Tuesday, February 26, 2013

Dye-Sensitized Solar Cells

DOE supports research and development projects aimed at increasing the efficiency and lifetime of dye-sensitized solar cells (DSSCs). Below are a list of current projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.

Background

DSSCs were developed in the early 1990s by Michael Grätzel of the Federal Polytechnic University in Lausanne, Switzerland. Although not yet in commercial production, dye-sensitized cells have achieved laboratory efficiencies of 12.3%. These solar cells rely on stable and easy-to-produce materials, making them an attractive alternative to polymer, bulk-heterojunction organic photovoltaics (OPV).
DSSCs have three separate components: the absorber dye, a metal-oxide electron-transfer network, and an electrolytic hole-transfer layer. The first component—the dye—is an organic or organometallic molecule with strong absorption in the visible spectrum that effectively absorbs a photon and generates an exciton. The second component—the metal-oxide electron-transfer layer—is a highly porous nanoscaffold that serves as a "home" for dye molecules and also acts as an electron highway between the excited dye molecule and the anode. The last component of the cell—the electrolyte or hole-transfer layer—replenishes these now electron-deficient dye molecules by ferrying away holes from the dye to the cathode.

Research Directions

Dye-sensitized cells have low efficiencies and limited durability compared to leading PV technologies. To increase durability, work is being done to convert the liquid hole-transport layer into a solid-state layer. Improved encapsulation will similarly help to mitigate reliability concerns.
Traditional dye-sensitized cells used dyes that incorporated ruthenium (Ru), an expensive metal. Significant research has been performed on zinc- or iron-based organometallic dyes and organic dyes, and the conversion efficiency using these dyes has recently surpassed that of ruthenium dyes. However, further improvements are still necessary to make dye-sensitized cells affordable and ready for market.

Benefits

The benefits of dye-sensitized solar cells include:

Low cost and ease of manufacturing: They hold the promise of being a very low-cost technology with great ease of manufacture.
Abundant materials: They rely on stable and abundant resource materials, especially if ruthenium-free dyes are used.

Production

Dye-sensitized cells are not currently produced in large-scale quantities.
Typically, the semiconductor framework anode is deposited first. This is a highly porous nanoscaffold typically made from titanium dioxide (TiO₂), although occasionally tin dioxide (SnO₂) or other metal oxides are used, as well. Typical deposition methods for nanoparticle networks include spin-coating or doctor-blade application of a colloidal nanoparticle solution, followed by sintering.
Dye is introduced into this network by soaking the metal-oxide anode in a dye solution. Dyes are often modeled after chlorophyll and are commonly porphyrins coordinated around a metal center. Originally, ruthenium was used, although recent efforts have successfully replaced ruthenium with zinc-based or metal-free dyes. During soaking, carboxylate, sulfonate, or phosphonate groups on the dye chelate to the metal oxide, providing strong adhesion.
The last production step is to create a hole transport layer. A liquid electrolyte redox couple, such as iodide/iodine (I^-/I₃^-), accomplishes this with a complicated series of redox reactions that ferry positive charge to the cathode. This process is effective, but the liquid nature of the redox couple makes these solar cells difficult to install in the field. Liquids have a tendency to expand and contract as the temperature changes, which can cause cracks in the cell over time. Solid-state remedies are being developed—such as polymer conductors, polymer-electrolyte gel frameworks, and small-molecule hole conductors. Electrolytes are injected into the metal-oxide framework so as to make contact with both the dye molecules and the cathode.

Organic Photovoltaics Research

DOE funds research and development projects related to organic photovoltaics (OPV) due to the unique benefits it offers. Below are a list of the projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.

Background

Organic photovoltaic (OPV) solar cells aim to provide an Earth-abundant and low-energy-production photovoltaic (PV) solution. This technology also has the theoretical potential to provide less expensive energy than first- and second-generation solar technologies. Because various absorbers can be used to create colored or transparent OPV devices, this technology is particularly appealing to the building-integrated PV market. Although OPV cells are fast approaching the 10% efficiency mark, efficiency limitations as well as long-term reliability remain significant barriers.
Unlike most inorganic solar cells, OPV cells use single-molecule absorbers. Therefore, an exciton is generated across a single molecule, instead of between bulk electronic states. These molecular absorbers are used in conjunction with an electron acceptor, such as a fullerene, which has molecular orbital energy states that facilitate electron transfer. Upon absorbing a photon, the resulting exciton migrates to the interface between the absorber material and the electron acceptor material. At the interface, the energetic mismatch of the molecular orbitals provides sufficient driving force to split the exciton and create free charge carriers (an electron and a hole).

Research Directions

The low efficiencies of OPV cells are related to their small exciton diffusion lengths (~ 10 nm) and low carrier mobilities (>5cm²/V^.s). These two characteristics ultimately result in the use of thin active layers that affect overall device performance. Furthermore, the operational lifetime of OPV modules remains significantly lower than for inorganic devices.
Current research focuses on increasing device efficiency and lifetime. Substantial efficiency gains have been achieved already by improving the absorber material, and research is being done to further optimize the absorbers and develop an organic multijunction architecture. Improved encapsulation and alternative contact materials are being investigated to reduce cell degradation and push cell lifetimes to industry-relevant values.
Learn more about the awardees and the projects involving organic PV below.

Benefits

The benefits promised by OPV solar cells include:

Low-cost manufacturing: Soluble organic molecules enable roll-to-roll processing techniques and allow for low-cost manufacturing.
Abundant materials: The wide abundance of building-block materials may reduce supply and price constraints.
Flexible substrates: The ability to be applied to flexible substrates permits a wide variety of uses.

Production

OPV cells are categorized into two classes:

Small-molecule OPV cells
Polymer-based OPV cells.

Small-molecule OPV cells use molecules with broad absorption in the visible and near-infrared portion of the electromagnetic spectrum. Highly conjugated systems are typically used for the electron-donating system such as phthalocyanines, polyacenes, and squarenes. Perylene dyes and fullerenes are often used as the electron-accepting systems. These devices are most commonly generated via vacuum deposition to create bilayer and tandem architectures. Recently, solution-processed small-molecule systems have been developed.
Polymer-based OPV cells use long-chained molecular systems for the electron-donating material (e.g., P3HT, MDMO-PPV), along with derivatized fullerenes as the electron-accepting system (e.g., PC₆₀BM, PC₇₀BM)) Like small-molecule OPV cells, these systems have small exciton diffusion lengths. However, this limitation is circumvented by a high interface surface area within the active device.
Dye-sensitized solar cells are a hybrid organic-inorganic technology that uses small-molecule absorber dyes. These dyes adsorb onto a suitable electron-accepting material, such as titanium dioxide or zinc oxide, along with an electrolyte to regenerate the dye. Unlike their OPV counterparts, dye-sensitized technologies have produced efficiencies beyond 10%; but currently, they require a liquid electrolyte within the solar cell, which limits their durability.

