Bhushan L. Sopori National Renewable Energy Laboratory
Bernhard Dimmler Würth Solar GmbH & Co. KG
Jeffrey Yang United Solar Ovonic LLC
Thomas Surek Surek PV Consulting
Q1: Crystalline Silicon Technologies
Monday AM, November 30, 2009
Room 306 (Hynes)
9:30 AM - **Q1.1
Hydrogen Passivation for Crystalline Silicon Solar Cells.
Michael Stavola 1 Show Abstract
1 Department of Physics, Lehigh University, Bethlehem, Pennsylvania, United States
The Si substrates that are often used for the fabrication of solar cells to reduce cost give rise to defect issues that must be addressed. Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency . A process that is used by industry to introduce hydrogen is by the post-deposition annealing of a hydrogen-rich SiNx layer that is used as an antireflection coating . A number of questions about this hydrogen introduction process and hydrogen’s subsequent interactions with defects have proved difficult to address because of the low concentration of hydrogen that is introduced into the Si bulk.Fundamental studies of hydrogen-containing defects in silicon provide a foundation for addressing issues of interest to the Si solar-cell community. Strategies have been developed by which hydrogen in silicon can be detected by IR spectroscopy with high sensitivity [3,4]. The introduction of hydrogen into Si by the post-deposition annealing of a SiNx coating has been investigated to reveal hydrogen’s concentration, diffusivity, and reactions with defects. The effect of processing variations on the concentration of hydrogen that is introduced into the Si bulk has also been studied. The contributions of F. Jiang, S. Kleekajai, V. Yelundur, A. Rohatgi, L. Carnel, J. Kalejs, and G. Hahn to our studies are gratefully acknowledged. This work has been supported by the Silicon Solar Research Center SiSoC Members through NCSU Subaward No. 2008-0519-02 and NSF Grant No. DMR 0802278. J. I. Hanoka, C. H. Seager, D. J. Sharp, and J. K. G. Panitz, Appl. Phys. Lett. 42,618 (1983). F. Duerinckx and J. Szlufcik, Sol. Energy Mater. Sol. Cells 72, 231 (2002). F. Jiang et al., Appl. Phys. Lett. 83, 931 (2003). S. Kleekajai et al., J. Appl. Phys. 100, 093517 (2006).
10:00 AM - Q1.2
A New, Ultrafast Technique for Mapping Dislocation Density in Large-area, Single-crystal and Multicrystalline Si Wafers.
Bhushan Sopori 1 , Przemyslaw Rupnowski 1 , Mathew Albert 2 , Chandra Khattak 2 , Mike Seacrist 3 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , GT Solar, Merrimack, New Hampshire, United States, 3 , MEMC, St. Peters, Missouri, United States
Average dislocation density and spatial distribution of dislocations are routinely used as a measure of crystal quality of single- and multicrystalline Si (mc-Si) wafers. A variety of techniques have been developed to generate dislocation maps, including X-ray imaging, Cu decoration, and chemical delineation. The most common method is to defect etch the wafer with a suitable chemical etchant and then count the etch pits using an optical microscope. Commercial camera systems, with image analysis software, are available as microscope attachments that can count etch pits within the field of view and combine that information to produce maps of dislocation distribution over a wafer. An improved technique uses light scattered by etch pits to statistically count dislocations. The wafer is illuminated by a laser beam and the total scattered light, which is proportional to the number of etch pits in the illuminated region, is measured. An instrument based on this technique takes 30–60 minutes to map a 6-in x 6-in wafer.This paper describes a new technique that uses scattering from a defect-etched wafer to map dislocation distribution of the entire wafer in a single image. The measurement is very fast and compatible with large-area wafers. In this technique, the single- or multicrystalline wafer is polished to produce a damage-free polished surface. The wafer is then defect etched using Sopori etch (HF:CH3COOH:HNO3 in a 36:15:1 ratio) for 30 s to produce etch pits at dislocation sites. The shape of the etch pit depends on the direction of dislocation at the surface and does not depend on the orientation of the wafer or grain (in mc-Si). The wafer is then placed in a reflectometer where a set of lights, symmetrically placed around the wafer, illuminate it at an oblique incidence. The light scattered normal to the wafer is collected by a camera and imaged. The image corresponds to the local reflectance of the defect-etched wafer. Because local scattering is proportional to the density of etch pits, the camera image is proportional to the local variation in the dislocation density of the wafer. The system is calibrated by using a reference sample to convert the reflectance map into a dislocation map. This technique allows a fast (< 1 s) mapping of dislocations. An interesting feature of this etch is that the scattering cross-section of all dislocations (which can have circular, elliptical, or comet shapes) is the same. Thus, all dislocations are counted. An instrument based on this technique is now commercially available. We will show results that demonstrate: (i) repeatability of defect etching of large mc-Si wafers, (ii) variation of dislocation patterns over selected parts of a mc-Si ingot, (iii) a correlation between defect maps and photocurrent maps of commercial Si solar cells, and (iv) a correlation between defect distribution and the solar cell performance.This abstract is subject to government rights.
10:15 AM - Q1.3
Low-cost, High Efficiency Solar Cells on Scrapped CMOS Silicon.
Daniel Inns 1 , Joel de Souza 1 , K. Saenger 1 , H. Hovel 1 , D. Sadana 1 Show Abstract
1 T. J. Watson Research Centre, IBM, Yorktown Heights, New York, United States
The cost of scrapped Si from the CMOS industry is extremely low which makes it an attractive material for solar industry. However, the minority carrier lifetime of this material is very low and variable, typically ~ 1 µs compared to the lifetime of the original prime-Si wafer which is > 500 µs. Solar cells made on scrapped wafers therefore result in efficiencies which are inferior to that from a prime CMOS grade Si. We have developed a novel and effective low-cost metal gettering anneal process which allows the minority lifetime of the scrapped wafer to recover to close to its original value, a 100-500 fold increase. Since the efficiency of a solar cell is directly impacted by the minority carrier lifetime, cell efficiency of the improved scrapped Si is nearly equivalent to that from a prime-Si wafer. In order to erase the processing history of the wafer, surface etching is performed to remove ~ 20 µm of surface Si. Following this is a unique impurity gettering step that is performed at > 1300°C with chlorine-containing gas to enable efficient gettering of metals out of the substrate. An efficiency of ~15% has been demonstrated on both prime and improved scrapped wafers using rudimental device design to study the validity of our unique metal gettering process. This efficiency is being improved to much higher values by refinements in device design, anti-reflection coating(s) and surface passivation schemes.
