CH7_Solar Cell_ M A Islam_ppt

Dr. Mohammad Aminul Islam
Assistant Professor
Dept. of Electrical & Electronic Engineering
International Islamic University Chittagong
SOLAR CELLS
M A Islam, EEE, IIUC 1

Introduction
Photovoltaic effect
Electron-hole formation
A solar panel (or) solar array
Types of Solar cell
Principle, construction and working of
Solar cell
Advantage, disadvantage and
application
Lecture Outline

3
o Solar cell fundamentals
o Requirements for an ideal solar cell
o Thin film materials for viable solar cells
o Strengths and Weaknesses of various thin film
cells
o Comparative production status of various cells
o New concepts to enhance cell conversion
efficiency
o Concluding Remarks
OUTLINE
M A Islam, EEE, IIUC

Recap : Photo means light in Greek and Volt is the name of a
pioneer in the study of electricity Alessandro Volta
Solar cell: Solar cell is a photovoltaic device that converts
the light energy into electrical energy based on
the principles of photovoltaic effect
Albert Einstein was awarded the 1921 Nobel Prize in physics
for his research on the photoelectric effect—a phenomenon
central to the generation of electricity through solar cells.
In the early stages, the solar cell was developed only with 4 to
6 % efficiency( because of inadequate materials and problems
in focusing the solar radiations). But, after 1989, the solar cells
with more than 50% efficiency was developed.
1. Introduction

Three generations of solar cells
First Generation
First generation cells consist of large-area, high quality and
single junction devices.
First Generation technologies involve high energy and
labour inputs which prevent any significant progress in
reducing production costs.

Second Generation
Second generation materials have been developed to address
energy requirements and production costs of solar cells.
Alternative manufacturing techniques such as vapour
deposition and electroplating are advantageous as they
reduce high temperature processing significantly

Materials for Solar cell
Solar cells are composed of various semiconducting materials
1. Crystalline silicon
2. Cadmium telluride
3. Copper indium diselenide
4. Gallium arsenide
5. Indium phosphide
6. Zinc sulphide
Note: Semiconductors are materials, which become
electrically conductive when supplied with light or heat, but
which operate as insulators at low temperatures

• Over 95% of all the solar cells produced worldwide are
composed of the semiconductor material Silicon (Si). As the
second most abundant element in earth`s crust, silicon has
the advantage, of being available in sufficient quantities.
• To produce a solar cell, the semiconductor is contaminated
or "doped".
• "Doping" is the intentional introduction of chemical
elements into the semiconductor.
• By doing this, depending upon the type of dopant, one can
obtain a surplus of either positive charge carriers (called
p-conducting semiconductor layer) or negative charge
carriers (called n-conducting semiconductor layer).

• If two differently contaminated semiconductor layers are
combined, then a so-called p-n-junction results on the
boundary of the layers.
• By doping trivalent element, we get p-type semiconductor.
(with excess amount of hole)
• By doping pentavalent element, we get n-type
semiconductor ( with excess amount of electron)
n-type semiconductor
p- type semiconductor
p-n junction layer

2. Photovoltaic effect
Definition:
The generation
of voltage across the
PN junction in a
semiconductor due
to the absorption of
light radiation is
called photovoltaic
effect. The Devices
based on this effect
is called photovoltaic
device.
Light
energy
n-type semiconductor
p- type semiconductor
Electrical
Power
p-n junction

Solar cell – Working Principle
 Operating diode in fourth quadrant generates power

3. electron-hole formation
• Photovoltaic energy conversion relies on the number
of photons strikes on the earth. (photon is a flux of
light particles)
• On a clear day, about 4.4 x 1017 photons strike a square
centimeter of the Earth's surface every second.
• Only some of these photons - those with energy in
excess of the band gap - can be converted into
electricity by the solar cell.
• When such photon enters the semiconductor, it may
be absorbed and promote an electron from the
valence band to the conduction band.

• Therefore, a vacant is created in the valence band and it is
called hole.
• Now, the electron in the conduction band and hole in
valence band combine together and forms electron-hole pairs.
hole
Valence band
Conduction band
electron
Photons

4. A solar panel (or) Solar array
Single solar cell
• The single solar cell constitute the n-typpe layer
sandwiched with p-type layer.
• The most commonly known solar cell is configured as a
large-area p-n junction made from silicon wafer.
• A single cell can produce only very tiny amounts of electricity
• It can be used only to light up a small light bulb or power a
calculator.
• Single photovoltaic cells are used in many small electronic
appliances such as watches and calculators

N-type
P-type
Single Solar cell

Solar panel (or) solar array (or) Solar module
The solar panel (or) solar array is the interconnection of
number of solar module to get efficient power.
• A solar module consists of number of interconnected
solar cells.
• These interconnected cells embedded between two
glass plate to protect from the bad whether.
• Since absorption area of module is high, more energy
can be produced.

