What is Organic Photovoltaics?
Organic photovoltaics (OPV) or also called organic solar cells are considered as the third generation of photovoltaic technology. These are made up of thin films of organic semiconductor materials with less than 100 nm width. These photovoltaics are also made up of compounds that can be dissolved in ink so it can be printed on thin rolls of plastics. Also, these materials can be curved and bent around the structures of the photovoltaic and it can be incorporated into clothing, as well.
It harvests energy from the sunlight both indoor and outdoor and then convert solar energy into electrical energy.
In addition, these solar cells have several layers such as a photoactive layer and two electrode layers. These solar cell layers are being printed using roll-to-roll manufacturing with thicknesses on the nanometer scale. Usually, these layers are printed on a thin plastic layer then these will be laminated with a protective and flexible foil.
While the absorbing layers used are based on organic semiconductors either small molecules or
Polymers. Organic materials become conducting or semiconducting with the use of a high level of conjugation–the alternating single and double bonds. Conjugation of the organic molecule causes the electrons to associate with the double bonds and then become delocalized across the entire conjugation’s length. These electrons contain higher energies compared to other electrons in the molecule and these are also equivalent to valence electrons in inorganic semiconductor materials.
On the other hand, inorganic materials electrons do not occupy a valence band but they are part of the highest occupied molecular orbital. Same with the inorganic semiconductors, energy levels are being unoccupied at higher energies.
Additionally, these organic photovoltaics (OPVs) become prominent due to promising qualities. These OPVs have tunable electronic properties, solution processability, low-temperature manufacture, and cheap lightweight materials. While other photovoltaic technologies have higher efficiencies, OPVs still remain preferable because of its environmental impact, low-cost and low material toxicity. They also have exceeded up to 13 percent efficiency even today which is close enough to the efficiency values obtained by the low-cost commercial silicon solar cells.
How does Organic Photovoltaics Work?
Just like the other photovoltaics, the main function of these OPVs is to harness solar energy, then generate it into electricity. This is attained when the solar energy is equal to or is greater than its bandgap. This leads to excitation and absorption of an electron (from the HOMO to the LUMO). The excited electron leaves behind the space containing a positive charge, which is called the ‘hole’. Because of the opposite charges of both the hole and electron, these two become attracted which causes the formation of the electron-hole pair or also known as an ‘exciton’. Then the exciton dissociation process will follow, wherein the pair of electron-hole will be separated to remove the charged particles from the solar cell.
Also, in an inorganic semiconductor, the attraction of electron and hole is small enough to be overcome by thermal energy at approximately 26 meV room temperature. Due to a high dielectric constant–the significant screening between the electron and hole, the attraction between them will be reduced. The separation of the electron and hole allows an easy exciton dissociation.
Whereas, Organic solar cells have low dielectric constants which did not allow the thermal energy alone to achieve the exciton dissociation. To repress this, there’s a need for two or more different OSCs within the OPV.
The organic photovoltaics are being classified as either a donor or acceptor depending on how the exciton dissociates. In most OPVs’ cases, the donor absorbs the most light, thus, the exciton will be generated. While at the interface with the acceptor, the exciton will be separated. Then, the electron will be donated to the material of the acceptor while the hole remains on the donor’s material.
Junction Types of Organic Photovoltaics
The single-layer type of organic photovoltaic cells is known as the simplest form. These cell layers are made by inserting a layer of organic electronic material between the two metallic conductors, which are typically a layer of indium tin oxide (ITO) containing high work functions and a layer with low work function metal like Magnesium, Aluminum or Calcium. The difference in the work function between these two conductors makes up an electric field in the organic layer. When the organic layer absorbs sunlight, the electrons will be excited to the LUMO and will leave holes in the HOMO, therefore excitons will be formed. The work potentials being created by the two different work functions help to break the exciton pairs by pulling electrons to the positively-charged electrode and holes to the negatively-charged electrode.
From the word itself, bilayer cells contain two layers between the conductive electrodes. These two layers have different amounts of energy being released when it was added to a neutral atom and have different ionization energies. Thus, electrostatic forces are being generated between the two layers’ interface. Also, the materials are chosen to make the potential differences large enough to make these local electric fields strong, which separate excitons more efficiently compared to the single-layer photovoltaic cells.
The layer with the higher released amount of energy and ionization potential becomes the electron acceptor, and the latter layer becomes the donor electron. And this structure is also known as the planar donor-acceptor heterojunction.
Discrete heterojunction is a three-layer fullerene-free stack junction composed of two electron acceptors and one electron donor. It can achieve a conversion efficiency of up to 8.4 percent. Also, the implementation can produce high open-circuit voltages and absorption in the visible spectra as well as high short-circuit currents. Between 400 nm and 720 nm wavelengths, the quantum efficiency can reach up to 75 percent, with around 1V open-circuit voltage.
