Plasmon

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Plasmon

In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantization of plasma oscillations, just like phonons are quantizations of mechanical vibrations. Thus, plasmons are collective (a discrete number) oscillations of the free electron gas density. For example, at optical frequencies, plasmons can couple with a photon to create another quasiparticle called a plasmon polariton.

Derivation

The plasmon was initially proposed in 1952 by David Pines and David Bohm and was shown to arise from a Hamiltonian for the long-range electron-electron correlations.

Since plasmons are the quantization of classical plasma oscillations, most of their properties can be derived directly from Maxwell's equations.

Explanation

Plasmons can be described in the classical picture as an oscillation of electron density with respect to the fixed positive ions in a metal. To visualize a plasma oscillation, imagine a cube of metal placed in an external electric field pointing to the right. Electrons will move to the left side (uncovering positive ions on the right side) until they cancel the field inside the metal. If the electric field is removed, the electrons move to the right, repelled by each other and attracted to the positive ions left bare on the right side. They oscillate back and forth at the plasma frequency until the energy is lost in some kind of resistance or damping. Plasmons are a quantization of this kind of oscillation.

Role

Plasmons play a huge role in the optical properties of metals and semiconductors. Frequencies of light below the plasma frequency are reflected by a material because the electrons in the material screen the electric field of the light. Light of frequencies above the plasma frequency is transmitted by a material because the electrons in the material cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny (reflective) in the visible range. Some metals, such as copper and gold, have electronic interband transitions in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color. In semiconductors, the valence electron plasmon frequency is usually in the deep ultraviolet, while their electronic interband transitions are in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color which is why they are reflective. It has been shown that the plasmon frequency may occur in the mid-infrared and near-infrared region when semiconductors are in the form of nanoparticles with heavy doping.

The plasmon energy can often be estimated in the free electron model as

${\displaystyle E_{\mathrm{p}}=}$${\displaystyle \hslash }$${\displaystyle \sqrt{\frac{n{e}^2}{m{ϵ}_0}}=}$${\displaystyle \hslash }$${\displaystyle {\omega }_{\mathrm{p}},}$

where ${\displaystyle n}$ is the conduction electron density, ${\displaystyle e}$ is the elementary charge, ${\displaystyle m}$ is the electron mass, ${\displaystyle {ϵ}_0}$ the permittivity of free space, ${\displaystyle \hslash }$ the reduced Planck constant and ${\displaystyle {\omega }_{\mathrm{p}}}$ the plasmon frequency.

Surface plasmons

Main article: Surface plasmon

Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in a polariton. They occur at the interface of a material exhibiting positive real part of their relative permittivity, i.e. dielectric constant, (e.g. vacuum, air, glass and other dielectrics) and a material whose real part of permittivity is negative at the given frequency of light, typically a metal or heavily doped semiconductors. In addition to opposite sign of the real part of the permittivity, the magnitude of the real part of the permittivity in the negative permittivity region should typically be larger than the magnitude of the permittivity in the positive permittivity region, otherwise the light is not bound to the surface (i.e. the surface plasmons do not exist) as shown in the famous book by Heinz Raether. At visible wavelengths of light, e.g. 632.8 nm wavelength provided by a He-Ne laser, interfaces supporting surface plasmons are often formed by metals like silver or gold (negative real part permittivity) in contact with dielectrics such as air or silicon dioxide. The particular choice of materials can have a drastic effect on the degree of light confinement and propagation distance due to losses. Surface plasmons can also exist on interfaces other than flat surfaces, such as particles, or rectangular strips, v-grooves, cylinders, and other structures. Many structures have been investigated due to the capability of surface plasmons to confine light below the diffraction limit of light. One simple structure that was investigated was a multilayer system of copper and nickel. Mladenovic et al. report the use of the multilayers as if its one plasmonic material. Oxidation of the copper layers is prevented with the addition of the nickel layers. It is an easy path the integration of plasmonics to use copper as the plasmonic material because it is the most common choice for metallic plating along with nickel. The multilayers serve as a diffractive grating for the incident light. Up to 40 percent transmission can be achieved at normal incidence with the multilayer system depending on the thickness ratio of copper to nickel. Therefore, the use of already popular metals in a multilayer structure prove to be solution for plasmonic integration.

Surface plasmons can play a role in surface-enhanced Raman spectroscopy and in explaining anomalies in diffraction from metal gratings (Wood's anomaly), among other things. Surface plasmon resonance is used by biochemists to study the mechanisms and kinetics of ligands binding to receptors (i.e. a substrate binding to an enzyme). Multi-parametric surface plasmon resonance can be used not only to measure molecular interactions but also nanolayer properties or structural changes in the adsorbed molecules, polymer layers or graphene, for instance.

Surface plasmons may also be observed in the X-ray emission spectra of metals. A dispersion relation for surface plasmons in the X-ray emission spectra of metals has been derived (Harsh and Agarwal).

Gothic stained glass rose window of Notre-Dame de Paris. Some colors were achieved by colloids of gold nano-particles.

More recently surface plasmons have been used to control colors of materials. This is possible since controlling the particle's shape and size determines the types of surface plasmons that can be coupled into and propagate across it. This, in turn, controls the interaction of light with the surface. These effects are illustrated by the historic stained glass which adorn medieval cathedrals. Some stained glass colors are produced by metal nanoparticles of a fixed size which interact with the optical field to give glass a vibrant red color. In modern science, these effects have been engineered for both visible light and microwave radiation. Much research goes on first in the microwave range because at this wavelength, material surfaces and samples can be produced mechanically because the patterns tend to be on the order of a few centimeters. The production of optical range surface plasmon effects involves making surfaces which have features <400 nm. This is much more difficult and has only recently become possible to do in any reliable or available way.

Recently, graphene has also been shown to accommodate surface plasmons, observed via near field infrared optical microscopy techniques and infrared spectroscopy. Potential applications of graphene plasmonics mainly addressed the terahertz to midinfrared frequencies, such as optical modulators, photodetectors, biosensors.

Possible applications

The position and intensity of plasmon absorption and emission peaks are affected by molecular adsorption, which can be used in molecular sensors. For example, a fully operational device detecting casein in milk has been prototyped, based on detecting a change in absorption of a gold layer. Localized surface plasmons of metal nanoparticles can be used for sensing different types of molecules, proteins, etc.

