Photovoltaics, or PV, is a way to change sunlight into electricity using special materials that react to light. This process, called the photovoltaic effect, is studied in science fields like physics and chemistry. It is used in products that create electricity and in devices that detect light.

A PV system uses solar modules, which are made up of many solar cells that produce electricity. These systems can be placed on the ground, rooftops, walls, or even on water. The way they are attached can be fixed or use a device that moves to follow the sun.

PV technology helps reduce climate change because it produces much less carbon dioxide than fossil fuels. Solar PV has advantages as an energy source: once built, it does not create pollution or greenhouse gases during use. It can be adjusted to meet different power needs, and silicon, a material used in PV systems, is widely available in Earth’s crust. However, other materials, like silver, may limit future growth. Using PV as a main energy source requires storage systems or special power lines, which add costs. Also, PV power can vary depending on weather, which must be managed. Making and installing PV systems causes some pollution and emissions, but much less than fossil fuels.

PV systems have been used for many years in special applications. Grid-connected PV systems have been used since the 1990s. Solar modules were first produced in large amounts in 2000, when the German government supported a program to install solar panels on 100,000 rooftops. Costs for PV have dropped over time, partly because of large investments in China to increase solar production and improve efficiency. Financial support, like special electricity prices for solar power, has helped PV grow in many countries. Panel prices dropped four times between 2004 and 2011, and module prices fell about 90% during the 2010s.

In 2022, global PV power reached more than 1 terawatt (TW), meeting about 2% of the world’s electricity needs. After hydro and wind power, PV is the third-largest renewable energy source by capacity. In 2022, experts predicted PV power would grow by more than 1 TW between 2022 and 2027. In some areas with strong sunlight, PV is the cheapest electricity source, with prices as low as 0.015 US dollars per kilowatt-hour in Qatar in 2023. In 2023, experts said that for projects with low costs and good sunlight, solar PV is now the cheapest electricity source in history.

Etymology

The word "photovoltaic" is made up of two parts. The first part, "phōs," comes from the Greek word for "light." The second part, "volt," is a unit used to measure electric force. This unit is named after Alessandro Volta, an Italian scientist who invented the battery, also called an electrochemical cell. The term "photovoltaic" has been used in English since the year 1849.

History

In 1989, the German Research Ministry began the first program to provide money for PV roofs, which included 2200 roofs. This program was led by Walter Sandtner in Bonn, Germany. In 1994, Japan followed this example and created a similar program, installing 539 residential PV systems. Since then, many countries have continued to produce and finance PV systems at an increasing rate.

Solar cells

Photovoltaics are a method for generating electric power by using solar cells to convert sunlight into electricity through the photovoltaic effect. Solar cells produce direct current electricity from sunlight, which can be used to power equipment or recharge batteries. The first practical use of photovoltaics was to power satellites and spacecraft, but today most photovoltaic modules are used in grid-connected systems for power generation. In these systems, an inverter is needed to change direct current to alternating current. There is also a smaller market for standalone systems used in remote homes, boats, recreational vehicles, electric cars, roadside phones, remote sensing, and pipeline protection.

Photovoltaic power generation uses solar modules made of solar cells containing a semiconductor material. Copper cables connect modules, arrays, and sub-fields. Because demand for renewable energy has grown, manufacturing of solar cells and photovoltaic arrays has improved significantly in recent years.

Solar cells need protection from the environment and are usually sealed tightly in solar modules. The power of a photovoltaic module is measured under standard test conditions (STC) in "Wp" (watts peak). The actual power output at a specific location may be less than or greater than this value, depending on geography, time of day, weather, and other factors. Solar photovoltaic array capacity factors are usually under 25% without storage, which is lower than many other electricity sources.

Solar-cell efficiency is the amount of sunlight energy that can be converted into electricity by the solar cell. The efficiency of solar cells, along with latitude and climate, determines the annual energy output of a system. For example, a solar panel with 20% efficiency and an area of 1 square meter produces 200 kWh/year under standard test conditions if exposed to 1000 W/m² of sunlight for 2.74 hours daily. Solar panels are usually exposed to sunlight longer than this in a day, but sunlight intensity is often less than 1000 W/m². Panels produce more energy when the sun is high in the sky and less in cloudy conditions or when the sun is low, such as in winter.