10:30 AM - **Q1.4
Crystalline Silicon Technology for Solar Applications.
Aditya Deshpande 1 , Mike Seacrist 1 , Steve Kimbel 1 , Gang Shi 1 , Jihong Chen 1 Show Abstract
1 , MEMC Electronic Materials, St Peters, Missouri, United States
The use of crystalline silicon in solar applications exceeds the silicon consumed in semiconductor applications. Further, the growth rate of silicon use in solar applications has been higher than the growth rate of use in semiconductor applications over the past several years. Many similarities and synergies exist between manufacturing silicon for semiconductor and solar applications. These include producing polysilicon raw material, growing silicon crystals, and converting crystals into silicon wafers by wire slicing. For these reasons there is a strong motivation for silicon suppliers to participate in the crystalline silicon solar market. Crystalline silicon solar cells are the workhorse of the photovoltaic industry and have a significant portion of the market share of the world production of solar cells. The key driver and challenge for crystalline silicon in solar is cost which is influenced by both the silicon material cost and silicon performance. For silicon to maintain and improve on solar cell market share, further reductions in production cost as well as improvements in solar cell efficiency are necessary. The approach of a vertically integrated silicon supplier to the challenge of improving solar cell efficiency performance while also improving silicon manufacturing productivity and reducing cost will be described. A technical roadmap for crystalline silicon material will be presented and discussed. Key components of the cost are silicon feedstock, crystallization, and slicing. The approaches for commercial production of all these steps will be contrasted with other available methods. The use of directional solidification (DS) methods to grow multi-crystalline silicon (mc-Si) is a large fraction of the crystalline silicon market. The efficiency of mc-Si solar cells is usually lower than for single crystal silicon because of a high degree of material defects that include dislocations, random grain orientations, grain boundaries, impurity precipitates, and inclusions. Typical defects and impurities in mc-Si wafers and their influence on the device performance are reviewed. Detailed characterization of these defects is not straightforward. Methods developed for characterization of these defects will be presented.
11:30 AM - **Q1.5
Developments in Crystalline Silicon-based Photovoltaic Product Architecture and Manufacturing.
Juris Kalejs 1 Show Abstract
1 , American Solar Technologies, Chelmsford, Massachusetts, United States
Solar electric (Photovoltaic) crystalline silicon (c-Si) product diversity has changed very little over three decades of development, including the last decade of unprecedented expansion of the industry. The dominant module product comprising over 90% of cumulative installations, which exceed 15 GW worldwide, still employs an ubiquitous configuration, a platform based on a planar laminate. This paper will review trends in module architecture and manufacturing methods for this currently dominant PV c-Si commodity module platform. The commodity flat-plate module contains typically 60-72 solar cells cut from multicrystalline blocks as 156 mm square areas, or 156 mm dimension pseudo-squares cut from single crystal boules. New module design and manufacturing approaches different from those of the commodity PV product are now in development and piloting. Developments which use innovations in manufacturing processes, i.e., stringing of cells and packaging in a laminate, will be discussed.
12:00 PM - **Q1.6
Contactless Measurement of Carrier Lifetime on As-Grown or Shaped Ingots, Sections, and Blocks.
Ronald Sinton 1 , Tanaya Mankad 1 , M. Forsyth 1 , James Swirhun 1 Show Abstract
1 , Sinton Instruments, Inc., Boulder, Colorado, United States
This work will describe recent developments in measurement techniques for assessing the bulk lifetime of ingots, sections, and blocks without surface preparation. This permits detailed characterization of the materials as they exist in the production environment. Prior to sawing into wafers, it is possible to characterize the quality of the feedstock and growth parameters of bulk silicon. This allows quick feedback in order to fully optimize the growth. This can also be used to qualify the quality and suitability of the crystalline silicon for particular solar cell processes. Measurement at this stage, compared to after wafering, is extremely useful and cost effective. The measurements are more sensitive to true bulk parameters before wafering, the entire ingot can be assessed quickly, and the cost of wafering can be adverted or modified if all or a portion of the piece “fails”. Therefore measurements in the ingots or blocks prior to sawing into wafers present an unusual combination of industrial and scientific advantages compared to measurements of wafers. After sawing, the unpassivated surfaces of the wafers can compromise electronic material measurements until at least the phosphorus-diffusion step in the process which acts to passivate the surface recombination.The parameters that are determined by Quasi-Steady-State Photoconductance, QSSPC, or transient photoconductance measurements on as-grown or shaped material are the bulk lifetime, the resistivity, and the “trapping” which can be a measure of crystalline quality. Patterns in any of these parameters, from top to bottom of the grown piece or across the diameter of a sectioned CZ ingot give valuable information concerning feedstock, external contamination during growth (from the crucible or growth furnace), and the thermal growth conditions.Three special cases will be described in some detail.1) Boron-doped CZ. The special characteristics of this material are the strongly injection-level-dependent lifetime and the B, O, and Fe spatial dependences that gives can give rise to strong lifetime variations in both the growth direction and radially. Typical ranges of bulk lifetime are 10-500 microseconds.2)B-doped multicrystalline silicon. This material, like B-CZ, has strong spatial dependence of the lifetime, ohm-cm, and trapping as a function of both the growth direction and the position of the block relative to the crucible.3)The highest efficiency cells in the industry use n-type CZ or FZ silicon. The desired sensitivity for these processes requires the accurate discrimination of differences between silicon in the 1-10 ms range. In this range, a significantly different measurement and analysis technique will be presented in detail.
12:30 PM - Q1.7
Commercial Production of Silicon Solar Cell Feedstock by Upgrade of Metallurgical Grade Silicon.