Plasmons are being considered as a means of transmitting information on computer chips, since plasmons can support much higher frequencies (into the 100 THz range, whereas conventional wires become very lossy in the tens of GHz). However, for plasmon-based electronics to be practical, a plasmon-based amplifier analogous to the transistor, called a plasmonstor, needs to be created.

Plasmons have also been proposed as a means of high-resolution lithography and microscopy due to their extremely small wavelengths; both of these applications have seen successful demonstrations in the lab environment.

Finally, surface plasmons have the unique capacity to confine light to very small dimensions, which could enable many new applications.

Surface plasmons are very sensitive to the properties of the materials on which they propagate. This has led to their use to measure the thickness of monolayers on colloid films, such as screening and quantifying protein binding events. Companies such as Biacore have commercialized instruments that operate on these principles. Optical surface plasmons are being investigated with a view to improve makeup by L'Oréal and others.

In 2009, a Korean research team found a way to greatly improve organic light-emitting diode efficiency with the use of plasmons.

A group of European researchers led by IMEC has begun work to improve solar cell efficiencies and costs through incorporation of metallic nanostructures (using plasmonic effects) that can enhance absorption of light into different types of solar cells: crystalline silicon (c-Si), high-performance III-V, organic, and dye-sensitized.  However, for plasmonic photovoltaic devices to function optimally, ultra-thin transparent conducting oxides are necessary. Full color holograms using plasmonics have been demonstrated.

Plasmon-soliton

Plasmon-soliton mathematically refers to the hybrid solution of nonlinear amplitude equation e.g. for a metal-nonlinear media considering both the plasmon mode and solitary solution. A soliplasmon resonance is on the other hand considered as a quasiparticle combining the surface plasmon mode with spatial soliton as a result of a resonant interaction. To achieve one dimensional solitary propagation in a plasmonic waveguide while the surface plasmons should be localized at the interface, the lateral distribution of the field envelope should also be unchanged.

Graphene-based waveguide is a suitable platform for supporting hybrid plasmon-solitons due to the large effective area and huge nonlinearity. For example, the propagation of solitary waves in a graphene-dielectric heterostructure may appear as in the form of higher order solitons or discrete solitons resulting from the competition between diffraction and nonlinearity.

Plasmonic solar cell

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Plasmonic_solar_cell

A plasmonic-enhanced solar cell, commonly referred to simply as plasmonic solar cell, is a type of solar cell (including thin-film or wafer-based cells) that converts light into electricity with the assistance of plasmons, but where the photovoltaic effect occurs in another material. 

A direct plasmonic solar cell is a solar cell that converts light into electricity using plasmons as the active, photovoltaic material.

The active material thickness varies from that of traditional silicon PV (~100-200 μm wafers) , to less than 2 μm thick, and theoretically could be as thin as 100 nm. The devices can be supported on substrates cheaper than silicon, such as glass, steel, plastic or other polymeric materials (e.g. paper). One of the challenges for thin film solar cells is that they do not absorb as much light as thicker solar cells made with materials with the same absorption coefficient. Methods for light trapping are important for thin film solar cells. Plasmonic-enhanced cells improve absorption by scattering light using metal nano-particles excited at their localized surface plasmon resonance. Plasmonic core-shell nanoparticles located in the front of the thin film solar cells can aid weak absorption of Si solar cells in the near-infrared region—the fraction of light scattered into the substrate and the maximum optical path length enhancement can be as high as 3133. On the other hand, direct plasmonic solar cells exploit the fact that incoming light at the plasmon resonance frequency induces electron oscillations at the surface of the nanoparticles. The oscillation electrons can then be captured by a conductive layer producing an electrical current. The voltage produced is dependent on the bandgap of the conductive layer and the potential of the electrolyte in contact with the nanoparticles.

There is still considerable research necessary to enable these technologies to reach their full potential and enable the commercialization of plasmonic solar cells.

History

Devices

There are currently three different generations of solar cells. The first generation (those in the market today) are made with crystalline semiconductor wafers, with crystalline silicon making "up to 93% market share and about 75 GW installed in 2016".  Current solar cells trap light by creating pyramids on the surface which have dimensions bigger than most thin film solar cells. Making the surface of the substrate rough (typically by growing SnO₂ or ZnO on surface) with dimensions on the order of the incoming wavelengths and depositing the SC on top has been explored. This method increases the photocurrent, but the thin film solar cells would then have poor material quality. 

The second generation solar cells are based on thin film technologies such as those presented here. These solar cells focus on lowering the amount of material used as well as increasing the energy production. Third generation solar cells are currently being researched. They focus on reducing the cost of the second generation solar cells.  The third generation SCs are discussed in more detail under the "Recent advancements" section.

Design

The design for plasmonic-enhanced solar cells varies depending on the method being used to trap and scatter light across the surface and through the material.

Nanoparticle cells

PSC using metal nano-particles.

A common design is to deposit metal nano-particles on the top surface of the solar cell. When light hits these metal nano-particles at their surface plasmon resonance, the light is scattered in many different directions. This allows light to travel along the solar cell and bounce between the substrate and the nano-particles enabling the solar cell to absorb more light. The concentrated near field intensity induced by localized surface plasmon of the metal nanoparticles will promote the optical absorption of semiconductors. Recently, the plasmonic asymmetric modes of nanoparticles have found to favor the broadband optical absorption and promote the electrical properties of solar cells. The simultaneously plasmon-optical and plasmon-electrical effects of nanoparticles reveal a promising feature of nanoparticle plasmon.

Recently, the core (metal)-shell (dielectric) nanoparticle has demonstrated a zero backward scattering with enhanced forward scattering on Si substrate when surface plasmon is located in front of a solar cell. The core-shell nanoparticles can support simultaneously both electric and magnetic resonances, demonstrating entirely new properties when compared with bare metallic nanoparticles if the resonances are properly engineered.

Despite these effects, the application of metal nanoparticles at the solar cells' front can bring considerable optical losses, chiefly due to partial shading and reflection of the impinging light. Instead, their integration at the rear side of thin-film devices, particularly in between the absorber layer and the rear metallic contact (acting as reflective mirror), can circumvent such issues since the particles interact only with the longer-wavelength light that is weakly-absorbed by the cell, for which the plasmonic scattering effects can allow pronounced photocurrent gains. Such so-called plasmonic back reflector configuration has allowed the highest PV efficiency enhancements, for instance as demonstrated in thin-film silicon solar cells.

