In physics, an entropic force acting in a system is an emergent phenomenon resulting from the entire system's statistical tendency to increase its entropy, rather than from a particular underlying force on the atomic scale.
Mathematical formulation
In the canonical ensemble, the entropic force associated to a macrostate partition is given by
where is the temperature, is the entropy associated to the macrostate , and is the present macrostate.
Examples
Pressure of an ideal gas
The internal energy of an ideal gas depends only on its temperature, and not on the volume of its containing box, so it is not an energy effect that tends to increase the volume of the box as gas pressure does. This implies that the pressure of an ideal gas has an entropic origin.
What is the origin of such an entropic force? The most general
answer is that the effect of thermal fluctuations tends to bring a
thermodynamic system toward a macroscopic state that corresponds to a
maximum in the number of microscopic states (or micro-states)
that are compatible with this macroscopic state. In other words,
thermal fluctuations tend to bring a system toward its macroscopic state
of maximum entropy.
Brownian motion
The entropic approach to Brownian movement was initially proposed by R. M. Neumann. Neumann derived the entropic force for a particle undergoing three-dimensional Brownian motion using the Boltzmann equation, denoting this force as a diffusional driving force or radial force. In the paper, three example systems are shown to exhibit such a force:
A standard example of an entropic force is the elasticity of a freely jointed polymer molecule.
For an ideal chain, maximizing its entropy means reducing the distance
between its two free ends. Consequently, a force that tends to collapse
the chain is exerted by the ideal chain between its two free ends. This
entropic force is proportional to the distance between the two ends. The entropic force by a freely jointed chain has a clear mechanical origin and can be computed using constrained Lagrangian dynamics.
With regards to biological polymers, there appears to be an intricate
link between the entropic force and function. For example, disordered
polypeptide segments – in the context of the folded regions of the same
polypeptide chain – have been shown to generate an entropic force that
has functional implications.
Another example of an entropic force is the hydrophobic
force. At room temperature, it partly originates from the loss of
entropy by the 3D network of water molecules when they interact with
molecules of dissolved substance. Each water molecule is capable of
Therefore, water molecules can form an extended three-dimensional
network. Introduction of a non-hydrogen-bonding surface disrupts this
network. The water molecules rearrange themselves around the surface,
so as to minimize the number of disrupted hydrogen bonds. This is in
contrast to hydrogen fluoride (which can accept 3 but donate only 1) or ammonia (which can donate 3 but accept only 1), which mainly form linear chains.
If the introduced surface had an ionic or polar nature, there
would be water molecules standing upright on 1 (along the axis of an
orbital for ionic bond) or 2 (along a resultant polarity axis) of the
four sp3 orbitals.
These orientations allow easy movement, i.e. degrees of freedom, and
thus lowers entropy minimally. But a non-hydrogen-bonding surface with a
moderate curvature forces the water molecule to sit tight on the
surface, spreading 3 hydrogen bonds tangential to the surface, which
then become locked in a clathrate-like
basket shape. Water molecules involved in this clathrate-like basket
around the non-hydrogen-bonding surface are constrained in their
orientation. Thus, any event that would minimize such a surface is
entropically favored. For example, when two such hydrophobic particles
come very close, the clathrate-like baskets surrounding them merge.
This releases some of the water molecules into the bulk of the water,
leading to an increase in entropy.
Another related and counter-intuitive example of entropic force is protein folding, which is a spontaneous process and where hydrophobic effect also plays a role. Structures of water-soluble proteins typically have a core in which hydrophobic side chains are buried from water, which stabilizes the folded state. Charged and polar
side chains are situated on the solvent-exposed surface where they
interact with surrounding water molecules. Minimizing the number of
hydrophobic side chains exposed to water is the principal driving force
behind the folding process,although formation of hydrogen bonds within the protein also stabilizes protein structure.
Colloids
Entropic forces are important and widespread in the physics of colloids, where they are responsible for the depletion force, and the ordering of hard particles, such as the crystallization of hard spheres, the isotropic-nematic transition in liquid crystal phases of hard rods, and the ordering of hard polyhedra. Because of this, entropic forces can be an important driver of self-assembly.
Entropic forces arise in colloidal systems due to the osmotic pressure
that comes from particle crowding. This was first discovered in, and is
most intuitive for, colloid-polymer mixtures described by the Asakura–Oosawa model.
In this model, polymers are approximated as finite-sized spheres that
can penetrate one another, but cannot penetrate the colloidal particles.
The inability of the polymers to penetrate the colloids leads to a
region around the colloids in which the polymer density is reduced. If
the regions of reduced polymer density around two colloids overlap with
one another, by means of the colloids approaching one another, the
polymers in the system gain an additional free volume that is equal to
the volume of the intersection of the reduced density regions. The
additional free volume causes an increase in the entropy of the
polymers, and drives them to form locally dense-packed aggregates. A
similar effect occurs in sufficiently dense colloidal systems without
polymers, where osmotic pressure also drives the local dense packing of colloids into a diverse array of structures that can be rationally designed by modifying the shape of the particles. These effects are for anisotropic particles referred to as directional entropic forces.
Cytoskeleton
Contractile forces in biological cells are typically driven by molecular motors associated with the cytoskeleton. However, a growing body of evidence shows that contractile forces may also be of entropic origin. The foundational example is the action of microtubule crosslinker Ase1, which localizes to microtubule overlaps in the mitotic spindle.
Molecules of Ase1 are confined to the microtubule overlap, where they
are free to diffuse one-dimensionally. Analogically to an ideal gas in a
container, molecules of Ase1 generate pressure on the overlap ends.
This pressure drives the overlap expansion, which results in the
contractile sliding of the microtubules. An analogous example was found in the actin cytoskeleton. Here, the actin-bundling protein anillin drives actin contractility in cytokinetic rings.
Controversial examples
Some forces that are generally regarded as conventional forces have been argued to be actually entropic in nature. These theories remain controversial and are the subject of ongoing work. Matt Visser, professor of mathematics at Victoria University of Wellington, NZ in "Conservative Entropic Forces" criticizes selected approaches but generally concludes:
There is no reasonable doubt
concerning the physical reality of entropic forces, and no reasonable
doubt that classical (and semi-classical) general relativity is closely
related to thermodynamics. Based on the work of Jacobson, Thanu Padmanabhan,
and others, there are also good reasons to suspect a thermodynamic
interpretation of the fully relativistic Einstein equations might be
possible.
In 2009, Erik Verlinde argued that gravity can be explained as an entropic force. It claimed (similar to Jacobson's
result) that gravity is a consequence of the "information associated
with the positions of material bodies". This model combines the
thermodynamic approach to gravity with Gerard 't Hooft's holographic principle. It implies that gravity is not a fundamental interaction, but an emergent phenomenon.
Other forces
In the wake of the discussion started by Verlinde, entropic explanations for other fundamental forces have been suggested, including Coulomb's law. The same approach was argued to explain dark matter, dark energy and Pioneer effect.
Links to adaptive behavior
It was argued that causal entropic forces lead to spontaneous emergence of tool use and social cooperation. Causal entropic forces by definition maximize entropy production
between the present and future time horizon, rather than just greedily
maximizing instantaneous entropy production like typical entropic
forces.
A formal simultaneous connection between the mathematical
structure of the discovered laws of nature, intelligence and the
entropy-like measures of complexity was previously noted in 2000 by
Andrei Soklakov in the context of Occam's razor principle.
