Layer by Layer: Exploring the Intricate Solar Cell Structure and Its Photovoltaic Function

Table of Contents

Solar cells, or photovoltaic (PV) cells, change sunlight into electricity. This happens through the photovoltaic effect. When materials like silicon are hit by sunlight, they create an electric current. Solar cells have layers of these materials, with an electric field that separates positive and negative charges. This separation creates electron flows, which we can use as electrical current. We often use solar cells in solar panels for homes, businesses, and satellites.

Solar cells are semiconductor devices that turn solar energy into electrical current. They have been developing rapidly for nearly 40 years. This growth is because we need to use renewable energy sources instead of fossil fuels. Also, people are more aware of the need for eco-friendly energy production methods.

Photovoltaic effect

The core of a solar cell’s working is the photovoltaic phenomenon. This is when a voltage is created inside a semiconductor material due to its interaction with light. Solar cells use the visible part of sunlight, which is why we also call them solar cells. The photovoltaic effect was first seen in 1839 by the French scientist Edmond Becquerel. He made a cell with platinum plates in a solution, some coated in AgCl. When light hit these plates, a voltage difference appeared in the circuit. Forty years later, the first solid-state device using this effect was made with selenium. This effect was also found in combinations like gold with selenium, copper oxide with copper, and thallium sulfide with lead. This research led to the development of photovoltaics.

In the 1950s, silicon-based electronics emerged, paving the way for silicon-based solar cell technology. Silicon, when exposed to a boron-chloride atmosphere, forms a p-type semiconductor layer. This creates a p-n junction, which was six times more efficient than earlier metal/semiconductor junctions. This discovery led to the rapid advancement of photovoltaics using various materials.

How do solar cells work

Solar cells work using a p-n junction, which separates light-excited electron-hole carriers. The illustration below shows the band structure in P-type and N-type semiconductors. When a p-n junction forms, it increases the carrier gradient, encouraging diffusion. Electron holes move from the P-type to the N-type semiconductor, and electrons go from the N-type to the P-type. This process creates a layer depleted of carriers. An electric field then forms, stopping further diffusion.

When the movement of carriers, pushed by the electric field, balances with the diffusion charge from the carrier concentration gradient, we reach an equilibrium temperature. At this point, the Fermi levels of the N-type and P-type semiconductors are the same.

solar cell structure — Band structure of p-type (a) and n-type (b) semiconductor

The differences in the potential of two semiconductors of the n-type type and p-type in thermal equilibrium is known as Vb, which is the term used to describe an embedded voltage. or Vb, and is connected to the different free energy between the p-side and the n-side of the junction. The embedded potential can be described in the following relation:

Where: kB – Bolztman’s constant, T – absolute temperature [K], NA and ND – concentration of acceptors and donors in p-type semiconductor and n-type semiconductor, e – elementary charge, ni – internal concentration of carriers.

The majority charge carriers in the depleted layer include donors and acceptors, making it distinctly different from other electrically neutral areas. The width of the depleted layer W can be described by the relation:

Where: ε – the dielectric constant of the semiconductor.

The width of the layer increases when the concentration of donors or acceptors is reduced. For example, when the concentration of acceptors is much greater than the concentration of donors Na>>Nd, the width of the depleted layer is described by the following relation:

The application of an external voltage on the junction, in such such that its positive pole connected to the semiconductor of the type p and the negative pole connects to the type semiconductor results in the potential difference of the layer that is depleted to be decreased. This polarity is a polarization that occurs in the direction of conduction, and it has the characteristic of reducing the electrons’ movement from the n-semiconductor towards the p-semiconductor-side and to increase the movement of electron holes that are in the other direction. It means that minor carriers – electrons – are injected into the p side and electron holes are injected onto the side of the n. Current density J in the event of positive polarization, can be defined by the equation:

where: VF – polarization in the conduction direction, J0 – saturation current density of the junction.