Metal film cells

Other methods utilizing surface plasmons for harvesting solar energy are available. One other type of structure is to have a thin film of silicon and a thin layer of metal deposited on the lower surface. The light will travel through the silicon and generate surface plasmons on the interface of the silicon and metal. This generates electric fields inside of the silicon since electric fields do not travel very far into metals. If the electric field is strong enough, electrons can be moved and collected to produce a photocurrent. The thin film of metal in this design must have nanometer sized grooves which act as waveguides for the incoming light in order to excite as many photons in the silicon thin film as possible. 

Principles

General

Thin film SC (left) and Typical SC (right).

When a photon is excited in the substrate of a solar cell, an electron and hole are separated. Once the electrons and holes are separated, they will want to recombine since they are of opposite charge. If the electrons can be collected prior to this happening they can be used as a current for an external circuit. Designing the thickness of a solar cell is always a trade-off between minimizing this recombination (thinner layers) and absorbing more photons (thicker layer).

Nano-particles

Scattering and Absorption

The basic principles for the functioning of plasmonic-enhanced solar cells include scattering and absorption of light due to the deposition of metal nano-particles. Silicon does not absorb light very well. For this reason, more light needs to be scattered across the surface in order to increase the absorption. It has been found that metal nano-particles help to scatter the incoming light across the surface of the silicon substrate. The equations that govern the scattering and absorption of light can be shown as:

• ${\displaystyle C_{scat}=\frac{1}{6\pi }{\left(\frac{2\pi }{\lambda }\right)}^4|\alpha {|}^2}$

This shows the scattering of light for particles which have diameters below the wavelength of light.

• ${\displaystyle C_{abs}=\frac{2\pi }{\lambda }\text{Im}[\alpha ]}$

This shows the absorption for a point dipole model.

• ${\displaystyle \alpha =3V\left[\frac{{ϵ}_p/{ϵ}_m-1}{{ϵ}_p/{ϵ}_m+2}\right]}$

This is the polarizability of the particle. V is the particle volume. ${\displaystyle {ϵ}_p}$ is the dielectric function of the particle. ${\displaystyle {ϵ}_m}$ is the dielectric function of the embedding medium. When ${\displaystyle {ϵ}_p=-2{ϵ}_m}$ the polarizability of the particle becomes large. This polarizability value is known as the surface plasmon resonance. The dielectric function for metals with low absorption can be defined as:

• ${\displaystyle ϵ=1-\frac{{\omega }_p^2}{{\omega }^2+i\gamma \omega }}$

In the previous equation, ${\displaystyle {\omega }_p}$ is the bulk plasma frequency. This is defined as:

• ${\displaystyle {\omega }_p^2=N{e}^2/m{ϵ}_0}$

N is the density of free electrons, e is the electronic charge and m is the effective mass of an electron. ${\displaystyle {ϵ}_0}$ is the dielectric constant of free space. The equation for the surface plasmon resonance in free space can therefore be represented by:

• ${\displaystyle \alpha =3V\frac{{\omega }_p^2}{{\omega }_p^2-3{\omega }^2-i\gamma \omega }}$

Many of the plasmonic solar cells use nano-particles to enhance the scattering of light. These nano-particles take the shape of spheres, and therefore the surface plasmon resonance frequency for spheres is desirable. By solving the previous equations, the surface plasmon resonance frequency for a sphere in free space can be shown as:

• ${\displaystyle {\omega }_{sp}=\sqrt{3}{\omega }_p}$

As an example, at the surface plasmon resonance for a silver nanoparticle, the scattering cross-section is about 10x the cross-section of the nanoparticle. The goal of the nano-particles is to trap light on the surface of the SC. The absorption of light is not important for the nanoparticle, rather, it is important for the SC. One would think that if the nanoparticle is increased in size, then the scattering cross-section becomes larger. This is true, however, when compared with the size of the nanoparticle, the ratio (${\displaystyle C{S}_{scat}/C{S}_{particle}}$) is reduced. Particles with a large scattering cross section tend to have a broader plasmon resonance range.

Wavelength dependence

Surface plasmon resonance mainly depends on the density of free electrons in the particle. The order of densities of electrons for different metals is shown below along with the type of light which corresponds to the resonance.

• Aluminum - Ultra-violet
• Silver - Ultra-violet
• Gold - Visible
• Copper - Visible

If the dielectric constant for the embedding medium is varied, the resonant frequency can be shifted. Higher indexes of refraction will lead to a longer resonant wavelength.

Light trapping

The metal nano-particles are deposited at a distance from the substrate in order to trap the light between the substrate and the particles. The particles are embedded in a material on top of the substrate. The material is typically a dielectric, such as silicon or silicon nitride. When performing experiment and simulations on the amount of light scattered into the substrate due to the distance between the particle and substrate, air is used as the embedding material as a reference. It has been found that the amount of light radiated into the substrate decreases with distance from the substrate. This means that nano-particles on the surface are desirable for radiating light into the substrate, but if there is no distance between the particle and substrate, then the light is not trapped and more light escapes.

The surface plasmons are the excitations of the conduction electrons at the interface of metal and the dielectric. Metallic nano-particles can be used to couple and trap freely propagating plane waves into the semiconductor thin film layer. Light can be folded into the absorbing layer to increase the absorption. The localized surface plasmons in metal nano-particles and the surface plasmon polaritons at the interface of metal and semiconductor are of interest in the current research. In recent reported papers, the shape and size of the metal nano-particles are key factors to determine the incoupling efficiency. The smaller particles have larger incoupling efficiency due to the enhanced near-field coupling. However, very small particles suffer from large ohmic losses.

Nevertheless, in certain types of nanostructured solar cells, such as the emerging quantum-dot intermediate band solar cells, the highly intense scattered near-field produced in the vicinity of plasmonic nanoparticles may be exploited for local absorption amplification in the quantum dots that are embedded in a host semiconductor.

Recently, the plasmonic asymmetric modes of nano particles have found to favor the broadband optical absorption and promote the electrical properties of solar cells. The simultaneously plasmon-optical and plasmon-electrical effects of nanoparticles reveal a promising feature of nanoparticle plasmon.

