solar cells research paper

INTRODUCTION

Currently 86% of the world’s energy comes from fossil fuels, which pollute the environment in

all phases of their lifecycles, from resource recovery to refinement processes to their final

consumption as fuel (Allen 2010). The combustion of hydrocarbons necessarily releases carbon

dioxide, the most prevalent of the set of greenhouse gases that have been determined to

contribute to global climate change (IPCC 2007). The use of coal, America’s leading source of

electric power (Allen 2010), is particularly deleterious to the environment; in addition to

contributing to greenhouse gas emissions, coal has been implicated in the annual global release

of tens of thousands of tons of radioactive isotopes into the atmosphere (Gabbard 1993). Almost

all power plants consume large amounts of fresh water from lakes and rivers to cool their heat-

intensive generators, a resource cost that is often overlooked when considering the impacts of

energy production (Smith et al. 2005).

Solar energy currently accounts for less than 0.1% of all American energy production (Allen

2010). While the sun’s energy is free, efficient solar energy devices are expensive; production

requires the investment of large capital costs, making it the most expensive source of energy on

the basis of cost per kilowatt-hour (Patel 2005). Therefore, without large government subsidies,

tax incentives, and/or carbon taxes, the market will continue to prefer less expensive—but more

environmentally harmful—traditional fossil fuels. Additionally, the availability of solar energy is

strongly a function of season, time of day, climate, and geography, so the electric grid and

transportation infrastructure must be adapted to allow for a higher degree of intermittency if solar

power is ever to supply a major portion of national and international energy demand.

If these problems of production and implementation are overcome, solar energy has the potential

to provide a potent alternative energy source and decrease humanity’s dependence on traditional

energy sources. Moving away from the use of hydrocarbon and nuclear fuels reduces the

emissions of harmful substances into all environmental media, while simultaneously providing a

more sustainable source of energy. The problems associated with fossil fuels are of growing

international concern when faced with the possibilities of global climate change and decreasing

supplies of fresh, unpolluted water. The depletion of the planet’s available deposits of fossil fuels

and nuclear fuel material is inevitable if no viable alternative source is found. If successful, solar

energy would be able to postpone, or even prevent, the exhaustion of these resources.

This report discusses two aspects of solar energy production: technology and implementation.

The section on technology provides details on currently available technology, focusing on the

variety of photovoltaic options and ending with a brief discussion of thermal solar systems, and

then discusses possible future technologies that are in development. The section on

Justin Chandler, Ariel Williams 2

implementation outlines the problems that have inhibited the widespread utilization of solar

energy, proposed solutions, and government and corporate programs to develop this technology

into an economically viable energy source.

Our discussion is focused primarily on integration efforts in the United States, with the exception

of our discussion on thermal solar energy storage, which uses a Spanish facility as a model.

While Europe leads the world in solar energy production (Patel 2005), the governmental policies

that have driven these endeavors are too numerous and variable to discuss in any depth in this

report.

In our discussions that follow, we mark words with asterisks to indicate that they are defined in

the glossary on page 14.

TECHNOLOGY

Solar energy production methods implement either thermal or photovoltaic processes.

Photovoltaic devices utilize what is known as the photoelectric effect—the phenomenon in

which photons* of light are absorbed in a material to excite ground-state electrons to higher-

energy conductive states, generating pairs of mobile charge carriers: excited electrons to carry

negative charge, holes* to carry the positive charge in solid state devices, or ion* pairs to carry

either charge in liquid state devices. Such a device is constructed in a manner that forces the

negative and positive charge carriers to flow preferentially in specific and opposing directions,

observed as an electric current that can be use to supply electricity to a home or to the electric

grid.

In thermal solar energy generation, the device is constructed such that the electrons flow equally

in all directions, producing no net electric current; the electrons simply emit their acquired

energy in the form of heat. This heat is captured by a storage medium such as water, concrete, or

molten salt that transfers energy to a closed loop of water, generating steam to power a turbine.

How Does a Solar Cell Work?

Figure 1 shows the general structure of a photovoltaic device. The conversion of solar energy to

electric power occurs in the two semiconductor layers that have each been mixed—or doped*—

with different electrically active impurities. The n-type material contains impurities that accept

electrons and donate holes, giving it a net negative charge. The p-type material is doped with

impurities that donate electrons and accept holes, giving it a net positive charge. The front

contact—often a mesh of thin silver wires—is mounted on top of the n-type semiconductor layer

to collect and transmit electrons to the circuit. The circuit is completed by mounting the

semiconductor layers to the back contact made of aluminum foil. The semiconductor layers are


covered by an anti-reflection coating to maximize the absorption of light, which is glued to a

layer of protective cover glass by a transparent adhesive (Patel 2005).

Figure 1: Standard Structure of a Photovoltaic Device (Patel 2005)

Characteristics of an ideal photovoltaic solar cell include the following:

1. High efficiency: the ratio of electricity produced to incident light for the standard

sunlight spectrum

2. Low band gap: the amount of energy required to excite an electron from its ambient

condition in the valence band* to an energetic condition in the conduction band*.

Lowering the band gap increases the portion of the spectrum that can be absorbed and

converted to electricity.

3. Long lifetime of operation

4. Very conductive materials: to avoid losing produced energy as dissipated heat

5. Inexpensive to produce: use the minimum amount of expensive materials, including

crystalline silicon and transition metals such as ruthenium, while still maintaining the

above characteristics.

Current Technologies

Below we discuss a variety of solar energy generation technologies, starting with photovoltaic

devices and ending with a brief discussion on thermal solar systems.


Silicon Wafers

Most silicon wafer cells exactly follow the standard structure in Figure 1. The general method of

production is relatively straightforward: a silicon wafer is first doped throughout with p-type

impurities using standard ion implantation techniques followed by implanting a thin layer on the

surface with an excess of n-type impurities. Aluminum and silicon intermix at temperatures

above 450˚C, so the wafer can be mounted to the aluminum back contact by heating to such a

temperature (Patel 2005). Below we discuss the two main varieties of silicon wafer solar cell

technology.

Crystalline Silicon Cells

Crystalline silicon cells are the most commonly used devices for large scale photovoltaic energy

production, with efficiencies of 16–20% (Patel 2005). Application of this technology is

hampered by the large costs associated with producing the required high-purity crystalline

material.

Amorphous Silicon Cells

Amorphous silicon cells are currently the least expensive and lowest quality cell (Patel 2005).

The bulk material is non-crystalline, which makes it far less expensive to produce than

crystalline silicon. In addition, the thickness of the active layer in these devices is 1% of that in

crystalline silicon devices, further decreasing material costs. Current devices have efficiencies of

approximately 8–10%, and are implemented in a variety of low-current applications, including

solar-powered calculators (Kadixy 2010).

Heterojunctions

A heterojunction is a device that interfaces two or more dissimilar semiconductors having unique

band gaps. When stimulated by light, electric fields are generated, causing charge carriers to

become mobile and electric current to flow. There are a variety of photovoltaic devices that

implement this architecture, and we discuss the most common varieties below. Figure 2 shows

the general structures of the three types of heterojunction devices we discuss, where the bottom

grey layer represents the conducting metal on which the active layer is mounted, the blue

represents the material that donates electrons, and the pink represents the material that accepts

electrons.

Bulk Heterojunction Cells

In a bulk heterojunction solar cell, the active layer is composed primarily of electron-accepting

n-type carrier material that can carry current a short distance to the conducting metal sheet on the

bottom. Intermixed in this material are regions of photoactive p-type material that can absorb

light and donate electrons to the carrier material. In this variety of device, the materials mix in a


disordered fashion to create microstructures with feature sizes determined by molecular-scale

interactions. Figure 2a shows the general structure of the bulk heterojunction.

Figure 2: Heterojunction Structures (Mayer et al. 2007)

(a) Bulk Heterojunction, (b) Ordered heterojunction, and (c) Multijunction (here a unijunction)

Ordered Heterojunction Cells

In ordered heterojunctions, the architecture of the interfaces between the two materials is much

more controlled than in bulk heterojunctions (see Figure 2b). The ordered microstructure

increases the probability that charge carrier pairs reach the correct material before recombining,

thus increasing the efficiency of the device (Patel 2005).

Multijunction Cells

All of the photovoltaic technologies that we have discussed so far have incorporated materials in

a way that gives the devices at most only two band gaps. Any incident photons that have

energies below the lowest band gap of a device will not be converted to electricity, but will

instead be reflected, transmitted, or absorbed as heat.

In contrast, multijunction cells—also known as tandem cells—incorporate multiple layers of

different materials, each with its own band gap. The layers are placed in order of decreasing

bandgap to maximize the absorption of high-energy photons. The most efficient solar cell device

ever produced, with an efficiency of 40.7%, is a bijunction cell incorporating three materials (in

order of highest band gap to lowest band gap): gallium indium phosphide, gallium arsenide, and

germanium (GaInP/GaAs/Ge). However, due to the high costs associated with controlling the

specifications of three alloys with a total of five metals, this particular device has been used only

for satellites and space applications, and is not likely to become a major component of the

terrestrial market in the near future (Patel 2005).

Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are composed of two elementary active components along

with some peripheral components, shown in Figure 3. An N3 dye is absorbed into a transparent

nanocrystalline layer of porous titanium oxide (TiO2), where it absorbs light and provides high-


energy electrons that are collected by the fluorinated tin oxide (SnO2:F) layer and sent to the (-)

terminal. The hole-transport material (HTM) is a polymer or an electrolyte* solution coated onto

the titanium oxide layer that carries holes* generated by the dye to a platinum wire mesh (Pt)

that carries them to the (+) terminal. The HTM also serves to return the electrons to the absorbed

dye to be re-excited (Shin et al. 2006).

Currently available DSSCs have efficiencies of 10–11%. As researchers continue to optimize

cell features and material selections, DSSCs become increasingly feasible as an economical

alternative to crystalline silicon cells (Grätzel 2004). While DSSCs have lower sunlight-to-

electricity efficiencies, the lower costs will make them more efficient on a kWh/$ basis.

Figure 3: Cross section of a DSSC (Shin et al. 2006)

Inorganic Thin-Film Cells

Inorganic thin-film cells offer another cost-effective alternative to crystalline silicon cells.

Examples include copper indium diselenide (CIS), cadmium telluride (CdTe), and copper indium

gallium selenide (CIGS). A number of companies have invested in the production of CIGS thin-

film cells. But General Electric (GE) announced that it plans to instead begin plans for the

manufacture of CdTe cells that, while have lower efficiencies than CIGS cells, are much easier

and less expensive to produce, since only two metals must be controlled instead of four. GE

plans to begin manufacture of CdTe cells in 2011 (LaMonica 2010).

Thermal Solar Energy

In thermal solar generation systems, solar energy is collected and used to heat a medium (usually

water or air) which does work on a turbine, producing electricity. It differs from all of the

photovoltaic technologies that we discussed above by virtue of the fact that thermal solar systems

convert sunlight to thermal energy, which is converted to mechanical energy and then to electric

power, whereas photovoltaic devices convert sunlight directly to electricity.

Thermal solar energy systems have an advantage over photovoltaics—they can produce

electricity during the nighttime. During the daytime, excess thermal energy can be collected and

stored in the form of high-pressure steam or molten salts. Sandia National Laboratory (2008)


reported a solar-to-grid conversion efficiency of 31.25% for a single-dish system, a new record

in thermal solar energy production.

Possible Future Technologies

Below we discuss new solar devices that have been the subjects of recent research.

Polymeric Thin-Layer Cells

Conjugated* polymers have received a great deal of attention in recent decades as possible

materials for use in thin-film solar cells because of their ability to absorb a large portion of

incident light. Thin film solar cells have the advantages of less expensive production methods

than conventional inorganic cells and of lacking the liquid components of conventional liquid

state dye-sensitized solar cells DSSCs, solving problems with leaks and evaporation that

decrease the operating lifetimes of the devices. A variety of devices have been developed,

including a DSSC in which a conjugated polymer is used as a hole-transport material (Shin et al.

2006).

Improved Amorphous Silicon Cells

Amorphous silicon has recently been a subject of research for possible alternatives to crystalline

silicon in photovoltaics. While amorphous silicon cells are already widely used for low-level

energy production—most notably in calculators—the devices are very inefficient, due to the low

conductivity of amorphous silicon. However, amorphous silicon is far less expensive to produce

than single-crystal or polycrystalline silicon. Therefore, if the problems in conversion efficiency

can be corrected, amorphous silicon cells will become an economically viable alternative to other

available devices (Patel 2005).

Photovoltaic Windows

Desilvestro and Hebting (2009) propose the application of semitransparent DSSCs to produce

photovoltaic windows. If inexpensive DSSCs are developed, then they are particularly suited to

such an application due to their ability to absorb light coming from all directions, unlike solar

cells mounted on layers of opaque crystalline silicon. If the absorption profile of the DSSC is

largely in the ultraviolet band of the sunlight spectrum, then a reasonable amount of energy can

be produced without decreasing the transparency of the window. And if the device partially

absorbs in the visible spectrum it could provide a useful replacement for tinted windows that

produces energy instead of wasting it reflection or heat losses.

IMPLEMENTATION

As of 2008, the rate of American solar energy production reached 8,775 MW, 792 MW of which

was grid-connected photovoltaic power. While solar energy currently accounts for less than


0.1% of energy production, the SEIA estimates that domestic photovoltaic production increased

by approximately 65% in 2008, making it the nation’s fastest growing energy source (SEIA

2009). Currently only 6.1% of American energy demand is met by renewable energy sources,

less than 1% of which coming from solar sources, being dominated instead by corn ethanol and

wind. (See Figure A-1 for a breakdown of energy sources and demand sectors.)

Below we discuss the challenges of utilizing solar as a major component of America’s energy

production portfolio, as well as solutions that have been offered to facilitate its incorporation into

the electric grid and transportation sector. We then discuss some examples of programs and

funding by government and the private sector to assist individuals and corporations in the

installation of solar generation capacity.

Challenges

Below we discuss the challenges that have been cited most often as barriers to the future

widespread implementation of solar energy.

High Capital Costs for Solar Energy

As we discuss in detail in the Technology section of this report, currently available photovoltaic

energy generation devices are made from expensive materials using costly production methods.

Thermal solar production processes are also prohibitively expensive. Table 1 summarizes the

generation and investment costs of the primary large scale production methods as of 2005,

alongside estimates of the societal and environmental damages each method incurs. The high

capital investment costs associated with solar energy have kept its overall generation costs far

above those of traditional fossil fuels, nuclear, wind, and biofuels, providing strong market

pressures that have prevented the widespread implementation of solar power generation.

Table 1: Generation, Investment, and External Costs for Large Scale Technologies in the U.S. (Patel 2005)

Technology Generation Cost

(cents/KWh) Investment Cost ($/W)

Societal and

Environmental Costs

(cents/kWh)

Coal, thermal 3–5 1.0–1.5 2.0–15

Nuclear 3–8 1.2–2.0 0.2–0.6

Gas combined cycle 3–5 0.5–0.7 1.0–4.0

Small hydro 5–10 0.8–1.2 –

Biomass, thermal 4–10 1.5–2.5 –

Wind 3–5 0.8–1.5 0.05–0.25

Solar, PV 20–35 6.0–8.0 0.05–0.25

Solar, thermal 15–30 4.0–6.0 –


Intermittency and Regional Variability

Solar energy has the particular disadvantage of being available for collection only during the

daytime, and even then suffering from cloud cover. Geographical and seasonal variations in

incident sunlight and climate limit available solar resources. The intermittent availability of this

resource creates challenges for meeting a large portion of the total national energy demand. See

Figure A-2 in the appendix for the geographical dependence of solar resource availability across

the United States.

Distribution to the Electrical Grid

High demand and irregular power flows already create a problem for the reliability of the

nation’s electrical grid, a problem that would be intensified by the addition of a large quantity of

intermittently available solar energy. While predicting availability using averages of data

collected over decades is fairly straightforward—and even sufficient while solar energy

generates a negligible percentage of total energy production—the problem becomes more

complex when considering instantaneous fluctuations. For example, while one cloudy hour on

an otherwise sunny day would have a very small effect on the average production for that month,

immediate action must be taken to alter production and distribution patterns in order to continue

supplying demanded power to the cloudy area. According to C.W. Forsberg of Oak Ridge

National Laboratories (2006) given the state of the current grid,

If the renewable component of the electric grid exceeds 10 to 15%, backup power is

required to provide electricity when the wind speed slows or when cloudy conditions

exist. The cost of this backup power creates a very large economic barrier to the large-

scale use of renewable electricity production.

Deregulation of the electrical utility industry and mergers of major providers have resulted in a

decrease of excess infrastructure in order to decrease operation costs. However, this additional

infrastructure once served as a useful buffer that helped to prevent widespread blackouts that can

result from a variety of power flow issues (Amin et al. 2005). Given these considerations, the

nation’s electrical distribution grid must be improved and problems with energy storage must be

overcome before solar energy can provide a major portion of American demand for electricity.

Use in the Transportation Sector

Currently the only nationally available infrastructure to incorporate solar energy directly into

fueling automobiles is the electric power grid, but only the small portion of vehicles that are

hybrids can utilize this energy. Even this limited application to electric vehicles is made

inconvenient by the low storage densities* of commercial batteries, which are on average only

1.5% that of gasoline (Agrawal 2007). Due to space limitations in a vehicle, the energy density

of the battery restricts total capable driving distance.


Increased use of battery-powered automotive travel would place additional strain on an energy

grid powered by a significant solar component. Most hybrid owners charge their cars in the

evening and though the night, when solar energy in unavailable for collection. Therefore,

demand must be met through other means.

Solutions

Below we discuss some of the solutions that have been offered to help solve the problems that

we discuss above.

Storage

One solution to the problems of portability for the transportation section and intermittent

availability of sunlight is to collect an excess amount of energy during the day, convert to

electricity only the portion that is demanded, and store the remainder for electrical generation

during the night and cloudy days. A variety of methods have been proposed, some of which we

discuss below.

Thermal Solar Storage

Photovoltaic cells cannot store solar energy, so their electrical generation capabilities are

restricted to daylight hours, and are particularly sensitive to cloud cover. However, the energy

collected by thermal solar systems is much less transient than photoelectric energy, so such

systems have the ability to store excess energy in a physical medium. Current facilities have the

capability to store thermal energy in the form of high-pressure steam, and are capable of

generating electricity for several hours without collecting sunlight.

A new facility in Spain was launched in 2007 that stores excess collected energy in the form of

molten salt, allowing it to generate electricity in the dark for as long as fifteen hours, due to salt’s

high heat capacity. Such a large storage time is a powerful solution to intermittency. It is the

first of its kind in the world, but if it is shown to be successful in the long term, then it is possible

that companies in other countries will construct similar systems (Martin 2007).

Batteries

Today’s batteries suffer from low storage density, making them inconvenient for use in the

transportation sector (Agrawal 2007). And even the largest battery in the world, employed in

Alaska for emergency power, is only capable of storing enough energy to serve a small city of

12,000 people for 7 minutes (Telegraph 2003).

However, recent research by MIT professor Donald Sadoway indicates the feasibility of a large-

scale liquid-metal battery system that can be used to store excess grid energy. It operates at

temperatures high enough to maintain the molten state of its constituent metals (currently


confidential), which Sadoway claims can be implemented without the need for external heating,

due to the high currents that would flow through the device. While it is not intended for portable

applications, we imagine that it would be helpful in the widespread implementation of

photovoltaic energy since it could provide a previously unavailable capacity for large-scale

energy storage (MIT News Office 2009).

Hydrogen

Compressed and liquefied hydrogen offer alternatives to battery storage as media to increase the

portability of solar energy for the transportation sector. Electrical power generated by solar

methods can be used to split water into hydrogen and oxygen gases, which are then separated.

Hydrogen is stored in a compressed state, or cooled to the extremely low temperatures (-

253°C or -423°F) required to liquefy it. Liquid hydrogen has a storage density comparable to

that of gasoline, making it a much more convenient energy storage medium than batteries.

However, the realization of a solar-powered hydrogen economy will require an improved

hydrogen distribution infrastructure and technological improvements in fuel cells to increase

efficiency and decrease associated capital investment costs (Forsberg 2006).

Unified Smart Grid

The power distribution infrastructure must be updated in order to allow for localized

intermittencies in solar generation rates, in order to decrease the probabilities of widespread

power outages. Amin et al. (2005) make the observation that many critical components of the

electric grid are still manually controlled by human operators. They suggest that, in order to

increase the reliability of the grid, the control systems should be updated to increase the

automated controls. Power flow fluctuations can create problems that cascade though the

network and cause regional blackouts in a time span of seconds, too fast for human operators to

take necessary action, but not too fast to be prevented by automated control systems.

In a proposed Unified Smart Grid system, the existing regional grids of the North American

Electric Reliability Corporation (NERC), which we map in Figure A-3, are increasingly

interconnected and improved with automated power flow infrastructure. Such an interconnected

system would serve the purpose of managing the risks associated with intermittent energy

sources; while it is somewhat probable that, at any given time, some regions will suffer from

decreased solar energy production as a result of cloud cover, it is highly improbable that the

entire national will simultaneously be suffering from such a problem. Expanding the lateral area

of the Smart Grid to cover the entire nation would also increase the number of hours of the day

that the national grid receives solar energy inputs, increasing the flexibility of America’s solar

resources (Amin et al. 2005, Patel 2005).


Assistance by Government and Corporations

In response to increasing public support of environmentally responsible and renewable energy

options that decrease foreign oil dependence, the Federal Government and some state

governments have provided funding for research and infrastructure improvements, as well as

monetary incentives to stimulate growth in solar implementation. Some energy companies, such

as British Petroleum Solar (BP Solar), have directed their business operations to pursue strong

positions in the solar energy market. Even companies in industries unrelated to energy

production—most notably Google—have invested in projects related to solar energy, even giving

grants to fund external projects. Below we give details of action that has been taken by

government and these two corporations.

The Federal Government

The American Recovery and Reinvestment Act of 2009 created federal tax credits that allow

individuals to claim a tax credit of up to 30% of the installation costs for new solar electricity

and solar water heaters. Obama’s proposed 2011 budget includes $302.4 million to support

research and development for solar energy research, of which 83% funds technological

development, 10% supports systems integration, and 7% funds market transformation. In

addition, it includes $144 million for research and development of the electricity distribution

grid, a $5 billion expansion of tax credits available to businesses to support domestic

manufacturing of clean energy technologies, and $500 million to fund loan guarantees for

projects related to energy efficiency and renewable energy (EERE 2010). Such a large allocation

of public resources will likely result in the development of devices with increased efficiencies

and decreases in the necessary capital costs associated with solar energy generation.

The State of California

State governments are also providing monetary incentives for their citizens, with California

currently leading the nation. In 2006, Governor Schwarzenegger signed the Million Solar Roof

Plan, which provides tax credits and financial assistance with the goal of installing solar power

systems in one million homes by the year 2018. In 2007, the State of California committed

$2.167 million to the California Solar Initiative (CSI), a ten-year program that works with private

utility companies to provide rebates for individuals who install home solar generation systems

and provide more energy to the grid than they consume (Go Solar California 2010).

Corporations

Below we provide two examples of corporations that are investing in and/or providing funding to

projects to help make solar power more accessible to the public.


BP Solar

BP Solar has partnered with Home Depot to provide all devices and services necessary to equip a

home or business with solar panels. BP Solar offers consultations and quotes, provides

installation services, and performs check-ups six months after the initial installation. BP Solar

also offers lease agreements in which its installs the panels and provides any necessary

maintenance on the system for a monthly fee, effectively eliminating the upfront capital costs

associated with solar power. Such available arrangements will likely encourage households and

businesses to install photovoltaic solar power systems (BP Solar 2009).

Google.org

In 2007 Google.org, Google’s philanthropic arm, initiated the largest solar panel installation in

the United States at their headquarters in California (see Figure A-4). Also in 2007, Google

launched the RechargeIT program to research electric and hybrid vehicles, important

components for realizing a solar-powered transportation sector. The first round of testing

concluded that these types of cars get approximately 90 mpg, with carbon dioxide emissions that

are 65% less than standard petroleum-powered vehicles. In addition to funding its own research,

Google.org issued $10 million in grants to assist external research and development of plug-in

vehicles, storage technologies, and other renewable energy applications (Google.org 2007).

CONCLUSION

Given the continuing increase in energy demand and decreasing fossil fuel reserves, solar energy

will likely serve a greater role in satisfying American energy demand. It is likely that ongoing

efforts in research and development will continue to increase the economic viability of solar

energy generation. Once the capital investment costs become sufficiently low, it is reasonable to

expect an explosion in solar cell manufacture to meet a high demand for inexpensive energy.

Anticipating this increase in intermittently available energy, and given the funding from the

Federal Government for this purpose, it is likely that electrical providers will upgrade the

infrastructure of the electrical distribution grid. Such improvements will increase the reliability

of the grid.

Solar energy is currently the fastest-growing source of energy production, and its growth rate

only shows signs of increasing. In the coming decades, if the necessary innovations take place, it

will likely begin to replace traditional fuel sources and have an important place in America’s

green revolution.


GLOSSARY

Band gap: in photovoltaics and semiconductors, the lowest amount of energy a particle of light

(see photon) must have in order to excite an electron in a specified material. In this context,

every material has one band gap that is determined by its identity.

Conduction Band: a high-energy network of mobile electron in an excited material

Conjugated: A chain of carbons bound with alternating double bonds, resulting in an extended

area in which electrons are free to move

Dope: to deposit an electrically active impurity (referred to as a dopant) into an otherwise pure

material

Electrolyte: An ion dissolved in a liquid solvent (see Ion)

Hole: The lack of an electron in a bond. It exhibits behavior that can be characterized as though

it were a positively charged particle.

Ion: An atom or molecule that possesses a net positive or negative charge.

Photon: The oscillating ―particle‖ that is the elementary constituent of light. It can be thought of

as a packet of electromagnetic energy, the amount of which is determined by the frequency of its

oscillations.

Storage density: maximum amount of energy that can be stored in a medium per unit mass of

that medium

Valence Band: a low-energy network of low-mobility ground-state electrons. It is highly

populated in unexcited materials.


APPENDIX

Figure A-1: Energy Sources and Demand Sector (Allen Lecture 2010)

Figure A-2: Photovoltaic Solar Resources of America (NREL 2008)


Figure A-3: NERC Wide Area Synchronous Regions (NERC 2010)

Figure A-4: Aerial View of Solar Installations at Google Headquarters in Mountain View, CA.


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