AM1 Technology Status and Critical Issues for Manufacturing High Volume Thin Film Photovoltaics: CdTe, Cu(InxGa1-x)Se2, Cu2ZnSn(SxSe1-x)4 and a-Si/nc-Si
Instructors: Dr. Tim Anderson, Dean of Engineering, University of Massachusetts Amherst, and Dr. Dave Albin, Senior Scientist, National Renewable Energy Laboratory
Thin film semiconductors have been investigated as absorber layers for large scale photovoltaic since the 1960's based on the promise of low manufacturing cost for large-area high-throughput PV modules. Three thin film absorber materials emerged in the 1980's as being technically and commercially viable for solar cells: CdTe, CuInSe2-based, and Si-based. More recently the Cu2ZnSn(SxSe1-x)4 kesterite material has received intense research as an earth-abundant absorber layer. The Si films are either amorphous or nanocrystalline while the other three are polycrystalline. Each is configured as a multilayer heterojunction device fabricated on glass, foil, or plastic substrates. Sustained worldwide research and development have significantly increased our understanding the materials chemistry and physics leading to advanced device structures and high-throughput processing at high yield. Each technology has the benefits common to thin films but each has specific challenges. In this tutorial, we will provide an overview of the basic processing sequence, device structures, manufacturing options, and key challenges for each thin film technology. Based on our 3 decades of direct involvement in thin film PV, we will review the evolution of each thin film technology into today's commercial success and identify critical issues limiting wider commercialization. The course is intended for graduate students, industrial researchers and technologists interested in an overview of the technologies as well has understanding the fundamental materials chemistry, device operation and characterization and process engineering.
Tim Anderson served on the faculty of the Chemical Engineering Department at the University of Florida for 35 years before joining the University of Massachusetts, Amherst in 2013, where he is a Distinguished Professor. He has an active program in thin film CuInxGa1-xSe2 and Cu2ZnSn(SxSe1-x)4 photovoltaics as well as growth of III-V materials for optoelectronic devices. His group is credited with over 230 publications and he has supervised over 65 Ph.D. graduates. Prof. Anderson is the inaugural editor-in-chief of the IEEE J. of Photovoltaics, inaugural Associate Editor (Solar Energy) of WIREs: Energy and Environment, member of the editorial advisory board of J. Energy Systems, and a Fellow of the American Institute of Chemical Engineers (AIChE).
AM2 Silicon Solar Cell Technology
Instructor: Dr. Stuart Bowden, Associate Professor Research, School of Electrical, Computer and Energy Engineering, Arizona State University
Crystalline silicon continues to be the dominant technology in solar cell production. There are also a number of advanced technologies which will dominate the next wave of large scale deployment on manufacturing lines and in field installation. This tutorial will cover all aspects of production in crystalline silicon from the present and into the future. We will delve into device physics of silicon solar cells and how the limitations in present devices can be overcome for both higher efficiency and higher throughput. The various aspects of production will also be covered: from crystallization to wafering, through cell production and finishing with module design and testing.
Stuart Bowden received his Ph.D. from the University of New South Wales (UNSW) in 1996 for work on static concentrators using silicon solar cells. Following graduation, he transferred the buried contact solar cell technology from UNSW to Samsung Advanced Institute of Technology (SAIT). In 1998, he joined the Inter-University Micro Electronics Centre (IMEC) in Belgium where he demonstrated rear surface passivation of multicrystalline silicon wafers using boron diffusions and inversion layers created by silicon nitride. In 2001, he joined Georgia Institute of Technology where he worked on molecular beam epitaxy and the characterization of interface states in a variety of oxide materials. From 2004 - 2008 he led the effort at the Institute of Energy Conversion at the University of Delaware, on the development of advanced silicon solar cell structures based around super-passivation and induced junctions. He presently heads the industrial collaboration laboratory at Arizona State University
AM3 Characterization: Advanced Electrical Characterization Techniques and Analysis
Instructors: Drs. Keith Emery (NREL), Sachit Grover (First Solar) and Jian V. Li (NREL)
If you find yourself asking the questions:
1) Why is the VOC of my device lower than expected?
2) What aspect of my solar cell should I be improving?
3) How do I identify/quantify losses in my solar cell?
