oC from a chemical bath [5]. These device structures show a high series resistance and a parallel resistance of the same magnitude, which are responsible for the low fill factor values, of < 0.35. Optimization of the thickness and processing of the Sb2(S/Se)3 layer and of the thickness of the PbS film is expected to lead to improved cell characteristics. From the point of easy up-scaling in chemical deposition and the availability of the materials, these cell structures are important for future work.[1] M. T. S. Nair, P. K. Nair, R. A. Zingaro, E. A. Meyers, J. Appl. Phys. 75 (1994) 1557[2] M. T. S. Nair, J. Campos, P. K. Nair, Thin Solid Films, 161 (1988) 21[3] K. Bindu, M. T. S. Nair, P. K. Nair, J. Electrochem. Soc. 153 (2006) C526[4] D. Y. Suarez-Sandoval, M. T. S. Nair, P. K. Nair, J. Electrochem. Soc. 153 (2006) C91[5] P. K. Nair, M. T. S. Nair, J. Phys. D – Appl. Phys. 23 (1990) 150
9:00 PM - Y12.25
Optical Properties of Strained CuInS2 Layers.
Jens Eberhardt 1 , Heinrich Metzner 1 , Rüdiger Goldhahn 2 , Florian Hudert 2 , Kristian Schulz 1 , Udo Reislöhner 1 , Thomas Hahn 1 , Janko Cieslak 1 , Wolfgang Witthuhn 1
1 Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Jena Germany, 2 Institut für Physik, Technische Universität Ilmenau, Ilmenau Germany
Show Abstract9:00 PM - Y12.26
ZnS/CdZnS as Alternative Buffer Layer for CIGS2.
Bhaskar Kumar 1 , Parag Vasekar 1 , NeelKanth Dhere 1
1 Florida Solar Energy Center, Univ. of central florida, Cocoa, Florida, United States
Show Abstract9:00 PM - Y12.27
Electronic and Optical Characterization of Defects in CuGaSe Devices.
J. Rembold 1 , Todd Curtis 1 , Jennifer Heath 1 , David Young 2 , Steve Johnston 2 , William Shafarman 3
1 Physics, Linfield College, McMinnville, Oregon, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractWe have studied the optical and electronic properties of CuGaSe (CGS) solar cells. Samples were grown in two different laboratories, and under varying conditions. This report focuses on four sample sets. Two were grown using the three stage process, one using the standard process and the other with the CGS surface modified by addition of a small quantity of In. The other two were grown using uniform elemental evaporation, with Cu/Ga ratios of 0.78 and 0.56, respectively. Device efficiencies ranged from 3% to 8.2%. Sample stochiometry and uniformity were determined using energy dispersive spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Electronic properties were studied using current-voltage-temperature measurements, quantum efficiency, drive level capacitance profiling (DLCP), and capacitance-voltage profiling. Optical properties were determined using transient photocapacitance spectroscopy (TPC) and transient photocurrent spectroscopy.The DLCP measurements distinguished high quality CGS films from those that would result in poorer devices. Films with defect densities (ND) above 2x1016 corresponded to low quality devices. Excluding these, our measurements confirm that a lower ND generally corresponds to higher short circuit current (Jsc), although at the expense of some open circuit voltage (Voc). The interface properties of the devices also differ and affect the ultimate efficiencies. Devices grown by uniform elemental evaporation have some near-interface defect response which seems to result in a lower Jsc despite good Voc. The devices grown using the three-stage process do not exhibit a near-interface defect response. These devices also have a distinctly different Na profile throughout the film, as studied with TOF-SIMS, which could be linked to the different interface properties. The device which is surface-modified with In has particularly high current collection, though its open circuit voltage is lower, likely due to the reduced surface bandgap. The TPC optical spectra generally look similar to those of CuIn1-xGaxSe2 (CIGS), with a broad defect response centered close to 0.8 eV. Urbach energies (EU) seem to correspond to stochiometric inhomogeneities that are not impacting the ultimate device performance. The surface-modified device has a particularly large value of EU=26 meV, likely due to variations in bandgap in the near surface region of this sample. Similarly the device with Cu/Ga=0.56 has EU=23 meV. Devices with Cu/Ga≈0.8 and no surface modifications have EU=17-19 meV, for either growth technique. Optical spectra of all CGS devices show an interesting peak around 1.4 eV whose origin and impact will require further study. This response is also present in wide-gap CIGS devices but not in CuInSe2-ySy or CuIn1-xAlxSe2, indicating it may relate to the presence of Ga in the matrix.
