Chapter 5- Pv Cell Technology

7/27/2019 Chapter 5- Pv Cell Technology

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Study of Solar Module & Its Efficient Use in Bangladesh

41PV Cell Technology

Chapter 05

PV CELL TECHNOLOGY

05.1 Introduction

Photovoltaic (PV) are clean alternative to conventional energy generation technologies.It is the cheapest method to generate electricity. This part aims at creating anunderstanding of the mechanism involved in the electricity generation and describingthe standard technologies.

Photovoltaic are relatively recent technology introduced for power generation, despitebeing known for quite some time. The name was introduced at late as 1948, and rapid

development followed.

05.2 Physics of Solar Cell

Solar cells are large semiconductor devices, which convert solar irradiation directly intoelectricity. In order to understand how a solar cell works, it is necessary to go back tosome basic physical concepts, which are reviewed in this section.

Semiconductor

In the simplest model of the atom, electrons orbit a central nucleus, composed ofprotons and neutrons. Each atom is electrically neutral, that is there is one electron foreach proton. Each atom has specific, discrete energy levels in which the electrons can befound. When bringing a large number of electrons closely together, these discrete levelsbecome blurred and they form so-called bands [14]. These bands are quasi-continuousregions where electrons can move. These bands are separated by forbidden zones, so-called band gaps, [14] where electrons cannot be. The electrons fill this energy level fromthe lowest upwards. These principles apply for all solid materials. The differencebetween the insulator and conductor is the position of the highest energy level which isfilled by electrons. If this Fermi level is in the middle of a band, the materials are metal.

If it is in forbidden region, the highest band is completely filled and next highest energylevel is forbidden, the material is an insulator or a semiconductor[14].The differencebetween insulator and semiconductor is the width of the band gap. The band with thehighest energy level containing electrons is called valence band, the first empty band iscalled the conduction band, because electrons in this band can move relatively easilyand conduction generally happens in this band.


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Figure 18: Difference between Metal, Semiconductor and Insulator

At room temperature and low illumination, pure so-called intrinsic semiconductorshave a high resistivity. But the resistivity can be greatly reduced by doping.

Doping

Doping means introducing a very small amount of impurity, of the order of one in amillion atoms [14].One can either introduce donors, i.e. atoms with excess electrons, oracceptors, i.e. an atom with a lack of electrons. Silicon generally prefers to share its fourvalance electrons with four other patterns as shown as figure.

Figure 19: Effects of Doping

Boron is a material with three valance electrons. Doping with its results as shown asfigure, in an acceptor, i.e. it can trap free electrons. It leaves so-called holes in thelattice. These holes are unsatisfied valence electrons. They act like positive charges.These charges can move through the material in exactly the same way as an electron.


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This P-type silicon has holes as majority carriers, i.e. a current is normally carriedthough hole transport [14].

On other hand Phosphorus has five valance electrons. It is a donor because it donatesthe unsatisfied electrons. Silicon so doped is called n-type. The majority carriers are

electrons [14].Holes, like electrons, will move under the influence of an applied voltagebut, as the mechanism of their movement is valence electron substitution from atom toatom, they are less mobile than free conduction electrons.

Direct and Indirect Band Gap

There are two kinds of semiconductors. Some have a direct band gap, while others havean indirect one.

Fig illustrates these two types of band gaps. The band boundaries of materials are

normally not constant across the material. They vary spatially as well as energetically.The band gap is assumed to be the minimum distance between valence band andconduction band. A direct band gap semiconductor has the minimum energy level ofthe conduction band in the same position as the maximum of the valance band [15].Anindirect band gap material has a spatial difference between these two energy levels [15].

Figure 20: Direct and Indirect Band Gaps

This has an influence on the absorption of light which can be observed for photonenergies similar to the band gap, as explained in the following section.


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Light absorption in a semiconductor

When light is absorbed in a semiconductor it interferes with the material, i.e. the photonhits the electron or nucleus. In this collision it transfers its energy. If the photon hits anelectron and has sufficient energy, it promoted to the conduction band, as in figure. The

energy transferred is normally larger than the band gap. The minimum energytransferred is normally larger than the band gap. If this is not possible, the material isinvisible for the photon and hardly any absorption will occur. If the energy is largerthan the band gap and electron is promoted to higher energy level within theconduction band, it will lose some of its energy almost immediately, passing on theexcess energy as heat. This process is called thermalisation. It will then exist for a whileon the lower edge of the band gap. Finally, after a certain life time, it will recombinewith the hole.