Metal film

As light is incident upon the surface of the metal film, it excites surface plasmons. The surface plasmon frequency is specific for the material, but through the use of gratings on the surface of the film, different frequencies can be obtained. The surface plasmons are also preserved through the use of waveguides as they make the surface plasmons easier to travel on the surface and the losses due to resistance and radiation are minimized. The electric field generated by the surface plasmons influences the electrons to travel toward the collecting substrate. 

Materials

First generation	Second generation	Third generation
Single-crystal silicon	CuInSe2	Gallium indium phosphide
Multicrystalline silicon	Amorphous silicon	Gallium indium arsenide
Polycrystalline silicon	Thin film crystalline Si	Germanium

Applications

There are many applications for plasmonic-enhanced solar cells. The need for cheaper and more efficient solar cells is considerable. In order for solar cells to be considered cost-effective, they need to provide energy for a smaller price than that of traditional power sources such as coal and gasoline. The movement toward a more green world has helped to spark research in the area of plasmonic-enhanced solar cells. Currently, solar cells cannot exceed efficiencies of about 30% (first generation). With new technologies (third generation), efficiencies of up to 40-60% can be expected. With a reduction of materials through the use of thin film technology (second Generation), prices can be driven lower.

Certain applications for plasmonic-enhanced solar cells would be for space exploration vehicles. A main contribution for this would be the reduced weight of the solar cells. An external fuel source would also not be needed if enough power could be generated from the solar cells. This would drastically help to reduce the weight as well.

Solar cells have a great potential to help rural electrification. An estimated two million villages near the equator have limited access to electricity and fossil fuels, and approximately 25% of people in the world do not have access to electricity. When the cost of extending power grids, running rural electricity and using diesel generators is compared with the cost of solar cells, in many cases the solar cells are superior. If the efficiency and cost of the current solar cell technology is decreased even further, then many rural communities and villages around the world could obtain electricity when current methods are out of the question. Specific applications for rural communities would be water pumping systems, residential electric supply and street lights. A particularly interesting application would be for health systems in countries where motorized vehicles are not overly abundant. Solar cells could be used to provide the power to refrigerate medications in coolers during transport.

Solar cells could also provide power to lighthouses, buoys, or even battleships out in the ocean. Industrial companies could use them to power telecommunications systems or monitoring and control systems along pipelines.

If the solar cells could be produced on a large scale and be cost effective, then entire power stations could be built in order to provide power to the electrical grids. With a reduction in size, they could be implemented on both commercial and residential buildings with a much smaller footprint. 

Other applications are in hybrid systems. The solar cells could help to power high-consumption devices such as automobiles in order to reduce the amount of fossil fuels used.

In consumer electronics devices, solar cells could be used to replace batteries for low-power electronics. This would save money and it would also reduce the amount of waste going into landfills.

Recent advancements

Choice of plasmonic metal nano-particles

Proper choice of plasmatic metal nanoparticles is crucial for the maximum light absorption in the active layer. Front surface located nanoparticles of silver and gold (Ag and Au) are the most widely used materials due to their surface plasmon resonances being located in the visible range, therefore interacting more strongly with the peak solar intensity. However, such noble metal nanoparticles always introduce reduced light coupling into Si at the short wavelengths below the surface plasmon resonance due to the detrimental Fano effect, i.e. the destructive interference between the scattered and unscattered light. Moreover, the noble metal nano-particles are impractical to use for large-scale solar cell manufacture due to their high cost and scarcity in the earth's crust. Recently, Zhang et al. demonstrated that low-cost and earth-abundant aluminium (Al) nano-particles can outperform the widely used Ag and Au nanoparticles. Al nanoparticles, with their surface plasmon resonances located in the UV region below the desired solar spectrum edge at 300 nm, can avoid the reduction and introduce extra enhancement in the shorter wavelength range.

Light trapping for absorption enhancement

As discussed earlier, being able to concentrate and scatter light from the surface or the back side of the plasmonic-enhanced solar cell will help to increase efficiencies, particularly when employing thin photovoltaic materials.

Recently, research at Sandia National Laboratories has discovered a photonic waveguide which collects light at a certain wavelength and traps it within the structure. This new structure can contain 95% of the light that enters it compared to 30% for other traditional waveguides. It can also direct the light within one wavelength which is ten times greater than traditional waveguides. The wavelength this device captures can be selected by changing the structure of the lattice which comprises the structure. If this structure is used to trap light and keep it in the structure until the solar cell can absorb it, the efficiency of the solar cell could be increased dramatically.

Another recent advancement in plasmonic-enhanced solar cells is using other methods to aid in the absorption of light. One method being researched is the use of metal wires on top of the substrate to scatter the light. This would help by utilizing a larger area of the surface of the solar cell for light scattering and absorption. The danger in using lines instead of dots would be creating a reflective layer which would reject light from the system. This is very undesirable for solar cells. This would be very similar to the thin metal film approach, but it also utilizes the scattering effect of the nano-particles.  Yue et al. used a type of new materials, called topological insulators, to increase the absorption of ultrathin a-Si solar cells. The topological insulator nanostructure has intrinsically core-shell configuration. The core is dielectric and has ultrahigh refractive index. The shell is metallic and support surface plasmon resonances. Through integrating the nanocone arrays into a-Si thin film solar cells, up to 15% enhancement of light absorption was predicted in the ultraviolet and visible ranges.

Third generation

The goal of third generation solar cells is to increase the efficiency using second generation solar cells (thin film) and using materials that are found abundantly on earth. This has also been a goal of the thin film solar cells. With the use of common and safe materials, third generation solar cells should be able to be manufactured in mass quantities, further reducing the costs. The initial costs would be high in order to produce the manufacturing processes, but after that they should be cheap. The way third generation solar cells will be able to improve efficiency is to absorb a wider range of frequencies. The current thin film technology has been limited to one frequency due to the use of single band gap devices.

Multiple energy levels

The idea for multiple energy level solar cells is to basically stack thin film solar cells on top of each other. Each thin film solar cell would have a different band gap which means that if part of the solar spectrum was not absorbed by the first cell then the one just below would be able to absorb part of the spectrum. These can be stacked and an optimal band gap can be used for each cell in order to produce the maximum amount of power. There are multiple options for how each cell can be connected, such as serial or parallel. The serial connection is desired because the output of the solar cell would just be two leads.

The lattice structure in each of the thin film cells needs to be the same. If it is not then there will be losses. The processes used for depositing the layers are complex. They include Molecular Beam Epitaxy and Metal Organic Vapour Phase Epitaxy. The current efficiency record is made with this process but doesn't have exact matching lattice constants. The losses due to this are not as effective because the differences in lattices allows for more optimal band gap material for the first two cells. This type of cell is expected to be able to be 50% efficient.