9:00 PM - Y12.28
CdS/CdTe Solar Cells Made by High-rate Magnetron Sputtering.
Victor Plotnikov 1 , Alvin Compaan 1
1 , University of Toledo, Toledo, Ohio, United States
Show Abstract9:00 PM - Y12.29
SnS Thin Films in Chemically Deposited Solar Cell Structures.
David Avellaneda 1 , M. T. Santhamma Nair 1 , P. Karunakaran Nair 1
1 Centro de investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show AbstractThe abundance of about 2 ppm of Sn on earth’s crust offers prospects for developing photovoltaic technologies in which tin-based thin film semiconductors take an important role. This aspect may be combined with the ease of scaling up of chemical bath deposition of SnS thin films to make them even more feasible in this technology. Here, SnS thin films of approximately 500 nm thickness were deposited using two different chemical baths. XRF studies on these materials have showed that they are of identical chemical composition. The film deposited from the bath as repoted in [1] has an orthorhombic crystalline structure similar to that of the mineral herzenbergite, and an optical band gap of 1.1 eV. It has a dark conductivity of 10-6 (Ω cm)-1 as deposited, which increases to 10-3 (Ω cm)-1 when heated for 30 min at 300 oC in nitrogen. The film deposited by the other method [2] has an optical band gap of 1.7 eV. It has a low electrical conductivity, 10-7 (Ω cm)-1. The crystalline structure of this material is yet to be determined. We combined these two types of SnS thin films in a solar cell structure by depositing them sequentially over a CdS thin film, which was chemically deposited on commercial TEC-8 or15 (Pilkington, Toledo) conductive tin oxide film. The structure is completed as follows: SnO2:F/CdS(100 nm)/SnS(1.1 eV, 300 nm and heated in nitrogen at 300 oC for 1 h)-SnS(1.7 eV, 100 nm)/CuS (80 nm). The finished structure is heated in air briefly (5 min) at 200 oC to increase the conductivity of the CuS film. Silver-print electrodes were applied to the conducting SnO2 film and the CuS thin films to serve as the contacts. This cell structure has the following characteristics when illuminated using 800 W/m2 (tungsten-halogen): open circuit voltage, 380 mV; short circuit current density, 7 mA/cm2; fill factor, 0.34; conversion efficiency, 1%. Using a curve-fitting program involving the Lambert W function for the I-V curve, the cell parameters were determined. We shall show that both types of SnS films may be converted to SnO2 by heating in air at 400-500 oC. For this, the material passes through intermediate compositions of SnS2/SnO2. Thus, there exists the possibility of adding this intermediate film over the conductive SnO2 film to improve upon the cell characteristics. Similarly, we shall report on the conversion of an SnS-CuS top layer into Cu2SnS3 by heating the structure at temperatures > 300 oC or the addition of a chemically deposited PbS thin film over the SnS thin film in the cell performance. Feasibility of producing small modules by this approach would be illustrated and the stability of the cells under direct solar radiation over a period of more than three months would be addressed. [1] M. T. S. Nair, P. K. Nair: Semicond. Scie. Technol. 6 (1991) 132[2] D. Avellaneda, G. Delgado, M. T. S. Nair, P. K. Nair, E-MRS Symp. O, Nice, 2006, accepted for publication in Thin Solid Films (Special Issue)
9:00 PM - Y12.3
Quantum Efficiency Measurements to Deduce Non-ideal Solar-Cell Features.