The highly energetic photons in the ultraviolet and blue parts of the spectrum are

absorbed very near the surface, while the less energetic longer wavelength photons inthe red and infrared are absorbed deeper the crystal. It also depends on the type ofsemiconductor.

p-n Junctions

A p-n junction is formed by bringing p- and n- doped material into conduct. Wherethese two meet, i.e. at the junction, areas with high electron densities are next to areaswith high hole densities. This is obviously not a stable state and the electrons and theholes will defuse to areas with lower concentration. By diffusing they leave a net chargeand thus create an electric field. This field opposes the diffusion. At some stage therewill be a balance between the electric field and the diffusion, and the system is inequilibrium [15].

The equilibrium then explains the alignment of the bands. Any physical system inthermal equilibrium can only have one Fermi level. Hence the system aligns as figure.


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Figure 21: Making of p-n Junction

Light Absorption in a p-n Junction

A photon being incident on a p-n junction may carry enough energy to generate anelectron hole pair. A hole created in a p-n area will not cause a large difference to theoverall space charge, while an electron certainly will. Thus only the minority carriersare important for the overall current.

The electron and the holes diffuse through the crystal in an effort to produce an evendistribution. This process is similar to gas always trying to occupy the largest possiblespace and by doing so creating an even pressure across this space. Some electron-holepairs recombine after a life time of the order of 1 s, neutralizing their charges andgiving up energy in the form of heat. Others will reach the junction, where they areseparated by the reverse field. The electrons are being accelerated towards the negativecontact and the holes towards the positive.

A different method of charge separation occurs if the electron-hole pair is generated inthe presence of an electric field. This leads to an immediate separation of the chargecarriers and the material can be significantly less pure than it needs to be if the devicerelies on diffusion for the initial transport of charge carriers. The separation of chargecarriers by means of an electric field is called drift current.


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If the cell is connected to load, electron will be pushed from the negative contactthrough the load to the positive contacts, where they will recombine with holes. Thisconstitutes an electric current.

05.3 Operation of Solar Cell

The efficiencies reported to date (2000) are well below the thermodynamic limits; asshown in the table. The best efficiencies were achieved with GaAs, which has nearlyideal properties for a photovoltaic device. It is however, prohibitively expensive and isthus used nearly exclusively in space application. It is followed by e-silicon which canbe expected as it was used exclusively in the past and all other technologies wereintroduced later. It is also the only other single- crystalline material, which causes thesuperior efficiencies. Dye sensitized cells, show a very low efficiencies.

Table 5Efficiencies of Non-Concentrating Devices

Material Best

efficiency

Research cell Module

c-Si 24.7 22.7

p-Si 19.8 15.3

a-Si (single j.) 12.7 _

a-Si (multi j.) 13.5 10.4

CdTe 16 10.7

CIGS 18.2 12.1

GaAs 25.1 _

Dye Sensitized 6.5 4.7

It is interesting to look at the reasons for the relatively low value given in table.


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General Losses

Losses divide into three categories. There are short circuit, open circuit, fill factor losses.The losses are briefly discussed here.

Short Circuit Losses

It is directly linked to the band gap and the spectrum used. The band gap defines theminimum energy needed for the generation of an electron-hole pair, because the photonmust carry enough energy to overcome the band gap. The maximum number ofelectron-hole pairs can be generated for a band gap zero, when every photon hasenough energy to produce an electron-hole pair.

A low quality material will cause shorter life times of electron-hole pairs. This willresult in less charge carrier making it across the junction and thus the short circuitcurrent will be reduced.

Open Circuit Losses

Assuming a minimum influence of the second diode and a high shunt resistor allowsone to estimate the open circuit voltages:

Voc kt/e ln(Iph/I01 +1)

This neglects some of the correction factors introduced in section Diode Model andthus is an approximation. It is apparent that the recombination within the bulk needs tobe as low as possible. Obviously a large photocurrent is beneficial as well. This

statement contradicts the simple idea that a low band gap is beneficial.

Fill Factor Losses

The fill factor is influenced by recombination within the bulk and the SCR of the device.Hence both need to be good quality in order to minimize the recombination withinthem. It is also influenced by parallel and series resistance. They determine the slope atIac and Voc respectively. Thus the series resistances need to be as low and the parallelresistance as high as possible.

