Solar Cell

Solar Cells

Solar cell, also called a photovoltaic cell, is a device that converts light energy into electrical energy. A single solar cell creates a very small amount of energy (about 0.5-0.6 volts DC) so they are usually grouped together in an integrated electrical panel called a solar panel. Sunlight is a somewhat diffuse form of energy and only a portion of the light captured by a solar cell is converted into electricity.

What is a Solar Cell

What are Solar CellsThin wafers of siliconSimilar to computer chipsMuch biggerMuch cheaperSilicon is in abundanceMade from sandNon-toxic, safeCells convert sunlight energy into electric current, however they do not store energySunlight is its source of fuel

DefinitionsCells:Basic photovoltaic device that is the building block for PV modulesModule:A group of PV cells connected in series and encapsulated in an environmentally protective laminatePanel:A group of modules that is the basic building block of a PV arrayArray:A group of panels that comprises the complete PV generating unit Panel

A Brief History Photovoltaic Technology1839 Photovoltaic effect discovered by Becquerel. 1870s Hertz developed solid selenium PV (2 %). 1905 Photoelectric effect explained by A. Einstein. 1930s Light meters for photography commonly employed cells of copper oxide or selenium. 1954 Bell Laboratories developed the first crystalline silicon cell (4 %).

Things Start To Get Interesting...1976 First amorphous silicon cell developed by Wronski and Carlson. 1980s - Steady progress towards higher efficiency and many new types introduced1990s - Large scale production of solar cells more than 10% efficient with the following materials:Ga-As and other III-Vs CuInSe2 and CdTeTiO2 Dye-sensitized Crystalline, Polycrystalline, and Amorphous Silicon Today prices continue to drop and new 3rd generation solar cells are researched.

Photovoltaic Materials

*Conductors, Insulators and SemiconductorsInsulatorSemiconductorValence Bandin redConduction Band: whiteBand gapNo gap

Band Theory There are 3 types of materials in Band Theory, which are differentiated by their electronic structure: insulators, conductors, and semiconductors.

EgMetal Insulator SemiconductorEfEfEf

Electronic Structure of Semiconductors SiliconGroup 4 elemental semiconductorSilicon crystal forms the diamond lattice Resulting in the use of four valence electrons of each silicon atom.

How do they work?

A solar cell is based upon the "photovoltaic effect" discovered in 1839 by Edmund Becquerel, a French Physicist. In his experiments he found that certain materials would produce small amounts of electric current when exposed to sunlight. Sunlight is made up of packets of energy called photons.

When the photons strike the semi-conductor layer (usually silicon) of a solar cell a portion of the photons are absorbed by the material rather than bouncing off on it or going through the material.

When a photon is absorbed the energy of that photon is transferred to an electron in an atom of the cell causing the electron to escape from its normal position.

This creates, in essence, a hole in the atom. This hole will attract another electron from a nearby atom now creating yet another hole, which in turn is again filled by an electron from another atom. This hole filling process is repeated and repeated, the result of which, an electric current is formed.How solar cell works

Energy Bands in a SemiconductorConduction Band Ec emptyValence Band Ev full of electrons

2/(3) Types of SemiconductorsIntrinsicExtrinsica) n-typeb) p-type

Types 2 (a) and 3 (b) are semiconductors that conduct electricity - How?by alloying semiconductor with an impurity, also known as dopingcarriers placed in conduction band or carriers removed from valence band.Note: Color Protocol

Type 1: IntrinsicPure semiconductor (intrinsic): contains equal no. of valancies or holes in the valence band and so in an intrinsic semiconductor no. of free electrons is always equal to the number of holes. Therefore, conduction band is empty.

Because electrons in full valence cannot move, the pure semiconductor acts like an insulator.

Type 2 (a) : n-Typen-type: current is carried by negatively charged electrons - How?Group 5 (Pentavalent e.g., As, Sb or P) ) impurity atoms added to silicon melt from which is crystal is grown4/5 of outer electrons used to fill valence band1/5 left is then put into conduction band. These impurity atoms are called donors.Within conduction band the electrons are moving, therefore, crystal becomes a conductor

Type 3 (b) : p-Typep-Type: current carried by missing electron holes which act as positively charged particles. How?Group 3 (Trivalent e.g., B, Ga In & Al) added to silicon meltneed 4 outer electrons but doping creates lack of electrons in valence band. missing electrons, are called as holes, are used to carry current.

What Carries the Current?Prevailing charges are called the majority carriersprevailing charge carrier in n-type: electronsprevailing charge carrier in p-type: holes

Creating a Junction

Generation of Electrical Energy

Semiconductor JunctionsAll the junctions contain strong electric fieldHow does the electric field occur?When two semiconductors come into contact, electrons near interface from n-type, transfer over to p-type, leaving a positively charged areaHoles from p-type by interface transfer over to n-type leaving a negatively charged area.Because electrons and holes are swapped, a middle potential barrier with no mobile charges, is formed.This potential barrier created does not let any more electrons or holes flow through.Electric field pulls electrons and holes in opposite directions.

n-type semiconductorp-type semiconductor + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Physics of Photovoltaic Generation

Depletion Zone

Theory of Solar Cell

Working PrincipleThe solar cell works on the principle of photovoltaic effect, which is the process of generating an EMF as a result of the absorption of ionizing radiation

When photons strike a transparent photovoltaic cell, it may be reflected pass through or absorbed by materialAbsorbed photons provide the thermal energy to excite the electrons to generate electricityWhen enough solar energy is absorbed by the material of cell electron break through from the atomsThe energy associated with photon is proportional to the frequency of radiations (As Plancks law)E = hv = h c / where h = Plancks constant;c = speed of light = wavelength of lightv = frequency (cycle/sec)

Equivalent circuit of a solar cell

whereI = output current (ampere) IL = photo generated current (ampere) ID = diode current (ampere) ISH = shunt current (ampere).

the current produced by the solar cell is The schematic symbol of a solar cell A PV device can be modeled by a current source in parallel with a diode, with resistance in series and parallel.