Timothy Nagle 1 , Alan Davies 1 , James Sites 1
1 Physics, Colorado State University, Fort Collins, Colorado, United States
Show AbstractAppropriate interpretation of Quantum Efficiency (QE) measurements made on non-ideal solar cells often reveal subtle features of the photodiodes. QE measurements on CuIn1-xGaxSe2 and CdTe thin-film solar cells with CdS buffer layers demonstrate some of these features, and in each case we identify the electrical processes responsible. One well-known complication in cells with CdS buffer layers is 'photodoping', where blue light exposure increases the n-type CdS carrier density. The resulting modification of the p-n junction alters the carrier collection for all wavelengths and can cause misinterpretation of results. The use of a white, DC-bias light during QE measurements generally solves the problem, but it has not been clear what bias light level is sufficient for accurate results. Measurements with varying intensities and spectra of DC-bias light show that if the CdS layer has approximately equal numbers of photoexcited electron-hole pairs and acceptor-like defects, then the complications due to CdS photoconductivity are mitigated. Using white light bias, and with acceptor-like defect density NA = 1018 cm-3, this condition is met near 0.05 sun intensity. Other factors which can cause misinterpretation of QE measurements include the presence of secondary barriers, and photoconductivity in the absorber layer. We demonstrate the effects of each of these factors with numerical simulations and AMPS modeling. Experimental Current-Voltage (J-V) and optical reflection data confirm the correct analysis. Short-circuit current values as determined by integration of QE-curves scaled to the one-sun spectrum agree with those determined during one-sun J-V measurements to within 3%. When the above complications are accounted for, voltage-biased QE measurements provide a clearer picture of a solar cell's band diagram. Finally, the analysis is used to suggest a standard set of measurement conditions for reliable QE analysis.
9:00 PM - Y12.30
Study of the Electronic Properties of Matched Na-containing and Na-free CuInGaSe2 Samples Using Junction Capacitance Methods.
Pete Erslev 1 , Adam Halverson 1 , William Shafarman 2 , J. Cohen 1
1 Physics, University of Oregon, Eugene, Oregon, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractSodium is well known to have a beneficial effect on CIGS solar cells. However, the precise mechanism for this improvement is still open to question. To gain more insight into this we carried out junction capacitance measurements to characterize the electronic properties of a matched set of Na-containing and Na-free CIGS devices. These samples were fabricated at the Institute of Energy Conversion by elemental evaporation in a homogeneous deposition process. The baseline (w/ Na) cells had efficiencies near 15%, whereas the Na-free cells, grown on soda-lime glass with a SiO2 diffusion barrier, had efficiencies of roughly 10% due to 20% lower values of Voc and fill factor. Our characterization methods included admittance, drive-level capacitance profiling (DLCP), as well as transient photocapacitance (TPC) and transient photocurrent (TPI) measurements. Both the admittance and the DLC profiles indicated marked differences between the two types of samples. For samples with Na the admittance measurements showed a clear activated capacitive step indicative of the well-known deep acceptor defect, but this was totally absent in the Na-free sample. The DLC profiles also clearly revealed much lower free carrier densities (by roughly an order of magnitude) in the Na-free samples compared to the Na-containing samples. The TPC and TPI measurements were used to examine the sub-bandgap optical spectra of the samples. Initial results show very little difference in either the TPC and TPI spectra between the Na-containing and Na-free devices. Because TPC measures the difference, p – n, in carrier collection, while TPI measures the sum, p + n, it appears that the minority carrier collection efficiencies are nearly the same in both types of samples. This may indicate that the loss in fill factor and VOC have an origin not directly related to bulk recombination in the CIGS absorber.
9:00 PM - Y12.31
Revisiting CdS-PbS Solar Cell Structure.
Harumi Moreno-Garcia 1 , Jose Campos 1 , M. T. Santhamma Nair 1 , P. Karunakaran Nair 1
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show AbstractCdS and PbS are well known semiconductor materials. The first one is among the best known photoconductive photodetectors, which in recent years has risen to prominence for its use as n-window or buffer layer in thin film solar cells. The second one is an equally well known photodetector, but in the near infrared region, which has also risen to prominence during the past three years due to the expectations of high quantum efficiencies due to carrier multiplication for incident photon energies in excess of times the optical band-gap energy in the nanocrystalline regime. These two are also among the most investigated materials by chemical deposition technique: PbS from the 1980’s and CdS from the 1960’s. Evaluation of the heterojunction parameters of a CdS-PbS junction predicts built-in voltages of up to about 1000 mV, depending on the material parameters chosen. Chemically deposited CdS-PbS solar cells reported in the 1970’s and 1980’s have shown open circuit voltage of up to 450 mV and short circuit current density of < 1 mA/cm2 . Here, we report photovoltaic structures: SnO2:F/CdS/PbS/silver print, in which the semiconductor thin films have been deposited sequentially over commercial (TEC-8/15) transparent conductive oxide films. These show open circuit voltages (Voc) of up to 560 mV and short circuit current density (Jsc) of 1.5 mA/cm2, under tungsten halogen lamp illumination of 850 W/m2. We have observed that hexagonal phase CdS film provides a relatively higher open circuit voltage compared with cubic-phase CdS film. However the cell characteristics remain stable in the structure using cubic-phase CdS film. We shall present the improvement in the cell characteristics through the use of interlayer films of CdSe, Bi2S3, SnS2, PbSe, all obtained by chemical deposition. We consider that photovoltaic structures developed through this methodology offers prospects in developing large area devices required in solar cell technology.