A conventional crystalline silicon solar cell (as of 2005). Electrical contacts made from busbars (the larger silver-colored strips) and fingers (the smaller ones) are printed on the siliconwafer.
A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect. It is a type of photoelectric cell, a device whose electrical characteristics (such as current, voltage, or resistance) vary when it is exposed to light. Individual solar cell devices are often the electrical building blocks of photovoltaic modules, known colloquially as "solar panels". Almost all commercial PV cells consist of crystalline silicon, with a market share of 95%. Cadmium telluridethin-film solar cells account for the remainder. The common single-junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts.
The separate extraction of those carriers to an external circuit.
There are multiple input factors that affect the output power of solar cells such as temperature, material properties, weather conditions, solar irradiance and more.
using light to excite electrons that can further be transported
by a semiconductor which delivers the energy (like that explored by Edmond Becquerel and implemented in modern dye-sensitized solar cells)
using light to split water directly into hydrogen and oxygen which can further be used in power generation
Electric vehicles that operate off of solar energy and/or sunlight are commonly referred to as solar cars. These vehicles use solar panels to convert absorbed light into electrical energy to be used by electric motors, with any excess energy stored in batteries. Batteries in solar-powered vehicles differ from starting batteries in standard ICE cars because they are fashioned to impart power towards electrical components of the vehicle for a long durations.
The first instance of photovoltaic cells within vehicular
applications was around midway through the second half of the 1900s. In
an effort to increase publicity and awareness in solar powered
transportation Hans Tholstrup decided to set up the first edition of the
World Solar Challenge in 1987.
It was a 3000 km race across the Australian outback where competitors
from industry research groups and top universities around the globe were
invited to compete.[citation needed]General Motors ended up winning the event by a significant margin with their Sunraycer vehicle that achieved speeds of over 40 mph. Contrary to popular belief however solar powered cars are one of the oldest alternative energy vehicles.
From a solar cell to a PV system. Diagram of the possible components of a photovoltaic system
Multiple solar cells in an integrated group, all oriented in one plane, constitute a solar photovoltaic panel or module.
Photovoltaic modules often have a sheet of glass on the sun-facing
side, allowing light to pass while protecting the semiconductor wafers. Solar cells connected in series creates an additive higher voltage, while connecting in parallel yields an additive higher current.
Parallel cells without bypass or shunt diodes that experience
shade can shut down the weaker (less illuminated) parallel string (each
string a number of series connected cells) causing substantial power
loss and possible damage because of the reverse bias applied to the
shaded cells by their illuminated partners.
Solar modules can be interconnected to create an array with a
desired peak DC voltage and loading current capacity. This functionality
can also be accomplished with various other solar devices that do more
than just create the desired voltages and currents, such as with MPPTs (maximum power point trackers) or module level power electronic (MLPE) units: microinverters or DC-DC optimizers.
Typical PV system prices in 2013 in selected countries (US$/W)
Source: IEA – Technology Roadmap: Solar Photovoltaic Energy report, 2014 edition Note: DOE – Photovoltaic System Pricing Trends reports lower prices for the U.S.
By 2020, the United States cost per watt for a utility scale system had declined to $0.94.
NASA used solar cells on its spacecraft from the beginning, their second successful satellite Vanguard 1 (1958) featured the first solar cells in space.
Solar cells were first used in a prominent application when they were proposed and flown on the Vanguard satellite in 1958, as an alternative power source to the primary battery
power source. By adding cells to the outside of the body, the mission
time could be extended with no major changes to the spacecraft or its
power systems. In 1959 the United States launched Explorer 6, featuring large wing-shaped solar arrays, which became a common feature in satellites. These arrays consisted of 9600 Hoffman solar cells.
By the 1960s, solar cells were (and still are) the main power
source for most Earth orbiting satellites and a number of probes into
the solar system, since they offered the best power-to-weight ratio.
The success of the space solar power market drove the development of
higher efficiencies in solar cells, due to limited other power options
and the desire for the best possible cells, up until the National Science Foundation "Research Applied to National Needs" program began to push development of solar cells for terrestrial applications.
In the early 1990s the technology used for space solar cells
diverged from the silicon technology used by terrestrial panels, with
the spacecraft application shifting to gallium arsenide-based III-V semiconductor materials, which then evolved into the modern III-V multijunction photovoltaic cell
used on spacecraft that are lightweight, compact, flexible, and highly
efficient. State of the art technology implemented on satellites uses
multi-junction photovoltaic cells, which are composed of different p–n
junctions with varying bandgaps in order to utilize a wider spectrum of
the sun's energy. Space solar cells additionally diverged from the
protective layer used by terrestrial panels, with space applications
using flexible laminate layers.
Additionally, large satellites require the use of large solar
arrays to produce electricity. These solar arrays need to be broken down
to fit in the geometric constraints of the launch vehicle the satellite
travels on before being injected into orbit. Historically, solar cells
on satellites consisted of several small terrestrial panels folded
together. These small panels would be unfolded into a large panel after
the satellite is deployed in its orbit. Newer satellites aim to use
flexible rollable solar arrays that are very lightweight and can be
packed into a very small volume. The smaller size and weight of these
flexible arrays drastically decreases the overall cost of launching a
satellite due to the direct relationship between payload weight and
launch cost of a launch vehicle.
In 2020, the US Naval Research Laboratory
conducted its first test of solar power generation in a satellite, the
Photovoltaic Radio-frequency Antenna Module (PRAM) experiment aboard the
Boeing X-37.
The photovoltaic effect was experimentally demonstrated first by French physicist Edmond Becquerel. In 1839, at age 19, he built the world's first photovoltaic cell in his father's laboratory. Willoughby Smith first described the "Effect of Light on Selenium during the passage of an Electric Current" in a 20 February 1873 issue of Nature. In 1883 Charles Fritts built the first solid state photovoltaic cell by coating the semiconductorselenium with a thin layer of gold to form the junctions; the device was only around 1% efficient. Other milestones include:
1946 – Russell Ohl patented the modern junction semiconductor solar cell, while working on the series of advances that would lead to the transistor.
1948 - Introduction to the World of Semiconductors states Kurt Lehovec may have been the first to explain the photo-voltaic effect in the peer-reviewed journal Physical Review.
1958 – Solar cells gained prominence with their incorporation onto the Vanguard I satellite.
Improved manufacturing methods post 1960’s
Pricing
and efficiency Improvements were gradual over the 1960s. One reason
that costs remained high was because space users were willing to pay for
the best possible cells, leaving no reason to invest in lower-cost,
less-efficient solutions. Also, price was determined largely by the semiconductor industry; their move to integrated circuits in the 1960s led to the availability of larger boules
at lower relative prices. As their price fell, the price of the
resulting cells did as well. These effects lowered 1971 cell costs to
some $100,000 per watt.
In late 1969 Elliot Berman joined Exxon's
task force which was looking for projects 30 years in the future and in
April 1973 he founded Solar Power Corporation (SPC), a wholly owned
subsidiary of Exxon at that time.
The group concluded that electrical power would be much more expensive
by 2000, and felt that the increase in price would make alternative
energy sources more attractive. He conducted a market study and
concluded that a price per watt of about $20/watt would create significant demand. To reduce costs, the team
eliminated the steps of polishing the wafers and coating them
with an anti-reflective layer, by relying on rough-sawn wafer surfaces.
replaced the expensive materials and hand wiring used in space applications with a printed circuit board on the back, acrylic plastic on the front, and silicone glue between the two, "potting" the cells.
used solar cells that could be made using cast-off material from the electronics market.
