design and engineering of solar photo voltaic cells

8/3/2019 Design and Engineering of Solar Photo Voltaic Cells

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DESIGN AND ENGINEERING OF PHOTOVOLTAIC

S YSTEMS

MEL 342

T

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APER

A BHISHEK M ALHOTRA

2008ME10477



SOLAR ENERGY : A N INTRODUCTION

The sun is the oldest source of energy known to man. It is the primary source of energy

for all processes on earth (with the exception of geothermal and nuclear energy) and this

energy manifests itself in various forms. It gets absorbed by the atmosphere and land

surfaces, gets converted to ocean currents, can be felt in the form of winds, drives thehydrological cycle, gets converted to chemical energy via photosynthesis etc. (Fig. 1)

Figure 1: Distribution of incoming solar radiation

For centuries man has been using the sun‟s energy indirectly in its various forms – starting with the energy in plants for food, then animals for food and work, trees for

shelter and fuel, the kinetic energy of wind and water for various purposes, and fossil

fuels for heat. Today, because of their relatively low cost and ease of use, coal, oil and

natural gas together account for 86.4% of the world‟s primary energy consumption.

However, with the increasing environmental impact of fossil fuel combustion and

because of the limited nature of their reserves, there is a need to reduce dependence on

them. This has led to increased interest in developing techniques to use solar radiation –

one of the most widely and readily available source of energy – economically and

efficiently.

THE R ESOURCE: SOLAR R ADIATION

The sun emits electromagnetic radiations at a mean temperature of 6000 K and the

distribution of radiation among different wavelengths is very similar to that of a black

body. Most of the electromagnetic radiation emitted by the sun lies in the visible band

centered at 500 nm, although the sun also emits significant energy in the ultraviolet and



Figure 4: Global variation of average annual insolation

In-depth knowledge of the solar resource is essential for its effective utilization.

Regional solar radiation maps at various scales are available which provide monthly

average daily total solar resource information on grid cells.

SOLAR PHOTOVOLTAIC CELLS: B ASIC PRINCIPLES

As the name suggests, solar photovoltaic cells are based on the photovoltaic effect. It isthe creation of voltage, or a corresponding electric current in a material, upon its

exposure to light. This phenomenon can be explained using the band theory of solids. A

semiconductor has a valence band and a conduction band with a small energy gap

between them. When it is exposed to solar radiation, the photons are absorbed and

impart energy to the electrons, exciting them from the valence band to the conduction

band and leaving a positive hole behind. The holes and electrons thus created are

responsible for the conduction of electricity within the semiconductor.

To create the potential difference, a junction between two types of semiconductors is

created – n-type having an excess of electrons and p-type having an electron deficit. N-type semiconductors are created by doping a tetravalent semiconductor with small

quantities of a pentavalent impurity, leading to an excess of electrons than usual in the

valence band. P-type semiconductors are created using a trivalent impurity, producing a

„hole‟ or an electron deficiency. When the two are brought in contact the excess electrons

from the n material diffuse into the p material. This results in the n-side becoming

positively charged and the p-side becoming negatively charged. The diffusion takes



place until a balance between the diffusion

and electromagnetic forces is attained. This is

called a p-n junction (Fig. 5).

When metallic contacts are placed on the n

and p regions, a diode is obtained. When aload is positioned at the cell‟s terminals,

because of the potential difference between

the two sides, the electrons from the n-side

migrate back to the holes in the p-side by way

of the outside connection, producing an

electric current.

A HISTORICAL PERSPECTIVE

E ARLY

EFFORTS

The photovoltaic effect, the direct conversion of solar energy to electricity, was first

documented by Edmond Becquerel in 1839. He demonstrated that a voltage was

produced when light was absorbed by an electrode immersed in an electrolyte. W. Smith

demonstrated an analogous effect in solids using trigonal selenium in 1873. Smith's

observation was confirmed in 1877 by W. G. Adams and R. E. Day, who also observed

the photovoltaic effects in selenium.

1940 – 1970: PRACTICAL A PPLICATION AND SLOW DEVELOPMENT

Nearly a century after its discovery, this laboratory phenomenon was considered as apotential source of electricity. Russell Ohl patented the modern junction semiconductor

solar cell in 1946, which was discovered while working on the series of advances that

would lead to the transistor. In 1954, workers at RCA and the Bell Telephone

Laboratories announced the construction of a silicon solar cell with an efficiency of 6%.