Two location-based factors affecting solar PV yield are the spread and strength of solar radiation. These factors vary greatly between countries. Regions with high radiation levels year-round include the Middle East, Northern Chile, Australia, China, and the Southwestern USA. In central Colorado, which receives 2000 kWh/m²/year of sunlight, a panel can produce 400 kWh/year. In Michigan, which receives 1400 kWh/m²/year, the same panel produces 280 kWh/year. In more northern European regions, such as southern England, annual energy yield is significantly lower, at 175 kWh/year under the same conditions.

Several factors influence a cell’s conversion efficiency, including reflectance, thermodynamic efficiency, charge carrier separation, charge carrier collection, and conduction efficiency. Because these parameters are hard to measure directly, other values are used instead, such as quantum efficiency, open-circuit voltage (VOC) ratio, and fill factor. Reflectance losses are measured by quantum efficiency. Recombination losses are measured by quantum efficiency, VOC ratio, and fill factor. Resistive losses are mainly measured by fill factor but also affect quantum efficiency and VOC ratio.

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

Performance and degradation

Module performance is usually tested under standard test conditions (STC): sunlight intensity of 1,000 W/m², a solar spectrum called AM 1.5, and a module temperature of 25 °C. The actual voltage and current output of the module changes depending on lighting, temperature, and load conditions, so there is no single voltage at which the module operates. Performance depends on location, time of day, season, sunlight intensity, direction and tilt of the module, cloud cover, shading, dirt, battery charge level, and temperature. Module performance can be measured at different times using tools like a DC clamp meter or shunt, and recorded, graphed, or charted with a data logger or chart recorder.

For best performance, solar panels should be made of similar modules arranged in the same direction, facing directly toward sunlight. Bypass diodes are used to avoid electrical loss from broken or shaded panels, helping to keep the system working efficiently. These diodes are usually placed in groups of solar cells to maintain a continuous flow of electricity.

Electrical characteristics include nominal power (P MAX, measured in watts), open-circuit voltage (V OC), short-circuit current (I SC, measured in amperes), maximum power voltage (V MPP), maximum power current (I MPP), peak power (watt-peak, W p), and module efficiency (%).

Open-circuit voltage (V OC) is the highest voltage a module can produce when not connected to a circuit. V OC can be measured directly with a voltmeter on the module’s terminals or disconnected cables.

The peak power rating (W p) is the maximum output under standard test conditions, not the highest possible output. Modules that are about 1 by 2 meters (3 ft × 7 ft) in size may have ratings from 75 W to 600 W, depending on their efficiency. During testing, modules are grouped based on their results, and manufacturers often rate them in 5 W increments, with ratings that may vary by ±3%, ±5%, +3/-0%, or +5/-0%.

The performance of a photovoltaic (PV) module depends on environmental conditions, especially the amount of sunlight (G) hitting the module. The temperature (T) of the p–n junction also affects key electrical parameters: short-circuit current (I SC), open-circuit voltage (V OC), and maximum power (Pmax). Generally, V OC decreases as temperature increases, while I SC increases slightly but not enough to offset the drop in V OC. As a result, Pmax decreases when temperature rises. This relationship depends on the semiconductor material and is caused by how temperature affects the movement and concentration of electrons and holes inside the PV cell.

Temperature sensitivity is described using temperature coefficients. These values, found in module data sheets, include:

• β: How much V OC changes with temperature (V OC per degree Celsius).
• α: How much I SC changes with temperature (I SC per degree Celsius).
• δ: How much Pmax changes with temperature (Pmax per degree Celsius).

Methods to calculate these coefficients from experimental data are available in scientific literature.

Solar modules are tested for their ability to withstand damage from rain, hail, heavy snow, and extreme temperature changes. Most solar panels sold in the U.S. are UL listed, meaning they have passed tests for hail resistance.

Potential-induced degradation (PID) is a type of performance loss in crystalline PV modules caused by stray electrical currents. This can reduce power output by up to 30%.

The biggest challenge for photovoltaic technology is the cost of electricity produced per watt. Improvements in PV technology involve "doping" silicon to lower activation energy, which increases the efficiency of converting sunlight into usable electricity.

Chemicals like boron (p-type) are added to semiconductor crystals to create energy levels closer to the valence and conduction bands. This reduces the activation energy from 1.12 eV to 0.05 eV, allowing boron to ionize at room temperature. This creates free energy carriers in the conduction and valence bands, improving the conversion of sunlight into electricity.