In the case of the reverse situation, the electrostatic potential of the depleted layer increases, the diffusion current decreases, and the diffusion current density of the polarization in the negative direction is described by the relation:

where: VR – polarization in the negative direction.

The illumination of the junction by sunlight leads to the formation of electron-hole pairs because of the absorption of light particles with higher energy than the energy from the excited gap in semiconductors and their quantity is proportional to the brightness of the sunlight. The electric field of the depleted layers causes shift of electrons towards semiconductor n , and electron holes towards semiconductor p . This happens inside the depleted layers. Charge separation creates current flow between the n-side and the p-side if the outer circuit is cut. The carriers that are generated in the diffusion range that originates near the edge of depleted layers are a part of the photocurrent because of the diffusion of carriers that are not needed into the charge region. Charge carriers created by light absorption can be separated by the electric field that is present in the charge region of space. Minority charge carriers which diffuse out to the boundary of the charge region before recommingling can also be a source of photocurrent.

For tiny grains (nanomaterials and thin films) The stretching of the bands is minimal, which means that excited electrons spread out towards the surface, and then recombine with electron holes, or are caught through trap levels. The time required for a charge-carrier to get to the surface from the semiconductor volume is strongly dependent on the size of the grain. This can be described as follows:

where: rz – grain diameter of the semiconductor, JD – electron diffusion coefficient.

Separation of charge carriers, as occurs in a p-n junction, can also occur for the junction of two semiconductors with the same type of conductivity, but differing in the value of the induced gap, which is used in dye-sensitized solar cells.

Solar cell structure

Solar cell structure is designed to maximize efficiency and durability. Here are the key components and their functions in a typical solar cell:

Front Glass or Plastic Layer: This transparent layer protects the cell and allows sunlight to pass through.
Anti-Reflective Coating: Applied to the front layer, it reduces the reflection of sunlight, ensuring more light enters the cell.
Semiconductor Layer (Usually Silicon): The core part of a solar cell where sunlight is converted into electricity. It’s typically made of silicon, which is doped with other elements to create two layers:
- N-type Layer: Doped with elements that have more electrons than silicon, creating extra free electrons.
- P-type Layer: Doped with elements having fewer electrons, creating “holes” or positive charge carriers.
P-N Junction: The interface between the N-type and P-type layers. When sunlight hits this junction, it creates an electric field that helps separate and move the electrons and holes.
Front and Back Electrical Contacts: These are metallic grids or layers on the top and bottom of the cell that collect and transport the separated charges, creating an electric current.
Back Sheet: The protective backing that supports the cell and can be made from various durable materials.
Encapsulation Materials: Used to protect the cell from environmental damage and provide mechanical stability.

These components work together to efficiently convert sunlight into usable electrical energy.

The diagram below illustrates the basic structure of a solar cell. The cell’s interior is comprised of two parts that are p-type that is called the base and an n type area that is known as the emitter. The p-type zone is generally coated with boron, while the n-type zone is doped is doped with the element phosphorus. The regions near the contact contain higher levels of dopants than the areas of p and n. They are classified as the p+ and n+ . A proper amount of doping prevents loss of charge at the contacts. The cell’s two surfaces are made up of metal contacts.

The fundamental principle behind the operation of the operation of a solar cell is the photovoltaic effect. It is the process of generating electromotive force in the course of the illumination of the p-n junction. The majority of photons don’t have enough energy to let an electron escape from the bonds of the semiconductor crystal. The main parameter that determines the effectiveness of photoconversion is the size of the space between the electrons. If a photon that has an energy higher than the the energy gap of silicon is positioned on the p-n junction area the energy is lost due to the formation of an electron-hole charge. Charges that result from the electric field at the junction are split and, after passing through the cell to reach the metal contacts. In the event that the circuit becomes shut the current of electricity flows through it. This is why solar energy is transformed into electricity within the cell in an electron-based method that is free of chemical reactions.

