How do solar panels work?
Energy demand and consumption nowadays has lead to great interest in the research and development of renewable energies. The world's primary energy consumption is expected to grow by 1.4% per year from 2014 to 2035, an overall increase of 34%. In 2015, 18% of the world's total energy demand was met by renewable energy, and a strong growth in renewable energy markets is predicted in the future. While this growth is encouraging, a larger growth rate is essential for future energy security and it is therefore vital that we continue to investigate alternative energy sources.
Despite several different green energies being necessary to meet global demands, solar energy is considered as one of the strongest alternatives to help replace finite resources. In 2014, renewables accounted for nearly half of all new power generation, with solar making up a third of this. In one year, the Sun supplies 3 x 1024 J to the Earth, which is 7500 times more than the global population currently consumes. Clearly, technologies that can harness even a fraction of this energy are worth researching. One of the most extensively researched solar technologies is the field of photovoltaics (PV), more commonly known as solar cells.
What is a solar cell?
A solar cell converts the Sun's energy into electricity. This happens via a process known as the photoelectric effect, where a material produces electrons when light is shone on it. Light can be thought of as tiny packets of energy known as photons. When photons strike the solar cell, they transfer their energy to the electrons in the material, knocking them off their atoms. The ability to extract these electrons as an electric current comes from a part of the solar cell called the p-n junction. These junctions are also crucial in many basic electronic devices, including transistors, LEDs and integrated circuits.
The key components of a solar cell are materials known as semiconductors. As the name suggests, semiconductors can conduct electricity more easily than insulating materials (like wood) but less so than conducting materials (like metals). Importantly, their conducting properties can be controlled by the addition of other elements known as dopants into the crystal structure, a process known as doping.
The majority of solar cells are made from silicon. When silicon is doped so that it contains slightly too few electrons, it is called p-type silicon (p for positive). When doped the opposite way to have slightly too many electrons, it is called n-type silicon (n for negative). Sandwiching together a piece of n-type silicon and a piece of p-type silicon forms the p-n junction. The extra electrons in the n-type material hop across the junction to fill the gaps in the p-type material. This process creates an electric field across the cell, which allows a current to be extracted.
Typically, the majority of photons from the Sun are absorbed in the p-type silicon layer. The electrons which are knocked off the atoms in this layer are transported into the n-type material by the electric field, where they flow out of the circuit. The more light that hits the solar cell, the more electrical current produced.
What are solar cells made from?
The first practical solar cell, made from crystalline silicon, was developed in 1954 at Bell Laboratories. Due to their expense, however, their only widespread use in the next few decades was in space applications. Luckily, prices dropped as it was found that the cells could be made using cast-off silicon from the electronics industry without much sacrifice of efficiency, and in the years following the oil crisis in 1973, many oil companies branched out and formed solar divisions, making solar feasible for terrestrial energy applications.
Crystalline silicon is still the most commonly used material in photovoltaic applications. Known as 1st generation cells, they comprise approximately 90% of the photovoltaic market, with their dominance most likely due to their high efficiencies and the technical know-how already developed by the semiconductor industry. Typically, crystalline silicon comes in two forms: monocrystalline, whereby the silicon wafers are one large single crystal, and polycrystalline, where the wafers are made up of many smaller crystals.
The efficiency of a 1st generation solar cell can never reach 100%. The maximum theoretical efficiency for these types of cells is 33.7%, which is referred to as the Shockley-Queisser limit. A solar cell's efficiency can be greatly diminished by defects in the material. Defects such as impurities and grain boundaries (the area where two crystals meet) can act as 'traps' for electrons. This means that the electrons generated by the sunlight get stuck in the traps, and cannot be extracted as an electric current.
To form high efficiency silicon solar cells, it is essential to use high purity (99.9999%) monocrystalline silicon. However, producing such silicon is an extremely energy intensive process, with temperatures reaching as high as 1900°C. The energy requirement can be reduced by instead using polycrystalline silicon, but with a sacrifice of efficiency.
