Third generation photovoltaic (PV) cells for eco-efficient buildings and other applications
Abstract:
Solar technology has developed from a basic idea to a technology that could help support the world’s increasing energy requirements. Currently, first and second generation panels are used widely in industry but do not offer the most efficient and cost effective product. It is hoped that the third generation can overcome these problems, either through multi-junction devices or the use of different materials/technologies. One area of interest is nanotechnology as this is something that could be used in photovoltaic panels and provide the higher efficiency levels demanded by industry at a reasonable cost; hence providing a feasible tool to support ever-expanding energy requirements. However, this is not yet commercially viable and a lot more research and development is needed.
12.1 Introduction
The sun is a source of practically unlimited energy and is essential for life on earth. Currently the sunlight that reaches the earth in a single hour is equivalent to what we use in one year and hence it will comfortably meet our future worldwide energy needs. The amount of sun that each country receives varies, and the Middle East, one of the sunbelt regions, receives nearly double that of European countries, highlighting where the geographical future focus of this technology could lie. The energy from the sun is clean, free and continuous during daylight hours. Solar energy is caused by a fusion reaction of hydrogen molecules, which occurs between the hydrogen and helium gases contained in the sun’s core. However, this reaction is not what provides our energy. It is the loss matter that is produced as a by-product of this reaction that provides the energy we use. As the saying goes, one man’s waste is another man’s gold, and the sun’s waste – radiant energy – is what can be used freely as our ultimate power source.
The sun is known for its extreme temperatures and high pressure, but this is not harmful to humans as it is located far away from earth and it is this that makes it such a useful power source. The output of the nuclear fusion that is harnessed for conversion into electricity is completed through a scientific innovation discovered in the nineteenth century. Previously we only optimized the heat and light from the sun, but this new innovation – the photovoltaic cell, which works using the principle of the photoelectric effect – also enables power to be harnessed. Explained in its simplest form, the photovoltaic cell is a device that enables the conversion of sunlight into electricity, which is known as solar power.
One question many people ask is whether all the radiation reaching the earth can be used for direct conversion. The simple answer is no, as primarily the human race does not currently need all the energy the sun provides (many thousand times daily more than we can cope with) and also the sun is used not only for producing electricity, but it also has many other functions. Nearly half of the radiation hitting the earth (which is already dramatically reduced due to filtering by the earth’s atmosphere), is either reflected directly back into space or used for water evaporation, and the remaining amount is free energy to provide warmth and light, grow food, and potentially supply as much energy as mankind could ever want. It is a little known fact that all energy sources come from the sun, except for nuclear, and therefore for years humans have been using this power, although not for direct conversion. Even the world’s current huge energy demand does not utilize all the potential energy provided and a lot of the sun’s free energy goes unused.
Although there are many advantages of solar energy, there are also many negative factors, such as:
• restriction of generation to daylight hours
• efficiency of solar cells can be affected by pollution and weather
• harnessing solar energy on an industrial scale can be extremely expensive
• efficient long-term storage of solar energy is limited and this area needs to be further investigated.
Solar energy has been hailed as the development for the future of mankind, but whether it will eventually live up to this accolade is very much still to be proven. One thing is certain: with dwindling supplies of traditional fuel reserves, we must keep on searching for ways to harness solar energy for the generations to come.
12.2 History of photovoltaic (PV) cells
Solar energy has already been in development in various forms for well over 150 years and this will continue and increase for at least the next 150 years. Alexandre Edmond Becquerel’s first discovery of the photoelectric effect in 1839 truly was the first step in relation to the development of solar energy. In 1873 Willoughby Smith, although not directly researching into solar energy, found that selenium had sensitivity to light, hence opening an interesting area of research, using selenium to produce solar cells. This principle was separately verified by Richard Day and William Adams in 1876 with only a slight modification of a platinum intersection. Adams continued in 1877 to produce an initial selenium solar cell, with Charles Fritts in 1883 building another solar cell. The device had poor efficiency of approximately 1–2% and it was produced by forming a junction of gold on the semiconductor selenium. This was just the start, and levels of efficiency and performance could only improve from this point. However, it was not until 1888 that the first patent in the US was filed by Edward Weston for a ‘Solar Cell’ (Weston, 1888).
After the initial discovery, there was little progress for nearly 20 years, but in the early twentieth century Nikola Tesla filed his patents on ‘Apparatus for the Utilization of Radiant Energy’ (Tesla, 1901a) and ‘Method of Utilizing Radiant Energy’ (Tesla, 1901b). This started a revival and some further publications were produced with the most commonly known by Albert Einstein in 1905 entitled ‘On a Heuristic Viewpoint Concerning the Production and Transformation of Light’ (Einstein, 1905), discussing the photoelectric effect which in 1921 would gain him the Nobel Prize, and be further verified in 1916 by Robert Milliken. During the past century a great deal of work has been ongoing on the photoelectric effect on different materials and also photovoltaic cell structures.
