What are Solar Cells?

Introduction

Solar cells, also called photovoltaic (PV) cells, are devices that convert sunlight directly into electricity. They are made of semiconducting materials that exhibit the photovoltaic effect – meaning when sunlight hits the cell, electrons are emitted that can be captured to produce an electric current. The photovoltaic effect was first observed in 1839 by French physicist Edmund Becquerel.

The development of solar cells grew slowly until the 1950s when they were commonly used to power space satellites. Continued advances have dramatically lowered costs and increased efficiencies, making solar PV systems a viable renewable energy source for home and commercial use.

Today, solar cells are widely used for electricity generation, powering everything from small electronics to entire buildings. Their modular design allows solar panels to scale easily to meet electricity needs. Solar PV offers a clean, renewable way to reduce fossil fuel dependence and carbon emissions.

How Solar Cells Work

Solar cells work by absorbing photons from sunlight and converting that light energy into electricity through a process called the photovoltaic effect. When the photons hit the solar cell, they transfer their energy to the semiconductor material in the cell, knocking electrons loose from their atoms. This generates electron-hole pairs, known as charge carriers, that can be captured within the cell. The electrons want to reunite with the holes, but they are forced apart by the internal electric field of the solar cell, which causes the electrons to flow in one direction and the holes to flow in the opposite direction. This movement of charge carriers generates an electrical current and voltage. Metal contacts on the top and bottom of the solar cell collect the electrons and transfer them to an external circuit where they can then power electrical devices or charge batteries. So in summary, solar cells absorb sunlight, generate charge carriers, and create usable electricity through the photovoltaic effect.

Solar Cell Materials

There are a few main materials used to make solar cells:

Silicon is the most common material used. It is an abundant, non-toxic semiconductor. Silicon cells can be monocrystalline (made from a single silicon crystal) or polycrystalline (made from many silicon crystals). Monocrystalline cells are more efficient but more expensive.

Cadmium telluride (CdTe) is a thin-film photovoltaic material. CdTe cells are easier and cheaper to manufacture than silicon cells but are less efficient. They work well in hotter temperatures.

Copper indium gallium selenide (CIGS) is another thin-film PV material. CIGS cells have a high absorption coefficient, allowing them to be made into flexible and lightweight solar panels. They are relatively efficient and stable.

Solar Cell Design

The most common design of solar cells is based on the semiconductor p-n junction. When light hits the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides of the p-n junction, forming an electrical circuit, the electrons can be captured in the form of an electric current.

Solar cells have two surfaces that collect the electrons – a front contact and a back contact. The front contact is designed to be transparent so light can enter the cell. This is usually made of a transparent conductive oxide material or a grid of thin metal lines. The back contact is made of a conductive metal and has a reflective surface to bounce light back into the cell.

Efficiency

The efficiency of a solar cell refers to what percentage of the sunlight striking the cell gets converted into electricity. There are two main measures of efficiency:

Record lab efficiency: This refers to the highest efficiency ever achieved for a certain type of solar cell under laboratory conditions. Scientists are constantly working to improve solar cell materials and designs to increase efficiency. The record lab efficiency gives a sense of the maximum potential of different solar cell technologies.

Commercial efficiency: The efficiency of solar panels you can actually buy. This is always lower than record lab efficiency, because commercial products have to balance performance with manufacturability and cost. Typical commercial solar panels on the market today have efficiencies around 15-22% for crystalline silicon panels.

Cost

The cost of solar cells has decreased dramatically over the past few decades as manufacturing techniques have improved and materials costs have declined. However, there are still costs beyond just the solar modules themselves.

Manufacturing costs for solar panels have dropped over 90% since 1977. This is partly due to economies of scale as production has increased. Technical innovations have also helped bring costs down. Further reductions are expected as technology continues to advance.

While the panels themselves are now relatively inexpensive, there are balance of system costs to consider. This includes inverters, wiring, labor, permitting and other expenses for installation and integrating solar into the electrical grid. Soft costs like customer acquisition, system design, and financing can also add to the overall price tag.

There are ongoing efforts to reduce balance of system costs through streamlined regulations, improved installation techniques, and cheaper supporting components like wiring and inverters. Bringing down these costs is key to making solar more affordable.

Challenges

While solar cells offer many benefits, there are some challenges that need to be addressed:

Intermittency

One of the main challenges with solar power is intermittency. Solar cells only produce power when the sun is shining. At night and on cloudy days, solar cell output is diminished. This intermittent nature can make solar power more challenging to integrate into the grid compared to more consistent power sources.

Storage

Because of the intermittent output, energy storage is an important consideration with solar power. Batteries or other storage solutions are often coupled with solar systems to store excess daytime energy for use at night. However, energy storage adds significant cost and complexity.

Toxic Materials

Some solar cell designs utilize materials like cadmium telluride, which can be toxic. There are concerns about the disposal of solar panels and potential leaching of toxic materials. However, solar manufacturers continue to improve designs to utilize less toxic materials.

Advantages

Solar panels provide several key advantages that make them an appealing renewable energy source:

• Renewable – Solar energy is a renewable resource that is abundantly available. Sunlight can be harnessed in all areas of the world via solar panels.
• Modular – Solar panels can be installed in modular components, allowing incremental capacity increases. Systems can start small and expand over time to meet growing energy needs.
• Low maintenance – Once installed, solar panels require very little maintenance compared to other energy systems. As there are no moving parts, the systems tend to be reliable and require only infrequent inspections and upkeep.

Disadvantages

Solar power comes with some downsides that limit its current adoption and growth. The main disadvantages of solar panels are:

High Upfront Cost

The initial purchase and installation cost of a solar panel system is quite high, typically ranging from $10,000 to $25,000 depending on system size. This substantial upfront investment deters some homeowners and businesses from switching to solar power, despite the long-term cost savings.

Low Efficiency

Most solar panels on the market today have efficiencies between 15-20%. This means they only convert that percentage of sunlight that hits them into usable electricity. The rest is lost as heat or light reflected away. Low efficiency requires larger and more panels to meet energy needs, driving up costs.

Land Use

Solar farms require significant land area since the panels cannot be stacked vertically. This could compete with land needed for agriculture or preservation. Rooftop systems also take up space that could otherwise be used for architectural elements. Careful land management is needed to minimize solar’s footprint.

Future Outlook

The future looks bright for solar cell technology and applications. Here are some key trends to watch:

Continued Cost Reductions: Manufacturing improvements and economies of scale will enable continued cost declines per watt. This will make solar power more cost competitive with conventional power sources.

Novel Materials: Advances in materials science are unlocking new high-efficiency solar cell designs. Perovskites, organic PV, quantum dots, and other novel materials hold promise to push efficiency further.

Emerging Applications: New form factors like solar shingles, solar windows, and solar paint could enable solar integration into building materials, vehicles, consumer electronics and more. Solar may become ubiquitous.

With these developments, solar has potential to become a mainstream energy source and play a leading role in the renewable energy transition.