The expense of crystalline silicon cells has led to extensive research into alternative 2nd generation cells in an attempt to achieve a better cost-efficiency balance. These cells generally try to reduce costs by using thin film technologies and low temperature solution based processing. Typically these cells are made from amorphous silicon (silicon with no regular crystal structure) or non-silicon materials such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Thin film cells are approximately 100 times thinner than 1st generation cells, thus saving on material usage and resulting in a flexible structure that can be incorporated into various types of architecture. To date, cadmium telluride solar cells have the shortest energy payback time of photovoltaic technologies, which is the time required for the cell to produce the energy used for its manufacture. The heavy metal cadmium, however is both a carcinogen and a genotoxin, meaning that it can cause inheritable mutations.
While the theoretical efficiency limit for 1st generation solar cells is an insurmountable barrier, it is possible to achieve efficiencies much higher than this through the development of new solar materials and cell designs. There are a variety of proposed cells that have this potential, all of which fall under the umbrella of 3rd generation cells.
Multi-junction cells are one type of 3rd generation cells which are already in commercial use, and by forming a cell made from several p-n junctions, efficiencies of 48.0% have been shown to be possible using concentrated sunlight. These cells use many layers of material to absorb a wider portion of the Sun's spectrum than 1st generation silicon solar cells, however the materials and processing methods required limit their use to space power applications where the desire for a high power-to-weight ratio is more important than cost.
A wealth of research into a host of other 3rd generation technologies has seen some promising advances, the most significant of which is the rapid improvement of the perovskite solar cell. Since the incorporation of a perovskite material into a solar cell in 2009, efficiencies have soared from 3.8% to 22.1%, making it the fastest advancing solar technology to date. Technically, perovskite is the mineral CaTiO3, although any compound that has the same crystal structure is often referred to as perovskite.
Typical perovskites used for solar applications consist of an organic molecule bound with a metal atom such as lead and a halogen atom such as chlorine. By varying the atoms and molecules used in the structure, thousands of chemical compositions are possible, giving great control over the properties of the material. Importantly, these materials can be made using liquid solution, without the need for high temperatures. While conventional silicon solar cells are heavy and rigid, perovskite solar cells have the advantage of consisting of lightweight thin films which can be deposited on surfaces of virtually any shape. Perovskite solar cells are therefore significantly cheaper to produce than silicon solar cells. In 2012, Dr Henry Snaith of Oxford University stated that perovskite has the potential to cut the cost of solar energy by three quarters.
Whilst perovskite technologies show a lot of promise, there are still problems which need to be addressed. To be truly commercially competitive with silicon, perovskite cells must be able to withstand the elements. Silicon cells are extremely resistant to the environment, whilst perovskites degrade in the presence of water, light and air. Another challenge lies in scaling up production to compete with the quantity of silicon wafers in the market.
The Future of Photovoltaics
Though the solar market is still young in comparison to that of the veteran fossil fuels, it is moving forward. Thanks to extensive research into PV materials and designs, in addition to lower production costs and higher production capacity, the solar industry is evolving and beginning to make its mark.
Today, more than 100 countries use photovoltaic technologies, and in 2014, solar generated electricity met 2% of the global electricity demand. Several countries exceed this value, with Germany currently the World's largest producer, meeting 7% of its electricity needs with photovoltaics. Technological advances and increased manufacturing scale have driven down the cost of the technology, making it more competitive than ever with conventional electricity sources. It is predicted that by 2050, over 20% of all electricity could be provided by photovoltaics.
Through continued materials research can we push the boundaries of photovoltaics and in order to take full advantage of this amazing technology, we must strive to rid ourselves of our dependence on fossil fuels. Solar energy has the potential to completely redefine electricity generation for the 21st century, and if we let it, the future of solar energy really is very bright.