Lower-quality materials that use cheaper deposition processes are being researched as well. These devices are not as efficient, but the price, size and power combined allow them to be just as cost effective. Since the processes are simpler and the materials are more readily available, the mass production of these devices is more economical.

Hot carrier cells

A problem with solar cells is that the high energy photons that hit the surface are converted to heat. This is a loss for the cell because the incoming photons are not converted into usable energy. The idea behind the hot carrier cell is to utilize some of that incoming energy which is converted to heat. If the electrons and holes can be collected while hot, a higher voltage can be obtained from the cell. The problem with doing this is that the contacts which collect the electrons and holes will cool the material. Thus far, keeping the contacts from cooling the cell has been theoretical. Another way of improving the efficiency of the solar cell using the heat generated is to have a cell which allows lower energy photons to excite electron and hole pairs. This requires a small bandgap. Using a selective contact, the lower energy electrons and holes can be collected while allowing the higher energy ones to continue moving through the cell. The selective contacts are made using a double barrier resonant tunneling structure. The carriers are cooled which they scatter with phonons. If a material has a large bandgap of phonons then the carriers will carry more of the heat to the contact and it won't be lost in the lattice structure. One material which has a large bandgap of phonons is indium nitride. The hot carrier cells are in their infancy but are beginning to move toward the experimental stage.

Plasmonic-electrical solar cells

Having unique features of tunable resonances and unprecedented near-field enhancement, plasmon is an enabling technique for light management. Recently, performances of thin-film solar cells have been pronouncedly improved by introducing metallic nanostructures. The improvements are mainly attributed to the plasmonic-optical effects for manipulating light propagation, absorption, and scattering. The plasmonic-optical effects could: (1) boost optical absorption of active materials; (2) spatially redistribute light absorption at the active layer due to the localized near-field enhancement around metallic nanostructures. Except for the plasmonic-optical effects, the effects of plasmonically modified recombination, transport and collection of photocarriers (electrons and holes), hereafter named plasmonic-electrical effects, have been proposed by Sha, etal. For boosting device performance, they conceived a general design rule, tailored to arbitrary electron to hole mobility ratio, to decide the transport paths of photocarriers. The design rule suggests that electron to hole transport length ratio should be balanced with electron to hole mobility ratio. In other words, the transport time of electrons and holes (from initial generation sites to corresponding electrodes) should be the same. The general design rule can be realized by spatially redistributing light absorption at the active layer of devices (with the plasmonic-electrical effect). They also demonstrated the breaking of space charge limit in plasmonic-electrical organic solar cell. Recently, the plasmonic asymmetric modes of nano particles have found to favor the broadband optical absorption and promote the electrical properties of solar cells. The simultaneously plasmon-optical and plasmon-electrical effects of nanoparticles reveal a promising feature of nanoparticle plasmon.

Ultra-thin plasmonic wafer solar cells

Reducing the silicon wafer thickness at a minimized efficiency loss represents a mainstream trend in increasing the cost-effectiveness of wafer-based solar cells. Recently, Zhang et al. have demonstrated that, using the advanced light trapping strategy with a properly designed nano-particle architecture, the wafer thickness can be dramatically reduced to only around 1/10 of the current thickness (180 μm) without any solar cell efficiency loss at 18.2%. Nano-particle integrated ultra-thin solar cells with only 3% of the current wafer thickness can potentially achieve 15.3% efficiency combining the absorption enhancement with the benefit of thinner wafer induced open circuit voltage increase. This represents a 97% material saving with only 15% relative efficiency loss. These results demonstrate the feasibility and prospect of achieving high-efficiency ultra-thin silicon wafer cells with plasmonic light trapping.

Direct plasmonic solar cells

The development of direct plasmonic solar cells that use plasmonic nanoparticles directly as light absorbers is much more recent than plasmonic-enhanced cells.

In 2013 it was confirmed that hot carriers in plasmonic nanoparticles can be generated by excitation of localized surface plasmon resonance. The hot electrons were shown to be injected into a TiO₂ conduction band, confirming their usability for light conversion to electricity. In 2019 another article was published describing how the hot electrons counterpart, the hot holes, can also be injected into a p-type semiconductor. This separation of charges enables direct use of plasmonic nanoparticles as light absorbers in photovoltaic cells.

A spin-off company from Uppsala university, Peafowl Solar Power, is developing direct plasmonic solar cell technology for commercial applications such as transparent solar cells for dynamic glass.

Copper indium gallium selenide solar cell

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Copper_indium_gallium_selenide_solar_cell

CIGS cell on a flexible plastic backing. Other architectures use rigid CIGS panels sandwiched between two panes of glass.

A copper indium gallium selenide solar cell (or CIGS cell, sometimes CI(G)S or CIS cell) is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper indium gallium selenide solid solution on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.

CIGS is one of three mainstream thin-film photovoltaic (PV) technologies, the other two being cadmium telluride and amorphous silicon. Like these materials, CIGS layers are thin enough to be flexible, allowing them to be deposited on flexible substrates. However, as all of these technologies normally use high-temperature deposition techniques, the best performance normally comes from cells deposited on glass, even though advances in low-temperature deposition of CIGS cells have erased much of this performance difference. CIGS outperforms polysilicon at the cell level, however its module efficiency is still lower, due to a less mature upscaling.

Thin-film market share is stagnated at around 15 percent, leaving the rest of the PV market to conventional solar cells made of crystalline silicon. In 2013, the market share of CIGS alone was about 2 percent and all thin-film technologies combined fell below 10 percent. CIGS cells continue being developed, as they promise to reach silicon-like efficiencies, while maintaining their low costs, as is typical for thin-film technology. Prominent manufacturers of CIGS photovoltaics were the now-bankrupt companies Nanosolar and Solyndra. Current market leader is the Japanese company Solar Frontier, with Global Solar and GSHK Solar also producing solar modules free of any heavy metals such as cadmium and/or lead. Many CIGS solar panel manufacturer companies have gone bankrupt.

Properties

CIGS is a I-III-VI₂ compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide, with a chemical formula of CuIn_xGa_(1−x)Se₂, where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).

Figure 1: Structure of a CIGS device. CdS is used optionally and some CIGS cells contain no cadmium at all.

