Semiconductors can be made from alloys that contain equal numbers of atoms from groups III and V of the periodic table, and these are called III-V semiconductors.
Group III elements include those in the column of boron, aluminium, gallium, and indium, all of which have three electrons in their outer shell.
Group V elements include those in the column of nitrogen, phosphorous, arsenic, and antimony, all of which have five electrons in their outer shell.
In a III-V semiconductor, atoms arrange into what’s called a zincblende crystal structure, also known as a face-centered cubic structure or cubic closest packing (CCP).
By construction, all the valence electrons in a III-V semiconductor are used up in bonding, so there aren’t any free to conduct. However, by doping in a similar manner to silicon, we introduce impurities that can then make it a semiconductor.
One of the main advantages of III-V semiconductors is that the crystal composition can be varied by replacing some group III atoms with other group III atoms. This changes the bonding and packing distances of the atoms. Why would we want to do this? The reason is that the crystal structure determines the band gap energy. III-V semiconductors, therefore, give us the ability to tune the band gap to our heart’s desire.
The methods by which III-V semiconductors are made include liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), metal organic chemical vapour deposition (MOCVD), and metal organic vapour phase epitaxy (MOVPE), all of which allow for fine control of the make-up and thickness of semiconductor layers. Unfortunately, these methods are also fairly expensive.
In all of these technologies, because of their mass-produced nature, the materials usually have more defects that will prevent the charge carriers from travelling as far as we’d like. To get around or compensate for this issue, materials usually need to be really good light absorbers.
Alternatively, several junctions are used, or the electric potential of the junction is extended to help carriers along, as is done in p-i-n semiconductors which have an intrinsic (undoped) semiconductor layer in the middle.
This solar cell technology produces cells with many defects, making them difficult to dope, and ultimately setting a limit on the junction potential that can be achieved. Defects also make the films more resistive, and overall make the cell’s performance dependent on the density of carriers present. To model this complex behaviour can be very challenging, and is a topic we’ll tackle in the Data Sets & Models section.
Probably the best-developed thin-film solar cell technology is amorphous silicon, which means silicon that isn’t arranged into a perfect crystal structure. It’s been in commercial production since 1980, and has the immediate advantage of not needing special crystal vibrations in order to absorb light (since the crystal lattices are all mismatched anyway). Therefore it has a direct band gap, and absorbs more strongly than monocrystalline silicon.
Cadmium telluride (CdTe) is made from the II-VI group elements, and has a direct band gap of 1.44 eV, making it one of the best-suited materials for photovoltaic applications. It has a wurtzite crystal structure shown below.
A Russian mineralogist named Lev A. Perovski discovered a class of materials that were, some time later in 2009, discovered to be useful in solar cells. Originally they were studied for ferroelectricity and superconductivity. These materials bear his name and are known as perovskites.
They follow the general formula ABX3, where A and B are both positive ions (cations) located in different parts of an octahedral configuration (where six atoms surround a central one). Usually A is divalent (with 2 electrons in its outer shell), while B is tetravalent (with 4 electrons in its outer shell). X, meanwhile, is a negative ion (anion) that is usually either oxygen or a halogen. The A atom is usually located in the center of a cube of an octahedral crystal near 12 anions, while the B atom is at one of the actual octahedral sites with 6 anions. The diagram shows this unique 3-dimensional structure.
What makes perovskites particularly interesting is that the band gap depends on how flat or two-dimensional the lattice is, so it gives researchers the ability to find the optimal band gap as is done with III-V semiconductors. In contrast to III-V semiconductors, however, perovskites are substantially cheaper. Because of these two combined factors, perovskite technology was selected as one of the biggest scientific breakthroughs of 2013 by the editors of both Science and Nature.
Perovskites already use some organic elements (which include carbon, hydrogen, nitrogen and oxygen), but there are other solar cells in development that fall purely on the organic side of the chemistry spectrum. The driving motivation behind organic solar cell development is that the materials would be cheap, as would the production, because mass-producing carbon chains is technology that is well developed in, for example, plastics.
These solar cells have benefited from advances in the development of LEDs based on similar technology, but they still have substantial development ahead in order to be competitive with silicon.
While there are a wide variety of organic solar cell materials, the majority rely on organic molecules with sp2 hybridization – that is, carbon double bonds. The electrons of these double bonds can move to fill in positive charge gaps, which makes the materials hole conductors. Usually they have a band gap around 2 eV, which isn’t ideal for solar absorption.
Recently there’s been a lot of focus around organic solar cells using fullerenes, which are a large series of carbon rings attached together in a configuration like a soccer ball (or football).
A particular type of organic material used in solar cells is worth discussing because of the particularly high research interest in it: graphene. Graphene is a form of carbon with alternating double-bonds that form a two-dimensional honeycomb sheet.
It was discovered in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester, who isolated it by peeling a layer off graphite (pencil lead) using scotch tape. They eventually won the Nobel prize for it in 2010.
While graphene is stronger than steel and highly conductive (due to the array of double bonds), it has a few other properties that make it particularly useful for solar cells. It is both very flexible and optically transparent (absorbing 2.3% of incident light from UV to IR), making it ideal for application in thin-film solar cells.
Remember that, in order to capture the current out of the absorption region of a solar cell, we have to run wires from the top to the bottom of the cell, passing through our load on the way. These wires shadow the front surface and decrease the overall light hitting our active area.
Graphene, on the other hand, can be applied as a collector to the front surface, and will serve to transmit much more of the light without shadowing, while still capturing and conducting the charge coming out of the absorption region. Its flexibility allows it to be used in thin-film solar cells, particularly in perovskites, where the main collector used is Indium Titanium Oxide (ITO), a brittle glass that cannot be bent without breaking. Graphene thus unlocks more of the potential benefits of perovskite flexibility.
Graphene has also been used to increase photon collection efficiency (PCE) in the perovskite active material itself, with some doped graphene allowing larger perovskite grains to form on the carbon network. In this role, it has been used as a carrier transport material.
Finally, it has also been used to protect the unstable perovskite films, because graphene has better physical, chemical, and thermal stability. While graphene by itself doesn’t make a solar cell, in combination with other material properties it unlocks a lot of potential advances.
As you might have already figured out, photovoltaics is a huge and interesting field of research that, as we’ve said, will play a major role in humanity’s energy future. We also mentioned above that there’s been so much development on monocrystalline silicon solar cells that there’s a steady trend of decreasing price, known as Swanson’s law.
The cost per watt is one of the bottom line metrics in the energy industry. The economies of manufacturing silicon have come very far since the invention of the first solar cell; so far, in fact, that much of the cost is in the installation and accompanying overhead rather than the cost of the devices themselves. The cost per watt for silicon dipped below a US-dollar years ago, and will continue to go down.
Silicon’s dominance in the market can make it very challenging for new photovoltaic technologies to gain traction. Therefore, it’s crucial that researchers get the most accurate data possible, as quickly as possible.