Solar Cell Project

Group members: Pan Zhe, Mathavan, Suresh, Kyaw Zin Naing, Phyo Thu Htet, Tin May Tun and Theint Theint Khaing.

A solar cell is a device that converts sunlight directly into electricity with no pollution. The other name for Solar cell is Photovoltaic cell. The term "photovoltaic" comes from the Greek : photo meaning "light", and "voltaic", from the name of the Italian physicist Volta, after whom the unit Volts is named. The modern age of solar power technology began in 1954 when Bell Laboratories, discovered that silicon doped with certain impurities was able to generate electricity . Today it is a rapidly growing and increasingly important renewable alternative to conventional fossil fuel to generate electricity.solar energy is a infinite source.

-Sunlight => electricity -Solar cell or photovoltaic cell -Modern age of solar cell began at 1950s -Impurities + silicon => Electrical characteristics -Renewable source -Pollution free

1. Cleaning the raw wafer. 2. Four- point sheet resistant measurement. 3. Doping the raw wafer. 4. Etching 5. Ellipsometer measurement 6. Photolithography 7. Metallization 8. Lift-off process 9. Anti- reflective layer coating 10. Open the window for the sunshine to shine at the solar cell.

Purpose: To remove contamination from the wafer.

Steps involved 5. Cleaning using chemical solutions(SC1,SC2) 6. Rinse using DI water 7. Spin dry (MAKE SURE during rinse and dry process the speed cannot over 2000rpm, if not wafer will fly away.)

Four-point probe sheet resistant measurement • The four point probe is use to measure the resistivity of wafers and crystal. • It is also used to measure the resistivity of thin layers of dopants added into the wafer surfaces by the dopant processes.

Sheet resistance formula Rs=4.53V/I 4.53 is constant(arises from the probe spacing. Unit; ohm per square

4-point probe measurement

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We have chosen 5 points in a wafer to measure sheet resistance to make sure all the bare wafers are in specification range.

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Our specific resistance range is around 100 to 200 ohm.

1 4

2 3

5

able of 4 point probe measurement of Wafer. Wafer 1

Point 1 (ohm) 0.9K

Point 2 (ohm) 0.9K

Point 3 (ohm) 0.9K

Point 4 (ohm) 0.8K

Point 5 (ohm) 0.8K

2

0.305K

0.3K

0.4K

0.305K

0.305K

3

0.4K

0.344K

0.345K

0.4K

0.35K

4

159.4

159.4

159.2

158

161.2

5

155.1

148.8

153.1

153.3

152.8

6

127.6

124.6

128.9

130.2

134

7

160

163.7

165.5

167.2

166.3

8

133.7

128.6

132.4

132.3

131.7

Dopant used: Phosphorous

Arrange the wafers and dopants alternatively in the quartz boat.

Equipment used: Furnace

Purpose: Doping is the process of using high temperature to distribute dopant from the region of high concentration to region of low concentration in the wafer to form p-n junction.

Sub-steps of doping are: •Pre-deposition •Drive-in

Purpose Pre-deposition: Saturate the expose wafer surface with lots of dopants. Drive-in: Drive the dopant to further diffuse deeper into the substrate.

Purpose: To selectively remove unwanted materials from the top layer of the wafer surface. Basic steps of etching: •

Dispersion

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Reaction

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Removal

Major types of etching: 7. Wet etching – Use liquid chemicals to remove unwanted materials. 8. Dry etching – Use a plasma to remove the unwanted materials.

Etching process The problems we are facing after doping process - There is a phosphorous glass layer will be formed on our wafer surface after doping process. - This layer will effect our process of making solar cell. - Therefore , we should etch away the phosphorous by etcher.

Ellipsometry is a technique for measuring the thickness and index of refraction of a wide range of films. Typically used for film thickness less than 100 angstroms in thickness.

Advantage: 5. Making non-contact 6. Nondestructive measurement.

Photolithography process •

Purpose of photolithography To transfer the circuit patterns from a mask to a light sensitive layer on the wafer using UV light. Photolithography defines the circuitry area on the wafer for future selective processing.

Three Basic Steps 1. Resist coating. 2. Exposure. 3. Developing.

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Resist coating - Resist coating is the first step of photolithography process. The wafer usually goes through priming before a layer of light sensitive resist is coated onto it using a spin bowl.

Exposure process The purpose of exposure is to transfer the image of the mask onto the surface of a resist coated wafer.

Developing It is one of the last steps in the photolithography process. The purpose of the develop step is to uncover the patterns from the mask exposure on the resist coated wafer.