CIGS has an exceptionally high absorption coefficient of more than 10⁵/cm for 1.5 eV and higher energy photons. CIGS solar cells with efficiencies around 20% have been claimed by the National Renewable Energy Laboratory (NREL), the Swiss Federal Laboratories for Materials Science and Technology (Empa), and the German Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW) (translated: Center for Solar Energy and Hydrogen Research), which is the record to date for any thin film solar cell.

All high performance CIGS absorbers in solar cells have similarities independent of production technique. First, they are polycrystalline α-phase which has the chalcopyrite crystal structure shown in Figure 3. The second property is an overall Cu deficiency. Cu deficiency increases the majority carrier (hole) concentration by increasing the number of (electron-accepting) Cu vacancies. When CIGS films are In rich (Cu deficient) the film's surface layer forms an ordered defect compound (ODC) with a stoichiometry of Cu(In,Ga)
₃Se
₅. The ODC is n-type, forming a p-n homojunction in the film at the interface between the α phase and the ODC. The recombination velocity at the CIGS/CdS interface is decreased by the homojunction's presence. The drop in interface recombination attributable to ODC formation is demonstrated by experiments which have shown that recombination in the bulk of the film is the main loss mechanism in Cu deficient films, while in Cu rich films the main loss is at the CIGS/CdS interface.

Figure 3: CIGS unit cell. Red = Cu, Yellow = Se, Blue = In/Ga

Sodium incorporation is necessary for optimal performance. Ideal Na concentration is considered to be approximately 0.1%. Na is commonly supplied by the soda-lime glass substrate, but in processes that do not use this substrate the Na must be deliberately added. Na's beneficial effects include increases in p-type conductivity, texture, and average grain size. Furthermore, Na incorporation allows for performance to be maintained over larger stoichiometric deviations. Simulations have predicted that Na on an In site creates a shallow acceptor level and that Na serves to remove In on Cu defects (donors), but reasons for these benefits are controversial. Na is also credited with catalyzing oxygen absorption. Oxygen passivates Se vacancies that act as compensating donors and recombination centers.

Alloying CIS (CuInSe₂) with CGS (CuGaSe₂) increases the bandgap. To reach the ideal bandgap for a single junction solar cell, 1.5 eV, a Ga/(In+Ga) ratio of roughly 0.7 is optimal. However, at ratios above ~0.3, device performance drops off. Industry currently targets the 0.3 Ga/(In+Ga) ratio, resulting in bandgaps between 1.1 and 1.2 eV. The decreasing performance has been postulated to be a result of CGS not forming the ODC, which is necessary for a good interface with CdS.

The highest efficiency devices show substantial texturing, or preferred crystallographic orientation. A (204) surface orientation is observed in the best quality devices. A smooth absorber surface is preferred to maximize the ratio of the illuminated area to the area of the interface. The area of the interface increases with roughness while illuminated area remains constant, decreasing open-circuit voltage (V_OC). Studies have also linked an increase in defect density to decreased V_OC. Recombination in CIGS has been suggested to be dominated by non-radiative processes. Theoretically, recombination can be controlled by engineering the film, and is extrinsic to the material.

Structure

The most common device structure for CIGS solar cells is shown in the diagram (see Figure 1: Structure of a CIGS device). Soda-lime glass of about of 1–3 millimetres thickness is commonly used as a substrate, because the glass sheets contains sodium, which has been shown to yield a substantial open-circuit voltage increase, notably through surface and grain boundary defects passivation. However, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils. A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber. Following molybdenum deposition a p-type CIGS absorber layer is grown by one of several unique methods. A thin n-type buffer layer is added on top of the absorber. The buffer is typically cadmium sulfide (CdS) deposited via chemical bath deposition. The buffer is overlaid with a thin, intrinsic zinc oxide layer (i-ZnO) which is capped by a thicker, aluminium (Al) doped ZnO layer. The i-ZnO layer is used to protect the CdS and the absorber layer from sputtering damage while depositing the ZnO:Al window layer, since the latter is usually deposited by DC sputtering, known as a damaging process. The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible.

The CuInSe₂-based materials that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for solar applications because of their high optical absorption coefficients and versatile optical and electrical characteristics, which can in principle be manipulated and tuned for a specific need in a given device.

Conversion efficiency

Solar cell efficiencies of various technologies as tracked by NREL, with CIGS progress in green

CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of September 2014 was 21.7%. A team at the National Renewable Energy Laboratory achieved 19.9%, a record at the time, by modifying the CIGS surface and making it look like CIS. These examples were deposited on glass, which meant the products were not mechanically flexible. In 2013, scientists at the Swiss Federal Laboratories for Materials Science and Technology developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%. These display both the highest efficiency and greatest flexibility.

The U.S. National Renewable Energy Laboratory confirmed 13.8% module efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules. In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6% on total module surface and 15.9% on aperture, which was produced on a mass production facility. MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1m² production module, and Solar Frontier claimed a 17.8% efficiency on a 900 cm² module.

Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. Gallium's relative abundance, compared to indium, lowers costs.

Lab record CIGS efficiencies by substrate

Substrate	Glass	Steel	Aluminium	Polymer
Efficiency	22.9%	17.7%	16.2%	20.4%
Institute	Solar Frontier	Empa	Empa	Empa

Comparison

Conventional crystalline silicon

Unlike conventional crystalline silicon cells based on a homojunction, the structure of CIGS cells is a more complex heterojunction system. A direct bandgap material, CIGS has very strong light absorption and a layer of only 1–2 micrometers (µm) is enough to absorb most of the sunlight. By comparison, a much greater thickness of about 160–190 µm is required for crystalline silicon.

The active CIGS-layer can be deposited in a polycrystalline form directly onto molybdenum (Mo) coated on a variety of several different substrates such as glass sheets, steel bands and plastic foils made of polyimide. This uses less energy than smelting large amounts of quartz sand in electric furnaces and growing large crystals, necessary for conventional silicon cells, and thus reduces its energy payback time significantly. Also unlike crystalline silicon, these substrates can be flexible.

In the highly competitive PV industry, pressure increased on CIGS manufacturers, leading to the bankruptcy of several companies, as prices for conventional silicon cells declined rapidly in recent years. However, CIGS solar cells have become as efficient as multicrystalline silicon cells—the most common type of solar cells. CIGS and CdTe-PV remain the only two commercially successful thin-film technologies in a globally fast-growing PV market.

