Solar Cell
A solar cell, also known as a photovoltaic (PV) cell, is an electronic device that converts the energy from light directly into electricity through the photovoltaic effect. This effect is a physical and chemical process where a material absorbs photons (particles of light) and, as a result, creates a flow of electrons, which is an electric current. Most commercial solar cells are made from crystalline silicon.
Structure of a Solar Cell
Solar cells are built as thin wafers composed of two types of silicon: n-type (electron-rich) and p-type (electron-deficient). These two layers are joined to form a p-n junction diode, which is the heart of the device. The top surface has metal grids or strips that collect the current but allow sunlight to reach the cell. Anti-reflective coatings and a protective glass layer enhance performance and durability.
How Solar Cells Work
A solar cell is the basic building block of a solar panel. A typical solar cell is made of semiconductor material, most commonly silicon. To create an electric field, silicon is treated with other elements via a doping process, producing two distinct layers:
n-type layer: Doped with phosphorus, this layer gains an excess of mobile electrons, which are the majority charge carriers. The material remains electrically neutral because each phosphorus atom that donates a free electron becomes a positively charged ion fixed in the crystal lattice, balancing the charge of the free electron.
p-type layer: Doped with boron, this layer has a deficit of electrons, creating an excess of holes (the majority charge carriers). The material remains electrically neutral because each boron atom that accepts an electron becomes a negatively charged ion fixed in the crystal lattice, balancing the charge of the holes it creates.
When sunlight (photons) strikes the solar cell, photons are absorbed by the semiconductor. If photons have enough energy (greater than the material's band gap), they excite electrons to jump from the valence band to the conduction band, creating electron-hole pairs. The built-in electric field at the p-n junction drives electrons toward the n-type side and holes toward the p-type side. This separation of charges or movement is what prevents them from immediately recombining, which would dissipate the energy as heat and not produce electricity.
As electrons accumulate on the n-type side and holes on the p-type side, a potential difference (voltage) develops across the junction. This voltage is known as the "open-circuit voltage" analogous to the voltage of a battery. When an external circuit is connected, the accumulated electrons flow from the n-type side, through the circuit, and back to the p-type side to recombine with the holes, creating a direct current (DC).
Since homes and most appliances runs on alternating current (AC), the DC electricity produced by the solar panels must be sent through an inverter, which converts it to usable AC electricity. The power can then be used by the building, stored in batteries, or sent back to the power grid.
Types of Solar Cells
Solar cells are generally categorized into three generations based on their technology and materials.
First Generation (Conventional)
Monocrystalline Silicon: Made from a single, continuous crystal of silicon. These cells are known for their high efficiency (around 17% to 22%) and long lifespan. They are more expensive to produce due to the complex manufacturing process (Czochralski method).
We can identify mono-crystalline solar cells by the empty space in their corners where the edge of the crystal column was. Each cell will also have a uniform pattern as all of the crystals are facing the same way.
Polycrystalline Silicon: Made from multiple silicon crystals. They are less efficient than monocrystalline cells but are more cost-effective to manufacture. This makes them a popular choice for many residential and commercial applications usually contain 60 solar cells.
The efficiency of this cell is around 15% to 17%. The low efficiency are due to how electrons move through the solar cell. Because polycrystalline cells contain multiple silicon cells, the electrons cannot move as easily and as a result, decrease the efficiency of the panel.
Second Generation (Thin-Film)
These cells are made by depositing one or more thin layers of photovoltaic material onto a substrate like glass, plastic, or metal. They are generally more flexible and lightweight than crystalline silicon cells.
Researchers have recently achieved 23.4% efficiency with thin film cell prototypes but thin-film panels that are commercially available generally have efficiency in the 10–13% range.
📌Thin film panels have excellent temperature coefficients!
Despite having lower performance of thin film panels tend to have the lowest temperature coefficient, which means as the temperature of a solar panel increases, the panel produces less electricity. Temperature coefficient tells us how much the power output will decrease by for every 1°C over 25°C the panel gets.
The standard temperature coefficient for mono and polycrystalline panels is in between -0.3% and -0.5% per °C. Thin film panels, on the other hand, are around -0.2% per °C, meaning thin film panels are much better at handling the heat than other panel types.
Comparison: Different Types of Solar Cell
| Type | Efficiency | Temperature Tolerance | Lifetime | Durability |
|---|---|---|---|---|
| Monocrystalline solar cells | 17%-22% solar cell efficiency | 0% to +5% | 25-30 year lifespan | Hail resistant 25 year P&M |
| Polycrystalline solar cells | 15%-17% solar cell efficiency | -5% to +5% | 20-25 year lifespan | 25 year P&M Warranty |
| Thin film solar cells | 10%-13% solar cell efficiency | -3% to +3% | 15-20 year lifespan | 25 year P&M Warranty |
Third Generation (Emerging)
These technologies are still largely in the research and development phase. They aim to combine the high efficiency of first-generation cells with the low manufacturing costs of thin-film cells.
Perovskite Solar Cells:
Perovskite solar cells are a promising, low-cost thin-film solar technology that uses a perovskite-structured material as the active layer. Perovskite solar cell work by converting sunlight into electricity using a layer of perovskite materials, through a process called the photovoltaic effect as silicon cell does.
According to data from the National Renewable Energy Laboratory, The efficiency of perovskite solar cell is nearing 27%. However, by layering perovskite on top of silicon (called ‘tandem solar cells’), this combines the best of both materials. Perovskite is better at absorbing a part of the light spectrum that silicon can't handle well, while silicon is more stable. The efficiency of this combination has recently reached a staggering 34.6%.
Perovskite solar cells are currently more sensitive to environmental factors like oxygen, moisture, and heat compared to traditional silicon cells, which impacts their long-term stability. While this has been the main challenge for commercialization, intense research is yielding significant breakthroughs in material and encapsulation techniques that are steadily improving their lifespan and durability.
Multi-Junction Cells:
These cells stack multiple layers of different semiconductor materials to absorb a broader spectrum of sunlight, achieving very high efficiencies (over 45%). They are currently very expensive and primarily used in specialized applications like space exploration.
Applications of Solar Cell
Solar cells are the fundamental building blocks of solar panels, which are then used to create solar arrays for a wide range of applications:
- Residential and Commercial: Rooftop solar panels on homes and businesses to generate electricity for self-consumption and to sell back to the grid.
- Agriculture Solar Pumps: Solar panels can be used for agricultural Solar Pumps which is useful for irrigation and powering equipments in farm.
- Large-Scale Power Plants: Solar farms that provide clean energy to thousands of homes and businesses, contributing to the national power grid.
- Off-Grid Solutions: Providing reliable electricity in remote areas where traditional power grids are not available.
- Consumer Electronics: Powering small devices like calculators, watches, and solar chargers.
- Space Exploration: Powering satellites, spacecraft, and space stations due to their reliability and lack of moving parts or fuel requirements.