Photovoltaics: from sunlight to electricity

Photovoltaic technologies – commonly known as solar panels – generate electricity using the energy coming from the sun. Research and adoption of solar panels are driven by sustainability, as photovoltaics is one of the lowest carbon footprint energy technologies.

In the midst of a climate and energy crisis, many governments are trying to integrate more renewable sources into their energy production. Solar photovoltaics (PV) is the?third largest renewable electricity supplier?behind hydropower and onshore wind.?Solar electricity grew by a record 23% in 2020 to 821 TWh,?driven by government policies in China, Vietnam, and the US(1).?

Discovered in 1839 by French physicist Edmond Becquerel, the photovoltaic effect found its first practical use in solar cells embedded on satellites in the 1960s, then on rooftops in the late 1980s. The cost of photovoltaic systems has?dropped by about 80% over the past decade,?making it the cheapest form of electricity generation in many parts of the world(2).?

Overview: how to harness solar energy(3)?

One way to exploit solar energy is to use photovoltaic devices that convert sunlight into electricity.?A single photovoltaic device is called a cell?and produces 1 to 2 watts (W) of power; by comparison, a phone charger uses 10 W, and a washing machine uses up to 1,000 W. To increase the power of PV cells, they are connected to form larger units called?modules or panels. Modules can be used individually or connected to form an?array: a typical residential rooftop array has about 30 modules. The whole is connected to the electrical grid as part of a?photovoltaic system. Because of their modular structure, PV systems can be adapted to meet any energy requirement, large or small.?

Solar cells are only a few microns thick, the equivalent o f a human hair. To withstand outdoor conditions, they are wedged between layers of protective materials such as glass and plastic.?

Solar cells contain a?semiconductor?that?creates an electrical current only when energy is provided by sunlight.?The primary used semiconductor is?silicon, which is the second?most abundant element on Earth after oxygen – and the most common semiconductor used in computer chips. Silicon is naturally found in sand but must undergo chemical refining to reach the level of purity required for solar cells. Once extracted from the semiconductor, the current flows through metal contacts into an inverter that adapts the current to supply the associated electrical system.

Exhibit 1 - How to make electricity from solar cells

Solar cells are made up of two layers of semiconductor, a material that can conduct electricity if it receives energy.?Each layer is “doped”, which means that?it contains additional atoms to increase its conductivity. One layer has a surplus of electrons (negative charges): it is an?n-type semiconductor. The other has a deficit of electrons, leaving behind vacant spaces, called holes, that are equivalent to a positive charge. This side is called a?p-type semiconductor.?When sunlight, made of particles called photons, hits the cell, some photons are absorbed by the semiconductor. If the absorbed photon brings the right amount of energy, it frees an electron and leaves behind a hole.?Electrons flow to the n-side of the semiconductor, while holes flow to the p-side.?The difference of charges between the two sides of the cell generates a voltage, just like a battery.?Electrons and holes are directed to the metal contacts on each side of the cell and flow to the external circuit as direct current. (Source: let's talk science)

Electrical conduction in a semiconductor(4)

An atom consists of a nucleus made of protons (positively charged particles) and neutrons (uncharged particles), surrounded by?electrons (negatively charged particles). The number of protons and electrons is equal, so that the atom is electrically neutral overall. In order to come together in a solid structure, atoms share electrons, forming what is called a?covalent bond.?

However,?this bond is only maintained at absolute zero; at higher temperatures (especially those at which solar cells operate), the electrons can gain enough energy to leave the bond and move through the semiconductor, creating electrical conduction. At room temperature, a semiconductor has enough free electrons to conduct current; near absolute zero, it acts as an insulator.?

When an electron gains enough energy to participate in conduction (i.e., the electron is “free”), it is in a high-energy state; similarly, when the electron is bound and cannot participate in conduction, it is in a low-energy state. The electron cannot have energy values in between those two levels:?it is either bound or free. The minimum energy required to free an electron is called the?band gap?of a semiconductor.?

Once an electron is free and moves along the semiconductor, it leaves behind an empty space called a?hole. An electron from a neighboring atom can move into this hole, leaving behind another hole, and so forth. The continuous movement of holes is equivalent to the movement of a positive charge through the structure. Both the electron and the hole participate in electrical conduction.

