Dr. Mohammad Aminul Islam Assistant Professor Dept. of Electrical & Electronic Engineering International Islamic University Chittagong SOLAR CELLS M A Islam, EEE, IIUC1

2 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, IIUC4 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. M A Islam, EEE, IIUC5

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 M A Islam, EEE, IIUC6

7 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

M A Islam, EEE, IIUC8 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).

M A Islam, EEE, IIUC9 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

M A Islam, EEE, IIUC10 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

M A Islam, EEE, IIUC12 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 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.

M A Islam, EEE, IIUC13 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

M A Islam, EEE, IIUC14 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

M A Islam, EEE, IIUC15 N-type P-type Single Solar cell

M A Islam, EEE, IIUC16 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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M A Islam, EEE, IIUC18 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

M A Islam, EEE, IIUC19 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 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.

M A Islam, EEE, IIUC20 Comparison of Types of solar cell MaterialEfficiency (%) Monocrystalline silicon14-17 Polycrystalline silicon13-15 Amorphous silicon5-7

M A Islam, EEE, IIUC21 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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M A Islam, EEE, IIUC24 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.

M A Islam, EEE, IIUC25 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.

M A Islam, EEE, IIUC26 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.

M A Islam, EEE, IIUC27 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:

M A Islam, EEE, IIUC28 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.

M A Islam, EEE, IIUC29 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 bandLow density E

M A Islam, EEE, IIUC30 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 bandLow density E junction

M A Islam, EEE, IIUC31 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 bandLow density E junction

M A Islam, EEE, IIUC32 Stage-4 When the PN junction is connected with external circuit, the current flows. Conduction band High density Valence bandLow density E junction Power

M A Islam, EEE, IIUC33 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

M A Islam, EEE, IIUC34 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

M A Islam, EEE, IIUC35 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$/watt)and Long Life  HIGH ABSORPTION COEFFICIENT > 10 5 cm- 1 with direct band gap ~1.5 eV  JUNCTION FORMATION ABILITY  HIGH QUANTUM EFFICIENCY  LONG DIFFUSION LENGTH  LOW RECOMBINATION VELOCITY  ABUNDANT,CHEAP & ECO-FRIENDLY MATERIAL o CONVENIENCE OF SHAPES AND SIZES o SIMPLE AND INEXPENSIVE INTEGRATED PROCESSING/MANUFACTURABILITY o MINIMUM MATERIAL / WATT o MINIMUM ENERGY INPUT/ WATT o ENERGY PAY BACK PERIOD < 2 YEARS o HIGH STABILTY and LONG LIFE (> 20 Years) o COST (< 1$/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

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