9:00 PM - Y12.32
Study of Nucleation and Growth Modes of InN films by MOCVD on Sapphire Substrate for Photovoltaic Applications.
Muhammad Jamil 1 , Yik-Khoon Ee 1 , Ronald Arif 1 , Hua Tong 1 , John Higgins 2 , Nelson Tansu 1
1 Center for Optical Technologies, Department of Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 , Air Products & Chemicals, Inc., Allentown, Pennsylvania, United States
Show Abstract Recently, it has been discovered that the bandgap of InN was at about ~0.65-0.7 eV. The narrow bandgap of InN materials corresponds to long absorption cut off wavelength at ~1850 nm, which is applicable for covering the solar spectrum up to ~1850 nm regime. The availability of high-quality InN materials allows the additional solar spectrum utilization in the ~1400-1850 nm regimes, applicable for achieving high-efficiency III-V thin film photovoltaic solar cells. In contrast to the more mature development of InN material systems by molecular beam epitaxy (MBE), the epitaxy of InN materials with metalorganic chemical vapor deposition (MOCVD) still requires further investigation, in particular on its nucleation condition. Here we present the experimental studies on nucleation and growth modes of InN thin films on c-plane sapphire utilizing metalorganic chemical vapor deposition (MOCVD). The MOCVD epitaxy of InN thin films were conducted utilizing a vertical reactor with TMIn and NH3 as the In- and N-precursors, respectively. The effects of growth temperature, V/III ratio and nitridation of sapphire substrate on the nucleation as well as the morphological evolution of the overgrown InN thin films were investigated. It was found that the epitaxy of InN thin film is particularly sensitive to growth temperature and V/III ratio. The deviation from optimum growth conditions results in Indium metallic droplets on the surface. Effect of temperature on the enhancement of lateral growth mode of 3D islands was also investigated for given V/III ratio. Film nucleation and density of the nuclei was observed to be affected by the nitridation process, which is conducted at 1050 °C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to monitor the nucleation and surface morphology of films. In our approach, we employed a low-temperature InN nucleation layer on c-plane sapphire after nitridation process of 5 minutes, followed by the higher temperature InN thin film epitaxy. The low temperature nucleation epitaxy was employed to promote improved surface coverage on the substrate as well as to suppress surface desorption of the thin film. After the nucleation process, the growth of InN film is conducted at higher temperature to promote the coalescence of the film, as well as to achieve improved surface morphology and higher crystalline quality. From our studies, we found that the optimum nucleation temperature and V/III ratio for the InN nucleation growth as 580 °C and 2.7 x 105, respectively. The optimum growth temperature and V/III ratio of the high temperature InN film are 610-650 °C and 2.25 x 105, respectively. Structural characterization by X-ray diffraction (XRD) and results of electrical and optical characterizations of the InN films will also be presented. 9:00 PM - Y12.33
Photoconductive Thin Films Obtained From Chemical Process.
Claudio Carvalho 1 , Fabio Freitas 1 , Victor Reynoso 1
1 Physics and Chemistry, Universidade Estadual Paulista - UNESP, Ilha Solteira, SP, Brazil
Show AbstractIn this work we have special interest in the titanium dioxide - TiO2 mainly in photoconductive applications. There are four polymorphs of titanium dioxide found in nature: rutile, anatase, brookite and TiO2 (B) that present crystalline phase, tetragonal, tetragonal, orthorhombic and monoclinic, respectively. Titanium dioxides are very important in industry as a white paint pigment and a strong, lightweight metal. In addition, titanium dioxide thin films are commonly used to support catalysts and can be used to photocatalyze reactions as well. Anatase is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system. There are some indications that anatase is more stable than rutile, due to surface energy effects. The temperature has a direct influence about this material, when strongly heated, anatase is converted into rutile, naturally occurring pseudomorphs of rutile after anatase are also known. Crystals of anatase have and continue to be artificially prepared using titanium tetrachloride, TiCl4, to produce TiO2 and HCl gas. Then, anatase may be used in the area of semiconductors and photovoltaic materials. However, a precursor solution of TiO2 has been prepared from a complex solution of titanium peroxide and it is deposited above glass substrate coated with SnO2-F conductor thin film using the spray pyrolysis technique. In this way, a porous titanium dioxide thin film with a thickness of few microns has been obtained through chemical process. The adherence and surface of the TiO2 nanoparticles was modified adjusting the solution acidity and controlling the temperature deposition that was maintained around 200 oC. After the deposition the samples are submitted thermal treatment in electric resistive furnace between 400 - 450 oC during one hour in ambient atmosphere The film presented fine TiO2 particles, approximately 30–50 nm in size as observed using a SEM. Measurements of X-ray diffraction has shown the presence of anatase crystalline phase. The forbidden band gap has also been analyzed using uv-vis spectroscopy by reflectance and transmitance. Measurement of AFM and efficiency are being carried out.
