Lab Partnering Service Discovery
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Iowa State University and Ames Laboratory researchers have developed a process for fabrication of solar cells with increased efficiency.
A 20% increase in efficiency has been observed experimentally, and ISU is seeking partners interested in commercializing this technology. Polymer-based photovoltaic devices have received intense interest in recent years because of their potential to provide low-cost solar energy conversion, flexibility, manufacturability, and light weight. However, the efficiency of organic solar cells is about 4-6%, and increasing this efficiency is critical for developing practical applications and commercially viable devices. One approach to increasing efficiency is to increase the light absorption on the organic film without increasing the thickness of the photoactive layer, and various light management techniques have been tried for enhancing optical absorption, such as collection mirrors, patterned substrates and microprism substrates. However, these approaches require extra processing steps or technically challenging coating technologies. To overcome these limitations, ISU and Ames Laboratory researchers have developed a process for conformal coating of polymer photovoltaic layers on microtextured substrates for increased light trapping. The light management architecture of these solar cells enables a high degree of light absorption in even very thin photoactive films and leads to improved power conversion efficiency.
Ali Javey and Zhiyong Fan at Berkeley Lab have invented a method for growing highly regular, single-crystalline nanopillar arrays of optically active semiconductors to produce efficient, 3D solar cells. The 3D configuration allows for less stringent requirements in terms of the quality and purity of the input materials, providing for a reduction in cost compared to other solar cell configurations.
The Berkeley Lab invention uses a "vapor-liquid-solid" process that produces large-scale modules of dense, ordered arrays of nanopillars. Researchers tested their method by producing a solar cell composed of electron-rich CdS nanopillars embedded in a polycrystalline thin film of hole-rich CdTe. The efficiency of this prototype was 6%, which may be readily improved with concentrators, more transparent top contacts, and optimization of the nanopillar dimensions. The technology was also used to produce solar modules on flexible substrates that offer more efficient light absorption and carrier collection than rigid arrays. These flexible arrays could be bent repeatedly without damage or loss of cell performance.
The ability to deposit single-crystalline semiconductors on support substrates is crucial in the development of efficient photovoltaics. However, the process, usually performed with epitaxial crystal growth, has been expensive and inefficient. In addition, when amorphous substrates have been used to grow single-crystalline nanowires non-epitaxially, at less expense, the nanowires have varied in size, alignment, and density giving the resulting arrays a limited efficiency of approximately 0.5%. The Berkeley Lab technology offers a significant improvement in efficiency and manufacturability.
Sandia National Laboratories has created a technology that produces an antireflective (matte) surface on a silicon photovoltaic solar cell. The process uses a randomly deposited metal catalyst followed by reactive ion etching (RIE) to produce nanoscale surface features. The texture of the cells is more effective in solar absorption and, therefore, storage of energy. This nanoscale texturing is also a cost effective and environmentally safe tool for a renewable energy source.
The subwavelength (nanoscale) roughness presents a gradual interface between the air and the photovoltaic cell which reduces reflection loss, for high overall solar energy collection efficiency. In contrast to a chlorine-based etch process, this nanoscale texturing process is a cost effective alternative that uses nontoxic materials.
Current oil and salt based heat transfer fluids have significant limitations such as usable temperature, high cost, and limited thermal conversion efficiency. To achieve the Department of Energy SunShot goal of high efficiency, low cost renewable power generation, a highly efficient and economical way to absorb solar heat and to store the thermal energy is important for broad deployment of concentrating solar power (CSP) plants as baseload power.
Engineers at the National Renewable Energy Laboratory (NREL) have developed a high-temperature “direct” supercritical CO2 (s-CO2) receiver for CSP applications. The direct s-CO2 receiver can be coupled with an s-CO2-Brayton power cycle to meet the DOE SunShot cost and performance goals. The near-blackbody (NBB) design employs a working mechanism resembling a blackbody furnace, and minimizes thermal losses from convection and radiation through reducing direct exposure of heated surfaces to the cool ambient surroundings. An ideal blackbody furnace design uses a well-known radiative mechanism and captures nearly all incoming radiation. The infrared (IR) re-radiation losses also behave as NBB emission, therefore a significant design emphasis is on minimizing IR emission. The NBB design maximizes solar energy collection efficiency while reducing IR re-radiation and convection losses for high performance.