The power output of a photovoltaic device decreases over time due to exposure to sunlight and other environmental factors. The degradation index, defined as the annual percentage of power loss, is critical for predicting long-term performance. The rate of degradation for each electrical parameter affects the overall performance of a solar system. Modules in the same installation may degrade at different rates, leading to variations in performance over time.

Studies show that crystalline silicon modules degrade at a steady rate of 0.8% to 1.0% per year. Thin-film modules, such as amorphous silicon, micromorphic silicon, or cadmium telluride, experience higher initial degradation (up to 3% to 4% annually) for the first few years, followed by a slower, more stable rate. Thin-film technologies also show greater seasonal changes due to their sensitivity to the solar spectrum. Other thin-film technologies, like CIGS, degrade more slowly even in the early years.

Manufacturing of PV systems

The manufacturing process for solar photovoltaics is simple because it does not require many complex or moving parts. Since PV systems are solid-state, they often last for a long time, from 10 to 30 years. To increase the electrical output of a PV system, the manufacturer can simply add more photovoltaic components. Because of this, economies of scale are important for manufacturers because costs decrease as production increases.

Crystalline silicon PV systems were responsible for about 90% of global PV production in 2013. Manufacturing silicon PV systems involves several steps. First, polysilicon is purified from mined quartz until it reaches semiconductor-grade purity. This material is then melted with small amounts of boron, a group III element, to create a p-type semiconductor with many electron holes. Using a seed crystal, a solid ingot is grown from the liquid polycrystalline material. The ingot may also be cast in a mold. Thin wafers are cut from the ingot using wire saws and then undergo surface etching and cleaning. Next, the wafers are placed in a phosphorus vapor deposition furnace, where a thin layer of phosphorus, a group V element, is added to create an n-type semiconducting surface. To reduce energy loss, an anti-reflective coating and electrical contacts are added to the surface. After finishing, the cells are connected via electrical circuits based on the specific application and prepared for shipping and installation.

Solar photovoltaic power is not entirely "clean energy" because its production releases greenhouse gas emissions. Materials used to build the cells may be unsustainable and could eventually run out. The technology also uses toxic substances that cause pollution, and there are no proven methods for recycling solar waste. Data about the environmental impact of PV systems are often uncertain. For example, the values of human labor and water consumption are not precisely measured due to a lack of systematic studies. It is also difficult to determine whether wastes are released into the air, water, or soil during manufacturing. Life-cycle assessments, which examine all environmental effects such as global warming, pollution, and water use, are not available for PV systems. Instead, studies have estimated the impact of PV systems by focusing on energy costs of manufacturing and transport, as the full environmental effects of components and disposal methods are unknown, even for commercially available solar cells.

Estimates of the environmental impact of PV systems focus on carbon dioxide equivalents per kilowatt-hour or energy pay-back time (EPBT). EPBT measures how long a PV system must operate to generate the same amount of energy used in its production. Some studies include transport energy costs in the EPBT calculation. EPBT has also been defined as "the time needed to compensate for the total energy required during the life cycle of a PV system," including installation costs. The lower the EPBT, the lower the environmental cost of solar power. EPBT depends on the location of the PV system (e.g., sunlight availability and grid efficiency) and the type of system used.

A 2015 review of EPBT estimates for first and second-generation PV systems found that variations in embedded energy were greater than variations in cell efficiency. This suggests that reducing embedded energy is key to lowering EPBT.

The most important component of solar panels, which uses much of the energy and causes most greenhouse gas emissions, is the refining of polysilicon. The percentage of EPBT attributed to silicon depends on the system type. A fully self-sufficient system requires additional components, such as power inverters and storage, which increase manufacturing energy costs. However, in a simple rooftop system, about 90% of the energy cost comes from silicon, with the rest from inverters and module frames.

EPBT is closely related to concepts like net energy gain (NEG) and energy returned on energy invested (EROI). These metrics compare the energy used to produce an energy source with the energy gained from it. NEG and EROI also consider the operating lifetime of a PV system, typically assumed to be 25 to 30 years. EPBT can be calculated using these metrics.