9:00 PM - Y12.34
Optical Absorption at Digitally and Continuously Graded Indium Gallium Nitride Schottky Barriers
Choudhury Praharaj 1 2
1 , Intel Corporation, Santa Clara, California, United States, 2 ( formerly with ) Department of Electrical and Computer Engineering, Cornell University, Ithaca, New York, United States
Show AbstractWe present numerical calculations of the optical absorption characteristics of graded Indium Gallium Nitride Schottky Barriers, and study their implications for photovoltaic and photodetector applications. We consider the two cases of digital and continuous grading because of their different effects on the collection of photo-generated carriers through band discontinuities. Composition grading can achieve desired spectral response between the ranges of 0.7 eV and 3.43 eV afforded by the Indium Gallium Nitride alloy system. However, the presence of spontaneous and piezoelectric polarizations in this material system adds bulk and/or interface bound charges in graded layers. This has a non-trivial effect on the band profile seen by the photo-generated carriers. The layer thicknesses needed for optimal absorption characteristics are well above the theoretical critical thickness limits reported in the literature for abrupt heterojunctions. However, experimental data about critical thicknesses is scarce, especially for graded compostions. Therefore, we calculate the characteristics of the Schottky barrier for the case of spontaneous polarization only and also for the case of both spontaneous and piezoelectric polarization assuming no relaxation. For photodetector applications, we calculate the effect of polarization on the carrier multiplication due to impact ionization and the transit time of carriers. The low or even negative Schottky barrier heights at low Gallium composition necessitates the use of high Gallium composition layers next to the metal, which lead to dark current suppression of 6 orders of magnitude.
9:00 PM - Y12.35
Optimization of Sodium Incorporation for CIGS Solar Cell by Na-doped Mo Electrode.
Jae Ho Yun 1 , Ki Hwan Kim 2 , Min Sik Kim 2 , Byung Tae Ahn 2 , Jin Seop Song 1 , Kyung Hoon Yoon 1
1 Photovoltaic Research Group, Korea Institute of Energy Research, Daejeon Korea (the Republic of), 2 Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractMany research groups have studied about role of Na diffusion from soda-lime glass or Na source material. However it has not yet been verified clearly. Moreover it is very difficult to estimate and control the quantity of Na diffusion from the soda-lime glass. In this work we formed Mo/Na-doped Mo bi-layer back contact by using the Na-doped Mo target for sputtering process. And we fabricated CIGS solar cell with bi-layer back contact on stainless steel and alumina, which means Na-free substrate. As the thickness of Na-doped Mo layer increased, (112) peak of CIGS films enhanced but grain size was reduced. In case of stainless steel(alumina)/ Na-doped Mo/Mo bi-layer/CIGS cells, the fill factor and open-circuit voltage increase compared to those of CIGS cells using the conventional Mo back contact. It seems that the Na-doped Mo acts as Na source and the content of Na in the CIGS layer can be controlled by the regulating the thickness of the Na-doped Mo layer. However the quantum efficiency decreased with Na-doped Mo thickness and leakage current increased. It seems that the excess Na diffusion is related to the formation of defect near to the CIGS/CdS interface. In case of 100 nm Na-doped Mo/1000 nm Mo bi-layer, the best conversion efficiency of CIGS solar cell on stainless steel and alumina were obtained with a values of 10.6% and 13.3%,respectively.
9:00 PM - Y12.37
Hg1-xCdxTe as the Bottom Cell Material in Tandem II-VI Solar Cells.