John Mott 1 , Julio Bragagnolo 1 , Michael Hayes 1 Show Abstract
1 , Ohio Solar Energy, LLC, Alliance, Ohio, United States
Introduction. The relationship between impurity content in Solar Grade Silicon (SGS) and solar cell quality is the subject of intensive research. The PV industry has developed around the use of silicon made by the Siemens process for the semiconductor industry, with impurity levels typically in the parts per billion by weight (ppbw) range. There is a growing consensus that SGS with impurities in the parts per million range (ppmw) can be obtained cost effectively from Metallurgical Grade Silicon (MGS) and used to yield solar cells with comparable performance (see for example ‘Beneficial Effects of Dopant Compensation on Carrier Lifetime in Upgraded Metallurgical Silicon’ by S. Dubois et al. in the 23rd European Photovoltaic Solar Energy Conference, Valencia, September, 2008). This provides insight on the success encountered by Timminco, an early SGS market entrant, in commercializing silicon material with [P] levels of the order of 2 ppmw. Current Work. Analysing data from 16 UDS runs on samples taken from the melt, before and after UDS, and a solid sample taken from the silicon frozen on the cold silicon collection surface, we note that the average values of [P] in the molten silicon samples increase from 11.9 ppmw before UDS to 15.9 ppmw after UDS. The average value of [P] in the solid silicon sample is 4.9 ppmw. This demonstrates an effective refining ratio of 0.41, even at a 50% solid fraction. This is important as UDS, by its nature, implies a loss of silicon, while little or no silicon is lost in B reduction. Performing a secondary UDS on silicon obtained from these primary UDS runs yields [P] around 2 ppmw.In addition to P and B reduction, in this paper we also discuss the hardware designed to implement this process in commercial production in volumes exceeding 4,000 MT per year. MB Scientific, the original process developer, and NC Consulting, an engineering company, have developed a plant design that can produce SGS at an estimated cost that will allow for profitable large scale production, and have joined in a new company, Ohio Solar Energy, to commercialize the large-scale production technology. Future Work. While the UDS equipment design is completed, we have so far succeeded in decreasing the B concentration to 30% of the initial value by using glass slagging, wherein molten glass devoid of boron is vigorously mixed with the silicon metal and made to ‘getter’ the boron in the silicon and is then removed from the silicon metal. Repeated with new glass each time, the number of steps is dependent on the starting concentration of the boron in the silicon. The difficulty with reducing B is related to the P levels in the glass constituents due to back-contamination with successive washes. A new furnace, with more powerful agitation and designed to prevent recontamination of the UDS-processed silicon, and purer glass will enable B removal to ≤1ppmw target levels.
12:45 PM - Q1.8
Efficient Single-crystal Black Silicon Solar Cells with Anti-reflection by a Nanocatalyzed One-step Etch.
Hao-Chih Yuan 1 , Vernon Yost 1 , Matthew Page 1 , Howard Branz 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
Without using silicon nitride or another dielectric anti-reflection (AR) layer, we have fabricated confirmed 16.8%-efficient prototype solar cells on 2.7 ohm-cm, 300 um p-type single-crystal Si (100) substrates. Aside from the use of an inexpensive single-step nanocatalyzed liquid etch [Branz, Appl. Phys. Lett. 94, 231121 (2009)] to produce a nanoporous black silicon surface layer, processing of these cells is nearly identical to the present practice in PV silicon manufacturing, including a POCl3-diffused emitter and aluminum back-surface field. Open-circuit voltage (612 mV) and fill factor (80%) of the single-crystal black silicon cells is comparable to a planar control. The weighted average reflectance from 350 to 1000 nm of the single-crystal black silicon cells is below 2% compared with 34.3% of the planar control with no AR. As a result, the short-circuit current of the black silicon solar cells is 38% higher than the planar control. Nonetheless, the 34.7 mA/cm2 short-circuit current density of the black silicon solar cells is about 3 mA/cm2 below that predicted by the reflectance reduction alone. Our modeling shows that the current deficit is due to high recombination in the nanoporous layer, which impacts the short-wavelength spectral response. We also study the optical properties of the nanoporous black silicon surface layer and our measurements reveal some scattering in the nanoporous surface layer, but little internal reflection (light trapping). The studies on the optoelectronic and optical properties enable us to describe not only possible improvements to our solar cells, but general design considerations for high-efficiency solar cells based on density-graded black-silicon surfaces. Finally, we estimate the potential cost advantage of eliminating the vacuum-coated silicon-nitride anti-reflection equipment from the PV manufacturing line.
Q2: CdTe and GaAs Based Technologies
Monday PM, November 30, 2009
Room 306 (Hynes)
2:30 PM - **Q2.1
Present Status of Research and Industrial Development of CdTe/CdS Solar Cells.
Ramesh Dhere 1 , David Albin 1 , Xiaonan Li 1 , Timothy Gessert 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
Thin-film solar cells have attracted considerable attention due to their potential for low-cost production. CdTe has been one of the main contenders in this arena because its bandgap of 1.5 eV is ideally matched to the solar spectrum and the binary compound allows the freedom to choose a variety of fabrication techniques. This presentation will highlight key developments during the last forty years that have been stepping stones for progress in device performance. Industrial activity began in the early 1980s when Matsushita introduced screen-printed modules, and there were several players in the field. The field has expanded tremendously in the last five years with First Solar leading the way. First Solar is already the largest producers of photovoltaics in the United States, and with a planned expansion to 1 GW by the end of 2009, they will be contending for the top spot worldwide. We will present an overview of different industries involved in the field and their approaches. In addition, we will present the ongoing research at our laboratory (NREL) and others and will analyze the status of the research. The efficiency of the champion CdTe cell is 16.5%, and module efficiency based on present knowledge is expected to reach around 12.5%, which is well below its potential. We will present the analysis of the device performance and the main parameters affecting the performance. Further improvement in module performance is unlikely without better understanding these parameters. Other area of emphasis is the long-term reliability and accelerated life testing that is necessary to understand the effect of processing changes on product reliability. NREL is developing the Process Development and Integration Laboratory (PDIL) to facilitate interaction among industry and research groups. The presentation will provide an overview of the CdTe tool being developed and provide some details about the capabilities of the tool, in particular, and PDIL facility, in general. This abstract is subject to government rights.
3:00 PM - Q2.2
Study of the Electrical Properties of Cross Sections of CdTe/CdS Solar Cells Measured with Scanning Kelvin Probe Microscopy.