Although Russell Ohl in 1946 patented the ‘modern solar cell’, the first research team to really bring photovoltaics to the general public was Pearson, Chapin and Fuller from Bell Laboratories, who in the 1950s worked on PV efficiency and patented their discoveries (Chapin et al., 1954, 1957). The first practical use of a photovoltaic (PV) was in 1958 when the US Signal Corps powered the Vanguard 1 satellite with solar energy for an eight-year period. For many years the implementation of PVs was limited to the space industry before terrestrial panels became more cost-effective.
Now, in the twenty-first century, photovoltaics are widely accepted as an efficient form of energy supply and can be used for both on-grid and off-grid electrical applications. They are not limited to this usage but can also be successfully used to power cars like the General Motors Sunracer vehicle in 1997 or even airplanes such as the Solar Challenger in 1981 or the Icar Plane in 1996. Initially the US led the way in using solar panels but Europe quickly followed, expanding the technology worldwide. In 2011, although the countries of Europe were still extremely strong contenders in the race to be the top PV country, the Japanese were also successfully throwing their hat in the ring (Fig.12.1). Worldwide, the capacity has expanded from the 58% increase which occurred from 2006 to 74% in 2011, showing how worldwide PV capacity has, in 2011, reached 70 GW.
However, the leader in total capacity as seen in Fig.12.1is unlikely to change dramatically for the next few years due to its current extensive lead and addition of new projects. Surprisingly, one thing that did change in 2011 is that Italy jumped ahead of Germany in the amount of PVs added followed by a new player – China – which came in third, showing that the number of countries embracing solar energy is expanding. PV systems are now successfully used in more than 120 countries with 25% of the market being utility-scale. Currently most of the manufacturing process has moved from the US to Asia, with 61% of 2011 global production located in this area.
The drive to continue worldwide embracement of PVs has not slowed down. In 2011 the main leaps forward took place in Europe and China. Germany has continued its love for solar energy but it is now joined by Italy, both rising by 57%, asserting themselves by the amount of the electric energy produced by PVs in 2011 as market leaders in the utilization of this energy source. In relation to the manufacturing element, China has risen as the frontrunner, not only housing the largest manufacturing PV company, Suntech Power Holding, but also being given the title of the country producing most (650 panel manufacturing companies) of the world’s PV panels. However, as the market expands, so does the interest in new utility-scale markets, which are emerging in Mali, China, Bulgaria, the United Arab Emirates, Egypt, Thailand and India, with the future being very much related to the regions of the Middle East, Africa, South America, India and China. Like many other countries that have embraced PVs, Europe has set some targets it wants to achieve in the next decade for the embracement of renewable energy. The continent states that by 2020 more than 33% of electricity will be produced by renewable energy sources, with up to 12% of this being specifically related to PVs.
12.3 Functions of a photovoltaic (PV) cell
Although technology has branched out in relation to the combination of materials used and the production methods selected, the basic operating principles are the same as they all aim to take the range of intensities from daylight and absorb the photons from it to create an electric current, hence the reason it was named ‘photovoltaic’ as it produces electricity from light (Fig.12.2).
The structure of the cell as shown in Fig.12.3 outlines simply how in the basic first generation crystalline silicon solar cell the sunlight is transferred into electrical energy, which is really the only energy source with no limit to its potential. These cells are manufactured from similar materials to those used in semiconductor wafers which create an electric field with one side being negative and the reverse positive. However, a PV panel is not a single cell, but rather a combination of series and parallel cells, which creates an array and are connected and housed in a structure encapsulated by a glass surface. With the cells connected in series providing a larger voltage and the increased current coming from parallel connections, although the cell voltage is not sensitive to light intensity, the current is affected. This structure enables a variety of voltage and current options to be available depending on the user’s requirements. The steps (Table 12.1) of the process that is undertaken in the cell are highlighted below but it needs to be noted that when sunlight hits the panel not all energy is directly converted to direct current (DC) electricity, as the light can either be absorbed, reflected or move straight through the cell.
Table 12.1
Steps in producing electricity from the sun
| Step | Description |
| 1 | Solar panel is struck by photons contained in sunlight |
| 2 | In the semiconductor material, negative electrons are knocked loose from their atoms |
| 3 | Enabling movement in a single direction in the material |
| 4 | Creating the possibility of an electric circuit from the electrons |
| 5 | Provided the negative and positive of the cell are connected, Advantages then DC electricity is generated |
A common assumption by many is that this principle will only work on sunny days. This is incorrect; solar power works to its maximum when there are no clouds in the sky and the sun is in its optimum position, but it will also operate without maximum sun, albeit slightly less efficiently. Out of the many renewable energy technologies, it is suspected that solar power could have the brightest future, not only because of potential advancements in materials and production methods but also because of some basic engineering principles embedded in the technology. PV cells currently produce approximately 1.5 W, dependent on size, so when they are placed together in a matrix form on a frame covered in glass, this is referred to as a module raising possible output. Several modules can be furthermore connected together in an array to provide any power combination required; hence in this configuration output power is not an issue. As with all innovations, PVs have positive and negative points as outlined in Table 12.2; however, researchers are constantly working on remedying, improving and eliminating the negative factors.