This receiver design performs at greater than 650°C operating temperature with less than 10% thermal loss (defined as the ratio of energy delivered to the heat transfer fluid divided by the total energy that enters the receiver aperture), while minimizing the thermal stress (and hence material requirements) of the receiver. Such a design enables use in a modular, small tower s-CO2 power system, where the s-CO2 power block may be directly integrated with the receiver on top of the tower, resulting in less piping requirements and parasitic consumptions.
A novel structure design for thin film organic photovoltaic (OPV) devices provides a system for increasing the optical absorption in the active layer. The waveguided structure permits reduction of the active layer thickness, resulting in enhanced charge collection and extraction, leading to improved power conversion efficiency compared to standard OPV devices.
High optical absorption in OPV devices demands an active layer thickness of about 150 nm, a thickness that results in inefficient charge collection. To decrease the thickness of the active layer a new structure has been designed that increases the optical absorption in very thin film OPV devices. The structure makes use of a slot-waveguide approach to confine the electromagnetic field in the thin active layer. The invention enables a stronger optical absorption in the active layers that are thin enough to have electrical transport gain leading to a significant increase in internal quantum efficiency (IQE) and a higher overall power conversion efficiency.
The scientists developing this capability welcome the opportunity to unite with industry and advance its potential.
Cost is a critical issue limiting widespread adoption of photovoltaic (PV) technology. Consequently, organic semiconductors represent an attractive direction for PV development, as they can be made into devices at very low cost by means of continuous processes such as solution printing and roll processing. In addition, organic semiconductors have unique attributes distinct from inorganic semiconductors such as strong infrared absorption with visible transparency, low weight, and weak intrinsic radiation interactions. However, the comparatively low efficiency and reliability of existing organic photovoltaic (OPV) technologies precludes their use as a surrogate for more expensive inorganic alternatives in a variety of applications.
For over a decade, Los Alamos National Laboratory (LANL) has worked with organic electronic materials and devices such as organic light-emitting diodes (OLEDs) using a theory-fabrication-measurement approach. LANL’s research produced a better understanding of how organic light emitting diodes work, particularly with regard to the processes controlling charge injection and transport; these same capabilities are now being applied to OPVs.
LANL researchers recognize the obstacles to high OPV efficiency and are focused on developing a fundamental understanding of organic semiconductor device physics with regard to interface, band structure, charge transport, and exciton diffusion in order to increase the effectiveness and reliability of OPVs. The ultimate objective is to optimize OPV device design across a number of key areas while preserving the low-cost fabrication schemes that make them an economically practicable alternative to inorganic photovoltaic devices.
To support this effort, the Laboratory has a number of unique capabilities designed specifically to meet the needs of current organic semiconductor research, development, processing, and fabrication:
- Argon glove boxes (~1 ppm 02, H2O) with integrated deposition chambers of various types (e.g., sputtering, e-beam, thermal, and Radak evaporators)
- SEM, EDX, XRD, XRF, and optical microscopy
- I-V, C-V, impedance, electroabsorption, EL, PL, spectroscopic ellipsometry
- General cw and time resolved (ns) optoelectronic measurements
- Sample cryostats from liquid He to ~500 C
Plasmonically-active surfaces developed by this group could potentially increase optical absorption and photocurrent generation over a broad range of visible wavelengths in photovoltaic devices, thereby increasing energy conversion efficiency. When integrated with traditional photovoltaic cell designs, these enhanced surfaces have the potential to reduce manufacturing costs by enabling the use of thinner PV materials. In addition, reducing the required thickness of PV materials will expand the number of materials that are suitable for photovoltaic devices. LANL is seeking partnership opportunities to utilize these capabilities and further advance this technology area.
An important component on the path to reducing the cost of photovoltaic (PV) cells is to improve energy efficiency with less photoactive material. An attractive possibility in this respect would be to use surface plasmons for initial light capture. Surface plasmons are oscillations of conducting electrons that trap optical waves near their surface. Los Alamos National Laboratory (LANL) investigators aim to integrate traditional PV designs with novel surfaces designed to create plasmonic activity at the PN junction of the cell. Plasmonically-enhanced photovoltaic cells have the potential to enhance optical absorption and enable the use of thinner materials for lower-cost manufacturing of solar panels.
