Solar energy is one of the cleanest and most abundant sources of energy. In a single sixty minute time period, our earth takes in enough solar energy from the sun to power our way of life for one year. These are just two of the numerous facts that make solar energy – and its utilization – a very attractive prospect.
Of the 174 petawatts (PW; 1 petawatt = 1 x 10E15 watts) of energy that reaches the earth, roughly 89 PW is absorbed by the surface. To power our way of life, we consume 0.015 PW; clearly, we receive far more than what is required and this is a good indication of the potential of harvesting solar energy to power our cities and lifestyles.
Utilizing solar energy is not new; in fact, Leonardo da Vinci predicted a solar industrialization centuries ago. However, to harness the true potential of solar energy, the technology has to be able to handle all this incoming energy: to achieve this, the technology has to be energy efficient and cost-effective. The current crop of solar energy technologies center around photovoltaic (PV) systems and solar thermal systems, and to a lesser extent, on concentrated photovoltaic (CPV) systems. This article will feature the developments found in PV.
Introduction to photovoltaics
Photovoltaic cells (or solar cells) are composed of material that allow for the conversion of sunlight to electricity. The theory behind this process is rather elegant: photons from the incident sunlight, after striking the solar cell, can either be absorbed by the material, reflected by the material, or will pass right through the material. If a photon is absorbed, the energy accompanying the photon is usually sufficient to displace an electron found in the semiconducting material of the solar cell. This displaced electron leaves behind a hole (which is basically describing a bond with one less electron) but searches for another hole to bond with. This photon-induced mobile electron-hole pair will then travel through the solar cell and exit the cell via connected wires, generating electricity as it does so.
Development of this technology has been rather steady, moving from first-generation monocrystalline PV cells to second-generation thin-film PV systems (such as those containing cadium telluride (CdTe) or those made of copper-indium-gallium-selenide (CIGS)) to very promising third-generation and fourth-generation solar cells. Third-generation and fourth-generation PV cells utilize innovative and/or revolutionary ideas and concepts to overcome the issues of cost and efficiency that have generally plagued first-generation and second-generation PV cells.
Examples of third-generation solar cells include nanocrystalline solar cells, gallium arsenide (GaA) solar cells and hybrid PV cells (where the traditional semiconducting material is mixed with a non-semiconducting material such as an organic polymer, or variants to this definition), and a promising example of a fourth-generation PV solar cell is a kesterite solar cell. First-generation and second-generation solar cells are fascinating but as the thrust of this article is on revolutionary concepts and ideas, the focus will (mainly) be on third-generation and fourth-generation solar cells.
The benefits of using carbon nanotubes in solar cells
Hybrid solar cells are typically one-part organic material and one-part inorganic assembled together in such a way as to increase the efficiency of the solar cell; the organic material absorbs the incident light, acting as a donor, while the inorganic material acts as the acceptor. Even though the efficiency rates are lower than first-generation solar cells (hybrids reach about 5-6% efficiency whereas first-generation solar cells can reach anything from 15% upwards), hybrid solar cells can be made at a relatively low cost and in a way that is environmentally-friendly, making hybrid solar cells an attractive option. One particular hybrid solar cell that has the potential to revolutionize the solar industry is the carbon nanotube (CNT) hybrid solar cell. CNTs are exceptional: the third form of carbon (after graphite and diamond) and with mechanical and electrical properties that are both numerous and substantial. One-ten thousandth the thickness of a single strand of human hair, CNTs possess a tensile strength fifty times greater than that of steel; being so small and being so strong allows for a very favourable strength-to-weight ratio, a feature that has opened doors to many new applications such as those in the aerospace industry.
In addition to the strength-to-weight point, CNTs are also efficient electron transporters and superb thermal conductors, and provide a high electric field at the interface between the organic polymer (the organic material) and CNTs. These properties, in concert with the many remaining properties of CNTs, make for an appealing prospect when considering next-generation solar cells. Incorporating CNTs into a hybrid solar cell would produce solar cells that are more durable (due to the high tensile strength) and have better conductivities (due to the high thermal and electrical conduction), producing solar cells that are generally more efficient at converting sunlight into electricity.
