April 26


A Deep Exploration into Solar Cell Structure and Operation

By hacheng1@gmail.com

April 26, 2023

This article offers a comprehensive overview of solar cells, delving into their basics, structure, operation, performance parameters, and factors affecting their performance. Throughout the article, you will learn about the types of solar cells, the principle of the photovoltaic effect, the composition of different solar cell structures, and key concepts such as efficiency, fill factor, and the current-voltage curve. Additionally, you will discover the impact of factors like temperature, light intensity, angle of incidence, material quality, and soiling on solar cell performance.

Solar Cell Basics

Definition of a Solar Cell

A solar cell, also known as a photovoltaic cell or PV cell, is an electrical device that directly converts sunlight into electricity by the photovoltaic effect. It is a semiconductor-based device that absorbs and converts light energy into direct current (DC) electricity. Multiple solar cells are connected to form a solar panel or solar module, which is then used in various applications, such as powering homes, businesses, or remote equipment.

Solar cells serve as a clean and renewable energy source. They are free from greenhouse gas emissions, and their primary raw material, sunlight, is abundant and inexhaustible. As solar technology advances, solar cells are becoming more efficient, cost-effective, and widely used in daily life.

Principle of Photovoltaic Effect

The photovoltaic effect is the operating principle behind solar cells. It is a physical-chemical process that occurs when a photovoltaic material absorbs photons (particles of light) and generates electron-hole pairs.

When the absorbed light energy is greater than the material’s bandgap energy, the electrons become excited and move from the valence band to the conduction band. This movement of electrons creates a voltage difference or potential, which drives the electron flow (electric current) through an external circuit. In a solar cell, the photovoltaic material is sandwiched between two layers of conductive material, often made of silicon.

The top layer is usually doped with phosphorus, creating an n-type semiconductor with an excess of free electrons. The bottom layer is often doped with boron, creating a p-type semiconductor with a lack of free electrons (holes). The interface between the n-type and p-type semiconductor is called the p-n junction.

When a photon of sunlight is absorbed, an electron is excited and leaves a hole in the p-type material. The electron is then attracted to the n-type material due to the electric field created by the p-n junction. This causes a flow of electrons (electric current) through the external circuit to fill the holes in the p-type material, generating electricity.

Types of Solar Cells

There are various types of solar cells, each with different materials, structures, and efficiencies. Some common types include monocrystalline, polycrystalline, thin-film, multi-junction, and perovskite solar cells.


Monocrystalline solar cells are made from a single crystal of high-purity silicon. They are characterized by their uniform dark appearance and rounded edges. Monocrystalline solar cells exhibit the highest efficiency among silicon-based solar cells, typically around 20%-25%. However, they are also more expensive to manufacture due to the complex production process and waste associated with cutting the cylindrical crystal into thin wafers.


Polycrystalline solar cells, also called multicrystalline, are made from multiple silicon crystals that have been melted together. These solar cells have a characteristic blue appearance and visible grain boundaries. Polycrystalline solar cells are less efficient than monocrystalline solar cells, usually having efficiency values between 15%-20%, but they are also less expensive to produce due to a simpler manufacturing process.


Thin-film solar cells are made by depositing thin layers of photovoltaic material onto a substrate, such as glass or plastic. Common materials used in thin-film solar cells include amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS). While thin-film solar cells are generally less efficient than crystalline silicon solar cells, ranging from 10%-12% efficiency, they have several advantages, including better performance in low-light conditions, flexibility, and lower manufacturing costs.


Multi-junction solar cells, also known as tandem solar cells or multilayer solar cells, are made by stacking multiple photovoltaic materials with different bandgaps. This allows the solar cell to absorb a wider range of the solar spectrum, thereby increasing its efficiency. Multi-junction solar cells are primarily used in high-concentration photovoltaic (HCPV) systems and for space applications where high efficiency is crucial. These solar cells can have efficiencies exceeding 40%, but they are also expensive to manufacture and face limited scalability.


Perovskite solar cells are an emerging type of solar cell made from perovskite-structured materials. Perovskite solar cells have demonstrated rapid improvements in efficiency since their introduction, with current values exceeding 25%. They have the potential to be manufactured using low-cost processes, making them a promising alternative to conventional solar cells. However, challenges related to stability and long-term durability remain, which are key factors for their commercial viability.

