How Does Solar Cell Produce Voltage?

Solar cells have been around for over half a century, but only in the past decade have they really become mainstream. Their basic operating principle was discovered in the 19th century, but it took the space race of the 1950s and 60s for the technology to be researched and developed into practical applications.

In simple terms, solar cells convert sunlight directly into electricity. They do this through the photovoltaic effect, which causes the generation of voltage in a material upon exposure to light. Solar cells are built around special materials that exhibit this effect.

Today, solar cells have become a critical source of renewable energy around the world. They are used in homes, businesses, utilities, satellites, and more. Their modular nature allows them to be scaled for systems of all sizes. And improvements in efficiency and cost have enabled their rapid adoption. The future is bright for solar cell technology to supply an increasing share of the world’s energy.

How Solar Cells Work

Solar cells are made of semiconductor materials such as silicon that are specially treated to form an electric field. When sunlight hits the solar cell, the energy from the photons of light excite the electrons in the semiconductor materials. This excitation gives the electrons enough energy to break free of their atoms and flow through the material, creating electricity. The special treatment of the semiconductor materials creates an imbalance where electrons want to move in one direction, from the p-type side to the n-type side. This movement of electrons from the p-type side to the n-type side generates an electric current, or electricity.

Photoelectric Effect

The photoelectric effect is a key part of how solar cells produce electricity. When photons (particles of light) strike the semiconductor material in a solar cell, they transfer energy to the electrons in the atoms of the semiconductor. If the photon transfers enough energy, it can excite an electron enough to break free of its atomic bond and flow freely.

The energy required to free an electron is called the “band gap energy”. It varies for different semiconductor materials. Photons that have more energy than the band gap energy knock electrons loose. Photons with less energy than the band gap pass through the material without interacting. This photoelectric effect depends on the wavelength and intensity of the light as well as the material’s band gap energy.

PN Junction

Solar cells contain a PN junction formed between two layers of semiconductor material, one layer with excess holes or positive charge (P-type) and one layer with excess electrons or negative charge (N-type). Silicon, the most common material used in solar cells, is naturally P-type. To create the N-type layer, impurities like phosphorus are added in a process called doping. This creates an electric field at the PN junction.

When sunlight hits the solar cell, the energy frees electrons in both the P-type and N-type layers. The built-in electric field across the PN junction causes these electrons to flow from the N-type side to the P-type side, generating an electrical current. The PN junction is key to converting light energy into electrical energy.

Electrical Contacts

Solar cells have electrical contacts on the front and back to collect and transport current. The front contact is designed to let in light but also conduct electricity. It is made very thin and from transparent conductive materials like indium tin oxide. This allows sunlight to pass through to the semiconductor while also conducting the electric current produced.

The back contact does not need to be transparent but has to be highly conductive. It is typically made from metals like aluminum or silver in the form of a full sheet or gridlines. The back contact collects the current transported to the back surface of the cell and connects it to the external circuit.

Voltage Generation

Voltage generation is the key principle that allows solar cells to convert sunlight into electricity. When light shines on a solar cell, the energy from the photons in the light excites electrons in the solar cell’s semiconductor material. This generates electron-hole pairs as electrons gain enough energy to break free from their atomic bonds.

The P-N junction within the solar cell separates these electron-hole pairs before they can recombine. The electrons accumulate on the N-type side, while the holes accumulate on the P-type side. This separation of charge across the P-N junction creates a voltage potential, similar to a battery. More intense sunlight generates more electron-hole pairs, resulting in a higher voltage.

The voltage generated is proportional to light intensity. Sunlight strikes the solar cell and transfers energy to excite electrons, generating more electron-hole pairs and current to flow across the P-N junction. More intense light results in more electrons excited to a higher energy state, which increases the voltage produced.

By connecting the front and back electrical contacts of the solar cell to an external load, we can harness the photocurrent and photovoltage produced by the solar cell to do useful work. The voltage provides the driving force to push current through the external load and power our devices and applications.

Connecting Solar Cells

While a single solar cell only produces a small amount of voltage and current, individual cells are connected in series and parallel configurations to produce higher voltages, currents, and power outputs. Connecting cells in series increases the voltage, while connecting in parallel increases the current.

When solar cells are connected in series, the positive terminal of one cell is connected to the negative terminal of the next cell, creating a string of cells. The voltages of each cell are additive, so the more cells connected, the higher the voltage. However, the current remains the same. For example, connecting 10 solar cells with 0.5 V in series produces 5 V.

Connecting cells in parallel means joining the positive terminals of multiple cells together and joining the negative terminals together. This increases the current while the voltage remains the same as an individual cell. For example, connecting two 0.5 V solar cells in parallel produces 0.5 V but doubles the current output.

By combining series and parallel connections, solar panel manufacturers can produce solar modules with specific voltages, currents, and power outputs tailored to different applications and power needs. Understanding these solar cell connections allows photovoltaic systems to be designed and sized appropriately.

Future Advancements

As solar technology continues to advance, researchers are focused on improving efficiency while lowering costs. Some key areas of innovation include:

Improving efficiency: Companies are exploring new materials and techniques to absorb more sunlight and convert it to electricity more effectively. For example, stacking multiple solar cell layers together can increase efficiency by absorbing different wavelengths of light.

New materials like perovskites: Perovskites are emerging as a promising material for solar cells. They are inexpensive to produce and can reach high efficiencies. However, perovskites are less stable than silicon, so more research is needed.

Novel manufacturing techniques: New methods like inkjet printing solar cell layers allow for precise, low-cost production. Flexible thin-film solar materials also enable lightweight, bendable solar panels ideal for new applications.

With continued research and development, solar power’s conversion efficiency will increase while costs decrease. More efficient, affordable solar technology will accelerate the global transition to clean renewable energy.

Applications

Solar cells have a wide variety of practical applications and uses, powering everything from small handheld gadgets to massive solar farms and satellites in space:

Solar farms and panels for homes and businesses

One of the most common uses of solar cells today is in solar panels mounted on rooftops or assembled into large solar farms to generate electricity. Homes and businesses can install solar panel systems to reduce their reliance on the main power grid and take advantage of clean, renewable solar energy.

Spacecraft and satellites

Solar cells are ideal for powering satellites and spacecraft, since solar energy is abundant in space. They provide a lightweight, reliable power source that can last for decades as satellites orbit the Earth.

Small electronics and devices

Solar cells can be used to provide power for small electronics like calculators, watches, and charging mats. Their decreasing cost makes solar appealing for powering a wide array of portable consumer gadgets and devices.

Conclusion

In summary, solar cells produce voltage through the photovoltaic effect. When sunlight strikes the semiconductor material, photons excite electrons into a higher state of energy which enables them to move freely. The p-n junction separates the free electrons and holes, creating an electric field that causes charge carriers to flow in one direction producing voltage and current. Connecting solar cells in modules and arrays produces higher voltages that can be used to power devices or feed into the electrical grid.

Solar photovoltaic energy provides a clean, renewable source of electricity that does not produce greenhouse gas emissions or pollution. As solar cell technology continues to advance and costs decrease, solar power will play an increasingly important role in transitioning the world’s energy systems to sustainability. Widespread adoption of solar photovoltaics is critical for reducing our reliance on fossil fuels and mitigating climate change.