Crystalline silicon PV systems, which are most commonly used, have high EPBT because silicon is produced by reducing high-grade quartz sand in electric furnaces at temperatures over 1000°C. This process is very energy-intensive, requiring about 11 kilowatt-hours (kWh) of energy per kilogram of silicon produced. The energy cost per unit of silicon is relatively fixed, meaning the process is unlikely to become more efficient in the future.

However, EPBT has decreased over time as crystalline silicon cells have become more efficient and wafer thickness has been reduced. Over the past decade, the amount of silicon used per watt-peak has dropped from 16 grams to 6 grams. The thickness of c-Si wafers has also decreased from 300 microns to about 160–190 microns. Improvements in sawing techniques have reduced kerf loss and made it easier to recycle silicon sawdust.

Crystalline silicon modules are the most studied PV type in life-cycle assessments (LCA) because they are the most widely used. Monocrystalline silicon (mono-Si) systems have an average efficiency of 14.0%. Their structure includes a front electrode, anti-reflective film, n-layer, p-layer, and back electrode. The sun hits the front electrode. EPBT ranges from 1.7 to 2.7 years. The cradle-to-gate CO2 equivalents per kilowatt-hour range from 37.3 to 72.2 grams when installed in Southern Europe.

Multi-crystalline silicon (multi-Si) systems are simpler and cheaper to produce than mono-Si systems but are less efficient, with an average efficiency of 13.2%. EPBT ranges from 1.5 to 2.6 years. The cradle-to-gate CO2 equivalents per kilowatt-hour range from 28.5 to 69 grams when installed in Southern Europe.

In 2020, it was estimated that a rooftop PV system in Ottawa, Canada, would take 1.28 years to produce the same amount of energy used to manufacture its silicon components (excluding silver, glass, mounts, and other parts). In Catania, Italy, the payback time was 0.97

Economics

Over the years, there have been major changes in the costs, industry structure, and market prices of solar photovoltaics technology. Understanding these changes is difficult because of the fast pace of cost and price changes, the complexity of the solar supply chain, which includes many manufacturing steps, and the costs of other system parts and installation. Differences in how solar systems are sold and regional market conditions also make it hard to track these changes. Additional challenges come from different government policies that support solar energy in various countries.

Renewable energy technologies, such as wind and solar power, have generally become less expensive since they were first developed. In many parts of the world, building renewable energy systems is now cheaper than building fossil fuel power plants.

Managing electricity bills and making energy investments depends on each customer’s unique situation, such as their energy needs, the type of electricity pricing they pay, and how they use energy. Electricity pricing can include daily fees, charges based on how much energy is used, and fees for using the most energy during peak times. Solar power can help reduce energy costs when electricity prices are high and rising, like in Australia and Germany. However, in places where customers are charged for using the most energy during late afternoon or early evening hours, solar power may not be as helpful. Energy investment decisions are usually based on carefully comparing different options, such as improving efficiency, using energy on-site, and storing energy.

In 1977, the price of crystalline silicon solar cells was $76.67 per watt. In the early 2000s, wholesale prices for solar modules stayed around $3.50 to $4.00 per watt because of high demand in Germany and Spain, supported by government subsidies and a shortage of materials. After subsidies in Spain ended in 2008, demand dropped, and prices fell to $2.00 per watt. Despite a 50% drop in income, manufacturers stayed profitable by improving technology and reducing costs. By late 2011, prices for solar modules fell below $1.00 per watt, surprising many in the industry and leading to the bankruptcy of some companies. The $1.00 per watt price is often seen as a key point where solar power becomes as cheap as regular electricity, but experts say this price is unlikely to last. Further price drops may happen due to better technology, improved manufacturing, and changes in the industry. In 2011, the average retail price of solar cells dropped from $3.50 per watt to $2.43 per watt. By 2013, wholesale prices had fallen to $0.74 per watt. This trend is similar to "Moore’s Law" for computers, known as "Swanson’s Law," which suggests solar prices drop 20% each time industry production doubles. Between 1980 and 2010, prices dropped about 25% as production increased. While module prices have fallen quickly, inverter prices have dropped much more slowly. In 2019, inverters cost over 61% of the total cost per kilowatt, compared to about 25% in the early 2000s.

The prices mentioned above are for solar modules alone. When installation costs are included, prices are higher. In the United States, the cost of installing solar panels on homes dropped from $9.00 per watt in 2006 to $5.46 per watt in 2011. Including costs for large industrial installations, the average national price was $3.45 per watt. This is higher than in Germany, where home installations averaged $2.24 per watt. These differences are likely due to stricter regulations and the lack of a national solar policy in the U.S.