Other thin films

In photovoltaics "thinness" generally is in reference to so-called "first generation" high-efficiency silicon cells, which are manufactured from bulk wafers hundreds of micrometers thick. Thin films sacrifice some light gathering efficiency but use less material. In CIGS the efficiency tradeoff is less severe than in silicon. The record efficiencies for thin film CIGS cells are slightly lower than that of CIGS for lab-scale top performance cells. In 2008, CIGS efficiency was by far the highest compared with those achieved by other thin film technologies such as cadmium telluride photovoltaics (CdTe) or amorphous silicon (a-Si). CIS and CGS solar cells offer total area efficiencies of 15.0% and 9.5%, respectively. In 2015, the gap with the other thin film technologies has been closed, with record cell efficiencies in laboratories of 21.5% for CdTe (FirstSolar) and 21.7% for CIGS (ZSW).

Production

Film production

The most common vacuum-based process is to co-evaporate or co-sputter copper, gallium, and indium onto a substrate at room temperature, then anneal the resulting film with a selenide vapor. An alternative process is to co-evaporate copper, gallium, indium and selenium onto a heated substrate.

A non-vacuum-based alternative process deposits nanoparticles of the precursor materials on the substrate and then sinters them in situ. Electroplating is another low cost alternative to apply the CIGS layer.

The following sections outline the various techniques for precursor deposition processing, including sputtering of metallic layers at low temperatures, printing of inks containing nanoparticles, electrodeposition, and a technique inspired by wafer-bonding.

Selenization

The Se supply and selenization environment is important in determining the properties and quality of the film. When Se is supplied in the gas phase (for example as H₂Se or elemental Se) at high temperatures, the Se becomes incorporated into the film by absorption and subsequent diffusion. During this step, called chalcogenization, complex interactions occur to form a chalcogenide. These interactions include formation of Cu-In-Ga intermetallic alloys, formation of intermediate metal-selenide binary compounds and phase separation of various stoichiometric CIGS compounds. Because of the variety and complexity of the reactions, the properties of the CIGS film are difficult to control.

The Se source affects the resulting film properties. H₂Se offers the fastest Se incorporation into the absorber; 50 at% Se can be achieved in CIGS films at temperatures as low as 400 °C. By comparison, elemental Se only achieves full incorporation with reaction temperatures above 500 °C. Films formed at lower temperatures from elemental Se were Se deficient, but had multiple phases including metal selenides and various alloys. Use of H₂Se provides the best compositional uniformity and the largest grain sizes. However, H₂Se is highly toxic and is classified as an environmental hazard.

Sputtering of metallic layers followed by selenization

In this method a metal film of Cu, In and Ga is sputtered at or near room temperature and reacted in a Se atmosphere at high temperature. This process has higher throughput than coevaporation and compositional uniformity can be more easily achieved.

Sputtering a stacked multilayer of metal – for example a Cu/In/Ga/Cu/In/Ga... structure – produces a smoother surface and better crystallinity in the absorber compared to a simple bilayer (Cu-Ga alloy/In) or trilayer (Cu/In/Ga) sputtering. These attributes result in higher efficiency devices, but forming the multilayer is a more complicated deposition process and did not merit the extra equipment or the added process complexity. Additionally, the reaction rates of Cu/Ga and Cu/In layers with Se are different. If the reaction temperature is not high enough, or not held long enough, CIS and CGS form as separate phases.

Companies currently that used similar processes include Showa Shell, Avancis, Miasolé, Honda Soltec, and Energy Photovoltaics (EPV). Showa Shell sputtered a Cu-Ga alloy layer and an In layer, followed by selenization in H₂Se and sulfurization in H₂S. The sulfurization step appears to passivate the surface in a way similar to CdS in most other cells. Thus, the buffer layer used is Cd-free, eliminating any environmental impact of Cd. Showa Shell reported a maximum module efficiency of 13.6% with an average of 11.3% for 3600 cm² substrates. Shell Solar uses the same technique as Showa Shell to create the absorber; however, their CdS layer comes from chemical vapor deposition. Modules sold by Shell Solar claim 9.4% module efficiency.

Miasole had procured venture capital funds for its process and scale up. A record 17.4% aperture efficiency module was confirmed by Fraunhofer in 2019.

EPV uses a hybrid between coevaporation and sputtering in which In and Ga are evaporated in a Se atmosphere. This is followed by Cu sputtering and selenization. Finally, In and Ga are again evaporated in the presence of Se. Based on Hall measurements, these films have a low carrier concentration and relatively high mobility. EPV films have a low defect concentration.

Chalcogenization of particulate precursor layers

In this method, metal or metal-oxide nanoparticles are used as the precursors for CIGS growth. These nanoparticles are generally suspended in a water based solution and then applied to large areas by various methods, such as printing. The film is then dehydrated and, if the precursors are metal-oxides, reduced in a H₂/N₂ atmosphere. Following dehydration, the remaining porous film is sintered and selenized at temperatures greater than 400 °C.

Nanosolar and International Solar Electric Technology (ISET) unsuccessfully attempted to scale up this process. ISET uses oxide particles, while Nanosolar did not discuss its ink. The advantages of this process include uniformity over large areas, non-vacuum or low-vacuum equipment and adaptability to roll-to-roll manufacturing. When compared to laminar metal precursor layers, sintered nanoparticles selenize more rapidly. The increased rate is a result of the greater surface area associated with porosity. Porosity produces rougher absorber surfaces. Use of particulate precursors allows for printing on a large variety of substrates with materials utilization of 90% or more. Little research and development supported this technique.

Nanosolar reported a cell (not module) efficiency of 14%, however this was not verified by any national laboratory testing, nor did they allow onsite inspections. In independent testing ISET's absorber had the 2nd lowest efficiency at 8.6%. However, all the modules that beat ISET's module were coevaporated, a process which has manufacturing disadvantages and higher costs. ISET's sample suffered most from low V_OC and low fill factor, indicative of a rough surface and/or a high number of defects aiding recombination. Related to these issues, the film had poor transport properties including a low Hall mobility and short carrier lifetime.

Electrodeposition followed by selenization

Precursors can be deposited by electrodeposition. Two methodologies exist: deposition of elemental layered structures and simultaneous deposition of all elements (including Se). Both methods require thermal treatment in a Se atmosphere to make device quality films. Because electrodeposition requires conductive electrodes, metal foils are a logical substrate. Electrodeposition of elemental layers is similar to the sputtering of elemental layers.