Viral Parikh 1 , Jie Chen 1 , S. Marsillac 1 , R. Collins 1 , A. Compaan 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show Abstract9:00 PM - Y12.38
CuGaSe2-based Solar Cells with High Open Circuit Voltage.
Raquel Caballero 1 2 , Susanne Siebentritt 1 , Christian A. Kaufmann 2 , Daniel Schweigert 2 , Hans-Werner Schock 2 , Martha Ch. Lux-Steiner 1
1 Heterogeneous Material Systems-SE2, Hahn-Meitner-Institut Berlin, Berlin Germany, 2 Technology-SE3, Hahn-Meitner-Institut, Berlin Germany
Show Abstract9:00 PM - Y12.39
Cu(In,Ga)Se2 Prepared from Electrodeposited CuGaSe2/CuInSe2 Bilayer for Solar Cell Applications.
Yusuke Oda 1 , Takashi Minemoto 1 , Hideyuki Takakura 1 , Yoshihiro Hamakawa 1
1 College of Science and Engineering, Ritsumeikan University, Kusatsu Japan
Show AbstractIt is useful to prepare Cu(In,Ga)Se2 (CIGS) thin-films by electrodeposition (ED) for a high used efficiency of materials and a solution process at low temperature. The Ga/III ratios for high efficiency CIGS thin-films solar cells range from 0.2 to 0.3. It is difficult to obtain ED-CIGS thin-films with the controlled Ga/III ratios from a single-step electrodeposition because the deposition potential of Ga is different from other elements. The performance of ED-CIGS thin-films solar cells with the controlled Ga/III ratios was inferior to the performance of ED-CuInSe2 (CIS) thin-films solar cells. In this study, CIGS thin-films were grown using a new two-step electrodeposition process. CuGaSe2 /CuInSe2 bilayers were prepared on Mo/glass substrates with a view to realizing the controlled Ga/III ratios and smooth layers of CIGS thin-films. CuGaSe2 (CGS) thin-films were electrodeposited using the potentiostatic electrochemical method, where the reference electrode was a Ag/AgCl electrode, the counter electrode was a Pt gauze and the working electrode was a soda-lime glass/ 0.8 μm Mo substrate. The 100 ml electrodeposition solution used for preparing CGS thin films consisted of 2.5 mM CuSO4-5H2O, 10 mM Ga2(SO4)3-19.3H2O, 20 mM H2SeO3, 5 mM H6TeO6, 0.15 M Li2SO4 and 100 mg gelatin. The pH of the solution was adjusted to 2.6 with the addition of NaOH. CGS thin-films were electrodeposited with the deposition condition of -590 mV vs. Ag/AgCl at 30 oC. A scanning electron microscope (SEM) and an energy dispersive X-ray spectrometry (EDS) showed that smooth CGS thin-films with nearly stoichiometric composition (Cu/III=0.82, Se/Metal=0.89) were realized. The solution for preparing CIS thin-films which contained 6.25-12.5 mM CuCl2-2H2O, 25 mM InCl3-4H2O and 25 mM H2SeO3 was adjusted to pH 1.5 with HCl. CIS thin-films were electrodeposited onto as-deposited CGS thin-films with the deposition condition of -450 mV vs. Ag/AgCl at 30 oC. The thickness control of each layer was realized with the coulomb meter. SEM images indicated smooth morphologies of ED-CIS layers which were deposited on ED-CGS layers. EDS analysis showed that Cu/III ratios were range from 1.25 to 1.36 and Se/Metal ratios were range from 0.84 to 0.95. These ED-CGS/CIS bilayers were annealed at several processes range between 550 oC and 650 oC in flowing Ar to form CIGS thin-films. The optimization of the annealing process and each deposition condition are expected to lead to the formation of the graded structure of CIGS thin-films and significant improvements in cell performance. The Cu/III ratios of CIGS thin-films were controlled from 0.9 to 1.0 with KCN etching (1.0 wt%, 1 min). We will report on the improvements in the electrodeposition condition and the annealing process for smooth ED-CIGS thin-films with the controlled Ga/III ratios and the electric/optical performance of solar cells using ED-CIGS thin-films as an absorber.