Solar Cell Structure

Basic Components of a Solar Cell

A solar cell, also known as a photovoltaic (PV) cell, is an electrical device that converts sunlight into electricity through the photovoltaic effect. The basic components of a solar cell include:

  1. Front Glass or Transparent Cover: This component serves as a protective layer to shield the solar cell from external damage while allowing sunlight to penetrate into the device. The front glass is usually made of a material with a high transmittance, such as low-iron tempered glass or transparent plastics. This cover also provides structural support to the solar cell.
  2. Anti-Reflective Coating: An anti-reflective (AR) coating is applied to the front glass to reduce the amount of sunlight reflected away from the solar cell. By minimizing reflection losses, AR coatings help maximize the amount of sunlight that enters the device, leading to higher energy conversion efficiency.
  3. Front and Back Contacts: These are metallic layers that allow the flow of electrical current generated by the solar cell to be collected and utilized. The front contact is typically patterned in a grid-like structure to minimize its shading effect on the solar cell, while the back contact is usually continuous, covering the entire surface area. Commonly used materials for these contacts include silver, aluminum, and copper.

Structure of Crystalline Silicon Solar Cells

Crystalline silicon is the most widely used material for solar cells due to its abundance, non-toxicity, and favorable electrical properties. The structure of a crystalline silicon solar cell is composed of:

  1. N-Type and P-Type Silicon Layers: A solar cell is made by connecting two different types of silicon layers. The N-type layer contains excess electrons, while the P-type layer contains excess positive charges called holes. Consequently, the N-type silicon has a negative electrical potential, whereas the P-type silicon has a positive electrical potential.
  2. PN Junction: The interface between the N-type and P-type silicon layers forms a PN junction. When sunlight penetrates the solar cell and is absorbed by the silicon, it generates electron-hole pairs. Under the influence of the electric field established by the junction, the electrons and holes are separated and driven towards the opposing electrodes, creating an electric current.

Structure of Thin-Film Solar Cells

Thin-film solar cells are an alternative to crystalline silicon-based solar cells, known for their low manufacturing costs and flexible properties. The main types of thin-film solar cells include:

  1. Cadmium Telluride (CdTe): CdTe solar cells have a layer of cadmium telluride as the light-absorbing material, sandwiched between transparent conductive oxide (TCO) layers. A layer of cadmium sulfide (CdS) is also included to form a junction with the CdTe layer.
  2. Copper Indium Gallium Selenium (CIGS): CIGS solar cells consist of a thin layer of CIGS material, which functions as the light-absorbing layer. The CIGS layer is placed between a TCO layer and a metal back contact, with a cadmium sulfide (CdS) or zinc oxide (ZnO) layer forming the junction with the CIGS material.
  3. Amorphous Silicon (a-Si): Amorphous silicon solar cells employ a thin, non-crystalline silicon layer as the light-absorbing material. These solar cells typically have a p-i-n junction, where the intrinsic (i) layer of silicon is sandwiched between the p-type and n-type layers.

Structure of Perovskite Solar Cells

Perovskite solar cells have attracted significant attention in recent years due to their rapid increase in energy conversion efficiency. The structure of a perovskite solar cell consists of:

  1. Electron Transport Layer (ETL): The ETL is responsible for efficiently transporting electrons generated in the active layer to the front contact. Common materials used for the ETL include titanium dioxide (TiO2) and zinc oxide (ZnO).
  2. Hole Transport Layer (HTL): The HTL functions to transport the holes generated in the active layer to the back contact. Common materials utilized for the HTL include spiro-OMeTAD, PEDOT:PSS, and copper iodide (CuI).
  3. Perovskite Active Layer: The perovskite active layer is the light-absorbing material that generates electron-hole pairs upon sunlight absorption. These materials have a unique crystal structure that consists of an organic cation (usually methylammonium or formamidinium), a metal cation (usually lead or tin), and an anion (such as iodide or bromide).