By 2012, Chinese manufacturers could produce the cheapest solar modules for about $0.50 per watt. In some markets, distributors buy modules at factory prices and sell them at higher prices based on what the market will pay. In California, solar power reached grid parity in 2011, meaning it became as cheap as regular electricity. By 2014, 19 markets worldwide had reached grid parity.

By 2024, increased production in China caused solar module prices to drop to as low as $0.11 per watt, a more than 90% decrease from 2011 prices.

The levelised cost of electricity (LCOE) is the average cost of electricity over the lifetime of a project. It is considered a better way to compare the cost-effectiveness of energy sources than the price per watt. LCOE varies depending on location. It represents the minimum price customers must pay for a power company to break even on a new power plant. Grid parity is reached when LCOE matches local electricity prices, though calculations are not always directly comparable. Large industrial solar installations in California reached grid parity in 2011, but rooftop systems were still far from achieving this. Many LCOE calculations are not accurate and depend on many assumptions. Future drops in module prices could further lower LCOE for solar energy.

Solar power is limited by the fact that the sun only shines during the day, so companies must account for the extra costs of providing stable energy or storing it to keep the grid stable. These costs are not included in LCOE calculations, nor are special subsidies that may make solar energy more attractive. The unpredictability of solar and wind power can cause problems for the stability of the entire energy grid.

As of 2017, solar farms in the United States could sell electricity for less than $0.05 per kilowatt-hour, and the lowest prices in some Persian Gulf countries were about $0.03 per kilowatt-hour. The U.S. Department of Energy aims to reduce the LCOE for solar energy to $0.03 per kilowatt-hour for utility companies.

Financial support, such as feed-in tariffs (FITs), has been used in some countries to help people install and operate solar energy systems. These subsidies were important for the early growth of solar power. Germany and Spain were key countries offering such support, and their policies helped increase demand for solar energy.

Some U.S. solar manufacturing companies have said that the drop in solar module prices is partly due to government support in China and the sale

Growth

Solar photovoltaics (PV) had the most research among seven types of sustainable energy studied in a global analysis of scientific work. The number of research papers published each year increased from about 9,000 in 2011 to 14,447 in 2019.

At the same time, the use of solar PV is growing quickly. By April 2022, the total solar PV power installed worldwide reached one terawatt. In one year, this power produces more than 500 terawatt-hours of electricity, which meets about 2% of the world’s electricity needs. Over 100 countries, including Brazil and India, use solar PV. China has the largest solar PV capacity, followed by the United States and Japan. Germany, once the largest producer, has seen slower growth in recent years.

In 2019, Honduras generated the highest share of its energy from solar power, at 14.8%. Vietnam had the largest solar PV capacity in Southeast Asia, about 4.5 gigawatts. Vietnam installs about 90 watts of solar power per person each year, placing it among the world’s top countries for solar growth. Government policies, such as financial incentives and tax breaks, helped Vietnam’s solar expansion. These policies aimed to improve energy independence and address public concerns about environmental quality.

A major challenge is the limited ability of electrical grids to carry large amounts of power.

China has the world’s largest solar power capacity, with 1,048 gigawatts installed in 2024. This is much larger than the European Union’s 339 gigawatts. Other countries with large solar power capacities include the United States, Japan, and Germany.

In 2017, experts predicted that global solar PV capacity could reach between 3,000 and 10,000 gigawatts by 2030. Greenpeace estimated in 2010 that 1,845 gigawatts of solar power worldwide could generate about 2,646 terawatt-hours of electricity each year by 2030. By 2050, solar power could supply over 20% of the world’s electricity.

Applications

Solar panels, also called photovoltaics, have many uses in different areas of technology. They help power irrigation systems in farming, provide electricity for refrigeration in remote medical facilities, and generate power in homes, public buildings, and large power plants. These systems use electrical devices called PV modules.

A photovoltaic system, or solar PV system, is a setup that turns sunlight into usable electricity. It includes solar panels to capture sunlight and convert it into electricity, a solar inverter to change the electricity from direct current (DC) to alternating current (AC), and parts like mounting hardware and wiring. These systems can be small, such as those on rooftops or built into structures, with power outputs from a few to tens of kilowatts. Larger systems, called utility-scale power stations, can produce hundreds of megawatts of electricity. Most solar systems today are connected to the power grid, while systems that work independently make up a small part of the market.