Table 12.2
Photovoltaic advantages and disadvantages
| Advantages | Disadvantages |
| No moving parts | High costs |
| Operates quietly | Low efficiency |
| No operating emissions | More expensive than fossil fuels |
| Long term use | |
| Limited-maintenance | |
| Modular – enabling gradual expansion |
One of the issues widely discussed in relation to the solar cell is that of efficiency. Initially the factors highlighted in Table 12.3 were defined as the most important elements reducing the efficiency of the solar cell, but it should be possible to address some of these in future generations of this technology. The efficiency ratio discussed considers the relationship between the output and optical powers of incident solar light and there are several factors that can affect this (especially the first generation PV) which are highlighted in Table 12.3.
Before it is identified which technology could potentially solve any of these issues, it is first important to have a broad understanding of the different technologies available and the differences between them.
12.4 Overview of photovoltaic (PV) technology: first, second and third generation cells
The PV industry started with one basic technology but has now expanded its search for the most optimal solution to convert solar energy into electricity. Currently there are three main areas that both the industrial and research communities are considering. The first and second generation PVs are more industry-focused, with some research being completed, and the third generation is still being heavily researched due to the drive from industry to develop a device that could be the solution to all our solar energy concerns. Although there have been many new innovations in materials over the years, the most common material for the production of solar cells to date is still silicon. Outlined below is an overview of the current technology used in solar modules, from the traditional mono-crystalline silicon to the second generation thin films, including the rest of the silicon family, polycrystalline and amorphous, but also not forgetting the semiconductor PVs that are manufactured from compound materials and the new and promising third generation developments. This section gives an overview of how and why the technology moved from the first to the second generation, and how it is hoped that the best of both can be used to provide a third generation that might solve all previously acknowledged issues. Figure 12.4 outlines the efficiency levels of some of the first, second and third generation PVs, recorded in 2010 (Saari, 2010).
Fig 12.4 Energy levels achieved by PVs in 2010 (Saari, 2010).
The following sections summarize where the technologies are now but also question how they got to this point and the historical steps that were taken to arrive at today’s technology, before providing a glimpse of where it could go in the future.
12.4.1 First generation
The first generation had one simple task, with no limitations, which was split into two steps:
1. Absorb light energy so positive and negative charges can be generated.
2. Create a potential difference by separating the positive and negative charges.
This generation of solar cells is commonly known to have, on the positive side, high efficiency, but on the flip side it also has a higher cost, to be expected with any new technology. It is estimated that the maximum theoretical efficiency that these single junction silicon cells could hope to reach is approximately 33%, limited only by thermodynamics (Shockley and Queisser, 1961). To date, first generation PVs account for the largest market share, but this is changing (Brown and Wu, 2009). One of the issues with this technology is its manufacturing process, which is expensive and based on the same principles as the computer industry, with the two major elements being the requirement for pure silicon (with the higher purity providing greater possible efficiency), and the single-junction photon energy extraction. Crystalline silicon PVs have to their advantage the fact that there is an abundance of raw material, and, due to the maturity of this technology, they have a long life, high module reliability, and are nearly at their maximum theoretical efficiency. However, developers have discovered that silicon is not the optimal material for PV cells and it is considered highly unlikely that the cost of this generation will ever drop below the equivalent energy cost of fossil fuels. There is no clear solution to the high labour and energy required for the production, again providing a barrier to reduction in cost. Table 12.4 summarizes the efficiency, degradation and the advantages/disadvantages of this first generation PVs for two models.
12.4.2 Second generation
The middle member of the PV family, which has been under constant research since the late twentieth century, is the second generation solar panel. It is considered as the smaller but fastest growing division (Brown and Wu, 2009). It has opposite traits to the first generation, and has both lower cost and energy – essentially the two issues it was created to address, following the shortcomings of the first technology (Hamakawa, 2004). The main reason for the difference in the two technologies is related to the smaller amounts of material, in addition to the lower manufacturing process cost. It was theorized that the ideal solar panel would not utilize the wafer principle as it uses a lot of material, hence the development of the thin film principle. The thin film was created by using various inexpensive deposition methods to produce thin layers of silicon, micrometres thick, which enable approximately 90–95% of the solar light spectrum to be absorbed, compared to the first generation which would need to be 200–400 μm thick for the same amount of absorption. The difference in this silicon material is it contains almost no crystal structure, hence can neither be referred to as crystalline nor multi-crystalline. Rather, it is named amorphous silicon (a-Si).
The main issue with this material is its low electrical properties, hence not improving overall efficiency. Many interesting structures and processes have enabled such PVs to reach approximately 10% efficiency, but this is anticipated to improve. One of the largest issues with the a-Si is the Staebler-Wronski phenomenon (Kolodziej, 2004), which basically causes degradation when the panel is exposed to sunlight. So, although this component of the second generation technology has good potential due to its low cost, more research and development is required to ensure a stable and efficient device. One of the most interesting principles associated with this technology is it has the ability to be either flexible or semi-transparent, leading the way to further installation options. If no flexibility is required, it is placed between two pieces of glass with no frame, but if flexibility is needed then it can be deposited onto plastic film. This thin film technology is normally connected with the second generation, but silicon is not the only material that can be linked with this group of PV technologies.
