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TERM PAPER ON

“SOLAR CELLS”

Submitted to

Amity University, Uttar Pradesh

In the fulfilment of the third semester term paper

ELECTRICAL AND ELECTRONICS ENGINEERING

By

SPARSH GOSWAMI

Enrolment No.: A7624615005

Under the guidance of

Dr.GANGARAM MISHRA

Department of Electrical and Electronics Engineering

Amity School of Engineering and Technology

Lucknow (U.P.)

SYNOPSIS

AMITY UNIVERSITY

-----UTTAR PRADESH-----

 

Amity School of Engineering & Technology, Lucknow

Term Paper

 

Student Name SPARSH GOSWAMI

Enrollment No A7624615005

Programme B.Tech (EEE)

Company's Name and Address AMITY UNIVERSITY UTTAR PRADESH

Amity University, Lucknow Campus, Nijampur, Malhaur, Gomti Nagar Extension

226028

Industry Guide

Name

Designation

 

Contact Number

  Ph.(O) :                 (R) :

  Mobile :                 

  Fax :                 

  E-mail :                 

________________________________________

Project Information

1) Project Duration : (45 Days)

           a)  Date of Summer Internship commencement (16/05/2016)

           a)  Date of Summer Internship Completion (30/06/2016)

2) Topic

 

SOLAR CELLS

 

3) Project Objective

 

The aim of this study is to explore, analyze, examine and determine the poten¬tiality of solar cells and understand the working of solar cells,its applications,constructions,efficiency.

 

4) Methodology to be adopted

 

First off,one should be able to identify a solar cell and its constituents,types.Photovoltaics (PV) covers the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry.A typical photovoltaic system employs solar panels, each comprising a number of solar cells, which generate electrical power. The first step is the photoelectric effect followed by an electrochemical process where crystallized atoms, ionized in a series, generate an electric current.[1] PV Installations may be ground-mounted, rooftop mounted or wall mounted.

 

5) Brief Summery of project(to be duly certified by the industry guide)

 

A solar cell, or photovoltaic cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light. They are used as a photodetector (for example infrared detectors), detecting light or other electromagnetic radiation near the visible range, or measuring light intensity.

 

 

 

 

Signature

(Student) Signature

(Industry Guide) Signature

(Faculty Guide)

DECLARATION

I, SPARSH GOSWAMI, student of B.Tech. Electrical and Electronics Engineering, third semester, hereby declare that the term paper titled “SOLAR CELLS” is submitted by me to the Department of Electrical and Electronics Engineering, Amity School of Engineering and Technology, Amity University Uttar Pradesh, Lucknow, in the fulfilment of third semester term paper. This is a comprehensive study based on the literature survey and brief review on existing knowledge, which is produced in the best possible manner.

Place – Lucknow

Date –

SPARSH GOSWAMI

Enrolment no.:A7624615005

B.Tech. E&EE

Third semester

CERTIFICATE

On the basis of declaration submitted by SPARSH GOSWAMI, student of B.Tech. Electrical and Electronics Engineering, third semester, batch 2015-2019, I hereby certify that the term paper titled “SOLAR CELLS” which is submitted to the Department of Electrical and Electronics Engineering, Amity School of Engineering and Technology, Amity University Uttar Pradesh, Lucknow, in the fulfilment of the third semester term paper is an original contribution with existing knowledge and faithful record of work carried out by him under my guidance and supervision.

To the best of my knowledge this work has not been submitted in part or full for any Degree or Diploma to this University or elsewhere.

Place – Lucknow

Date –

Dr. GANGARAM MISHRA

Department of Electrical and Electronics Engineering

Amity School of Engineering and Technology

Amity University Uttar Pradesh, Lucknow

Professor O.P. Singh

Head of Department

Department of Electrical and Electronics Engineering

Amity School of Engineering and Technology

Amity University Uttar Pradesh, Lucknow

ACKNOWLEDGEMENT

It has been my earnest endeavour to deal with this project on “SOLAR CELLS”. I take this opportunity to thank my teachers and friends who have helped me throughout the project. First and foremost I would like to thank our Director ASET –Wing Commander Dr. ANIL TIWARI, Deputy Director - Brig. UMESH K. CHOPRA, Head of Department - Prof. O.P. SINGH and my guide Dr. ganga ram mishra for their valuable advice and time during development of this project. I thank them for providing me this opportunity. I would also like to thank my family and friends; they have been a constant support to me throughout this project.