Energy Loss

Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation isnot monochromatic - it is made up of a range of different wavelengths, and thereforeenergy levels.

Light can be separated into different wavelengths, and we can see them in the form of arainbow. Since the light that hits our cell has photons of a wide range of energies, itturns out that some of them won't have enough energy to form an electron-hole pair.


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They'll simply pass through the cell as if it were transparent. Still other photons havetoo much energy. Only a certain amount of energy, measured in electron volts (eV) anddefined by our cell material (about 1.1 eV for crystalline silicon), is required to knock anelectron loose. We call this the band gap energy of a material. If a photon has moreenergy than the required amount, then the extra energy is lost (unless a photon has

twice the required energy, and can create more than one electron-hole pair, but thiseffect is not significant). These two effects alone account for the loss of around 70percent of the radiation energy incident on our cell.

The band gap also determines the strength (voltage) of our electric field, and if it's toolow, then what we make up in extra current (by absorbing more photons), we lose byhaving a small voltage. Remember that power is voltage times current. The optimalband gap, balancing these two effects, is around 1.4 eV for a cell made from a singlematerial.

We have other losses as well. Our electrons have to flow from one side of the cell to theother through an external circuit. We can cover the bottom with a metal, allowing forgood conduction, but if we completely cover the top, then photons can't get through theopaque conductor and we lose all of our current (in some cells, transparent conductorsare used on the top surface, but not in all). If we put our contacts only at the sides of ourcell, then the electrons have to travel an extremely long distance (for an electron) toreach the contacts. Remember, silicon is a semiconductor -- it's not nearly as good as ametal for transporting current. Its internal resistance (called series resistance) is fairlyhigh, and high resistance means high losses. To minimize these losses, our cell is

covered by a metallic contact grid that shortens the distance that electrons have to travelwhile covering only a small part of the cell surface. Even so, some photons are blockedby the grid, which can't be too small or else its own resistance will be too high.

Now that we know how a solar cell operates, let's see what it takes to power a housewith the technology.

Influence of Temperature

The influence of temperature is mainly on the band gap and on the recombination. Theband gap effectively shrinks with increasing temperature. Thus the short circuit current

increasing with temperature and open circuit voltage decreases.Recombination increase, as diode saturation currents are proportional to the band gap.

[I01, 02 ~ exp [ eEgap / kT]

Thus the recombination increases with decreasing band gap, resulting in a furtherdecrease in the open circuit voltage. The result is a net loss of efficiency, as the changein open circuit voltages much more pronounced than the change in short circuit current.


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Influence of Irradiation

An increase in radiation will result in a linear increase in a short circuit current. Thiswill, according to equation, result in an increase of open circuit voltage.An empirical relationship of the theoretical efficiency with an idealized fill factor FFi as

t ~ VF . FFi

The voltage factor VF depends on the band gap and the open circuit voltage. It is givenas

FF = eVoc/ Egap

Thus increase in irradiation will result in an increase in efficiency.

Figure 22: Influence of Irradiation on the Device Performance

05.4 Silicon Cells

Cell Manufacturing

Silicon is the second most abundant element in Earths crust, it is essentially sand, theform used for cell production is quartzite, the crystalline form of silicon dioxide. In anarc furnace, carbon is added, resulting in silicon and carbon dioxide. The resulting Si isabout 98% pure, it is called metallurgical grade silicon. This silicon is finely ground anddissolved in HCl. This solution is distilled several times. The boiling temperature of theSiHCL3, which results from dissolving Si in HCl, as is as low as 310C. The liquid is thenreduced by hydrogen and electrically heated. Semiconductor grade silicon is thencollected is on silicon rod. The problems with the last step are that the yield is low( ~37% ) and that the temperatures needed are 1000-12000C.


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Mono crystals are then produced, if crystalline Silicon cells (c-Si) are to be produced.The main industrial process for doing this is called the Czochralski process. The semiconductor grade silicon is melted in a crucible, using an inert gas atmosphere. Tracelevel of the desired dopant are added.