By 1973 they announced a product, and SPC convinced Tideland Signal to use its panels to power navigational buoys, initially for the U.S. Coast Guard.
Research and industrial production post 1970’s
Research
into solar power for terrestrial applications became prominent with the
U.S. National Science Foundation's Advanced Solar Energy Research and
Development Division within the "Research Applied to National Needs"
program, which ran from 1969 to 1977,
and funded research on developing solar power for ground electrical
power systems. A 1973 conference, the "Cherry Hill Conference", set
forth the technology goals required to achieve this goal and outlined an
ambitious project for achieving them, kicking off an applied research
program that would be ongoing for several decades. The program was eventually taken over by the Energy Research and Development Administration (ERDA), which was later merged into the U.S. Department of Energy.
Following the 1973 oil crisis,
oil companies used their higher profits to start (or buy) solar firms,
and were for decades the largest producers. Exxon, ARCO, Shell, Amoco
(later purchased by BP) and Mobil all had major solar divisions during
the 1970s and 1980s. Technology companies also participated, including
General Electric, Motorola, IBM, Tyco and RCA.
Energy
volume of silicon solar cells and oil harnessed by human beings per
dollar; Carbon intensity of some key electricity generation
technologies.
Adjusting for inflation, it cost $96 per watt for a solar module in
the mid-1970s. Process improvements and a very large boost in production
have brought that figure down more than 99%, to 30¢ per watt in 2018
and as low as 20¢ per watt in 2020.
Swanson's law is an observation similar to Moore's Law
that states that solar cell prices fall 20% for every doubling of
industry capacity. It was featured in an article in the British weekly
newspaper The Economist in late 2012. Balance of system
costs are now higher than the solar panels alone, where in 2018
commercial arrays could be built at below $1.00 a watt, fully
commissioned.
Over decades, costs for solar cells and panels has declined for may reasons:
Larger boules. When the semiconductor industry moved to ever-larger boules, older equipment became inexpensive.
Larger physical solar cell size. Sizes grew as surplus semiconductor equipment became available. ARCO
Solar's original panels used cells 2 to 4 inches (50 to 100 mm) in
diameter. Panels in the 1990s and early 2000s generally used 125 mm
wafers; since 2008, almost all new panels use greater than 156mm cells, and by 2020 even larger 182mm ‘M10’ cells.
Availability of large, high-quality glass sheets to cover the panels. Widespread introduction of flat screen televisions in the late 1990s and early 2000s led to such availability.
Thinner solar cells. High silicon prices in 2004–2008 encouraged
silicon solar cell manufacturers to reduce silicon consumption by making
them thinner; whereby 2008, according to Jef Poortmans, director of IMEC's
organic and solar department, cells used 8–9 grams (0.28–0.32 oz) of
silicon per watt of power generation, with typical wafer thicknesses in
the neighborhood of 200 microns.
During the 1990s, polysilicon
("poly") cells became increasingly popular. These cells offer less
efficiency than their monosilicon ("mono") counterparts, but are grown
in large vats that reduce cost. By the mid-2000s, poly was dominant in
the low-cost panel market, but more recently the monosilicon cells have
returned to widespread use due to the efficiency gains.
Crystalline silicon
panels dominate worldwide markets and are mostly manufactured in China
and Taiwan. By late 2011, a drop in European demand dropped prices for
crystalline solar modules to about $1.09 per watt down sharply from 2010. Prices continued to fall in 2012, reaching $0.62/watt by 4Q2012.
It was anticipated that electricity from PV will be competitive
with wholesale electricity costs all across Europe and the energy
payback time of crystalline silicon modules can be reduced to below 0.5
years by 2020.
Falling costs are considered one of the biggest factors in the rapid growth of renewable energy, of 2016, solar PV is growing fastest in Asia, with China and Japan currently accounting for half of worldwide deployment.
Costs of solar photovoltaic electricity fell by ~85% between 2010 (when
solar and wind made up 1.7% of global electricity generation) and 2021
(where they made up 8.7%). Global installed PV capacity reached at least 301 gigawatts in 2016, and grew to supply 1.3% of global power by 2016. In 2019 solar cells accounted for ~3 % of the world's electricity generation at 720 Tw-hr.
Subsidies and grid parity
Solar-specific feed-in tariffs
vary by and within country countries. Such tariffs can encourage the
development of solar power projects and to achieve grid parity. Grid parity, the point at which photovoltaic electricity is equal to or cheaper than grid power without subsidies, is expected to be first achieved in areas with abundant sun and high electricity costs such as in California and Japan. In 2007 BP claimed grid parity for Hawaii and other islands that otherwise use diesel fuel to produce electricity. George W. Bush set 2015 as the date for grid parity in the US. The Photovoltaic Association reported in 2012 that Australia had reached grid parity (ignoring feed in tariffs).
The price of solar panels fell steadily for 40 years, interrupted
in 2004 when high subsidies in Germany drastically increased demand
there and greatly increased the price of purified silicon (which is used
in computer chips as well as solar panels). The recession of 2008
and the onset of Chinese manufacturing caused prices to resume their
decline. In the four years after January 2008 prices for solar modules
in Germany dropped from €3 to €1 per peak watt. During that same time
production capacity surged with an annual growth of more than 50%. China
increased solar panel production market share from 8% in 2008 to over
55% in the last quarter of 2010. In December 2012 the price of Chinese solar panels had dropped to $0.60/Wp (crystalline modules). (The abbreviation Wp stands for watt peak capacity, or the maximum capacity under optimal conditions.)
As of the end of 2016, it was reported that spot prices for assembled solar panels (not cells) had fallen to a record-low of US$0.36/Wp. The second largest supplier, Canadian Solar
Inc., had reported costs of US$0.37/Wp in the third quarter of 2016,
having dropped $0.02 from the previous quarter, and hence was probably
still at least breaking even. Many producers expected costs would drop
to the vicinity of $0.30 by the end of 2017.
It was also reported that new solar installations were cheaper than
coal-based thermal power plants in some regions of the world, and this
was expected to be the case in most of the world within a decade.
Theory
Schematic
of charge collection by solar cells. Light transmits through
transparent conducting electrode creating electron hole pairs, which are
collected by both the electrodes.Working mechanism of a solar cell
A solar cell is made of semiconducting materials, such as silicon, that have been fabricated into a p–n junction. Such junctions are made by doping one side of the device p-type and the other n-type, for example in the case of silicon by introducing small concentrations of boron or phosphorus respectively.
In operation, photons in sunlight hit the solar cell and are absorbed by the semiconductor. When the photons are absorbed, electrons are excited from the valence band to the conduction band (or from occupied to unoccupied molecular orbitals in the case of an organic solar cell), producing electron-hole pairs.
If the electron-hole pairs are created near the junction between p-type
and n-type materials the local electric field sweeps them apart to
opposite electrodes, producing an excess of electrons on one side and an
excess of holes on the other. When the solar cell is unconnected (or
the external electrical load is very high) the electrons and holes will ultimately restore equilibrium by diffusing back across the junction against the field and recombine
with each other giving off heat, but if the load is small enough then
it is easier for equilibrium to be restored by the excess electrons
going around the external circuit, doing useful work along the way.