Photosensors are devices that detect light or other types of electromagnetic radiation. A photodetector uses a special part called a p–n junction to turn light particles, called photons, into electrical current. When photons are absorbed, they create pairs of electrons and holes in a specific area of the device. Examples of photodetectors include photodiodes and phototransistors. Solar cells use some of the light energy they absorb to produce electricity.

Experimental technology

Crystalline silicon photovoltaics are one type of solar cell. Even though they are the most common type of solar cell made today, there are other new and promising technologies that could help meet future energy needs. In 2018, crystalline silicon was used to make several types of solar panels, including monocrystalline, multicrystalline, mono PERC, and bifacial.

Thin-film photovoltaics are made by placing thin layers of semiconducting materials, such as perovskite, onto a surface like glass or stainless steel in a vacuum. These layers can be made from materials like cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), or amorphous silicon (a-Si). After the layers are placed, they are separated and connected using electrical circuits created by lasers. Perovskite solar cells are very efficient at converting sunlight into electricity and have good properties for solar energy use. However, making them larger for real-world use is still being studied. Thin-film solar cells may become more popular in the future because they use less material and cost less to make than silicon-based cells. In 2019, university labs at Oxford, Stanford, and other places reported perovskite solar cells with efficiencies of 20-25%.

Copper indium gallium selenide (CIGS) is a type of thin-film solar cell based on copper indium diselenide (CIS), a type of semiconductor. CIGS and CIS are often used together in research. The structure of a CIGS solar cell includes soda lime glass as the base, a layer of molybdenum (Mo) as the back contact, CIGS or CIS as the layer that absorbs light, a buffer layer made of cadmium sulfide (CdS) or zinc (Zn), and a front contact made of zinc oxide with aluminum (ZnO:Al). CIGS cells are about 1/100 the thickness of traditional silicon solar cells. Materials needed to make CIGS cells are available and less expensive per watt of electricity produced. CIGS solar cells resist damage over time and remain stable in real-world conditions.

The global warming impact of CIGS solar cells ranges from 20.5 to 58.8 grams of CO₂-equivalent per kilowatt-hour of electricity generated, depending on sunlight and efficiency. The time it takes for the energy used to make CIGS cells to be recovered (EPBT) ranges from 0.2 to 1.4 years, with an average of 1.393 years. Toxicity is a concern in CIGS cells because they contain cadmium and gallium. CIS cells do not use heavy metals.

A perovskite solar cell (PSC) is a type of solar cell that uses a perovskite compound, often a mix of organic and inorganic materials like lead or tin halides, as the layer that captures light. Perovskite materials are inexpensive and easy to make. Solar cell efficiencies using perovskite materials have increased from 3.8% in 2009 to 27% in 2025 for single-junction cells, and up to 34.85% in silicon-based tandem cells. Perovskite solar cells have improved the fastest since 2016. They are becoming commercially attractive because of their high efficiency and low cost. However, challenges include their stability over time, sensitivity to moisture, and the use of lead, which is toxic. Managing lead in PSCs is important because it can harm health, including causing neurological problems. Because PSCs are still new, lead toxicity remains a major challenge for widespread use.

Dye-sensitized solar cells (DSCs) are a type of thin-film solar cell. They work well in low light conditions compared to other solar technologies. They use a dye that absorbs light between two materials that carry electrical charges. The dye covers titanium dioxide (TiO₂) nanoparticles arranged in a network. TiO₂ acts as a conductor in a semiconductor and helps transfer electricity when light is absorbed. For TiO₂ DSCs, preparing samples at high temperatures improves their structure. Another type of DSC uses copper with a redox shuttle called TMBY. DSCs perform well under artificial and indoor light, reaching up to 29.7% efficiency at 2,000 lux.

However, DSCs have issues with their liquid electrolyte. The solvent used is harmful and can leak through plastics. The liquid is unstable in extreme temperatures, causing the cell to fail in cold or hot conditions. DSCs are not ideal for large-scale use because of their lower efficiency. Benefits include working in various light levels, low manufacturing costs, and longer lifetimes compared to other thin-film cells.