Copper Indium Gallium Diselenide

DOE-sponsored research on copper indium gallium diselenide [Cu(In_xGa_1-x)Se₂], or CIGS, solar cells focuses on developing better film qualities, and thus, higher efficiencies. A list of current projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology are below.

Background

Since its initial development, copper indium diselenide (CuInSe₂) thin-film technology has been considered promising for solar cells because of its favorable electronic and optical properties. It was later found that by substituting gallium (Ga) for indium (In), the bandgap can be increased from about 1.04 electron-volts (eV) for copper indium diselenide (CIS) films to about 1.68 eV for copper gallium diselenide (CGS) films. Optimal devices have been fabricated with only a partial substitution of Ga for In, leading to a substantial increase in overall efficiency and more optimal bandgap. These solar cells are commonly known as a copper indium gallium diselenide [Cu(In_xGa_1-x)Se₂], or CIGS, cells.
Although laboratory-scale cell efficiencies have exceeded 20%, commercial CIGS modules typically have efficiencies between 12% and 14%.

Research Directions

Historically, cadmium sulfide (CdS) has provided an adequate window layer, but its low bandgap prevents many high-energy photons from reaching the CIGS absorber layer. A suitable replacement for the CdS window layer is being investigated as a way to increase voltage and current.
Learn more about the DOE SunShot awardees and the projects involving CIGS below.

Benefits

The benefits of CIGS solar cells include:

High absorption: This direct-bandgap material can absorb a significant portion of the solar spectrum, enabling it to achieve the highest efficiency of any thin-film technology.
Tandem design: A tunable bandgap allows the possibility of tandem CIGS devices.
Protective buffer layer: The grain boundaries form an inherent buffer layer, preventing surface recombination and allowing for films with grain sizes of less than 1 micrometer to be used in device fabrication.

Production

Two of the low-cost deposition methods that produce the highest device and module efficiencies were developed in the 1980s. These methods are:

Co-evaporation, in which precursor elements are allowed to sublimate in a high-vacuum environment and then re-deposit on a heated substrate.
Precursor Reaction Processes, in which a precursor containing Cu and In/Ga is deposited at a low temperature by any of several processes, such as sputtering or electroplating. This is followed by a reactive annealing step in a Se compound, such as hydrogen selenide (H₂Se) or gaseous selenium (Se), to form CIGS films. This is also commonly known as two-stage deposition; a variant of this technique, three-stage deposition, is also commonly used.

After the CIGS deposition, the junction is formed by chemical-bath deposition of the n-type CdS layer. To finish the solar cell, a high-resistance zinc oxide (ZnO) layer and a high-conductivity n+-type ZnO layer are deposited by either sputtering or chemical-vapor deposition. Laser-scribing processes at different steps in the production process create the individual solar cells connected in series.
Alternative manufacturing techniques have been explored, such as reactive sputtering, magnetron sputtering (Cu, In, and Ga are sputtered while Se is evaporated), and electrodeposition. However, co-evaporation and precursor reaction processes still remain the most popular.
A major increase in device performance was achieved when the ceramic or borosilicate glass substrate was replaced by soda-lime glass. Although soda-lime glass was chosen because it has closer thermal expansion properties to CIGS, it was ultimately determined that the primary advantage of using soda-lime glass results from the diffusion of sodium (Na) ions from the glass into the CIGS absorber layer. Work is currently being done to identify the role of Na in improving CIGS performance and what tolerances CIGS has to the inclusion of Na. Current manufacturing techniques incorporate Na either from soda-lime glass or a separate Na source. Soda-lime glass has an added advantage of being less expensive than previous glass substrates.
All high‐efficiency CIS and CIGS devices use molybdenum (Mo) as the back contact primarily because of its work function and the high reflectivity of the Mo film. These films are typically deposited through direct-current (DC) sputtering. The sputtering deposition process requires precise pressure to control the stress in the film. Because of some inherent problems with the Mo back-contact, such as the possibility of a hole-blocking Schottky diode effect at the interface, other metals have been investigated to replace Mo, but have had limited success.

Cadmium Telluride

DOE supports innovative research focused on overcoming the current technological and commercial barriers for cadmium telluride (CdTe) solar cells. Below are a list of current projects, summary of the benefits, and discussion of the production and manufacturing techniques used for this solar technology.

Background

CdTe solar cells are the second most abundant photovoltaic (PV) technology in the world marketplace after crystalline silicon, currently representing 6% of the 2011 world market. CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing a lower-cost alternative to conventional silicon-based technologies. The record efficiency for a laboratory CdTe solar cell is 17.3%, which is well above current commercial CdTe modules, which have efficiencies between 10% and 12.4%.

Research Directions

Current projects seek higher cell efficiencies by increasing crystal quality, improving doping control, and increasing the minority lifetime. Manufacturers are also investigating the possibility of materials reuse and recycling as a way to mitigate concerns on toxicity and materials scarcity.
Learn more about the DOE SunShot awardees and the projects involving CdTe below.

Benefits

The benefits of CdTe thin-film solar cells include:

High absorption: Cadmium telluride is a direct-bandgap material with bandgap energy of about 1.45 defined (eV), which is well matched to the solar spectrum and nearly optimal for converting sunlight into electricity using a single junction.
Low-cost manufacturing: Cadmium telluride solar cells use low-cost manufacturing technology to produce low-cost cells.

Production

The most common CdTe solar cells consist of a simple p-n heterojunction structure containing a p-doped CdTe layer matched with an n-doped cadmium sulfide (CdS) layer, which acts as a window layer. This structure is similar to the heterojunction in CIGS cells. As with most thin-film solar technologies, carrier collection is accomplished by drift, or field-assisted collection.
Typical CdTe thin-film deposition techniques include: close-spaced sublimation, vapor-transport deposition, physical-vapor deposition, sputter deposition, electrodeposition, metal-organic chemical-vapor deposition, spray deposition, and screen-print deposition.
CdTe solar cells are completed by adding a high-quality transparent conductive oxide (TCO)—usually fluorine-doped tin oxide (SnO₂:F)—and a back electrical contact—typically a metal or carbon paste with copper (Cu). One disadvantage to using Cu in the back contact is the gradual diffusion of Cu atoms into the CdTe and CdS layers, which creates defects and promotes Cu accumulation at the CdTe/CdS junction.
Overall CdTe solar cell performance was significantly improved after the discovery of a cadmium chloride (CdCl₂) vapor treatment. This annealing process is accomplished in the presence of oxygen at temperatures near 390°C after the CdTe layer is grown on the CdS layer and prior to the back-contact deposition. The CdCl₂ treatment has positive effects on the CdTe solar cell, such as growth of larger CdTe grains and the passivation of defects.