What is the photovoltaic effect?(4,5)

The photovoltaic effect is the process by which solar energy is transformed into electrical energy: the word comes from the Greek “photo”, meaning light, and “Volt”, the unit of electrical potential energy. - Sunlight is composed of photons, particles of solar energy. - Photons have a different amount of energy depending on their?wavelength?(i.e., their position in the solar spectrum). When they hit the solar cell, some are reflected, and others are absorbed. If the energy of an absorbed photon is less than the band gap, the electron cannot be released and the photon passes through the semiconductor; if the energy is well above the band gap, an electron is released but the excess energy is converted to heat and lost. Only photons whose energy matches the band gap can free electrons without producing excess heat.?

The movement of freed electrons is random but, in order to generate electricity, electrons must flow in the same direction. This is achieved by using?two layers of silicon?and a technique called?doping, which involves disrupting the balance between holes and electrons by adding atoms in the semiconductor.?

The semiconductor layer that is exposed to the sun is doped with phosphorus atoms that have one more electron than silicon. This layer has a surplus of electrons (therefore a surplus of negative charges) and becomes the?negative terminal (n). The other side is doped with boron atoms that have one electron less. This layer has a surplus of holes (thus a surplus of positive charges) and becomes the?positive terminal (p).?

When released, an electron is swept to the n-side while the hole drifts to the p-side.?Electrons exit the semiconductor in the form of current, while the charge imbalance between the front and back of the cell creates a voltage potential like the positive and negative terminals of a battery.

How to build solar panels(3)

To be used in solar panels, raw silicon (usually quartz sand) must be processed into a?high-purity silicon product called polysilicon. Solar-grade polysilicon must be 99.999999% pure.?The primary method of purification is to pass a hydrogen-chloride-silicon gas compound over a heated silicon filament. The silicon atoms break away from the gas and deposit on the filament, which gradually grows into a large u-shaped polysilicon rod.

Daqo New Energy (ticker: DQ US) is a Chinese company specializing in the manufacture of purified silicon intended for photovoltaic panels and electronic circuits. Using an optimized version of this purification process, the company produces large blocks of solar-grade polysilicon. It has a total polysilicon nameplate capacity of 105,000 metric tons (with a three-year plan to expand it to 270,000 metric tons) and is one of the world’s lowest cost producers of high-purity polysilicon.

The polysilicon rods are then heated until they become liquid, then gradually cooled until they solidify, forming a large ingot. Silicon ingots are then cut into thin wafers, about 200 microns thick, using diamond wire saws. The wafers are polished to eliminate saw damage and increase the amount of light that penetrates the surface. An anti-reflective coating is added on top of the cell to minimize photon loss.?

To build panels, cells are assembled using copper wire. The sheet of interconnected cells is placed between two sheets of adhesive film, and the whole is placed between two sheets of glass. The whole is laminated in an oven to sea l the module and make it waterproof, then mounted in a frame and connected to electrical cables.

Exhibit 2 - Combining solar and hydrogen: the future of green energy(2)?

Storage of solar energy is essential to using it efficiently. Storage technologies such as lithium-ion batteries or pumped storage hydropower – where water is pumped to a higher reservoir using excess solar electricity and later released to generate electricity – are common.?The use of hydrogen as a source of electricity has been researched in recent years. It is particularly sought-after for long-distance freight transportation in the aviation and marine sectors, as well as for industrial applications in hydrogen-based steelmaking. However, electricity is required to perform the electrolysis of water and extract hydrogen. This electricity currently comes from fossil fuels or nuclear energy. To make hydrogen completely green, scientists are working on its production using photovoltaic technologies.?The idea would be to either dedicate solar panels to hydrogen production, or to use hydrogen as storage for the excess electricity generated during hours of high sunlight.?Due to the expected decrease in hydrogen production costs and the already observed decrease in solar energy costs, it is estimated that?solar hydrogen will be competitive throughout Europe in 2030.?It is important to note that the peak power of the PV system should always be largely superior to the peak power of the electrolyzer, as the PV system rarely produces electricity at its peak power.?Success will therefore depend on the future availability of more sophisticated solar cells, such as multijunction cells, which have much higher power than traditional silicon cells.?(Source: National Renewable Energy Laboratory)

Panels alone are not enough: the photovoltaic system(3)

Photovoltaic modules are the part where electricity is generated; but for them to be useful, several structures must be installed. Together, they form a?photovoltaic system.?PV panels require?mounting structures. Steel structures provide a stable, durable support that withstands harsh environmental conditions (rain, wind, corrosion, etc.) for decades. Often, these structures tilt the panels at a?fixed angle determined by their location: in the northern hemisphere, panels face south at an angle equal to the local latitude to maximize sun exposure.?More advanced mounted structures have?tracking mechanisms that automatically move the panels to follow the sun during the day, providing more energy. Sophisticated tracking systems involve higher initial costs and require more maintenance. PV panels can also be directly?integrated into buildings, often on roofs.?

