1. Chapter summary

Electron energy materials are semiconducting materials that are able to directly convert the energy of electrons into electrons, light, heat and other forms of energy. What makes electron energy materials different from ordinary electronic materials is that they are semiconducting materials that are used for energy devices. Electron energy materials are attracting a lot of attention for their applications in renewable energy and their potential to make energy use more efficient, and are seen by many being advanced materials that hold the key to solving environmental and energy problems. Materials that convert mechanical energy into electron energy or vice-versa will be introduced in Chapter 6 that discusses mechanical response materials.

Section 3.2 of this Chapter will discuss solar cell materials that convert light energy into electron energy. People all over the world are interested in these materials that can produce electrical energy from the inexhaustible supply of sunlight, and the competition between developers is fierce. The current research trends and technical themes can be summarized and classified into the following categories: silicon (Si), CIGS, dye-sensitized, organic thin-film, and quantum dot systems.

Section 3.3 will look at LED materials that convert electron energy into light energy. Japan leads the world in the development and application of LED materials, and this section will discuss current themes from a technical perspective.

Section 3.4 will discuss (power) electronic materials that convert electron energy to electron energy. Electronic materials are used in devices that are an intimate part of our daily lives. Section

3.4 will use the PC and digital consumer electronics as examples and give an overview of the current situation and the future, from the viewpoint of the environment and energy.

Section 3.5 will touch on thermoelectric conversion materials that can convert heat energy into electron energy and vice-versa, and will discuss their applications as well as present and future technical issues. Compared with other electron energy materials, thermoelectric conversion materials will come into their own more in the future.

2. Key words from material science

All the electron energy materials discussed in this Chapter are semiconductors that have a band gap. To organize the material science factors that determine the conversion efficiencies of solar cell materials, LED materials, (power) electronic materials and thermoelectric conversion materials, the issues important for the raising of efficiency are listed under the “key items” and areas where they apply are listed under the title “keywords.” Regardless of whether the material is organic or inorganic, these are abstracted material science topics. The results are shown in Table 3.1.1.

There are six “key items”: material structure control,” “impurity control,” “band gap control,” “conduction mechanism control,” “space-charge layer control,” and “heat management.” In this Chapter, we will attempt to explain each type of material in terms of the factors in the “key items.” This is to provide a wideranging view of different types of materials and scientific fields. The keywords used in this paper are shown in bold.

1. Fundamentals of solar cell materials

Solar cells are devices that convert directly the energy of light into electrical energy by utilizing the light absorption, photoconductive effect and photovoltaic effect of materials. The most typical of these are crystalline silicon (Si)-based solar cells. Structurally, these are pn-junction diodes that have a large surface area to capture sunlight. Fig. 3.2.1 illustrates the principle of the pn-junction solar cell. Light absorption within the semiconductor creates electron-hole carrier pairs. The internal electric field at the pn-junction causes a drift effect which polarizes the carriers. The polarized carriers are collected at the electrodes, which causes a photoelectromotive force. The increase in conductivity of a semiconductor due to an incident light, ∆σph, can be expressed by the following equation 1):

,

where e and h are the mobility of electrons and holes; τe and τh are the lifetime of electrons and holes, respectively, g is the generation rate of photo-generated electronhole pairs in unit time, e is the elementary electric charge. The absorption of light in a semiconductor occurs basically by transition of photo-excited electrons from the valence band into the conduction band. Therefore, incident light with photon energy higher than the energy band gap is absorbed. Fig. 3.2.2 shows the absorption coefficient spectra of the semiconductors used for prevailing solar cells. The performance of a solar cell is also closely related to the band gap. The smaller the band gap, the greater the short-circuit current density and the lower the open-circuit voltage. At a band gap of some 1.4 eV where both of them attain a proper balance, the theoretical limit energy conversion efficiency (i.e., ideal solar cell efficiency) of a single-junction solar cell for the solar spectrum is obtained around 30%, which is the maximum value.

Fig. 3.2.1 The operating principle of a pn-junction solar cell

Fig. 3.2.2 Absorption coefficient spectra 1)

2. Research trends

World solar cells market has rapidly expanded in the last few years, propelled by environmental concerns, skyrocketing crude oil prices and government policies. At present, bulk crystalline Sibased solar cells, such as monocrystalline Si solar cells and polycrystalline Si (poly-Si) solar cells, comprise 90% of the market. However, market growth has resulted in a Si raw material shortage, and there is an urgent need to develop manufacturing technologies for mass-production of solar-grade poly-Si at low cost.