Layer by Layer: Dissecting the Complex Structure of Modern Solar Cells

Introduction to the Multi-Layered World of Solar Cells

In the quest for sustainable energy solutions, solar cells stand at the forefront of technological innovation. These devices, which convert sunlight into electrical energy, are more than just panels basking in the sun; they are intricate assemblies of multiple layers, each with a specific function. This article delves into the complex structure of modern solar cells, exploring each layer and its role in harnessing solar power.

The Protective Outer Layer: Ensuring Durability and Transparency

At the very top is the protective outer layer, typically made of glass or transparent plastic. This layer serves a dual purpose: it protects the delicate inner components from environmental elements while allowing sunlight to pass through unimpeded.

The Anti-Reflective Coating: Maximizing Light Absorption

Beneath the protective outer layer lies the anti-reflective coating. This crucial layer reduces the amount of light reflected away from the cell, thereby increasing the amount of light absorbed. Enhanced light absorption directly correlates to better solar cell performance.

The Heart of the Cell: The Semiconductor Layer

The semiconductor layer is where the magic of the photovoltaic effect takes place. This layer, usually made of silicon, is doped with other elements to create two types of regions: the N-type and P-type. The N-type has extra electrons, while the P-type has extra ‘holes’ or spaces for electrons.

The P-N Junction: The Site of Energy Conversion

Where the N-type and P-type regions meet, a P-N junction is formed. This junction is critical as it creates an electric field, facilitating the movement of electrons and holes, which is essential for generating electrical energy.

Conductive Contacts: From Solar Cells to Power Grids

The top and bottom of the solar cell have conductive contacts, typically made of metal. These contacts allow the flow of electrons out of the cell, generating a current. The top contact is designed to be thin or mesh-like to let sunlight through to the semiconductor layer.

Back Sheet and Encapsulation: Protection and Insulation

Finally, the back sheet and encapsulation materials seal and protect the solar cell. These layers provide insulation, mechanical support, and ensure the longevity of the cell by preventing moisture and other environmental damages.

Organic solar cells

Photovoltaic cells turn sunlight into electricity using small organic molecules and electroactive polymers. These cells have an active layer between two electrodes: a conductive transparent glass and a metal contact. In the simplest form, this layer has two materials with different electrical properties. One is a donor with a positive charge, and the other is an acceptor with a negative charge. In organic solar cells, sunlight hits the donor layer and creates an exciton (an electron-hole pair). These excitons move to where the two materials meet, and here, free charge carriers separate. Free electrons move from the acceptor to the donor layers towards the negative electrode. Likewise, positive charge carriers (holes) move across the donor layer to the positive electrode. This movement of charges creates a photovoltage between the electrodes.

Organic solar cells could be used in various ways, like in portable electronics or as window coatings that generate electricity while letting light through. But, their efficiency and durability need to be better to match conventional silicon-based solar cells.

Organic solar cells, or organic photovoltaic cells, are gaining attention in renewable energy because of their unique features compared to traditional silicon solar cells. Next, we will look at some key advantages of organic over silicon solar cells.

Flexibility Flexible: Organic solar cells are typically constructed from lightweight and flexible materials like polymers. This makes them more adaptable and user-friendly compared to traditional silicon-based solar cells which tend to be stiff and difficult to work with. This flexibility makes organic solar cells ideal for applications such as wearable electronics and building-integrated photovoltaics.
Cost Effective Manufacturing Organic solar cells offer major advantages due to their production through cost-effective techniques like roll-to-roll printing. This suggests that organic solar cells could potentially be less costly to manufacture than silicon-based ones, which require costly and intricate manufacturing procedures.
Efficiency: Organic solar cells are still not as efficient as traditional silicon-based ones, but their efficiency has seen dramatic improvements over the last several years. From less than 4% to over 17 percent in just two decades, organic solar cells are now more competitive than their traditional counterparts. Furthermore, since organic solar cells can be manufactured in various dimensions and shapes for specific applications, their efficiency could potentially increase even further.
Eco-Friendly Organic Solar Cells Eco-friendly Organic solar cells are produced using abundant and sustainable ingredients. Unlike traditional silicon-based solar cells, organic solar cells don’t need toxic heavy metals or chemicals in their manufacturing process – making them a greener and sustainable option for the generation of renewable energy sources.
Increased Durability Enhance Durability: Organic solar cells have long had problems with durability due to the organic materials they use in production being sensitive to moisture and oxygen. But recent advances have enabled the development of novel materials and techniques for encapsulation that have significantly enhanced both durability and performance of organic solar cells.