Amorphous Silicon

DOE has a proven track record of funding successes in amorphous silicon (a-Si)research. A list of current projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology are below.

Background

Thin-film a-Si solar cells are commonly known as hydrogenated amorphous silicon, or a-Si:H. Currently, laboratory-scale cells achieve conversion efficiencies of 12.5%, whereas cells manufactured in high-volume processes have efficiencies ranging from 6% to 9%. Although these efficiencies are significantly lower than those of crystalline silicon solar cells, these thin-film cells are lighter, more flexible, and less expensive to produce. Amorphous silicon solar cells represented about 3% of the 2011 world market.

Research Directions

The efficiency of amorphous silicon solar cells decreases rapidly on its first exposure to sunlight, reaching a relatively steady state after about 1,000 hours of illumination. This phenomenon, first described in 1977 by D.L. Staebler and C.R. Wronski, results from the creation of additional dangling bonds that act as recombination centers.
Current research is focused on improving thin-film quality and reducing the Staebler-Wronski effect, by improved manufacturing techniques, as well as developing thin, flexible, waterproof roof shingles.
Learn more about the DOE SunShot awardees and the projects involving amorphous silicon below.

Colorado School of Mines (Next Generation Photovoltaics II Projects)

Benefits

The benefits of amorphous silicon solar cells include:

Less material: Amorphous silicon is a direct-bandgap material, which means that less silicon is needed for a-Si cells.
Inexpensive substrates: Amorphous silicon can be deposited on inexpensive substrates, such as glass, stainless steel, or even plastic (compared to bulk silicon wafers), which lowers costs.
Manufacturing options: Amorphous silicon can be deposited at temperatures below 300°C, making it a good candidate for flexible substrates and roll-to-roll manufacturing processes.

Production

By nature, amorphous silicon contains numerous crystal defects and requires hydrogen atoms to passivate the intrinsically high concentration of dangling bonds. Doping is detrimental to the quality of the absorber layer because it may lead to silicon dangling bonds—and therefore, to recombination centers. The minority-carrier diffusion lengths are only about 0.1 micrometer and amorphous silicon has a low carrier mobility; therefore, diffusion alone does not provide sufficient carrier collection. To increase the carrier collection, the concept of the "p-i-n solar cell" was introduced by Carlson and Wronski in 1976. The p-i-n structure generates an electric field across the thicker intrinsic (i) layer of the device, which possesses the best characteristics for absorption, photogeneration, and carrier lifetime. Carrier collection across the intrinsic region is via a drift mechanism—i.e., assisted by the electric field.
The most efficient commercial amorphous silicon PV cells are typically produced by silane-based glow discharge induced by radio-frequency (RF) voltages, or plasma-enhanced chemical-vapor deposition (PECVD), with other gases added for doping and alloying. The silane decomposes spontaneously at temperatures above 450°C, forming polycrystalline silicon. The concentration of hydrogen in silane can be used to control the growth of the material. Although there are multiple methods of producing amorphous silicon, such as hot-wire glow-discharge deposition and indirect microwave deposition, the most-common method remains RF PECVD.

Crystalline Silicon Photovoltaics Research

DOE supports crystalline silicon photovoltaic (PV) research and development efforts that lead to market-ready technologies. Below are a list of the projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.

Background

Crystalline silicon PV cells are the most common solar cells used in commercially available solar panels, representing 87% of world PV cell market sales in 2011. Crystalline silicon PV cells have laboratory energy conversion efficiencies as high as 25% for single-crystal cells and 20.4% for multicrystalline cells. However, industrially produced solar modules currently achieve efficiencies ranging from 18%–24%.

Research Directions

Current DOE research efforts focus on innovative ways to reduce costs. Research and development is being done to reduce raw material requirements, including pioneering ultra-thin crystalline silicon absorber layers, developing kerf-free wafer production techniques (kerf is silicon dust that is wasted when silicon ingots are cut into thin wafers), and optimizing growth processes.
Learn more about the DOE SunShot awardees and the projects involving crystalline silicon below.

Benefits

The benefits of crystalline silicon solar cells include:

Maturity: There is a considerable amount of information on evaluating the reliability and robustness of the design, which is crucial to obtaining capital for deployment projects.
Performance: A standard industrially produced silicon cell offers higher efficiencies than any other mass-produced single-junction device. Higher efficiencies reduce the cost of the final installation because fewer solar cells need to be manufactured and installed for a given output.
Reliability: Crystalline silicon cells reach module lifetimes of 25+ years and exhibit little long-term degradation.
Abundance: Silicon is the second most abundant element in Earth's crust (after oxygen).

Production

Typical crystalline silicon solar cells are produced from monocrystalline (single-crystal) silicon or multicrystalline silicon. Monocrystalline cells are produced from pseudo-square silicon wafers, substrates cut from boules grown by the Czochralski process, the float-zone technique, ribbon growth, or other emerging techniques. Multicrystalline silicon solar cells are traditionally made from square silicon substrates cut from ingots cast in quartz crucibles. More information on these production techniques and the types of silicon used in photovoltaics can be found at the Energy Basics website.
To reduce the amount of light reflected by the solar cell—and therefore not used to generate current—an antireflective coating (ARC), often titanium dioxide (TiO₂) or silicon nitride (SiN), is deposited on the silicon surface. To increase light trapping and absorption, the top of the solar cell can be textured with micrometer‐sized pyramidal structures, formed by a chemical etch process.
To create a p-n junction, typically a phosphorus-doped n⁺ region is created on top of a boron-doped p-type silicon substrate. A metal electrode, such as aluminum, forms the back contact, whereas the front contact is most often formed using screen-printed silver paste applied on the top of the ARC layer.
Charge-carrier collection in a crystalline silicon solar cell is achieved by minority-carrier diffusion within the p‐doped and n‐doped layers. Long diffusion lengths (> 200 micrometers) assist carrier collection over the entire range of the solar cell thickness where the optical absorption occurs.

Photovoltaic Supply Chain and Cross-Cutting Technologies

Four projects are working to accelerate the development of revolutionary products or processes for the photovoltaic (PV) industry through the High Impact Supply Chain R&D for PV Technologies/Systems program, which represents the second round of PV Supply Chain and Cross-Cutting Technologies funding. These projects encourage innovation in companies across the solar energy supply chain and develop PV-specific solutions from non-solar innovations, including:

Processing steps that improve throughput, yield, or diagnostics
Materials that improve reliability or enhance optical, thermal, or electrical performance
System components that streamline installation.

On Feb. 4, 2011, DOE announced $20.3 million to fund these projects. The awardees are targeting manufacturing and product cost reductions with the potential to have an impact within 2 to 6 years on a substantial segment of the PV industry.