I-V Curve for Solar Cells Fourth quadrant (i.e., power quadrant) of the illuminated I-V characteristic defining fill factor (FF) and identifying Jsc and Voc

A current versus power versus voltage curve

Voltage increases rapidly up to about 200 W/m2, and then is almost constant

Current and maximum power increase proportionally with irradiance.

Current in a Solar CellOutput current = I = Il-Io [ exp(qV/kT)-1]Il=light generated current I0= Diode currentq = electric chargeV = voltage across the junction k = Boltzmanns constant = 1.3807 10-23 J/KWhen in open circuit (I=0) all light generated current passes through diodeWhen in short circuit (V=0) all current passes through external loadImportant points:1) During open circuit the voltage of open circuit,Voc = (kT/q) ln( Il/Io +1)2) No power is generated under short and open circuit - but Pmax = VmIm=FFVocIsc

Solar Cell Materials

Solar cell is made up of different materials and Si is the element which is using in nearly 92%-95% solar cells.

Various materials display varying efficiencies and have varying costs.

Materials for efficient solar cells must have characteristics matched to the spectrum of available light.

Some cells are designed to efficiently convert wavelengths of solar light that reach the Earth surface. Light absorbing materials can often be used in multiple physical configurations to take advantage of different light absorption and charge separation mechanisms.

Materials presently used for photovoltaic solar cells include mono crystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride and copper indium.

Most of the solar cells are made from bulk materials that are cut into wafers between 180 to 240micrometers thick that are then processed like other semiconductors.

Other materials are made as thin-films layers, organic dyes, and organic polymers that are deposited on supporting substrates.

A third group are made from nanocrystals and used as quantum dots (electron-confined nanoparticles).Introduction

PV Classification

Silicon Crystalline Technology Thin Film Technology

Mono Crystalline PV Cells Amorphous Silicon PV Cells Multi Crystalline PV Cells Poly Crystalline PV Cells ( Non-Silicon based) Cadmium telluride (CdTe)Copper indium gallium selenide (CIGS) Gallium arsenide (GAs) Nanocrystalline silicon (microcrystalline silicon)

CrystallineSiliconAmorphous Silicon

Mono Crystalline PV CellsMonocrystalline, as the name suggests, is constructed from a single crystal of silicon, by cutting from ingots.This gives the solar panel a uniform appearance across the entire module. This crystal is then cut into thin wafers between 0.2mm and 0.3mm thick, which then form the basis of the solar PV cell.These solar PV cells are the most efficient, however, they also tend to be the most expensive to produce.They are rigid and are mounted in a rigid frame for protection. They are still more expensive than polycrystalline, but can be up to 3% more efficient. It is made using the Czochralski process. These large single crystals are exceedingly rare, and the process of 'recrystallising' the cell is more expensive to produce.

Multicrystalline PV CellsPolycrystalline (or multicrystalline) modules are composed of a number of different crystals, fused together to make a single cell (hence the term 'multi'). Hence, PV cells contain multiple silicon crystals.

This gives them a blue marbled appearance (rather than the much darker colour of monocrystalling solar PV cells).

Polycrystalline solar PV cells are slightly less efficient than monocrystalline cells, however, they are also less expensive to produce.

These solar PV cells also need to be stored in a rigid protective frame.

Silicon Crystalline TechnologyCurrently makes up 86% of PV market Very stable with module efficiencies 10-16%Mono crystalline PV CellsMade using saw-cut from single cylindrical crystal of SiOperating efficiency up to 15%

Multi Crystalline PV CellsCaste from ingot of melted and recrystallised siliconCell efficiency ~12%Accounts for 90% of crystalline Si market

Thin Film Technology Silicon deposited in a continuous on a base material such as glass, metal or polymersThin-film crystalline solar cell consists of layers about 10m thick compared with 200-300m layers for crystalline silicon cells

PROS Low cost substrate and fabrication process

CONS Not very stable

Amorphous Silicon PV CellsAmorphous solar PV cells, are a type of thin film solar cells and are made from a thin film of amorphous (non-crystalline) silicon.

Silicon is sprayed onto the substrate as a gas (called 'vapour deposition'), which means that the silicon wafer is approx 1 micron thick (compared to approx 200 microns for mono and poly). This means that the panel uses less energy to produce. And, it also means that the panels are far less efficient than mono or poly (approx 5-6% efficient).

This can be placed on a wide range of different surfaces and, because the amorphous silicon layer is flexible, if placed on a flexible surface, then the whole solar PV cell can be flexible.

These panels are the least expensive to produce but are also the least efficient than crystalline panels, and a greater number is required for the same output. On average, a thin film solar array will need 2.5 times more roof area than mono or poly.

Amorphous Silicon PV Cells The most advanced of thin film technologies Operating efficiency ~6% Makes up about 13% of PV market

PROS Mature manufacturing technologies available

CONS Initial 20-40% loss in efficiency

Poly Crystalline PV CellsCadmium telluride

A cadmium telluride solar cell uses a cadmium telluride (CdTe) thin film, a semiconductor layer to absorb and convert sunlight into electricity.

Cadmium telluride PV is the only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness (Rs. 44 per watt-peak, with the lowest crystalline silicon (c-Si) module at Rs. 55 per watt-peak).

The cadmium present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during res in residential roofs. A square meter of CdTe contains approximately the same amount of Cd as a single C cell Nickel-cadmium battery, in a more stable and less soluble form.

Poly Crystalline PV CellsCadmium Telluride ( CdTe)Unlike most other II/IV material CdTe exhibits direct band gap of 1.4eV and high absorption coefficient

PROS 16% laboratory efficiency6-9% module efficiency CONSImmature manufacturing process

Non Silicon Based Technology

Poly Crystalline PV CellsCopper Indium Gallium Selenide (CIGS)

CIGS (CuIn1-xGaxSe2 or CIGS) is a direct band gap semiconductor. Because the material strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. The CIGS absorber is deposited on a glass backing, along with electrodes to collect current.CIGS's absorption coefficient is higher than any other semiconductor used for solar modules. It has the highest efficiency (~20%) among thin film materials. The market for thin-film PV grew at a 60% annual rateTherefore, a strong incentive exists to develop and improve deposition methods for these films that will allow lower cost and increased throughput.

