The fundamental physical origins of the photogeneration process in organic photovoltaics can be simply understood in the context of the kinetics of polaron-pair dissociation and recombination at donor-acceptor heterojunctions that depends critically on the local nanostructure of the organic films. Nanostructure control demands that the properties of the donor-acceptor interface and that of the bulk thin films be independently optimized, whereby there needs to be disorder at the interface to reduce the polaron-pair dissociation rate and order in the film bulk to reduce series resistance. This level of control can be achieved during vapor phase deposition of the donor and acceptor thin films. In this talk, I will discuss recent progress in materials and their morphological control that has led to high efficiency single and multiple junction organic photovoltaic cells deposited by both conventional and roll to roll deposition processes.
Area 1: Lars Samuelson, Vice-Director, Center for Nanoscience, Lund University
From basic nanowire science to opportunities in photovoltaics
Nanowire (NW) growth permits the combination of materials having vastly different atomic spacing, such as InAs and InP, attributed to efficient radial relaxation in narrow NWs . Another attractive NW feature is their ability to absorb close to 100% of the incoming solar flux even when only around 10% of the surface area is covered by the nanowires. This phenomenon, ascribed to a nano-photonic antenna effect, was first demonstrated for InP NWs where an efficiency of 13.8% was measured . More recently were reported, first at IEEE-PNSC 42, optimally grown and radially passivated GaAs NW array solar cells having an efficiency of 15.3% . The highly efficient angular in-coupling of light into the nanowires was recently demonstrated experimentally as well as theoretically . In the highly competitive space of photovoltaics the cost of epitaxial growth of III-V films as well as NWs still seem to prohibit large-scale terrestrial applications. I will describe a radically different method of fabrication of single-crystalline NWs, labeled Aerotaxy , a method in which NWs of, say, GaAs are growing out of Au metallic particles, while these are being transported as an aerosol through the growth furnace. This growth mode enables ultra-fast growth (about 1µm/s) and very cost efficient fabrication of device quality NWs. Finally I will speculate on methods for up-scaling by 6 - 7 orders of magnitude, from the dimension of individual NWs to square-meter-sized panels.  M.T. Björk et al., “One-dim steeplechase for electrons realized”, Nano Lett 2, 642 (2002)  J. Wallentin et al., “InP NW array solar cells achieving 13.8%..”, Science 339, 1057 (2013)  I. Åberg et al., “A GaAs NW array solar cell with 15.3% effici..”, IEEE J of PV 6, 185 (2016)  O.M. Ghahfarokhi et al., “Performance of GaAs NW array..”, IEEE J of PV 6, 1502 (2016)  M. Heurlin et al., “Continuous gas-phase synthesis of NWs with..” Nature 492, 90 (2012)
Area 2: Paul Haney, Project Leader in the Energy Research Group, National Institute of Standards and Technology
Analytic description of the impact of grain boundaries on Voc (invited)
Despite decades of research, the role of grain boundaries in the photovoltaic behavior of thin film polycrystalline solar cells remains poorly understood. The high defect density of grain boundaries generally promotes recombination and reduces photovoltaic efficiency. However, thin film polycrystalline photovoltaics such as CdTe exhibit high efficiencies despite a large density of grain boundaries. Previous numerical studies illustrate the detrimental effect of grain boundaries, but the large number of system parameters and nonlinearities of the problem make it difficult to develop intuition from numerical studies alone. I’ll present our analysis of recombination in a 2-d model of a pn+ junction containing columnar grains with positively charged grain boundaries. We restrict our attention to dark recombination at forward bias and derive the dark J-V relation. We formulate a physical picture of the electron/hole currents and recombination, and translate this picture into a simplified effective model which describes the essential features of the full system. This model is amenable to simple analytic solutions. Under reasonable assumptions, we find that these analytic solutions successfully describe the results of full 2-d numerical simulations for a range of grain boundary orientations and defect structures. Numerical simulations further show that the dark J-V relation can be used to determine the open-circuit potential Voc of an illuminated junction for a given short-circuit current density Jsc. We can therefore provide a precise relation between grain boundary properties and Voc. This relation enables one to rationalize the (relatively) low Voc observed in polycrystalline CdTe and to formulate strategies to optimize grain boundary properties. I conclude by discussing approaches to experimentally test our results.