At first, solar cells using diffused silicon p-n junctions were used for toys and other

minor uses because of their high cost. The first major successful use was in the satellite

Vanguard I in 1958, the fourth artificial satellite to be launched.

Technological progress in photovoltaics has been slow since its inception mainly due to

the availability of cheap fossil fuels. For much of the 1960's and 1970's solar cells wereused for providing electrical power for space vehicles. These cells used a p-type

substrate into which a thin n-type layer was introduced by high temperature diffusion of

phosphorous impurities. The cells had an efficiency of only about 10%; however this

increased to 12% in terrestrial applications because of less light having shorter

wavelengths (Fig. 6(a)). This was another reason for their slow development and high

cost because cost was not a major issue for space applications and they did not try to

Figure 5: P-N Junction



look for lower cost solutions if they implied lower efficiencies. However, since 1975 the

use of terrestrial-based solar cells has surpassed their level of use in space programs and

this reflects the rapidly increasing interest in using solar cells as an alternate energy

source.

1970 – 1980: COST R EDUCTION AND HIGHER EFFICIENCY

A major development in solar photovoltaics was the price reduction brought about by

the efforts of Elliott Berman of the Solar Power Corporation in the early 70‟s. Earlier,

manufacture of solar cells was based on the manufacturing process used for electronic

semiconductor devices. It was realized that instead of polishing the wafers and then

coating them with anti-reflective coating, it was easier to directly use rough-sawn

wafers, which do not reflect light. Also, the silicon being used for electronics was too

pure for photovoltaic applications, and hence further cost reductions were achieved by

using lower quality silicon. Through these innovations, the cost of solar power was

brought down from $100/watt to $20/watt within two years. Since then, there has also been an improvement in the efficiency of solar cells by use of techniques such as alloying

aluminum into the rear cell surface, use of better antireflection materials (Fig. 6(b)),

surface texturing (in which selective chemical etching is used to form randomly located

pyramid structures across the cell surface to reduce reflection) (Fig. 6(c)) and

innovations in surface passivation and contact design.

Figure 6: Significant developments in solar cell design in the 60's and 70's



1980 – 1990: THE EMERGENCE OF THIN-FILM TECHNOLOGY

There have been recent developments in

photovoltaic technology with the emergence

of thin-film solar cells (TFSCs). The first

TFSC having efficiency exceeding 10% wasproduced in 1980 at The Institute of Energy

Conversion at University of Delaware. A

thin-film solar cell, also called a thin-film

photovoltaic cell (TFPV), is a solar cell that is

made by depositing one or more thin films of

amorphous photovoltaic material on a

substrate (Fig. 7). The thickness range of

such a layer is wide and varies from a few

nanometers to tens of micrometers. Eventhough the efficiency of such solar cells is

lower than silicon-wafer cells, they are much more economical, lightweight, durable and

easy to use. However, they are still not being used in large scale commercial applications

due to lower efficiency and larger area requirement per watt produced.

1990 – PRESENT: COST VS. EFFICIENCY

Another major change has been the introduction of polycrystalline silicon or polysilicon

for solar cells in the late 90‟s. It is a material consisting of small silicon crystals, which

can be recognized by a visible grain, a “metal flake effect”. This material has less

efficiency, but is less expensive to produce in bulk. For the first time, in 2006, over half of the world's supply of polysilicon is being used for production of renewable electricity

solar power panels.

However, the highest efficiencies achieved so far for any photovoltaic technology have

been those for multi-junction solar cells. In 2008, scientists at the U.S. Department of

Energy's National Renewable Energy Laboratory (NREL) set a world record in solar cell

efficiency with an inverted metamorphic triple-junction solar cell that converts 40.8

percent of incident light into electricity.