Panels are also commonly associated with materials that include cadmium telluride and cadmium sulphide layers (CdTe/CdS) or members of the chalcopyrite family, copper indium gallium selenide and copper indium diselenide (CIGS/CIS), which offer high-efficiency possibilities. As can be seen, the difference between these two initial types of PVs is the material used and the manufacturing methods, rather than a difference in the method of conversion.
One further technology that is sometimes considered under this second generation umbrella is the PVs made from organic materials, which currently (2012) have a very low efficiency of 1–8%, but have potential because, not only is the material accessible, but it is reasonably priced to further support easy and cheap manufacturing methods. These organic devices, which operate in a similar way to the photosynthesis process, could in the future offer an alternative to inorganic materials.
It has been anticipated that not only will this second generation technology have the market share by 2015 but also by this year it will be able to provide power at a lower cost than fossil fuel. It is only with time that we will be able to assess if their lifetime will be proven compatible with first generation technology. Table 12.5 summarizes the efficiency, degradation and the advantages/disadvantages of second generation PVs for two models.
12.4.3 Third generation
Third generation PVs are still heavily under research. Although development of this technology has been ongoing for about 20 years, they are only now starting to emerge on to the market. It is expected that third generation photovoltaics will combine the best of both the first and second generation technologies; hence they should have high efficiency and low cost. The original definition of ‘third generation’ was that the technology had to have an intrinsically higher efficiency than a single junction device, had to use thin-film technology, as well as abundant, non-toxic, durable materials (Green, 2009). However, this has been developed as researchers have worked on this area and Table 12.6 outlines some of the third generation solar conversion methods and their approximate associated efficiency (Green, 2009).
Table 12.6
Third generation PV potential efficiencies (Green, 2009)
| Solar conversion methods | Efficiency (%) |
| Circulators | 74 |
| Tandem (n = infinity) | 68 |
| Tandem (n = 6) | 58 |
| Thermal, thermo PV, thermionic | 54 |
| Tandem (n = 3) | 50 |
| Impurity PV & band, up-converters | 49 |
| Impact ionization | 45 |
| Tandem (n = 2) | 44 |
| Down-converters | 39 |
| Single cell (n = 1) | 31 |
Researchers expect that this generation will have the ability to overcome the Shockley–Queisser boundary of 31–41% efficiency which was seen by previous generations. The Shockley–Queisser limit is most simply put as a basic physics boundary on the maximum possible solar cell efficiency, and aims to maintain low production costs while trying to enhance performance. One of the options being considered through development of the second generation thin films is incorporating novel approaches to increase their efficiency range to 30–60%.
Figure 12.5 shows a multi-junction device with three layers, sometimes referred to as cascaded or tandem. Each of the three materials has three varying band gap energies which are normally formed from lowest to highest. The highest band gap energies (Eg) are stopped by Cell 2 with the rest flowing to this cell which stops the mid-energy photons as it has the middle Eg. Cell 3 is the final layer and it absorbs the photons with low energy. This type of technology has already proved interesting with a practical efficiency so far of 32%. This technology expands upon the developments of the second generation thin films as most of them are not only multi-junction but also thin film.
The reasoning behind the interest in this technology is that, although there is a wide spectrum range, most of this does not reach the PV panel. The two main types of light that are of interest are red and blue. The red light and silicon band gap are similar at approximately 1.1 eV. Unfortunately, the blue light, which has a higher band gap and approximately three times the energy, is not absorbed as the band gap is too low – this is an issue under research, hence the interest in multi-junctions. Table 12.7 outlines four different layers and the possible theoretical efficiency with each highlighting the band gaps that they include.
The second part of the third generation focuses on the development of nanostructures, which obviously are in the size of nanometres. It must be understood that there are vast differences between normal and nanostructures because at that small scale the quantum physical effect takes over the device. The most important feature of the nanostructure is that their optical features change as the size varies providing the option of changing the optical properties by optimizing the size of the nanostructure. This provides many more options with regard to how the capture of the photons can be optimized, enabling more of the energy of the solar spectrum to be used, thus providing maximum efficiency.
The market suggests that there is still some time to wait before this technology will be commercially viable, and it is likely it will be towards the end of the first quarter of the twenty-first century before it is achieved. It is anticipated that the third generation will shatter the current price of $1/W to around $0.20/W (Green, 2003).
12.5 The use of nanotechnology in photovoltaic (PV) technology
Nanotechnology is related to the direct control of matter on the atomic scale and it can also be used to describe material, devices or structures with one of their dimensions being between 1 and 100 nanometres. An atom is considered the basic building block and has a diameter of 0.1 nm with its nucleus being smaller at 0.00001 nm. It is difficult to actually picture this size – one nanometre (nm) is equal to one billionth or 10- 9 of a metre. The size of atoms are not the only factor to consider – as with every industry, it has its own specific assembly methods and in nanotechnology it is split into two main areas, the bottom-up and top-down approaches, which are summarized in Table 12.8 (Rodger, 2006).
