Sunday, 4 October 2015

Organic Photovoltaic

Organic Photovoltaics

Organic Photovoltaic (OPV) devices convert solar energy to electrical energy. A typical OPV device consists of one or several photoactive materials sandwiched between two electrodes. Figure 1 depicts a typical bilayer organic photovoltaic device.
Figure 1. Structure of a bilayer organic photovoltaic device
In a bilayer OPV cell, sunlight is absorbed in the photoactive layers composed of donor and acceptor semiconducting organic materials to generate photocurrents. The donor material (D) donates electrons and mainly transports holes and the acceptor material (A) withdraws electrons and mainly transports electrons. As depicted in Figure 2, those photoactive materials harvest photons from sunlight to form excitons, in which electrons are excited from the valence band into the conduction band (Light Absorption). Due to the concentration gradient, the excitons diffuse to the donor/acceptor interface (Exciton Diffusion) and separate into free holes (positive charge carriers) and electrons (negative charge carriers) (Charge Separation). A photovoltaic is generated when the holes and electrons move to the corresponding electrodes by following either donor or acceptor phase (Charge Extraction).
Figure 2. Functional mechanism of a bilayer organic photovoltaic (D = donor, A = acceptor)
A primary advantage of OPV technology over inorganic counterparts is its ability to be utilized in large area and flexible solar modules, specially facilitating roll-to-roll (R2R) production. Additionally, manufacturing cost can be reduced for organic solar cells due to their lower cost compared to silicon-based materials and the ease of device manufacturing. However, to catch up with the performance of silicon based solar cells, both donor and acceptor materials in an OPV need to have good extinction coefficients, high stabilities and good film morphologies. Since the donor plays a critical role as the absorber to solar photon flux, donor materials require wide optical absorption to match the solar spectrum. Another basic requirement for ideal donor/acceptor is a large hole/electron mobility to maximize charge transport. The significant improvement of OPV device performance has been accomplished by introducing various OPV architectures, such as bulk-heterojuction (BHJ) and inverted device structures, and developing low band gap conjugated polymers and innovative organic small molecules as donor materials.
Aldrich Materials Science offers extensive organic donor and acceptor materials to help enable innovative OPV research.

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.


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 (Voc) with the top cell. To achieve tandem power conversion efficiencies greater than 25%, the CdSe top cell must have a Voc 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 CdxZn1-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.
Learn more about the third round of SunShot's Next Generations Photolvoltaics funding program, or find other PV competitive award programs.

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