Figure 7: Structure of a thin-film solar cell



ENGINEERING A SPECTS OF SOLAR PHOTOVOLTAICS

MONOCRYSTALLINE SILICON W AFER CELLS AND SOLAR MODULES

Because the voltage of an individual silicon wafer solar cell is quite small (typically a

maximum voltage of 0.7V, with maximum power produced at 0.4V), a number of cellsare connected together to form a solar panel (or module), the number and arrangement

of cells depending on the application and required output. These cells sit on a tough

backing plate, while the grid of electrical connections lies above and below the surface of

the cells. The wafer has a full area metal contact made on the back surface, and a grid-

like metal contact made up of fine "fingers" and larger "busbars" are screen-printed onto

the front surface using a silver paste. The rear contact is also formed by screen-printing

a metal paste, typically aluminium. Usually this contact covers the entire rear side of the

cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at

several hundred degrees celsius to form metal electrodes in ohmic contact with the

silicon. Some companies use an additional electro-plating step to increase the cellefficiency.

After the metal contacts are made, the solar cells are interconnected in series (and/or

parallel) by flat wires or metal ribbons, and assembled into modules. Electrical

connecting strips go from the bottom of one cell to the top of the next, connecting cells

in series. The connections are made in series to achieve the desired output voltage and

in parallel to achieve the desired current. The cells are connected to each other and to

the rest of the system using thin strips which may contain silver, copper or other

conductive but non-magnetic transition metals.

Over this is a non reflective layer (silicon is naturally reflective), to increase light

absorption. Silicon nitride has gradually replaced titanium dioxide as the antireflection

coating because of its excellent surface passivation qualities. It prevents carrier

recombination at the surface of the solar cell. It is typically applied in a layer several

hundred nanometers thick using plasma-enhanced chemical vapor deposition (PECVD).

Some solar cells have textured front surfaces that, like antireflection coatings, serve to

increase the amount of light coupled into the cell. Such surfaces can usually only be

formed on single-crystal silicon, though in recent years methods of forming them on

multicrystalline silicon have been developed. Finally on top will be a layer of tough

tempered glass, and the whole structure is usually within an aluminum frame, sealedagainst the weather (Fig. 8).



Figure 8: Structure of a silicon wafer solar cell

THIN FILM SOLAR CELLS

The construction of modules using thin-film solar cells is slightly different. Thin-film

solar cells can be made from a variety of materials, including amorphous silicon (which

has no crystalline structure), gallium arsenide (GaAs), copper indium gallium diselenide

(CIGS) and cadmium telluride (CdTe).

Thin-film modules can be classified into two types: rigid thin-film modules and flexible

thin-film modules. In rigid thin-film modules, the cell is created on a glass substrate,and the electrical connections are created in situ, a so called "monolithic integration".

The substrate is laminated with an encapsulant to a front sheet, usually another sheet of

glass.

Flexible thin film cells and modules are created by depositing the photoactive layer and

other necessary layers on a flexible substrate. If the substrate is an insulator (e.g.

polyester or polyimide film) then monolithic integration can be used. If it is a conductor

then another technique for electrical connection must be used. The cells are assembled

into modules by laminating them to a transparent colourless fluoropolymer on the front

side (typically Ethylene tetrafluoroethylene (ETFE) or Fluorinated ethylene propylene(FEP)) and a polymer suitable for bonding to the final substrate on the other side. The

only commercially available flexible module uses amorphous silicon triple junction.

SILICON: In the thin-film silicon cell, the silicon is deposited on a substrate (glass, plastic

or metal coated with a layer of transparent conducting oxide (TCO)) using plasma-

enhanced chemical vapor deposition, from silane gas and hydrogen gas. The p-type layer

is kept on top where light intensity is higher because the mobility of electrons is much

higher as compared to the holes. So, the collection rate of electrons moving from the p-

to n-type contact is better than holes moving from p- to n-type contact.

COPPER INDIUM G ALLIUM DISELENIDE: There are two basic configurations of a CIGS thin

film solar cell – CIGS-on-glass (Fig. 9(a)) and CGIS-on-foil (Fig. 9(b)). The CIGS-on-

glass cell requires an extra layer of molybdenum to create an electrode. This is not

required in the CIGS-on-foil cell because the foil acts as the electrode. In both types, a

thin layer of zinc oxide (ZnO) on the other side acts as the other electrode. Two layers of



the semiconductor material and cadmium sulphide are sandwiched in between, and

these act as the n-side and p-side of the solar cell respectively.

Figure 9(a): CGIS-on-glass, (b): CGIS-on-foil

C ADMIUM TELLURIDE: The CdTe solar cell has a similar structure. One electrode is made

from a layer of carbon paste infused with copper, the other from tin oxide (SnO2) or

cadmium stannate (Cd2SnO4). The semiconductor in this case is cadmium telluride

(CdTe), which, along with cadmium sulfide (CdS), creates the n-type and p-type layers

required for the PV cell to function.