SPARSH GOSWAMI

Enrolment no.: A7624615005

B.Tech. E&EE

Third Semester

CONTENTS

i. Synopsis………………………………………………………………………………………….page 2

     ………………………………………………………………………………………….page 3

ii. Declaration……………………………………………………………………………………page 4

iii. Certificate…………………………………………………………………………………….page 5

iv. Acknowledgement……………………………………………………………………….page 6

v. Contents……………………………………………………………………………………….page 7

vi. Abstract………………………………………………………………………………………..page 8

1. Introduction……………………………………………………………………….page 9

2. Why Solar…………………………………………………………………………..page 10

3. Solar Cell Structure…………………………………………………………….page 11

4. Solar Cell Operation………………………………………………………….page 12

                           ………………………………………………………….page 13

                         ……………………………………………………………page 14

5. The Photovoltaic Effect………………………………………………………page 15

6. Solar Cell Parameter…………………………………………………………..page 16

                          …………………………………………………………..page 17

                          …………………………………………………………..page 18

                          …………………………………………………………..page 19

7. Resistive Effects………………………………………………………………….page 20

                   ………………………………………………………………….page 21

                   ………………………………………………………………….page 22

8. Design Of Silicon Cells…………………………………………………………page 23

                             ………………………………………………………..page 24

9. Types Of Solar Cells…………………………………………………………….page 25

                        .…………………………………………………………….page 26

                        ……………………………………………………………..page 27

vii. Conclusion……………………………………………………………………………………page 28

viii. References…………………………………………………………………………………..page 29

     

CONTENTS

(FIGURE CONTENTS)

• Cross section of solar cell.....................................................page 12

• The impact of surface passivation and diffusion length on collection probability............................................................page 17

• Quantum efficiency of a silicon solar cell............................page 18

• Photovoltaic effect ...............................................................page 19

• IV curve of solar  cell showing short circuit current.............page 20

• IV curve of solar cell showing open circuit voltage.............page 21

• Cell with low fill factor.........................................................page 22

• Characteristic resistance........................................................page 24

• Parasitic series and shunt resistance in a solar cell circuit....page 25

• Schematic of solar cell with series resistance.......................page 26

• Schematic of solar cell with shunt resistance........................page 26

• Source of optical loss in a solar cell......................................page 27

ABSTRACT

This term paper provides us with a brief description on “SOLAR CELLS. A solar cell, or photovoltaic cell is an electrical device that converts the energy of lightdirectly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels.

Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light. They are used as a photodetector (for example infrared detectors), detecting light or otherelectromagnetic radiation near the visible range, or measuring light intensity.

The operation of a photovoltaic (PV) cell requires 3 basic attributes:

• The absorption of light, generating either electron-hole pairs or excitons.

• The separation of charge carriers of opposite types.

• The separate extraction of those carriers to an external circuit.

INTRODUCTION

A solar cell (or a "photovoltaic" cell) is a device that converts photons from the sun (solar light) into electricity.In general, a solar cell that includes both solar and nonsolar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell.Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity.This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.Solar cells have many applications.

Historically solar cells have been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, consumer systems, e.g. handheld calculators or wrist watches, remote radio-telephones and water pumping applications.

Solar cells are regarded as one of the key technologies towards a sustainable energy supply.

WHY SOLAR

While a majority of the world's current electricity supply is generated from fossil fuels such as coal, oil and natural gas, these traditional energy sources face a number of challenges including rising prices, security concerns over dependence on imports from a limited number of countries which have significant fossil fuel supplies, and growing environmental concerns over the climate change risks associated with power generation using fossil fuels. As a result of these and other challenges facing traditional energy sources, governments, businesses and consumers are increasingly supporting the development of alternative energy sources and new technologies for electricity generation. Renewable energy sources such as solar, biomass, geothermal, hydroelectric and windpower generation have emerged as potential alternatives which address some of these concerns. As opposed to fossil fuels, which draw on finite resources that may eventually become too expensive to retrieve, renewable energy sources are generally unlimited in availability.

Solar power generation has emerged as one of the most rapidly growing renewable sources of electricity. Solar power generation has several advantages over other forms of electricity generation:

• REDUCED DEPENDENCE ONFOSSIL FUELS.    Solar energy production does not require fossil fuels and is therefore less dependent on this limited and expensive natural resource. Although there is variability in the amount and timing of sunlight over the day, season and year, a properly sized and configured system can be designed to be highly reliable while providing long-term, fixed price electricity supply.

• ENVIRONMENTAL ADVANTAGES.    Solar power production generates electricity with a limited impact on the environment as compared to other forms of electricity production.