9:00 PM - Y12.4
Electron Backscatter Diffraction of Photovoltaic Thin Films
Helio Moutinho 1 , Ramesh Dhere 1 , Chun-Sheng Jiang 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractElectron-backscatter diffraction (EBSD) uses the electron beam of a scanning electron microscope (SEM) to investigate the crystalline structure of a sample. In EBSD, electrons from the SEM beam are diffracted from a small volume close to the sample surface. The diffraction forms Kikuchi patterns on the screen of a special detector that are a signature of the crystalline structure and orientation of the crystal. The advantage of EBSD over other diffraction techniques, such as X-ray diffraction, is the high spatial resolution. Orientation maps are formed by measuring the Kikuchi patterns while the electron beam scans the sample surface. In these maps, regions with different orientations are represented with different colors. Crystalline features such as twins and grain boundaries are revealed, and the misorientation between grains can be measured, as well as pole figures and inverse pole figures. A requirement for EBSD analysis is a flat sample. On rough samples, the diffracted electrons are shadowed from the detector by surface features. In this work, we investigate CdTe samples deposited by close-spaced sublimation (CSS) and physical vapor deposition (PVD) using EBSD. PVD samples are relatively smooth, and good-quality EBSD data are produced without any surface treatment. However, CSS samples are rough, resulting in most regions producing a weak diffracted signal, which cannot be indexed by the software. We processed the CSS samples using three methods: mechanical polishing, chemo-mechanical polishing, and ion-beam milling. Mechanical polishing produced flat samples, but low-quality EBDS data. Although the chemo-mechanical polished samples produced better EBSD data, the quality was still poor. We believe that the top surface of the samples flattened by these two methods had poor crystallinity, and, because the EBSD data come from the top layers (top few nanometers), no good-quality data were obtained. In contrast, ion-beam milling proved to be suitable to produce flat samples with minimal surface damage, which produced good EBSD data. In this case, we optimized the milling parameters, such as ion-beam angle and milling time. We also analyzed the samples with atomic force microscopy, and correlated the quality of the EBSD data with the roughness and height of surface features. We were able to optimize the ion-milling process and routinely produce samples that provide excellent EBSD patterns. In this work, we will provide details on this procedure. We also investigate the crystalline structure of CdTe films grown by PVD and CSS, and show how this technique can be used to characterize semiconductor materials and the relevant information that can be obtained, such as texture, grain-boundary structure, and intragrain boundaries, such as twins. We also show results of EBSD measurements performed on CIGS samples grown by a three-stage process.This abstract is subject to government rights.
9:00 PM - Y12.5
Optical Properties of CuInGaSe2 Thin Films with Quantum-scale Surface Formations
Peter Gorley 1 , Yuri Vorobiev 2 , Jesus Gonzalez-Hernandez 3 , Viktor Makhniy 1 , Sergiy Bilichuk 1 , Olena Galochkina 1 , Volodymyr Khomyak 1 , Paul Horley 1
1 Dept. of Electronics and Energy Engineering, Chernivtsi National University, Chernivtsi Ukraine, 2 , CINVESTAV-IPN, Unidad Queretaro, Queretaro Mexico, 3 , CIMAV, Chihuahua Mexico
Show AbstractThin films (TF) of CuInxGa1-xSe2 (CIGS) are one of the promising materials for efficient solar cells. The optical properties of TF CIGS determining the performance of the solar cells manufactured on their base significantly depend on the material obtaining technology. In this paper we report the investigation results of structural, electrical and optical properties of CIGS films, deposited by magnetron sputtering (MS) and thermal evaporation (TE). The latter was performed in the quasi-closed volume from the CIGS targets (obtained by grinding of the previously-grown bulk crystals of the composition x=0.0, 0.2, 0.5, and 1.0) at the constant temperatures of the evaporator and the substrate, which were kept within the ranges 1423-1573 K and 573-673 K, respectively. Thin films deposited with high-frequency MS were obtained from the same initial materials in argon atmosphere under the constant power at the cathode. Substrate temperature in this case was within the ranges of 473-573K. The authors performed film deposition over glass, sital and silicon substrates. Technology optimization was performed empirically, adjusting the working gas pressure, target-substrate distance, substrate temperature, etc. Obtained refined technological regimes for both MS and TE methods allowed to obtain TF CIGS with reproducible and time-stable electrical and optical properties. X-ray diffractometry, AFM and SEM investigations have shown that the obtained TF CIGS do not feature any dominating orientation and are polycrystalline; their continuous surface appeared to be covered with nano-scale formations (NSF), the linear dimensions of which can be controlled by film deposition technology and/or the type and temperature of the substrate. It was found that the typical size of the NSF for the CIGS films obtained by MS was 0.1-0.2 μm, while the corresponding values for thermally-evaporated film were 0.7-0.8 μm. Specific resistivity of TF CIGS depends significantly on their deposition technology. In particular, for MS, the temperature of the substrate can controllably change the resistivity of the resulting film in the ranges 10-106 Ωcm. Films with the thickness close to 1 μm featured comparatively high transmission (up to 50%) in the transparency region. Presence of NSF leads to the shift of TF optical transmission edge towards the higher energies; the authors determined the dependence of such blue-shift magnitude on the linear dimensions of the surface nano-structure.