Helio Moutinho 1 , Ramesh Dhere 1 , Chun-Sheng Jiang 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
We apply scanning Kelvin probe microscopy (SKPM) to analyze cross sections of working CdTe/CdS solar cells under different bias conditions. This technique is performed inside a scanning probe microscope (SPM), and it provides the distribution of the electrical potential inside the device with high spatial resolution. The SKPM and topographic images are compared to associate variations of the potential with topographic features. For instance, it is possible to compare the position of the p-n junction with the metallurgical junction between CdTe and CdS. In SKPM, we apply AC and DC bias between the tip of the SPM and the sample. The signal measures the difference between the work functions of the tip and sample surface, and it is proportional to the surface potential of the sample. By biasing the cell during measurement (reverse and forward polarizations), we avoided artifacts such as Fermi-level pinning, and we were able to investigate the distribution of the electrical potential inside a live device polarized under different conditions. By taking the derivative of the potential, we determined the distribution of the electric field, and by locating the maximum of the electric field, we located the position of the p-n junction.This work complements our other work presented in the 2009 Spring MRS Meeting that investigated different ways to prepare cross sections of samples, compared two different SKPM measurement procedures (using the first and second resonance cantilever peaks), and determined the position of the junction for a standard CdTe/CdS solar cell. In this work, we investigate the change in the electrical potential using different bias, forward and reverse, showing the change in the width of the depletion region. By calculating the derivative of the potential, we observed the distribution of the electric field inside the device and noticed the following: the field has a strong value concentrated on a thin layer of the device at the junction, and a much smaller value moving away from the junction, showing that there is a change on the electrical properties of the device at the interface. To investigate whether interdiffusion of Te and S is responsible for this effect, we analyzed solar cells without CdCl2 heat treatment, as well as cells produced without the CdS layer. In both cases, the distribution of the electrical potential and electric field was different than for a standard device. In this work, we will also present results of the distribution of the electrical potential on the back contact of the solar cells. In our case, we use graphite paste deposited on the CdTe film followed by the deposition of Ag film. We will show that, in general, the potential drop between the CdTe/graphite interface is much smaller than in the junction region. This abstract is subject to government rights.
3:15 PM - Q2.3
Finite Element Model to Understand the Effect of O2 on Closed Space Sublimation of CdTe.
Nirav Vora 1 , Ramesh Dhere 1 Show Abstract
1 National Center for Photovoltaics, National Renewable Energy laboratory, Golden, Colorado, United States
Incorporating O2 in the closed space sublimation (CSS) of CdTe thin film has resulted in improved cell efficiencies. Many studies have been undertaken to understand this effect on cell efficiency. In this work we study the effect of oxygen on lateral uniformity of the deposited CdTe film. A finite element model has been developed to represent the mass and heat transfers involved in the CSS process. The model takes into consideration the effect of O2 by modeling its reaction with Cd vapors in the space between the source and the substrates. This reaction can decrease the amount of Cd available for condensation near the substrate if the diffusion of Cd from the source to the substrate is not fast enough. One of the factors affecting this reaction rate is the concentration of O2. So a gradient of O2 from the edges to the center of the substrate can result in a laterally non-uniform film. This gradient can be formed if the rate of diffusion of O2 is lower than that of its reaction with Cd. A steady state model will be solved at various temperatures, pressures, and separation distances to determine the optimum conditions for depositing a CdTe film with uniform thickness. Experiments will be carried out at these conditions and results compared with the simulation results. The comparison will help in determining the reaction rate constants, as there is a lot of variation in the values reported in the literature. A transient model will also be developed to better represent the experiments. Finally the model will be modified to represent the vapor transport deposition of CdTe. This abstract is subject to government rights.
3:30 PM - Q2.4
CdTe Thin Film Growth Using High Rate Sputtering for Photovoltaic Applications.
John Walls 1 , Paresh Nasikkar 1 , Hari Upadhyaya 1 Show Abstract
1 Electronic and Electrical Engineering, Loughborough University, Loughborough United Kingdom
Magnetron sputtering has a number of important advantages for the deposition of thin films for use in photovoltaic devices. Sputtering provides control over thin film thickness with sub-nanometre precision using time only. This allows the thickness of the CdTe absorber layer to be optimized thereby minimizing materials usage and process manufacturing time. Using the closed field configuration, the thin films are super-smooth (< 1 nm rms roughness). This is especially important in the TCO base layer since roughness of the TCO can break through the CdS layer and cause “shunting” across cells. This paper describes a flexible reactive sputtering process in which adjacent unbalanced magnetrons are constructed of opposite magnetic polarity. The resulting closed magnetic field maintains a high density reactive plasma. In contrast to previous reactive sputtering strategies, the process does not require an auxiliary ion or plasma source and the associated use of high voltage ion acceleration. As a result, the deposition energy is optimized and insufficient to cause damage in the growing thin film. The substrate temperature is typically maintained below 100°C without the need for direct cooling. The thin films exhibit bulk optical properties, they are also dense and super-smooth. The thin films also have typically low compressive stress. The magnetron targets are simple metals or semiconductors for high rate deposition and are converted to compound thin films when required by using the appropriate reactive gas. This paper provides data derived from a medium throughput batch system with a 0.4 m diameter drum substrate carrier and four 0.6m linear magnetrons. However, the process geometry is scalable and adaptable to in-line deposition. The optical and electrical performance of each layer in the CdTe thin film photovoltaic stack will be presented together with preliminary device performance.
3:45 PM - Q2.5
Fabrication and Modeling of Three-Dimensionally Structured CdTe Thin Film Photovoltaic Devices with Self-Aligned Back-Contacts.
Jonathan Guyer 1 , Daniel Josell 1 , Carlos Beauchamp 1 , Suyong Jung 2 , Behrang Hamadani 2 , Lee Richter 3 , John Bonevich 1 , Nikolai Zhitenev 2 , Tom Moffat 1 Show Abstract
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 2 Center for Nanoscale Science and Technology, NIST, Gaithersburg, Maryland, United States, 3 Surface and Microanalysis Science Division, NIST, Gaithersburg, Maryland, United States
Our goal is to provide industry with test structures and models ofnext-generation photovoltaics, with an initial focus on CdTe andCuInxGa1-xSe2 (CIS or CIGS) materials. These tools will enableinterpretation of measured external properties affected by geometry, grainstructure, and nanoscale phase separation, which will support improvedprocessing and design of Second Generation (thin film) and Third Generation(nanostructured) photovoltaic devices.