• MATCHING PEAK TIME OUTPUT WITH PEAK TIME DEMAND.    Solar energy can effectively supplement electricity supply from an electricity transmission grid, such as when electricity demand peaks in the summer

• MODULARITY AND SIMILARITY.    As the size and generating capacity of a solar system are a function of the number of solar modules installed, applications of solar technology are readily scalable and versatile.

• FLEXIBLE LOCATIONS.    Solar power production facilities can be installed at the customer site which reduces required investments in production and transportation infrastructure.

SOLAR CELL STRUCTURE

A solar cell is an electronic device which directly converts sunlight into electricity. Light shining on the solar cell produces both a current and a voltage to generate electric power. This process requires firstly, a material in which the absorption of light raises an electron to a higher energy state, and secondly, the movement of this higher energy electron from the solar cell into an external circuit. The electron then dissipates its energy in the external circuit and returns to the solar cell. A variety of materials and processes can potentially satisfy the requirements for photovoltaic energy conversion, but in practice nearly all photovoltaic energy conversion uses semiconductor materials in the form of a p-n junction.

Figure 1. Cross section of a solar cell

                                                                                      

TYPES OF SOLAR CELLS

1. FIRST-GENERATION

About 90 percent of the world's solar cells are made from wafers of crystalline silicon (abbreviated c-Si), sliced from large ingots, which are grown in super-clean laboratories in a process that can take up to a month to complete. The ingots either take the form of single crystals (monocrystalline or mono-Si) or contain multiple crystals (polycrystalline, multi-Si or poly c-Si). First-generation solar cells work like we've shown in the box up above: they use a single, simple junction between n-type and p-type silicon layers, which are sliced from separate ingots. So an n-type ingot would be made by heating chunks of silicon with small amounts of phosphorus, antimony, or arsenic as the dopant, while a p-type ingot would use boron as the dopant. Slices of n-type and p-type silicon are then fused to make the junction. A few more bells and whistles are added (like an antireflective coating, which improves light absorption and gives photovoltaic cells their characteristic blue color, protective glass on front and a plastic backing, and metal connections so the cell can be wired into a circuit), but a simple p-n junction is the essence of most solar cells. It's pretty much how all photovoltaic silicon solar cells have worked since 1954, which was when scientists at Bell Labs pioneered the technology: shining sunlight on silicon extracted from sand, they generated electricity.

2. SECOND- GENERATION

Classic solar cells are relatively thin wafers—usually a fraction of a millimeter deep (about 200 micrometers, 200μm, or so). But they're absolute slabs compared to second-generation cells, popularly known as thin-film solar cells (TPSC) or thin-film photovoltaics (TFPV), which are about 100 times thinner again (several micrometers or millionths of a meter deep). Although most are still made from silicon (a different form known as amorphous silicon, a-Si, in which atoms are arranged randomly instead of precisely ordered in a regular crystalline structure), some are made from other materials, notably cadmium-telluride (Cd-Te) and copper indium gallium diselenide (CIGS). Because they're extremely thin, light, and flexible, second-generation solar cells can be laminated onto windows, skylights, roof tiles, and all kinds of "substrates" (backing materials) including metals, glass, and polymers (plastics). What second-generation cells gain in flexibility, they sacrifice in efficiency: classic, first-generation solar cells still outperform them. So while a top-notch first-generation cell might achieve an efficiency of 15–20 percent, amorphous silicon struggles to get above 7 percent, the best thin-film Cd-Te cells only manage about 11 percent, and CIGS cells do no better than 7–12 percent. That's one reason why, despite their practical advantages, second-generation cells have so far made relatively little impact on the solar market.

3. THIRD-GENERATION

The latest technologies combine the best features of first and second generation cells. Like first-generation cells, they promise relatively high efficiencies (30 percent or more). Like second-generation cells, they're more likely to be made from materials other than "simple" silicon, such as amorphous silicon, organic polymers (making organic photovoltaics, OPVs), perovskite crystals, and feature multiple junctions (made from multiple layers of different semiconducting materials). Ideally, that would make them cheaper, more efficient, and more practical than either first- or second-generation cells.

SOLAR CELL OPERATION

The basic steps in the operation of a solar cell are :-

• the generation of light-generated carriers;

• the collection of the light-generated carries to generate a current;

• the generation of a large voltage across the solar cell; and

• the dissipation of power in the load and in parasitic resistances.