Inverters are the device that convert the direct current (DC) generated by the modules into alternating current (AC), which is used for local power transmission and appliances.?Finally, coupling?solar power and storage technologies?is critical because electricity is not generated when it is most needed: peak electricity consumption occurs in the afternoon and evening, when people who work during the day return home and start using electricity to heat/ cool their homes, cook, and run appliances.?This is the time when solar generation fails.

The benefits of storage are numerous: it?balances electrical loads?(i.e., avoids losing electricity when the panel produces too much, or not having enough at night or when the weather is not favorable); ensures that quick variations in electricity production do?not disrupt the flow of electricity (i.e., a household appliance does not shut down when a cloud passes); and?provides resiliency?(i.e., serves as a backup in the event of an electrical disruption). The technology most frequently coupled to photovoltaic power plants is electrochemical storage (batteries), particularly lithium-ion.

A name that we have been including for a long-time in our battery-oriented portfolios is Samsung SDI (ticker: 006400 KS), a global leader in lithium-ion energy storage systems (ESS). Initially intended for the automotive industry, these technologies were adapted to make large-scale systems built to store large amount of renewable energy, hence their crucial role in the energy transition nowadays. Spanning from the size of kWh to MWh, Samsung SDI supplies various ESS solutions – residential, utility, commercial, UPS and base transceiver station – applicable to everyday life. Samsung SDI ranked #1 in the global ESS market in 2020 with a 31% market share (usage: 6.2 GWh) , followed by LG Energy Solution (4.8 GWh)(7). In 2021 for example, LG Energy Solution and Samsung SDI were selected as battery suppliers in connection with a large-scale power and ESS project being promoted in California, actually the world’s largest solar-ESS project. When completed, the project will be able to provide electricity to more than 158,000 households in California. LG Energy Solution and Samsung SDI will supply a total of 2,445 MWh batteries.

How much energy can a PV panel convert??

The efficiency is the ratio of electrical energy produced by the cell to the amount of sunlight it receives. To measure efficiency, cells are assembled into a one-square-meter panel placed under a solar simulator that mimics ideal sunlight conditions: 1,000 W of light per cubic meter at an ambient temperature of 25°C.?The electrical power produced by the panel, or peak power, is a percentage of the incoming solar energy: if the panel produces 200 W of electrical power, it has an efficiency of 20%.?The theoretical maximum efficiency, known as the Shockley-Queisser limit, is about 33%. This limit is calculated for a conventional two-layer silicon semiconductor; with more layers, or with concentrated light (using mirrors, for example), the limit is higher.?High quality commercial panels have efficiencies of about 18-22%.

In reality, the amount of electricity produced by a cell depends of its efficiency but also of the average amount of sunshine in the area, which varies drastically by region. For example, the Paris region receives 1 megawatt-hour per square meter per year (MWh/m2/year) of sunlight, while the Sahara Desert receives 3 MWh/m2/year. A solar panel with an efficiency of 15% would generate 150 kWh/m2/year in Paris and three times this amount in the Sahara(5).

In addition, many factors contribute to reducing the efficiency of a solar panel.?

• High temperatures?reduce the voltage of solar cells: conventional rooftop solar modules can lose up to 30% of their power on hot summer days. Extreme temperature increases can also damage the cells and other materials in the module, shortening their lifespan. Since a large portion of solar energy is converted into heat, cells require proper thermal management.
• When light is reduced, so is power: a 10% overall shading can result in a 50% reduction in power output. Sometimes,?light is partially blocked?by an obstacle (for example, a utility pole that shades part of the module), and a single shaded cell limits the power output of all cells in a system.?Fouling of the panels?can also induce efficiency losses of 10%.
• The efficiency of a cell can be improved by minimizing the amount of light reflected from the cell surface.?Untreated silicon reflects more than 30% of the light. It is therefore coated with an anti-reflective solution that makes high-efficiency cells appear dark blue or black.