4) How do I connect device physics and material science?
Then Advanced Electrical Techniques and Analysis is the right tutorial for you!
This tutorial starts by revisiting the physics of a solar cell and textbook theoretical models used for analyzing J-V and QE measurements. Impact of recombination mechanisms and defects on these measurements; Temperature and voltage dependence of current conduction mechanisms; Influence of surface recombination on QE are some of the topics that will be covered in the first part. The second part of this tutorial will help connect macroscopic observables such as Voc to recombination mechanisms active in the solar cell. A back to basics approach for semiconductor recombination physics is used to derive new equations that empower quantitative separation of recombination channels. The third part of this tutorial will review capacitance-based measurement including C-V, admittance spectroscopy, and DLTS. These techniques enable the characterization of electrical properties of the junction, absorber, and back contact.
Keith Emery established and has managed the Cell and Module Performance Characterization team at NREL since 1980. The team established the procedures for calibrating cells and modules that have since been codified in standards and adopted by the international PV community. He received his B.S. physics and M.S.E.E. from Michigan State in 1979 and worked on a PhD at Colorado State in 1979-1980 and 1982. His graduate thesis work was in the area of comprehensive modeling of the pulsed hydrogen fluoride laser system, electron and laser beam vapor phase epitaxy of oxides and nitrides, and ion beam sputtering of tin oxide on Si. He has 340 publications and 5 chapters in PV books to date and one patent for a laser photoresponse mapper. His ISO 9001 and ISO 17025 PV accredited calibration group provides the community with reference cell calibrations and efficiency certification. He is also active in PV standards development and consulting on PV performance rating hardware, solar simulation, current versus voltage measurement software and procedures. He is the recipient of the 2007 NCPV Paul Rappaport, 2009 NREL Harold M. Hubbard award, 2012 World Renewable Energy Network Pioneer Award, and the 2013 IEEE William R. Cherry award.
Sachit Grover is Device Physicist at First Solar working on characterization and modeling of CdTe materials and devices. In 2014 he was Senior Device Scientist at Scifiniti, Inc. where he researched on improving low-cost kerfless-silicon substrates for solar cells. He previously worked as Postdoctoral Researcher with the Silicon Group at NREL where he investigated kerfless and high-efficiency silicon solar cells, and developed a unique approach for analyzing recombination in heterojunction cells. He received a Ph.D. in electrical engineering from University of Colorado Boulder and a B.E. in electrical engineering from Indian Institute of Technology Delhi.
Jian V. Li is research scientist at National Renewable Energy Laboratory since 2007. He works on electrical characterization of semiconductor materials and energy conversion devices. He previously worked on infrared lasers and photodetectors at Jet Propulsion Laboratory, NASA/Caltech and on measurement electronics at National Instruments Co. He received a Ph.D. in electrical engineering from the University of Illinois at Urbana-Champaign and a B.E. in modern physics from the University of Science and Technology of China.
AM4 Photovoltaic System Performance Modeling
Instructors: Instructor: Drs. Cliff Hansen, Joshua Stein and Dan Riley, Photovoltaic Modeling and Analysis Team, Sandia National Laboratory
This tutorial will provide attendees with a basic understanding of the modeling steps required to predict PV system performance from weather information. The series of modeling steps may include irradiance translation to plane-of-array, irradiance to DC energy conversion, DC to AC energy conversion, cell temperature estimation, and shading or reflection effects. Explanations of modeling procedures will be demonstrated using Sandia National Laboratories' PV_LIB photovoltaic modeling toolbox for MATLAB (provided at no cost through http://pvpmc.org); knowledge of MATLAB programming is not required, but will be very useful in understanding the demonstrations.
Dr. Joshua Stein is a Distinguished Member of the Technical Staff at Sandia National Laboratories working in the area of Photovoltaics and Grid Integration. Dr. Stein's specialty is modeling and analysis of complex natural and engineered systems, including assessments of uncertainty and sensitivity using stochastic methods. He currently develops and validates models of solar irradiance, photovoltaic system performance, reliability, and PV interactions with the grid. He leads the PV Performance Modeling Collaborative (http://pvpmc.org). He has a Ph.D. from the University of California, Santa Cruz in Earth Sciences.