Based on the types of crystal used, soar cells can be classified as,
1. Monocrystalline silicon cells
2. Polycrystalline silicon cells
3. Amorphous silicon cells
1. The Monocrystalline silicon cell is produced from
pure silicon (single crystal). Since the Monocrystalline
silicon is pure and defect free, the efficiency of cell will be
higher.
2. In polycrystalline solar cell, liquid silicon is used as raw material
and polycrystalline silicon was obtained followed by solidification
process. The materials contain various crystalline sizes. Hence,
the efficiency of this type of cell is less than Monocrystalline cell.
5. Types of Si Solar cell

3. Amorphous silicon was obtained by depositing silicon
film on the substrate like glass plate.
•The layer thickness amounts to less than 1µm – the
thickness of a human hair for comparison is 50-100 µm.
•The efficiency of amorphous cells is much lower than that
of the other two cell types.
• As a result, they are used mainly in low power
equipment, such as watches and pocket calculators,
or as facade elements.

Comparison of Types of solar cell
Material Efficiency (%)
Monocrystalline silicon 14-17
Polycrystalline silicon 13-15
Amorphous silicon 5-7

6. Principle, construction and working of Solar cell
Principle: The solar cells are based on the principles
of photovoltaic effect.The photovoltaic effect is the
photogeneration of charge carriers in a light absorbing
materials as a result of absorption of light radiation.
Construction
• Solar cell (crystalline Silicon) consists of a n-type
semiconductor (emitter) layer and p-type semiconductor
layer (base). The two layers are sandwiched and hence
there is formation of p-n junction.
• The surface is coated with anti-refection coating to avoid the
loss of incident light energy due to reflection.

• A proper metal contacts are made on the n-type and p-
type side of the semiconductor for electrical connection
Working:
• When a solar panel exposed to sunlight , the light energies
are absorbed by a semiconduction materials.
• Due to this absorded enrgy, the electrons are libereted
and produce the external DC current.
• The DC current is converted into 240-volt AC current using
an inverter for different applications.

Mechanism:
• First, the sunlight is absorbed by a solar cell in a solar
panel.
• The absorbed light causes electrons in the material to
increase in energy. At the same time making them free to
move around in the material.
• However, the electrons remain at this higher energy for
only a short time before returning to their original lower
energy position.
• Therefore, to collect the carriers before they lose the
energy gained from the light, a PN junction is typically
used.

• A PN junction consists of two different regions of a
semiconductor material (usually silicon), with one side
called the p type region and the other the n-type region.
• During the incident of light energy, in p-type material,
electrons can gain energy and move into the n-type region.
• Then they can no longer go back to their original low
energy position and remain at a higher energy.
• The process of moving a light- generated carrier from
p-type region to n-type region is called collection.
• These collections of carriers (electrons) can be either
extracted from the device to give a current, or it can remain in
the device and gives rise to a voltage.

• The electrons that leave the solar cell as current give
up their energy to whatever is connected to the solar
cell, and then re-enter the solar cell. Once back in the
solar cell, the process begins again:

The mechanism of electricity production- Different stages
Conduction band High density
Valence band Low density
E
The above diagram shows the formation of p-n junction in a solar
cell. The valence band is a low-density band and conduction
band is high-density band.

Stage-1
Therefore, the hole
(vacancy position left
by the electron in the
valence band) is
generates. Hence, there
is a formation of
electron-hole pair on
the sides of p-n
junction.
When light falls on the semiconductor surface, the electron
from valence band promoted to conduction band.
E

Stage-2
In the stage 2, the electron and holes are diffuse across the
p-n junction and there is a formation of electron-hole pair.
E
junction

Stage-3
In the stage 3, As electron continuous to diffuse, the negative
charge build on emitter side and positive charge build on the
base side.
E
junction

Stage-4
When the PN junction is connected with external circuit, the
current flows.
E
junction
Power

7. Advantage, disadvantage and application of Solar cell
Advantage
1. It is clean and non-polluting
2. It is a renewable energy
3. Solar cells do not produce noise and they are totally
silent.
4. They require very little maintenance
5. They are long lasting sources of energy which can be
used almost anywhere
6. They have long life time
7. There are no fuel costs or fuel supply problems

Disadvantage
1. Soar power can be obtained in night time
2. Soar cells (or) solar panels are very expensive
3. Energy has not be stored in batteries
4. Air pollution and whether can affect the production
of electricity
5. They need large are of land to produce more
efficient power supply

Applications
1.Soar pumps are used for water supply.
1.Domestic power supply for appliances include
refrigeration, washing machine, television and lighting
1.Ocean navigation aids: Number of lighthouses and
most buoys are powered by solar cells
1.Telecommunication systems: radio transceivers on
mountain tops, or telephone boxes in the country can
often be solar powered
1.Electric power generation in space: To providing
electrical power to satellites in an orbit around the Earth

36
• Direct Conversion of light into electrical energy is called
PHOTOVOLTAICS (PV)
• Photovoltaic devices which convert solar energy into electricity are
called SOLAR CELLS
• Two electronically dissimilar materials (with different free electron
densities) brought together to form a junction with a barrier form
a PV device. Typical examples are :
metal1-oxide-metal2
metal-semiconductor (Schottky)
p-type semiconductor-n-type semiconductor (Homojunction)
n+-n semiconductor
p-type semiconductor(1)-n-type semiconductor(2) (Heterojunction)
p- (Insulator)-n
(p-i-n)1-(p-i-n)2- p-i-n)3 ………. (Multijunction)
Jct 1/Jct 2 /Jct 3 ………(Tandem)
SOLAR CELL: PHOTOVOLTAICS