Because of the shortage of Si raw materials, material saving thin-film solar cells including thin-film Si-based solar cells and thin-film Cu (InGa) Se2 (CIGS) compound solar cells, have attracted much attention as the next generation solar cells suitable for popularization of photovoltaic system. They have also the advantage of requiring lower energy input and causing lower levels of greenhouse gas emissions in their manufacture. Researches into thin-film Si-based materials have been promoted in Japan as part of national projects “Sunshine Project” (from 1974) and “New Sunshine Program” (from 1993) that were prompted by the Oil Shock of the 1970s. Studies to elucidate the mechanism of photo-induced deterioration phenomena of amorphous Si (Staebler-Wronski effect) have been ongoing. Actually, researchers are trying to control nanostructure associated with this effect by controlling film growth process. The conversion efficiency has been improved by techniques that include using amorphous SiC in the p-type window layer of the pin structure, light confinement techniques and tandem-junction structure. Recently in Japan, amorphous Si cell / microcrystalline Si cell tandem junction modules with a conversion efficiency approaching 12% have been commercialized. For development of multiple junction solar cells, researches on new materials such as microcrystalline SiGe and microcrystalline SiC are underway. Flexible

1.The operating principle of LEDs and the materials features

A p-n junction diode is formed from an n-type semiconductor with electron conducting properties and a p-type semiconductor with hole conducting properties. Minority carriers are injected across the junction in a forward direction, resulting in the recombination of electrons and holes. If energy is released in the form of light at this point, then the diode can be used as a light emitting diode (LED). Fig. 3.3.1 shows the structure of an LED. Essentially, the LED is a p-n diode, but with modifications near the light emitting layer to improve the efficiencies of recombination luminescence and the optical output. LEDs have a chip size of about 300 microns, where a current ranging from several dozen to several hundred mA is consumed. The driving voltage varies, depending on the LED material, but is in the range of about 2 to 4 volts. Compared with a conventional incandescent light bulb, the energy efficiency of an LED is far greater, and could be as high as 70% or more (the external quantum efficiency of a blue-light LED) with structural optimization. Nowadays, with the drive to conserve energy, applications for LEDs are being found in various kinds of indicators of electrical equipments, traffic lights, Christmas decorations and other uses. The key to use in these kinds of applications has been the success of the blue LEDs. LED “light bulbs” that use white LEDs and are as bright as conventional 40or 60-watt bulbs are already being marketed. LEDs will also find greater use in fields where they can take advantage of their superior monochromatic qualities.

The first thing to consider in an LED material is the energy band structure of electrons. The wavelength of the emitted light depends on the band gap. The luminous efficiency is determined by whether or not the phonon process is required for the recombination of electrons and holes (whether the semiconductor is a direct or indirect transition type). At the practical application stage of an LED, the impurity doping techniques (to modify the electronic properties of the semiconductor), the level of impuri-

Fig.3.3.1 Basic structure of double heterojunction LEDs

Fig. 3.3.2 An example of the relationship between peak wavelength and external quantum efficiency of a LED using a wide-gap material

ties and the control of crystal imperfections are important factors. Fig. 3.2.2 shows an example of a plot of the external quantum efficiency in charge injection light emission vs. peak wavelength. Basically, the wider the gap, the lower the luminous efficiency. This is because trap levels are easily formed within the gap accompanying crystal imperfections, recombination luminescence is hindered, and the deep impurity levels common to wide-gap materials become a serious problem, causing increased losses due to electrical resistance.

2. Research trends

The development of gallium nitride ultra-violet LEDs used in blue light diodes, as well as deep ultra-violet LEDs that use widegap materials, are still in the research stage however it is already attracting a lot of attention. Wide-gap materials include gallium- aluminum-nitride 1), aluminium-nitride 2), boron-nitride 3) and diamond 4), 5). Ultra-violet LEDs emitting light with wavelengths in the 355 to 375 nm range have already been partially commercialized and are used in paper currency recognition devices, air purifiers and other devices. It is hoped that the deep ultra-violet LEDs, with tunable wavelengths and abilities to kill bacteria, will be used for a wide range of devices, including high-quality whitelight sources and the next generation of high-density data storage. At present, the external quantum efficiency of these LEDs at a peak wavelength of around 260 nm is no more than 1%, and near 240 nm it is very low –around 10-4 %. Nevertheless, diamond LEDs utilize light that is emitted by the recombination of free