Introduction to photovoltaics. Part 1: Solar cells
Due to climate change and the need for sustainable development, renewable energy solutions – in this case solar energy – has become one of the most important phenomenon of the XXI century. Nowadays solar technology is widely used not only by industries, but by households too – it’s quite common to see solar panels mounted on a facade of a house or somebody carrying autonomous solar power bank. But what exactly solar cells and solar modules are? First part of introduction to photovotaics covers history of photovoltaics, what solar cell is made of and differences between crystalline silicon solar cell technologies.
History of photovoltaics
Scientists use the term photovoltaics (PV) to talk about solar cells – the smallest fraction of the solar technology. A combination of several solar cells creates solar module and several modules – solar panel. However, panel is often used as synonym for module.
Researchers from early XIX century first attempted to work with photovoltaic effect and Charles Fritts made a first, though quite low efficiency, solar cell in 1884. No longer than 70 years later, in 1954, Bell Laboratories researchers Calvin Souther Fuler and Gerald Pearson demonstrated the first practical PV cell, the device that converts light (photons) to electricity (current) by the photovoltaics effect.
Nowadays cells are mostly made of silicon due to few reasons: charge in silicon is created when it is exposed to sunlight, because it is relatively easy to create pn junction in this material, which is the basis of solar cell, and as silicon is found in the sand, there’s plenty of it.
Let’s dive right into the world of photovoltaics and get familiar with commonly used terms in the solar business. Experienced Metsolar researchers and engineers will be your guide.
Light is an electromagnetic wave with frequencies around 10^15 Hz. Word “Spectrum” means that we have many frequencies of light. White (or yellowish) light from the sun, consists rainbow of colors and even a part of light that cannot be seen with an eye (ranging from blue to ultraviolet and in other direction from red to infrared). So, when we build a solar cell, we want it to absorb as much energy as possible. However, because of the laws of physics, highest amount of energy transferred to electricity from a simple cell (single pn junction) is 33%. Not to mention it is quite difficult to reach even this number. A record of single junction solar cell is only around 25,8%, this is due to optical loses, material impurities and technological imperfections.
Generations of solar cells
Solar cells are usually categorized into 3 generations:
- First generation solar cells are mainly based on silicon technology with moderate performance of 15-20% efficiency and is most commonly used nowadays.
- Second generation solar cells are based on amorphous silicon, CIGS or CdTe, where efficiency of such cells is low.
- Third generation solar cells use organic materials and has potential to overcome efficiency above 30%. Recently researchers of perovskite solar cells scored above 20% of efficiency.
What solar cell is made of?
1. Light absorbing material
It is a semiconductor material, the main part of solar cell, which is used to absorb solar light. And as mentioned before – the most common material for solar cells is silicon, mainly because it is one of the most abundant minerals on Earth.
Absorbed light results in a creation of charge carriers, which later are moved to contacts. In order to achieve high solar energy conversion efficiency to electricity some properties of this material should be considered: absorption, difficulty of technological processing, price and availability.
2. Solar light trapping layers
These layers have a purpose to decrease light reflections from the front side of the cell or, in other words, to increase transmission through the front surface of the solar cell and to decrease light escape from the cell, when it is already inside:
- Special layers called – Antireflection coatings (most common is silicon nitride (SiN), which causes solar cells to look blueish).
- Surface texture (chemically etched or laser assisted surface structure, what resembles pyramids – see image below).
Often these two means are combined in front surface. On the back side of the cell only texture is present.
3. Charge separation (pn junction)
Solar cell has a built-in potential voltage which is a force for electrons created by solar light to move to contacts. This built-in potential is formed with special type of junction called – pn junction. This type of junction became a fundamental part of nowadays electronics industry.
4. Charge collection (contacts)
As an electricity generating device, solar cell needs to have contacts to move charge to terminals and accumulate it or to be used immediately with, for example, household appliance. There are several contact structure variations used in photovoltaics. Most of silicon solar cells have contact structure that can be called standard and for sure everyone has seen it (see picture below). It changes a little bit in time, when further device optimization occurs, but looks similar as well.
This metallization type contains of metal (most commonly silver paste is used) grid on upper side of a cell, and a full layer of metal (aluminum is most common) on the back side of cell. Why metal can’t be used to cover all the surface of the cell? Logically, seems that more metal is better as there will be lower resistance and charge carrier will have a shorter path. However, metals absorb all the light and shadow the cells, causing cells not to work in optimal regime.
Polycrystalline vs Monocrystalline solar cell
Silicon can be made with different types of crystallography. It generally means the quality of structure, how well atoms are ordered in a structure. Crystallites with lower grain sizes result in higher recombination therefore higher loss, but also such solar cells are cheaper to produce. Silicon solar cells have few common types of crystallography defined by crystallite grain size and growth mechanism.
Single crystal or monocrystalline solar cells are more expensive to produce (Float Zone, Czochralski growth processes are used), but results in higher purity and higher efficiencies (commercial efficiencies 19-21%). These solar cells have even dark blue color (poly solar cells has more sparkly-like color, where different shades of blue are visible) and higher efficiency, therefore are more often used in applications, where limited space for solar integration is present.
Polycrystalline cells are cheaper, but lower energy conversion to electricity efficiencies are achieved (18-19%). Polycrystalline solar cells are mainly used in mainstream solar industry – solar power plans, households and applications where price per watt is more important than generated energy from poly cell solar module per area.
As research for new and more efficient solar technologies are already accelerating, novel technologies such as organic solar cells, perovskite or tandem cells is expected to shake the solar world with highly increased efficiency and widen solar technology application spectre even further.