Photovoltaic cells Session 5 DAMON FYSON. Photovoltaic cells Session 5 Photovoltaic cells are made...

DAMON FYSON

Photovoltaic cellsSession 5

DAMON FYSON

Photovoltaic cells Session 5

•Photovoltaic cells are made of high-grade silicon, a semi-conductor.

•When sunlight shines on a PV cell electrons break free and create an electrical current.

•When light strikes the cell, some energy is absorbed by the semiconductor and energy is transferred.

•The energy dislodges electrons allowing them to move freely.

•PV cells have one or more electrical fields that force freed electrons to flow only in one direction.

•By placing metal contacts in the top and bottom if a PV cell electric current can be drawn off.

How PV Cells Work: Photons to Electrons

DAMON FYSON


2 electrons in inner shell

2 electrons in outer shell

8electrons in second shell

Nucleus 14 protons & 14electrons

Atomic structure of silicon

• Under stable conditions Si: 14 protons,14 neutron,& 14 electrons

• When a photon of solar radiation strikes the outer shell ,an electron is released

• The incoming photon loses an amount of energy required to eject an electron from its shell (photovoltaic effect)

• Note 1: if the photon’s energy is more than the electron’s binding energy, the remainder of the energy will appear as heat in the silicon

• Therefore not all solar radiation is used to produce free electrons

• Note 2: max. conversion efficiency of silicon PV cells is less than 40%

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The photoelectric effect

DAMON FYSON


There is N-type silicon and P-type silicon, standing for negative and positive respectively. They are created by using different sorts of impurities in the silicon to produce different electrical charges. The N-type has free electrons in the atomic structure, and the P-type has free spaces (or holes) for electrons. Therefore, when they are put together, an electrical field is produced by the free electrons in the N-type silicon going to fill the gaps in the P-type. Not all the gaps are filled however, but an equilibrium is reached at the junction. At the point when photons hit the solar cell, if it has the right amount of energy, it pops free an electron. Then, because of the electrical field, an electron will flow between the N-type and the P-type, creating voltage and current and hence electrical power.

DAMON FYSON


http://upload.wikimedia.org/wikipedia/commons/7/74/PVeff(rev100921).jpg

DAMON FYSON


For traditional PV solar panels two halves of one pure silicon crystal are doped with two different dopants (e.g. arsenic, gallium, aluminium, phosphorus). One half of the crystal is left electron deficient - i.e. the atoms it contains are short of electrons. This is called the p-type layer. The other half of the crystal has an excess of electrons - this is called the n-type layer

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PN junction,

• the half labelled p- has a shortage of electrons so contains acceptor atoms each with a hole which could be 'filled' by an electron. • the half labelled n- has excess electrons and so contains donor atoms which have an extra electron

• electrons have moved from the n-type (negative) side to the p-type (positive) side of the crystal recombining with holes. • likewise holes have moved from the p-type side to the n-type side.

•Material close to the junction in the n-type side is positive and material close to the junction in the p-type side is negative

• a potential between the two sides of around 0.6-0.7 volts in a silicon PN junction.

DAMON FYSON


solar cells work with a semiconductor that has been doped to produce two different regions separated by a p-n junction.

Across this junction, the two types of charge carrier – electrons and holes – are able to cross and transfer their charge to the new region.

This migration of charge results in a potential gradient or electrical slope

When sunlight strikes a solar cell, atoms are bombarded with particles of light called photons, and give up electrons. When an electron is kicked out of an atom, it leaves behind a hole, which has an equal and opposite (positive) charge.

Summary PN junction

http://www.daviddarling.info/encyclopedia/D/AE_doping.html

http://www.daviddarling.info/encyclopedia/P/AE_p-n_junction.html

http://www.daviddarling.info/encyclopedia/E/AE_electron.html

http://www.daviddarling.info/encyclopedia/H/AE_hole.html

http://www.daviddarling.info/encyclopedia/P/AE_photon.html

DAMON FYSON


, as described in the sections below.