Awardees

1366 Technologies, Inc. ($3 million)
Lexington, Massachusetts
Silicon wafers remain the single largest cost component in the manufacture of silicon PV modules. The high cost stems from inefficient ingot casting and sawing, which can result in 50% of purified and crystallized silicon wasted as kerf. In turn, scaling production is slowed by the high capital costs of silicon refining, ingot making, and sawing facilities. The 1366 Direct Wafer manufacturing process reduces wafer costs by 60%, eliminating the cost barrier imposed by sawn wafers. The kerf-free, 156 mm standard silicon wafers allow high throughput for very low capital expense and rapid scale up. The heat transfer, which is perpendicular to the plane of the wafer, creates thin, flat, low stress, high lifetime wafers.
3M Company ($4.4 million)
St. Paul, Minnesota
3M’s objective is to develop and commercialize a flexible, highly transparent ultra barrier topsheet (UBT) that enables successful commercialization and growth of flexible solar modules manufactured from second and third generation photovoltaic technologies. This UBT is being manufactured with a proprietary high volume, low cost roll-to-roll process that has the potential to meet or exceed the technical requirements for these solar technologies. Successful commercialization of the UBT could have a dramatic impact on the solar industry by reducing total costs for installation in current markets such as commercial rooftop and residential building integrated photovoltaics (BIPV). Furthermore, it could also grow the range of applications to newer markets, such as consumer products and automotive applications.
PPG Industries, Inc. ($3.1 million)
Cheswick, Pennsylvania
The goal of this project is to develop the materials, coating designs, and manufacturing processes necessary to commercialize a new glass article for the cadmium telluride (CdTe) module manufacturing industry. This new glass article combines an improved transparent conductive oxide plus buffer layer, a high transmission glass substrate, and a low-soiling anti-reflective (LSAR) coating into one product offering. The combination of these various technologies into a single product results in performance gains and improvements in module cost. The cost reductions stem from the choice of deposition technology, scaling to high volume manufacturing, and systems integration of multiple coating operations.
Varian Semiconductor Equipment Associates, Inc. ($4.8 million)
Gloucester, Massachusetts
The goal of this project is to reduce the cost of manufacturing interdigitated back contact (IBC) cells—the most efficient solar cells on the market. Although IBC cells typically achieve 23% efficiency, they have gained only a small market share because of the high cost of creating the interdigitated doped regions by diffusion. This project is replacing diffusion with in situ patterned ion implantation, thereby reducing the number of steps in IBC fabrication from 26 to 10 and reducing the associated manufacturing cost per wafer

Solar Incubator Program

Photo of two men talking in front of a poster display.

William Parish from Solar Mosaic, one of nine solar startups chosen to participate in SunShot Incubator 6, discusses his company's project with Energy Secretary Steven Chu at the SunShot Grand Challenge Summit in Denver, Colorado. Photo by John De La Rosa

Fifty-four startups have participated in the SunShot Incubator program since it began in 2007. These DOE solar projects are accelerating technological innovation for:

Photovoltaic (PV) technologies
Concentrating solar power (CSP) technologies
Power electronics
Balance-of-system (BOS) hardware
Balance-of-system non-hardware (Soft Costs).

The Incubator program provides early-stage assistance to help startup companies cross technological barriers to commercialization while encouraging private sector investment. Since the program was launched in 2007, $92 million in government funds has leveraged more than $1.7 billion in venture capital and private equity investment, demonstrating a ratio of nearly $20 in subsequent private sector support for every $1 of federal support.

Objectives

The SunShot Incubator program aims to shorten the time between laboratory-scale proof of concept and prototype development and accelerate the process for companies to transition pre-commercial prototypes through the pilot stage into full-scale manufacture. Most projects are cooperative agreements that last from twelve to eighteen months with payment made upon completion and verification of aggressive project deliverables.

Approach

The SunShot Incubator Program uses a two-tiered approach to accomplish its objectives.

Tier 1 speeds the development of innovative solar hardware and soft cost concepts to the prototype stage. Generally, Tier 1 awards are provided to applicants that have a proof-of-concept or early prototype design or device and need to advance their design or assembly process to produce a commercially-relevant prototype.
Tier 2 aims to shorten the timeline for awardees to transition innovative materials, devices, systems, or ideas into pilot and eventually full-scale manufacturing, production, or deployment. Successful participation in this program accelerates the transition to full commercial production or product release.

The Incubator program was originally created to support innovative solar startups working to develop and launch transformative PV technologies. Over the past five years, the program has evolved to take an all-inclusive approach to significantly lower the total installed cost of solar energy systems. Visit the Financial Opportunities page for more information about future funding rounds.

Awardees

Current Projects

Incubator 7 (2012)

AmberWave, Inc. (Salem, New Hampshire)
Bandgap Engineering (Woburn, Massachusetts)
Enki Technology (San Jose, California)
Infinite Invention, LLC (Philadelphia, Pennsylvania)
Princeton Power Systems (Lawrenceville, New Jersey)
Qado Energy, Inc. (Summit, New Jersey)
QBotix, Inc. (Menlo Park, California)
REhnu, Inc. (Tucson, Arizona)
Seeo (Hayward, California)
Solaflect Energy (Norwich, Vermont)
Stion (San Jose, California)

Incubator 6 (2012)

Clean Energy Experts (Manhattan Beach, California)
Clean Power Finance (San Francisco, California)—Tier 1 and 2 awards
concept3D
Distributed Energy Research & Solutions (Cambridge, Massachusetts)
Genability (San Francisco, California)
Simply Civic (Parker, Colorado)
Solar Mosaic (Berkeley, California)
Tigo Energy (Los Gatos, California)
Urban Glue (Deephaven, Minnesota)

Incubator 5 (2011)

Halotechnics (Emeryville, California)
Tigo Energy (Los Gatos, California)

Incubator 4 (2010)

Caelux (Pasadena, California)
Crystal Solar (Santa Clara, California)

Past Projects

Incubator 5 (2011)

Renewable Power Conversion (San Luis Obispo, California)
Solaflect Energy (Norwich, Vermont)

Incubator 4 (2010)

Solexant (San Jose, California)
Stion (San Jose, California)

Incubator 3 (2009)

Alta Devices, Inc. (Santa Clara, California)
Semprius, Inc. (Durham, North Carolina)
Solar Junction (San Jose, California)
Tetra Sun (Saratoga, California)

Pre-Incubator (2009)

1366 Technologies, Inc. (Lexington, Massachusetts)
Ascent Solar Technologies, Inc. (Littleton, Colorado)
Banyan Energy, Inc. (Kensington, California)
Crystal Solar, Inc. (Santa Clara, California)
EPIR Technologies, Inc. (Bolingbrook, Illinois)
Lightwave Power, Inc. (Cambridge, Massachusetts)
Luna Innovations, Inc. (Danville, Virginia)
MicroLink Devices (Niles, Illinois)
SpectraWatt, Inc. (Hillsboro, Oregon)
TiSol, LLC (Pasadena, California)
Vanguard Solar, Inc. (Sudbury, Massachusetts)