LANL researchers are leveraging the following unique Laboratory capabilities to develop enhanced surfaces and advanced nanoparticle architectures to produce surface plasmons in photovoltaic devices:
- Robust plasmonic testing platforms
- Modeling and testing of plasmonic enhancement at surface versus interface
- Fabrication-free plasmonic substrates through polymer-directed electroless deposition
- Seed-mediated reduction of gold nanoshells
- Field-assisted particle assembly
- Acid-directed synthesis of silver particles
- Laser-structured silicon surfaces
The scientists developing this capability welcome the opportunity to unite with industry and advance its potential.
As the solar industry works to build the infrastructure necessary to make electricity from photovoltaic (PV) technologies cost-competitive with grid electricity by 2015, many technical challenges emerge along the way. Los Alamos National Laboratory (LANL) researchers are working to anticipate and solve these challenges by modeling multi-scale, light-harvesting processes in nanomaterials.
The design of efficient PV cells necessitates an optimization of the device’s energy band structure to the solar spectrum; this, in turn, calls for the engineering of materials with controlled energy band structures. The energy band structure of a PV device can be determined by a myriad of aspects including the composition, surface chemistry, and lattice strain effects of the material itself. To support this effort, LANL researchers are using theoretical modeling to study a variety of effects including:
- Modeling of new materials for PV applications
- Impacts of amorphous optically-active conjugated polymers on light-harvesting properties
- Effects of ligands on semiconductor quantum dot (QD) functionality
- Contributions of direct photogeneration and population relaxation on the total quantum efficiency of carrier multiplication in lead selenide QDs
- Effects of plasmonically-enhanced nanoparticles on energy transport
- Impact of carrier transport phenomena on photovoltaic efficiency
- Effects of conformational disorder in bulk polymeric materials on electronic transport
The ultimate goal of this research is to determine the optimal composition of semiconductor structures in order to engineer materials with the electronic and optical properties necessary to increase power-conversion efficiency in solar cells.
"Third-generation" PV technologies are being actively pursued in academic research labs. These include nano-optics, multi-junction architectures, multi-exciton, plasmonics, and lower cost tandem cells. The goal is a module cost of less than $0.60/watt. Many of these technologies are in exploratory or early research stages but still can be evaluated according to their material requirements, processing complexity, and potential scalability. For example, concepts that utilize GaAs or CIGS may have cost issues or material availability issues. Similarly, complex cell designs or designs that feature nano-architectures such as quantum wires may not be easily scaled.
Robert Kostecki and Sam Mao of Berkeley Lab have invented a new approach to affordable, third generation PV. The technology builds upon previous research that uses surface chemical reactions to generate and inject hot electrons into a metal catalyst. In the recent invention, light is absorbed by the PV material and hot electrons are generated. Charges are then separated across a Schottky barrier.
The technology is a simple, low cost, scalable solution that addresses the concerns of high cost materials, materials availability and toxicity, complexity of cell design, and large-scale manufacturability. It relies on cheap, abundant materials that do not require ultra-high purity processing. Various existing deposition technologies can be used for cell manufacturing, none of which rely on ultra-high vacuum or expensive MOCVD/MBE equipment. Based on the initial laboratory conceptual demonstration, greater than 10% efficiency is projected with process development on existing tool sets. The technology uses a simple cell design that, together with the processing flexibility, should provide a path to scalable manufacturing.
Ramesh Jasti and Carolyn Bertozzi of Berkeley Lab have developed a technique to build carbon-ring “nanohoops,” molecular building blocks for the formation of carbon nanotubes. Carbon nanohoops might serve as seeds, or templates, for the efficient and large-scale synthesis of nanotubes of exceptional precision and uniformity. Nanohoops are cycloparaphenylenes, carbon-ring structures that are the smallest sub-units of “armchair” nanotubes, which have unique electrical and optical properties. The armchair form is characterized by thin walls and a much higher conductivity than copper.
Current nanotube production techniques result in low yields. Bulk scale production has not been feasible, because desired structures must be sorted from batches of randomly varied nanotube architectures and traits. This mixture of atomic arrangements produces carbon nanotubes with vastly different electrical and optical properties, a significant hurdle for their use in advanced nanotechnology applications. The successful production of cycloparaphenylene nanohoops could lead to the preparation of carbon nanotubes with a predefined arrangement of atoms. This rational design process would sidestep the arduous and inefficient separation methods currently used to harvest carbon nanotubes of a desired kind.