Research carried out in the US at Notre Dame University showed that by integrating carbon nanotubes into a nanostructured semiconductor the efficiency rate could effectively be doubled. In conventional solar cells, the electron that is ejected after the incident sunlight strikes the solar cell eventually travels through the semiconductor material. With a CNT support, the ejected electron can travel faster through the semiconductor material, thus increasing the efficiency dramatically (albeit, from a low starting point as organic solar cells offer low efficiency rates). In spite of the current lower efficiency rate, the prospects look good for integrated CNT-solar cells and once the efficiency rate reaches a comparable level to crystalline silicon, then it is quite feasible that these specific solar cells might replace traditional first-generation solar cells.
Graphene as an alternative solar cell
A slight twist to an integrated CNT-hybrid solar cell involves the use of graphene, a single sheet of carbon with a thickness of one atom. Basically, graphene can be considered as being nothing more than an unfolded CNT with thermal, electrical and mechanical properties that are also very unique. For example: graphene is inert in an oxygen environment and also inert against water vapour; graphene is an excellent transporter of electrons; graphene has a large surface area and is transparent (hence, incoming sunlight will not be blocked); and graphene is very strong, two hundred times stronger than steel. Unfortunately, the issue preventing the widespread use of graphene is adhesiveness: graphene is a material that is reluctant to stick to any surface.
However, recent progress in research has solved this problem and has allowed organic photovoltaics (another name for organic hybrid solar cells) to once more come back to the fore. Researchers at the University of Southern California in America have investigated the use of graphene in solar cells and produced a rather unique organic hybrid: graphene integrated into a Gräetzel solar cell, (a Gräetzel cell is a thin-film dye-sensitized solar cell).
By producing a graphene-Gräetzel solar cell, they were able to make solar cells that were more flexible but this feature came at a cost: the efficiency was reduced significantly. While a traditional solar cell could generate 14 watts of electricity per square meter for every 1000 watts of sunlight that struck, these flexible graphene-based solar cells could only generate 1.3 watts. Such an output makes the technology unsuitable for applications such as rooftop solar systems, but acceptable for applications with a low-energy requirement such as mobile phone chargers.
Late 2010, researchers at Massachusetts Institute of Technology (MIT) created a novel way of bonding graphene to a solar cell with the sole purpose of replacing one of the two electrodes currently used in a solar cell, namely, the inflexible and brittle indium tin oxide (ITO). The MIT researchers managed to ‘glue’ the graphene sheet to the solar cell by introducing defects to the surface of the solar structure, creating a solar cell that possessed different characteristics to the conventional solar cell but losing none of its performance characteristics.
An unexpected discovery was the rate of electrical conductivity: the graphene-topped solar cell showed a higher electrical conductivity, indicating a possible solar cell with a higher efficiency rate being produced. What is also rather encouraging about the graphene-solar cell from MIT is that due to its flexibility the solar cell can be used to cover or mount uneven surfaces, such as non-flat rooftops or even structures containing multi-angled façades.
As the graphene sheet is transparent, a possible solar-based application might involve coating the exterior of all windows in a building with this graphene-solar material, thereby allowing a building to generate electricity for powering the internal lighting system or even the air-conditioning. While issues still remain, not least on the manufacturing side, the prospect of a mainstream graphene coated-solar cell with much higher efficiencies, lower costs (as carbon is cheap and abundant) and being able to harvest more of this free energy around us is indeed very exciting.
Increasing the efficiency by using pure-carbon solar cells
Another type of hybrid solar cell is not really a hybrid solar cell at all: a pure carbon nanotube solar cell. In the fall of 2009, researchers at Cornell University in the US replaced the silicon in a solar cell with a carbon nanotube, producing a photodiode.