Area 2: Wyatt Metzger, Research Staff, CdTe Project Lead, National Renewable Energy Laboratory
CdTe photovoltaic technology is now competing with mc-Si at costs competitive with conventional electrical energy sources. Further performance advances are possible by overcoming fundamental material challenges to increase lifetime, hole density, open-circuit voltage, and fill factor. This talk will review recent work to understand and improve CdTe properties using single crystals, varying grain size, stoichiometry, different doping approaches, advanced characterization, and modeling.
Area 2: Lars Stolt , Chief Technology Officer, Solibro GmbH
CIGS PV module technology at 17% efficiency and beyond
Thin film technology have held the promise of a low cost PV technology for decades. Although fundamentally with same efficiency potential as silicon wafer technologies, it has been hard to reach the efficiency level of polycrystalline solar cells which dominates the PV module market. In the last couple of years First Solar has reported remarkable efficiency progress with their CdTe-based thin film technology reaching, or even surpassing the best polycrystalline silicon modules with prototype modules. In this paper Solibro will present the status of their CIGS technology with a champion result of 17.0 % total area efficiency for a prototype full size module fabricated in the production facility in Germany. The module has an area of 0.94 m2 and the efficiency has been confirmed by TÜV Rheinland. Further performance enhancement will be outlined based on R&D results from Solibro Research in Sweden, demonstrated in 30 x 30 cm2 submodules with higher than 18 % aperture area efficiency. The technology advancements to reach these efficiency levels will be described. Key elements are a new front contacting technology using metal grid and TCO in combination, and a post CIGS deposition treatment using alkaline elements.
Area 2: Xuanzhi Wu , Founder, Advanced Solar Power
Commercialization of Polycrystalline CdTeThin-Flim Solar Cell Technology in ASP
Polycrystalline CdTe thin-film PV technologies have demonstrated high performance and the ability to attract production-scale capital investment, and to produce the PV modules with high efficient﹑high throuput and low cost. In this presentation, we review the commercialization of poly-CdTe thin-film PV technology in ASP. The R&D line, pilot and manufacturing line of CdTe solar cell have been developed in ASP. A series of new materials, key-equipments and novel manufacturing processes have been developed to build an fully-automated 40MW/Y CdTe thin-film PV module manufacturing line. The production line has demonstrated high throughput and excellent reproducibility. The 0.6 x 1.2 m2 CdTe thin-film PV modules with an average efficiency of more than 13% and good yield have been fabricated. The 3KW PV power station including ASP CdTe modules and commercial Si modules has demonstrated that annual electricity generated by CdTe thin-film modules is about 5% more than Si solar modules in Hangzhou area for four years, due to its low temperature coefficient、excellent low irradiance performance and high stability. This presentation will also show novel ASP＇s PV products and its application examples for the distributed PV power stations and BIPV applications, which make a big difference with the large-scale ground PV power station. Finally, it will propose several directions for further improving the level of commercialization of CdTe thin-film solar cell technology.
* E-mail: email@example.com Address: 801 Lingyun Street, Hangzhou Economic & Technological Development District, Hangzhou, Zhejiang Province 310018, China
Area 3: Frank Dimroth, Head of Department, Fraunhofer Institute for Solar Energy
2-terminal III-V/Si Tandem Solar Cells with > 30 % AM1.5g Efficiency
Solar cells from III-V compounds hold all efficiency records for single-junction as well as multi-junction PV devices. But due to the high manufacturing cost, applications are so far limited to concentrator photovoltaics and satellites. A question which is often asked is: What needs to be done to reduce the cost of III-V multi-junction solar cells to the point where they are competitive with conventional flat plate photovoltaics? One path is the combination of III-V absorbers with a low-cost silicon bottom cell. This requires only 2-6 µm of III‑V material to reach realistic efficiencies beyond 35% (AM1.5g), significantly above what is achievable with silicon single-junction devices. But a number of technological challenges have to be overcome: thermal expansion difference, impurity diffusion into silicon, lattice mismatch and the transition between diamond and zincblende lattices. These are real technological challenges that have been researched for decades. We have recently succeeded in developing GaInP/(Al)GaAs/Si triple-junction solar cells using two approaches: direct growth on silicon and wafer bonding. Our 2-terminal devices with an area of 4 cm2 reach AM1.5g efficiencies of 19.7% and 30.2% respectively. This is the first demonstration of a monolithic silicon-based solar cell with an efficiency greater than the theoretical limit of silicon (29.4%). In this presentation we will discuss the development of a suitable Si bottom solar cell, III-V on silicon epitaxy and we will show a pathway towards higher efficiency at lower cost.