Dr. Clifford Hansen is a Distinguished Member of the Technical Staff at Sandia National Laboratories working in the area of photovoltaics and solar resource modeling. Dr. Hansen's work includes development of methods for accurately calibrating PV performance models to measured system data, and specializes in characterization and analysis of uncertainty in model predictions. He has a Ph.D. from The George Washington University in Mathematics.
Mr. Daniel Riley is a Senior Member of the Technical Staff at Sandia National Laboratories working in the area of Photovoltaics characterization and modeling. Mr. Riley's work includes designing tests to characterize and evaluate PV and CPV modules, and developing and implementing model algorithms for PV-related models. He has an M.S. from the University of Missouri - Rolla in Electrical Engineering.
PM1 Third Generation Photovoltaics: Advanced Concepts to Boost Efficiencies Beyond the Schockley-Queisser Limit
Instructor: Dr. Gavin Conibeer, Professor, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, and Dr. Nicoleta Sorloaica-Hickman, Associate Professor, Florida Solar Energy Center, University of Central Florida
While the conventional photovoltaic technologies are currently dominating the market and advancing, there are still many exciting research efforts in developing novel materials and device architectures in this area. This tutorial will provide a perspective of the new technologies and materials, as well as novel device concepts which are actively being investigated globally to overcome the current limit of photovoltaic conversion efficiency and further drive down the cost of PV electricity generation. Topics covered will include major losses in single junction PV and approaches to reduce loss such as tandem; IBSC; up-conversion; down-conversion; MEG; TPV; hot carrier cell. In addition, we will present perspectives for future PV devices - further experimental proofs of concept, development to industry and other new concepts (quantum antennae, non-reciprocating circulators).
Professor Gavin Conibeer received his BSc degree from Queen Mary College, London University in Materials Science; MSc from the University of North London in Polymer Science; and PhD from Southampton University, UK, in III-V semiconductors for tandem PV cells. Conibeer has held research positions at Monash, Southampton, Cranfield and Oxford Universities and moved to his current location at the University of New South Wales, Sydney in 2002. He has worked on III-V, II-VI, group IV and nanostructure materials for solar cells as well as PV systems and policy. He has published more than 150 refereed papers and been successful in securing more than A$12M of research funding. He is an editor of Progress in Photovoltaics. Conibeer is currently Professor and Australian Research Council Future Fellow at the University of New South Wales where he leads the Third Generation Photovoltaics research group.
Nicoleta Sorloaica-Hickman is currently a Research Associate Professor in the Solar Technologies Research Division and the leader of the Laboratory for Photovoltaic and Thermoelectric Materials and Devices at Florida Solar Energy Center (FSEC) - University of Central Florida. She received a B.S. and M.S in Physics from "Al. I. Cuza" University - Romania in 1998 and 1999, respectively. She received her second M.S. and Ph.D. in Physics from Clemson University in 2004 and 2006, respectively. At FSEC, Dr. Sorloaica-Hickman's group performs research at the leading edge of advances in electronic, and thermal materials, and devices for applications in photovoltaic, thermoelectric, and solar energy integrated systems, developing inexpensive hybrid constructions of Photovoltaic and Thermoelectric cell to improve the solar cell efficiency and longevity. In 2012 Dr. Sorloaica-Hickman co-founded HybridaSol, which has the mission to combine the innovations in silicon cell and thermoelectric architectures with inexpensive and fast manufacturing process to bring solar cells to cost parity with coal-based electricity. She has (co)written over thirty papers in the areas of Photovoltaic and Thermoelectric and organized topical symposia, workshops and taught short courses on these topic as well as speaking at numerous national and international conferences.
PM2 Photovoltaic Module Reliability
Instructor: Dr. John Wohlgemuth, Principal Scientist, Photovoltaic Reliability Research and Development, National Renewable Energy Laboratory
As PV has grown into a large business, reliability of the products and systems become even more important. This combined with pressure to reduce production costs means that ensuring PV component reliability is critical. This tutorial will cover:
- Types of solar cells and modules commercially available or under development.
- History of field failures and how they led to the development of accelerated stress tests.
- Methods of failure analysis for PV modules.