• The I-V curve of an illuminated PV cell has the shape shown in Figure 3 as the voltage across the measuring load is swept from zero to VOC, and many performance parameters for the cell can be determined from this data,

• At the open circuit situation R=Rmax an open circuit voltage Voc is measured and circuit current is zero• As the resistance decreases the electrical current increases and voltage decreases• Maximum current is called the short circuit current Isc and is measured s/cct situation R=0 and the voltage is zero• Every solar cell has a characteristic-V curve , I sc and Voc are quoted to help characterise a cell

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Short Circuit Current (ISC)The short circuit current ISC corresponds to the short circuit condition when the impedance is low and is calculated when the voltage equals 0. I (at V=0) = ISC ISC occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant. For an ideal cell, this maximum current value is the total current produced in the solar cell by photon excitation.ISC = IMAX = Iℓ for forward-bias power quadrantOpen Circuit Voltage (VOC)The open circuit voltage (VOC) occurs when there is no current passing through the cell. V (at I=0) = VOC VOC is also the maximum voltage difference across the cell for a forward-bias sweep in the power quadrant.VOC= VMAX for forward-bias power quadrant

Courtesy National Instruments

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Maximum Power (PMAX), Current at PMAX (IMP), Voltage at PMAX (VMP)The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation P=IV. At the ISC and VOC points, the power will be zero and the maximum value for power will occur between the two. The voltage and current at this maximum power point are denoted as VMP and IMP respectively.

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The Fill Factor (FF) is essentially a measure of quality of the solar cell. It is calculated by comparing the maximum power to the theoretical power (PT) that would be output at both the open circuit voltage and short circuit current together. FF can also be interpreted graphically as the ratio of the rectangular areas

A larger fill factor is desirable, and corresponds to an I-V sweep that is more square-like. Typical fill factors range from 0.5 to 0.82. Fill factor is also often represented as a percentage.


DAMON FYSON


Efficiency (η)Efficiency is the ratio of the electrical power output Pout, compared to the solar power input, Pin, into the PV cell. Pout can be taken to be PMAX since the solar cell can be operated up to its maximum power output to get the maximum efficiency.

Pin is taken as the product of the irradiance of the incident light, measured in W/m2 or in suns (1000 W/m2), with the surface area of the solar cell [m2]. The maximum efficiency (ηMAX) found from a light test is not only an indication of the performance of the device under test, but, like all of the I-V parameters, can also be affected by ambient conditions such as temperature and the intensity and spectrum of the incident light. For this reason, it is recommended to test and compare PV cells using similar lighting and temperature conditions


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During operation, the efficiency of solar cells is reduced by the dissipation of power across internal resistances. These parasitic resistances can be modeled as a parallel shunt resistance (RSH) and series resistance (RS),

For an ideal cell, RSH would be infinite and would not provide an alternate path for current to flow, while RS would be zero, resulting in no further voltage drop before the load.Decreasing RSH and increasing Rs will decrease the fill factor (FF) and PMAX as shown in Figure 6. If RSH is decreased too much, VOC will drop, while increasing RS excessively can cause ISC to drop instead.

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If incident light is prevented from exciting the solar cell, the I-V curve shown in Figure 8 can be obtained. This I-V curve is simply a reflection of the “No Light” curve from Figure 1 about the V-axis. The slope of the linear region of the curve in the third quadrant (reverse-bias) is a continuation of the linear region in the first quadrant, which is the same linear region used to calculate RSH in Figure 7


DAMON FYSON


It follows that RSH can be derived from the I-V plot obtained with or without providing light excitation, even when power is sourced to the cell. It is important to note, however, that for real cells, these resistances are often a function of the light level, and can differ in value between the light and dark tests.


DAMON FYSON


Temperature Measurement ConsiderationsThe crystals used to make PV cells, like all semiconductors, are sensitive to temperature. Figure 9 depicts the effect of temperature on an I-V curve. When a PV cell is exposed to higher temperatures, ISC increases slightly, while VOC decreases more significantly.

For a specified set of ambient conditions, higher temperatures result in a decrease in the maximum power output PMAX


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I-V Curves for ModulesFor a module or array of PV cells, the shape of the I-V curve does not change. However, it is scaled based on the number of cells connected in series and in parallel. When n is the number of cells connected in series and m is the number of cells connected in parallel and ISC and VOC are values for individual cells, the I-V curve shown in Figure 10 is produced.

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irradiance

Irradiance is measured in the units of watts per square meter (W/m2) and milli watts per square centimeter (m W/cm2). “Watts” is a measure of the power of the light or how bright the light is. In a very clear weather at midday (12 p.m.), the irradiance reaching a surface that faces the sun is about 1000 W/m2 (or 100 m W/cm2). This irradiance of 1000 W/m2 is called full sun, one sun or AM1 intensity

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Figure above shows I-V curves for 100, 500 and 1000 W/m2 . The I-V curve at 1000 W/m2 is for a module that faces the sun directly. When the sun is exactly overhead in tropical countries, the module should be horizontal for maximum current. When the sun is low in the sky at 30° above the horizon, the module should be tilted towards the sun at an angle of 60° from horizontal for maximum current

DAMON FYSON


For the 1000 W/m2 I-V curve, figure above shows the effect of cell temperature. As the temperature rises above 0 °C, Voc falls while Isc gets slightly higher. However, the graph shows that the current at 16 V decreases because of the decrease of Voc. Therefore, to get the maximum current output, modules should be mounted so that air can circulate around them freely to keep the cell cool.

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DAMON FYSON


There are three basic types of solar cellCrystalline solar cells are wired in series to produce solar panels. As each cell produces a voltage of between 0.5 and 0.6 Volts, 36 cells are needed to produce an open-circuit voltage of about 20 Volts. This is sufficient to charge a 12 Volt battery under most conditions

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Monocrystalline Solar CellsImage 1 of 3

Monocrystalline – made from a single large crystal, cut from ingots. Most efficient, but also the most expensive. Somewhat better in low light conditions (but not as good as some advertising hype would have you believe).

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Polycrystalline – basically cast blocks of silicon which may contain many small crystals. This is probably the most common type right now. Slightly less efficient than single crystal, but once set into a frame with 36 or so other cells, the actual difference in watts per square foot is not much.Although the theoretical efficiency of monocrystalline cells is slightly higher than that of polycrystalline cells, there is little practical difference in performance. Crystalline cells generally have a longer lifetime than the amorphous variety.

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Amorphous - technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications. They are made by depositing a thin film of silicon onto a sheet of another material such as steel. The panel is formed as one piece and the individual cells are not as visible as in other types.The efficiency of amorphous solar panels is not as high as those made from individual solar cells, although this has improved over recent years to the point where they can be seen as a practical alternative to panels made with crystalline cells. Their great advantage lies in their relatively low cost per Watt of power generated. This can be offset, however, by their lower power density; more panels are needed for the same power output and therefore more space is taken up.

DAMON FYSON


Vaporware – this is a 4th type – one that pops up in the news once in a while proclaiming to be the next major breakthrough that will make plastic spray on solar cells that will cost around 5 cents a watt, or some similar claim. None have reached production yet as of this writing.

DAMON FYSON


Cause of performance loss Explanatiom Typical loss

Grid coverage The surface of the cell has to be covered with metallic grid to collect electrons produced by photovoltaic effect

4.0%

Reflection loss Some of the incoming solar radiation is reflected from the front surface of the cell. 2.0%

Spurious absorption Some of the electron rejected from their electron shell will be absorbed by impurity atoms in the crystal

1.0%

Photon energy less than required absorption energy(hv>Eg)

Some of the incoming solar radiation does not have sufficient energy to eject an electron from its solar shell

19%

Photon energy greater than required absorption energy(hv>Eg)

Some of the incoming solar radiation has more than enough energy to eject an electron from its solar shell The extra energy is dissipated as heat in the crystal

28%

Quantum efficiency Of the photons with the correct energy to eject an electron from its electron shell only approx.90% will actually strike an electron and eject it

4.5%

Absorption not near junction Some photon are absorbed by the crystal far from the junction. The photons create electrons hole pairs which do nothing but immediately recombine, leaving only a little heat as their legacy

19%

Electrical resistance(fill factor)

The solar cell and its circuit have a small but significant electrical resistance 4.75

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Take home questions

1) Why can’t all solar radiation be used to produce free electrons when is impinges on a solar cell?

2) What is meant by;a) p-type siliconb) N-type silicon3) What is a pn junction? What happens when a n-type and p type semiconductor are

fused together4) Describe what happens at the depletion region5)What causes the free electrons to move when there are produced by the

photoelectric effect near the pn junction in a solar cell?6)Briefly explain the follwing termc) Vocd) Isce) Vmpf) Imp

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Take home questions continued………7) Why is the output of PV cells recorded and rated at 25°C

8) Which characteristic of a solar cell is reasonably constant under varying irradiance

9) What factors affect the efficiency of solar cells?

10) List the types of solar PV cell technology currently available.

Photovoltaic cells Session 5 DAMON FYSON. Photovoltaic cells Session 5 Photovoltaic cells are made...

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