Solar Cell(Photovoltaics)

발표자 : 20075418 Ju Dae-Hyun

Solar Cell (PV)

LightLight ElectricityElectricity

Nonrenewable EnargyNonrenewable Enargy

재생에너지원 재생에너지원

(Renewable (Renewable Energy)Energy)

일회용에너지원 일회용에너지원

(Nonrenewable (Nonrenewable Energy)Energy)

RenewableRenewable

What is a Solar Cell?

• A structure that converts solar energy directly to DC electric energy.– It supplies a voltage and a current to a resistive load

(light, battery, motor).– Power = Current x Voltage=Current2 x R= Voltage2/R

• It is like a battery because it supplies DC power.• It is not like a battery because the voltage supplied by the

cell changes with changes in the resistance of the load.

Basic Physics of Solar Cells

• Silicon (Si) is from group 4 of the period table. When many Si atoms are in close proximity, the energy states form bands of forbidden energy states.

• One of these bands is called the band gap(Eg) and the absorption of light in Si is a strong function of Eg.

• The Sun daily provides about 10 000 times more energy to the Earth than we consume

• Photovoltaic technology directly converts solar energy into electricity

• No moving parts – no noise – no emissions – long lifetime• Large industrial potential - cost reductions needed• Feedstock for PV industry is silicon - the second most abundant

element in the crust of the Earth

The Sun as Energy Source

Solar Energy status

• Market is exploding• The solar industry is very profitable• Lack of highly purified silicon (polysilicon)• Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

Actual Growth vs. Historic Forecasts

Actual market

development

Solar Energy status

• Market is exploding• The solar industry is very profitable• Lack of highly purified silicon (polysilicon)• Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

159

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- Gross revenue development

Solar Energy status

• Market is exploding• The solar industry is very profitable• Lack of highly purified silicon (polysilicon)• Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

Solar Grade Silicon Supply-Demand (MT/year)

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SOG Polysilicon supply SOG Polysilicon demand

Solar Energy status

• Market is exploding• The solar industry is very profitable• Lack of highly purified silicon (polysilicon)• Cost of solar electricity is too high, R&D

focus on reducing cost and increasing efficiency

Cost reductions – existing technologies

• Thinner wafers- Wire sawing- Laser cutting and etching

• Higher efficiencies- Semiconductor technologies on single crystal wafers (examples Sanyo / SunPower)

• Thin film technologies (flat panel display)

Public incentives are important

Cost goals for third generation solar cells

Efficiency and cost projections for first-, second- and third generation photovoltaic technology (wafers, thin-films, and advanced thin-films, respectively)Source: University of New South Wales

Next generation technology

• Silicon nanostructures

Bandgap engineering of silicon. Applications could be tandem solar cells and energy selective contacts for hot carrier solar cells. Fabrication of silicon nanostructures consisting of quantum well and quantum dot super lattices to achieve band gap control

• Up/Down converters

Luminescent materials that:EITHER absorb one high energy photon and emit more than one low energy photon just above the bad gap of the solar cell (down-conversion) OR that absorb more than one low energy photon below the band gap of the cell and emit one photon just above the band gap (up-conversion).

Next generation technology (cont.)

Understanding cell efficiency

SOLAR SPECTRUM AM 1,5(1000 watt/m2)

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wavelength, nm

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Irradiation AM 1,5

Useful irradiation (c-Si)

• Hot carrier Cells

This concept tackles the major PV loss mechanism of thermalisation of carriers.

The purpose is to slow down the rate of photoexcited carrier cooling caused by phonon interaction in the lattice to allow time for the carriers to be collected whilst they are still hot, and hence increasing the voltage of a cell.

Next generation technology (cont.)

• Thermoelectric solar cells

Application of the concept of energy –selective electron transport used in hot carrier solar cells, to develop thermo electrics and thermo-ionics devices.

Next generation technology (cont.)

Polysilicon Wafer Solar Cell Solar Module

Chemical Process(purification)

CastingCutting

Surface Treatment Assembly

Systems

InstallationOperation

The PV Value Chain (multi-crystalline)

Prices are actually increasing

How does solar energy work?

Solar Electric or Photovoltaic Systems convert some of the energy in sunlight directly into electricity. Photovoltaic (PV) cells are made primarily of silicon, the second most abundant element in the earth's crust, and the same semiconductor material used for computers. When the silicon is combined with one or more other materials, it exhibits unique electrical properties in the presence of sunlight. Electrons are excited by the light and move through the silicon. This is known as the photovoltaic effect and results in direct current (DC) electricity. PV modules have no moving parts, are virtually maintenance-free, and have a working life of 20 - 30 years.

Silicon Solar cell

Photovoltaics

Most current solar cells are photovoltaicTypically made from silicon or amorphous silicon.

Typical efficiency ~ 12%.Best efficiency ever in laboratory: ~30%.Theoretical maximum, including concentrating light: 43%

Generic design: doped pn junction.Photons come in and photoionize donors.Built-in electric field at junction causes carriers to flow,building up a potential (voltage) btw the p and n sides.Clearly one can play with different band gap systems toarrive at materials with different absorption spectra.Also, good mobility of charge essential for this to work well -trapping of charge or poor mobility will kill efficiency.

Principle p-n Junction Diode.

Silicon Solar cell

Ref. Soft Condensed Matter physics group in univ. of Queenland

p n접합

앞면전극

p 층

n 층

반사방지막

전자정공

뒷면전극

전기

부하

정공

전자

기판준비 : Si ingot 330m 2cm x 2cm Surface cleaning

Texturing : chemical v-groove

p-n junction : POCl3 (900ºC)

ITO increasing minorty carrier correction, ARC

Poly-Si Solar cell Making process

Back Surface FieldDeposition Al and Ag ohmic-contact

Forward surface Electrode

Anti-reflection coating (ARC) TiO2 deposition

H2 diffusion dangling bond H2 bonding Decreasing recombination

Measure

H2 H2 H2 H2

- An individual PV cell typically produces between 1 and 2 watts

Solar Cell, Module, Array

• decrease the area of solar cell material being used in a system

Concentrator collectors

• Flat-plate collectors typically use large numbers or areas of cells that are mounted on a rigid, flat surface.

substrate ; metal, glass, plastic

• They are simpler to design and fabricate.

• They do not require special optics, specially designed cells, or mounting structures that must track the sun precisely. plus, flat-plate collectors can use all the sunlight

Flat-Plate Systems

Uses for Solar Energy

Solar Home Systems

Space

Water Pumping

Telecom

Main Application Areas – Off-grid

Residential HomeSystems (2-8 kW)

PV Power Plants( > 100 kW)

Commercial BuildingSystems (50 kW)

Main Application AreasGrid Connected

• Solar energy will become the most important and cost-efficient energy source in the future.

• The present lack of silicon feedstock is promoting a rapid development of next generation technology.

• Immediate actions are taken to cut thinner wafers and increase cell efficiencies for crystalline silicon.

• New thin film technologies are being developed• Stronger influence from semiconductor industry will accelerate the

development of better technologies• Nanosilicon and other third generation technologies may offer a

long-term solution for the future solar energy technology.

Conclusions