- Update on the work being done by PVQAT (PV Quality Assurance Task Force) to develop i) Comparative Tests to assess module wear out, ii) Module manufacturing QA System Guidelines and iii) an IEC conformity assessment system for PV power plants.
- How do we develop module service life predictions?
Dr. Wohlgemuth joined the National Renewable Energy Laboratory as Principle Scientist in PV Reliability in 2010. He is responsible for establishing and conducting research programs to improve the reliability and safety of PV modules. Before joining NREL he worked at Solarex/BP Solar for more than 30 years. His PV experience includes cell processing and modeling, Si casting, module materials and reliability, and PV performance and standards. Dr. Wohlgemuth has been an active member of working group 2 (WG2), the module working group within TC-82, the IEC Technical Committee on PV since 1986 and has been convenor of the group for more than 15 years. Dr. Wohlgemuth is a member of the Steering Committee of PVQAT and he chairs Task Group 3 on Humidity, Temperature and Voltage. Dr. John Wohlgemuth earned a Ph.D. in Solid State Physics from Rensselaer Polytechnic Institute.
PM3 High Efficiency Multijunction Solar Cells based on III-V Semiconductors
Instructor: Dr. Frank Dimroth, Head of Department III-V - Epitaxy and Solar Cells, Fraunhofer Institute for Solar Energy
This tutorial will give a general introduction to the field of III-V high efficiency solar cells and the benefits of using multiple junctions to overcome the Schockley-Queisser limit of single-junction solar cells. III-V materials have proven to reach the highest conversion efficiencies of all PV technologies peaking at 46 % efficiency for cells operated under concentrated sunlight illumination. This success story is significantly related to the high material and crystal quality which can be obtained with III-V compounds today. The tutorial will give an introduction to high efficiency photovoltaics, discuss theoretical considerations and successful practical approaches. It will discuss material combinations of modern cell architectures including: upright growth, inverted growth, metamorphic growth, wafer bonding, quantum/nano structures and advanced materials like dilute nitrides or antimonides. The tutorial will further discuss fundamental requirements for reaching high efficiency like minimizing resistance and shadow losses. Photon recycling and photon coupling are introduced with their potential impact on cell performance. III-V solar cells are used in space and terrestrial concentrators and both applications will be covered with their specific requirements. Besides these established technologies, the tutorial will talk about III-V on silicon devices which are seen as a potential future technology for reaching low cost photovoltaics with cell efficiencies beyond 30 % at 1-sun. The tutorial will introduce the current stat-of-the-art and discuss material needs and cost challenges for large scale deployment of III-V solar cells.
Frank Dimroth is heading the "III-V Epitaxy and Solar Cells" department at Fraunhofer ISE in Freiburg, Germany. He joined the institute in 1996 and since performed research on high efficiency III-V solar cells for space and concentrator photovoltaics. The institute has a core focus on applied research and works with many companies in this field. In 2005, Frank was co-founder of Concentrix Solar (today SOITEC Solar), a leading manufacturer for CPV systems. His team demonstrated record efficiencies for III-V multi-junction solar cells and recently presented a wafer bonded device with 46 % efficiency.
PM4 Silicon Hetero-Junction PV Technology, Basic Principles, Manufacturing and Prospects
Instructor: Professor Christophe Ballif, director Photovoltaics and Thin Film Electronics Laboratory) IMT Neuchatel, EPFL, Switzerland.