Multi junction solar cell

Multi-junction solar cells consist of multiple layers of semiconductors with different Eg energy gap values.

To maximize the efficiency of a multijunction cell, the thickness of the cell should be chosen so that the largest portion of the radiation is absorbed. Multi-junction cells are optically an arrangement of layers, each of which absorbs a different part of the solar spectrum. It is therefore reasonable to design successive layers of semiconductors so that, counting from the transparent layer, successive layers have a smaller energy gap than the previous layer. An example is the GaInP / GaAs/Ge system, where the energy gaps are GaInP (1.9 eV), GaAs (1.4 eV) and Ge (0.7 eV), respectively, with an efficiency of approx. 33%, or AlGaInP/AlGaAs/GaInP/GaInAs/GaInNAs, with energy gaps of AlGaInP (2.2eV), AlGaAs (1.6 eV), GaInP (1.7 eV), GaInAs (1.2 eV), GaInNAs (1.0-1.1 eV). Depending on the percentage composition, the above systems achieve yields of 30-40%.

To maximize the flow of charge in a multi-junction cells, two junctions consecutively linked to one another using tunnel junctions (junctions where particles are able to traverse a barrier with a potential that is higher than its energy of motion). It is composed of highly doped layers, with lower absorption, low voltage drop , and high peak current. Its thickness should be approximately 10mm. Without tunnel junctions photovoltaic energy from junctions that have opposite signification could compensate.

With many connectors, high efficiency could be reached, achieving the limit of Shockley-Queisser.

The efficiency of multi-junction cell will continue to increase. Based on the amount of semiconductors in the cell, simulations have shown efficiencies that exceed than 60 percent. The highest efficiency, even with an infinite number of layers, could be 86.8 percent. This is known as the thermodynamic limit.

Multilayer systems are costlier and usually constructed by using gallium arsenide, which toxicity and the possibility of disposal in a safe manner ongoing research are being conducted. According to NREL research Multi-junction solar cells have achieved 47 percent efficiency. In the beginning, they were utilized exclusively in space because of their cost. In the present, efforts are launched to bring them onto the market.

Quantum dot solar cell

Researchers at the University of Toronto in Canada and KAUST in Saudi Arabia have made a highly efficient solar cell. This cell uses colloidal quantum dots and has about 7% efficiency, which is 40% better than earlier versions.

Quantum dot solar cells use tiny particles to boost efficiency. These particles, just a few nanometers big, are made of materials like cadmium selenide or lead sulfide.

In these cells, tiny particles sit in a thin semiconductor layer. When light hits these quantum dots, it excites electrons, similar to traditional solar cells. These excited electrons can then be used to produce electricity.

A key benefit of quantum dot cells is their ability to absorb different light colors by changing their size and material. This means they can be fine-tuned for certain parts of sunlight, possibly increasing efficiency.

Though still in early research stages, quantum dot solar cells are promising for better efficiency and cost-effective solar power.

Here’s how they work: Light hits the quantum dot cell and is absorbed, exciting electrons and creating pairs of electrons and holes. These can be used to make electric current.

The size of the quantum dots determines the light they absorb. They can be customized to absorb specific light wavelengths, using more of the solar spectrum. By stacking different quantum dots, these cells can absorb more light and increase efficiency.

In the end, the separated electrons and holes are collected by electrodes and sent to a circuit. This electricity can power devices or be stored in batteries.