The most commonly known solar cell is configured as a large-area p–n junction
made from silicon. Other possible solar cell types are organic solar
cells, dye sensitized solar cells, perovskite solar cells, quantum dot
solar cells, etc. The illuminated side of a solar cell generally has a transparent conducting film
for allowing light to enter into the active material and to collect the
generated charge carriers. Typically, films with high transmittance and
high electrical conductance such as indium tin oxide, conducting polymers, or conducting nanowire networks are used for the purpose.
Efficiency
The Shockley-Queisser limit for the theoretical maximum efficiency of a solar cell. Semiconductors with band gap between 1 and 1.5eV
(827 nm to 1240 nm; near-infrared) have the greatest potential to form
an efficient single-junction cell. (The efficiency "limit" shown here
can be exceeded by multijunction solar cells.)
Solar cell efficiency may be broken down into reflectance efficiency,
thermodynamic efficiency, charge carrier separation efficiency and
conductive efficiency. The overall efficiency is the product of these
individual metrics.
The power conversion efficiency of a solar cell is a parameter which is defined by the fraction of incident power converted into electricity.
A solar cell has a voltage dependent efficiency curve, temperature coefficients, and allowable shadow angles.
Due to the difficulty in measuring these parameters directly, other parameters are substituted: thermodynamic efficiency, quantum efficiency, integrated quantum efficiency, VOC ratio, and fill factor. Reflectance losses are a portion of quantum efficiency under "external quantum efficiency". Recombination losses make up another portion of quantum efficiency, VOC
ratio, and fill factor. Resistive losses are predominantly categorized
under fill factor, but also make up minor portions of quantum
efficiency, VOC ratio.
The fill factor is the ratio of the actual maximum obtainable power to the product of the open-circuit voltage and short-circuit current.
This is a key parameter in evaluating performance. In 2009, typical
commercial solar cells had a fill factor > 0.70. Grade B cells were
usually between 0.4 and 0.7. Cells with a high fill factor have a low equivalent series resistance and a high equivalent shunt resistance, so less of the current produced by the cell is dissipated in internal losses.
Single p–n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of 33.16%, noted as the Shockley–Queisser limit in 1961. In the extreme, with an infinite number of layers, the corresponding limit is 86% using concentrated sunlight.
In 2014, three companies broke the record of 25.6% for a silicon
solar cell. Panasonic's was the most efficient. The company moved the
front contacts to the rear of the panel, eliminating shaded areas. In
addition they applied thin silicon films to the (high quality silicon)
wafer's front and back to eliminate defects at or near the wafer
surface.
In 2015, a 4-junction GaInP/GaAs//GaInAsP/GaInAs solar cell
achieved a new laboratory record efficiency of 46.1% (concentration
ratio of sunlight = 312) in a French-German collaboration between the Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE), CEA-LETI and SOITEC.
In September 2015, Fraunhofer ISE announced the achievement of an efficiency above 20% for epitaxial wafer cells. The work on optimizing the atmospheric-pressure chemical vapor deposition
(APCVD) in-line production chain was done in collaboration with NexWafe
GmbH, a company spun off from Fraunhofer ISE to commercialize
production.
For triple-junction thin-film solar cells, the world record is 13.6%, set in June 2015.
In 2016, researchers at Fraunhofer ISE announced a GaInP/GaAs/Si triple-junction solar cell with two terminals reaching 30.2% efficiency without concentration.
In 2017, a team of researchers at National Renewable Energy Laboratory (NREL), EPFL and CSEM (Switzerland)
reported record one-sun efficiencies of 32.8% for dual-junction
GaInP/GaAs solar cell devices. In addition, the dual-junction device was
mechanically stacked with a Si solar cell, to achieve a record one-sun
efficiency of 35.9% for triple-junction solar cells.
Global photovoltaics market share by technology 1980–2021.[67]: 24, 25
Solar cells are typically named after the semiconducting material of which they are composed. These materials have varying characteristics to absorb optimal available sunlight spectrum. Some cells are designed to handle sunlight that reaches the Earth's surface, while others are optimized for use in space. Solar cells can be made of a single layer of light-absorbing material (single-junction) or use multiple physical configurations (multi-junctions) to take advantage of various absorption and charge separation mechanisms.
Solar cells can be classified into first, second and third generation:
Third generation of solar cells
includes a number of thin-film technologies often described as emerging
photovoltaics—most of them have not yet been commercially applied and
are still in the research or development phase. Many use organic
materials, often organometallic
compounds as well as inorganic substances. Despite the fact that their
efficiencies had been low and the stability of the absorber material was
often too short for commercial applications, there is research into
these technologies as they promise to achieve the goal of producing
low-cost, high-efficiency solar cells.
As of 2016, the most popular and efficient solar cells were those
made from thin wafers of silicon which are also the oldest solar cell
technology.
By far, the most prevalent bulk material for solar cells is crystalline silicon (c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon or wafer. These cells are entirely based around the concept of a p–n junction. Solar cells made of c-Si are made from wafers between 160 and 240 micrometers thick.
The roof, bonnet and large parts of the outer shell of the Sion are equipped with highly efficient monocrystalline silicon cells
Monocrystalline silicon
(mono-Si) solar cells feature a single-crystal composition that enables
electrons to move more freely than in a multi-crystal configuration.
Consequently, monocrystalline solar panels deliver a higher efficiency
than their multicrystalline counterparts.
The corners of the cells look clipped, like an octagon, because the
wafer material is cut from cylindrical ingots, that are typically grown
by the Czochralski process. Solar panels using mono-Si cells display a distinctive pattern of small white diamonds.
Epitaxial silicon development
Epitaxial wafers of crystalline silicon can be grown on a monocrystalline silicon "seed" wafer by chemical vapor deposition
(CVD), and then detached as self-supporting wafers of some standard
thickness (e.g., 250 μm) that can be manipulated by hand, and directly
substituted for wafer cells cut from monocrystalline silicon ingots.
Solar cells made with this "kerfless" technique can have efficiencies approaching those of wafer-cut cells, but at appreciably lower cost if the CVD can be done at atmospheric pressure in a high-throughput inline process. The surface of epitaxial wafers may be textured to enhance light absorption.
In June 2015, it was reported that heterojunction
solar cells grown epitaxially on n-type monocrystalline silicon wafers
had reached an efficiency of 22.5% over a total cell area of 243.4 cm.
Polycrystalline silicon,
or multicrystalline silicon (multi-Si) cells are made from cast square
ingots—large blocks of molten silicon carefully cooled and solidified.
They consist of small crystals giving the material its typical metal flake effect.
Polysilicon cells are the most common type used in photovoltaics and
are less expensive, but also less efficient, than those made from
monocrystalline silicon.
Ribbon silicon
Ribbon silicon is a type of polycrystalline silicon—it is formed by drawing flat thin films from molten
silicon and results in a polycrystalline structure. These cells are
cheaper to make than multi-Si, due to a great reduction in silicon
waste, as this approach does not require sawing from ingots. However, they are also less efficient.
Mono-like-multi silicon (MLM)
This
form was developed in the 2000s and introduced commercially around
2009. Also called cast-mono, this design uses polycrystalline casting
chambers with small "seeds" of mono material. The result is a bulk
mono-like material that is polycrystalline around the outsides. When
sliced for processing, the inner sections are high-efficiency mono-like
cells (but square instead of "clipped"), while the outer edges are sold
as conventional poly. This production method results in mono-like cells
at poly-like prices.