Other future solar technologies include organic, dye-sensitized, and quantum-dot photovoltaics. Organic photovoltaics (OPVs) are thin-film cells with efficiencies around 12%, lower than silicon-based cells (12–21%). OPVs require very pure materials and must be sealed, which increases costs and limits large-scale production. Dye-sensitized cells have similar efficiency to OPVs but are easier to make. However, their liquid electrolyte is toxic and can leak. Quantum-dot solar cells are made using a solution process, which could allow scaling, but their current efficiency is around 12%.

Organic and polymer photovoltaics (OPVs) are a newer area of research. Traditional OPV cells have layers like a semi-transparent electrode, electron blocking layer, tunnel junction, and electrode. OPVs use carbon instead of silver for electrodes, reducing costs and making them more eco-friendly. OPVs are flexible, lightweight, and suitable for mass production using roll-to-roll printing. They use common elements and require low energy during manufacturing. Current efficiencies range from 1–6.5%, but research suggests higher efficiencies may be possible.

Many OPV designs use different materials for each layer. OPVs compare well to other solar technologies in terms of energy payback time, even though they currently have shorter lifespans. A 2013 study found that OPVs with 2% efficiency had energy payback times ranging from 0.29 to 0.52 years for 1 square meter of solar cells. The average CO₂-equivalent per kilowatt-hour for OPVs is 54.922 grams.

Thermophotovoltaic (TPV) energy conversion turns heat directly into electricity using photons. A basic TPV system includes a hot object that emits light and a photovoltaic cell similar to a solar cell but tuned to specific wavelengths.

Advantages

• Pollution and Energy in Production

The amount of sunlight reaching Earth’s surface is very large—about 122 quadrillion watts. This is nearly 10,000 times more than the 13 trillion watts of energy humans used on average in 2005. Because of this, scientists believe solar energy may soon become the main energy source worldwide. Solar energy also has the highest power density (170 watts per square meter on average) among all renewable energy types.

Solar energy does not create pollution during use. This helps reduce pollution when solar power replaces other energy sources, like coal. For example, a study by MIT found that about 52,000 people in the U.S. die earlier each year from pollution caused by coal power plants. Nearly all of these deaths could be avoided if solar panels replaced coal. Waste and emissions from making solar panels can be managed with current pollution control methods. Recycling technologies for solar panels are being developed, and rules are being created to encourage recycling.

Solar panels are usually guaranteed to last about 25 years, though some parts may need replacing sooner. After installation, solar power systems require little maintenance, making their operating costs very low compared to other energy sources. One way to improve solar panels’ environmental impact is by designing them to be easily taken apart for recycling.

Using solar panels on rooftops can reduce energy loss during transport because the electricity is used where it is produced.

• Solar Cell Research Investment

Compared to fossil fuels and nuclear energy, very little money has been spent on research to improve solar cells. This means there is still much room for improvement. However, some experimental solar cells already convert over 40% of sunlight into electricity, and their efficiency is increasing quickly. At the same time, the cost to produce these cells is decreasing rapidly.

• Housing Subsidies

In some U.S. states, homeowners may lose money if they install solar panels on their homes and later sell the house. This happens because the new buyer might not value the solar system as much as the seller. To solve this, the city of Berkeley created a program called PACE (Property Assessed Clean Energy). This program adds a special tax to the home’s value, which is passed to the new owner. The tax helps pay for the solar panels. Now, 30 U.S. states use this method to support solar energy.

Disadvantages

• Impact on electricity network

Rooftop solar panels that generate electricity for use at the location where they are installed can send power back to the grid when they produce more electricity than is needed locally. This two-way flow of electricity allows for net metering, where excess electricity is shared with the grid. However, electricity networks were not built to handle power moving in both directions, which can cause technical problems. For example, too much electricity flowing back into the grid from solar panels can cause voltage levels to rise too high. Solutions to manage this issue include adjusting the power factor of solar inverters, installing new voltage and energy control devices, upgrading power lines, and managing electricity use by consumers. These solutions often have limits and can be expensive.

When solar panels produce a lot of electricity during the middle of the day, the overall need for electricity from the grid decreases. However, as the sun sets, the need for electricity from the grid increases quickly, requiring power plants to rapidly increase their electricity production. This creates a specific pattern of electricity demand known as the duck curve.