Gallium arsenide multijunctionHigh-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration.

Multi-junction solar cells or tandem cells are solar cells containing several p-n junctions. Each junction is tuned to a different wavelength of light, reducing one of the largest inherent sources of losses, and thereby increasing efficiency.

These multijunction cells consist of multiple thin films produced using metal organic vapour phase.

A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2.

Each type of semiconductor will have a characteristic band gap energy. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.

GaAs based multijunction devices are the most efficient solar cells till date. (a record high efficiency of 44%).

Nanocrystalline silicon (nc-Si)Nanocrystalline silicon (nc-Si), also known as microcrystalline silicon (c-Si), is a form of porous silicon.

It is an allotropic form of silicon with paracrystalline structure. And it is similar to amorphous silicon (a-Si), in that it has an amorphous phase.

Where they differ, however, is that nc-Si has small grains of crystalline silicon within the amorphous phase. And it is in contrast to polycrystalline silicon (poly-Si) which consists solely of crystalline silicon grains, separated by grain boundaries.

nc-Si has many useful advantages over a-Si, one being that if grown properly it can have a higher electron mobility, due to the presence of the silicon crystallites. It also shows increased absorption in the red and infrared wavelengths, which make it an important material for use in a-Si solar cells. One of the most important advantages of nanocrystalline silicon, however, is that it has increased stability over a-Si.

Semiconductor Material Efficiencies

Emerging Technologies Electrochemical solar cells have their active component in liquid phase

Dye sensitizers solar cells (DSSCs) are used to absorb light and create electron-hole pairs in nanocrystalline titanium dioxide semiconductor layer

Cell efficiency ~ 7%

DSSCs are made of low-cost materials. It can be engineered into flexible sheets, and although its conversion efficiency is less than the best thin film cells, However, its price/performance ratio should be high enough to allow them to compete with fossil fuel electrical generation.

Discovering new realms of Photovoltaic Technologies Electrochemical solar cells

Environmental AspectsExhaustion of raw materials CO2 emission during fabrication processAcidificationDisposal problems of hazardous semiconductor materialIn spite of all these environmental concerns, Solar Photovoltaic is one of the cleanest form of energy

Solar Cells BackgroundFirst Generation Single Junction Silicon Cells

89.6% of 2007 Production

45.2% Single Crystal Si 42.2% Multi-crystal SI

Large-area, high quality and single junction devices.

High energy and labor inputs significant progress in reducing production costs.

Single junction silicon devices theoretical limit efficiency of 33%.Payback period 57 years.

Single crystal silicon - 16-19% efficiencyMulti-crystal silicon - 14-15% efficiencySilicon Cell Average Efficiency

Solar Cells BackgroundSecond Generation Thin Film Cells

CdTe 4.7% & CIGS 0.5% of 2007 ProductionNew materials and processes to improve efficiency and reduce cost.

As manufacturing techniques evolve, production costs will be dominated by constituent material requirements, whether this be a silicon substrate, or glass cover. Thin film cells use about 1% of the expensive semiconductors compared to First Generation cells.

The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon and micromorphous silicon.

Trend toward second gen., but commercialization has proven difficult. 2007 - First Solar produced 200MW of CdTe solar cells, 5th largest producer in 2007 and the first to reach top 10 from of second generation technologies alone. 2007 - Wurth Solar commercialized its CIGS technology producing 15MW. 2007 - Nanosolar commercialized its CIGS technology in 2007 with a production . capacity of 430MW for 2008 in the USA and Germany. 2008 - Honda began to commercialize their CIGS base solar panel.

CdTe 8 11% efficiency (18% demonstrated) CIGS 7-11% efficiency (20% demonstrated)Payback time < 1 year in Europe

Solar Cells BackgroundThird Generation Multi-junction Cells

Third generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs.

Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques. They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, 31% under 1 sun illumination and 40.8% under the maximal artificial concentration of sunlight (46,200 suns).

Approaches to achieving these high efficiencies including the use of multijunction photovoltaic cells, concentration of the incident spectrum, the use of thermal generation by UV light to enhance voltage or carrier collection, or the use of the infrared spectrum for night-time operation.

Typically use fresnel lens (3M) or other concentrators, but cannot use diffuse sunlight and require sun tracking hardware

Multi-junction cells 30% efficiency (40-43% demonstrated)

How Solar Cells are Made

Solar Cell ConstructionMaterialsCrystalline SiliconThin Film Gallium Arsenide (more expensive)Grown into large single-crystal ingotsSawed into thin wafers2 wafers are bonded together (p-n junction)Wafers grouped into panels or arrayshttp://en.wikipedia.org/wiki/Solar_panel

Creating Silicon Wafers

Growing Silicon IngotsCzochralski Process

Drawing a Silicon Ingot

Silicon Ingots & Wafershttp://www.sumcosi.com/english/products/products2.html

Polycrystalline silicon wafers are sawn from cast rectangular ingots.

Creating PV Cells

Solar Modules and Arrays

PV Modules have efficiencies approaching 17%

Modules are constructed from PV cells that are encapsulated by several layers of protective materials.

Solar Panel

An array is a group of PV modules integrated as a single power-generating unit.

Several modules may be connected together to form a panel, which is installed as a preassembled unit.

A junction box on the back of a module provides a protected location for electrical connections and bypass diodes.

PV cells or modules are connected in series strings to build voltage.

The overall I-V characteristics of a series string are dependent on the similarity of the current outputs of the individual PV devices.

Strings of PV cells or modules are connected in parallel to build current.

The overall I-V curve of PV devices in parallel depends on the similarity of the current outputs of the individual devices.

Modules are available in several sizes and shapes, including squares, rectangles, triangles, flexible units, and shingles.

Bypass diodes allow current to flow around devices that develop an open-circuit or high-resistance condition.

Modules are added in series to form strings or panels, which are then combined in parallel to form arrays.

Output current of the ArrayFor increasing the necessary output current and power output, a number of modules having same nominal voltage to be connected in parallel:

Let V = Operating voltage of solar generatorI = Operating current of solar generatorVn = Nominal voltage of a moduleNs = Number of modules connected in seriesNp = Number of modules connected in parallelthen NS = V / Vnand operating current I = P / V NP = I / In

Hence, the total number of modules in the solar generator:N = NS NP

Solar Power Plant

Types of solar power plant

1.Photovoltaic solar energy plant

Grid Independent SPP Grid Connected SPP

2.Solar thermal energy plant3.Concentrating power plant

Photovoltaic solar energy plantSolar Cells produce DC power which fluctuates with the sunlight's intensity. For practical use this usually requires conversion to certain desired voltages or alternating current (AC), through the use of inverters.

Multiple solar cells are connected inside modules. Modules are wired together to form arrays, then tied to an inverter, which produces power at the desired voltage, and for AC, the desired frequency/phase.

Many residential systems are connected to the grid wherever available, especially in developed countries with large markets. In these grid-connected PV systems, use of energy storage is optional.

Block Diagram of Solar Power Plant

Mithapur Solar Power PlantIt is a 25 MW solar power plant located in Mithapur , Gujarat. It is expected to produce 40,734 MWh/year. The power plant is spread over an area of 100 acres (40.5ha). The 108,696 modules of polycrystalline silicon photovoltaic technology were used. The developer of the solar power plant, is Tata Power Ltd. The project estimated cost is Rs. 365 crores. The power plant was commissioned on 25 January 2012.

MonthMWhkWh/kW/dayTotal Revenue (Rs crore)January845.9731.269February3,937.7255.6257.176March4,259.3035.49613.565April3,693.5444.92519.105201212,736.54519.105

Solar Thermal energy plantA solar thermal energy plant will be used for creating solar generated heaters which can be used for heating water and also as an indoor heating system.

Thermal cells will be used to capture the energy which has been generated by the sun and then convert it into heat energy.

It is also possible to make use of this energy for cooking purposes and also for drying clothes.

Low temperatures can be used for heating water as well as swimming pools.

Medium heat is used for heating up the inside of homes as well as office buildings.

Concentrating power plantThe systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. Various techniques are used to track the Sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.

A parabolic trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The receiver is a tube positioned right above the middle of the parabolic mirror and is filled with a working fluid.

The presence of solar power plants in many parts of the world has made it possible for the energy from the sun to be utilized in the right manner.

Advantages:

Solar energy is renewable unlike the conventional resources (coal, oil) which will inevitably run out.Non-polluting, no carbon dioxide like fossil fuels (Free except for capital expenses)Environmentally friendly because the conversion of energy doesn't produce any carbon dioxide.It comes from the sun, which, unless you are in The South or North pole, comes out almost everydaySolar power is better for the environment, compared to burning fossil fuels and other electrical power.sun is renewableYou get clean energy without harming the environment [in term of carbon emissions], in certain countries, excessive power generated can be sold back to local electricity provider educes pollutionreduced dependence on fossil fuelsIt is environmentally friendly and no pollution is associated with solar powerIt can be installed anywhereBatteries can be used to store power for use at night It does no damage to the earth or its atmosphereIt produces no carbon dioxideIt doesn't have to be dug up from the ground like coal, oil, natural gas, or uraniumIt doesn't have to be cut down, like wood from forests.It produces clean, green power in the form of electricity and can be used to power just about everything we need.Solar cells last a long time, typically guaranteed for 20 or 25 years.

PVnomics .Module costs typically represents only 40-60% of total PV system cost and the rest is accounted by inverter, PV array support, electrical cabling and installation

Most PV solar technologies rely on semiconductor-grade crystalline-silicon wafers, which are expensive to produce compared with other energy sources

The high initial cost of the equipment they require discourages their large-scale commercialization

The Other Side

Use newer and cheaper materials like amorphous silicon , CuInSe2 , CdTe.

Thin-film solar cells use less than 1% of the raw material (silicon) compared to wafer based solar cells, leading to a significant price drop per kWh.

Incentives may bring down the cost of solar energy down to 10-12 cents per kilowatt hour - which can imply a payback of 5 to 7 years.

Applications @ PVWater Pumping: PV powered pumping systems are excellent, simple, reliable life 20 yrs

Commercial Lighting: PV powered lighting systems are reliable and low cost alternative. Security, billboard sign, area, and outdoor lighting are all viable applications for PV

Consumer electronics: Solar powered watches, calculators, and cameras are all everyday applications for PV technologies.

Telecommunications

Residential Power: A residence located more than a mile from the electric grid can install a PV system more inexpensively than extending the electric grid

(Over 500,000 homes worldwide use PV power as their only source of electricity)

Present PV Scenario in IndiaIn terms of overall installed PV capacity, India comes fourth after Japan, Germany and U.S. (With Installed capacity of 110 MW)

In the area of Photovoltaics India today is the second largest manufacturer in the world of PV panels based on crystalline solar cells. (Industrial production in this area has reached a level of 11 MW per year which is about 10% of the worlds total PV production)

A major drive has also been initiated by the Government to export Indian PV products, systems, technologies and services (Solar Photovoltaic plant and equipment has been exported to countries in the Middle East and Africa)

Indian PV Era Vision 2012 Arid regions receive plentiful solar radiation, regions like Rajasthan, Gujarat and Haryana receive sunlight in plenty. Thus the Potential availability - 20 MW/km2 (source IREDA)

IREDA has electrified 18,000 villages by year 2012 mainly through solar PV systems

Targets have been set for the large scale utilization of PV technology by different sectors within the next five years

A Step towards achieving the VisionThe Delhi Government has decided to make use of solar power compulsory for lighting up hoardings and for street lighting

By the year 2030, India should achieve Energy Independence through solar power and other forms of renewable energy

Dr. A. P. J. Abdul Kalam President of India Independence Day Speech, 2005

AdvantagesNon polluting: no noise, harmful or unpleasant emmisions or smellsReliable: most solar panels have a 25 year warranty and even longer life expectancySolar modules over their lifetime produce more power per gram of material than nuclear power but without the problem of large volumes of environmentally hazardous materialSolar panels produce more power within 5 years than the power consumed in their productionSolar power is a renewable energy source. It cannot be used up thus is effective in reducing the usage of fossil fuelsSave more money in the long run

DisadvantagesWe are unable to utilize the power of the sun at night or cloudy daysSolar panels are expensive to buy and hard to set up

Production and Disposal ConcernsProduction - Worker Health and SafetyAmorphous silicon -Silane, an explosive gas, is used in making amorphous silicon. Toxic gases such as phosphine and diborane are used to electronically "dope" the material. Copper indium diselenide -Toxic hydrogen selenide is sometimes used to make copper indium diselenide, a thin-film PV material.Cadmium telluride -Cadmium and its compounds, which are used in making cadmium telluride PV cells, can be toxic at high levels of lung exposure. DisposalModule lifespan typically around 30 yearsSome material classified as hazardous wasteRecycling process not yet perfected

Why not?Expensive for Consumers and ProducersTwo years output needed to just equal the amount of energy used in productionLarge land areas needed to produce energy on a power plant scaleLimited by intermittence. Stable grids require traditional generating facilities or costly backup to ensure uninterrupted supply.Due to PV efficiency and low market demand, technological progression is slow. Environmental concerns in production and disposalLack of subsidies and tax credits (In the U.S.)

Cost AnalysisUS retail module price = ~$5/W (2005)Installation cost = ~$3.50/W (2005)Cost for a 4 kW system= ~$17,000 (2006)Without subsidiesTypical payback period is ~24 years (warranty)

CostPrecise calculation of solar electricity costs depend on the location and the cost of finance available to the owner of the solar installationWith the best PV electricity prices (in the sunniest locations) approaching 30 cents/kWh and the highest tariffs now exceeding 20 cents/kWhFunding programs that bridge this gap are causing rapid growth in sales of solar PV, especially in Japan and Germany.

House of the Future?This zero-energyhouse in theNetherlands has30m2 of PV panelsfor powergeneration and12m2 of solarcollectors for waterand space heating

Solar PV DependenciesLocation, Location, Location!LatitudeLower latitudes are better than higher latitudesWeatherClear sunny skies are better than cloudy skiesHowever the temperature is not importantDirection solar arrays faceSouth is preferred, east and west are acceptableHowever, solar panels are more effective if they are arranged like treesAbsence of shadeTrees, flatirons, etc

Emerging PV TechnologiesCells made from gallium arsenideMolecular beam epitaxy35% more efficiency has been observedNon-silicon panels using carbon nanotubesQuantum dots embedded in special plasticsMay achieve 30% efficiencies in timePolymer (organic plastics) solar cellsSuffer rapid degration to date

Thin film Solar CellsUse less than 1% of silicon required to make wafersSilicon vapour deposited on a glass substrateAmorphous crystalline structureMany small crystals vs. one large crystal

New roof integrated PV productsFlexible PV Cells

Thank YOU!!

How Solar Cells are madeMaterialsCrystalline SiliconGallium Arsenide (which are more expensive)StepsGrown into large single-crystal ingotsSawed into thin wafers2 wafers are bonded together (p-n junction)Wafers grouped into panels or arrays

Creating Silicon wafers

Creating a PV Cell

Solar PV systemCellsBuilding block of PV systemTypically generates 1.5-3 watts of powerModules36 cells connected together have enough voltage to charge 12 volt batteries and run pumps and motorsAka PanelsMade up of multiple cellsArraysMade up of multiple modulesCosts about $5-$6/wattHowever, a typical system costs about $8/watt

Types of mounted arraysStandoff-Mounted ArraysRack-and Pole-Mounted ArraysCalifornia Patio Cover

Solar PV ApplicationsSpacecraft

Solar PV ApplicationsResidential

Solar PV ApplicationsCommercial

Large Scale Solar Power PV-generated electricity still costs more than electricity generated by conventional plants in most places, and regulatory agencies require most utilities to supply the lowest-cost electricity.Output dependent on weatherMostly used in SouthwestSolar furnace project in CaliforniaDish collector focuses heat to drive generator150 MW solar power facility in California the worlds largest

Solar Cell EfficienciesTypical module efficiencies ~12%Screen printed multi-crystalline solar cellsEfficiency range ~6-30%6% for amorphous silicon-based PV cells20% for best commercial cells30% for multi junction research cellsTypical power of 120W/m2

Solar Panel Efficiency~1 kW/m2 (sunny day)~20% efficiency 200W/m2 electricityDaylight and weather in northern latitudes100 W/m2 in winter; 250 W/m2 in summerOr 20 to 50 W/m2 from solar cellsValue of electricity generated at $0.08/hWh$0.10/m2/day or $83,000 km2/day

Worlds Largest PV Solar Plants

World Solar Power Production

Current energy demand in the worldAround 0.1% of primary energy demand Solar electric installations totalled 200MW in 1999, 280MW in 2000 and 340MW by 2001 and 427MW in 2002.

The growth rate is among the fastest in energy sources. Most of the growth is driven by the growth in Germany, Japan, and USA. From IEA

AdvantagesNon polluting: no noise, harmful or unpleasant emmisions or smellsReliable: most solar panels have a 25 year warranty and even longer life expectancySolar modules over their lifetime produce more power per gram of material than nuclear power but without the problem of large volumes of environmentally hazardous materialSolar panels produce more power within 5 years than the power consumed in their productionSolar power is a renewable energy source. It cannot be used up thus is effective in reducing the usage of fossil fuelsSave more money in the long run

DisadvantagesWe are unable to utilize the power of the sun at night or cloudy daysSolar panels are expensive to buy and hard to set up

*This presentation will cover the fundamental semiconductor physics that makes solar photovoltaics possible and review the materials needed to make solar cells.

For a basic understanding of solar photovoltaic physics see Solar Electricity, 2nd Edition by Tomas Markvart (Editor), 2000.

For a more advanced discussion of solar photovoltaic physics see The Physics of Solar Cells by Jenny Nelson, Imperial College Press, 2003.

For a detailed discussion of the future of solar photovoltaic physics see Third Generation Photovoltaics : Advanced Solar Energy Conversion by Martin Green, Springer, 2003.*In the mid 1970s people were getting worried because of the oil crisis and OPEC. Because people were worried, more thought and money was given to find new and cheaper ways of making solar cells. One development that came out of this research push was the first amorphous silicon cell developed by Chris Wronski and Dave Carlson. The discovery of the amorphous silicon solar cells caused a lot of excitement because it was a fundamentally cheaper material than standard crystalline silicon. This material is also used for the thin film transistors that drive modern flat panel displays. They have been mass produced real cheap, and over the years the price for a flat panel has gone down significantly. The same is happening for solar cell prices.

In the 1980s the solar photovoltaic community made steady progress towards higher efficiency and many new types of solar cells were introduced so that by the 1990s there were large scale production of solar cells more than 10% efficient with the following materials: Ga-As and other III-Vs, CuInSe2 and CdTe, TiO2 Dye-sensitized, and finally the largest producers Crystalline, Polycrystalline, and Amorphous Silicon solar cells. Today prices continue to drop and new 3rd generation solar cells are researched.

*Here is the timeline of photovoltaic technology. Solar cell technology is actually pretty old. The photovoltaic effect was discovered in the late 1830s. An explanation of the photoelectric effect won Einstein the Nobel prize. There was a big break through in 1954 because the first crystalline silicon solar cell was developed. Four years later it was used on a space satellite. The good news is that it worked, the bad news is that the solar cells kept working past when NASA expected so then the satellite kept sending data to earth when it was no longer needed and took up E and M bandwidth.

*In the mid 1970s people were getting worried because of the oil crisis and OPEC. Because people were worried, more thought and money was given to find new and cheaper ways of making solar cells. One development that came out of this research push was the first amorphous silicon cell developed by Chris Wronski and Dave Carlson. The discovery of the amorphous silicon solar cells caused a lot of excitement because it was a fundamentally cheaper material than standard crystalline silicon. This material is also used for the thin film transistors that drive modern flat panel displays. They have been mass produced real cheap, and over the years the price for a flat panel has gone down significantly. The same is happening for solar cell prices.


*Solar cells are made from semiconductors - the most important being silicon. Semiconductors have special electronic properties which allow them to be insulating or conducting depending on their composition. In photovoltaic materials you are dealing with the semiconductors (yellow). Most of the doping comes from boron and phosphorous for silcion solar cellsate. There are different types of solar cells such as cadmium telluride (CdTe) made from Cadmium and Tellurium or copper indium diselenide or gallium aresenide. Source:http://www.mse.cornell.edu/courses/engri111//semicon.htm2*Band theory models the behavior of electrons in solids, by postulating the existence of energy bands, continuous ranges of energy which electrons may occupy, and gaps, which they may not. It successfully explains many physical properties of solids, such as electric resistivity and optical absorption. For the conductor, the valence band is above conduction so there are free electrons to conduct electricity. In the insulator, the two bands are far apart from each other and its very hard to get charge carried from the bottom (valence) to top band (conduction band). For the semiconductor there is a gap between the valence and conduction bands this is known as the bandgap and is denoted Eg (Energy of the gap). The Fermi Energy (Ef) is also shown for the insulator and semiconductor. The ground state of a non-interacting fermion system (like one made up of electrons here) is constructed by starting with an empty system and adding particles one at a time, consecutively filling up the lowest-energy unoccupied quantum states. The Fermi-energy is the level (if there is one) where half of the states are occupied with electrons. *Most solar cells are made from silicon. Silicon has the diamond structure -a perfect tetrahedral coordinated crystal. Each silicon atom in the lattice is in an identical position to every other atom and each has four nearest neighbors. The 4 electrons in Si form bonds to the nearest neighbors (as seen by the red bonds in the figure). Silicon is a semiconductor and is the element largely responsible for the integrated circuit, which makes modern computers possible.*In the mid 1970s people were getting worried becauSse of the oil crisis and OPEC. Because people were worried, more thought and money was given to find new and cheaper ways of making solar cells. One development that came out of this research push was the first amorphous silicon cell developed by Chris Wronski and Dave Carlson. The discovery of the amorphous silicon solar cells caused a lot of excitement because it was a fundamentally cheaper material than standard crystalline silicon. This material is also used for the thin film transistors that drive modern flat panel displays. They have been mass produced real cheap, and over the years the price for a flat panel has gone down significantly. The same is happening for solar cell prices.


*Energy of an electron in a semiconductor must fall between two defined bands. The valence band are energies of valence orbitals which form bonds between atoms. The conduction band, next higher up, is separated from the valence band by and energy gap, or bandgap. The width of the energy gap, Eg, is defined by the difference in energy of the conduction and valence bands: Ec- Ev. This is the energy band structure for a typical semiconductor in detail like silicon. This is your valence band and conduction band. There is an energy gap noted Eg between the two. The valence band is filled with electrons the energy of electrons is going up. A hole is an absence of an electron. The conduction band is empty at absolute zero but at room temperature some valence electrons are excited into it and free to travel. For the remainder of the presentation, if something is full (almost full) the color will be dark and if something is empty (or almost empty) the color will be light. For example, dark green means full valence band and light green means empty conduction band.*There are three types of semiconductors. There is the intrinsic, which we just discussed (green). There are no extra electrons in conduction band and the valence band is full at absolute zero. Then there is the n-type (orange). It has extra electrons, in the conduction band, which are denoted by the blue circles. The extra electrons pull the Fermi level (black line) up towards the conduction band. For clarity the Fermi energy was not shown for the intrinsic semiconductor although in this case it would be in the middle of the gap. The p-type (purple) is when you have extra holes in valence band and the Fermi level is closer to the valence band. You can create n and p type semiconductors by placing atoms of a different element inside the lattice. For example, you can place elements in row three and row five (B and P) and remove electrons or give extra ones respectively for silicon.*In semiconductors, electrons and holes are solely created by a thermal excitation. On thermal excitation , it leaves equal no. of vacancies or holes in the valence band and so in an intrinsic semicondutor no. of free electrons is always equal to the number of holes. Therefore, the conduction band is empty. Because of that you cannot have any movement of electrons in the valence band. The pure intrinsic type basically acts like an insulator. You have to dope it if you want useful electronic properties.*For the n-type you have the group 5 impurities and you are adding an electron. It is also doping with a donor because now there are extra electrons in the conduction band which have room to move. This acts like a conductor, for example, a copper wire. In n-type the current is carried by negatively charged electrons - How? By using group 5 impurity atoms added to silicon melt from which is crystal is grown so 4/5 of outer electrons used to fill valence band and 1/5 left is then put into conduction band. These impurity atoms are called donors.

*In the p-type semiconductor electrons are removed by adding group 3 elements to the silicon. This creates holes or positive charges. There is now room for charge carriers to move around so now you have a hole conductor. Holes are the moving absence of electrons. *What carries the current?In the n-type material its the electrons. In the p-type material its the holes. This is true for all semiconductors. *You can create devices by putting two similar semiconductors together. The device is called a junction and there are four different types. There is the p-n junction, p-i-n junction, Schottcky barrier, and the heterojunction. Each has a built in potential (a built in voltage) associated with them.

**All the junctions already shown contain strong electric fields. What is actually happening is that if you have n and p semiconductors, that come into contact, electrons near the interface in the n-type material will be transferred over to the p-type material. What you do is deplete a region of electrons in the n-type right beside the p-type. The same works for holes. The holes move from p-type to n-type and it ends up that there is a region where the negative electrons become positive and positive holes become negative. The changing of charges gives the barrier. Electric field pulls electrons and holes in opposite directions.

*This is the same graph as before only flipped around to highlight the 4th quadrant of the I-V plot (Current density vs. Voltage). As V=0, you have maximum current, which is called the short circuit current. If you increase voltage, the current decreases. If you continue to increase voltage, the current goes to zero, and this is called an open circuit. The dashed line signifies power density and is shown on the right axis. The maximum current density and voltage is at the maximum power point, because after that, the power drops off, as well as the current. Maximum power for current= Jmp. Maximum power for voltage= Vmp. Fill factor is defined as: FF= (Imax)( Vmax) / short circuit open circuit. A 100% fill factor would have a square angle in the figure.Chapter 5-*The operating point at which a PV device produces its maximum power output lies between the open-circuit and short-circuit condition, when the device is electrically loaded at some finite resistance. The maximum power point (Pmp) is the operating point on an I-V curve where the product of current and voltage is at maximum. A variation of the I-V curve plots power against voltage, which clearly shows the maximum power point. See Figure 5-13. Maximum power is often called peak power and the parameter may be designated by Wp for peak watts.Chapter 5-*Changes in solar irradiance have a small effect on voltage but a significant effect on the current output of PV devices. The current of a PV device increases proportionally with increasing solar irradiance. Consequently, since the voltage remains nearly the same, the power also increases proportionally. See Figure 5-19. Chapter 5-*Changes in solar irradiance have a small effect on voltage but a significant effect on the current output of PV devices. The current of a PV device increases proportionally with increasing solar irradiance. Consequently, since the voltage remains nearly the same, the power also increases proportionally. See Figure 5-19. *In order to calculate current in a solar cell use the following formula: I = Il-Io [ exp(qV/kT)-1]. Where Il=light generated current. Io= dark generated current, q = electric charge, V = voltage, and k = Boltzmanns constant = 1.3807 10-23 J/K. Just by looking at this equation you can see one of the problems that occur in solar cells. As temperature increases, the current in the solar cell decreases. You dont get as much energy output from a solar cell if the temperature is higher. This occurs in all types but it hurts the diffusion type of solar cell the most. In amorphous silicon drift type of solar cells, however, you decrease the defects by increasing the temperature and that effect somewhat balances the loss in performance due to temperature increases. The open circuit is defined when I=0. Short circuit conditions occur is when V=0. Rearranging the output current equation and solving for voltage gives the open circuit equation. Voc = (kT/q) ln( Il/Io +1). In addition, no power is generated under short or open circuit but, Pmax = VmIm=FFVocIsc.

*Lets take a look at silicon technology first because it makes up the majority of the current photovoltaic market. Here is the graphical difference between crystalline silicon and amorphous silicon. The crystalline silicon lattice is perfect, like the swan. The amorphous is messed up with defects (strained and broken bonds), like the ugly duckling. These physical defects lead to electronic defects and lower solar cell performances.