Thin-film technologies reduce the amount of active material in a
cell. Most designs sandwich active material between two panes of glass.
Since silicon solar panels only use one pane of glass, thin film panels
are approximately twice as heavy as crystalline silicon panels, although
they have a smaller ecological impact (determined from life cycle analysis).
Cadmium telluride is the only thin film material so far to rival
crystalline silicon in cost/watt. However cadmium is highly toxic and tellurium (anion: "telluride") supplies are limited. The cadmium
present in the cells would be toxic if released. However, release is
impossible during normal operation of the cells and is unlikely during
fires in residential roofs. A square meter of CdTe contains approximately the same amount of Cd as a single C cell nickel-cadmium battery, in a more stable and less soluble form.
Copper indium gallium selenide (CIGS) is a direct band gap material. It has the highest efficiency (~20%) among all commercially significant thin film materials (see CIGS solar cell). Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. Recent developments at IBM and Nanosolar attempt to lower the cost by using non-vacuum solution processes.
Amorphous silicon is the most well-developed thin film technology
to-date. An amorphous silicon (a-Si) solar cell is made of
non-crystalline or microcrystalline silicon. Amorphous silicon has a
higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which
means it absorbs the visible part of the solar spectrum more strongly
than the higher power density infrared
portion of the spectrum. The production of a-Si thin film solar cells
uses glass as a substrate and deposits a very thin layer of silicon by plasma-enhanced chemical vapor deposition (PECVD).
Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open-circuit voltage.
Nc-Si has about the same bandgap as c-Si and nc-Si and a-Si can
advantageously be combined in thin layers, creating a layered cell
called a tandem cell. The top cell in a-Si absorbs the visible light and
leaves the infrared part of the spectrum for the bottom cell in nc-Si.
Gallium arsenide thin film
The semiconductor material gallium arsenide (GaAs) is also used for single-crystalline thin film solar cells. Although GaAs cells are very expensive, they hold the world's record in efficiency for a single-junction solar cell at 28.8%. Typically fabricated on crystalline silicon wafer with a 41% fill factor,
by moving to porous silicon fill factor can be increased to 56% with
potentially reduced cost. Using less active GaAs material by fabricating
nanowires is another potential pathway to cost reduction. GaAs is more commonly used in multijunction photovoltaic cells for concentrated photovoltaics (CPV, HCPV) and for solar panels on spacecraft, as the industry favours efficiency over cost for space-based solar power.
Based on the previous literature and some theoretical analysis, there
are several reasons why GaAs has such high power conversion efficiency
for three main reasons:
GaAs bandgap is 1.43ev which is almost ideal for solar cells.
Gallium is a by-product of high temperature smelting of other
metals, allowing GaAs cells to be relatively insensitive to heat and it
can keep high efficiency when temperature is quite high in application.
Using GaAs as the active layer in a solar cell, engineers can have
multiple choices of other layers which can more efficiently generate
electrons and holes in GaAs.
Dawn's 10 kW triple-junction gallium arsenide solar array at full extension
Multi-junction cells consist of multiple thin films, each essentially a solar cell grown on top of another, typically using metalorganic vapour phase epitaxy. Each layer has a different band gap energy to allow it to absorb electromagnetic radiation over a different portion of the spectrum. Multi-junction cells were originally developed for special applications such as satellites and space exploration, but are now used increasingly in terrestrial concentrator photovoltaics
(CPV), an emerging technology that uses lenses and curved mirrors to
concentrate sunlight onto small, highly efficient multi-junction solar
cells. By concentrating sunlight up to a thousand times, High concentration photovoltaics (HCPV) has the potential to outcompete conventional solar PV in the future.
Tandem solar cells based on monolithic, series connected, gallium
indium phosphide (GaInP), gallium arsenide (GaAs), and germanium (Ge)
p–n junctions, are increasing sales, despite cost pressures.
Between December 2006 and December 2007, the cost of 4N gallium metal
rose from about $350 per kg to $680 per kg. Additionally, germanium
metal prices have risen substantially to $1000–1200 per kg this year.
Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and
7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing
crystals, and boron oxide, these products are critical to the entire
substrate manufacturing industry.
A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP 2. Triple-junction GaAs solar cells were used as the power source of the Dutch four-time World Solar Challenge winners Nuna in 2003, 2005 and 2007 and by the Dutch solar cars Solutra (2005), Twente One (2007) and 21Revolution (2009).
GaAs based multi-junction devices are the most efficient solar cells to
date. On 15 October 2012, triple junction metamorphic cells reached a
record high of 44%.
In 2022, researchers at Fraunhofer Institute for Solar Energy Systems
ISE in Freiburg, Germany, demonstrated a record solar cell efficiency of
47.6% under 665-fold sunlight concentration with a four-junction
concentrator solar cell.
GaInP/Si dual-junction solar cells
In 2016, a new approach was described for producing hybrid photovoltaic wafers combining the high efficiency of III-V multi-junction solar cells
with the economies and wealth of experience associated with silicon.
The technical complications involved in growing the III-V material on
silicon at the required high temperatures, a subject of study for some
30 years, are avoided by epitaxial growth of silicon on GaAs at low
temperature by plasma-enhanced chemical vapor deposition (PECVD).
Si single-junction solar cells have been widely studied for
decades and are reaching their practical efficiency of ~26% under 1-sun
conditions.
Increasing this efficiency may require adding more cells with bandgap
energy larger than 1.1 eV to the Si cell, allowing to convert
short-wavelength photons for generation of additional voltage. A
dual-junction solar cell with a band gap of 1.6–1.8 eV as a top cell can
reduce thermalization loss, produce a high external radiative
efficiency and achieve theoretical efficiencies over 45%.
A tandem cell can be fabricated by growing the GaInP and Si cells.
Growing them separately can overcome the 4% lattice constant mismatch
between Si and the most common III–V layers that prevent direct
integration into one cell. The two cells therefore are separated by a
transparent glass slide so the lattice mismatch does not cause strain to
the system. This creates a cell with four electrical contacts and two
junctions that demonstrated an efficiency of 18.1%. With a fill factor
(FF) of 76.2%, the Si bottom cell reaches an efficiency of 11.7% (± 0.4)
in the tandem device, resulting in a cumulative tandem cell efficiency
of 29.8%. This efficiency exceeds the theoretical limit of 29.4%
and the record experimental efficiency value of a Si 1-sun solar cell,
and is also higher than the record-efficiency 1-sun GaAs device.
However, using a GaAs substrate is expensive and not practical. Hence
researchers try to make a cell with two electrical contact points and
one junction, which does not need a GaAs substrate. This means there
will be direct integration of GaInP and Si.
Perovskite solar cells are solar cells that include a perovskite-structured
material as the active layer. Most commonly, this is a
solution-processed hybrid organic-inorganic tin or lead halide based
material. Efficiencies have increased from below 5% at their first usage
in 2009 to 25.5% in 2020, making them a very rapidly advancing
technology and a hot topic in the solar cell field. Researchers at University of Rochester reported in 2023 that significant further improvements in cell efficiency can be achieved by utilizing Purcell effect.
Perovskite solar cells are also forecast to be extremely cheap to
scale up, making them a very attractive option for commercialisation.
So far most types of perovskite solar cells have not reached sufficient
operational stability to be commercialised, although many research
groups are investigating ways to solve this.
Energy and environmental sustainability of perovskite solar cells and
tandem perovskite are shown to be dependent on the structures.
Photonic front contacts for light management can improve the perovskite
cells' performance, via enhanced broadband absorption, while allowing
better operational stability due to protection against the harmful
high-energy (above Visible) radiation. The inclusion of the toxic element lead in the most efficient perovskite solar cells is a potential problem for commercialisation.
Bifacial solar cell plant in Noto (Senegal), 1988 - Floor painted in white to enhance albedo.
With a transparent rear side, bifacial solar cells can absorb light
from both the front and rear sides. Hence, they can produce more
electricity than conventional monofacial solar cells. The first patent
of bifacial solar cells was filed by Japanese researcher Hiroshi Mori,
in 1966. Later, it is said that Russia was the first to deploy bifacial solar cells in their space program in the 1970s. In 1976, the Institute for Solar Energy of the Technical University of Madrid, began a research program for the development of bifacial solar cells led by Prof. Antonio Luque.
Based on 1977 US and Spanish patents by Luque, a practical bifacial
cell was proposed with a front face as anode and a rear face as cathode;
in previously reported proposals and attempts both faces were anodic
and interconnection between cells was complicated and expensive.
In 1980, Andrés Cuevas, a PhD student in Luque's team, demonstrated
experimentally a 50% increase in output power of bifacial solar cells,
relative to identically oriented and tilted monofacial ones, when a
white background was provided. In 1981 the company Isofoton was founded in Málaga
to produce the developed bifacial cells, thus becoming the first
industrialization of this PV cell technology. With an initial production
capacity of 300 kW/yr of bifacial solar cells, early landmarks of
Isofoton's production were the 20kWp power plant in San Agustín de Guadalix, built in 1986 for Iberdrola, and an off grid installation by 1988 also of 20kWp in the village of Noto Gouye Diama (Senegal) funded by the Spanish international aid and cooperation programs.
Due to the reduced manufacturing cost, companies have again
started to produce commercial bifacial modules since 2010. By 2017,
there were at least eight certified PV manufacturers providing bifacial
modules in North America. The International Technology Roadmap for
Photovoltaics (ITRPV) predicted that the global market share of bifacial
technology will expand from less than 5% in 2016 to 30% in 2027.
Due to the significant interest in the bifacial technology, a
recent study has investigated the performance and optimization of
bifacial solar modules worldwide. The results indicate that, across the globe, ground-mounted bifacial
modules can only offer ~10% gain in annual electricity yields compared
to the monofacial counterparts for a ground albedo coefficient
of 25% (typical for concrete and vegetation groundcovers). However, the
gain can be increased to ~30% by elevating the module 1 m above the
ground and enhancing the ground albedo coefficient to 50%. Sun et al. also derived a set of empirical equations that can optimize bifacial solar modules analytically.
In addition, there is evidence that bifacial panels work better than
traditional panels in snowy environments as bifacials on dual-axis
trackers made 14% more electricity in a year than their monofacial
counterparts and 40% during the peak winter months.
An online simulation tool
is available to model the performance of bifacial modules in any
arbitrary location across the entire world. It can also optimize
bifacial modules as a function of tilt angle, azimuth angle, and
elevation above the ground.
Intermediate band photovoltaics in solar cell research provides methods for exceeding the Shockley–Queisser limit
on the efficiency of a cell. It introduces an intermediate band (IB)
energy level in between the valence and conduction bands. Theoretically,
introducing an IB allows two photons with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency.
Luque and Marti first derived a theoretical limit for an IB device with one midgap energy level using detailed balance.
They assumed no carriers were collected at the IB and that the device
was under full concentration. They found the IB maximum efficiency to be
63.2%, for a bandgap of 1.95eV with the IB 0.71eV from either the
valence or conduction band ans compared to the under one sun
illumination limiting efficiency of 47%.[116]
Several means are under study to realize IB semiconductors with such
optimum 3-bandgap configuration, namely via materials engineering
(controlled inclusion of deep level impurities or highly mismatched
alloys) and nano-structuring (quantum-dots in host hetero-crystals).
In December 2022, it was reported that MIT
researchers had developed ultralight fabric solar cells. These cells
offer a weight one-hundredth that of traditional panels while generating
18 times more power per kilogram. Thinner than a human hair, these
cells can be laminated onto various surfaces, such as boat sails, tents,
tarps, or drone wings, to extend their functionality. Using ink-based
materials and scalable techniques, researchers coat the solar cell
structure with printable electronic inks, completing the module with screen-printed electrodes.
Tested on high-strength fabric, the cells produce 370
watts-per-kilogram, representing an improvement over conventional solar
cells.
Upconversion and downconversion
Photon upconversion is the process of using two low-energy (e.g., infrared) photons to produce one higher energy photon; downconversion is the process of using one high energy photon (e.g.,
ultraviolet) to produce two lower energy photons. Either of these
techniques could be used to produce higher efficiency solar cells by
allowing solar photons to be more efficiently used. The difficulty,
however, is that the conversion efficiency of existing phosphors exhibiting up- or down-conversion is low, and is typically narrow band.
One upconversion technique is to incorporate lanthanide-doped materials (Er3+ , Yb3+ , Ho3+ or a combination), taking advantage of their luminescence to convert infrared radiation to visible light. Upconversion process occurs when two infrared photons are absorbed by rare-earthions
to generate a (high-energy) absorbable photon. As example, the energy
transfer upconversion process (ETU), consists in successive transfer
processes between excited ions in the near infrared. The upconverter
material could be placed below the solar cell to absorb the infrared
light that passes through the silicon. Useful ions are most commonly
found in the trivalent state. Er+ ions have been the most used. Er3+ ions absorb solar radiation around 1.54 μm. Two Er3+
ions that have absorbed this radiation can interact with each other
through an upconversion process. The excited ion emits light above the
Si bandgap that is absorbed by the solar cell and creates an additional
electron–hole pair that can generate current. However, the increased
efficiency was small. In addition, fluoroindate glasses have low phonon energy and have been proposed as suitable matrix doped with Ho3+ ions.
Typically a rutheniummetalorganicdye (Ru-centered) is used as a monolayer of light-absorbing material, which is adsorbed onto a thin film of titanium dioxide. The dye-sensitized solar cell depends on this mesoporous layer of nanoparticulatetitanium dioxide (TiO2) to greatly amplify the surface area (200–300 m2/g TiO 2, as compared to approximately 10 m2/g
of flat single crystal) which allows for a greater number of dyes per
solar cell area (which in term in increases the current). The
photogenerated electrons from the light absorbing dye are passed on to
the n-type TiO 2 and the holes are absorbed by an electrolyte on the other side of the dye. The circuit is completed by a redox
couple in the electrolyte, which can be liquid or solid. This type of
cell allows more flexible use of materials and is typically manufactured
by screen printing or ultrasonic nozzles,
with the potential for lower processing costs than those used for bulk
solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light and the cell casing is difficult to seal
due to the solvents used in assembly. Due to this reason, researchers
have developed solid-state dye-sensitized solar cells that use a solid
electrolyte to avoid leakage. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations.
Quantum dot solar cells (QDSCs) are based on the Gratzel cell, or dye-sensitized solar cell architecture, but employ low band gapsemiconductornanoparticles, fabricated with crystallite sizes small enough to form quantum dots (such as CdS, CdSe, Sb 2S 3, PbS,
etc.), instead of organic or organometallic dyes as light absorbers.