Figure 23: Manufacturing Steps of PV Modules

05.5 Thin Film Cells

The production of crystalline silicon devices is quite labor and energy intensive andthus expensive. A further cost reduction of photovoltaic energy production is only


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possible if the manufacturing costs per Wp can be reduced. The way forward seems tobe at the moment the introduction of thin film solar cells. They typically are direct bandgap materials, which need a material thickness in the range of 1m only. This reducesthe amount of semiconductor material significantly, and thus reduces the energy costwith producing the devices. The cost of the semiconductor material is, in fact, not the

main concern any more. The cost of glass and TCO (transparent conducing oxide) usedfor the top contacts tends to dominate the device prices.

This device has an area ratio nearly 100%. The device covers the whole module and nonoticeable gaps exist between single cells. This gives the typical uniform look.

Figure 24: Sectional Structure of Typical Thin Film

Figure 25: Thin Film PV ModulesGeneral Structure

The huge advantages of thin film devices is that production can be nearly completelyautomated. The materials are directly deposited on glass substrate, as illustrated in thefigure. Interconnect and all deposition steps can be done automatically and hardly any


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human interaction is needed. This is main reason why thin film devices are supposedlycheaper than c-Si devices.

Figure 26: General Structure of a Thin Device

Module Manufacturing

Solar cells are fragile objects, which must be shielded from the environment. They alsohave to be supported mechanically. This is done typically in photovoltaic modules,which are typically around 30-40 cells mounted in one single unit. This number of cellsstems from the fact that so far the majority of modules are used for charging batteries.The cells are largely connected in series, because of the voltage generated by single cells(~0.5-0.6 volts) is rarely sufficient for any application. A typically arrangement forpackaging solar cells into modules is shown in figure.

Figure 27: Schematic Diagram of PV Module

The window is typically a low iron tempered glass. Glass is used because it is arelatively cheap and weather proof material. Tempered glass is also quite a tough


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material. The low iron content normally results in high transparency. The backing canbe glass, plastic or metal.

The single cells within a module are normally contacted manually. They must beencapsulated in a potting compound which allows for thermal expansion, is transparent

and UV resistant and allows low cost production.Ethy1 Viny1 Acetate (EVA) is used inthe majority of todays modules. A glass fiber separator is normally introduced betweenthe cells and the back protection in order to avoid any puncturing of the plastic by highspots of solder and any other soil trapped in the encapsulate.

The cells are laminated. A laminate of glass/EVA/fiberglass/plastic foil is placed in alaminator. Firstly a vacuum is generated in order to remove the air between thedifferent sheets. Then the laminate is subjected to a temperature of around 200C, wherethe EVA softens. Applying the mechanical pressure forces the EVA between the cellsand into the weave of the fiber glass. It also wets the glass and the foil, giving them

good mechanical bond. Finally the EVA is back to cure and then allowed to cool down.After cleaning and sealing the edge the laminate is placed in a frame, which addsmechanical strength.

One of the danger spots is the point of where the electrical connection from thelaminated cells feeds out through the back protective cover. Often an additionalprotection in the form of terminal box is included.

Moisture penetration is responsible for the majority of long term module failures, withcondensation on the cells and circuitry censing short circuiting or corrosion. Hence the

encapsulation system must be highly resistant to the permeation or ingress of gases,vapors and liquid. It should be carefully chosen to be able to maintain adhesion underextreme operating condition.

Contrary to c-Si production, there are no separate steps for producing a cell and then amodule. The materials are directly deposited on the glass substrate. The different cellsare then formed using laser scribing before each deposition step. Figure illustrates theproduction using the example of an amorphous Si device


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Glass substrate

Deposit TCO

Laser Scribe TCO

Deposit Active Materials

Laser Scribe Active Materials

Deposit back contact

Form cells in back contacts

Attach Al Contact Strip

Laminate + Frame

Finished

Figure 28: Steps to Produce Thin Film Devices

The process does not need any manual soldering as for c-Si devices. The thermalexpansion cannot cause the module connectors to come loose. It is important to keepmoisture out because other-wise it can happen that the thin semiconductor layerliterally dissolves.


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Materials

Amorphous Silicon

Amorphous silicon was initially seen as an ideal candidate for thin film devices and

significant investments where under taken. It is an abundant material, environmentbenign and potentially very cheap to produce. Early p-n devices suffered from adegradation of about 40%.