Long before carbon nanotubes were conceived of, organic chemists were intrigued by the striking symmetrical structure of cycloparaphenylenes. Since the early 1930’s, attempts to synthesize them have failed due to the tendency of carbon atoms to organize in planar, rather than curved structures. The Berkeley Lab team solved this longstanding structural chemistry challenge by discovering a method to bend carbon components into arcs that subsequently can be assembled into hoops.
Berkeley Lab researchers Peter Agbo and Rebecca Abergel have improved the efficiency of downconverting lanthanide nanoparticles by using the ligand 3,4,3-LI (1,2-HOPO) as an ultraviolet photosensitizer of NaGd1-xEuxF4 nanoparticles to create a UV downconverting material emitting light at wavelengths where the photocurrent response of silicon photovoltaics is higher. The construct is an effective downconverter of UV light into the red-frequency emissions
Berkeley Lab researchers Peter Agbo and Rebecca Abergel have improved the efficiency of downconverting lanthanide nanoparticles by using the ligand 3,4,3-LI (1,2-HOPO) as an ultraviolet photosensitizer of NaGd1-xEuxF4 nanoparticles to create a UV downconverting material emitting light at wavelengths where the photocurrent response of silicon photovoltaics is higher. The construct is an effective downconverter of UV light into the red-frequency emissions. The inventors estimate a 1,000x-improvement in UV absorption efficiency with this ligand, and potentially greater improvement with optimized ligands.
In the Berkeley Lab technology, the ligand acts like antennas, increasing the UV absorption cross-section of the nanoparticles, and therefore the UV absorption efficiency, without affecting the downconversion or reemission processes. Testing resulted in 610 nm downconversion luminescence upon 317 nm excitation.
The problem of spectral mismatch between semiconductor band gaps and the Earth’s terrestrial solar spectrum limits the efficiency of modern photovoltaics. While a number of methods have been implemented to focus on the problem of light upconversion, little work has addressed the challenge of UV downconversion towards the low-energy visible and near-infrared regimes, where the photocurrent response for bulk silicon is highest. Maturation of downconverter prototypes has been inhibited by the low molar absorptivities of f?f transitions. In addition, most lanthanide nanoparticles have very low absorption efficiency, so researchers often resort to high-powered lasers to test absorption, downconversion, and reemission.
The global photovoltaic (PV) industry has seen approximately 30% growth each year for the past 15 years, leading to a $10 billion industry. The PV industry is looking to lower the high costs and increase the efficiency of solar power. Scientists at NREL have designed a new process, Black Silicon Etching, that has a confirmed conversion efficiency of 16.8% in a cell without conventional antireflection coatings. This surpasses the previous black silicon record by 2.9%. Additionally, this new technology could lower the Levelized Cost Of Energy (LCOE), or the lifetime cost per unit of energy, by 2.5%.
The winner of a 2010 R&D 100 Award, Black Silicon Etching is an innovative technology that both increases the efficiency of solar cells and decreases the capital costs of producing them. Black Silicon Etching is a one-step process using gold nanoparticle catalysis to etch nanopores into the surface of a silicon wafer, producing a density-graded black surface. The near-complete lack of reflection from the surface of the material means it can absorb more sunlight, and more electricity can be produced. Current technologies first texturize the silicon wafer and then coat the cells with an antireflective layer, which takes 8 to 30 minutes; this two-step process produces cells that reflect 3-7% of the light. NREL’s Black Silicon Etch process requires less than three minutes to complete at room temperature, and creates PV cells that absorb more than 98% of the incident light. Additionally, the cells created will have improved performance during morning, evening, and diffuse light conditions, since the black material can absorb light at incident angles which conventional cells reflect. Black Silicon Etching has been proven to be 16.8% efficient for single-crystalline silicon cells and 14.9% efficient for multi-crystalline cells, eclipsing the best previous density-graded antireflection solar cells which are 13.9% efficient.
Solar cell manufacturing is an expensive, capital-intensive process. Black Silicon Etching is a scalable technology that reduces the capital costs of PV manufacturing. This new process reduces the LCOE by 2.5% which includes $6-15 million capital savings on standard 100MW PV lines. Additionally, this novel process reduces the use of hazardous gases, can be performed with any simple wet-chemistry equipment, consumes less power and generates fewer greenhouse gases, and is easily inserted into 85% of the market’s current manufacturing processes. Black Silicon Etching could greatly improve current PV technologies by increasing the conversion efficiency and decreasing the costs of solar power while integrating smoothly into legacy manufacturing systems.