Table 12.8
| Approaches | Description |
| Bottom-up | Self-assembly by chemical principles of molecular |
| components using the principle of molecular recognition | |
| Top-down | No atomic-level control as objects are built from larger parts |
Currently, nanotechnology has been accepted for use in applications related to surface science, organic chemistry, semiconductor physics, modular biology and micro-fabrication, but in the future it is expected that this will expand to include electronics, medicine, biomaterials and energy production.
If PV technology is to become a good competitor to fossil fuels, it must be cost competitive. Currently, silicon-based PVs are actually more expensive per kilowatt hour, hence the search for another solution. The nanostructure configuration is known to enable control of the electronics, structural and optical factors, therefore providing it as a possible solution for PVs. With this under consideration, development of nanotechnology PVs (Anon., 2010) is expected to increase in the coming years and ameliorate the problems seen in the first and second generation aiming at lower cost (Anai et al., 2010) by not only reducing material but also ensuring that the manufacturing process is efficient. Most importantly, the PV industry considers that nanotechnology offers flexibility and also can overcome the limitations with band gap issues (Serrano et al., 2009).
The main reasons for opting to try nanotechnology as a third generation solution (Fthenakis et al., 2009) are shown in Fig. 12.6. Although it is currently known that it requires more energy for the material and manufacturing, it is hoped that solutions to these drawbacks can be found.
Nanotech PVs offer a solar option that not only could be more efficient (Hamakawa, 2004) but also ‘greener’ as the product offers the possibility of recycling at the end of its lifetime (Shockley and Queisser, 1961), something not often initially considered when assessing renewable energy technologies. Other advantages of nano-PVs are as follows:
One of the main issues that has been seen in this area relates to problems with PV cell design, but as the sector develops, it is hoped that this can be overcome.
Although nanotechnology is still primarily in the research and development phase there are some companies at the manufacturing stage. One of the benefits of nano-tech PV design is that the dimensions of the nanocrystals identify which part of the solar spectrum is absorbed with their specific band gap, hence solving some previous issues. Table 12.9 presents a nanosolar technology company example, briefly outlining some of the positive points of the product. It is expected that over the coming years many more companies producing this technology will emerge.
Table 12.9
Nanotechnology company example
| Company | Nanosolar |
| Country | USA |
| Types | CIGS nanoparticles |
| Substrate | Metal foil or glass (low cost) |
| Manufacturing | Non-vacuum printing of nanoparticle ink to a substrate. These are transferred to electronic film of high quality using the rapid thermal processing (RTP) method |
| Efficiency | 14% |
| Cell cost | $0.36 per watt |
| Panel cost | $0.99 per watt |
| Throughput | 1 panel per 10 seconds |
Currently PVs made from nanoscale materials are not suitable for large-scale installations because of issues not only with their efficiency – which it is hoped will be improved – but also with their long-term stability. These problems are due to the fact that there are some factors that are difficult to experimentally understand in the nanoscale range, hence high power computation is required, which until recently was a problem (Anai et al., 2010). Currently, because of the development of high performance computing and advances in the calculation of electronic structures, it is easier to consider material properties using atomistics information, enabling researchers to develop more materials with specific properties through calculation.
The current nanotechnology PV research and development focus is on (Manna and Mahajan, 2007):
The next sections will explain each of these areas in detail.
12.5.1 Quantum dots
The principle related to this technology was first discovered by Ekimov in the 1980s (Ekimov and Onushchenko, 1981) and the technology was initially applied to LED technology. The quantum dot, which was first defined by Reed et al. (1988), has properties between discrete modules and semiconductors (Norris, 1995; Bawendi et al., 2000; Brus, 2007).
Quantum dots (QDs) are made up from nanostructures of semiconductor materials which are categorized by their confinement dimensions. Figure 12.7 (Nozik, 2010) outlines samples of quantization in nanoscale materials and where the quantum dot falls in this band. Quantum dots are normally in the range of 2–10 nanometres in diameter, containing hundreds to thousands of atoms. As defined in Fig.12.7, QD or nanoparticles are semiconducting nanometre-sized crystals (Manna and Mahajan, 2007). There is still a lot of research ongoing into the various shapes that nanocrystals come in, but for renewable energy currently the most focus is being placed on spheres, cubes, rods, wires, tubes and tetrapods. As with the traditional PV market, QD developers have also decided to consider semiconductor materials; however, it is not limited to this and research could be completed into metals or organic materials and they could be combined with porous films or dyes, but as semiconductors are currently the number one PV material, this is the obvious starting point. Semiconductor materials are made from the following mixture of periodic groups: II–VI, III–V and IV–VI.
QDs are expected to be ideal for use in solar panels as they can overcome one of the previously identified issues of the limited band gap (BG). The adjustable band gap they offer means that the larger and wider BG equals more light absorbed which in turn provides more output voltage. A smaller BG provides more current but less output voltage. Hence to optimize this phenomenon, ideally tuning the QD to different band gaps would enable absorption of different wavelengths (Manna and Mahajan, 2007), therefore removing the previous limited PV efficiency which is seen with the earlier generations. This, however, is not the only reason for the use of QDs: the QD also enables material moulding into different forms as well as being cheaper due to the fact that they use basic chemical reactions (MIT, 2007) and they also reduce wasteful heat seen in the previous generations, in addition to maximizing the amount of light to electricity conversion.