CdTe and CIGS are some of the most stable and efficient photovoltaicmaterials. A wide variety of deposition methods enable novel devicestructures at previously unobtainable dimensions, but optimal structuresand dimensions are unknown. Sensitivity of the microstructure (and,ultimately, the device efficiency) to deposition methods and processingconditions varies, and potential new and cheaper fabrication methods havenot been verified.
We are adapting our experience with the electrochemical "superfill" ofmetal in sub-micrometer-scale trenches and vias  to the fabrication ofnovel photovoltaic structures. We have fabricated electrodeposited CdTedevices that use interdigitated back-contact electrodes both forindependent electrode-position of n- and p-type material during fabricationand for collecting the photo-generated current in the fabricated devices. The devices enable quantitative evaluation of bulk and interfaceproperties of 3-d devices through controlled variation of length scales,independent of the absorber thickness, including electrode pitch andcross-section.
To guide and interpret the experimental measurements, we have implemented aphotovoltaic device model that gives us complete control over geometry andmicrostructure. We are using the model, in conjunction with experimentalmeasurements, to extract discrete materials properties from complex structures.We implemented our device model in theFiPy partial differential equation solver package . This freely available package allows us to easilydistribute our photovoltaic codes to researchers in industry and elsewhereas they are validated.
 D. Josell, D. Wheeler, W. H. Huber & T. P. Moffat Phys. Rev. Lett. 87 (2001) 016102
 D. Josell, C. Beauchamp, S. Jung, B.H. Hamadani, A. Motayed, L. Richter, M. Williams, J.E. Bonevich, A. Shapiro, N. Zhitenev & T.P. Moffat J. Electrochem. Soc., in press
 J. E. Guyer, D. Wheeler & J. A. Warren Comput. Sci. Eng. 11 (2009) 6 http://www.ctcms.nist.gov/fipy
4:30 PM - Q2.6
Thin-film ITO/InP Solar Cells on Flexible Plastic Substrates.
Kuen-Ting Shiu 1 2 , Jeramy Zimmerman 2 , Hongyu Wang 2 , Stephen Forrest 2 Show Abstract
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Materials Science and Engineering, Electrical Engineering and Computer Science and Physics, University of Michigan, Ann Arbor, Michigan, United States
Photovoltaic cells made with amorphous silicon, II-VI compounds such as CdTe , and copper-indium-gallium-selenide (CIGS)  thin-films have been adopted for use in high specific power, light-weight solar cell panel applications. In this work, we demonstrate single-crystal thin-film InP Schottky-type solar cells mounted on flexible plastic substrates by cold-welding a metallic film on the solar cell to one deposited on the plastic sheet. The lightly p-doped InP cell is grown epitaxially on an InP substrate via gas source molecular beam epitaxy. The InP substrate is removed via selective chemical wet-etching after the epitaxial layers are bonded to a metal layer pre-deposited onto the surface of an 25 m thick Kapton sheet, and indium tin oxide (ITO) top contacts are then deposited. The power conversion efficiency under 1 sun is up to 10.2±1.0% and its specific power is 2.0±0.2 kW/kg. Stress tests indicate that the thin film solar cells can tolerate both tensile and compressive stress by bending over a 1 cm radius without damage. Full materials and device characterization will be presented. This work provides an alternative method for adopting III-V semiconductor solar cells for portable and space applications where very high specific power efficiency is required. 1. Romeo, et.al, Solar Energy Materials & Solar Cells, 90, 3407 (2006).2. Otte, et.al, Thin Solid Films, 511-512, 613 (2006)
4:45 PM - Q2.7
Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells.
Tingyi Gu 1 , Kai Yang 1 , Mohamed El-Emawy 1 , Andreas Stintz 1 , Luke Lester 1 Show Abstract
1 Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico, United States
Recent interest in using InAs quantum dots (QDs) in the absorbing region of solar cells has focused primarily on the predicted increase in quantum efficiency due to the intermediate band effect or simply larger short circuit current density. However, the three-dimensional carrier confinement inherent to QDs endows them with unique carrier transport capabilities that have not been previously explored in the context of solar cells. In this work, it is observed that InAs/InGaAs dots-in-a-well (DWELL) structures efficiently suppress lateral carrier diffusion. Therefore, not only do the DWELL structures enhance photocurrent by extending the absorption edge, but they should also inhibit the spreading of current to the perimeter of a device where edge recombination can dominate. In this paper, we examine this premise by comparing the dark current behavior of DWELL cells and GaAs control cells of varying area. The results are promising for applications such as concentration and flexible surfaces where shrinking the size of the device while maintaining high charge collection efficiency are of paramount importance.The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode structure with six layers of InAs QDs embedded in InGaAs quantum wells placed within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the same pin configuration but without the DWELL structure. The typical DWELL solar cell exhibits higher short current density while maintaining nearly the same open-circuit voltage for different scales, and the advantage of higher short current density is more obvious in the smaller cells. In contrast, the smaller size cells, which have a higher perimeter to area ratio, make edge recombination current dominant in the GaAs control cells, and thus their open circuit voltage and efficiency severely degrade. The open-circuit voltage and efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85% to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mmsq to 2*2mmsq, compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%, respectively, in the DWELL solar cells.The lower open-circuit voltage in the smaller GaAs control cells is caused by strong Shockley-Read-Hall (SRH) recombination on the perimeter, which leads to a shoulder in the semi-logarithmic dark IV curve. However, despite the fact that the DWELL and GaAs control cells were processed simultaneously, the shoulders on the dark IV curve disappear in all the DWELL cells over the whole processed wafer. As has been discussed in previous research on transport in QDs, it is believed that the DWELL cells inhibit lateral diffusion current and thus edge recombination by collection first in the InGaAs quantum well and then trapping in the embedded InAs dots. This conclusion is further supported by the almost constant current densities of the different area DWELL devices as a function of voltage.
5:00 PM - Q2.8
Virtual Single Crystalline GaAs Epitaxial Thin Films on Flexible Polycrystalline Metallic Substrates.
