Thin Film Solar Cells

1. Energy demand and its resources / 19
2. Solar energy for electricity / 35
3. Basics of semiconductor physics / 61
4. P-N junction and analysis / 111
5. Thin film solar cells / 149
6. Analysis and characterization methods for solar cells / 195
7. Limitations and future directions / 235
Index / 257

Chapter 1 Energy demand and its resources

1.1. Introduction
A large number of energy sources are available to human being on this earth to provide their day-by-day energy needs. However, the use of particular energy source should be made in each case on the basis of economic, environmental and safety considerations. In this chapter, we will consider briefly the world energy consumption that depends on these number of available energy resources, the use of these energy sources to provide our daily needs, the problems associated with these energy sources and some several scenarios for future.

1.2. Energy resources
There are two types of energy sources available to mankind, which can be categories as: (1) non-renewable energy sources, i.e. those sources of energy which cannot be replaced or produced on any time scale less than million years once used; and (2) renewable energy sources, i.e. those sources of energy which are available naturally for millions of years. Table 1.1 shows a listing of these energy sources according to their category.
In Table 1.1, the energy sources are listed as per their use by mankind to provide its daily uses. The non-renewable energy sources, which are the prime sources of energy for mankind, will eventually become exhausted, while the renewable energy sources will not. Therefore, we should consider the global energy consumption in order to know how long will be the non-renewable energy sources last.

Table 1.1. Different types of available energy sources to mankind

1.3. Energy economy

1.3.1. Global energy consumption
Global energy consumption has about doubled in the last three decades of the past century. The most recent estimate of the world energy consumption was 5.67×1020 joules, or 157,481 TWh, in 2013. According to the Energy Information Administration (EIA), the total world energy consumption was 143,851 TWh in 2008, 133,602 TWh in 2005, 117,687 TWh in 2000, and 102,569 TWh in 1990. In 2013, about 81.4% of the primary energy consumption was from fossil fuels (oil 31.1%, coal 28.9%, natural gas 21.4%), 4.8% from nuclear fuels, 10.2% from biofuels and waste and the remaining 3.6% from renewable energy sources (out of which 2.4% from hydro and 1.2% from ‘other’ (solar, wind, geothermal, heat, etc.)). Oil, coal, and natural gas were the most popular energy fuels [1]. The world demand for energy is expected to increase steadily until 2030 according to many scenarios. Global primary energy demand is projected to increase by 1.7% per year from 2000 to 2030, reaching an annual level of 15.3×109 tons of oil equivalent (toe). The projected growth is, nevertheless, slower than the growth over the past 30 years, which ran at 2.1% per year. The global oil demand is expected to increase by about 1.6% per year from 75×106 barrels per day to 120×106 barrels per day [2]. The steady increase in world’s population and to strive for better development and comfort are the two main reasons for the increase in the energy consumption at all times. The world population is expected to almost double in the next 50 years. Thus, energy demand could double or triple as population increases and developing countries expand their economies and overcome poverty by 2050. With this increase in the population, the per capita consumption of energy is also increasing. This rise in population as well as in the economy are the two main parameters that will cause to increase the energy demand in the coming decades. With such increasing energy demand, it is accepted that there will be energy crises in the near future due to limited amount of readily available fossils fuels.

1.4. Fossils fuels and its impact on environment
The fossil fuels such as coal, oil, and natural gas (also known as non-renewable energy source), are limited in reserves and it is predicted that the demand will soon exceed the supply based on the continues increasing energy consumption. Therefore, it is necessary to reduce their exploitation rate by partial replacements, especially through the use of renewable energy sources. In addition, the increased use of these non-renewable energy sources resulted into several negative environmental impacts [3]. Burning of fossils fuels has caused global-scale increase in CO2 in the atmosphere, which may lead to global climatic change in the near future. The use of fossils fuels in automobiles industries produces exhaustive gases that increase the air pollution as well as the surface ozone concentration, which is very dangerous to human health and environment. Therefore, it is today’s need to opt the available renewable energy sources to get rid of or/ reduce these unwanted and damaging effects to environment as well human health.

1.5. Renewable energy sources

1.5.1. Wind energy
Wind is a form of solar energy. The uneven heating of the atmosphere by the sun, the irregularities of the earth’s surface, and rotation of the earth causes the wind energy [4]. The flow patterns of wind are modified by the earth’s terrain, bodies of water, and vegetative cover. The modern wind turbines, which are used to generate electricity by harvesting this wind flow, or motion energy [4].

