Dr. Mohammad Aminul Islam

Assistant Professor

Dept. of Electrical & Electronic Engineering

International Islamic University Chittagong

SOLAR CELLS

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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

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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

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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.

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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

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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

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• 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).

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• 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

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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

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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.

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• 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

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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

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N-type

P-type

Single Solar cell

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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.

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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

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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.

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Comparison of Types of solar cell

Material

Efficiency (%)

Monocrystalline silicon 14-17

Polycrystalline silicon 13-15

Amorphous silicon 5-7

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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.

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• 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.

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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.

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• 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.

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• 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:

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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.

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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.

Conduction band High density

Valence band Low density

E

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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.

Conduction band High density

Valence band Low density

E junction

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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.

Conduction band High density

Valence band Low density

E junction

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Stage-4

When the PN junction is connected with external circuit, the

current flows.

Conduction band High density

Valence band Low density

E junction

Power

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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

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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

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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

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• 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

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Appealing Characteristics

• Consumes no fuel

• No pollution

• Wide power-handling capabilities

• High power-to-weight ratio

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Solar cell – Working Principle

Operating diode in fourth quadrant generates power

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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 ?

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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

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Modeled Losses in a Standard Solar Cells

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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

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• 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

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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

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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

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Single, Tandem & Triple Junction Si Solar Cells

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Triple Junction a-Si:H/SiGe:H/nc-Si:H Solar Cells

Initial efficiency: 15.1%; Stable efficiency:

13.3%

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

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Gretzel- Dye Sensitive Solar Cell

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Organic Solar Cell

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History of Organic Solar Cell

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Summary of Exitonic-Organic Solar Cell

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Efficiency Enhancement by Fundamental Processes

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Thank You

Q & A

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