Alex Freundlich 1 , Venkat Selvamanickam 1 Show Abstract
1 , University of Houston, Houston, Texas, United States
Development of high quality III-V epitaxial layers on inexpensive flexible substrates is a desirable feature to many civilian and military applications. In particular it may be a game-changing enabler toward significantly reducing the cost and increasing the efficiency of thin film solar cells, as it offers the possibility of combining the unsurpassed performance of GaAs based multi-junction technologies (1 sun efficiency >36%) with a conventional roll to roll processing standard of thin film industry as afforded by polycrystalline metallic foil technology.Here we report our recent results on the development of virtual single crystalline GaAs thin film on thin (50 microns) flexible polycrystalline metallic substrates. The flexible poly-crystalline Ni-based substrates were coated with a few hundred nm oxide-ceramic epitaxial buffer, adapted from a previously developed structure for high Tc superconductor wire technology, followed by a very thin (<50nm) Ge epilayer. After introduction in the molecular beam epitaxy chamber Ge native oxide was thermally removed and a subsequent high temperature annealing was implemented resulting in the formation of clear c(2x2) (mixed (2x1)(1x2) ) reconstruction, typical of (001) Ge surface. The GaAs growth was initiated at relatively low temperature (~400C) and a thin nucleation layer of GaAs was deposited , followed by an annealing step under As2, subsequently 1 micron-thick GaAs was deposited in standard growth conditions (growth rate ~ 1ML/sec T~550C). The entire growth sequence was monitored by reflection high energy electron diffraction (RHEED). The self -annihilation of anti-phase boundaries (mixed 2x4, 4x2 RHEED diagram), was observed for thicknesses exceeding 100 nm where a 2x4 RHEED diagram typical of a single domain (001) GaAs was recorded. Epilayers exhibited a specular morphology. High resolution X-ray diffraction analysis confirmed the single crystalline (001) nature of GaAs. Temperature dependent photoluminescence (PL) analysis revealed a strong PL in as-grown samples (Fig. 2). The low temperature photoluminescence was found to be dominated with DA –eA like bands in the 1.4-1.5 eV range and a relatively broad deeper luminescence band at 1.3- 1.35 eV. At low temperature the GaAs excitonic emission was detected at 1.526 eV (FWHM~20 meV) and was found to be slightly red shifted compared to the typical A0X exciton (1.512) in homoepitaxial GaAs. The magnitude of this red-shift (~14 meV) suggested the absence of any significant thermoelastic/lattice mismatch strain in the epilayers. In summary the development of high quality single crystalline (001) GaAs on flexible metal substrates is demonstrated. Samples exhibit high optical and structural quality as stressed by the RHEED, X-ray and luminescence properties of the as-grown epilayers. Development of GaAs based thin film single junction solar cells is underway and preliminary device results will be presented at the meeting.
5:15 PM - Q2.9
Neodymium Luminescent Solar Concentrator.
Phil Reusswig 1 , Carmel Rotschild 1 , Marc Baldo 1 Show Abstract
1 EECS, MIT, Cambridge, Massachusetts, United States
Luminescent solar concentrators (LSCs) are promising in photovoltaic applications because they do not need to track the sun to obtain high optical concentration factors. However, loss mechanisms that are associated with optical self-absorption decrease the efficiency of LSCs at increasing values of the geometric gain, G, which is defined as the ratio of the facial area to the edge area. In this work, we demonstrate LSCs based on a lanthanide infrared emitter, neodymium (Nd3+). Neodymium is nearly the optimal infrared LSC material: inexpensive, abundant, efficient, and spectrally well matched to high-performance silicon solar cells. Neodymium is a natural four level system, making it reasonably transparent to its own emission enabling high optical concentrations and geometric gain. Neodymium’s one disadvantage is its absorption. It has relatively poor overlap with the visible spectrum, meaning that it will require sensitization. LG-760 and APG-1 phosphate glass was purchased from Schott with Nd3+ doping concentrations of 1.0%, 2.0% and 3.0% with thicknesses of 1mm and 5mm. For the neodymium glass without sensitizer, peak optical quantum efficiencies (OQE) of 50% have been measured with an estimated power efficiency of 2.5%. The neodymium glass was sensitized by spin casting thin films doped with BASF Lumogen Violet 570, Gelb 083, and Orange 240 in a matrix of poly(methyl methacrylate) (PMMA). Together the dyes exhibit broad absorption below λ = 550 nm. Foerster energy transfer was used to couple the dyes non-radiatively such that all radiative emission was matched to the neodymium absorption lines at approximately λ = 580 nm. Initial results indicate peak OQE of 45% with an increase in estimated power efficiency of 3.5%. We will also discuss the sensitization of neodymium glass using nanocrystals in a polymer matrix.
Q3: Poster Session I
Monday PM, November 30, 2009
Exhibit Hall D (Hynes)
9:00 PM - Q3.1
Electron Reflector Strategy on Micron-Thickness CdTe Solar Cells.
Kuo-Jui Hsiao 1 , James Sites 1 Show Abstract
1 Physics, Colorado State University, Fort Collins, Colorado, United States
Incorporation of an electron reflector is a proposed strategy to improve Voc for CdTe thin-film solar cells. An electron reflector is a conduction-band barrier at the back surface, which can reduce the recombination resulting from the electron flow to the back surface. It should be particularly valuable at sub-micron thicknesses. For optimal improvement with an electron reflector, reasonable carrier lifetime (1 ns or above) and full depletion are required. Numerical simulation is used to investigate the electron reflector strategy for thin CdTe cells. Theoretically, a 200 mV increase in voltage and 3% in efficiency should be possible for a micron-thickness CdTe cell with a 0.2-eV electron reflector barrier, assuming 2x1014-cm-3 hole density and 1-ns lifetime, which are currently achieved. For the electron reflector to be beneficial, the CdTe needs to be fully depleted at its typical operating voltage. With the electron reflector, good CdTe cell performance at thicknesses as low as 0.3 μm should be possible.
9:00 PM - Q3.10
Mechano-chemical Synthesis, Deposition and Structural Characterization of CIGS.