Solar Cell Operation

Solar cells, also known as photovoltaic (PV) cells, are semiconductor devices that convert sunlight into electricity. The operation of a solar cell is based on the photovoltaic effect, which causes the generation of electric current when photons of light strike the cell’s surface. This process can be broken down into four key steps: generation of electron-hole pairs, separation of charges, creation of electric current, and occlusion and recombination.

Generation of Electron-Hole Pairs

The first step in the operation of a solar cell is the generation of electron-hole pairs. When sunlight, composed of photons, strikes the surface of a solar cell, some of these photons are absorbed by the semiconductor material. The energy from the absorbed photons is then transferred to the electrons in the atoms of the semiconductor. This transfer of energy enables the electrons to be excited and jump from their original energy levels, called valence bands, to higher energy levels, called conduction bands.

When an electron moves from the valence band to the conduction band, it leaves behind a vacant space or a “hole” in the valence band. The combination of the excited electron in the conduction band and the hole in the valence band is referred to as an electron-hole pair. The generation of electron-hole pairs is the primary process that initiates the conversion of sunlight into electricity in a solar cell.

Separation of Charges

The next step in the solar cell operation is the separation of charges. To create a current, the excited electrons in the conduction band and the holes in the valence band must be separated. This separation is achieved using layers of semiconductor materials with different electrical properties, known as p-type and n-type.

The p-type material has a high concentration of positively charged “holes,” while the n-type material has a high concentration of negatively charged electrons. When sunlight hits the solar cell, some of the photons will reach the junction between the p-type and n-type semiconductor layers. At this junction, the electron-hole pairs are separated due to the electric field created by the difference in the charge between the two layers.

The electric field pushes the electrons in the conduction band towards the n-type material, while the holes are pushed towards the p-type material. This separation of charges generates voltage across the solar cell.

Creation of Electric Current

Once the charges have been separated, the solar cell can generate an electric current. To create a current, an external electrical circuit needs to be connected between the p-type and n-type semiconductor layers. The electrons in the n-type layer flow to the positive terminal of the external circuit, while the holes in the p-type layer flow to the negative terminal.

As the electrons flow through the external circuit, they create a continuous flow of electrical current. This flow of electrons is harnessed and converted into usable electrical power by various devices and systems, such as inverters and transformers.

Occlusion and Recombination

The final step in the operation of a solar cell is occlusion and recombination. Occlusion refers to the reflection of sunlight off the surface of the solar cell instead of absorbing it, which reduces the amount of energy that can be converted into electricity. To minimize occlusion, solar cells often have anti-reflective coatings and textured surfaces to increase the chances of photon absorption.

Recombination occurs when excited electrons and holes in the semiconductor material recombine without generating electricity. Recombination is an undesirable phenomenon as it results in the loss of energy that could have been converted into electrical power. To minimize recombination, various techniques are employed in solar cell design, such as passivation layers and improved material quality.

In summary, the operation of a solar cell involves the generation of electron-hole pairs, separation of charges, creation of electric current, and occlusion and recombination. These processes contribute to the overall efficiency and performance of solar cells, enabling the transformation of sunlight into clean, renewable electricity for various applications.

Key Performance Parameters

Solar cell performance is affected by many factors. In order to evaluate a solar cell’s efficiency and overall performance, several key metrics have been established. These parameters are critical for researchers, manufacturers, and users interested in determining how well a solar cell or module performs under various conditions. In this section, we will discuss three major performance parameters: efficiency, fill factor, and the current-voltage (IV) curve.


Efficiency is the most commonly cited metric when discussing solar cell performance. It refers to the ratio of electrical energy generated by the solar cell to the amount of solar energy incident on its surface. Efficiency can be expressed as a percentage, with higher values indicating better performance. Efficiency values for commercial solar cells typically range from 15% to 22%, while record-breaking research prototypes may achieve efficiencies above 25%.

There are several factors that can affect the efficiency of a solar cell, including its material composition, construction, and operating temperature. Different semiconductor materials have varying levels of light absorption and conversion efficiencies, while material impurities and defects can introduce recombination centers that decrease the overall efficiency. Additionally, solar cells generally exhibit decreased performance as temperatures rise due to a reduction in the open-circuit voltage and an increase in the rate of electron-hole recombination.