Colloidal quantum dots are tiny semiconductor particles, just a few nanometers wide, created by crystallizing semiconductor solutions. Their production method allows them to be easily placed on various substrates. One use for colloidal quantum dots is in organic solar cells, where they act as light absorbers. Their main advantage in photovoltaics is the ability to absorb light across a wide range of wavelengths. This is because changing their size adjusts their energy gaps.

However, making solar cells with colloidal quantum dots faces a challenge. Bare spots on their surfaces can trap electrons, reducing cell efficiency. To solve this, the dots are passivated. This means covering them with organic compounds or a semiconductor layer with a wider energy gap.

Edward Sargent’s team at the University of Toronto treated the dots’ surfaces with chloride. This filled the bare spots and improved the surface quality.

In the manufacturing process, these quantum dots are placed on a transparent glass surface, then covered with a conductive layer. They are linked together using an organic binder, creating a dense dot agglomerate that absorbs more light than looser layers. This dense structure was confirmed by X-ray scattering experiments at KAUST.

“Most solar cells today use heavy crystals,” Sargent said. “Our research shows that lighter, flexible materials like colloidal quantum dots could compete, especially in cost, with traditional solar cells.

Understanding Solar Cell Efficiency: What Makes the Perfect Photovoltaic

The Quest for Maximum Efficiency

Solar cell efficiency is a measure of how well a photovoltaic (PV) cell can convert sunlight into electricity. The quest for the perfect photovoltaic involves increasing this efficiency, enabling more power generation from the same amount of sunlight. Researchers continuously explore new materials and designs to achieve this goal.

Fundamentals of Solar Cell Efficiency

Efficiency in solar cells is determined by several key factors:

Light Absorption: The ability of the cell to absorb as much sunlight as possible is crucial. Thinner cells can now achieve high absorption rates due to advanced materials and structuring.
Minimizing Losses: Losses occur due to reflection, resistance, and recombination of charge carriers. Anti-reflective coatings, improved electrical contacts, and better semiconductor quality help reduce these losses.
Maximizing Voltage and Current: The efficiency of a solar cell also depends on its ability to generate higher voltages and currents from the absorbed light. This involves optimizing the band gap of the semiconductor material.

Innovative Materials and Technologies

Recent advancements focus on materials like perovskites, which offer high efficiency and lower manufacturing costs. Tandem solar cells, which layer multiple types of materials, are another avenue for increasing efficiency beyond the limits of traditional silicon cells.

The Role of Silicon

Despite new materials, silicon remains the cornerstone of solar cell technology due to its abundance and proven performance. Enhancements in silicon cell technology, like passivated emitter and rear cell (PERC) design, continue to push its efficiency boundaries.

Real-World Factors Affecting Efficiency

While laboratory efficiencies have reached impressive levels, real-world factors like temperature, angle of sunlight, and dust accumulation can impact performance. Therefore, advancements are also focused on making cells more resilient and adaptable to different environments.

The Future of Solar Cell Efficiency

The future of solar cell efficiency lies in both material science breakthroughs and practical enhancements to existing technologies. Balancing cost, durability, and efficiency will remain key in making solar energy more accessible and sustainable.

Challenges and Opportunities: The Future of Solar Cell Research and Development

Embracing the Challenges

The journey of solar cell technology is marked by both challenges and opportunities. As we look to the future, addressing these challenges is key to unlocking the full potential of solar energy.

Material Shortages and Costs: Traditional silicon, used in most solar cells, is abundant but not without cost and environmental concerns. Research is focused on finding more sustainable and less expensive materials without compromising efficiency.
Efficiency Limitations: Current solar cells have an efficiency ceiling, known as the Shockley-Queisser limit, which restricts how much solar energy can be converted into electricity. Overcoming this limit is a major research goal.
Environmental Impact and Sustainability: The full lifecycle impact of solar cells, from manufacturing to disposal, poses environmental challenges. Developing more eco-friendly production processes and recyclable materials is a priority.