Finally, the?photon wavelength?is also important. Photons have a wide range of wavelength (and, therefore, energy): from ultraviolet to infrared, through the visible range. Only a fraction of these photons correspond to the band gap of the semiconductor and can initiate an electron movement(3).

Exhibit 3 - End-of-Life management of photovoltaics(6)

Photovoltaics are a clean, renewable energy source designed to last about 30 years. Their energy payback time (the time after which the energy used for manufacture equals to the energy produced under normal operating conditions) is 5 years for a multijunction cell vs. 7 years for a single junction. However, the problem of renewability occurs at end-of-life.?End-of-Life management refers to the process that occurs when solar systems, especially panels, are retired from operation.?Hundreds of millions of PV panels are in use, most of which are young – about 70% of solar energy systems have been deployed in the past five years.?PV modules are recyclable: materials such as glass sheets and aluminum frames, that surround the cells, can be recovered, and reused.

Similarly, the semiconductors that make up the cells can be extracted individually from the arrays. However, the cost of recycling is often higher than landfilling. Extracting each cell and separating all materials requires multiple processes, and the resulting materials contain many impurities. While the EU provides guidelines to ensure specific waste treatment, countries such as China, India, and the US still lack specific regulations and treat PV waste as regular non-hazardous waste, even though some panels such as thin film contain toxic materials (tellurium, cadmium).?Research is currently funded to make PV systems easier to recycle. Addressing this issue would significantly reduce the environmental impact of PV systems. (Source: Let’s talk science, SETO)

Innovations on photovoltaic technologies(3)

According to the Solar Energy Technologies Office, research is focused on three main aspects: improving reliability and durability, increasing efficiency, and lowering materials and process costs.

Multijunction solar cells?overcome the wavelength and efficiency limitations. The idea is to layer multiple semiconductor materials (as opposed to conventional cells that have only one semiconductor). Each layer is designed to have a different band gap so it can absorb photons from different parts of the solar spectrum. Multijunction cells use a high band gap top cell to absorb high-energy photons while allowing lower-energy photons to pass through. A?lower band gap semiconductor is placed underneath to absorb the lower-energy photons. Multijunction cells have demonstrated record efficiencies higher than 45%, but they are expensive and difficult to manufacture. So far, they have been dedicated to space exploration.

Thin-film solar cells?are a less expensive and easier-to-manufacture option than silicon cells; they currently account for about 10% of the market. They are built by applying a thin layer of a semiconductor material to a sheet of glass, plastic, or metal foil called a substrate, rather than building and slicing a crystal ingot. The substrate can be flexible, which allows for greater versatility. The main thin-film cell developed is the Perovskite solar cell, which has an efficiency of about 20%. However, thin-film cells currently lack the robustness to withstand outdoor conditions.

Finally,?organic photovoltaics?are lightweight solar cells made of carbon compounds that can be dissolved, making them easier to manufacture. Like plastic, they can be made transparent, which is particularly relevant to the building-integrated market to make solar windows.

Providers of residential & commercial solar/storage solutions

One of the companies we used to include in our U.S. infrastructure-oriented portfolios is Sunrun (ticker: RUN US), a provider of customized residential solar systems, as well as home batteries that serve as electricity backups. It also provides electric vehicle (EV) chargers, battery retrofits, re-powered or expanding systems, home energy management services, and other home electrification products. In 2021, Sunrun was the top U.S. residential solar provider for the fifth year in a row. Another company is Enphase (ticker: ENPH US), which develops micro-inverters, a system that uses one inverter per module, instead of using one to convert the current of the whole array. Micro-inverters optimize electricity production and avoid losing energy when only some cells are shaded. Enphase was the first company to successfully commercialize microinverters on a large scale and remains the leader in this field.

References: (1) International Energy Agency - (2) E. Vartiainen, True Cost of Solar Hydrogen, Solar RRL, vol. 6, 2022 - (3) Solar Energy Technologies Office - (4) C. B. Honsberg, S. G. Bowden, Photovoltaics Education, 2019 - (5) Planète énergies - (6) M. Monteiro Lunardi, et al., A Review of Recycling Processes for Photovoltaic Modules, in Solar Panels and Photovoltaic Materials, London, 2018 - (7) SNE Research

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