Incubator 2 (2008)

1366 Technologies (Lexington, Massachusetts)
Innovalight (Sunnyvale, California)
Skyline Solar (Mountain View, California)
Solasta (Newton, Massachusetts)
Solexel (Milpitas, California)
Spire Semiconductor (Hudson, New Hampshire)

Incubator 1 (2007)

Abound Solar (Fort Collins, Colorado)
Blue Square Energy (North East, Maryland)
CaliSolar (Menlo Park, California)
Enfocus Engineering (Sunnyvale, California)
MicroLink Devices, Inc. (Niles, Illinois)
Plextronics (Pittsburgh, Pennsylvania)
PrimeStar Solar (Golden, Colorado)
Solaria (Fremont, California)
SolFocus (Palo Alto, California)
SoloPower (Milpitas, California)

Earth-Abundant Materials

DOE funds research into Earth-abundant materials for thin-film solar applications in response to the issue of materials scarcity surrounding other photovoltaic (PV) technologies. Below are a list of the projects, summary of the benefits, and discussion on the production and manufacturing of this solar technology.

Background

Currently, the most promising alternative to resource-intensive photovoltaic technologies such as CIGS and CdTe is copper zinc tin sulfoselenide (Cu₂ZnSnSe_xS_4-x, or simply CZTS). Other alternatives, such as lead sulfide (PbS) and pyrite (FeS₂)-based materials, have also garnered attention. This section focuses on CZTS as a model system for Earth-abundant chalcogenide absorbers.
CZTS is very similar to CIGS in optoelectronic and crystallographic properties, as well as in methods of fabrication. However, CZTS has a laboratory efficiency of just above 10%, which is about half that of CIGS cells. Kesterite CZTS is very similar to the chalcopyrite crystal structure, but with the Group-III indium (In) and gallium (Ga) ions replaced in an ordered manner with an equal number of Group-II zinc ions and Group-IV tin ions. This maintains many of the optoelectronic properties of CIGS, but eliminates the need for the expensive In and Ga metals.

Research Directions

For CZTS to be commercially viable, many improvements are needed. For example, a better understanding of the defect physics of CZTS is essential, because defect-based recombination at the interfaces appears to be the limiting factor in efficiency. Currently, work is being done to determine the primary recombination pathways in CZTS and to what extent the CZTS defect structure behaves like that of CIGS.
Additionally, similar to CIGS cells, CZTS cells would benefit from an optimized back contact and window layer. Researchers are investigating replacements for the CdS window layer in CZTS cells, and an optimal back-contact material.
Researchers are also developing other Earth-abundant materials, such as iron sulfide (FeS₂), commonly known as pyrite, and lead sulfide (PbS). These materials exhibit solar-relevant properties and have similarities to other solar absorber materials.
Learn more about the DOE SunShot awardees and the projects involving CZTS below.

Learn more about the awardees and the projects involving non-silicon Earth-abundant solar cell technologies below.

Benefits

The benefits of CZTS solar cells include:

Abundant, low-cost materials: Because of its similar thermal expansion and crystal lattice constants, CZTS can be put in a solar cell with a CdS (cadmium sulfide) window layer and molybdenum (Mo) back contact and derive about the same functionality as a CIGS cell, while using more abundant and less costly materials for the absorber layer.
Tunable bandgap: CZTS has a tunable bandgap that can be modified from ~1 electron-volt (eV) for copper zinc tin selenide to ~1.5 eV for copper zinc tin sulfide by modulating the ratio of S to Se. Moreover, by doping the film with Ga, researchers have reached bandgaps as high as 2.25 eV.

Production

Neither non-silicon Earth-abundant nor CZTS solar cells are currently produced in large quantities. Record CZTS cells have been produced using a CZTS solution in hydrazine. This solution is then deposited onto a substrate and annealed. Using this process, efficiencies of greater than 10% have been achieved.
Similar to CIGS, including sodium appears to be important to CZTS performance, but more work is needed to fully understand this phenomenon in CZTS.

SunShot Initiative

Next Generation Photovoltaics II

Twenty-three solar projects are investigating transformational photovoltaic (PV) technologies with the potential to meet SunShot cost targets. The projects' goals are to:

Increase efficiency
Reduce costs
Improve reliability
Create more secure and sustainable supply chains.

On Sept. 1, 2011, the U.S. Department of Energy (DOE) announced $24.5 million to fund the Next Generation Photovoltaics II projects over a performance period of either two years or four years. This early-stage applied research investment seeks to not only demonstrate new photovoltaic concepts, but also to train the next generation of graduate students and post-doctoral fellows who will ultimately lead the development and commercialization of PV technologies in future years.

Awardees

Bandgap Engineering ($750,000)
Woburn, Massachusetts
In this project, silicon (Si) nanowire arrays are being used to engineer an intermediate band solar cell (IBSC). The IBSC has a theoretical efficiency of up to 60%; however, the goal is to engineer a 36% efficient solar cell made only with Si. Bandgap Engineering is seeking the early phase demonstration of an IBSC material produced by growing the Si nanowires epitaxially on the surface of an oriented Si wafer to achieve accurate control over crystallographic orientation and faceting of the nanowires, which will selectively increase coupling between specific electronic states.

California Institute of Technology ($750,000)
Pasadena, CA
The goal of this project is to develop a waferless, flexible, low-cost, tandem, multijunction, wire-array solar cell that combines the efficiency of wafered crystalline silicon (c-Si) technologies with the cost and simplicity of thin-film technologies. The approach synthesizes tandem solar cells by conformal epitaxial growth of III-V compound, semiconductor, wide-bandgap absorber layers to form dual-junction and triple-junction wire array tandem solar cells. Such high-efficiency multijunction wire arrays represent a transformational, and as-yet unrealized, opportunity for low-cost, high-efficiency photovoltaics.

Colorado School of Mines ($1,484,364)
Golden, Colorado
Researchers on this project are developing a new approach to the synthesis of hydrogenated nanocrystalline silicon (nc-Si:H), which exploits hot-carrier collection as a way of boosting conversion. By using a novel gas-phase plasma process, the research team is creating engineered films that incorporate quantum-confined Si nanocrystals with tailored surface termination. These engineered composites of amorphous and nanocrystalline Si have the potential to dramatically increase the efficiency of single junction and multijunction thin-film Si solar cells by mitigating photo-induced degradation, allowing increased absorption, and offering the realistic possibility of hot-carrier devices.