Crystalline silicon and gallium arsenide are typical choices of materials for solar cells. Gallium arsenide crystals are grown especially for photovoltaic use, but silicon crystals are available in less-expensive standard ingots, which are produced mainly for consumption in the microelectronics industry. Polycrystalline silicon has lower conversion efficiency but also lower cost.When exposed to direct sunlight a 6-centimeter diameter silicon cell can produce a current of about 0.5 ampere at 0.5 volt (equivalent to about 90 W/m average, range is usually between 50-150 W/m, depending on sun brightness and solar cell efficiency). Gallium arsenide is more efficient than silicon, but also more expensive.Crystalline ingots are sliced into wafer-thin disks, polished to remove slicing damage, dopants are introduced into the wafers, and metallic conductors are deposited onto each surface: a thin grid on the sun-facing side and usually a flat sheet on the other. Solar panels are constructed of these cells cut into appropriate shapes, protected from radiation and handling damage on the front surface by bonding on a cover glass, and cemented onto a substrate (either a rigid panel or a flexible blanket). Electrical connections are made in series-parallel to determine total output voltage. The cement and the substrate must be thermally conductive, because the cells heat up from absorbing infrared energy that is not converted to electricity. Since cell heating reduces the operating efficiency it is desirable to minimize the heating. The resulting assemblies are called solar panels or solar arrays.A solar panel is a collection of solar cells. Although each solar cell provides a relatively small amount of power, many solar cells spread over a large area can provide enough power to be useful. To get the most power, solar panels have to be pointed directly at the sun.

http://en.wikipedia.org/wiki/Solar_panelDiagram outlining the various states of Silicon during the Czochralski process. Created by Rob Church

The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platinum, silver, gold) and salts. Crucibles used in Czochralski methodThe most important application may be the growth of large cylindrical ingots, or boules, of single crystal silicon. High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible , which is usually made of Quartz. Dopant impurity atoms such as boron or phosphorus can be added to the molten intrinsic silicon in precise amounts in order to dope the silicon, thus changing it into n-type or p-type extrinsic silicon. This influences the electrical conductivity of the silicon. A seed crystal, mounted on a rod, is dipped into the molten silicon. The seed crystal's rod is pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. This process is normally performed in an inert atmosphere, such as argon, and in an inert chamber, such as quartz.This picture shows the drawing of a silicon ingot in a fiery furnace containing molten silicon. Special high-speed saws slice the ingots into wafers about the thickness of a dime. They are then ground thinner and polished mirror smooth. (Image courtesy of Texas Instruments, Inc.)

Finished Ingots*Chapter 5-*A module is a PV device consisting of a number of individual cells connected electrically, laminated, encapsulated, and packaged into a frame. See Figure 5-21. The PV cells are laminated within a polymer (plastic) substrate to hold them in place and to protect the electrical connections between cells. The cell laminates are then encapsulated (sealed) between a rigid backing material and a glass cover. Some thin-film laminates use flexible materials such as aluminum or stainless steel substrate and polymer encapsulation instead of a glass cover.

***Solar panels are devices for capturing the energy in sunlight. The term solar panel can be applied to either solar hot water panels (usually used for providing domestic hot water) or solar photovoltaic panels (providing electricity). Solar panel by BP solar at a German autobahn bridge. pic taken 4/04 by Thomas Springer.

Chapter 5-*An array is a complete PV power-generating unit consisting of a number of individual electrically and mechanically integrated modules with structural supports, trackers, or other components. See Figure 5-22. Chapter 5-*The term panel is also used in relation to modules and arrays. Sometimes panel is used as an alternate term for a module. More commonly, the term panel refers to an assembly of two or more modules that are mechanically and electrically integrated into a unit for ease of installation in the field. See Figure 5-23.

Chapter 5-*All modules include some means for making intermodule electrical connections, through the use of either pre-wired connectors or a junction box. The junction box may also include bypass diodes and the ability to change the series or parallel configuration of the module cells with certain jumper arrangements. See Figure 5-24. For example, a module might be changed from 36 series-connected cells to two parallel strings of 18 series-connected cells. This doubles the current and halves the voltage, but the power output remains the same.

Chapter 5-*Individual cells are connected in series by soldering thin metal strips from the top surface (negative terminal) of one cell to the back surface (positive terminal) of the next. Modules are connected in series with other modules by connecting conductors between the negative terminal of one module to the positive terminal of another module. When individual devices are electrically connected in series, the positive connection of the whole circuit is made at the device on one end of the string and the negative connection is made at the device on the opposite end. See Figure 5-25.

Chapter 5-*Only PV devices having the same current output should be connected in series. When similar devices are connected in series, the voltage output of the entire string is the sum of the voltages of the individual devices, while the current output for the entire string remains the same as for a single device. Correspondingly, the I-V curve for a string of similar PV devices is the sum of the I-V curves of the individual devices. See Figure 5-26.

Chapter 5-*Parallel connections are not generally used for individual PV devices, especially cells, but for series strings of cells and modules. Parallel connections involve connecting the positive terminals of each string together and all the negative terminals together at common terminals or busbars. See Figure 5-27.

Chapter 5-*When similar devices are connected in parallel, the overall circuit current is the sum of the currents of individual devices or strings. The overall voltage is the same as the average voltage of all the devices connected in parallel. See Figure 5-28.Chapter 5-*When similar devices are connected in parallel, the overall circuit current is the sum of the currents of individual devices or strings. The overall voltage is the same as the average voltage of all the devices connected in parallel. See Figure 5-28.Chapter 5-*PV modules are available in a range of sizes and designed for a variety of applications. See Figure 5-29. Smaller modules of less than 50 W are typically used individually for low-power battery charging applications, such as navigational aids, accent lighting, motorist-aid call boxes, and small circulation pumps. Smaller modules are often more expensive per unit watt output than larger ones, and are not typically used to build large arrays due to the large number of intermodule connections and mechanical attachments that would be required.Chapter 5-*A bypass diode is a diode used to pass current around, rather than through, a group of PV cells. The current is allowed to pass around groups of cells that are shaded or develop an open-circuit or other high resistance condition, preventing an interruption of the continuity of the string. This allows the functional cells or modules in the string to continue delivering power. The consequence, however, is that the string will operate at a lower voltage. See Figure 5-30.

Chapter 5-*The modules or groups of modules are then integrated to form a complete array, using additional series or parallel connections. The result is a complete array that integrates all the modules into a single power-generating unit, with one positive terminal and one negative terminal for connection to other components. See Figure 5-34.Chapter 5-*The modules or groups of modules are then integrated to form a complete array, using additional series or parallel connections. The result is a complete array that integrates all the modules into a single power-generating unit, with one positive terminal and one negative terminal for connection to other components. See Figure 5-34.*******

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