Seizing the Opportunities

The future of solar cells is not just about overcoming challenges but also about seizing new opportunities.

Perovskite Solar Cells: These next-generation solar cells offer higher efficiencies and lower production costs than traditional silicon-based cells. Their potential to revolutionize the solar industry is immense.
Building-Integrated Photovoltaics (BIPV): Integrating solar cells directly into building materials offers a dual-purpose solution—structures that both shelter and generate power.
Tandem Solar Cells: Layering different types of solar cells can capture a broader range of the solar spectrum, significantly boosting efficiency.

Advancements in Storage and Integration

Improving solar cell technology is only part of the equation. Advancements in energy storage and grid integration are critical for the widespread adoption of solar energy.

Energy Storage Solutions: Developing more efficient and affordable energy storage systems will allow for the better utilization of solar energy, making it a more reliable power source.
Smart Grid Integration: Integrating solar power with smart grids enables more efficient energy distribution and usage, adapting to varying energy demands and availability.

Collaboration and Policy Support

The future of solar cell research also hinges on collaboration between governments, industries, and academia. Policies that support renewable energy development and usage are crucial.

Looking Ahead

As we move forward, the future of solar cell technology is not just about scientific and technical advances but also about creating a sustainable and energy-secure world. With continued innovation and support, solar cells will play a pivotal role in our energy landscape.

Recent researches about Solar Cell

“The way to predict outdoor lifetime” – Discusses modeling the outdoor ageing behavior of perovskite solar cells using temperature-dependent degradation rates. (Published on 19 Dec 2023 in Nature Energy)

The recent research titled “The way to predict outdoor lifetime” by Mark Khenkin and Steve Albrecht, published in Nature Energy in December 2023, focuses on the operational stability of perovskite solar cells. This study is crucial as it addresses a key challenge in the commercialization of perovskite solar cells: predicting their outdoor lifespan. While perovskite solar cells have shown promising efficiency and cost-effectiveness, their real-world operation and durability under various environmental stress factors remain uncertain. The research introduces a model utilizing temperature-dependent degradation rates from laboratory stability tests, involving both heat and light stressors, to simulate and predict the outdoor ageing behavior of these devices. This approach is a significant step in bridging the gap between laboratory conditions and real-world operation, potentially leading to more reliable and durable solar cell technology

“Fixed charge passivation in perovskite solar cells” – An interlayer of aluminium oxide with fixed charges is shown to boost perovskite solar cell performance. (Published on 04 Dec 2023 in Nature Energy)

The research titled “Fixed charge passivation in perovskite solar cells,” published in Nature Energy in December 2023, presents a significant advancement in enhancing the performance of perovskite solar cells. This study demonstrates the use of an interlayer of aluminium oxide with fixed charges to boost the performance of these cells. Notably, this method resulted in an increase in the open-circuit voltage by 60 meV. Additionally, the solar cells showed impressive stability, with no significant efficiency drop after 2,000 hours under one sun illumination at 85 °C. This research represents a notable step forward in the development of more efficient and stable perovskite solar cells, potentially contributing to their wider adoption in solar technology.

“Machine-learning-accelerated selection of perovskite passivants” – A machine-learning model accelerates the selection of bifunctional pseudo-halide passivators for perovskite photovoltaics. (Published on 28 Nov 2023 in Nature Materials).

The research titled “Machine-learning-accelerated selection of perovskite passivants,” published in Nature Materials in November 2023, represents a significant advancement in the field of perovskite photovoltaics. The study introduces a machine-learning model that significantly accelerates the process of identifying effective bifunctional pseudo-halide passivators for perovskite solar cells. These passivating agents, identified through the model, have been experimentally demonstrated to enhance the performance of perovskite solar cells. This development is a substantial leap forward in optimizing perovskite solar cells, making the process more efficient and time-effective.

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