In order to improve the efficiency of solar cells, researchers may employ various techniques such as developing new materials with better light absorption properties, constructing tandem or multi-junction cells capable of harnessing a broader range of the solar spectrum, and utilizing light trapping methods to direct more incident light toward the active layers of the cell.

Fill Factor

The fill factor (FF) is another important performance parameter that directly influences the power output of a solar cell. It is defined as the ratio of the maximum power point (MPP) on the IV curve to the product of the short-circuit current (I_SC) and the open-circuit voltage (V_OC). This represents the “squareness” of the IV curve, with higher fill factors indicating a more rectangular shape, which corresponds to better energy conversion and power generation performance.

Fill factor values for high-quality solar cells typically range between 75% and 85%. The fill factor is affected by factors such as series resistance, shunt resistance, and the recombination process within the cell. Reducing series resistance and increasing shunt resistance can lead to an improvement in the fill factor, while minimizing recombination losses can also contribute to higher FF values.

To improve the fill factor, researchers and manufacturers often focus on optimizing the design of the solar cell, such as optimizing the thickness and uniformity of semiconductor layers, improving the quality of materials used, and minimizing contact resistance at the metal-semiconductor interface.

Current-Voltage (IV) Curve

The current-voltage (IV) curve is a graphical representation of a solar cell’s output in terms of current and voltage under various illumination conditions. It is useful for understanding the performance of a solar cell under a wide range of operating conditions and can reveal important characteristics such as the maximum power point (MPP), the open-circuit voltage (V_OC), and the short-circuit current (I_SC).

Several features of an IV curve can be influenced by factors such as the cell’s temperature, incident light intensity, and semiconductor material properties. The slope of the curve near the open-circuit voltage (V_OC) is related to the cell’s series resistance, while the slope near the short-circuit current (I_SC) is indicative of the cell’s shunt resistance. Furthermore, the MPP and fill-factor, both crucial performance parameters, can be derived from the IV curve.

The IV curve is an essential tool for researchers and manufacturers to optimize the performance of solar cells and to diagnose potential issues affecting their efficiency. By plotting an IV curve for a solar cell under various conditions, it is possible to identify and adjust parameters that are having a negative impact on the cell’s output. This enables a deeper understanding of the fundamental processes occurring within the solar cell, leading to further innovation and advancement in solar technology.

Factors Affecting Solar Cell Performance

Solar cell performance can be impacted by several factors, all of which should be taken into account during the design and installation of a solar power system. The key factors that affect the performance of a solar cell include temperature, light intensity, angle of incidence, material quality, and soiling and degradation. By understanding the importance of these factors, one can optimize the efficiency and output of a solar power system. In this section, we will discuss each of these factors in detail.


Temperature is a two-faced factor influencing solar cell operation. On the one hand, high temperature can increase the efficiency of some solar panel materials, and on the other hand, it may decrease the overall efficiency of the solar panel combined with other materials.

Most solar cells, such as those made from crystalline silicon, experience a decrease in efficiency as temperature increases. This occurs because high temperatures cause the charges present in the material’s atomic lattices to scatter the electrons generated by the cell, thereby reducing the net electricity produced. The temperature coefficient, typically expressed as a percentage per degree Celsius (%/°C), quantifies this effect. For example, a solar cell with a temperature coefficient of -0.5%/°C will experience a 0.5% decrease in efficiency for each degree Celsius increase in temperature.

Conversely, some thin-film solar cells, such as those made from cadmium telluride (CdTe) or amorphous silicon, may display a slight increase in efficiency due to temperature annealing – though that incremental benefit is relatively small when compared to the drawbacks. To mitigate the effects of temperature on solar cell performance, engineers often design solar systems with adequate ventilation and cooling systems to manage heat dissipation.

Light Intensity

The amount of sunlight, or light intensity, is directly proportional to the output of a solar cell. Therefore, maximizing the exposure of the solar panel to sunlight is critical in achieving optimal performance. Generally, the performance of a solar cell is determined by measuring its efficiency under standard testing conditions, where the solar irradiance is fixed at 1,000 watts per square meter (W/m²).