Simultaneous deposition employs a working electrode (cathode), a counter electrode (anode), and a reference electrode as in Figure 4. A metal foil substrate is used as the working electrode in industrial processes. An inert material provides the counter electrode, and the reference electrode measures and controls the potential. The reference electrode allows the process to be performed potentiostatically, allowing control of the substrate's potential.

Figure 4: CIGS electrodeposition apparatus

Simultaneous electrodeposition must overcome the fact that the elements' standard reduction potentials are not equal, causing preferential deposition of a single element. This problem is commonly alleviated by adding countering ions into solution for each ion to be deposited (Cu²⁺, Se⁴⁺, In³⁺, and Ga³⁺), thus changing that ion's reduction potential. Further, the Cu-Se system has a complicated behavior and the film's composition depends on the Se⁴⁺/Cu²⁺ ion flux ratio which can vary over the film surface. This requires the precursor concentrations and deposition potential to be optimized. Even with optimization, reproducibility is low over large areas due to composition variations and potential drops along the substrate.

The resulting films have small grains, are Cu-rich, and generally contain Cu_2−xSe_x phases along with impurities from the solution. Annealing is required to improve crystallinity. For efficiencies higher than 7%, a stoichiometry correction are required. The correction was originally done via high temperature physical vapor deposition, which is not practical in industry.

Solopower is currently producing cells with >13.7% conversion efficiency as per NREL.

Precursor combination by wafer-bonding inspired technique

Figure 5: Schematic of wafer-bonding inspired technique

In this process, two different precursor films are deposited separately on a substrate and a superstrate. The films are pressed together and heated to release the film from the reusable superstrate, leaving a CIGS absorber on the substrate (Figure 5). Heliovolt patented this procedure and named it the FASST process. In principle, the precursors can be deposited at low temperature using low-cost deposition techniques, lowering module cost. However, the first generations of products use higher temperature PVD methods and do not achieve full cost cutting potential. Flexible substrates could eventually be used in this process.

Typical film characteristics are not known outside of the company, as no research has been conducted by independently funded laboratories. However, Heliovolt claimed a top cell efficiency of 12.2%.

Coevaporation

Coevaporation, or codeposition, is the most prevalent CIGS fabrication technique. Boeing's coevaporation process deposits bilayers of CIGS with different stoichiometries onto a heated substrate and allows them to intermix.

NREL developed another process that involves three deposition steps and produced the current CIGS efficiency record holder at 20.3%. The first step in NREL's method is codeposition of In, Ga, and Se. This is followed by Cu and Se deposited at a higher temperature to allow for diffusion and intermixing of the elements. In the final stage In, Ga, and Se are again deposited to make the overall composition Cu deficient.

Würth Solar began producing CIGS cells using an inline coevaporation system in 2005 with module efficiencies between 11% and 12%. They opened another production facility and continued to improve efficiency and yield. Other companies scaling up coevaporation processes include Global Solar and Ascent Solar. Global Solar used an inline three stage deposition process. In all of the steps Se is supplied in excess in the vapor phase. In and Ga are first evaporated followed by Cu and then by In and Ga to make the film Cu deficient. These films performed quite favorably in relation to other manufacturers and to absorbers grown at NREL and the Institute for Energy Conversion (IEC). However, modules of Global Solar's films did not perform as well. The property in which the module most obviously under-performed was a low V_OC, which is characteristic of high defect density and high recombination velocities. Global Solar's absorber layer outperformed the NREL absorber in carrier lifetime and hall mobility. However, as completed cells the NREL sample performed better. This is evidence of a poor CIGS/CdS interface, possibly due to the lack of an ODC surface layer on the Global Solar film.

Disadvantages include uniformity issues over large areas and the related difficulty of coevaporating elements in an inline system. Also, high growth temperatures raise the thermal budget and costs. Additionally, coevaporation is plagued by low material utilization (deposition on chamber walls instead of the substrate, especially for selenium) and expensive vacuum equipment. A way to enhance Se utilisation is via a thermal or plasma-enhanced selenium-cracking process, which can be coupled with an ion beam source for ion beam assisted deposition.

Chemical vapor deposition

Chemical vapor deposition (CVD) has been implemented in multiple ways for the deposition of CIGS. Processes include atmosphere pressure metal organic CVD (AP-MOCVD), plasma-enhanced CVD (PECVD), low-pressure MOCVD (LP-MOCVD), and aerosol assisted MOCVD (AA-MOCVD). Research is attempting to switch from dual-source precursors to single-source precursors. Multiple source precursors must be homogeneously mixed and the flow rates of the precursors have to be kept at the proper stoichiometry. Single-source precursor methods do not suffer from these drawbacks and should enable better control of film composition.

As of 2014 CVD was not used for commercial CIGS synthesis. CVD produced films have low efficiency and a low V_OC, partially a result of a high defect concentration. Additionally, film surfaces are generally quite rough which serves to further decrease the V_OC. However, the requisite Cu deficiency has been achieved using AA-MOCVD along with a (112) crystal orientation.

CVD deposition temperatures are lower than those used for other processes such as co-evaporation and selenization of metallic precursors. Therefore, CVD has a lower thermal budget and lower costs. Potential manufacturing problems include difficulties converting CVD to an inline process as well as the expense of handling volatile precursors.

Electrospray deposition

CIS films can be produced by electrospray deposition. The technique involves the electric field assisted spraying of ink containing CIS nano-particles onto the substrate directly and then sintering in an inert environment. The main advantage of this technique is that the process takes place at room temperature and it is possible to attach this process with some continuous or mass production system like roll-to-roll production mechanism.

Rear surface passivation

Concepts of the rear surface passivation for CIGS solar cells shows the potential to improve the efficiency. The rear passivation concept has been taken from passivation technology of Silicon solar cells. Al2O3 and SiO2 have been used as the passivation materials. Nano-sized point contacts on Al2O3 layer and line contacts on SiO2 layer  provide the electrical connection of CIGS absorber to the rear electrode Molybdenum. The point contacts on the Al2O3 layer are created by e-beam lithography and the line contacts on the SiO2 layer are created using photolithography. It is also seen that the implementation of the passivation layers does not change the morphology of the CIGS layers.