9:00 PM - Y12.6
Effect of N on Photocorrosion Resistance of III-V Semiconductors in PEC Hydrogen Production
Theanne Schiros 1 2 , Jennifer Leisch 1 , Lars-Ake Naslund 1 , Hirohito Ogaswara 1 , Todd Deutsch 3 , Sarah Kurtz 3 , John Turner 3 , Anders Nilsson 1 2
1 , Stanford Synchrotron Radiation Laboratory, Menlo Park, California, United States, 2 Fysikum, Stockholm University, Stockholm Sweden, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract9:00 PM - Y12.7
Epitaxial Growth of CdTe Films Via Closed-Space Sublimation.
Stella Quinones 1 , Arev Escobedo 1 , Jose Luis Cruz-Campa 1 , Brandon Aguirre 1 , Adame Michelle 1 , John McClure 2 , David Zubia 1
1 Electrical and Computer Engineering, The University of Texas at El Paso, El Paso, Texas, United States, 2 Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, Texas, United States
Show AbstractClosed-space sublimation, a relatively inexpensive deposition technique typically used for polycrystalline thin-films, was used to produce high-quality epitaxial films of CdTe on CdTe(111) and CdTe(211)/Si(211) single crystal substrates. Device quality CdTe and its related alloys are of interest for solar cell and infrared detector applications. For example, CdZnTe(211) on Si(211) is of particular interest as a template for epitaxial growth of HgCdTe-based infrared devices. Special interest in using closed-space sublimation is due to its much lower cost and faster growth rates (1 to 50 micron/hr) compared to molecular beam epitaxy. In this study, closed-space sublimation reactor parameters that yielded epitaxially smooth layers were developed for the CdTe(111) and CdTe(211)/Si(211) substrates. A single crystal CdTe wafer was used as a sublimation source. The films were examined by profilometry, scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), atomic force microscopy (AFM) and x-ray diffraction (XRD). In particular, electron microscopy showed that smooth continuous films can be achieved. Furthermore, EBSD and XRD rocking curves show that highly oriented, single crystal films can be grown at high growth rates (> 1 micron/hr). Closed-space sublimation is a very promising low-cost technique for fast-growth-rate deposition of high-quality CdTe thin films.
9:00 PM - Y12.8
Highly As Doped CdTe Tunnelling Layer for Back Contact of CdTe Solar Cells.
Vincent Barrioz 1 , Eurig Jones 1 , Stuart Irvine 1
1 School of Chemistry, University of Wales Bangor, Bangor, Gwynedd, United Kingdom
Show AbstractLow resistance back contacts on CdTe PV solar cells rely on p+ doping of the surface to induce tunnelling through the Shottky barrier. The p+ doping is normally achieved through coating the surface of the CdTe with the dopant, such as copper, and then annealing to achieve diffusion into the near surface region. This paper reports on a study using an alternative of a highly arsenic doped CdTe layer prior to the deposition of the back metal contact. The CdTe:As layer was grown by metal organic chemical vapour deposition (MOCVD) as an additional step in the MOCVD deposition of the CdS/CdTe structure. By this process it was possible to achieve arsenic concentrations in excess of 1 x 1019 at.cm-3. This method does not require any post-growth annealing other than the usual CdCl2 diffusion anneal treatment of the p-n structure and does not require any wet etching before the deposition of the metal contact. Various metals have been successfully tried as back contacts. Using gold contacts, the series resistance was reduced from 10 Ω down to 4.2 Ω, for a contact area of 24.5 mm2. Further characterisation of this new contacting procedure will be presented.
9:00 PM - Y12.9
Growth and Characterization of Transparent Conducting ZnO Thin Films.
Natalya Suvorova 1 , L. Stan 1 , I. Usov 1 , R. DePaula 1 , A. Dattelbaum 1 , P. Arendt 1 , Q. Jia 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract