Assistive technology in sport

From Wikipedia, the free encyclopedia

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

A New York City Marathon competitor uses a racing wheelchair.

Assistive technology in sport is an area of technology design that is growing. Assistive technology is the array of new devices created to enable sports enthusiasts who have disabilities to play. Assistive technology may be used in disabled sports, where an existing sport is modified to enable players with a disability to participate; or, assistive technology may be used to invent completely new sports with athletes with disabilities exclusively in mind.

An increasing number of people with disabilities are participating in sports, leading to the development of new assistive technology. Assistive technology devices can be simple, "low-tech", or they may use highly advanced technology, with some even using computers. Assistive technology for sports may also be simple or advanced. Accordingly, assistive technology can be found in sports ranging from local community recreation to elite Paralympic games. More complex assistive technology devices have been developed over time, and as a result, sports for people with disabilities "have changed from being a clinical therapeutic tool to an increasingly competition-oriented activity".

Assistive devices

Assistive devices can enable exercise and training, on top of enabling participation in a sport. Below are some of the assistive devices currently available for different impairments:

• Mobility impairments:
  • Light-weight wheelchairs for basketball, tennis, and racing
  • All-terrain wheelchairs with rugged frames and wheels for rolling over unpaved surfaces, like hiking trails, snow, or beach sand
  • High performance power wheelchairs for powerchair football (power soccer) and power hockey
  • Handcycles, or recumbent bicycles, which are like bicycles with pedals and steering using only the rider's arms
  • Cross-country sit skis that allow skiers to sit down and push along the trail with tips that dig into the snow
  • Weights that users strap onto their wrists rather than having to hold them with the hands
  • Gym equipment that lets users stay in a wheelchair while using arm exercise machines
  • Mitts with Velcro straps that help users to hold onto an exercise machine if their grip isn't strong enough
  • Elastic band or tubes that exercise muscles through resistance instead of weight
  • Paramobile devices such as specialized golf carts with support for standing assist players with mobility disabilities
  • Bowling balls with hand grips assist bowlers with limited use of their hands
  • One-handed fishing rods assist fishers who have limited mobility
• Visual impairments:
  • Softballs that beep, so that people with visual problems can locate the ball to hit and catch it
  • Basketballs with jingle bells inside for people who have limited or no eyesight

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  A golfer with a leg amputation uses an adaptive golf cart that has a chest strap to help him maintain his balance while standing on one leg.
   
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  Wheelchair curling uses a specially adapted long stick to launch the "rock" down the ice. These players are delivering rocks in the 2010 Vancouver Paralympics.
   
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  Australian Paralympic athletes using a two-seated tandem racing bicycle; the visually impaired cyclist pedals in rear, while a sighted "pilot" sits in the front.

Sports using assistive technology

Sports that use assistive technology may include the following:

• Accessible Fishing & Hunting
• Accessible Sailing / Boating / Kayaking
• Adaptive Archery
• Adaptive Bowling
• Adaptive Flying
• Adaptive Golf
• Adaptive Horseback Riding/Equestrian
• Adaptive Scuba Diving
• Adaptive Shooting
• Adaptive Skiing & Snowboarding
• Adaptive Table Tennis
• Adaptive Water Sports
• Extreme Wheelchair Sports
• Handcycling
• Power Soccer
• Quad Rugby
• Sled, Floor & Power Hockey
• Wheelchair Basketball
• Wheelchair Curling
• Wheelchair Fencing
• Wheelchair Lacrosse
• Wheelchair Pool and Billiards
• Wheelchair Racing and Field Sports
• Wheelchair Baseball
• Wheelchair Softball
• Wheelchair Table Tennis
• Wheelchair Tennis
• Wheelchair Volleyball
• Wheelchair Weightlifting

Many of the sports listed above have attained international elite sport status, being included in the Paralympic Games.

Sports requiring assistive technology

Some sports have developed with the goal of creating a challenge that players with a disability could enjoy. These sports require assistive technology for all players as part of the game. Some examples are: Sledge (sled) hockey; wheelchair basketball; adaptive sailing, with boats designed especially for sailors with disabilities; Nordic (cross-country) skiing with "sit-ski" buckets; and handcycling races.

Sport wheelchair design

Sport wheelchairs are designed for the requirements of specific sports. Power chairs can also be fitted with assistive devices that are temporary adaptations to the demands of a sport, such as a kick plate attached to a power chair for powerchair football (power soccer).

Light-weight frames are a necessity for wheelchairs used in sports requiring sharp, fast turns and overall agility, such as tennis, basketball, and racing.

Chairs with reinforced frames and impact protection are required for contact sports, such as wheelchair rugby or basketball.

Racing chairs are designed with bucket seats, angled wheels for improved stability, and a t-frame with a third wheel in front, allowing precision steering and improved balance. The athlete and wheelchair are viewed together by some sport researchers as a unified performance system. Improvements can be made to chairs by evaluating the chair and athlete separately or in performance conditions together.

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  A recumbent racing handcycle with a streamlined frame and thin road racing wheels.
   
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  Wheelchair rugby players use wheelchairs with strong frames, foot protection and wheel covers, to prevent injury in this contact sport.
   
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  Wheelchair tennis requires agility. These Paralympic players use highly manoeuvrable chairs that can change direction very rapidly
   
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  Powerchair football, also called "power soccer", uses an attachment over the footrest of a power wheelchair to bump or push the ball.

Prosthetics

Prosthetic devices come in a variety of designs suited to different athletic purposes. Prosthetic legs may be designed for rock climbing, running, or jumping. The technology is designed to attain goals, such as greater gait efficiency when running. The technology is constantly improving to meet the demands of athletes who set ever-higher sports challenges for themselves.

In 2008, the International Association of Athletics Federations (IAAF) began a worldwide debate when they established Rule 144.2(e), prohibiting the use of technical devices that offer a competitive advantage. South African athlete Oscar Pistorius, using energy-storing prosthetic legs, fought for the right to run against able-bodied athletes in the 2008 Olympic and Paralympic Games, and won the right to compete, although he did not meet the qualifying time required for the traditional able-bodied competitions.

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  A Paralympic track athlete walks with a prosthetic leg designed for racing.
   
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  A prosthetic leg designed for bicycle racing. Adaptations can attach the prosthetic foot to the pedal during riding.
   
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  The Prosthetics Division at Naval Medical Center San Diego, configures a custom-built Cheetah high-performance carbon fiber foot designed for sprinting.
   
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  Climbing a rock wall is made more accessible with specially designed prosthetic legs.

Functional classification

Functional classification systems are used to evaluate and categorize athletes in elite sports. The classification determines the type and extent of assistive technology use by the athlete.

Organizations

Organizations and associations at the national and international level support the development of adaptive sport and recreation, often through the use of assistive technology for players. These organizations are "growing in number and scope".

• International Paralympic Committee - the global body responsible for many sports up to international elite level competition.
• Disabled Sports USA - The national organization that provides over 40 different adaptive sports to over 60,000 people with disabilities in the US through a network of over 100 community-based chapters.
• Adapted Physical Activity Council (APAC), a council of the American Association for Physical Activity and Recreation

Obstacles

Assistive technology has made sports accessible to many athletes who would have otherwise not been able to play. However, it has its downfalls. This technology is expensive, so many people will never have access to it. It can be subject to abuse, as some people use the technology when they don't actually need it. There have been rulings that athletes who use assistive technology have an advantage over "able-bodied" athletes.

Quantum triviality

From Wikipedia, the free encyclopedia

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

 

Quantum field theory

Feynman diagram In a quantum field theory, charge screening can restrict the value of the observable "renormalized" charge of a classical theory. If the only resulting value of the renormalized charge is zero, the theory is said to be "trivial" or noninteracting. Thus, surprisingly, a classical theory that appears to describe interacting particles can, when realized as a quantum field theory, become a "trivial" theory of noninteracting free particles. This phenomenon is referred to as quantum triviality. Strong evidence supports the idea that a field theory involving only a scalar Higgs boson is trivial in four spacetime dimensions, but the situation for realistic models including other particles in addition to the Higgs boson is not known in general. Nevertheless, because the Higgs boson plays a central role in the Standard Model of particle physics, the question of triviality in Higgs models is of great importance.

This Higgs triviality is similar to the Landau pole problem in quantum electrodynamics, where this quantum theory may be inconsistent at very high momentum scales unless the renormalized charge is set to zero, i.e., unless the field theory has no interactions. The Landau pole question is generally considered to be of minor academic interest for quantum electrodynamics because of the inaccessibly large momentum scale at which the inconsistency appears. This is not however the case in theories that involve the elementary scalar Higgs boson, as the momentum scale at which a "trivial" theory exhibits inconsistencies may be accessible to present experimental efforts such as at the Large Hadron Collider (LHC) at CERN. In these Higgs theories, the interactions of the Higgs particle with itself are posited to generate the masses of the W and Z bosons, as well as lepton masses like those of the electron and muon. If realistic models of particle physics such as the Standard Model suffer from triviality issues, the idea of an elementary scalar Higgs particle may have to be modified or abandoned.

The situation becomes more complex in theories that involve other particles however. In fact, the addition of other particles can turn a trivial theory into a nontrivial one, at the cost of introducing constraints. Depending on the details of the theory, the Higgs mass can be bounded or even calculable. These quantum triviality constraints are in sharp contrast to the picture one derives at the classical level, where the Higgs mass is a free parameter. Quantum triviality can also lead to a calculable Higgs mass in asymptotic safety scenarios.

Triviality and the renormalization group

Modern considerations of triviality are usually formulated in terms of the real-space renormalization group, largely developed by Kenneth Wilson and others. Investigations of triviality are usually performed in the context of lattice gauge theory. A deeper understanding of the physical meaning and generalization of the renormalization process, which goes beyond the dilatation group of conventional renormalizable theories, came from condensed matter physics. Leo P. Kadanoff's paper in 1966 proposed the "block-spin" renormalization group. The blocking idea is a way to define the components of the theory at large distances as aggregates of components at shorter distances.

This approach covered the conceptual point and was given full computational substance in Wilson's extensive important contributions. The power of Wilson's ideas was demonstrated by a constructive iterative renormalization solution of a long-standing problem, the Kondo problem, in 1974, as well as the preceding seminal developments of his new method in the theory of second-order phase transitions and critical phenomena in 1971. He was awarded the Nobel prize for these decisive contributions in 1982.

In more technical terms, let us assume that we have a theory described by a certain function ${\displaystyle Z}$ of the state variables ${\displaystyle \{s_i\}}$ and a certain set of coupling constants ${\displaystyle \{J_k\}}$. This function may be a partition function, an action, a Hamiltonian, etc. It must contain the whole description of the physics of the system.

Now we consider a certain blocking transformation of the state variables ${\displaystyle \{s_i\}\to \{{\tilde{s}}_i\}}$, the number of ${\displaystyle {\tilde{s}}_i}$ must be lower than the number of ${\displaystyle s_i}$. Now let us try to rewrite the ${\displaystyle Z}$ function only in terms of the ${\displaystyle {\tilde{s}}_i}$. If this is achievable by a certain change in the parameters, ${\displaystyle \{J_k\}\to \{{\tilde{J}}_k\}}$, then the theory is said to be renormalizable. The most important information in the RG flow are its fixed points. The possible macroscopic states of the system, at a large scale, are given by this set of fixed points. If these fixed points correspond to a free field theory, the theory is said to be trivial. Numerous fixed points appear in the study of lattice Higgs theories, but the nature of the quantum field theories associated with these remains an open question.

Historical background

The first evidence of possible triviality of quantum field theories was obtained by Lev Landau, Alexei Abrikosov, and Isaak Khalatnikov by finding the following relation of the observable charge g_obs with the "bare" charge g₀,

${\displaystyle g_{\text{obs}}=\frac{g_0}{1+{\beta }_2{g}_0\ln \mathrm{\Lambda }/m},}$

1

where m is the mass of the particle, and Λ is the momentum cut-off. If g₀ is finite, then g_obs tends to zero in the limit of infinite cut-off Λ.

In fact, the proper interpretation of Eq.1 consists in its inversion, so that g₀ (related to the length scale 1/Λ) is chosen to give a correct value of g_obs,

${\displaystyle g_0=\frac{g_{\text{obs}}}{1-{\beta }_2{g}_{\text{obs}}\ln \mathrm{\Lambda }/m}.}$

2

The growth of g₀ with Λ invalidates Eqs. (1) and (2) in the region g₀ ≈ 1 (since they were obtained for g₀ ≪ 1) and the existence of the "Landau pole" in Eq.2 has no physical meaning.

The actual behavior of the charge g(μ) as a function of the momentum scale μ is determined by the full Gell–Mann–Low equation (by Murray Gell-Mann and Francis E. Low)

${\displaystyle \frac{dg}{d\ln \mu }=\beta (g)={\beta }_2{g}^2+{\beta }_3{g}^3+\dots ,}$

3

which gives Eqs.(1),(2) if it is integrated under conditions g(μ) = g_obs for μ = m and g(μ) = g₀ for μ = Λ, when only the term with ${\displaystyle {\beta }_2}$ is retained in the right hand side.

The general behavior of ${\displaystyle g(\mu )}$ relies on the appearance of the function β(g). According to the classification by Nikolay Bogolyubov and Dmitry Shirkov, there are three qualitatively different situations:

1. if ${\displaystyle \beta (g)}$ has a zero at the finite value g^*, then growth of g is saturated, i.e. ${\displaystyle g(\mu )\to g^{\ast }}$ for ${\displaystyle \mu \to \mathrm{\infty }}$;
2. if ${\displaystyle \beta (g)}$ is non-alternating and behaves as ${\displaystyle \beta (g)\propto g^{\alpha }}$ with ${\displaystyle \alpha \le 1}$ for large ${\displaystyle g}$, then the growth of ${\displaystyle g(\mu )}$ continues to infinity;
3. if ${\displaystyle \beta (g)\propto g^{\alpha }}$ with ${\displaystyle \alpha >1}$ for large ${\displaystyle g}$, then ${\displaystyle g(\mu )}$ is divergent at finite value ${\displaystyle {\mu }_0}$ and the real Landau pole arises: the theory is internally inconsistent due to indeterminacy of ${\displaystyle g(\mu )}$ for ${\displaystyle \mu >{\mu }_0}$.

The latter case corresponds to the quantum triviality in the full theory (beyond its perturbation context), as can be seen by reductio ad absurdum. Indeed, if g_obs is finite, the theory is internally inconsistent. The only way to avoid it, is to tend ${\displaystyle {\mu }_0}$ to infinity, which is possible only for g_obs → 0.

Conclusions

As a result, the question of whether the Standard Model of particle physics is nontrivial remains a serious unresolved question. Theoretical proofs of triviality of the pure scalar field theory exist, but the situation for the full standard model is unknown. The implied constraints on the standard model have been discussed.

Nerve injury

From Wikipedia, the free encyclopedia

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

 

Nerve injury
Micrograph of a nerve with a decrease in myelinated nerve fibres (pink) and an abnormal increase in fibrous tissue (yellow), as may be seen in nerve injuries. HPS stain.

Nerve injury is an injury to a nerve. There is no single classification system that can describe all the many variations of nerve injuries. In 1941, Herbert Seddon introduced a classification of nerve injuries based on three main types of nerve fiber injury and whether there is continuity of the nerve. Usually, however, nerve injuries are classified in five stages, based on the extent of damage to both the nerve and the surrounding connective tissue, since supporting glial cells may be involved.

Unlike in the central nervous system, neuroregeneration in the peripheral nervous system is possible. The processes that occur in peripheral regeneration can be divided into the following major events: Wallerian degeneration, axon regeneration/growth, and reinnervation of nervous tissue. The events that occur in peripheral regeneration occur with respect to the axis of the nerve injury. The proximal stump refers to the end of the injured neuron that is still attached to the neuron cell body; it is the part that regenerates. The distal stump refers to the end of the injured neuron that is still attached to the end of the axon; it is the part of the neuron that will degenerate, but the stump remains capable of regenerating its axons.

The study of nerve injury began during the American Civil War and greatly expanded during modern medicine with such advances as use of growth-promoting molecules.

Types

To assess the location and severity of a nerve injury, clinical assessment is commonly combined with electrodiagnostic tests. Injuries to the myelin are usually the least severe (neuropraxia), while injuries to the axons and supporting structures are more severe (axonotmesis is moderate injury, while neurotmesis is severe injury). It may be difficult to differentiate the severity by clinical findings due to common neurological impairments, including motor and sensory impairments distal to the lesion.

Neurapraxia

Nerve compression in neurapraxia

Neurapraxia is the least severe form of nerve injury, with complete recovery. In this case, the axon remains intact, but there is myelin damage causing an interruption in conduction of the impulse down the nerve fiber. Most commonly, this involves compression of the nerve or disruption to the blood supply (ischemia). There is a temporary loss of function which is reversible within hours to months of the injury (the average is 6–8 weeks). Wallerian degeneration does not occur, so recovery does not involve actual regeneration. There is frequently greater involvement of motor than sensory function with autonomic function being retained. In electrodiagnostic testing with nerve conduction studies, there is a normal compound motor action potential amplitude distal to the lesion at day 10, and this indicates a diagnosis of mild neurapraxia instead of axonotmesis or neurotmesis.

Axonotmesis

Axonotmesis is a more severe nerve injury with disruption of the neuronal axon, but with maintenance of the epineurium. This type of nerve damage may cause paralysis of the motor, sensory, and autonomic functions, and is mainly seen in crush injury.

If the force creating the nerve damage is removed in a timely fashion, the axon may regenerate, leading to recovery. Electrically, the nerve shows rapid and complete degeneration, with loss of voluntary motor units. Regeneration of the motor end plates will occur, as long as the endoneural tubules are intact.

Axonotmesis involves the interruption of the axon and its covering of myelin, but with preservation of the connective tissue framework of the nerve (the encapsulating tissue, the epineurium and perineurium, are preserved). Because axonal continuity is lost, Wallerian degeneration occurs. Electromyography (EMG) performed 2 to 4 weeks later shows fibrillations and denervation potentials in musculature distal to the injury site. Loss in both motor and sensory spines is more complete with axonotmesis than with neurapraxia, and recovery occurs only through regenerations of the axons, a process requiring time.

Axonotmesis is usually the result of a more severe crush or contusion than neurapraxia, but can also occur when the nerve is stretched (without damage to the epineurium). There is usually an element of retrograde proximal degeneration of the axon, and for regeneration to occur, this loss must first be overcome. The regeneration fibers must cross the injury site and regeneration through the proximal or retrograde area of degeneration may require several weeks. Then the neuritis tip progresses down the distal site, such as the wrist or hand. Proximal lesion may grow distally as fast as 2 to 3 mm per day and distal lesion as slowly as 1.5 mm per day. Regeneration occurs over weeks to years.

Neurotmesis

Neurotmesis is the most severe lesion with no potential of full recovery. It occurs on severe contusion, stretch, or laceration. The axon and encapsulating connective tissue lose their continuity. The last (extreme) degree of neurotmesis is transsection, but most neurotmetic injuries do not produce gross loss of continuity of the nerve but rather internal disruption of nerve structures sufficient to involve perineurium and endoneurium as well as axons and their covering. Denervation changes recorded by EMG are the same as those seen with axonotmetic injury. There is a complete loss of motor, sensory and autonomic function. If the nerve has been completely divided, axonal regeneration causes a neuroma to form in the proximal stump. For neurotmesis, it is better to use a new more complete classification called the Sunderland System.

Overview of peripheral regeneration

Wallerian degeneration is a process that occurs before nerve regeneration and can be described as a cleaning or clearing process that essentially prepares the distal stump for reinnervation. Schwann cells are glial cells in the peripheral nervous system that support neurons by forming myelin that encases nerves. During Wallerian degeneration Schwann cells and macrophages interact to remove debris, specifically myelin and the damaged axon, from the distal injury site. Calcium has a role in the degeneration of the damage axon. Bands of Büngner are formed when uninnervated Schwann cells proliferate and the remaining connective tissue basement membrane forms endoneurial tubes. Bands of Büngner are important for guiding the regrowing axon.

At the neuronal cell body, a process called chromatolysis occurs in which the nucleus migrates to the periphery of the cell body and the endoplasmic reticulum breaks up and disperses. Nerve damage causes the metabolic function of the cell to change from that of producing molecules for synaptic transmission to that of producing molecules for growth and repair. These factors include GAP-43, tubulin and actin. Chromatolysis is reversed when the cell is prepared for axon regeneration.

Axon regeneration is characterized by the formation of a growth cone, which has the ability to produce a protease that digests any material or debris that remains in its path of regeneration toward the distal site. The growth cone responds to molecules produced by Schwann cells such as laminin and fibronectin.

Neuron-intrinsic changes

Immediately following injury, neurons undergo a large number of transcriptional and proteomic changes which switch the cell from a mature, synaptically active neuron to a synaptically silent, growth state. This process is dependent on new transcription, as blocking the ability of cells to transcribe new mRNA severely impairs regeneration. A number of signaling pathways have been shown to be turned on by axon injury and help to enable long distance regeneration including BMP, TGFβ, and MAPKs. Similarly, a growing number of transcription factors also boost the regenerative capacity of peripheral neurons including ASCL1, ATF3, CREB1, HIF1α, JUN, KLF6, KLF7, MYC, SMAD1, SMAD2, SMAD3, SOX11, SRF, STAT3, TP53, and XBP1. Several of these can also boost the regenerative capacity of CNS neurons, making them potential therapeutic targets for treating spinal cord injury and stroke.

Role of Schwann cells

Guillain–Barré syndrome – nerve damage

Schwann cells are active in Wallerian degeneration. They not only have a role in phagocytosis of myelin, but they also have a role in recruitment of macrophages to continue the phagocytosis of myelin. The phagocytic role of Schwann cells has been investigated by studying the expression of molecules in Schwann cells that are typically specific to inflammatory macrophages. Expression of one such molecule MAC-2, a galactose-specific lectin, is observed in not only degenerating nerves that are macrophage-rich but also degenerating nerves that are macrophage-scarce and Schwann cell-rich. Furthermore, the effects of MAC-2 in degenerating nerves are associated with myelin phagocytosis. There was a positive correlation between the amount of MAC-2 expression and the extent of myelin phagocytosis. A deficiency in MAC-2 expression can even cause inhibition of myelin removal from injury sites.

Schwann cells are active in demyelination of injured nerves before macrophages are even present at the site of nerve injury. Electron microscopy and immunohistochemical staining analysis of teased nerve fibers shows that before macrophages arrive at the injury site, myelin is fragmented and myelin debris and lipid droplets are found in the cytoplasm of Schwann cells, indicating phagocytic activity before macrophages arrive.

Schwann cell activity includes recruitment of macrophages to the injury site. Monocyte chemoattractant protein (MCP-1) plays a role in recruiting monocytes/macrophages. In tellurium-induced demylenation with no axon degeneration, nerve crush with axon degeneration, and nerve transection with axon degeneration an increase in MCP-1 mRNA expression followed by an increase in macrophage recruitment occurred. In addition varying levels of MCP-1 mRNA expression also had an effect. Increased MCP-1 mRNA levels correlated positively with an increase in macrophage recruitment. Furthermore, in situ hybridation determined that the cellular source of MCP-1 was Schwann cells.

Schwann cells play an important role in not only producing neurotrophic factors such as nerve growth factor (NGF) and ciliary neurotrophic factor (CNTF), which promote growth, of both the damaged nerve and supporting Schwann cells, but also producing neurite promoting factors, which guide the growing axon, both of which are discussed below.

Role of macrophages

The primary role of macrophages in peripheral regeneration is demylenation during Wallerian degeneration. Immunohistochemical analysis showed that in tellurium demylenated, crushed, and cut nerves, expression of lysozyme, which is a marker for myelin phagocytosis, and of ED1, which is a marker for macrophages, occurred in the same region. Lysozyme was also investigated with respect to the temporal progression of myelin phagocytosis by macrophages in nerve injury. Northern blotting showed that peak lysozyme mRNA expression occurred at an appropriate time with respect to temporal models of myelin phagocytosis. Macrophages do not phagocytose all cellular debris at the nerve injury site; they are selective and will salvage certain factors. Macrophages produce apolipoprotein E which is involved in rescuing cholesterol in damaged nerves. In the same investigation, temporal levels of apolipoprotein E mRNA expression in the three models for demylenation and nerve damage were consistent with respect to models for cholesterol salvage in nerve injury. Macrophages play a role in salvaging cholesterol during nerve injury.

Macrophages also play a role in inducing the proliferation of Schwann cells that occurs during Wallerian degeneration. Supernatant has been collected from medium in which macrophages are active in myelin phagocytosis where lysosomal processing of the myelin occurs within the macrophage. The supernatant contains a mitogenic factor, a mitosis promoting factor, that is characterized heat and trypsin sensitivity, both of which characterize it as a peptide. Treatment of Schwann cells with the collected supernatant shows that it is a mitogenic factor and thus plays an important role in the proliferation of Schwann cells.

Macrophages are also involved in the secretion factors that promote nerve regeneration. Macrophages secrete not only interleukin-1, a cytokine that induces expression of nerve growth factor (NGF) in Schwann cells but also an interleukin-1 receptor antagonist (IL-1ra). Expression of IL-1ra in mice with transected sciatic nerves via implantation of a tube releasing IL-1ra showed the regrowth of fewer myelinated and unmyelinated axons. Macrophage secretion of interleukin-1 is involved in stimulation of nerve regeneration.

Role of neurotrophic factors

Several signaling pathways are upregulated following a nerve injury.

Neurotrophic factors are those that promote survival and growth of neurons. A trophic factor can be described as a factor that is associated with providing nourishment to allow for growth. In general they are protein ligands for tyrosine kinase receptors; binding to the specific receptor yields autophosphorylation and subsequent phosphorylation of tyrosine residues on proteins that participate in further downstream signaling to activate proteins and genes involved in growth and proliferation. Neurotrophic factors act through retrograde transport in neurons, in which they are taken up by the growth cone of the injured neuron and transported back to the cell body. These neurotrophic factors have both autocrine and paracrine effects, as they promote growth of the damaged neurons as well as the adjacent Schwann cells.

Nerve growth factor (NGF) typically has a low level of expression in nerves that are healthy and not growing or developing, but in response to nerve injury NGF expression increases in Schwann cells. This is a mechanism to increase growth and proliferation of Schwann cells at the distal stump in order to prepare for reception of the regenerating axon. NGF has not only a trophic role but also a tropic or guiding role. The Schwann cells that form the bands of Bungner at the distal injury site express NGF receptors as a guiding factor for the regenerating axon of the injured neuron. NGF bound to the receptors on Schwann cells provides the growing neurons that are contacted with a trophic factor to promote further growth and regeneration.

Ciliary neurotrophic factor (CNTF) typically has a high level of expression in Schwann cells associated with nerves that are healthy, but in response to nerve injury CNTF expression decreases in Schwann cells distal to the injury site and remains relatively low unless the injured axon begins to regrow. CNTF has numerous trophic roles in motor neurons in the peripheral nervous system including the prevention of atrophy of dennervated tissue and the prevention of degeneration and death of motor neurons after nerve injury. (frostick) In sciatic motor neurons both CNTF receptor mRNA expression and CNTF receptor is increased after injury for a prolonged time frame compared to the short time frame in the central nervous system suggesting a role for CNTF in nerve regeneration.

Insulin-like growth factors (IGFs) have been shown to increase the rate of peripheral nervous system axon regeneration. IGF-I and IGF-II mRNA levels are significantly increased distal to the site of crush injury in rat sciatic nerves. At the site of nerve repair, locally delivered IGF-I can significantly increase the rate of axon regeneration within a nerve graft and help expedite functional recovery of a paralyzed muscle.

Role of neurite-promoting factors

Neurite promoting factors include many extracellular matrix proteins produced by Schwann cells at the distal stump including fibronectin and laminin. Fibronectin are components of the basal lamina and promote neurite growth and adhesion of the growth cone to the basal lamina. In regenerating neural cells, neurite promoting factors play a role in adhesion of the axon and include neural cell adhesion molecule (N-CAM) and N-cadherin.

Treatment

Unless otherwise demonstrated, nerve injuries are commonly irreversible, and therefore complete treatment is rather difficult, though still possible and hence lifelong management of disabilities arising due to nerve injuries is necessary.

Nerve regeneration therapies

Main article: Neuroregeneration

Electrical stimulation can promote nerve regeneration. The positive effect of electrical stimulation on nerve regeneration is due to its molecular influence on the damaged neuron and Schwann cells. Electrical stimulation can directly accelerate the expression of cyclic adenosine monophosphate (cAMP) both in neurons and Schwann cells. cAMP is a molecule that stimulates multiple signaling pathways that aid nerve regeneration by enhancing the expression of several neurotrophic factors. Electrical stimulation also results in the influx of calcium ions, which further triggers multiple regeneration pathways.

The frequency of stimulation is an important factor in the success of both quality and quantity of axon regeneration as well as growth of the surrounding myelin and blood vessels that support the axon. Histological analysis and measurement of regeneration showed that low frequency stimulation had a more successful outcome than high frequency stimulation on regeneration of damaged sciatic nerves.

Other studies have used both oscillating current (AC) and non-oscillating direct current (DC) stimulation to regenerate mammalian nerves. Mammalian neurons preferentially orient and grow towards the cathode in DC electric fields.

Surgery can be done in case a nerve has become cut or otherwise divided. Recovery of a nerve after surgical repair depends mainly on the age of patients. Younger the patients, better the prognosis, because of better healing capacity of young tissues. Young children can recover almost normal nerve function. In contrast, a patient over 60 years old with a cut nerve in the hand would expect to recover only protective sensory function, that is, the ability to distinguish hot/cold or sharp/dull; recovery of motor function would be likely incomplete. Many other factors also affect nerve recovery. The use of autologous nerve grafting procedures that involve redirection of regenerative donor nerve fibers into the graft conduit has been successful in restoring target muscle function. Localized delivery of soluble neurotrophic factors may help promote the rate of axon regeneration observed within these graft conduits.

An expanding area of nerve regeneration research deals with the development of scaffolding and bio-conduits. Scaffolding developed from bio-compatible material would be useful in nerve regeneration if they successfully exhibit essentially the same role as the endoneurial tubes and Schwann cells do in guiding regrowing axons.

Prevention of nerve injuries

Methods to help prevent nerve injuries include injection pressure monitoring.  The presence of a high opening injection pressure (> 20 PSI) is a sensitive sign of intrafascicular/intraneural needle tip placement.  Extrafascicular needle tip placement is associated with low pressures (< 20 PSI).  Also, high pressure injection was associated with neurologic deficits and severe axonal damage after the block.  Other methods of preventing nerve injury include electrical nerve stimulation and ultrasonography. Electrical stimulation with a motor response at < 0.2 mA only can occur with an intraneural/intrafasciular needle tip location.

Solar-cell efficiency

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Solar-cell_efficiency

Reported timeline of research solar cell energy conversion efficiencies since 1976 (National Renewable Energy Laboratory)

Solar-cell efficiency is the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity by the solar cell.

The efficiency of the solar cells used in a photovoltaic system, in combination with latitude and climate, determines the annual energy output of the system. For example, a solar panel with 20% efficiency and an area of 1 m² produces 200 kWh/yr at Standard Test Conditions if exposed to the Standard Test Condition solar irradiance value of 1000 W/m² for 2.74 hours a day. Usually solar panels are exposed to sunlight for longer than this in a given day, but the solar irradiance is less than 1000 W/m² for most of the day. A solar panel can produce more when the Sun is high in Earth's sky and produces less in cloudy conditions, or when the Sun is low in the sky. The Sun is lower in the sky in the winter.

Two location dependent factors that affect solar PV yield are the dispersion and intensity of solar radiation. These two variables can vary greatly between each country. The global regions that have high radiation levels throughout the year are the Middle East, Northern Chile, Australia, China, and Southwestern USA. In a high-yield solar area like central Colorado, which receives annual insolation of 2000 kWh/m²/year, a panel can be expected to produce 400 kWh of energy per year. However, in Michigan, which receives only 1400 kWh/m²/year, annual energy yield drops to 280 kWh for the same panel. At more northerly European latitudes, yields are significantly lower: 175 kWh annual energy yield in southern England under the same conditions.

Schematic of charge collection by solar cells. Light transmits through transparent conducting electrode creating electron hole pairs, which are collected by both the electrodes. The absorption and collection efficiencies of a solar cell depend on the design of transparent conductors and active layer thickness.

Several factors affect a cell's conversion efficiency, including its reflectance, thermodynamic efficiency, charge carrier separation efficiency, charge carrier collection efficiency and conduction efficiency values. Because these parameters can be difficult to measure directly, other parameters are measured instead, including quantum efficiency, open-circuit voltage (V_OC) ratio, and § Fill factor. Reflectance losses are accounted for by the quantum efficiency value, as they affect external quantum efficiency. Recombination losses are accounted for by the quantum efficiency, V_OC ratio, and fill factor values. Resistive losses are predominantly accounted for by the fill factor value, but also contribute to the quantum efficiency and V_OC ratio values.

As of 2024, the world record for solar cell efficiency is 47.6%, set in May 2022 by Fraunhofer ISE, with a III-V four-junction concentrating photovoltaic (CPV) cell. This beat the previous record of 47.1%, set in 2019 by multi-junction concentrator solar cells developed at National Renewable Energy Laboratory (NREL), Golden, Colorado, USA, which was set in lab conditions, under extremely concentrated light. The record in real-world conditions is held by NREL, who developed triple junction cells with a tested efficiency of 39.5%.

Factors affecting energy conversion efficiency

The factors affecting energy conversion efficiency were expounded in a landmark paper by William Shockley and Hans Queisser in 1961. See Shockley–Queisser limit for more detail.

Thermodynamic-efficiency limit and infinite-stack limit

Main article: Thermodynamic efficiency limit

The Shockley–Queisser limit for the efficiency of a single-junction solar cell under unconcentrated sunlight at 273 K. This calculated curve uses actual solar spectrum data, and therefore the curve is wiggly from IR absorption bands in the atmosphere. This efficiency limit of ~34% can be exceeded by multijunction solar cells.

If one has a source of heat at temperature T_s and cooler heat sink at temperature T_c, the maximum theoretically possible value for the ratio of work (or electric power) obtained to heat supplied is 1-T_c/T_s, given by a Carnot heat engine. If we take 6000 K for the temperature of the sun and 300 K for ambient conditions on earth, this comes to 95%. In 1981, Alexis de Vos and Herman Pauwels showed that this is achievable with a stack of an infinite number of cells with band gaps ranging from infinity (the first cells encountered by the incoming photons) to zero, with a voltage in each cell very close to the open-circuit voltage, equal to 95% of the band gap of that cell, and with 6000 K blackbody radiation coming from all directions. However, the 95% efficiency thereby achieved means that the electric power is 95% of the net amount of light absorbed – the stack emits radiation as it has non-zero temperature, and this radiation must be subtracted from the incoming radiation when calculating the amount of heat being transferred and the efficiency. They also considered the more relevant problem of maximizing the power output for a stack being illuminated from all directions by 6000 K blackbody radiation. In this case, the voltages must be lowered to less than 95% of the band gap (the percentage is not constant over all the cells). The maximum theoretical efficiency calculated is 86.8% for a stack of an infinite number of cells, using the incoming concentrated sunlight radiation. When the incoming radiation comes only from an area of the sky the size of the sun, the efficiency limit drops to 68.7%.

Ultimate efficiency

Normal photovoltaic systems however have only one p–n junction and are therefore subject to a lower efficiency limit, called the "ultimate efficiency" by Shockley and Queisser. Photons with an energy below the band gap of the absorber material cannot generate an electron-hole pair, so their energy is not converted to useful output, and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity. Traditional single-junction cells with an optimal band gap for the solar spectrum have a maximum theoretical efficiency of 33.16%, the Shockley–Queisser limit.

Solar cells with multiple band gap absorber materials improve efficiency by dividing the solar spectrum into smaller bins where the thermodynamic efficiency limit is higher for each bin.

Quantum efficiency

Main article: Quantum efficiency

When a photon is absorbed by a solar cell it can produce an electron-hole pair. One of the carriers may reach the p–n junction and contribute to the current produced by the solar cell; such a carrier is said to be collected. Or, the carriers recombine with no net contribution to cell current.

Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions. The two types of quantum that are usually referred to when talking about solar cells are external and internal. External quantum efficiency (EQE) relates to the measurable properties of the solar cell. The "external" quantum efficiency of a silicon solar cell includes the effect of optical losses such as transmission and reflection. Measures can be taken to reduce these losses. The reflection losses, which can account for up to 10% of the total incident energy, can be dramatically decreased using a technique called texturization, a light trapping method that modifies the average light path.

The internal quantum efficiency (IQE) gives insight into the internal material parameters like the absorption coefficient or internal luminescence quantum efficiency. IQE is mainly used to aid the understanding of the potential of a certain material rather than a device.

Quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others, spectral measurements of quantum efficiency can yield valuable information about the quality of the semiconductor bulk and surfaces.

Quantum efficiency is not the same as overall energy conversion efficiency, as it does not convey information about the fraction of power that is converted by the solar cell.

Maximum power point

Dust often accumulates on the glass of solar modules - highlighted in this negative image as black dots - which reduces the amount of light admitted to the solar cells

A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes).

The maximum power point of a solar cell is affected by its temperature. Knowing the technical data of certain solar cell, its power output at a certain temperature can be obtained by ${\displaystyle P(T)=P_{STC}+\frac{dP}{dT}(T_{cell}-T_{STC})}$, where ${\displaystyle P_{STC}}$ is the power generated at the standard testing condition; ${\displaystyle T_{cell}}$ is the actual temperature of the solar cell.

A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 V open-circuit (V_OC). The cell temperature in full sunlight, even with 25 °C air temperature, is probably close to 45 °C, reducing the open-circuit voltage to 0.55 V per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (I_SC). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 V in this case) and 90% of the short-circuit current. This output can be up to 70% of the V_OC x I_SC product. The short-circuit current (I_SC) from a cell is nearly proportional to the illumination, while the open-circuit voltage (V_OC) may drop only 10% with an 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 V_OC at 1/2 I_SC. The usable power output could thus drop from 70% of the V_OC x I_SC product to 50% or even as little as 25%. Vendors who rate their solar cell "power" only as V_OC x I_SC, without giving load curves, can be seriously distorting their actual performance.

The maximum power point of a photovoltaic varies with incident illumination. For example, accumulation of dust on photovoltaic panels reduces the maximum power point. Recently, new research to remove dust from solar panels has been developed by utilizing electrostatic cleaning systems. In such systems, an applied electrostatic field at the surface of the solar panels causes the dust particles to move in a "flip-flop" manner. Then, due to gravity and the fact that the solar panels are slightly slanted, the dust particles get pulled downward by gravity. These systems only require a small power consumption and enhance the performance of the solar cells, especially when installed in the desert, where dust accumulation contributes to decreasing the solar panel's performance. Also, for systems large enough to justify the extra expense, a maximum power point tracker tracks the instantaneous power by continually measuring the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so the maximum power is always transferred, regardless of the variation in lighting.

Fill factor

Another defining term in the overall behaviour of a solar cell is the fill factor (FF). This factor is a measure of quality of a solar cell. This is the available power at the maximum power point (P_m) divided by the open circuit voltage (V_OC) and the short circuit current (I_SC):

${\displaystyle FF=\frac{P_m}{V_{OC}\times I_{SC}}=\frac{\eta \times A_c\times G}{V_{OC}\times I_{SC}}.}$

The fill factor can be represented graphically by the IV sweep, where it is the ratio of the different rectangular areas.

The fill factor is directly affected by the values of the cell's series, shunt resistances and diodes losses. Increasing the shunt resistance (R_sh) and decreasing the series resistance (R_s) lead to a higher fill factor, thus resulting in greater efficiency, and bringing the cell's output power closer to its theoretical maximum.

Typical fill factors range from 50% to 82%. The fill factor for a normal silicon PV cell is 80%.

Comparison

Main article: Photovoltaics

Energy conversion efficiency is measured by dividing the electrical output by the incident light power. Factors influencing output include spectral distribution, spatial distribution of power, temperature, and resistive load. IEC standard 61215 is used to compare the performance of cells and is designed around standard (terrestrial, temperate) temperature and conditions (STC): irradiance of 1 kW/m², a spectral distribution close to solar radiation through AM (airmass) of 1.5 and a cell temperature 25 °C. The resistive load is varied until the peak or maximum power point (MPP) is achieved. The power at this point is recorded as Watt-peak (Wp). The same standard is used for measuring the power and efficiency of PV modules.

Air mass affects output. In space, where there is no atmosphere, the spectrum of the Sun is relatively unfiltered. However, on Earth, air filters the incoming light, changing the solar spectrum. The filtering effect ranges from Air Mass 0 (AM0) in space, to approximately Air Mass 1.5 on Earth. Multiplying the spectral differences by the quantum efficiency of the solar cell in question yields the efficiency. Terrestrial efficiencies typically are greater than space efficiencies. For example, a silicon solar cell in space might have an efficiency of 14% at AM0, but 16% on Earth at AM 1.5. Note, however, that the number of incident photons in space is considerably larger, so the solar cell might produce considerably more power in space, despite the lower efficiency as indicated by reduced percentage of the total incident energy captured.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 44.0% with multiple-junction production cells and 44.4% with multiple dies assembled into a hybrid package. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14–19%. The highest efficiency cells have not always been the most economical – for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide produced at low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while delivering only about four times the output.

However, there is a way to "boost" solar power. By increasing the light intensity, typically photogenerated carriers are increased, increasing efficiency by up to 15%. These so-called "concentrator systems" have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6–400 times the Sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%.

A common method used to express economic costs is to calculate a price per delivered kilowatt-hour (kWh). The solar cell efficiency in combination with the available irradiation has a major influence on the costs, but generally speaking the overall system efficiency is important. Commercially available solar cells (as of 2006) reached system efficiencies between 5 and 19%.

Undoped crystalline silicon devices are approaching the theoretical limiting efficiency of 29.43%. In 2017, efficiency of 26.63% was achieved in an amorphous silicon/crystalline silicon heterojunction cell that place both positive and negative contacts on the back of the cell.

Energy payback

See also: Energy payback time by technology and location

The energy payback time is the recovery time required for generating the energy spent for manufacturing a modern photovoltaic module. An 2008 estimate puts it at from 1 to 4 years depending on the module type and location. With a typical lifetime of 20 to 30 years, this means that modern solar cells would be net energy producers, i.e., they would generate more energy over their lifetime than the energy expended in producing them. Generally, thin-film technologies—despite having comparatively low conversion efficiencies—achieve significantly shorter energy payback times than conventional systems (often < 1 year).

A study published in 2013 found that energy payback time was between 0.75 and 3.5 years with thin film cells being at the lower end and multicrystalline silicon (multi-Si) cells having a payback time of 1.5–2.6 years. A 2015 review assessed the energy payback time and EROI of solar photovoltaics. In this meta study, which uses an insolation of 1,700 kWh/m²/year and a system lifetime of 30 years, mean harmonized EROIs between 8.7 and 34.2 were found. Mean harmonized energy payback time varied from 1.0 to 4.1 years. Crystalline silicon devices achieve on average an energy payback period of 2 years.

Like any other technology, solar cell manufacture is dependent on the existence of a complex global industrial manufacturing system. This includes the fabrication systems typically accounted for in estimates of manufacturing energy; the contingent mining, refining and global transportation systems; and other energy intensive support systems including finance, information, and security systems. The difficulty in measuring such energy overhead confers some uncertainty on any estimate of payback times.

Technical methods of improving efficiency

Choosing optimum transparent conductor

The illuminated side of some types of solar cells, thin films, have a transparent conducting film to allow 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. There is a trade-off between high transmittance and electrical conductance, thus optimum density of conducting nanowires or conducting network structure should be chosen for high efficiency.

Promoting light scattering

Diagram of the characteristic E-field enhancement profiles experienced in thin photovoltaic films (thickness t_PV) patterned with front features. Two simultaneous optical mechanisms can cause light-trapping: anti-reflection and scattering; and two main spectral regions can be distinguished for each mechanism, at short and long wavelengths, thus leading to the 4 types of absorption enhancement profiles illustrated here across the absorber region. The main geometrical parameter of the photonic structures influencing the absorption enhancement in each profile is indicated by the black arrows.

The inclusion of light-scattering effects in solar cells is a photonic strategy to increase the absorption for the lower-energy sunlight photons (chiefly in near-infrared range) for which the photovoltaic material presents reduced absorption coefficient. Such light-trapping scheme is accomplished by the deviation of the light rays from the incident direction, thereby increasing their path length in the cells' absorber. Conventional approaches used to implement light diffusion are based on textured rear/front surfaces, but many alternative optical designs have been demonstrated with promising results based in diffraction gratings, arrays of metal or dielectric nano/micro particles, wave-optical micro-structuring, among others. When applied in the devices' front these structures can act as geometric anti-reflective coatings, simultaneously reducing the reflection of out-going light.

For instance, lining the light-receiving surface of the cell with nano-sized metallic studs can substantially increase the cell efficiency. Light reflects off these studs at an oblique angle to the cell, increasing the length of the light path through the cell. This increases the number of photons absorbed by the cell and the amount of current generated. The main materials used for the nano-studs are silver, gold, and aluminium. Gold and silver are not very efficient, as they absorb much of the light in the visible spectrum, which contains most of the energy present in sunlight, reducing the amount of light reaching the cell. Aluminium absorbs only ultraviolet radiation, and reflects both visible and infra-red light, so energy loss is minimized. Aluminium can increase cell efficiency up to 22% (in lab conditions).

Anti-reflective coatings and textures

Anti-reflective coatings are engineered to reduce the sunlight reflected from the solar cells, therefore enhancing the light transmitted into the photovoltaic absorber. This can be accomplished by causing the destructive interference of the reflected light waves, such as with coatings based on the front (multi-)layer composition, and/or by geometric refractive-index matching caused by the surface topography, with many architectures inspired by nature. For example, the nipple-array, a hexagonal array of subwavelength conical nanostructures, can be seen at the surface of the moth's eyes. It was reported that utilizing this sort of surface architecture minimizes the reflection losses by 25%, converting the additional captured photon to a 12% increase in a solar cell's energy.

The use of front micro-structures, such as those achieved with texturizing or other photonic features, can also be used as a method to achieve anti-reflectiveness, in which the surface of a solar cell is altered so that the impinging light experiences a gradually increasing effective refractive-index when travelling from air towards the photovoltaic material. These surfaces can be created by etching or using lithography. Concomitantly, they promote light scattering effects that further enhance the absorption, particularly of the longer wavelength sunlight photons. Adding a flat back surface in addition to texturizing the front surface further helps to trap the light within the cell, thus providing a longer optical path.

Radiative cooling

See also: Passive daytime radiative cooling

An increase in solar cell temperature of approximately 1 °C causes an efficiency decrease of about 0.45%. To prevent this, a transparent silica crystal layer can be applied to solar panels. The silica layer acts as a thermal black body, which emits heat as infrared radiation into space, cooling the cell up to 13 °C. Radiative cooling can thus extend the life of solar cells. Full-system integration of solar energy and radiative cooling is referred to as a combined SE–RC system, which have demonstrated higher energy gain per unit area when compared to non-integrated systems.

Rear surface passivation

See also: Surface passivation

Surface passivation is critical to solar cell efficiency. Many improvements have been made to the front side of mass-produced solar cells, but the aluminium back-surface is impeding efficiency improvements. The efficiency of many solar cells has benefitted by creating so-called passivated emitter and rear cells (PERCs). The chemical deposition of a rear-surface dielectric passivation layer stack that is also made of a thin silica or aluminium oxide film topped with a silicon nitride film helps to improve efficiency in silicon solar cells. This helped increase cell efficiency for commercial Cz-Si wafer material from just over 17% to over 21% by the mid-2010s, and the cell efficiency for quasi-mono-Si to a record 19.9%.

Concepts of the rear surface passivation for silicon solar cells has also been implemented for CIGS solar cells. The rear surface passivation shows the potential to improve the efficiency. 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. Also, the implementation of the passivation layers does not change the morphology of the CIGS layers.

Thin film materials

See also: Thin-film solar cell

Although not constituting a direct strategy to improve efficiency, thin film materials show a lot of promise for solar cells in terms of low costs and adaptability to existing structures and frameworks in technology. Since the materials are so thin, they lack the optical absorption of bulk material solar cells. Attempts to correct this have been demonstrated, such as light-trapping schemes promoting light scattering. Also important is thin film surface recombination. Since this is the dominant recombination process of nanoscale thin-film solar cells, it is crucial to their efficiency. Adding a passivating thin layer of silicon dioxide could reduce recombination.

Tandem cells

See also: Multi-junction solar cell

Tandem solar cells combine two materials to increase efficiency. In 2022 a device was announced that combined multiple perovskite with multiple layers of silicon. Perovskites demonstrate a remarkable ability to efficiently capture and convert blue light, complementing silicon, which is particularly adept at absorbing red and infrared wavelengths. This unique synergy between perovskites and silicon in solar cell technologies allows for a more comprehensive absorption of the solar spectrum, enhancing the overall efficiency and performance of photovoltaic devices. The cell achieved 32.5% efficiency.