Massachusetts Institute of Technology ($750,000)
Cambridge, Massachusetts
In this project, MIT researchers are developing c-Si thin-film solar cells with a thickness of less than 10 microns at efficiencies greater than 20%. Typical c-Si wafers are about 180–250 micrometers thick and account for approximately 30%–40% of the total module cost. By dramatically reducing the size through nanostructuring surfaces, developing high-performance transparent conductors, and identifying low-cost manufacturing processes, this research effort aims to open a new pathway for meeting cost targets.

Massachusetts Institute of Technology ($1,500,000)
Cambridge, Massachusetts
MIT is using systematic defect engineering to advance thin-film PV cells based on tin sulfide (SnS), which offers high optical absorption, high carrier mobilities, and long minority lifetimes. Because tin and sulfur are earth-abundant and require processing temperatures below 400°C, use of these materials in thin-film PV cells has the potential to lead to low-cost fabrication. A rapid ramp-up of efficiency is also possible by leveraging the decades of development of similar thin-film materials.

National Renewable Energy Laboratory ($750,000)
Golden, Colorado
NREL, together with MIT, is developing a novel class of earth-abundant materials for single-junction, tandem-junction, or multijunction thin-film PV applications that can be synthesized with low-cost, scalable methods. A team of NREL researchers has completed proof-of-concept synthesis and characterization of these novel materials. Preliminary results have demonstrated facile synthesis of materials with independent tunability of key material properties, which has the potential to reduce costs. The team is performing exploratory research on these promising materials with a goal to fabricate baseline PV devices by the end of the project.

National Renewable Energy Laboratory ($750,000)
Golden, Colorado
The research team for this project, which includes partners from Colorado School of Mines and Cornell University, is working to establish a new solar cell paradigm of ternary copper nitride absorbers (Cu-M-N). These absorbers are expected to have favorable properties because of the large valence-band dispersion that results from a nearly perfect energy match of Cu and N energy levels. This match may lead to a defect immunity similar to that exhibited by copper indium gallium diselenide (CIGS) devices. The primary objectives of this project are to identify earth-abundant, thermodynamically stable, and nonreactive Cu-M-N materials, determine their exact chemical stoichiometry and crystallographic structure, and study their physical properties related to photovoltaic applications.

National Renewable Energy Laboratory ($750,000)
Golden, Colorado
The aim of this project is to develop a novel (Zn,Mg)Cu oxysulfide solar absorber material with the potential to reach and exceed 20% energy conversion efficiency. The research team is substantially modifying the Cu₂O base material by alloying with sulfur, zinc, and magnesium. This, in effect, is tailoring the band-structure properties to match the solar spectrum. In developing a novel optimized solar absorber material, the team is employing both theoretical modeling with electronic structure methods and combinatorial thin-film synthesis and characterization.

National Renewable Energy Laboratory ($750,000)
Golden, Colorado
NREL seeks to dramatically improve solar photoconversion efficiency in amorphous silicon (a-Si) and organic-based photovoltaic (PV) technologies by breaking the Shockley-Queisser limit. The research team is implementing a strategy that allows single-junction solar cells to harvest a wider portion of the solar spectrum effectively with a system that can convert low-energy (red to near-infrared) photons to higher-energy (visible) photons. This molecular upconversion approach allows for a substantial increase in photocurrent and a high open-circuit voltage with only marginal cost increases.

PLANT PV ($750,000)
Berkeley, California
PLANT PV is studying the feasibility of using cadmium selenide (CdSe) as the wide band-gap top cell and Si as the bottom cell in a monolithically integrated tandem architecture. The greatest challenge in developing efficient tandem solar cells is achieving a high open circuit voltage (V_oc) with the top cell. To achieve tandem power conversion efficiencies greater than 25%, the CdSe top cell must have a V_oc greater than 1.1 V. Through this project, PLANT PV seeks to determine whether it is possible to epitaxially grow CdSe films with sufficient minority carrier lifetimes and with p-type doping levels necessary to produce an open-circuit voltage greater than 1.1 V using close-space sublimation.

Princeton University ($1,476,609)
Princeton, New Jersey
Researchers on this project are developing silicon/organic heterojunctions (SOH) as a new class of high-efficiency, low-cost photovoltaic technology. In SOH cells, the light is absorbed in silicon just like in conventional crystalline and multi-crystalline silicon photovoltaics, but there is no p-n junction. Instead, the carriers are separated by the field in the silicon created by a silicon/organic heterojunction. These devices are fabricated by spin-coating or spraying a thin layer of an organic semiconductor on silicon. This low-cost room-temperature process eliminates the need for any expensive high-temperature diffusion steps required to fabricate p-n junctions.

Purdue University ($750,000)
West Lafayette, Indiana
This project combines earth-abundant copper zinc tin sulfide (CZTS) semiconductor technology with the low-cost and scalable nanocrystal ink technique. This approach has several inherent benefits that contribute to lower module cost, including the ability to uniformly coat large area substrates, automate manufacturing, and reduce labor with faster throughput. The resulting thin-film solar cells are expected to offer high optical absorption coefficients with significantly reduced material and processing costs.

Sandia National Laboratories ($749,853)
Livermore, California
The goal of this research is to introduce a new photovoltaic material—crystalline nanoporous framework (CNF)—that allows detailed control of key interactions at the nanoscale level. This approach can overcome the disorder and limited synthetic control inherent in conventional bulk heterojunction photovoltaic materials. The research team is designing and synthesizing semiconducting CNFs—infiltrating their pores with a complimentary donor or acceptor—and fabricating prototype photovoltaic cells using CNF-composite active layers. This research is reducing the distance that excitons travel before meeting a charge-separating heterojunction, creating a tunable donor-acceptor offset, and maximizing exciton splitting and carrier mobility by eliminating disorder and defects that inhibit charge transport.

Stanford University ($1,380,470)
Stanford, California
This effort aims to develop an efficient upconverting medium capable of converting low-energy transmitted photons to higher-energy photons, which can then be absorbed by any type of commercial solar cell. The research team is using electrodynamic simulations and ab-initio quantum computations to optimize existing upconversion processes and design new molecular complexes for high-efficiency photovoltaic upconversion. This technology promises broadband upconversion at low incident power in a solution-processable, scalable platform.

University of California, Berkeley ($1,500,000)
Berkeley, California
Researchers on this project are developing a unique method to grow defect-free, III-V compound micro-pillar structures on single- and poly-crystalline silicon substrates. This approach combines the high conversion efficiencies of compound semiconductor materials with the low costs and scalability of silicon-based materials. The dense forest of micron-sized indium gallium arsenide/ indium phosphide (InGaAs/InP) pillars is excellent for omnidirectional, broadband light trapping and for reducing the amount of rare-earth materials required.

