Energy, the quality of life, and health
StarCraft, one of the main flagship games of Blizzard Entertainment features three different species that struggle to extract resources from the outer world. After taking up additional resources (energy sources) through battles, managements, and operations, the tribes overpower or destroy the opponent's camps.
Figure 1. A scene from StarCraft 2
Except for a few cases that the opponents are defeated with a special strategy in the very early stage, the most important thing in this game is securing the main resources, minerals, and vespene gas. One of them, which successfully secures the energy sources faster than the other parties, would hold a dominant position in terms of the population and technologies. In other words, as more energy sources are secured, the higher the chances of winning.
Similarly, securing energy resources is one of the most important things a country should ensure to maintain and develop its economic community and strengthen its competitiveness worldwide. Since the Industrial Revolution, a wide range of machines have contributed to the explosive improvement of productivity and convenience. Despite a steadily growing number of the world population, this has allowed humanity to avoid suffering from the acute shortage of materials. Furthermore, more people have been enjoying leisure and cultural activities because of surplus resources incomparably than ever before.
On the other hand, energy consumption used for productivity and mobility has drastically increased. In particular, fossil fuels, which are conventional energy sources, will certainly run out someday, given their finite and nonrenewable properties. All of a sudden, faced with the depletion of fossil fuels, humankind will have to give up the lives they are enjoying now. This may result in severe damage to their livelihoods and health, including the economy.
In addition, fossil fuels cause serious environmental issues, such as air and water pollution. Thus, to ensure health and well-being, human beings have dedicated themselves to research on the production of eco-friendly alternative energy, also known as renewable energy sources, since decades ago. Along with the scale of studies, the renewable energy market has rapidly been growing through hydropower, wind power, and solar power.
Efficiency, shapes, and economic feasibility of photovoltaic cells
Photovoltaics (PV), among others, is an appealing alternative energy technology compared to wind power with unstable output and hydropower that causes concern over the destruction of the ecosystem. This technology is based on a clean and infinite energy source from the sun. It also has constant annual output.
Photovoltaics is the conversion of light into electricity through the PV effect. When semiconducting materials inside PV cells absorb the energy of light, electron-hole pairs are created, both of which are then separated in the opposite directions. A semiconductor material mainly used in PV cells is silicon, which features an appropriate conversion efficiency (approximately 15%) compared to its affordable price.
A boost in PV sales depends on the question, "How much can PV cells save on electricity bills?" The installation cost will be offset by the saved electricity costs when used, for example, for about six years in a detached house in a nonurban area. However, the initial installation cost is significant, considering that the lifespan of solar panels is 5 to 10 years. This inhibits more sales and availability of PV cells.
If solar panel efficiency increases from 15% to 20%, the breakeven point will be shortened from 6 to 4.5 years on average, which will have a positive influence on sales. However, this is also challenging because efficiency increase requires additional production costs.
Either compound semiconductors instead of silicon or tandem solar cells with multiple junctions can be used to produce high-efficiency PV cells. However, the latter demands multiple processes, which, in turn, increases production costs. The former also brings about demanding issues: it emits harmful substances, such as cadmium (Cd) and indium (In), and uses costly raw materials.
Therefore, even if high-efficiency PV materials can be used, they must be harmless to the human body. In addition, the process cost needs to be reduced so that the cost increase can be less than the improved efficiency. In recent years, researchers have been working on the development of new materials combined with elements, such as copper (Cu) and sulfur (S), or tandem cells of junctions combined with those materials.
PV materials research and computational science
Research and development of PV materials is largely concentrated on the methods of improving the absorption coefficient of PV materials and the efficiency of charge separation in tandem cells. Determinants of the improved absorption coefficient include the appropriate band gap size, the orbital shapes of the valence band maximum and conduction band minimum, and the absence or presence of a direct band gap. The orbital shape or band gap size can be changed by adding heteroatoms to basic PV materials, such as gallium arsenide (GaAs) or copper-indium-gallium-diselenide (CIGS).
The key technology for a multi-junction system lies in the selection of materials with high mobility that can facilitate charge transport by controlling the arrangement and selection of adjacent materials and interfaces. The introduction of impurities (doping) to reduce the band gap may decrease the mobility, or doping to increase the mobility may cause the change in the orbital shapes of the valence band maximum and conduction band minimum, thereby decreasing the absorption coefficient. In either case, the PV efficiency can be reduced.
As such, it is difficult to develop PV materials and process control systems that meet all the requirements mentioned. The bottom line is that changes in the electronic structure should be induced by altering the process conditions and composition of the materials. An effective method for doing so is to use Ab initio quantum chemistry methods rather than numerous experiments. The density functional theory (DFT) can also be used to predict the thermodynamic equilibrium composition and the resulting absorption coefficient, thereby understanding the influences of defects or compounds on the charge separation and mobility.
In general, however, the approach of Ab initio quantum chemistry methods is technically difficult, so insufficient experience may cause incorrect parameters to be set, leading to miscalculations. Therefore, it is recommended to commission a consultancy or utilize professional programs well-organized for the research and development of each material.
Photovoltaics (PV) is a technology that should be continuously developed and used for humankind to enjoy the beautiful continuation of present and future life. Its economic feasibility depends on the efficiency of PV materials. In addition, the more PV systems are used, the more jobs can be created in new technology fields in conjunction with environmental conservation. As the PV material efficiency is determined by the bulk and interfacial electronic structure, Ab initio quantum chemistry methods can be effective in resolving difficulties with experiments and speed problems.
Group Leader, Institute of Inorganic Chemistry, University of Cologne