The real-world solar radiation intensity will fluctuate throughout the day and the year, affecting the amount of energy a solar panel can generate. Cloud cover, geographic location, and time of day are some factors that influence light intensity on solar panels. By designing a solar system with a proper mounting angle and orientation, one can optimize solar cell output based on local light intensity variations.

Angle of Incidence

The angle at which sunlight strikes a solar panel, known as the angle of incidence, greatly impacts the performance of the solar cells. When sunlight strikes a solar panel at a perpendicular angle, the light travels the shortest path through the panel’s material, maximizing the energy absorbed and converted into electricity. As the angle deviates from perpendicular, the energy absorption decreases, diminishing the solar cell’s efficiency.

To address this issue, solar system designers often use tracking systems that adjust the angle of the solar panels to follow the sun. This helps to ensure that the sunlight strikes the panels at an optimal angle throughout the day. Alternatively, solar panels can be mounted at a fixed angle that maximizes yearly energy production based on the geographical location and the sun’s path.

Material Quality

The quality of materials used in the fabrication of solar cells can directly impact their performance. Imperfections or impurities in the materials can increase the number of charge traps in a solar cell, decreasing the amount of electricity generated. Lower-quality materials may also have lower electron mobility or shorter carrier lifetimes, affecting the solar cell’s overall efficiency.

In choosing a solar panel or module, it is essential to consider factors such as the solar cell’s efficiency, the manufacturing process, and the associated warranties. High-quality solar cells from well-established manufacturers tend to have better performance and longer lifetimes, providing a higher return on investment.

Soiling and Degradation

Soiling refers to the accumulation of dirt, dust, and other debris on the surface of a solar panel, which can inhibit the transmission of sunlight through the glass, affecting the performance of the solar cells. Regular cleaning, either through manual or automated methods, can maintain the energy output of a solar panel.

Degradation is the reduction in the performance of a solar cell over time, primarily due to factors such as exposure to UV radiation, temperature changes, and humidity. Solar cells typically have a warranty guaranteeing at least 80% of their initial performance after 25 years, and ongoing research efforts are aimed at minimizing degradation to extend the lifespan of the cells.

In summary, optimizing solar cell performance requires considering factors such as temperature, light intensity, angle of incidence, material quality, and cleanliness. By understanding these factors, both system designers and solar panel users can work towards maximizing the efficiency and output of their solar power systems.

Frequently Asked Questions

1. What is the basic structure of a solar cell?

A solar cell generally consists of a semiconductor material, most commonly silicon, that’s doped to create p-type (positive) and n-type (negative) layers. Light-absorbing materials are sandwiched between electrodes that facilitate current flow when photons are absorbed and generate an electric current (Fahrenbruch & Bube, 1983).

2. How does a solar cell convert sunlight into electricity?

Sunlight contains photons, which carry energy. When these photons strike the solar cell’s semiconductor material, they transfer their energy to electrons, liberating them from atoms in the n-type and p-type layers. The built-in electric field drives these electrons to external circuits, creating electrical power (Chopra, Paulson & Dutta, 2004).

3. What is the importance of the PN junction in a solar cell?

The PN junction is crucial to solar cell operation. It separates the p-type and n-type layers, producing an electric field that enables the flow of charge carriers, namely electrons and holes. This junction minimizes recombination, ensuring optimal energy conversion from absorbed photons into electrical power (Fahrenbruch & Bube, 1983).

4. What factors influence solar cell efficiency?

Solar cell efficiency depends on numerous factors, including the material’s light absorption properties, quality of the semiconductor material, surface reflection losses, and temperature. Optimizing these factors allows for increased conversion of sunlight energy into electricity (Green, 2005).

5. Why is silicon predominantly used in solar cells?

Silicon is widely used in solar cells due to its high abundance on Earth’s crust, ideal semiconducting properties, and light-absorbing characteristics. Additionally, well-established manufacturing processes make it economically feasible for large-scale production of solar cells (green, 1982).

6. What are the alternatives to silicon-based solar cells?

Alternative materials to silicon include cadmium telluride (CdTe), copper indium gallium selenide (CIGS), gallium arsenide (GaAs), and organic polymers. These materials offer unique properties like flexibility, cost-effectiveness, and high efficiency, opening possibilities for novel solar cell applications (Saliba et al., 2016).


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