Area 3: Michael Haney, Program Director, Advanced Research Projects Agency-Energy, US Department of Energy
Can Micro-CPV Technology Change the Paradigm of Flat-Plate PV?
This talk presents the goals and status of ARPA-E’s “Microscale Optimized Solar Cell Arrays with Integrated Concentration” (MOSAIC) program, whose overall objective is to cost-effectively combine the enhanced harvesting efficiency of CPV with the low cost and form-factor of flat panel 1-sun PV. A variety of concepts that concentrate the solar energy within an array of micro-optical and associated micro-PV elements are being investigated. Techniques for embedding actuation elements for solar tracking within the array are also being evaluated, as well as methods for harvesting the direct and diffuse elements of the solar resource within common planar structures. The technical approaches and key advancements for the various projects of the MOSAIC program are reviewed, as well as potential future steps needed to enhance the ability of the new solar harvesting technologies to be transitioned into the marketplace.
Area 4: Weiwei Deng, State Key Laboratory of PV Science and Technology, Trina Solar
22.61% Efficient Fully Screen Printed PERC Solar Cell
Passivated emitter and rear solar cells (PERC) on p-type Cz Si wafers are currently being migrated to mainstream production, with ongoing improvements in recent years. We describe and characterize our recent batch of PERC cells, fabricated on 156x156 mm2 wafers with fully screen printed technology and an industrial type process. The champion cell efficiency has reached 22.61%, with a Voc of 684.4 mV and a fill factor of 81.49%, confirmed by Fraunhofer CalLab. The co-optimization of the front diffusion and Ag-Si contact increases the fill factor, a lower J0 and higher lifetime increase Voc, the two main contributors to our recent cell efficiency improvements.
Area 5: Uwe Rau, Director, IEF5-Photovoltaics, Forschungszentrum JÃ¼lich
Characterization of photovoltaic materials and devices from the atomistic to the module level
The post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) absorbers with alkali-fluorides has led to a significant increase in solar cell efficiencies over the recent years. The effect of the alkali elements and the mechanisms leading to the efficiency improvements are currently heavily investigated. We have studied the influence of the alkali-treatments on the CIGSe surface and on the absorber/buffer interface. Using nanoscale spatially resolved imaging of the optoelectronic properties by Kelvin probe force microscopy, we investigated different alkali-elements and different buffer layers. For increasing deposition times of the typical CdS buffer layer (by chemical bath deposition) on RbF-treated CIGSe, we observe an initial increase in work function, followed by the expected decrease only after 3 min CdS deposition. At the same time, an increase in the surface photovoltage to >150 mV indicates the formation of a charge-separating pn-junction. However, inhomogeneities in the surface potential on the scale of 5-10 micrometers are still observed after 3 min CdS deposition. Thus, the frequently reported possibility of reduced chemical bath deposition times of the CdS buffer layer has to be carefully considered to avoid incomplete or inhomogeneous coverage potentially leading to efficiency losses. We also studied the effect of alkali-fluoride PDT on the electronic properties of grain boundaries (GBs). For the case of the wide-spread KF-treated CIGSe surface, we observe that the band bending at the GBs is increased by ~70% with respect to the untreated absorber. Moreover, the variation of the magnitude of the band bending is reduced, indicating more homogeneous GB electronic properties. These results reveal that the KF-PDT also has a beneficial effect on the GBs, which presumably contributes to the improved efficiency values observed for alkali-treated CIGSe thin-film solar cells.