LIGHT GENERATED CURRENT

The generation of current in a solar cell, known as the "light-generated current", involves two key processes. The first process is the absorption of incident photons to create electron-hole pairs. Electron-hole pairs will be generated in the solar cell provided that the incident photon has an energy greater than that of the band gap. However, electrons (in the p-type material), and holes (in the n-type material) are meta-stable and will only exist, on average, for a length of time equal to the minority carrier lifetime before they recombine. If the carrier recombines, then the light-generated electron-hole pair is lost and no current or power can be generated.

A second process, the collection of these carriers by the p-n junction, prevents this recombination by using a p-n junction to spatially separate the electron and the hole. The carriers are separated by the action of the electric field existing at the p-n junction. If the light-generated minority carrier reaches the p-n junction, it is swept across the junction by the electric field at the junction, where it is now a majority carrier. If the emitter and base of the solar cell are connected together (i.e., if the solar cell is short-circuited), the the light-generated carriers flow through the external circuit.

COLLECTION PROBABILITY

The "collection probability" describes the probability that a carrier generated  by light absorption in a certain region of the device will be collected by the p-n junction and therefore contribute to the light-generated current, but probability depends on the distance that a light-generated carrier must travel compared to the diffusion length. Collection probability also depends on the surface properties of the device. The collection probability of carriers generated in the depletion region is unity as the electron-hole pair are quickly swept apart by the electric field and are collected. Away from the junction, the collection probability drops. If the carrier is generated more than a diffusion length away from the junction, then the collection probability of this carrier is quite low. Similarly, if the carrier is generated closer to a region such as a surface with higher recombination than the junction, then the carrier will recombine. The impact of surface passivation and diffusion length on collection probability is illustrated below.

Figure 2. The impact of surface passivation and diffusion length on collection probability

 The equation for the light-generated current density (JL), with an arbitrary generation rate (G(x))and collection probability (CP(x)), is shown below:-

where:

q is the electronic charge;

W is the thickness of the device;

α(λ) is the absorption coefficient;

H0 is the number of photons at each wavelength.

QUANTUM EFFICIENCY

The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell. The quantum efficiency may be given either as a function of wavelength or as energy. If all photons of a certain wavelength are absorbed and the resulting minority carriers are collected, then the quantum efficiency at that particular wavelength is unity. The quantum efficiency for photons with energy below the band gap is zero. A quantum efficiency curve for an ideal solar cell is shown below.

Figure 3. The quantum efficiency of a silicon solar cell

THE PHOTO VOLTAIC EFFECT

The collection of light-generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the "photovoltaic effect". The collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type side and holes to the p-type side of the junction. Under short circuit conditions, there is no build up of charge, as the carriers exit the device as light-generated current.

However, if the light-generated carriers are prevented from leaving the solar cell, then the collection of light-generated carriers causes an increase in the number of electrons on the n-type side of the p-n junction and a similar increase in holes in the p-type material. This separation of charge creates an electric field at the junction which is in opposition to that already existing at the junction, thereby reducing the net electric field. Since the electric field represents a barrier to the flow of the forward bias diffusion current, the reduction of the electric field increases the diffusion current. A new equilibrium is reached in which a voltage exists across the p-n junction. The current from the solar cell is the difference between IL and the forward bias current. Under open circuit conditions, the forward bias of the junction increases to a point where the light-generated current is exactly balanced by the forward bias diffusion current, and the net current is zero. The voltage required to cause these two currents to balance is called the "open-circuit voltage". The following animation shows the carrier flows at short-circuit and open-circuit conditions.

Figure 4. The Photovoltaic Effect

SOLAR CELL PARAMETERS

 IV CURVE

The IV curve of a solar cell is the superposition of the IV curve of the solar cell diode in the dark with the light-generated current. The light has the effect of shifting the IV curve down into the fourth quadrant where power can be extracted from the diode. Illuminating a cell adds to the normal "dark" currents in the diode so that the diode law becomes:

where IL = light generated current.

 SHORT CIRCUIT CURRENT

The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). Usually written as ISC. The short-circuit current is shown on the IV curve below.

Figure 5. IV Curve of a solar cell showing short circuit current

The short-circuit current depends on a number of factors which are described below:

• THE AREA OF THE SOLAR CELL. To remove the dependence of the solar cell area, it is more common to list the short-circuit current density (Jsc in mA/cm2) rather than the short-circuit current;

• THE NUMBER OF PHOTONS .(i.e., the power of the incident light source). Isc from a solar cell is directly dependant on the light intensity.