Resist coating application •

The spinning vacuum chuck is one of the central components of a resist dispensing system. • The diameter of the chuck should be slightly smaller than the diameter of the wafer. Spin speed Higher speeds produce thinner coatings. Slower speeds result in thinker coatings. Extremely high spin speeds may also cause excessive drying which can cause striations in the resist. Extremely slow speeds may result in a less uniform coating. Spin time The wafer should be spun for an additional 15-20 seconds to obtain optimum uniformity and to further dry the resist.

-spinning speed is around 1000-1500 rpm -acceleration is 150 r/s -timing 30 seconds

Resist Baking There are three baking steps, namely Soft bake- Immediately after resist coating (to avoid particles contamination) to drive out solvents and to improve adhesion. Post Exposure Bake – Immediately after exposure of resist. To drive out residual solvents. Hard Bake – Immediately after developing of resist. To drive out remaining solvents. For our project specification range for baking is -temperature set to 90`C -timing 1.5-2 mins.

Exposure process •

One of the most important steps in the photolithography process is mask alignment. • The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. There are three primary exposure methods: • Contact • Proximity • Projection For exposure our specification timing is 25-30seconds.

Developing •

Once the wafer has been cleaned, dried, primed, coated and exposed, the next step is to develop the pattern on the wafer surface. Today, there are several different methods used for developing • Immersion. • Spray. • Puddle. For our specification range for developing is- for new chemical will take about 20 seconds but for old chemical meaning that is almost 2 days using for developing process, so we need to change the time setting depend on the chemical solution weakness.

Metallization Process Purpose of metallization during making solar cell: To deposit aluminum layer on both side of solar cell. the reason: semiconductor is high resistance material and poor conductivity so we should deposit both side aluminum to conduct current.

Thermal Evaporator It is to depositing Aluminum layer into the wafer.

Aluminum should be place here.

Wafer should be place firmly on this position.

What is lift-off process "Lift-off" is a simple, easy method for patterning films that are deposited. A pattern is defined on a substrate using photo-resist. A film, usually metallic, is blanket-deposited all over the substrate, covering the photo-resist and areas in which the photo-resist has been cleared. During the actual lifting-off, the photo-resist under the film is removed with solvent, taking the film with it, and leaving only the film which was deposited directly on the substrate .

Ideal Liftoff Process • Substrate • Spin photo-resist • Pattern photo-resist • Deposit • Remove photo-resist

Problem in process: •

1. If sidewalls are not perpendicular, the metal is likely to be continuous. There is no separation between the undesired and desired metal.

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2. Metal on substrate doesn’t fall perfectly flat. Parts can also pile up and cling to the resist sidewalls. After the lift, unpredictable results will occur.

• Solution Create an overhang in the resist profile using chlorobenzene.

Structure of 2 types semiconductor (impure silicon) P-type silicon

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N-type silicon

For P-type silicon these blue color are holes ,and unstable because there are no electrons inside, so very easy to absorb electrons and form P-type semiconductor. For N-type silicon there is one extra electron around yellow atoms that means these electrons are very active, and form N-type semiconductor.

How the solar cell work? A solar cell consists of two layers of semiconductor, one p-type and the other n-type, sandwiched together to form a 'p - n junction'.

When particles of light ('photons') are absorbed by the semiconductor the electrons and holes near the p - n junction are swept across in opposite directions by the action of the electric field and others diffuse towards the junction to replace them. This separation of charge induces a voltage across the device. By connecting the device to an external circuit, the electrons are able to flow and this flow of electrons is what we call electricity.

As the surface of solar cell can easily reflected lights so a anti-reflection later must be coat on this surface to reduce photons losses.

Anti-reflection coating Anti-reflection coating is a type of optical coating applied to surface of solar cell to reduce reflection , light reflection reduce ,means optical losses reduce that increase the absorption .

Normally there are 2 types surface: 1.flat surface 2.texturing surface

A square based pyramid which forms the surface of an appropriately textured crystalline silicon solar cell.

In SEM photo

Surface texturing Surface texturing, either in combination with an anti-reflection coating or by itself, can also be used to minimise reflection.

Calculate the efficiency of a solar cell. a) Put the solar cell in a circuit that results in maximum (maximum volts x amps). b) Measure the cell and calculate the surface area in c) Calculate the power output per m squared (PS) PS = Power output Area

Calculate the efficiency of the cell using the equation %%Efficiency = Energy input x 100% Energy output % Efficiency = PS x 100% P1 Example Our Wafer Voltage is 0.35 V While the Current is 2.14 mA. How do we find the Power Power = Voltage X Current P = V X I Power ?? = 0.35V X 0.0000214 A = 0.00000743 W

THE END