Issues with non-utilization of PVs extra electron energy (‘hot carrier’ or ‘hot excitation’) have been previously highlighted. Normally it is kinetic free energy which is lost in pico or subpicoseconds through a process of electron–phonon scattering creating heat from the kinetic energy. Some of these issues have been previously solved by tandem PVs, but not all. A positive aspect is that the quantum dot holds on to the hot carrier for a longer time, thus enabling an extended period for it to cool, extending the lifetime of the hot electrons by as much as 1,000 times. Part of this is the three-dimensional array of the QD, which enables strong electronic coupling producing an extended lifetime for excitons. This provides more movement of hot carriers and the possibility of more electric generation, enabling more charge from one photon (Manna and Mahajan, 2007).
Quantum dots are very interesting and exciting as they have optical properties that are not seen in typical materials and enable a good spectrum control of emitted light, and these properties come from confinement of the electron–hole pairs. The QD offers a varied emission and absorption spectrum which is linked to the various particle sizes, although it is important to remember that the QDs will only stay in a confined space if their wavelengths are associated.
The relationship between the size of dot with wavelength and energy can be summarized as a smaller dot equalling shorter wavelength fit, also equalling higher energy of the electron, with the larger dots having the opposite relationship. Also different from traditional PVs is the QDs’ photovoltaics’ ability to obtain three electrons from one high energy photon of sunlight where normally it was only a maximum of one (Nozik, 2001; Ellingson et al., 2005; ISIS Press Release, 2006). This is referred to as multiple exciton generation, or MEG for short.
Extended research is ongoing with regard to progression in QDs. Conibeer (2007) discusses a crystalline material tandem thin film QD which obviously enables the wider band gap (BG) and is constructed of silicon placed between layers of Si-based dielectric compound with a QD diameter of 2 nm and a BG of 1–7 eV. The process to produce this QD is as follows:
This method means that issues with lattice mismatching are not suffered as the framework is lacking a definite shape. However, Conibeer (2007) highlights that there is still work to be done on the creation of junctions, passivation of defects and connection to silicon cells. One of the real ongoing problems is linked to spectral sensitivity. Green (2009) also discusses silicon QD, defining the link between changing QD size to control the optical band gap. Also discussed in this reference is the research that is ongoing into increasing the voltage by increasing optical BG. The silicon option is expected to provide not only a higher efficiency but also a lower cost and it will also use material that is abundant and, most importantly in this era, environmentally friendly. More work on exciton splitting and collection of resulting free electrons and holes could further improve the devices.
An obvious factor also to consider is the materials that could be and are currently being used in the development of this technology. Materials used to date include, but are not limited to, cadmium selenide, cadmium sulphide, cadmium telluride, indium phosphide, indium arsenide, lead selenide and lead sulphide.
Another example of a QD is a lead selenide semiconductor compound combined with titanium oxide which is used to remove the hot carriers, hence enabling new electrons to be embraced. The issue with arrangement is linked with the fact that, although one wants to induce electronic transfer, no chemical interaction between the layer/material can be allowed.
In the next few years it is thought that the quantum dot efficiency could be increased by not only improving the electrode and layer structure but also by increasing the QD density. At the start of this century, Nozik predicted that the use of QDs in PVs would increase efficiency, enabling 65% of the sun’s energy to be converted into electricity. There have been many predictions but only time and practical development will truly lay the building blocks to future technology.
The size of the QD as discussed previously is a consideration that can improve efficiency and so how the size is controlled is important and includes the following factors:
Therefore the method of QD fabrication is important and currently there are two main options. The first is the suspension in liquid of ultrafine particles, referred to as colloids. This colloidal chemistry is not expensive and does not require any expensive equipment; rather it only needs the proper chemicals and room temperature as a requirement to produce. Basically it is the linking of one metal ion with another. The second fabrication method is epitaxial growth. In this process, crystals are grown on a semiconductor material on the surface of another with the structural orientation being the same.
12.5.2 Carbon nanotubes (CNT)
Carbon nanotubes (CNTs), or nanotubes (NTs) as they are often referred to, are cylinders constructed of carbon atoms (nanostructure) and are not only organic but are good photosensitive material. Basically they consist of a single sheet of graphite with a hexagonal lattice (Manna and Mahajan, 2007) which is constructed from linked carbon rings forming the tube, capped at each end with a pentagonal carbon ring.
Nanotubes are made up of n rows and m columns and the link between them defines exactly how they are formed. The structure has a length-to- diameter ratio of 132,000,000 : 1 (Wang et al., 2009), although this can vary, which is significantly larger than any other material. An NT is very thin, being approximately 10,000 times thinner than a human hair and hollow in the middle. Nanotubes are normally divided into two specific types, which are specifically related to the amount of walls contained in the structure; SWNTs (single-walled nanotubes) which have good light absorption properties and efficiency, and MWNTs (multi-walled nanotubes) which have slightly different properties. The SWNT, although any length, normally has a diameter of approximately 1 nm, as the structure is basically a one atom- thick layer of graphite which is wrapped until both ends meet, forming a tube. Obviously it is harder to define the size of the multi-walled nanotube as this depends on the amount of walls. The main reason that the solar industry is interested in nanotubes as a third generation PV is related to the properties associated with its materials and structure. The following points are interesting facts about nanotubes commonly known in the industry:
• They are as elastic as a rubber band.