Area 6: David J Jones, Senior Research Associate, University of Melbourne
High Performance Molecular Donors For Organic Solar Cells, Materials Design And Device Optimization
We demonstrate the development of high performance molecular donors for the use in organic solar cells. We modified the known BTR small molecule donor to reduce diffusion rates during thermal annealing by chromophore extension to generate BQR. OPV devices containing BQR are thermally stable, with average PCEs of around 10%. Active layers containing BQR can be deposited from industrially relevant solvents using slot die printing. We demonstrate that simple slot die printed 1 cm2 OPV devices using toluene as a solvent with around 6% PCE.
Area 6: Alex Zunger, Professor, University of Colorado Boulder
Developing an understanding-based selection of hybrid-perovskite compounds and the Cu-In hybrid perovskite (CHIPs) Family
Developing an understanding-based selection of hybrid-perovskite compounds and the Cu-In hybrid-perovskite (CIHPs) family Alex Zunger*
Renewable and Sustainable Energy Institute (RASEI) University of Colorado Boulder, Boulder, Colorado 80309 Email: firstname.lastname@example.org; Tel: 303 492-7084
The long-term chemical instability and the presence of toxic Pb in otherwise stellar solar absorber APbX3made of organic molecules on the A site and halides for X have hindered their large-scale commercialization. Previously explored ways to achieve Pb-free halide perovskites involved replacing Pb2+ with other similar M2+ cations in ns2 electron configuration, e.g., Sn2+ or by Bi3+ (plus Ag+), but unfortunately this showed either poor stability (M = Sn) or weakly absorbing oversized indirect gaps (M = Bi), prompting concerns that perhaps stability and good optoelectronic properties might be contraindicated. Herein, we exploit the electronic structure underpinning of classic Cu[In,Ga]Se2 (CIGS) chalcopyrite solar absorbers to design Pb-free halide perovskites by transmuting 2Pb to the pair [BIB + CIII] such as [Cu + Ga] or [Ag + In] and combinations thereof. The resulting group of double perovskites with formula A2BCX6 (A = K, Cs; B = Cu, Ag; C = Ga, In; X = Cl, Br, I) benefits from the ionic, yet narrow-gap character of halide perovskites, and at the same time borrows the advantage of the strong and rapidly rising Cu(d)/Se(p)→Ga/In(s/p) valence-to-conduction-band absorption spectra known from CIGS. This constitutes a new group of CuIn – based Halide Perovskite (CIHP). Our first-principles calculations guided by such design principles indicate that the CIHPs class has members with clear thermodynamic stability, showing rather strong direct-gap optical transitions, and manifesting a wide-range of tunable gap values (from zero to about 2.5 eV) and combination of light electron and heavy-light hole effective masses. Materials screening of candidate CHIPs then identifies the best-of-class Cs2[AgIn]Br6 and K2[AgIn]Br6, having direct band gaps of 1.50 and 1.44 eV, and a theoretical spectroscopic limited maximal efficiency comparable to chalcopyrites and CH3NH3PbI3. Our finding offers new routine for designing new-type stable, Pb-free halide perovskite solar absorbers. _________________________________________________________________________ *In close collaboration with L.B Abdalla, L.L. Kazmerski, and G. Dalpian at CU Boulder and L. Zhang and his students at College of Materials Science and Engineering, Jilin University, Changchun, China. Work at CU supported by USA Department of Energy jointly by EERE Sun Shot and by the Office of Science, Basic Energy Science, MSE Division under Grant No. DE-FG02-13ER46959.
Area 10: Mahesh Morjaria, Vice President PV Systems Development, First Solar
Advances in Utility-Scale PV Plants: Key Lessons Learned
Advances in utility-scale PV plant design and balance of system (BOS) components, along with PV module cost reduction and efficiency improvement, have played a significant role in making solar energy more cost-effective than conventional resources. The industry has already achieved a significant PV plant cost reduction by adopting the next generation plant architecture that increases direct current (DC) system voltage from 1,000VDC to 1,500VDC. The BOS savings result from greater scale efficiencies by enabling higher power throughput in DC components such as cables, combiner boxes, and inverters. There are various other BOS innovations in structures and DC wiring that have been noteworthy too. Further cost reductions are possible with adoption of cost-effective string inverters and adopting advanced Medium Voltage DC (MVDC) plant architecture. Another enabler in integrating large amount of PV generation into the electric power grid, is the capability of utility-scale PV plants to address grid reliability and stability concerns. PV plants with “grid-friendly” features such as voltage regulation, active power controls, ramp rate controls, fault ride-through, frequency droop control and others have alleviated these concerns. The viability of PV plants to provide important ancillary services to the grid was recently demonstrated in a test conducted with NREL and CAISO on a 300MW utility-scale PV plant. The results showed that the PV plant value can be extended to provide services such as spinning reserves, load following, ramping, frequency response, variability smoothing and frequency regulation. The results showed that a PV plant can regulate to 4-second Automated Generator Control (AGC) signal 24-30 points more accurately than even fast gas turbines.