Due to the toxicity associated with Cd and Pb based compounds there are
also a series of "green" QD sensitizing materials in development (such
as CuInS2, CuInSe2 and CuInSeS). QD's size quantization allows for the band gap to be tuned by simply changing particle size. They also have high extinction coefficients and have shown the possibility of multiple exciton generation.
In a QDSC, a mesoporous layer of titanium dioxide nanoparticles forms the backbone of the cell, much like in a DSSC. This TiO 2 layer can then be made photoactive by coating with semiconductor quantum dots using chemical bath deposition, electrophoretic deposition
or successive ionic layer adsorption and reaction. The electrical
circuit is then completed through the use of a liquid or solid redox couple. The efficiency of QDSCs has increased to over 5% shown for both liquid-junction and solid state cells, with a reported peak efficiency of 11.91%. In an effort to decrease production costs, the Prashant Kamat research group demonstrated a solar paint made with TiO 2 and CdSe that can be applied using a one-step method to any conductive surface with efficiencies over 1%. However, the absorption of quantum dots (QDs) in QDSCs is weak at room temperature. The plasmonic nanoparticles can be utilized to address the weak absorption of QDs (e.g., nanostars). Adding an external infrared pumping source to excite intraband and interband transition of QDs is another solution.
They can be processed from liquid solution, offering the
possibility of a simple roll-to-roll printing process, potentially
leading to inexpensive, large-scale production. In addition, these cells
could be beneficial for some applications where mechanical flexibility
and disposability are important. Current cell efficiencies are, however,
very low, and practical devices are essentially non-existent.
Energy conversion efficiencies achieved to date using conductive
polymers are very low compared to inorganic materials. However, Konarka Power Plastic reached efficiency of 8.3% and organic tandem cells in 2012 reached 11.1%.
The active region of an organic device consists of two materials,
one electron donor and one electron acceptor. When a photon is
converted into an electron hole pair, typically in the donor material,
the charges tend to remain bound in the form of an exciton,
separating when the exciton diffuses to the donor-acceptor interface,
unlike most other solar cell types. The short exciton diffusion lengths
of most polymer systems tend to limit the efficiency of such devices.
Nanostructured interfaces, sometimes in the form of bulk
heterojunctions, can improve performance.
In 2011, MIT and Michigan State researchers developed solar cells
with a power efficiency close to 2% with a transparency to the human
eye greater than 65%, achieved by selectively absorbing the ultraviolet
and near-infrared parts of the spectrum with small-molecule compounds.Researchers at UCLA more recently developed an analogous polymer solar
cell, following the same approach, that is 70% transparent and has a 4%
power conversion efficiency. These lightweight, flexible cells can be produced in bulk at a low cost and could be used to create power generating windows.
In 2013, researchers announced polymer cells with some 3% efficiency. They used block copolymers,
self-assembling organic materials that arrange themselves into distinct
layers. The research focused on P3HT-b-PFTBT that separates into bands
some 16 nanometers wide.
Adaptive cells
Adaptive
cells change their absorption/reflection characteristics depending on
environmental conditions. An adaptive material responds to the intensity
and angle of incident light. At the part of the cell where the light is
most intense, the cell surface changes from reflective to adaptive,
allowing the light to penetrate the cell. The other parts of the cell
remain reflective increasing the retention of the absorbed light within
the cell.
In 2014, a system was developed that combined an adaptive surface
with a glass substrate that redirect the absorbed to a light absorber
on the edges of the sheet. The system also includes an array of fixed
lenses/mirrors to concentrate light onto the adaptive surface. As the
day continues, the concentrated light moves along the surface of the
cell. That surface switches from reflective to adaptive when the light
is most concentrated and back to reflective after the light moves along.
Surface texturing
Solar Impulse aircraft are Swiss-designed single-seat monoplanes powered entirely from photovoltaic cells
Incident light rays onto a textured surface do not reflect out to
the air as opposed to rays onto a flat surface, but rather some light
rays are bounced back onto the other surface again due to the geometry
of the surface; increasing light absorption and light to electricity
conversion efficiency. Surface texturing is one technique used to reduce
optical losses, primarily in cost-effective, low light absorption thin-film solar cells. In combination with anti-reflective coating,
surface texturing technique can effectively trap light rays within a
thin film silicon solar cell. Consequently, at the same power output,
thickness for solar cells can decrease with the increased absorption of
light rays.
Surface texture geometry and texturing techniques can be done in multiple ways. Etching c-Si substrates can produce randomly distributed square based pyramids on the surface using anisotropic etchants. Studies show that c-Si
wafers could be etched down to form nano-scale inverted pyramids. In
2012, researchers at MIT reported that c-Si films textured with
nanoscale inverted pyramids could achieve light absorption comparable to
30 times thicker planar c-Si.
While easier to manufacture, but with less efficiency, multicrystalline
solar cells can be surface-textured through isotopic etching or
photolithography methods to yield solar energy conversion efficiency
comparable to that of monocrystalline silicon cells.
This texture effect as well as the interaction with other
interfaces in the PV module is a challenging optical simulation task,
but at least one efficient method for modeling and optimization that
exists is the OPTOS formalism.
Encapsulation
Solar
cells are commonly encapsulated in a transparent polymeric resin to
protect the delicate solar cell regions for coming into contact with
moisture, dirt, ice, and other environmental conditions expected during
operation. Encapsulants are commonly made from polyvinyl acetate
or glass. Most encapsulants are uniform in structure and composition,
which increases light collection owing to light trapping from total
internal reflection of light within the resin. Research has been
conducted into structuring the encapsulant to provide further collection
of light. Such encapsulants have included roughened glass surfaces, diffractive elements, prism arrays, air prisms, v-grooves, diffuse elements, as well as multi-directional waveguide arrays. Prism arrays show an overall 5% increase in the total solar energy conversion.
Arrays of vertically aligned broadband waveguides provide a 10%
increase at normal incidence, as well as wide-angle collection
enhancement of up to 4%, with optimized structures yielding up to a 20% increase in short circuit current. Active coatings that convert infrared light into visible light have shown a 30% increase.
Nanoparticle coatings inducing plasmonic light scattering increase
wide-angle conversion efficiency up to 3%. Optical structures have also
been created in encapsulation materials to effectively "cloak" the
metallic front contacts.
Solar cells share some of the same processing and manufacturing
techniques as other semiconductor devices. However, the strict
requirements for cleanliness and quality control of semiconductor
fabrication are more relaxed for solar cells, lowering costs.
Polycrystalline silicon
wafers are made by wire-sawing block-cast silicon ingots into 180 to
350 micrometer thick wafers. The wafers are usually lightly p-type-doped. A surface diffusion of n-type dopants is performed on the front side of the wafer. This forms a p–n junction a few hundred nanometers below the surface.
Anti-reflection coatings are then typically applied to increase the amount of light coupled into the solar cell. Silicon nitride
has gradually replaced titanium dioxide as the preferred material,
because of its excellent surface passivation qualities. It prevents
carrier recombination at the cell surface. A layer several hundred
nanometers thick is applied using plasma-enhanced chemical vapor deposition.
Some solar cells have textured front surfaces that, like
anti-reflection coatings, increase the amount of light reaching the
wafer. Such surfaces were first applied to single-crystal silicon,
followed by multicrystalline silicon somewhat later.
A full area metal contact is made on the back surface, and a
grid-like metal contact made up of fine "fingers" and larger "bus bars"
are screen-printed onto the front surface using a silver
paste. This is an evolution of the so-called "wet" process for applying
electrodes, first described in a US patent filed in 1981 by Bayer AG.
The rear contact is formed by screen-printing a metal paste.To maximize
frontal surface area available for sunlight and improve solar cell
efficiency, manufacturers use various rear contact electrode techniques:
Passivated emitter rear contact (PERC) uses a solid aluminum rear contact surface and adds a polymer film to capture light
Tunnel oxide passivated contact (TOPCon) uses a grid pattern of
increasingly smaller silver rear bus bars and adds an oxidation layer to
the PERC film to capture more light
Interdigitated back contact (IBC)
The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact
with the silicon. Some companies use an additional electroplating step
to increase efficiency. After the metal contacts are made, the solar
cells are interconnected by flat wires or metal ribbons, and assembled
into modules or "solar panels". Solar panels have a sheet of tempered glass on the front, and a polymer or glass encapsulation on the back.
Different types of manufacturing and recycling partly determine
how effective it is in decreasing emissions and having a positive
environmental effect. Such differences and effectiveness could be quantified for production of the most optimal types of products for different purposes in different regions across time.
The IEA's 2022 Special Report highlights China's dominance over the solar PV supply chain,
with an investment exceeding US$50 billion and the creation of around
300,000 jobs since 2011. China commands over 80% of all manufacturing
stages for solar panels. This control has drastically cut costs but also
led to issues like supply-demand imbalances and polysilicon
production constraints. Nevertheless, China's strategic policies have
reduced solar PV costs by more than 80%, increasing global
affordability. In 2021, China's solar PV exports were over US$30
billion.
Meeting global energy and climate targets necessitates a major
expansion in solar PV manufacturing, aiming for over 630 GW by 2030
according to the IEA's "Roadmap to Net Zero Emissions by 2050". China's
dominance, controlling nearly 95% of key solar PV components and 40% of
the world's polysilicon production in Xinjiang, poses risks of supply
shortages and cost surges. Critical mineral demand, like silver, may
exceed 30% of 2020's global production by 2030.
In 2021, China's share of solar PV module
production reached approximately 70%, an increase from 50% in 2010.
Other key producers included Vietnam (5%), Malaysia (4%), Korea (4%),
and Thailand (2%), with much of their production capacity developed by
Chinese companies aimed at exports, notably to the United States.
As of September 2018, sixty percent of the world's solar photovoltaic modules were made in China. As of May 2018, the largest photovoltaic plant in the world is located in the Tengger desert in China. In 2018, China added more photovoltaic installed capacity (in GW) than the next 9 countries combined. In 2021, China's share of solar PV module production reached approximately 70%.
In the first half of 2023, China's production of PV modules
exceeded 220 GW, marking an increase of over 62% compared to the same
period in 2022. In 2022, China maintained its position as the world's
largest PV module producer, holding a dominant market share of 77.8%.
Vietnam
In
2022, Vietnam was the second-largest PV module producer, only behind
China, with its production capacity rising to 24.1 GW, marking a
significant 47% increase from the 16.4 GW produced in 2021. Vietnam
accounts for 6.4% of the world's photovoltaic production.
In 2022, Malaysia was the third-largest PV module producer, with a
production capacity of 10.8 GW, accounting for 2.8% of global
production. This placed it behind China, which dominated with 77.8%, and
Vietnam, which contributed 6.4%.
Solar energy production in the U.S. has doubled from 2013 to 2019. This was driven first by the falling price of quality silicon, and later simply by the globally plunging cost of photovoltaic modules.In 2018, the U.S. added 10.8GW of installed solar photovoltaic energy, an increase of 21%.
Latin America: Latin America has emerged as a promising
region for solar energy development in recent years, with over 10 GW of
installations in 2020. The solar market in Latin America has been driven
by abundant solar resources, falling costs, competitive auctions and
growing electricity demand. Some of the leading countries for solar
energy in Latin America are Brazil, Mexico, Chile and Argentina.
However, the solar market in Latin America also faces some challenges,
such as political instability, financing gaps and power transmission
bottlenecks.
Middle East and Africa: The Middle East and Africa has
also experienced significant growth in solar energy deployment in recent
years, with over 8 GW installations in 2020. The solar market in the
Middle East and Africa has been driven by the low-cost generation of
solar energy, the diversification of energy sources, the fight against
climate change and rural electrification are motivated. Some of the
notable countries for solar energy in the Middle East and Africa are
Saudi Arabia, United Arab Emirates, Egypt, Morocco and South Africa.
However, the solar market in the Middle East and Africa also faces
several obstacles, including social unrest, regulatory uncertainty and
technical barriers.
Materials sourcing
Like many other energy generation technologies, the manufacture of
solar cells, especially its rapid expansion, has many environmental and
supply-chain implications. Global mining may adapt and potentially
expand for sourcing the needed minerals which vary per type of solar
cell.Recycling solar panels could be a source for materials that would otherwise need to be mined.
Disposal
Solar
cells degrade over time and lose their efficiency. Solar cells in
extreme climates, such as desert or polar, are more prone to degradation
due to exposure to harsh UV light and snow loads respectively. Usually, solar panels are given a lifespan of 25–30 years before decommissioning.
The International Renewable Energy Agency estimated that the amount of solar panel electronic waste
generated in 2016 was 43,500–250,000 metric tons. This number is
estimated to increase substantially by 2030, reaching an estimated waste
volume of 60–78 million metric tons in 2050.
The most widely used solar cells in the market are crystalline solar
cells. A product is truly recyclable if it can harvested again. In the 2016 Paris Agreement,
195 countries agreed to reduce their carbon emissions by shifting their
focus away from fossil fuels and towards renewable energy sources.
Owing to this, Solar will be a major contributor to electricity
generation all over the world. So, there will be a plethora of solar
panels to be recycled after the end of their life cycle. In fact, many
researchers around the globe have voiced their concern about finding
ways to use silicon cells after recycling.
Additionally, these cells have hazardous elements/compounds, including lead (Pb), cadmium
(Cd) or cadmium sulfide (CdS), selenium (Se), and barium (Ba) as
dopants aside from the valuables silicon (Si), aluminum (Al), silver
(Ag), and copper (Cu). The harmful elements/compounds if not disposed of
with the proper technique can have severe harmful effects on human life
and wildlife alike.
There are various ways c-Si can be recycled. Mainly thermal and chemical separation methods are used. This happens in two stages
PV solar cell separation: in thermal delamination, the ethylene vinyl acetate (EVA) is removed and materials such as glass, Tedlar®, aluminium frame, steel, copper and plastics are separated;
cleansing the surface of PV solar cells: unwanted layers
(antireflection layer, metal coating and p–n semiconductor) are removed
from the silicon solar cells separated from the PV modules; as a result,
the silicon substrate, suitable for re-use, can be recovered.
The First Solar panel recycling plant opened in Rousset, France in
2018. It was set to recycle 1300 tonnes of solar panel waste a year, and
can increase its capacity to 4000 tonnes.
If recycling is driven only by market-based prices, rather than also
environmental regulations, the economic incentives for recycling remain
uncertain and as of 2021 the environmental impact of different types of
developed recycling techniques still need to be quantified.