MULTI-JUNCTION SOLAR CELLS

Multi-junction cells use two or more layers of different materials with different band

gaps. The main motivation behind this is the fact that conventional photovoltaic cells

are unable to extract all the energy from a photon, and that they cannot extract any

energy at all from photons of certain wavelengths. If the photon has less energy than the

bandgap, it is not collected at all. This is a major consideration for conventional solar

cells, which are not sensitive to most of the infrared spectrum, although that represents

almost half of the power coming from the sun. Conversely, photons with more energy

than the bandgap, say blue light, initially eject a photon with much more energy than

the bandgap, but this extra energy is lost through a process known as "relaxation". This

lost energy turns into heat in the cell, which has the side-effect of further increasing blackbody losses. This problem limits the theoretical maximum efficiency of solar cells

to about 34%.

This problem is overcome in multi-junction solar cells by having more than one junction

with different band gaps. Depending on the substance, photons of varying energies are

absorbed. So by stacking higher band gap material on the surface to absorb high-energy

photons, while allowing lower-energy photons to be absorbed by the lower band gap

http://en.wikipedia.org/wiki/Infrared

http://en.wikipedia.org/wiki/Infrared



material beneath, much higher efficiencies can be attained. Theoretically, a solar cell

having infinite number of junctions can have efficiency as high as 87%. Commercial

samples having efficiency up to 30% are now available.

The majority of multi-junction cells that have been produced to date use three layers,

tuned to blue (on top), yellow and red (on the bottom). These cells require the use of semiconductors that can be tuned to specific frequencies, which has led to most of them

being made of gallium arsenide (GaAs) compounds, often germanium for red, GaAs for

yellow, and GaInP2 for blue. To date, their higher price and lower price-to-performance

ratio have limited their use to special roles, notably in aerospace where their high

power-to-weight ratio is desirable. The cost is mainly due to the complex structure and

the high pice of materials. In terrestrial applications these solar cells are used in

concentrated photovoltaics (CPV) with operating plants all over the world.

Table 1 shows a comparison of the different photovoltaic technologies available, with Fig

10 showing the relative market share for each.

Technology η (%) V OC (V) I SC (A) W/m² t (µm)

u c-Si 24.7 0.5 0.8 63 100

p c-Si 20.3 0.615 8.35 211 200

a-Si 11.1 6.3 0.0089 33 1

CdTe 16.5 0.86 0.029 – 5

CIGS 19.5 – – – 1

MJ 40.7 2.6 1.81 476 140

Table 1: Performance characteristics of different photovoltaic technologies

Figure 10: Relative market share for different photovoltaic technologies

http://en.wikipedia.org/wiki/Gallium_arsenide

http://en.wikipedia.org/wiki/Gallium_arsenide



Once the solar cells have been linked together to form solar modules, these are further

linked together to form a collection known as a solar array. The connections are made a

rationale similar to that followed for the connections among individual cells – they are

connected in series to achieve the desired voltage (each such set in series is called a

„string‟) and the individual strings are connected in parallel to allow the system to

produce more current.

B ALANCE OF S YSTEM

The balance of system or BOS includes all components of the solar system apart from

the solar modules. This includes the battery (in the case of off-grid installations), the

control unit and the inverter, the mechanical support structure, the electric cabling, and

protection devices such as fuses, grounding rods, and disconnect switches. Fig. 11

illustrates the costs incurred in the various components of a photovoltaic installation

with storage.

Figure 11: Cost breakup of a photovoltaic installation with storage

B ATTERIES: Photovoltaic systems can be broadly divided into two types – standalone

systems and grid-connected systems. Grid connected systems vary in size from

residential (2-10kWp) to solar power stations (up to 10s of GWp). Standalone systems

vary in scale and application from toys and calculators to buildings and space stations.

In many cases such as PV-powered pumps batteries are not required as water can be

pumped and stored during sunlight hours. If power is to be supplied independent of

solar insolation, it has to be stored using batteries. These comprise a significant portion

of the cost of a photovoltaic system, especially over the course of its life cycle because

batteries need to be replaced every four to eight years due to their relatively short

lifetime.