Vidhya Bhojan 1 , Velumani Subramaniam 1 , Jesus A.Arenas-Alatorre 2 , Rene Asomoza 1 Show Abstract
1 Electrical Engineering, CINVESTAV, Mexico,D.F. Mexico, 2 Institute of Physics, Universidad Nacional Autónoma de México, Mexico ,D.F. Mexico
CuInGaSe2 (CIGS) is a prominent thin-film photovoltaic material. However, commonly used physical vapour deposition and sputtering techniques to fabricate CIGS thin-film photovoltaic (PV) devices are complex and expensive. Therefore non-vacuum deposition techniques such as paste coating, spray pyrolysis and electro deposition are gaining more attention in recent years. Our intention is to choose a low cost non-vacuum technique like mechano-chemical synthesis of CIGS powder, followed by screen printing. Mechano chemical synthesis is a process that induces physical and/or chemical change in the compounds by mechanical energy, such as pulverization, friction or compression. This method has some advantages for the mass production of CIGS solar cells, high productivity and short processing cycle time. In the present work CIGS powders suitable for screen-printing ink has been prepared by ball milling. High purity elemental copper granules (>99.9% pure), selenium and indium powders (>99.9% pure) and fine chips of gallium (>99.9% pure) were used as the starting materials. Ball milling was carried out for an optimized composition of CuIn0.75Ga0.25Se2 using a SPEX-8000 mixer/mill at 1200 rpm for 1.5 hours. X-ray diffraction analysis of the milled powder shows the presence of (112), (220)/ (204), (312)/ (116), (400) and (332) peaks corresponding to CIGS chalcopyrite structure with a preferential orientation along (112) peak. The average grain size calculated by Scherrer’s formula is about 13.8 nm. Crystallographic structure of the prepared CIGS powder was analyzed by Rietveld analysis using X-ray powder diffraction data. Geometry optimization of the structure was performed and the basic structural properties have been evaluated by density functional approximation and compared with experimental results. Density of states has been simulated for the same structure inorder to have a better understanding of the distribution of atomic orbitals. FESEM analysis shows the agglomeration of nano particles. Particle size varied from 11 to 30nm. Final composition of the milled powder studied by Energy dispersive X-ray analysis gives 24.39 at% Cu, 21.42 at% In, 7.22 at% Ga and 46.97 at% Se. HRTEM analysis reveals the presence of nano crystalline particles. The interplanar distance (d-spacing) corresponding to (112), (220)/ (204), (512)/ (417) and (620)/ (604) diffraction peaks has been estimated and compared with standard values and the corresponding diffraction pattern has been simulated with simulaTEM software. CIGS ink is prepared by proper mixing of the powder with a suitable organic binder (ethyl cellulose). Screen printing is carried out on glass substrates, followed by annealing at 5 different temperatures of 300,350,400,450 and 500 degrees, in order to form a porous CIGS film. XRD and SEM analysis were carried out to study the structure and morphology of screen printed CIGS .Cross section of the screen printed CIGS thin film has been analyzed by HRTEM.
9:00 PM - Q3.11
Photo-patternable Polythiophenes Having Methacrylate Pendant Group for Organic Photovoltaics.
Yuna Kim 1 , Jeonghun Kim 1 , Sehwan Kim 1 , Eunkyoung Kim 1 Show Abstract
1 Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul Korea (the Republic of)
New series of poly(3-hexylthiophene) copolymer functionalized with methacrylate group were synthesized for the application to organic photovoltaics. The regioregularity of the polythiophenes was controlled by the ratio of thiophene methacrylate to 3-hexylthiophene. As synthesized copolymer showed good solution processible and photo-patternable properties. Photoconversion of the side chain cross-linking of the polymer was examined by isothermal photo-DSC and FT-IR study under UV irradiation. By the photo cross-linking reaction, the ordering of poly(3-hexylthiophene) copolymer itself and the phase separation in fullerene blended films were significantly changed as observed by UV-Vis absorbance, PL, AFM and XRD. The photo-induced charge transport of polymer films were also changed by the photo cross-linking reaction. The organic photovoltaics were fabricated to show different photo conversion efficiency from the polythiophenes depending on the degree of the side chain cross-linking.
9:00 PM - Q3.13
Thermally and Chemically Stable Transparent Conducting Multi-layered Al-doped ZnO and Its Applications for Dye Sensitized Solar Cells.
Jun Hong Noh 1 , Hyun Soo Han 1 , Sangwook Lee 1 , Hyun Suk Jung 2 , Kung Sun Hong 1 Show Abstract
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Advanced Materials Engineering, Kookim University, Seoul Korea (the Republic of)
Thermally and chemically stable NTO/AZO multilayered TCO fabricated by pulsed laser deposition for application in DSSC. Nb doped TiOx (NTO) layer was deposited at room temperature with amorphous and the layer crystallized to anatase and Ti3+ ions in the TiOx layer oxidized Ti4+ ions during air annealing at 450 oC. Owing to the crystallization and oxidation, NTO layer prevented from penetrating oxygen into AZO layer and consequently the AZO layer was in quasi-reducing atmosphere. As if the AZO layer is annealed under reducing atmosphere, oxygen vacancy was produced and substitutional Al atoms were activated in the AZO, thereby the conductivity of NTO/AZO was enhanced. In addition, the NTO layer prevented from forming the aggregate Zn2+-dye molecule on surface of AZO so that high transmittance of the NTO/AZO was maintained. The DSSC using the stable NTO/AZO TCO showed twice efficiency as high as the AZO-DSSC. However, the NTO/AZO-DSSC showed lower fill factor compared to the conventional FTO-DSSC. The cause for low fill factor might be electronic barrier which was formed by the non-degenerated thin AZO:Oi layer.
9:00 PM - Q3.14
Fabrication, Characterization and Optical Studies of Cu(In1-xGax)3Se5 Bulk Compounds.