• Impressive thermal conduction which is twice that of diamond.
• Withstand temperatures of 2700°C.
• Do not react with other atoms.
• Better electrical conductor than copper (100 times).
• Depending on their structure, their electrical properties will either be metallic or semiconductor.
• Conduct at 109 amperes per square cm.
• Thermal properties vary depending on the location – along the tube there are good thermal properties whereas other locations in the tube provide good insulation.
• Electrical properties change depending on what material is attached to them.
Although nanotubes have all these advantageous properties, their practical implementation is still ongoing, with research and development looking into ways to ensure they reach their maximum potential. Carbon nanotubes are of interest in nanotechnology but are not limited to this field; it is expected that electronics, optics and material science amongst others would also be considered.
Currently, two main areas where nanotubes are being used which have been published are:
• NASA’s decision to develop a composite material from NT and another material to be used in spacecraft.
• The delivery of medical drugs by attaching the drugs to the nanotubes.
Although these two examples are not in the photovoltaic field, they demonstrate how broad the use of nanotubes could be. However, this chapter is specifically interested in their relationship to support and enhance the development of the third generation PV and its continuous aim to develop a panel that uses materials that have a higher conversion efficiency rate.
Nanotubes are widely believed to be one of the forerunners in third generation PVs and it is expected that in the next decade their efficiency and cost will improve significantly (Anon., 2010). If this development happens, it will be due to several advancements that are linked together to provide the ideal material for photovoltaic panels. In the future, with these improvements it is expected that panels will not only withstand extended sunlight but also be able to be bent into any form due to their physical characteristics. They also accept high currents but withstand the effect of the heat generated. A very important point for any material being introduced into a system is that there should be no reaction with its surroundings and the NT complies with this. Finally, but importantly, it must provide good conduction without loss of energy to friction.
One of the most important environmental aspects is that the NT is both easy to reuse (Scharber, n.d.) and is biodegradable, hence it can be decomposed, answering one often-posed question about how to deal with the end-of-life cycle of renewable energy products. However, more pressing issues are related to problems and obstacles that need to be overcome to ensure that NT PV have a chance of succeeding in this industry. The issue of cost effectiveness needs to be addressed, as does the improvement of mass production to ensure quality products. In addition, the high temperature needed to produce the nanotubes could affect other components used in the solar cell/panel, hence this needs to be considered. One technical issue that needs more research is the problem with low fill factor which is only adding to the low energy conversion efficiency problem.
Carbon nanotubes are currently used in several areas of the PV device. One such area is the photoactive layers, which are expected to provide more efficient PV devices due to the nanotube and polymer junctions. The high electric field created here enables splitting up of the excitons and also in this structure the SWNT allows a pathway for the electrons to travel along (Kymakis et al., 2003). Unfortunately, this is currently very inefficient and issues with the combination of the NTs are still occurring, leaving the way open for further improvements.
Another approach is to use CNT as a transparent electrode to replace the currently used indium tin oxide (ITO) which could solve issues with its lack of compatibility with polymers, poor mechanical properties, expensive production costs and material cost and availability. Currently the option of ITO over CNT provides comparable efficiency but again more work is required to make it a practical option.
Finally, one further option to enhance the flow of current between the cells is to use graphene as a possible electrode, again as an alternative to ITO. Graphene offers an inexpensive, transparent and flexible omnipresent carbon substrate in a flat chicken-wire form with the major problem to date being the joining of the graphene to the panel due to it not accepting water solutions. However, it has been suggested by Park et al. (2010) that this issue could be overcome by doping the cell surface, hence including impurities, which in turn enables the graphene to be accepted. Added to this is the fact that the overall conductivity is improved and this could in theory solve some of the issues seen in the previous generation, offering a lightweight, flexible, transparent PV cell, thus leading the way to other PV installation options.
Research has discussed many aspects related to carbon nanotubes. Not only can they be combined with polymers and bucky balls, which is a hollow spherical module completely made of carbon, to form a painted PV product, the NT can also be coated by both p and n type semiconductors which provides a p-n junction for electricity generation. In 2002 SWNT and polymer PV devices were seen to provide a doubled photocurrent, but this was not the last advancement, rather the start of this development. Also the substrates which the CNT are placed on are currently polyethylene tere- phthalate, glass, polymethyl methacrylate and silicon, but this research is only in the primary stages and could be further expanded. A specific example is naphthalocyanine (NaPc) dye-sensitized nanotubes which use nanotubes as electrons and polymer as holes and provide increased absorption of both red and ultraviolet as well as larger short circuit current (Kymakis and Amaratunga, 2003). It is not only the quantum dot that enables more output from the electron; the NT also offers this possibility.