University of California Irvine ($1,422,130)
Irvine, California
The goal of this project is to build a prototype solar cell made from nontoxic, inexpensive, and earth-abundant iron pyrite (FeS₂), also known as Fool's Gold, with an efficiency of 10% or greater. The research team is developing a stable p-n heterojunction using innovative solution-phase pyrite growth and defect passivation techniques. A pyrite-based device offers a clear pathway to meeting SunShot cost targets, 20% module efficiency, and terawatt scalability using a proven, manufacturable geometry that is suitable for rapid scale-up by a U.S. thin-film photovoltaic industrial partner.

University of California, Los Angeles ($1,500,000)
Los Angeles, California
The primary objective of this project is to identify and develop an appropriate III-Sb quantum dot absorbing medium for intermediate band solar cells (IBSC) via thorough experimental analysis supported by sophisticated band structure modeling. The limiting efficiency of IBSC is on par with three solar cells operating in tandem, though it may have reduced complexity and cost. Supported by a team of internationally recognized experts, both graduate and undergraduate students are working on band structure calculations, quantum dot solar cell device design, materials development, and in-depth experimental analysis.

University of Chicago ($1,500,000)
Chicago, Illinois
Researchers on this project are developing solution-processed, all-inorganic photovoltaic absorber layers composed of colloidal nanocrystals, such as cadmium telluride (CdTe) and lead sulfide (PbS). These are being electronically coupled through novel molecular metal chalcogenide ligands, which provide band-like carrier transport while preserving advantageous quantum confinement effects. At the end of the project, the research team anticipates delivering an inexpensive tandem cell using nanocrystals of CdTe for the top and PbS for bottom junctions with 20% efficiency.

University of Delaware ($1,278,110)
Newark, Delaware
Through this project, the university research team is addressing the efficiency limit and high fabrication cost of current light-trapping methods by developing novel low-symmetry gratings (LSG) for next-generation thin crystalline silicon (c-Si) and copper indium gallium selenide (Cu(InGa)Se₂ or CIGS) photovoltaic solar cells. The LSG design achieves light-trapping enhancement exceeding the 4n² Lambertian limit within a specified range of photon wavelengths and can be fabricated using a low-cost, single-step nano-imprint/molding technique. The researchers are also using deposited high-refractive-index glass materials for low-temperature LSG processing, which enables direct imprint/molding sculpting of even complex grating geometries without requiring an additional pattern transfer step.

University of Michigan ($1,500,000)
Ann Arbor, Michigan
This research effort addresses efficiency, reliability, and scalability (i.e., cost) issues that must be resolved to transform organic photovoltaics into a competitively viable solution. The methods for accomplishing this are based primarily on small molecular-weight organic nanocrystalline cells that are stacked to form a high-efficiency tandem architecture. The research team, which includes doctoral candidates as well as undergraduates, is using low-cost light in-coupling schemes to enhance efficiency, exploring deposition by the scalable and manufacturing-ready technologies of liquid phase and organic vapor phase deposition, and subjecting prototype devices to realistic reliability testing.

University of Minnesota ($1,500,000)
Minneapolis, Minnesota
Researchers on this project are aiming to demonstrate the first functional copper indium aluminum gallium diselenide / copper indium gallium diselenide (CIAGS/CIGS) tandem solar cell through the use of novel materials and processes. The researchers are combining aluminum with both gallium and indium to form a wide bandgap absorber. They are also developing a novel tunnel junction using thermal and air-stable oxides. Finally, they are introducing a graded Cd_xZn_1-xS layer using a novel continuous flow chemical bath deposition system. This system can better control process conditions, reduce particle loading, and allow well-controlled graded films.

University of Washington ($492,865)
Seattle, Washington
The University of Washington research team is employing the solution-phase chemistry methods used for CZTSSe device fabrication to conduct high-throughput experiments with a novel combinatorial deposition platform. This approach allows for the discovery of alloying and doping strategies that produce a back surface field and defect passivation strategies that can dramatically decrease recombination and increase the minority carrier lifetime. By using photoluminescence, current-voltage, capacitance-voltage, and external quantum efficiency analysis, the researchers hope to rapidly converge on practical routes to high-efficiency CZTSSe-based solar cells.

University of Wisconsin–Madison ($462,508)
Madison, Wisconsin
In this project, the research team is developing nanostructures of pyrite (FeS₂) semiconductor to overcome material bottlenecks and allow for application in high-performance solar PV devices. This effort is aimed at developing effective doping methods and improved surface passivation strategies, as well as suitable nanoscale heterostructures of pyrite with other semiconductors. This exploratory research is demonstrating the proof-of-concept of a novel earth-abundant solar material while developing the understanding, materials, and processes needed for its deployment.

Sunday, February 24, 2013

Motorola's solar LCD to charge your cell phone

Requires relatively small amount of light

By Sumner Lemon

IDG News Service - Motorola Inc. has patented a way of keeping your mobile phone charged using only sunlight.

The company was recently issued a patent for an LCD (liquid crystal display) that includes solar cells capable of charging the battery in a mobile phone or other portable device.

The basic premise has been proposed before: a display screen is stacked over one or more solar cells, which are charged by the light passing through the display. But earlier designs allowed a relatively small amount of light to reach the solar cells, resulting in very little power being generated even under the best light conditions, Motorola researchers said in the patent.

The ultimate goal is to develop a device that could remain charged indefinitely, without requiring users to plug into a socket or carry external chargers with them when they travel. Until now, the major obstacle has been the LCD's polarizer and reflective screen, which sends light back to the viewer. In earlier designs, the reflective screen allowed less than 6 percent of the available light to reach the solar cells, Motorola said.

To solve this problem, Motorola proposed using either cholesteric liquid crystal or polymer-disbursed liquid crystal in the display, instead of super-twisted nematic liquid crystals. This change in materials eliminates the need for both a reflective screen and polarizer in the LCD screen. As a result, Motorola claims as much as 75 percent of available light is able to reach the solar cells, providing a sufficient amount of power to charge the battery of a mobile device.

Motorola also found a way to increase the amount of light that passes through screens based on super-twisted nematic liquid crystals, by using a selective color reflector. These reflectors only reflects one color, such as green for a green display, and allows other colors to pass through, the patent said. While not as effective as designs using cholesteric liquid crystal or polymer-disbursed liquid crystal, these displays still allow around 30 percent of available light to reach the solar cells, Motorola said.

The patent, which offered no hint of commercial product plans, also outlines how solar cells can be added to OLED (organic light-emitting diode) and touchscreen displays.