1.5.2. How wind power is generated
The kinetic energy of wind can be changed into other forms of energy, either mechanical or electrical energy and this process can be termed as “wind energy” or “wind power”. Wind turbine is a device that converts the kinetic energy from the wind into mechanical or electrical power. The mechanical power can be used for specific tasks (such as grinding grain or pumping water).

1.5.3. Wind turbine types

Fig. 1.1. Photo images of horizontal axis and vertical axis wind turbine designs.
(Sources: httpss://en.wikipedia.org/wiki/Wind_power and httpss://en.wikipedia.org/wiki/Troposkein)

Wind turbines power an electric generator by turning in the moving air and deliver an electric current. In simple words, a wind turbine is the opposite of a fan. Wind turbines use wind to make electricity, whereas fan uses electricity to make wind.
Modern wind turbines are categorized into two basic groups; the horizontal-axis design, like the traditional farm wind mills, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.
1.5.4. Turbine components

Fig. 1.2. Schematic representation of wind turbine with its each component.

Fig. 1.2 shows the schematic representation of horizontal axis wind turbine with its different components. blade or rotor, which converts the energy in the wind to rotational shaft energy; adrive train, usually including a gearbox and a generator; a tower that supports the rotor and drive train; and other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment.
Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants.

1.5.5. Wind turbine size and power ratings
Wind turbines are available in a variety of sizes, and therefore power ratings. The largest machine has blades that span more than the length of a football field, stands 20 building stories high, and produces enough electricity to power 1,400 homes. A small home-sized wind machine has rotors between 8 and 25 feet in diameter and stands upwards of 30 feet and can supply the power needs of an all-electric home or small business. Utility- scale turbines range in size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.

1.5.6. Solar thermal technologies
Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy or electrical energy for use in industry, and in the residential and commercial sectors. The first installation of solar thermal energy equipment occurred in the Sahara approximately in 1910 by Frank Shuman when a steam engine was run on steam produced by sunlight. Because liquid fuel engines were developed and found more convenient, the Sahara project was abandoned, only to be revisited several decades later [1].
Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high- temperature collectors. Low-temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for heating water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for fulfilling heat requirements up to 300 deg ℃ / 20 bar pressure in industries, and for electric power production. Two categories include Concentrated Solar Thermal (CST) for fulfilling heat requirements in industries, and Concentrated Solar Power (CSP) when the heat collected is used for power generation. CST and CSP are not replaceable in terms of application. The largest facilities are located in the American Mojave Desert of California and Nevada. These plants employ a variety of different technologies. The largest examples include, Ivanpah Solar Power Facility (377 MW), Solar Energy Generating Systems installation (354 MW), and Crescent Dunes (110 MW). Spain is the other major developer of solar thermal power plant. The largest examples include, Solnova Solar Power Station (150 MW), the Andasol solar power station (150 MW), and Extresol Solar Power Station (100 MW).

1.5.7. Biomass energy technologies
There are many types of biomass ― organic matter such as plants, residue from agriculture and forestry, and the organic component of municipal and industrial wastes ― that can now be used to produce fuels, chemicals, and power. Wood has been used to provide heat for thousands of years. This flexibility has resulted in increased use of biomass technologies. According to the Energy Information Administration, 53% of all renewable energy consumed in the United States was biomass-based in 2007.

Fig. 1.3. A cogeneration plant in Metz, France. The station uses waste wood biomass as an energy source, and provides electricity and heat for 30,000 dwellings. (Source: Wikipedia)

Biomass technologies break down organic matter to release stored energy from the sun. The process used depends on the type of biomass and its intended end-use.

1.6. Global warming and climate change
Global warming and climate change are terms for the observed century-scale rise in the average temperature of the Earth’s climate changeand its related effects. Global warming and climate change refer to an increase in average global temperatures. Activities including natural events and human activities are responsible to be contributing to an increase in average global temperatures. This is caused primarily by increases in greenhouse gases such as Carbon Dioxide (CO2). Six main greenhouse gases are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), plus three fluorinated industrial gases: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Water vapor is also considered a greenhouse gas [1].
Energy from the sun drives the earth’s weather and climate, and heats the earth’s surface. In turn, the earth radiates energy back into space. Some atmospheric gases (water vapor, carbon dioxide, and other gases) trap some of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse. These gases are therefore known as greenhouse gases. The greenhouse effect leads to rise in temperature on earth as certain gases in the atmosphere trap energy.