Appealing Characteristics
• Consumes no fuel
• No pollution
• Wide power-handling capabilities
• High power-to-weight ratio

Solar cell – Working Principle
 Operating diode in fourth quadrant generates power

40
1.Cheap,Simple and Abundant Material
2.Integrated Large Scale Manufacturabilty
3.Cost (< 1$/watt)and Long Life
 HIGH ABSORPTION COEFFICIENT > 105 cm-1 with direct band gap ~1.5 eV
 JUNCTION FORMATION ABILITY
 HIGH QUANTUM EFFICIENCY
 LONG DIFFUSION LENGTH
 LOW RECOMBINATION VELOCITY
 ABUNDANT,CHEAP & ECO-FRIENDLY MATERIAL
o CONVENIENCE OF SHAPES AND SIZES
o SIMPLE AND INEXPENSIVE INTEGRATED PROCESSING/MANUFACTURABILITY
o MINIMUM MATERIAL / WATT
o MINIMUM ENERGY INPUT/ WATT
o ENERGY PAY BACK PERIOD < 2 YEARS
o HIGH STABILTY and LONG LIFE (> 20 Years)
o COST (< 1$/Watt)
What is essential for an ideal Solar Cell ?

Figure: Loss processes in a standard solar cell: (1) non absorption
of below bandgap photons; (2) lattice thermalization loss; (3) and
(4) junction and contact voltage losses; (5) recombination loss
(radiative recombination is unavoidable)
Loss Processes in a Standard Solar Cell

Modeled Losses in a Standard Solar Cells

43
Single Elements:
Si ( bulk, mc, nc, mixed)
Carbon (nanotubes, DLC)
Binary alloys / Compounds:
Cu2S, Cu2O Cu-C, CdTe, CdSe,
GaP, GaAs, InP,ZnP , a-Si :H, Dye coated TiO2
Ternary (+) Alloys / Compounds:
Cu-In-S, Cu-In-Se,Cu-Zn-S, CdZnSe , CdMnTe,
Bi-Sb-S, Cu-Bi-S, Cu-Al-Te, Cu-Ga-Se, Ag-In-S,
Pb-Ca-S, Ag-Ga-S, Ga-In-P, Ga-In-Sb ,and so on.
Organic-Inorganic Materials & Organic Materials:
Semiconducting Organics / Polymers and Dyes
POSSIBLE SOLAR CELL MATERIALS

44
• Crystalline Silicon solar cells
- Single, Multi, Ribbon
• Thin Film solar cells
- Silicon, Cu2 S , a-Si, m-Si,n-Si, CdTe,
CIGS, CNTS
• Concentrating solar cells
- Si, GaAs
• Dye, Organic ,Hybrid & other emerging
solar cells
• New Ideas
Solar Cell Technologies

45
o Efficiency of single crystal Si cells (Laboratory) has been rising steadily
to ~ 25% as a result of better understanding of the junction properties
and innovations in cell design and fabrication technologies.
o The world PV production of ~ 7900 MW in FY 2009 is primarily (~ 93%)
based on single, crystal and polycrystalline silicon.
o With increasing production of Si-PV from 200 kW in 1976 to 6900 MW in
2008, the cost of solar cells has decreased from $100 to about $3/Wp.
o With the existing technology and the material cost, the cost
of Si cells can not be decreased
significantly unless major innovations in the production of
appropriate quality silicon thin sheets take place.
Crystalline Silicon :Present Scenario

46
o Present day technology uses 8”or larger pseudo square of ~
200μ m thickness, with an efficiency of ~ 15-16%. The energy
(16-5 kWH/Wp) pay back period of such cells is ~3-4 years. The
module life is about 25 years.
o Specially designed silicon solar cells with efficiency ~ 18-20%
are being manufactured on a limited scale for special
applications (e.g for concentration).
o Polycrystalline silicon solar cells with efficiency ~ 12-14% are
being produced on large scale.
o Specially designed thin(~ 20 m) films silicon solar cells with
efficiency ~ 12% have been fabricated on a lab scale .
Production of hybrid thin film Si cells on MW scale is being
pursued
Crystalline Silicon :Present Scenario

Single, Tandem & Triple Junction Si Solar Cells

Triple Junction a-Si:H/SiGe:H/nc-Si:H Solar Cells
Initial efficiency: 15.1%; Stable efficiency:
13.3%

Amorphous Silicon (a-Si-H) : A Review

Gretzel- Dye Sensitive Solar Cell

Organic Solar Cell

History of Organic Solar Cell

Summary of Exitonic-Organic Solar Cell

Efficiency Enhancement by Fundamental Processes

Thank You
Q & A

CH7_Solar Cell_ M A Islam_ppt

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