• THE SPECTRUM OF THE INCIDENT LIGHT. For most solar cell measurement, the spectrum is standardised to the AM1.5 spectrum;

• THE OPTICAL PROPERTIES (absorption and reflection) of the solar; and

• THE COLLECTION PROBABILITY of the solar cell, which depends chiefly on the surface passivation and the minority carrier lifetime in the base.

 OPEN CIRCUIT VOLTAGE

The open-circuit voltage, VOC, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. The open-circuit voltage is shown on the IV curve below.

Figure 6. IV curve of a soar cell showing open circuit voltage

 FILL FACTOR

The short-circuit current and the open-circuit voltage are the maximum current and voltage respectively from a solar cell. However, at both of these operating points, the power from the solar cell is zero. The "fill factor", more commonly known by its abbreviation "FF", is a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve.

Figure 7. Cell with low fill factor

As FF is a measure of the "squareness" of the IV curve, a solar cell with a higher voltage has a larger possible FF since the "rounded" portion of the IV curve takes up less area. The maximum theoretical FF from a solar cell can be determined by differentiating the power from a solar cell with respect to voltage and finding where this is equal to zero. Hence:

giving:

However, the above technique does not yield a simple or closed form equation. The equation above only relates Voc to Vmp, and extra equations are needed to find Imp and FF. A more commonly used expression for the FF can be determined empirically as

 EFFICIENCY

Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. In addition to reflecting the performance of the solar cell itself, the efficiency depends on the spectrum and intensity of the incident sunlight and the temperature of the solar cell. Therefore, conditions under which efficiency is measured must be carefully controlled in order to compare the performance of one device to another. Terrestrial solar cells are measured under AM1.5 conditions and at a temperature of 25°C. Solar cells intended for space use are measured under AM0 conditions.

The efficiency of a solar cell is determined as the fraction of incident power which is converted to electricity and is defined as:

where Voc is the open-circuit voltage;

where Isc is the short-circuit current; and

where FF is the fill factor

where η is the efficiency.

 DETAILED BALANCE

Detailed balance provides a technique to calculate the maximum efficiency of photovoltaic devices. Originally the method was proposed by Shockley and Queisser in 1961. An extended version was published in 1984 by Tiedje et al.

Detailed balance in its simplest and most common implementation makes several fundamental assumptions:

1. The mobility is infinite, allowing collection of carriers no matter where they are generated.

2. Complete absorption of all photons above the band gap.

The calculations for detailed balance calculations involve calculating the particle flux for different configurations of the Plank’s equation. The general form of the equation is:

RESISTIVE EFFECTS

 CHARACTERISTIC RESISTANCE

The characteristic resistance of a solar cell is the output resistance of the solar cell at its maximum power point. If the resistance of the load is equal to the characteristic resistance of the solar cell, then the maximum power is transferred to the load and the solar cell operates at its maximum power point. It is a useful parameter in solar cell analysis, particularly when examining the impact of parasitic loss mechanisms. The characteristic resistance is shown in the figure below.

Figure 8. Characteristic Resistance

The characteristic resistance of a solar cell is the inverse of the slope of the line, shown in the figure above, which, after Green , can be given as:

It can alternately be given as an approximation where:

 EFFECT OF PARASITIC RESISTANCES

Resistive effects in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances. The most common parasitic resistances are series resistance and shunt resistance. The inclusion of the series and shunt resistance on the solar cell model is shown in the figure below.

Figure 9. Parasitic series and shunt resistances in a solar cell circuit.

In most cases and for typical values of shunt and series resistance, the key impact of parasitic resistance is to reduce the fill factor. Both the magnitude and impact of series and shunt resistance depend on the geometry of the solar cell, at the operating point of the solar cell. Since the value of resistance will depend on the area of the solar cell, when comparing the series resistance of solar cells which may have different areas, a common unit for resistance is in Ωcm2. This area-normalized resistance results from replacing current with current density in Ohm's law as shown below:

 SERIES RESISTANCE

Series resistance in a solar cell has three causes: firstly, the movement of current through the emitter and base of the solar cell; secondly, the contact resistance between the metal contact and the silicon; and finally the resistance of the top and rear metal contacts. The main impact of series resistance is to reduce the fill factor, although excessively high values may also reduce the short-circuit current.

Figure 10. Schematic of a solar cell with series resistance.

I=IL−I0exp[q(V+IRS)nkT]I=IL−I0exp[q(V+IRS)nkT]

where: I is the cell output current, IL is the light generated current, V is the voltage across the cell terminals, T is the temperature, q and k are constants, n is the ideality factor, and RS is the cell series resistance.