In most cases, lead-acid batteries are used because of their low cost. While automobile

batteries are optimized for providing strong current for a short period to start the car,

the ideal batteries for PV systems are so-called “deep cycle” batteries that can yield a

large fraction of their charge (deep discharge cycle) and must operate with high



efficiency and long duration. Yet, many PV applications use standard “shallow

discharge” auto batteries due to their ubiquitous availability and lower initial cost. Some

of the modern batteries such as those based on lithium ion or lithium polymer used in

laptop computers or mobile phones could be used in solar applications but they are too

expensive and are not significantly better than properly managed lead-acid batteries.

Alternative methods of storage exist but they will not replace lead-acid battery, at least,not in the next ten years.

INVERTER : The next requirement is an inverter, which converts the DC produced by the

photovoltaic module into AC. For large photovoltaic plants, this may either be a

centralized inverter, which is theoretically cheaper, or one may have a solar micro-

inverter on the back of every module, which increases the reliability of the system. The

inverter has an additional and important role: to vary the electrical operating point of

the PV array to maintain its output at the maximum value. Changes of temperature and

insolation change the voltage where maximum power extraction occurs, so the

electronics of the inverter typically include maximum power tracking.

TRACKING S YSTEM: To gather solar energy effectively, a solar panel should be within

about twenty degrees either side of perpendicular to the sun. An angle of more than

thirty-five degrees from the perpendicular results in a significant portion of sunlight

being reflected off the solar panel surface. In order to maximize the amount of solar

radiation incident on the solar modules, large photovoltaic installations that approach

or exceed one megawatt have tracking systems. These use optical sensors to track the

position of the sun and make sure that the panel is aimed directly at the sun or at the

brightest portion of a partially cloudy sky, improving the performance by upto 100%.

CONCENTRATED PHOTOVOLTAIC TECHNOLOGY

Solar cells may also be used along with solar radiation concentrating systems. This

offers two main advantages. The first is that since fewer solar cells are required to collect

the sunlight falling on a given area, higher cost technology can be used than in cases

where non-concentrated radiation is used to generate power, and therefore solar cells

having higher efficiency (such as multi-junction solar cells) can be used. The second is

that concentration of solar radiation enables more radiation per unit area to be incident

upon the solar cell, producing more power and hence reducing the area of the

photovoltaic cells, replacing it with less expensive optical material.

The concentrators used for this purpose can be either refractive or reflective. The lenses

used can have point focus, or may have a linear focus (Fig. 12(a)) and are usually made

of acrylic plastic (PMMA), which is much more durable than glass.



Figure 12(a): Refractive concentrators, (b): Reflective concentrators

Reflective concentrators use parabolic mirrors to concentrate sunlight from a large area

onto a single point or along a line (Fig. 12(b)).

Most concentrated photovoltaic applications use some kind of tracking mechanism to

maximize the amount of solar power being concentrated. With point concentration, two-

axis tracking is required, which is more difficult and expensive. However, they are able

to achieve a higher concentration ratio, reducing the cost of the solar cells. With linear

concentrators, one axis tracking can be used, which is less expensive but concentrates

the solar radiation over a larger area.

For high concentration ratios, the module also requires cooling systems because the

efficiency of solar cells deteriorates at higher temperatures. This further increases the

cost of the system, making it suitable for only large scale applications. So, before

installing any such system, there needs to be an in depth analysis of the improvements

in cost and efficiency achieved by using higher levels of concentration versus the

additional costs and complexities involved in doing the same.



R EFERENCES

Renewable energy: sources for fuels and electricity, T. B. Johansson, H. Kelly, A. K. N. Reddy, R.

H. Williams, L. Burnham (eds.), Island Press (1993)

Handbook of Photovoltaic Science and Engineering, A. Luque, S. Hegedus (eds.), John Wiley

and Sons Ltd., UK (2003)

A Guide to Photovoltaic (PV) System Design and Installation, California Energy Commission,

June 2001, 500-01-020

C. D. Mickey, Solar Photovoltaic Cells (1981), Journal of Chemical Education, Vol. 58 No. 5 pp.

418-423

A. G. Aberle, Thin Film Solar Cells (2009), Thin Solid Films, Vol. 517, Issue 17 pp. 4706-4710

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