There can be no doubt that photovoltaics is growing in popularity and with good reason. It’s an efficient, clean and sustainable way to generate electricity. But there is still some way to go in increasing the efficiency of PV solar panels and reducing their initial cost. And when it comes to materials selection,
In this article, we will look at the top four most commonly used materials for solar panels:
- Crystalline silicon (c-Si)
- Amorphous silicon (a-Si)
- Gallium arsenide (GaAs)
- Organometallics (soluble platinum)
Basic principles of a photovoltaic solar energy
Before we discuss the developments in PV material choice and efficiency and the challenges that lie ahead, let’s take a quick look at the basic concepts and construction of a PV system.
In principle, the photovoltaic effect is the direct conversion of light into electric energy.
Solar panels consist of a number of layers, typically glass, then a protection layer and a front contact layer covering individual solar cells switched in series. Beneath those, there are metal back contacts which conduct the electricity and are laminated to waterproof the cells and insulate it from excess heat. Finally, there is a protective back layer of glass, metal or plastic.
Solar cell properties
The solar cell is based on semiconducting materials which vary from system to system. Most commonly, solar cells contain two different types of semiconducting materials: a p-type and an n-type semiconductor, leading to a p-n-junction.
When the light of appropriate wavelength impinges on the solar cell, energy is absorbed promoting electrons to the conduction band of the semiconductor and leaving behind a hole in the valence band.
Yet, this is only possible in case the photon energy exceeds the bandgap of the semiconductor. The charge carriers then move according to the electrical field to the outer contacts.
Silicon – the material of choice for solar panels
While according to Shockley and Queisser, the maximum theoretical solar conversion efficiency for a single p-n junction photovoltaic cell is achieved at a bandgap of 1.34 eV, silicon with a bandgap of 1.1 eVis used in approximately 90% of solar cell semiconductors sold today. Silicon PV comes in a number of different forms:
- Crystalline silicon
Crystalline silicon is the most common material used in solar cells. The lifespan of crystalline silicon cells is more than 25 years without deterioration, making it ideal for industrial solar power generation. It yields an energy conversion efficiency of up to 22%, the highest of all the currently mass-produced panels.
To improve efficiency even further by reducing reflected light, crystalline silicon is coated with chemicals such as silicon nitride or titanium dioxide.
- Amorphous silicon
Amorphous silicon is silicon without a crystalline structure. It is used to create a thin-film solar cell and is commonly found in smaller solar panels such as those on calculators or to power private homes.
The cells are manufactured by vapour depositing silicon in a very thin film (approximately 1µm) onto a metal or glass frame. Amorphous silicon solar panels only achieve an efficiency of around 7%, due to a degradation of the material when first exposed to sun rays.
Interested in silicon? Read the newest article “Making cheaper silicon for photovoltaics” by our guest author – Mathieu Vadon, Ph.D. in Process Engineering. Discover two main silicon production methods and understand how energy consumption differs throughout them.
Gallium arsenide in solar cells
Gallium arsenide (GaAs) has been growing in popularity as a solar panels semiconductor in recent years. It’s a compound mixture of gallium and arsenic. It’s highly effective as a semiconductor and produces a high energy yield for a small amount of material. It has a bandgap of 1.49 eV, which is better than silicon.
However, there are two main drawbacks: firstly, gallium is rarer than gold and is costly, and secondly arsenic is poisonous creating safety issues when manufacturing the solar cells.
Big blue panels that somehow silently generate clean electricity. But what materials are they composed of? Why do the individual cells have such a specific shape? Why do they take up so much area? And why are they blue? Find answers here.
Other less common semiconductor materials include organometallics such as soluble platinum, which is a metal conjugated polymer, containing polymers with band-gaps ranging between 1.4 to 3.0eV. This could be a solar cell material of the future. It is lightweight, relatively cheap to produce, and it’s an efficient semiconductor.
Growth of photovoltaics
The global picture for the PV industry is one of strong growth over the past few years, which forecasts predict will continue to accelerate. A recent report from the Fraunhofer Institute for Solar Energy Systems (ISE) revealed that the global compound annual growth rate of PV installations between 2010 and 2016 was 40%, which makes it an extremely fast-growing market.
Europe still leads the way in the overall number of PV installations, contributing 33% to the global total in 2016, with China at 26%. Germany, in particular, has embraced the technology providing 13% of the global PV energy yield in 2016, and producing 7% of their national energy requirement through solar power.
A major factor that has driven the growth of global solar energy production has been a drastic reduction in the cost of installation over the past 25 years. In Germany, the price of a typical roof-mount PV system fell from 14,000 €/kWp in 1990 to 1,270 €/kWp in 2016.
The reduction in cost has been seen globally and is in large part due to improvements in material efficiency and price.
The challenges ahead
As solar energy production increases across the globe, there will be an increasing demand for higher efficiency and lower cost.
Fortunately, some new materials are showing some promising signs. Solar cells containing thin-film metal halide perovskites are now showing a high solar cell efficiency of 22.7%, and work is being done to increase their cell stability.
Perovskite solar panels seem to be the material of the future as they have the potential to cost less than silicon and they are also more lightweight.
Recent research at MIT and Stanford has combined perovskites with silicon cells, creating a hybrid that is efficient and could be less costly than traditional silicon only panels. Several companies are starting up to manufacture and sell perovskite-silicon tandem cells at the moment, so watch this space.
For further reading, we recommend this article. It covers these points in-depth:
- What are solar panels
- How do solar panels work?
- How do solar panels produce electricity?
- And what is the installation process?