Polymer solar cell
Encyclopedia
Organic solar cell
An organic photovoltaic cell is a photovoltaic cell that uses organic electronics--a branch of electronics that deals with conductive organic polymers or small organic molecules for light absorption and charge transport....
(also called plastic solar cell), or organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
photovoltaic cell that produce electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
from sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
using polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s. There are also other types of more stable thin-film semiconductors that can be deposited on different types of polymers to create solar cells. This technology is relatively new, being actively researched by universities, national laboratories and several companies around the world.
Currently, commercial solar cells are made from a refined, highly purified silicon crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
, similar to the material used in the manufacture of integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s and computer chips (wafer silicon). The high cost of these silicon solar cells, and their complex production process has generated interest in developing alternative photovoltaic technologies.
Compared to silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), potentially disposable and inexpensive to fabricate (sometimes using printed electronics
Printed electronics
Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and...
), flexible, and customizable on the molecular level, and they have lower potential for negative environmental impact. An example device is shown in Fig. 1. The disadvantages of polymer solar cells are also serious: they offer about 1/3 of the efficiency of hard materials, and they are relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.
Polaron A polaron is a quasiparticle composed of a charge and its accompanying polarization field. A slow moving electron in a dielectric crystal, interacting with lattice ions through long-range forces will permanently be surrounded by a region of lattice polarization and deformation caused by the moving... surrounded by fullerene Fullerene A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes... molecules |
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Device physics
The following discussion is based primarily on Mayer et al.'s review, cited below. Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
s. The molecules forming the electron donor region of organic PV cells, where exciton
Exciton
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids...
electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electron
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
s that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy bandgap between these orbitals determines which wavelength of light can be absorbed
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
.
Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbs the photon is the donor, and the material acquiring the electron is called the acceptor. In Fig. 2, the polymer chain is the donor and the fullerene is the acceptor. After dissociation has taken place, the electron and hole may still be joined as a geminate pair and an electric field is then required to separate them.
After exciton dissociation, the electron and hole must be collected at contacts. However, charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
mobility now begins to play a major role: if mobility is not sufficiently high, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other. In that case, space-charge limited photocurrent (SCLP) hampers device performance.
As an example of the processes involved in device operation, organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron transfer from the polymer chain to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle
Quasiparticle
In physics, quasiparticles are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space...
, or polaron
Polaron
A polaron is a quasiparticle composed of a charge and its accompanying polarization field. A slow moving electron in a dielectric crystal, interacting with lattice ions through long-range forces will permanently be surrounded by a region of lattice polarization and deformation caused by the moving...
(P+), occurs on the polymer chain and the fullerene becomes an ion-radical C60- Polarons are highly mobile along the length of the polymer chain and can diffuse away. Both the polaron and ion-radical possess spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
S= ½, so the charge photoinduction and separation processes can be controlled by the Electron Paramagnetic Resonance
Electron paramagnetic resonance
Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...
method.
Architectures
This section is derived largely from Mayer's review, referenced below. The simplest architecture that may be used for an organic PV device is a planar heterojunctionHeterojunction
A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
, shown in Fig. 1. A film of active polymer (donor) and a film of electron acceptor are sandwiched between contacts in a planar configuration. Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor. However, planar heterojunctions are inherently inefficient; because charge carriers have diffusion lengths of just 3-10 nm in typical organic semiconductor
Organic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
s, planar cells must be thin to enable successful diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
to contacts, but the thinner the cell, the less light it can absorb.
Bulk
Bulk
-Industry:* Bulk cargo* Bulk liquids* Bulk mail* Bulk material handling* Bulk pack, packaged bulk materials/products* Bulk purchasing- Physics :*Bulk density*Bulk modulus...
heterojunctions (BHJs) address this shortcoming. In a BHJ, the electron donor and acceptor materials are blended together and cast as a mixture that then phase-separates. Regions of each material in the device are separated by only several nanometers, a distance optimized for carrier diffusion. Although devices based on BHJs are a significant improvement over planar designs, BHJs require sensitive control over materials morphology on the nanoscale. A great number of variables, including choice of materials, solvents, and the donor-acceptor weight ratio can dramatically affect the BHJ structure that results. These factors can make rationally optimizing BHJs difficult.
The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). This paradigm eliminates much of the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO2. Holes still must diffuse along the length of the pore through the polymer to a contact, so OHJs do have thickness limitations. Mitigating the hole mobility bottleneck will thus be key to further enhancing OHJ device performance, but controlling morphology inside the confines of the pores is challenging.
Engineers at the University of California, San Diego (UCSD) have employed "nanowires" to boost the efficiency of organic solar cells.
Conclusion
At the moment, an open question is to what degree polymer solar cells can commercially compete with silicon solar cells and the other thin-film cells. The silicon solar cell industry has the important industrial advantage of being able to leverage the infrastructure developed for the computer industry. Besides, the present efficiency of polymer solar cells lies near 10 percent, much below the value for silicon cells. Polymer solar cells also suffer from environmental degradation. Good protective coatingCoating
Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wetability, corrosion resistance, wear resistance, and scratch resistance...
s are still to be developed.
Still, organic PV devices show great promise for decreasing the cost of solar energy to the point where it may become widespread in the decades ahead. While great progress has been made in the last ten years with respect to understanding the chemistry, physics, and materials science of organic photovoltaics, work remains to be done to further improve their performance. Specifically, novel nanostructures must be optimized to promote charge carrier diffusion; transport must be enhanced through control of order and morphology; and interface engineering must be applied to the problem of charge transfer across interfaces. Novel molecular chemistries and materials offer hope for revolutionary, rather than evolutionary, breakthroughs in device efficiencies in the future.
Current commercial status
For the reasons described above, polymer solar cells are not generally produced commercially today. One exception is the company Konarka Technologies, which in 2008 opened a factory with the capacity to produce a gigawatt's worth of polymer-fullerene solar cells each year. The initial cells from the factory are 3-5% efficient, and only last a couple years, but the company has stated that it would eventually be able to improve both the efficiency and durability. The company expects to initially sell the cells for a number of niche applications, such as in laptop-recharging briefcases; put into tents, umbrellas, and awnings; and as window tinting (since the cells can be made semi-transparent).Other third generation solar cells
- Dye-sensitized solar cell
- Hybrid solar cellHybrid solar cellHybrid solar cells combine advantages of both organic and inorganic semiconductors. Hybrid photovoltaics have organic materials that consist of conjugated polymers that absorb light as the donor and transport holes. Inorganic materials in hybrid cell are used as the acceptor and electron...
- Nanocrystal solar cellNanocrystal solar cellQuantum dot solar cells are an emerging field in solar cell research that uses quantum dots as the photovoltaic material, as opposed to better-known bulk materials such as silicon, copper indium gallium selenide or CdTe. Quantum dots have bandgaps that are tunable across a wide range of energy...
- Photoelectrochemical cellPhotoelectrochemical cellPhotoelectrochemical cells or PECs are solar cells which generate electrical energy from light, including visible light. Some photoelectrochemical cells simply produce electrical energy, while others produce hydrogen in a process similar to the electrolysis of water.-Photogeneration cell:In this...
See also
- Layer (electronics)Layer (electronics)A layer is the deposition of molecules on a substrate or base .High temperature substrates includes stainless steel and polyimide film and PET ....
- Substrate