Work is ongoing and, in 2011, the University of Surrey Advanced Technology Institute published work on the combination of organic solar cells and MWNT, showing that both SWNT and MWNT are progressing (Miller et al., 2006). Also the development of the CNT will include matching to the related solar spectrum, improved optical absorption and the reduction of carrier scattering. To date there are three main production methods for nanotubes. The method that is currently used most is arc discharge (Iijima, 1991; Ebbesen and Ajayan, 1992) which basically consists of a high current being placed between two graphite electrodes in a helium atmosphere producing a nanotube that has a maximum length of 50 micrometres (Collins and Avouris, 2000) with a product yield of 30%.
The second is laser ablation, which is used for single- and multi-walled nanotubes (Guo et al., 1995a, 1995b) but mostly SWNTs. In the basic process, the laser changes a graphite piece into vapour in a reactor, while at the same time gas is drawn into the chamber and the nanotube forms on the cold surfaces in the reactor. Although this yields 70%, which is more than arc discharge, it is also the most expensive method (Collins and Avouris, 2000). The final method, which was also mentioned in the quantum dot section, is CVD (chemical vapour deposition) (Bunch et al., 2005). It is expected that CVD will probably be the most commonly used method in the commercial production of carbon nanotubes in the future and it is expected that it could reach industrial scale during this century.
In the future, with improved production, or by varying nanotubes, efficiency will improve (Landi et al., 2005; Cataldo et al., 2012), costs will reduce, the expected efficiency of 71% will be closer to reality (Anon., 2006) and properties such as lifetime can be realistically considered.
12.6 Future trends
The future development of PVs is principally split into three areas – countries’ governments, the scientists/researchers, and consumers, with each of them having a varied amount of responsibility. Governments are responsible for encouraging and supporting the research, development and integration of its progression into each country and they could introduce subsidies and incentives as encouragement for consumers and companies. In developing countries specifically there are many options for the use of solar power in remote locations that could enable up to two billion more people to have electricity; for example, the kerosene lamps that are currently used in villages could be replaced by a 5 W system with a battery backup for approximately $500 per village.
In developed nations energy demands are high so solar energy is an additional power resource to support the traditional sources. In many large developed countries, power accessibility is not the problem, and rather the issue is the installation of distribution lines, which can cost up to $30,000 per km. Therefore, for remote locations, a PV array would be a more suitable and cheaper alternative.
Governments worldwide are definitely making plans for solar power to be part of future energy strategies, and this is supported by the fact that in the last five years the cost of this technology has reduced by about one-third, and with all the research and development this can only improve. Europe is one of the leaders in this field and it is hoped that by 2020 it can produce 688 GW of power, with expectations nearly double this target by 2030. This would ensure that by 2020 Europe would produce 12% of its power from the sun, with the sunbelt countries potentially hitting the same target by 2030. Worldwide, 9% of all power could feasibly be provided by solar energy by 2030, with this increasing to 20% by 2050. However, it is not just about the energy produced; countries are also embracing it as a sustainable clean energy source, hence a way of cutting their carbon dioxide emissions and helping impact positively on climate change. If solar energy continues to increase, it is predicted that by 2050 4,047 million tonnes of CO2 could be cut worldwide every year. The last but equally important factor, due to the global financial climate, is the added bonus of jobs using this technology and Fig. 12.8 shows the possibility for this over the next 20 years. All these compelling arguments should continue to drive governments to keep striving to enhance their implementation, research and development of solar technologies.
The most major obstacle to implementation of the technology is that, despite silicon being the second most abundant element in the earth’s crust, for it to be used in PVs it must be purified to a high level to produce better efficiency. However, to achieve the correct purity the cost is $40 per kg. The simplest solution to this problem may be to use a lower purity of silicon but unfortunately to date this is not an option that has been researched to an acceptable level.
There is also a lot of work ongoing in the solar technology field to challenge other technologies in the race against thin film to beat the $1 per watt production cost. Rivalry is not always a bad thing. On the development side, advancements in other industries are having an impact on PVs; the flat screen TV has meant that larger sheets of glass are readily available and progress in the semi-conductor industry has provided second-hand machinery that can produce thicker cells. All these factors can help to reduce the cost of production, but we must also keep in the forefront of our minds that the real aim is about refining it to be more efficient.
The greatest advancement to date might be the emergence of screen printing on solar cells as it allows the option of PV roofing shingles that could easily be added to any current or future home. This new method will also enable different PV mounting frames and positions to be considered, a factor that has always impacted on the full PV system. However, in the future it is expected that the greatest advancements will be in third generation technology, specifically related to nanotechnology.
Finally, the consumer must be encouraged and educated in various ways such as advertising, literature, etc., to make the choice to embrace this clean source of energy, even if it is currently more expensive than traditional energy. Its use in devices such as a simple solar powered calculator or garden light is broad, but further devices that we use on a daily basis need to be considered to ensure less effect on the environment than in previous generations. More consumers purchasing PVs will obviously have an effect as it will introduce more competition in the market, thus not only lowering cost but also challenging further and new developments, which can only be a positive step forward for all.
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