The bulk heterojunction is a type of OPV junction that has an absorption layer consisting of a nanoscale blend of acceptor and donor materials. The domain sizes of this blend can be interpreted by nanometers. This allows the excitons with short lifetimes to reach the interface and to dissociate because of the vast interfacial area of the donor-acceptor.
Bulk heterojunctions are most commonly built by the formation of a solution containing the two components, drop-casting and spin coating. This allows the two phases to split, typically with the aid of an annealing procedure. The two components will be self-assembled into a merging network, which connects the two electrodes. Generally, they are composed of a conjugated fullerene-based acceptor and molecule-based donor. The nanostructural morphology of these bulk heterojunctions can be difficult to control but it is significant when it comes to performance of photovoltaic solar cells.
In graded heterojunction, the electron donor and electron acceptor are being mixed moderately to make the gradual transition. This architecture combines the travel distance of a short electron in the diffuse heterojunction with the aid of the gradient charge of the bilayer technology.
The concept of continuous junction is similar to the graded heterojunction concept which aims to apply a restrained transition between the electron donor to an electron acceptor. Although, the electron acceptor material is directly being prepared from the donor polymer in a post-polymerization modification procedure.
Cost of Organic Photovoltaics (OPV)
Organic photovoltaic is perceived as the next step for solar energy, which will be made cheaper than the other solar technology to encourage more people to use solar energy instead of the normal electric utility-scale. Although organic solar cell technology is still new and just developing, the estimated cost of purely organic solar cells will range between 37 USD and 100 USD per square meter.
Progress of Organic Photovoltaics
In 1986, Tang proposed the first two-component OPV but its efficiency remained very low for a few years because of the reliance on bilayer cells. Excitons can only be dissociated at the donor and acceptor interface and can generally be diffused at approximately 10nm before it turns back to its ground state. On the other hand, a total active layer with 100nm above thickness is typically required to absorb light efficiently. Bilayer cells are either, too thick for effective exciton dissociation or too thin to be absorbed, properly.
While In 1995, the solution to this problem was introduced which is known as the “bulk heterojunction (BHJ) cell”. With this, the donor and acceptor materials are being intimately mixed at the nanoscale level which allows the interfaces at an appropriate diffusion distance to be separated across the active layer while maintaining the necessary thickness of the cell for effective absorption.
The Future of Organic Photovoltaics
Large majority of researches and studies about OPV remain focused on its efficiency values, while the main issues why OPV is being restricted in commercialization are due to its scalability and long-term stability. In fact, some researches have recommended that the current efficiency values obtained could be competitive with the aid of other technologies if only it will be scaled appropriately. Currently, there is little consideration of the synthetic complexity of materials being used and the suitability of its scalable deposition techniques and these are likely to be the main focus of researches in the next few years together with various greener solvent systems. On the other hand, OPV has a struggle with long-term stability, mostly the reason is the havoc from water and oxygen access.
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Organic photovoltaics (OPVs) reached a new milestone this week whenKonarka announced that it achieved a world-record 8.3 percent efficiency rating from the National Renewable Energy Laboratory. OPVs are a novelform of photovoltaics (PVs) that are finding a home in portable,flexible applications and also building-integrated applications.
The gains in efficiency are more impressive considering that in May2009, the world-record for OPV was 6.4 percent efficient, which was also held by Konarka at the time. But OPV still remains far less efficientthan other forms of PV—like silicon, which commonly reaches averageefficiencies of 14 percent and has been produced at efficiency levelshigher than 20 percent.
On the other hand, OPV is flexible, uses less expensive materials tomanufacture and can be easily incorporated into fabrics andbuilding-integrated PVs.
“In OPV, the manufacturing process is so much cheaper and greener,and they’re using cleaner materials,” said Tracy Wemett, a spokespersonfor Konarka.
The technology continues to face obstacles, however.
For instance, in addition to lower efficiencies, the materials also have a shorter lifespan.
“The goal has always been 10 percent efficiency and a 10-year lifetime,” said Wemett.
For comparison, silicon cells are expected to last at least 25 years. She said the company is close to the first goal, but the material inwhich the OPVs are encased—a plastic—limits their viable life to about 5 years at this point.
“If you put a different barrier between it, it should last longer,” she said.
At this point, Konarka is focused on getting the technology into more hands, Wemett said.
Konarka is primarily an OPV manufacturer that’s selling the productto other companies for their uses and makes its OPV Power Plastic tocustomers’ specifications, which include multiple colors. In fact,Konarka has developed camouflaged-patterned materials for use by themilitary, Wemett said.
Since the materials Konarka uses can also be transparent orsemi-transparent, they’re ideal for building-integrated PV applicationslike windows or curtain walls.
Perhaps the most readily available application of Konarka’s PowerPlastics is now found in travel ware, like laptop bags and even personal coolers. Applications that take advantage of the material’s lightweight while providing power for laptops or other portable electricdevices.
Image courtesy of NREL.
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