Device structures were changed to p-i-n structures, as shown in figure. This structuregenerates an electrical field that stretches nearly completely across the whole device,especially as it is a good assumption that the thickness of the p and the n layer isnegligible compared to the thickness of the i layer. This has the advantages that thelower materials quality, which would normally cause increased recombination, isacceptable. As the materials ages, and thus recombination increase, the electrical field

helps to reduce the impact on the reduction on the overall efficiency.

Figure 29: Typical Single Junction Structure

Amorphous silicon devices are very promising because of their low thermal energylosses. It is the only material for which a positive temperature co-efficient was reported.

Cadmium Telluride

It is ideal material for photovoltaic application. A typical structure is shown in figure.The active junction is formed by a CdS-CdTe hetero junction, which is by small grainpolycrystalline materials.


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Figure 30: Cd-Te Device Structure

It is not possible to n-dope CdTe, thus a thin CdS layer is introduced to form thejunction with the p-CdTe. This layer must be thin enough to ensure a high transmission

of photon to the CdTe materials.

Dye Sensitized Cells

It is relatively a new device. This new technology is developing very quickly and manydifferent device structures are discussed at the moment. One possible device structure isshown in Figure-31. The actual cell is enclosed between two sheets of TCO coated glass.The active material, dye-sensitized TiO2 is surrounded by an electrolyte which allowsan electron transport.

Figure 31: Structure of Dye Sensitized Solar Cell

Here we explain the basic mechanism involved in electricity generation by photovoltaic.This starts with the absorption of a photon in a materials, the generation of electron hole


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pair. This pair is separated by diffusion and across a barrier introduced by the p-njunction. A certain % reaches the node with a voltage defined by the band gap andengineering factors. A suitable model for common device is described with the twodiode model. The limits to the efficiency are discussed on the basis of the performanceindicators they affect. This helps in understanding the good engineering can have on

the device performance. Todays structure and production methodology of thecommonly available c-Si and p-Si cells is discussed. The strengths and the problems inthe expansion of this technology to much higher volumes are highlighted. In contrast tothis, newer thin film device are discussed and their production is investigated. Thus alltechnologies available to date are discussed and an overview of cell technology is given.

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05.6 Applications

Professional Applications

Telecommunicationssolid state microwave techniques over the past twenty years have considerablyreduced the power consumption of telecommunication equipment. In thiscircumstance, PV has proved to be the cheapest and most reliable source of powerand many systems have been installed all over the world.

Cathodic Protection

This system provide essential corrosion control for pipe lines, well heads, bridge andother metallic structures. A small direct is impressed on the structure at regular

intervals to inhabit electro chemical corrosion. Distributed PV sources have provedto be an ideal way of providing this current, as they eliminate the problems oftransmission loss, fuel supply and maintenance with diesel generators.

Navigational AidsMarine beacons and navigation light on buoys were traditionally powered byacetylene, pressurized kerosene or batteries. Maintenance was difficult and costly. Inmany countries simple PV generator are providing cheaper and more reliable forthis application.

Remote Air Craft BeaconsPV can be used to advantage to power radio and light beacons near air ports, wherethe cost of running cables or maintaining batteries would be prohibitive.

Alarm System and Traffic Warning LightsIndependence from main supplies and saving battery charging make PV attractivefor railway signals, alarm systems, fog, fire and flood hazard warnings, trafficcontrol warning, lights and highway breakdown telephones.


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Automatic Weather StationsSolar powered automatic weather stations enable networks to be expanded atrelatively low cost, with a consequent improvement in forecasting accuracy.

Social Applications

Rural Electrification

About 80% of population of developing countries live in small villages and scatteredhouses without electricity. Electric power is necessary to improve the health andeducation of this people to provide opportunities for the development of localindustries, which would help to arrest the damaging drift of populations to the towns.As cost continues to fall PV is becoming increasingly accessible for rural electrification,in the industrialized as well as in the developing countries.

Water pumping

The lack of ready supply of clean water is a major problem in many developingcountries. Sources are often contaminated and some distance from the dwellings. PVpumps are playing an important role in the efforts to overcome this problem. They haveproved to be reliable and there are thousands in use around the world.

Vaccine Refrigeration

To remain effective, vaccines used in programs to improve health standards and reduceinfant mortality have to be kept refrigerated during transportation. Special pv poweredportable refrigerators have been developed to help maintain cold chain from the factoryto user.

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Chapter 5- Pv Cell Technology