 SHUNT RESISTANCE

Significant power losses caused by the presence of a shunt resistance, RSH, are typically due to manufacturing defects, rather than poor solar cell design. Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The effect of a shunt resistance is particularly severe at low light levels, since there will be less light-generated current. The loss of this current to the shunt therefore has a larger impact. In addition, at lower voltages where the effective resistance of the solar cell is high, the impact of a resistance in parallel is large.

Figure 11. Schematic diagram of solar cell with shunt resistance

Circuit diagram of a solar cell including the shunt resistance.

The equation for a solar cell in presence of a shunt resistance is:

I=IL−I0exp[qVnkT]−VRSHI=IL−I0exp[qVnkT]−VRSH

where: I is the cell output current, IL is the light generated current, V is the voltage across the cell terminals, T is the temperature, q and k are constants, n is the ideality factor, and RSH is the cell shunt resistance.

DESIGN OF SILICON CELLS

SOLAR CELL DESIGN PRINCIPLES

Solar cell design involves specifying the parameters of a solar cell structure in order to maximize efficiency, given a certain set of constraints. These constraints will be defined by the working environment in which solar cells are produced. For example in a commercial environment where the objective is to produce a competitively priced solar cell, the cost of fabricating a particular solar cell structure must be taken into consideration. However, in a research environment where the objective is to produce a highly efficient laboratory-type cell, maximizing efficiency rather than cost, is the main consideration.

In designing such single junction solar cells, the principles for maximizing cell efficiency are:

• increasing the amount of light collected by the cell that is turned into carriers;

• increasing the collection of light-generated carriers by the p-n junction;

• minimising the forward bias dark current;

• extracting the current from the cell without resistive losses.

 OPTICAL PROPERTIES

 OPTICAL LOSSES

Optical losses chiefly effect the power from a solar cell by lowering the short-circuit current. Optical losses consist of light which could have generated an electron-hole pair, but does not, because the light is reflected from the front surface, or because it is not absorbed in the solar cell. For the most common semiconductor solar cells, the entire visible spectrum (350 - 780 nm) has enough energy to create electron-hole pairs and therefore all visible light would ideally be absorbed.

Figure 12. Sources of optical loss in a solar cell.

There are a number of ways to reduce the optical losses:

• Top contact coverage of the cell surface can be minimised (although this may result in increased series resistance). This is discussed in more detail in Series Resistance;

• Anti-reflection coatings can be used on the top surface of the cell.

• Reflection can be reduced by surface texturing.

• The solar cell can be made thicker to increase absorption (although light that is absorbed more than a diffusion length from the junction has a low collection probability and will not contribute to the short circuit current).

• The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping.

The reflection of a silicon surface is over 30% due to its high refractive index. The reflectivity, R, between two materials of different refractive indices is determined by:

R=(n0−nSin0+nSi)2R=(n0−nSin0+nSi)2

where n0 is the refractive index of the surroundings and nSi is the complex refractive index of silicon.

CONCLUSION

A solar cell, or photovoltaic cell (in very early days also termed "solar battery" – a denotation which nowadays has a totally different meaning, see here), is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Solar cells are the building blocks of photovoltaic modules, otherwise known as solar panels.

Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light. They are used as a photodetector (for example infrared detectors), detecting light or other electromagnetic radiation near the visible range, or measuring light intensity.

The operation of a photovoltaic (PV) cell requires 3 basic attributes:

• The absorption of light, generating either electron-hole pairs or excitons.

• The separation of charge carriers of opposite types.

• The separate extraction of those carriers to an external circuit.

References

1. Fei Yuan, “solar cells”, Analog Circuits and Signal Processing, 2007

2. Kimmo Koli, “operation of solar cells”, Helsinki University of Technology Electronic Circuit Design Laboratory, Report 30, Espoo 2000.

3. A. Sedra and K.C. Smith, “working of solar cells”, IEEE Transactions on Circuit Theory, vol.17, pg. 132-133.

4. Petri Eloranta (Nokia), Prof Chris Toumazou. “silicon- History, Theory, Applications and Implementation”, Slides.

5. J.C.G. Lesurf, “Electronics”, lecture notes, 2010.

6. B. Razavi, “resistive effects”, McGraw-Hill, Inc. 2001.

7. http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/Analog.html

8. http://ellabedu.physics.upatras.gr/Pages/Single%20stage%20amps/Current/Current%20amplifier.htm

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