Silicon heterojunction technology (HJT) consist of thin hydrogenated amorphous silicon layers (a-Si:H) deposited on mono-crystalline silicon wafers (c-Si) and allow for photovoltaic devices with energy-conversion efficiencies above 21 %, also at industrial-production level. In this tutorial we review the present status of this technology, and point out new trends. We first discuss how the properties of thin amorphous silicon films can be exploited to fabricate passivating contacts, enabling devices with very high open circuit voltages. This is the key to high-efficiency HJT solar cells. With this approach, an increasing number of groups are reporting devices with solar cell efficiencies well over the 20 % mark on n-type wafers; Panasonic leading the field with efficiencies as high as 24.7 %. Exciting results have also been obtained on p-type wafers. Usually, open-circuit voltages well above 720 mV are reached. To improve these results further, the current focus is centered on additional sophistications of existing processes, on the improved understanding of carrier transport at the device interfaces, but also on reducing parasitic absorption in HJT solar cells. Several routes can be taken for this: Short-wavelength parasitic absorption losses are reduced by replacing the front amorphous silicon with wider-bandgap window layers. Conversely, long-wavelength losses are mitigated by introducing new high-mobility TCO’s such as hydrogenated indium oxide, and also by designing new rear reflectors. Optical shadow losses caused by the front metalisation grid are significantly reduced by replacing printed silver electrodes with plated copper contacts. This approach may lead also to interesting material cost advantages. A different approach to minimize current losses is the implementation of alternative architectures such as back-contacted HJT cells, as no grid shadowing losses are present here at all and parasitic absorption in the front layers can be minimized irrespective of electrical transport requirements. Here, several groups have already obtained results that approach or overcome the performance of the best ‘standard’ HJT devices, even with simple processes. Indeed, Panasonic obtained 25.6% with this architecture in 2014, setting a new world-record for crystalline silicon single-junction solar cells. Finally, given the virtually perfect surface passivation and excellent red response of HJT solar cells, we anticipate they will become the preferred bottom cell in ultra-high efficiency c-Si-based tandem devices, exploiting better the solar spectrum. A combination with a perovskite solar cell as top cell seems to be particularly attractive.
Christophe Ballif is director of the Photovoltaics and Thin Film Electronics Laboratory) (PV-Lab at the institute of microengineering (IMT) in Neuchâtel (part of the EPFL since 2009). The lab is focusing on thin film silicon, high efficiency heterojunction crystalline cells, module technology and storage application. It also works on novel detector and macroelectronics application. The PV-Lab has strongly contributed to technology transfer and industrialization of novel devices.
Christophe Ballif graduated as a physicist from the EPFL in 1994, where he also obtained in 1998 his Phd degree working on novel PV materials. He accomplished his postdoctoral research at NREL (Golden, US) on compound semiconductor solar cells (CIGS and CdTe). He worked then at the Fraunhofer ISE (Ge) on crystalline silicon photovoltaics (monocrystalline and multi-crystalline) until 2003 and then at the EMPA in Thun (CH) before becoming full professor at the University of Neuchâtel IMT in 2004, taking over the chair of Prof. A. Shah. He (co-) authored over 200 journal and technical papers, as well as several patents.
Since 2013, C.Ballif is also the director of the new CSEM PV-Center, also located in Neuchâtel. The CSEM PV-Center is focusing more on industrialization and technology transfer in the field of solar energy, including solar electricity management and storage.
SC1 Short Course on Thin Film Deposition (offered on Saturday)
Instructor: Dr. Angus Rockett, Professor, Department of Materials Science and Engineering, University of Illinois Urbana-Champaign
This full day short course introduces students to the fundamentals of vapor phase deposition processes, thin film nucleation and growth, epitaxy, and the specific processes of evaporation, sputtering, and chemical vapor deposition (including a brief introduction to atomic layer deposition). The vapor phase deposition portion includes an introduction to vacuum, mean-free-paths, flux incident on surfaces, etc. Adsorption, desorption, surface diffusion, classical nucleation theory, wetting, surface energy effects, strain, and mechanisms of ion modification of materials are covered. Special issues related to epitaxial growth, critical thickness and mechanisms of strain relief are described. Discussion of evaporation include types of evaporation sources, flux distributions from evaporation sources and effusion cell design, and flux monitoring techniques including RHEED oscillations. Sputter deposition describes the basic mechanisms of sputtering, fundamentals of magnetron and rf sputtering processes, sputter yields, cosputtering, reactive sputtering, and methods of obtaining energetic fluxes of ions at a substrate to produce ion modification of the growth processes. Finally, basic chemical vapor deposition is described. This includes reaction and gas transport rate limited growth, hot and cold wall reactors, reactant selection issues, gas handling and safety, gas phase reaction processes, illustration of various CVD methods based on examples from GaAs deposition, selective CVD, and plasma-enhanced CVD are discussed. Finally a brief overview of the concepts of atomic layer deposition is provided.
Students should be aware that this is a very large range of topics to cover in the time available so the coverage of individual topics will necessarily be limited. Complete one or two day short courses on most of these topics are available from the American Vacuum Society and other organizations.