Area 11: Anastasios Golnas, Systems Integration, DoE SunShot
Accurate solar irradiance forecasting holds significant promise in lowering the cost of integrating ever greater amounts of solar power into the electric networks. The SunShot Initiative has been supporting relevant R&D in order to not only overcome the inherent challenges associated with predicting the hours-to-days-ahead evolution of atmospheric variables, but to also ensure that improvements can be practically leveraged in the energy management operations of utilities and balancing authorities. This presentation will discuss the funding opportunities announced by the DOE in this space and the challenges associated with their design.
Area 12: Vasilis Fthenakis, Director of Center for Life Cycle Analysis, Columbia University
Producing fresh water via water desalination and water reuse is essential for arid, water-scarce regions, but it is expensive and energy-intensive. The cost of energy is a significant contributor to this high cost and the use of fossil fuels that currently power desalination plants causes emissions of greenhouse gases and other hazardous pollutants. The recent cost reductions of photovoltaic systems create opportunities for developing low-cost and emission-free desalination technologies. However, their adoption is hampered by the lack of concepts and designs that are integral to the variability of solar resources. This paper presents a holistic approach and collaborative work within a newly founded Global Clean Water Desalination Alliance (GCWDA)-H2O without CO2- for developing friendly water desalination and reuse technologies world-wide. We envision the culmination of collaborative work to provide viable solutions by 1) hybridization of existing and emerging desalination and water reuse technologies, 2) integration of solar electricity generation with advanced desalination systems, 3) increasing the reliability and longevity of such systems. There are options currently available which can be readily implemented and longer-term solutions that require process design studies, plant techno-economic modeling and field testing. The first include partial replacement of fuel-based electricity in current desalination plants with lower cost solar electricity while accommodating load peaks and decreased middle of the day thermo-electric plant generation PV-RO. Longer term, high RE penetration solutions include but are not limited to modeling and developing operational strategies that add benefits to variable renewable energy (VRE) integration with desalination and water reuse. For example, plant designs that facilitate variable operation may allow for time-shifting of energy use, demand response, utilization of over-generation by solar and wind power, and other functions.
Area 12: Chinho Park, Managing Director, Office of Strategic R&D Planning, South Korean Ministry of Trade, Industry and Energy (MOTIE)
With the emergence of PV as a competitive, mainstream energy source, stakeholders are looking for confirmation that the environmental performance of PV is keeping pace with the technology. Over the past decade, the PV industry has seen several sustainability-related milestones such as the establishment of commercial recycling technologies, methodology guidelines for life cycle assessment, and corporate sustainability scorecards. However, the past year has seen an effort to take these individual efforts and standardize them across the industry to promote transparency and credibility when measuring environmental performance. These include a sustainability leadership standard for PV modules (NSF 457) developed by NSF International, product environmental footprint category rules developed by the European Commission, and a national PV recycling program developed by the U.S. Solar Energy Industries Association. The motivation, current status, and implementation requirements for each initiative will be presented. More generally, principles of sustainability when applied to industry involve an approach of creating sustainable value by balancing between internal and external priorities and meeting today's and tomorrow's challenges. Industry must perform well simultaneously in all four quadrants of this framework in order to maximize value over time. Despite the considerable progress of the photovoltaic industry, it remains a volatile industry with primary focus on today's internal challenges. The emerging PV industry sustainability initiatives aim to rebalance the industry's trajectory toward also addressing external concerns and future trends to help promote the industry's long-term viability.