Monday, February 18, 2013

Leap Frog Algorithm

The leapfrog algorithm
The first method used is the leapfrog algorithm, which is a modified version of the Verlet algorithm. The Verlet algorithm uses the positions and accelerations at the time t and the positions at the time $t-\Delta t$ to predict the positions at the time $t + \Delta t$ , where $\Delta t$ is the integration step. From a Taylor expansion of the 3-rd order, we obtain

$\begin{displaymath}{\bf r}_i(t + \Delta t) = 2 {\bf r}_i(t) - {\bf r}_i(t - \Delta t) + \ddot{\bf r}_i(t) \Delta t^2. \end{displaymath}$

(8.3)

The error in the atomic positions is of the order of $\Delta t^4$ . The velocities are obtained from the basic definition of differentiation

$\begin{displaymath}\dot{\bf r}_i(t) = \frac{{\bf r}_i(t + \Delta t)-{\bf r}_i(t - \Delta t)} {2 \Delta t}, \end{displaymath}$

(8.4)

with an error of the order of $\Delta t^2$ . To obtain more accurate velocities, the leapfrog algorithm is used, using velocities at half time step

$\begin{displaymath}\dot{\bf r}_i(t + \frac{\Delta t}{2}) = \dot{\bf r}_i(t - \frac{\Delta t}{2}) + \ddot{\bf r}_i(t) \Delta t. \end{displaymath}$

(8.5)

The velocities at time t can be also computed from

$\begin{displaymath}\dot{\bf r}_i(t) = \frac{\dot{\bf r}_i(t + \frac{\Delta t}{2}) +\dot{\bf r}_i(t - \frac{\Delta t}{2})}{2}. \end{displaymath}$

(8.6)

This is useful when the kinetic energy is needed at time t, as for example in the case where velocity rescaling must be carried out (see below). The atomic positions are then obtained from

$\begin{displaymath}{\bf r}_i(t + \Delta t) = {\bf r}_i(t) + \dot{\bf r}_i(t + \frac{\Delta t}{2}) \Delta t. \end{displaymath}$

(8.7)

The leapfrog algorithm is computationally less expensive than the Predictor-Corrector approach for example, and requires less storage. This could be an important advantage in the case of large scale calculations. Moreover, the conservation of energy is respected, even at large time steps. Therefore, the computation time could be greatly decreased when this algorithm is used. However, when more accurate velocities and positions are needed, another algorithm should be implemented, like the Predictor-Corrector algorithm. We will show in the following the effects of this algorithm on the results of the calculations.

Simulations For Solar Cells

FDTD or TransMatrix?

FDTD-Simulation Box:

illustration of a standard Cartesian Yee cell used for FDTD, about which electric and magnetic field vector components are distributed

Strengths of FDTD modeling
Every modeling technique has strengths and weaknesses, and the FDTD method is no different.

FDTD is a versatile modeling technique used to solve Maxwell's equations. It is intuitive, so users can easily understand how to use it and know what to expect from a given model.

FDTD is a time-domain technique, and when a broadband pulse (such as a Gaussian pulse) is used as the source, then the response of the system over a wide range of frequencies can be obtained with a single simulation. This is useful in applications where resonant frequencies are not exactly known, or anytime that a broadband result is desired.

Since FDTD calculates the E and H fields everywhere in the computational domain as they evolve in time, it lends itself to providing animated displays of the electromagnetic field movement through the model. This type of display is useful in understanding what is going on in the model, and to help ensure that the model is working correctly.

The FDTD technique allows the user to specify the material at all points within the computational domain. A wide variety of linear and nonlinear dielectric and magnetic materials can be naturally and easily modeled.

FDTD allows the effects of apertures to be determined directly. Shielding effects can be found, and the fields both inside and outside a structure can be found directly or indirectly.

FDTD uses the E and H fields directly. Since most EMI/EMC modeling applications are interested in the E and H fields, it is convenient that no conversions must be made after the simulation has run to get these values.

Weaknesses of FDTD modeling

Since FDTD requires that the entire computational domain be gridded, and the grid spatial discretization must be sufficiently fine to resolve both the smallest electromagnetic wavelength and the smallest geometrical feature in the model, very large computational domains can be developed, which results in very long solution times. Models with long, thin features, (like wires) are difficult to model in FDTD because of the excessively large computational domain required. Methods such as Eigenmode Expansion can offer a more efficient alternative as they do not require a fine grid along the z-direction.^[64]

There is no way to determine unique values for permittivity and permeability at a material interface.

Space and time steps must satisfy the CFL condition, or the leapfrog integration used to solve the partial differential equation is probable to become unstable.

FDTD finds the E/H fields directly everywhere in the computational domain. If the field values at some distance are desired, it is likely that this distance will force the computational domain to be excessively large. Far-field extensions are available for FDTD, but require some amount of postprocessing.^[5]

Since FDTD simulations calculate the E and H fields at all points within the computational domain, the computational domain must be finite to permit its residence in the computer memory. In many cases this is achieved by inserting artificial boundaries into the simulation space. Care must be taken to minimize errors introduced by such boundaries. There are a number of available highly effective absorbing boundary conditions (ABCs) to simulate an infinite unbounded computational domain.^[5] Most modern FDTD implementations instead use a special absorbing "material", called a perfectly matched layer (PML) to implement absorbing boundaries.^[39]^[42]

Because FDTD is solved by propagating the fields forward in the time domain, the electromagnetic time response of the medium must be modeled explicitly. For an arbitrary response, this involves a computationally expensive time convolution, although in most cases the time response of the medium (or Dispersion (optics)) can be adequately and simply modeled using either the recursive convolution (RC) technique, the auxiliary differential equation (ADE) technique, or the Z-transform technique. An alternative way of solving Maxwell's equations that can treat arbitrary dispersion easily is the Pseudospectral Spatial-Domain method (PSSD), which instead propagates the fields forward in space.

Transfer Matrix:

The transfer-matrix method is a method used in optics and acoustics to analyze the propagation of electromagnetic or acoustic waves through a stratified (layered) medium.^[1] This is for example relevant for the design of anti-reflective coatings and dielectric mirrors.

The reflection of light from a single interface between two media is described by the Fresnel equations. However, when there are multiple interfaces, such as in the figure, the reflections themselves are also partially transmitted and then partially reflected. Depending on the exact path length, these reflections can interfere destructively or constructively. The overall reflection of a layer structure is the sum of an infinite number of reflections, which is cumbersome to calculate.

The transfer-matrix method is based on the fact that, according to Maxwell's equations, there are simple continuity conditions for the electric field across boundaries from one medium to the next. If the field is known at the beginning of a layer, the field at the end of the layer can be derived from a simple matrix operation. A stack of layers can then be represented as a system matrix, which is the product of the individual layer matrices. The final step of the method involves converting the system matrix back into reflection and transmission coefficients.

Sunday, February 17, 2013

Ph.D. in ChemE

Should start an awesome project to continue my Ph.D. journey.
What's the next big breakthrough in this area?

Plasmonic Solar Cell?
Polymer Solar Cell?
Transparent Solar Cell?