The photovoltaic effect, discovered by Frenchman Edmond Becquerel in 1839, is a physical phenomenon that converts light energy, particularly solar radiation, into electrical energy. This principle lies at the heart of the photovoltaic cells that make up solar panels, enabling electricity to be generated fromsolar energy, the renewable energy with the greatest potential today.
Today, photovoltaic panels are widely deployed in Europe and around the world, contributing to the energy transition towards clean, renewable energy sources. The Vanguard satellite, launched in 1958, was the first to use solar cells to power its electronic systems. Since then, photovoltaic technology has evolved, offering a sustainable and environmentally-friendly alternative for generating electricity from solar energy.
The principle of the photovoltaic effect
The semiconductor material most commonly used in photovoltaic cells is silicon. When the photons that make up sunlight strike the surface of a photovoltaic cell, they transfer their energy to the material’s electrons. The electrons in the valence layer gain enough energy to switch to the conduction band, creating electron-hole pairs.
In a photovoltaic cell, a PN junction is formed by bringing together an N-type conductor and a P-type conductor. This creates an electric field at the junction, separating electrons and holes. The flow of electrons generates an electric current that can be used to power electrical appliances or fed into the grid.
The efficiency of photovoltaic panels depends on the available light power, the efficiency of the photovoltaic cells and environmental factors such as solar radiation intensity and temperature. By optimizing these parameters, the electrical power produced by solar panels can be maximized.
The photovoltaic effect is a phenomenon that occurs when a semiconductor material absorbs photons of light and generates an electric current by converting light energy into electrical energy. Here’s a detailed description of the photovoltaic effect from an engineering perspective.
Photovoltaic cells
Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators. Crystalline silicon is the most commonly used semiconductor material for photovoltaic cells, but other materials, such as cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and perovskites, are also used or being researched.
Semiconductors have an electronic structure characterized by energy bands. The valence band is the highest energy band, completely filled with electrons, while the conduction band is the lowest energy band, partially filled or empty. The energy gap between these two bands is called the band gap.
Electricity generation process
When a photon of light with an energy equal to or greater than the width of the band gap strikes the semiconductor material, it can transfer its energy to an electron in the valence band. The electron then gains enough energy to switch to the conduction band, leaving behind a hole in the valence band. This electron-hole pair is called an exciton pair.
A photovoltaic cell generally consists of a PN junction, formed by bringing together a P-type semiconductor (rich in holes) and an N-type semiconductor (rich in electrons). The PN junction creates an internal electric field that separates the electrons and holes generated by photon absorption.
Under the influence of this electric field, electrons are attracted to the N-type region, while holes are attracted to the P-type region. This charge separation creates an electrical voltage across the photovoltaic cell. When an external circuit is connected, an electric current flows, producing electrical energy.
Optimizing the photovoltaic cell
The efficiency of a photovoltaic cell depends on several factors, such as the absorption capacity of the semiconductor material, the width of the band gap, the quality of the PN junction, the reflection and transmission of light at the cell surface, and resistive losses in the material and electrical contacts. The performance of photovoltaic cells generally decreases with increasing temperature. Engineers are therefore looking for solutions to manage heat dissipation, for example by using materials with low temperature coefficients, or by integrating passive or active cooling systems.
Engineers work on the design and optimization of photovoltaic cells to improve their efficiency and reliability. This involves choices of materials, doping techniques, cell structures (monocrystalline, multicrystalline, thin or heterojunction) and anti-reflective coatings. Tandem cells are an approach aimed at improving the efficiency of photovoltaic cells by stacking several cells with different bandgaps. This makes it possible to absorb a wider spectrum of light and convert more light energy into electrical energy. Photovoltaic cells are usually grouped together in modules and solar panels to form photovoltaic systems of different sizes and capacities.
The history of the photovoltaic effect
The history of the photovoltaic effect dates back to the 19th century and extends to the present day, with key scientific and technological advances shaping its development. Here’s a chronological overview of the milestones in the history of the photovoltaic effect:
- 1839: Discovery of the photovoltaic effect – French physicist Alexandre Edmond Becquerel discovers the photovoltaic effect by observing that light incident on an electrode immersed in an electrolyte causes the production of an electric current.
- 1873: Discovery of the photoconductive properties of selenium – Willoughby Smith, a British engineer, discovers that the resistivity of selenium decreases when exposed to light.
- 1883: First selenium solar cell – Charles Fritts, an American inventor, makes the first solar cell by coating a selenium plate with a thin layer of gold. The cell had a low efficiency of around 1%.
- 1905: Explanation of the photoelectric effect – Albert Einstein publishes a theory on the photoelectric effect, explaining how photons of light can release electrons from a material. Einstein received the Nobel Prize in Physics in 1921 for his work on the photoelectric effect.
- 1941: Invention of the transistor – John Bardeen, Walter Brattain and William Shockley invent the transistor at Bell Laboratories. The transistor, based on semiconductor materials, was essential for the subsequent development of photovoltaic cells.
- 1954: Development of the first silicon solar cell – Bell Laboratories researchers Daryl Chapin, Calvin Fuller and Gerald Pearson develop the first silicon solar cell. This cell has an efficiency of 6%, much higher than that of selenium cells.
- 1958: First satellite powered by solar cells – The Vanguard 1 satellite is launched with solar cells to power its electronic systems. This marks the first use of photovoltaic solar energy in space.
- 1970-1980: Growth of the solar industry and improved efficiency – The energy crises of the 1970s stimulate research and development in renewable energies, including photovoltaic solar energy. Solar cell efficiency increases and production costs gradually fall.
- 1990-present: Expansion and widespread adoption of solar energy (continued ) – Researchers continue to work on new materials and innovative manufacturing techniques to improve photovoltaic cell efficiency and reduce production costs. Materials such as perovskites and quantum dots are being intensively researched to create next-generation solar cells.