The interesting aspect to the photodiode is that the diode has the ability to generate multiple pairs of election-holes; in a conventional solar cell, incident sunlight generates a single electron-hole pair. Thus, with the creation of multiple electron-hole pairs, a solar cell could theoretically increase the electrical output. Traditionally, excess energy was lost as heat but with the ability to generate multiple electron-hole pairs the excess energy that would otherwise have been lost will now be absorbed. This breakthrough has led to a considerable focus on researching CNT-based solar cells. In September 2010, researchers at MIT created a carbon nanotube ‘antenna’ (a nanofiber or rope consisting of thirty million carbon nanotubes) that was one hundred times greater at concentrating solar energy than traditional solar cells.
In early 2011, a team of researchers at Fudan University in China successfully managed to produce a flexible and very strong solar cell using tunable (tailor-made) carbon nanotubes. According to Professor Peng, the lead researcher, the technique utilized to make these carbon nanotubes allowed for an enhancement of the mechanical and electrical properties of the carbon nanotubes; Professor Peng stated that even after being tied, bent, or folded, the carbon nanotube did not break.
Peng also described how the semiconducting properties found in carbon nanotubes could make electron transportation more efficient; such enhancements will only improve the efficiency of solar cells. Taken together, the research described in this article gives an indication of the potential of integrating carbon nanotubes in a solar cell. The expected increase in efficiency, coupled with the lower manufacturing costs due to the abundance of the carbon-based raw material, makes CNT-based hybrid solar cells a potential candidate to revolutionize the solar industry.
Less toxic and less expensive thin-films
The concept of using non-conventional material does not solely extend to using carbon nanotubes; while there is a notable amount of interest in integrating CNTs into solar cells, there is also a sizeable push to produce thin-film solar cells where the expensive and rare (and sometimes toxic) elements such as indium (from copper-indium-gallium-selenium thin-films), cadmium and tellurium (from cadmium telluride thin-films) are replaced with cheaper options such as zinc and tin.
The so-called kesterite photovoltaic devices are composed of copper, tin, zinc, sulphur and selenium. With a first-generation solar cell, an efficiency rate of around 20% can be obtained; very pure versions reaching 40% efficiency can also be obtained, however, due to the costs involved, highly efficient solar cells are used predominantly in the aerospace industry and not generally suitable for the mass deployment of solar energy technology. With second-generation solar cells, the thin-film solar cells, the efficiency rate is reduced compared to first-generation cells but recent advances have led to a thin-film (CIGS) solar device with an efficiency rate of 19.3%.
With kesterite photovoltaic devices, IBM Research very recently recorded an efficiency rate of 9.6%; not at all comparable to either first-generation or second-generation solar cells but an indication of what is possible using alternative raw materials. With time, however, kesterite photovoltaic cells are expected to improve their efficiency rates and one day provide an ideal replacement for first-generation and second-generation solar cells.
Implications for the solar energy industry
The solar energy industry has steadily evolved over the years, producing silicon-based solar cells with a progressive improvement in efficiency. A revolutionary moment for the solar industry was with the production of flexible thin-film solar cells; the loss in efficiency was balanced by the general lower costs associated with using thin-films.
The developments listed in this article point to the production of solar cells with both lower costs and higher efficiencies, eliminating the trade-off associated with thin-film technology. As such, commercialization of this technology will benefit not only consumers (in accelerating the roll-out of solar technology for commercial and residential purposes) but also manufacturers and material suppliers: manufacturers will benefit as the overall costs of manufacturing solar cells will be reduced due to the lower cost of the raw material, and indirectly, due to alternative manufacturing processes being developed and utilized; and material suppliers will see an uptick in their trade as alternative raw materials are selected for use, such as less toxic and more abundant raw materials.
Based on the significant progress of the last few years, the anticipation is that progress over the next the 5-10 years will be rapid due to the raised awareness of moving away from obtaining energy from burning carbon-based sources. By bringing frontier science to the solar energy industry, the industry will once again be revolutionized by producing appreciably more efficient solar cells with lower manufacturing costs.