Dayane Habib 1 , Georges El Haj Moussa 1 , Roy Al Asmar 1 , Michael Ibrahim 1 , Mario El Tahchi 1 , Claude Llinares 2 Show Abstract
1 LPA, Lebanese University, Jdeidet Lebanon, 2 Centre Electronique et Micro-optoélectronique de Montpellier (CEM2), University of Montpellier 2, Montpellier France
In this paper we present the structural and optical properties of Cu(In1-xGax)3Se5 ternary and quaternary compounds crystals fabricated by horizontal Bridgman technique. The Cu(In1-xGax)3Se5 materials were characterized by Energy Dispersive Spectrometry (EDS), hot point probe method, X-ray diffraction, Photoluminescence (PL), and Optical response (Photoconductivity). The Cu(In1-xGax)3Se5 have an Ordered Vacancy Chalcopyrite-type structure with lattice constants varying as a function of the x composition. A good stœchiometry given by the EDS characterization method is well observed in our samples and its magnitude deviation Δy is slight; so, our samples present a nearly perfect stœchiometry (Δy = 0) . X-Ray diffraction patterns show the presence of many preferential orientations according to the planes (112), (220) and (312) of all the samples . Also, it shows a linear shifting of peaks towards the higher magnitudes of 2θ when the x composition increases. These compounds can be of stanite structure  or an Ordered Vacancy Chalcopyrite structure (OVC)  or Ordered Defect Chalcopyrite Structure (ODC).We observe a large shift of the main PL and optical response emission peak versus x composition. The band gap energy of Cu(In1-xGax)3Se5 compounds is found to vary from 1.23 eV to 1.85 eV as a function of x. Migual A. Contreras, Holm Wiesner, Rick Mtson, John Tuttle, Kanna Ramanathan, Rommel Noufi, Mat. Res. Soc. Symp. Proc. Vol. 426 (1996) 243-254. Ariswan, G. El Haj Moussa, M. Abdelali, F. Guastavino, C. Llinares, Solid State Communications 124 (2002) 391-396.  M. Suzuki, T. Uenoyama, T. Wada, T. Hanada, Y. Nakamura, Jpn. J. Appl. Phys. 36 L1139 (1997). Kristjan Laes, Sergei Bereznev, A. Tverjanovich, E.N. Borisov, Tiit Varema, Olga Volobujeva , Andres Öpik. Thin Solid Films 517 (2009) 2286–2290.
9:00 PM - Q3.15
Reactive Sputtering of Magnesium Hydride Thin Films for Photovoltaic Applications.
Charlotte Platzer-Bjorkman 1 , Smagul Karazhanov 1 , Jan-Petter Maehlen 1 , Erik Marstein 1 , Arve Holt 1 Show Abstract
1 , Institute for Energy Technology, Kjeller Norway
Metal hydrides have been intensively studied for hydrogen storage , battery  and smart window  applications. For these purposes, fast and repeatable switching of material properties and hydrogen content are crucial. Many metal hydrides are also semiconducting or insulating and have recently been suggested for application in photovoltaic devices either as transparent conducting layers, antireflective coatings or even absorber layers . The advantage of using metal hydrides for these applications are the large abundance of the constituent elements, high hydrogen content possibly improving bulk and surface passivation of silicon-based devices and suitable band gap range of several of the materials. In contrast to the applications based on hydrogenation/dehydrogenation, high stability is required for photovoltaic applications. In this work, we investigate the possibilities for in-situ deposition of MgH2 using reactive sputtering from a metallic target in Ar/H plasma. For low H2/Ar ratio, crystalline MgH2 is formed together with metallic Mg while for increasing ratio films are amorphous, partly transparent and insulating. Previous studies of in-situ deposition of MgHx films by activated reactive evaporation showed difficulties in obtaining single phase MgH2 . In the present study, sputtering process parameters such as RF power, gas ratio, pressure and substrate temperature are varied. Film properties are monitored using x-ray diffraction, resistivity measurements and optical characterization. Depositions on both glass and silicon substrates are reported as well as studies of stability under annealing and light exposure.References1P. Chen, Z. Xiong, and J. Luo, Nature 420, 302 (2002).2L. Schlapbach and A. Zuttel, Nature 414, 535 (2001).3J. Huiberts, R. Griessen, and J. Rector, Nature 380 (1996).4S. Karazhanov, A. Ulyashin, P. Vajeeston, and P. Ravindran. Phil. Mag. 88(16), 2461 (2008).5R. Westerwaal, C. Broedersz, R. Gremaud, et al., Thin Solid Films 516, 4351 (2008).
9:00 PM - Q3.16
Wide Energy Bandgap Materials (LiF and Al2O3) as Hole Blocking Layers in Organic Solar Cells.
Chen Cheng Hong 1 , Wu Jui Chi 1 , Chen Chun Wei 1 , Chen Jen Sue 1 Show Abstract
1 Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Materials of different energy bandgaps are investigated as hole blocking layers in organic solar cells with P3HT:PCBM heterojunction active layer and Al cathode. In this study, by inserting LiF (Eg: 14.6 eV) or Al2O3 (Eg: 10.8 eV) between P3HT:PCBM and Al as the hole blocking layer, we demonstrate the power conversion efficiency (PCE) of the solar cell devices reaches 3.05% or 2.57%, respectively, which is much higher than that of the solar cell without the interlayer (only 1.32%). The enhancement of efficiency is associated with the increase of short circuit current and fill factor. The mechanism for the high efficiency is based on the tunneling effect. Owing to the wide energy bandgap of the hole blocking layer, the band offset between HOMO of PCMB and valance band edge of hole blocking layer is significantly large and the barrier height for tunneling of holes will be subsequently elevated. Consequently, the electron-hole recombination at the Al cathode will be appreciably reduced. Therefore, the use of wide bandgap materials as the hole blocking layer in solar cell devices will achieve higher efficiency.
9:00 PM - Q3.17
Ti-doped Gallium Phosphide Layers with Concentrations Above the Mott Limit.
David Pastor 1 , Javier Olea 1 , Maria Toledano-Luque 1 , Ignacio Martil 1 , German Gonzalez-Diaz 1 , Jordi Ibanez 2 , Ramon Cusco 2 , Luis Artus 2 Show Abstract
1 Dpto. de Física Aplicada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid Spain, 2 Institut Jaume Almera, Consejo Superior de Investigaciones Científicas (CSIC), Barcelona Spain
Intermediate band solar cell (IBSC) materials exhibit highly attractive properties that could allow exceeding the solar conversion efficiency limit for single junction solar cells. An intermediate band (IB) material is formed by the introduction of a new band inside the bandgap of a semiconductor. In these compounds, photons with energy below the bandgap can be absorbed, pumping electrons from the valence band to the conduction band with the IB as an intermediary step. To form the IB material, deep center impurities with a concentration above the Mott limit (∼5x1019 cm-3) have to be introduced. Theoretical calculations have predicted Ti-doped Gallium Phosphide as one of the