Angus Rockett is a professor of materials science and engineering at the University of Illinois. He received a Ph.D from the same department and a B.S in Physics from Brown University. He is a past President of the American Vacuum Society and was the 2012 Program Chair for the IEEE PVSC. He has an extensive history of activities with the Materials Research Society, the AVS, and the IEEE. He is a short course instructor for the AVS for their Sputter Deposition of Thin Films and Photovoltaics short courses and has given tutorial lectures for the IEEE PVSC and the MRS as well as at numerous international conferences and universities. He is the author of The Materials Science of Semiconductors and has over 120 publications in refereed journals. He currently serves as a member of the AVS Publications Committee and as an Associate Editor of the Journal of Photovoltaics.
SC2 Fundamentals of PV (offered on Saturday)
Instructor: Dr. Steven Hegedus, Scientist, Institute of Energy Conversion, University of Delaware
This comprehensive class will span the entire field of photovoltaics (PV). Learn about the technology that can produce energy for 25 years with no moving parts or emissions and can be deployed at scales to charge a cell phone or power a major city. First, we will cover the critical issues in PV cell operation, manufacturing, cost and performance. Then, we will move into applications by considering the very different design criteria off-grid vs on-grid systems. Finally, actual outdoor performance will be discussed in terms of module orientation and tilt, available sunlight, installation method. Issues of testing, outdoor reliability, impact of policy and grid connection will be briefly covered as well. A limited knowledge of solid state electronics (p-n diode) and simple circuits is assumed (Associate or bachelor level engineering or physics). A common theme throughout the class will be the trade-off between cost and performance whether applied to selecting materials, moving advanced cell designs into production, and module installation options such as tracking or concentration.
Topics to be covered will include
- What are the advantages and disadvantages of PV for producing electricity?
- What semiconductor properties are important for solar cells: why are only a limited number of materials promising for high efficiency solar devices?
- How the pn junction diode functions and how it becomes a solar cell when illuminated: simple solar cell circuit model for current and voltage and power
- Basic definitions of solar cell performance: efficiency, power, short circuit current, open circuit voltage, fill factor
- Learn a simple algorithm to accurately calculate the monthly or annual energy output of a solar module for any locations and orientation using the information on a module data sheet and available sunlight and temperature data.
- Manufacturing process flow from start to finish: design and manufacturing of today’s standard Si solar cell - from purifying and crystallizing raw Si feedstock into wafers into metalized solar cells - and encapsulating them to make a PV module
- Advanced Si solar cell concepts for higher efficiency
- What are thin film PV technologies? Focus on two with strong commercial interest: CdTe and Cu(InGa)Se2
- Analyze off-grid stand-alone application in terms of using battery storage to balance seasonal energy production and demand
- Compare qualities of 3 primary grid connected applications: residential (< 10kW), commercial rooftop (10-1000 kW), and centralized utility-scale power systems (> 1MW).
- A typical residential rooftop installation procedure
- Balance of systems: the non-PV components like inverter, wiring, and fuses necessary to functionally and safely connect your modules to deliver useful energy to a load
- What limits how much PV connected to the grid? Utility concerns. Critical role of battery storage to allow PV to make a significant contribution (>10%) to national electric generation by shifting peak PV supply and providing grid support services.
Dr. Steven Hegedus has been involved in solar cell research for over 30 years at the Institute of Energy Conversion (IEC) at the University of Delaware (UD), the world's oldest photovoltaic research laboratory. He is a Senior Scientist who has worked on all of the commercially active solar cell technologies - a-Si, CdTe, Cu(InGa)Se2, organic, and c-Si. Areas of his research have included a-Si and a-SiGe device fabrication, textured TCOs, thin film device analysis and characterization, a-Si/c-Si heterojunctions, back contact back junction Si solar cells, and accelerated stability studies of thin film devices. Dr Hegedus co-edited the 1st and 2nd editions of the "Handbook of Photovoltaic Science and Engineering" (Wiley 2003, 2011). He has been teaching a comprehensive graduate-level PV class for over 10 years. He completed the Solar Energy International Grid Connected PV System Installation class and has had a PV system on his roof since 2007.