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Part V. Solar CellsPart V. Solar Cells

Introduction Basic Operation Mechanism Materials for Solar Cells Design Considerations of Solar Cell Various of Device Configurations Optical Concentration

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Introduction to Solar CellsIntroduction to Solar Cells

The solar cell is a semiconductor device that convert directly the solar energy into the electric energy by a photovoltaic (PV) effect.

Solar cells are useful for both space and terrestrial applications. Long-duration power supply for satellites. An alternative terrestrial energy source (safe, convenient and clear)

Advantages of solar power generation: Nearly permanent natural power source (~ 1010 years) Low operating cost (fuel and transportation cost are not needed) Virtually non-polluting Flexible module size Highly distributive

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Solar RadiationSolar Radiation

• Solar radiation is primarily as electro-magnetic radiation in the ultraviolet to infrared region (0.2 ~ 3 m), from a nuclear fusion reaction in the sun.

• Solar constant– The intensity of solar radiation in free space at

the average distance of the Earth from the sun.– The value of the solar constant is 1353 W/m2

• Air mass (AM)– The degree to which the atmosphere affects the

sunlight received at the Earth’s surface.– AM0 : the solar spectrum outside the Earth’s

atmosphere (1353 W/m2).AM1: the sunlight at the Earth’ s surface when the sun is overhead (at which point the incident is about 925 W/m2.

• Atmospheric attenuation of sunlight– Ultraviolet absorption in the ozone– Infrared absorption in the water vapor– Scattering by airborne dust and aerosols.

• GaAs solar cells are better matched to the solar spectral and provide greater efficiencies than the Si solar cells.

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Comparison between the Solar Cell and the PhotodiodeComparison between the Solar Cell and the Photodiode

1) For a photodiode only a narrow wavelength range centered at the optical signal wavelength is important,whereas for a solar cell, high spectral responses over a broad solar wavelength range are required.

2) Photodiodes are small to minimized junction capacitance,while solar cells are large-area devices

3) One of the most important figures of merit for photodiodes is the quantum efficiency,whereas the main concern for solar cells is the power conversion efficiency.

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Basic Operation PrinciplesBasic Operation Principles

A p-n junction device operating at the 4th quadrant of I-V curve under illumination . In the 4th quadrant, the junction voltage

is positive and the current is negative. Hence power is delivered to the external circuit.

Photovoltaic effect : The appearance of a forward voltage

across an illuminated junction. Ideal I-V characteristics:

I = Is ( eqV/kT – 1) – IL

Open-circuit voltage (Voc)

Short-circuit current (Isc)• Isc = IL

S

Loc I

Iq

kTV ln

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Maximum Output Power and The Maximum Output Power and The FillFill Factor Factor

• The output power P• P = Is V( eqV/kT – 1) – ILV

• Pm is obtained when dP/dV = 0

• The Fill Factor (FF)

• The fill factor is an important figure of merit for the solar cell design.

• The fill factor is about 0.7 ~ 0.83 for a Si cell and 0.8 ~ 0.9 for a GaAs cell.

kTqV

qkTVV m

ocm 1ln

kTqVII

mLm

11

qkT

kTqV

qkTVIVIP m

ocLmmm 1

ocL

mm

VIVIFF

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Conversion EfficiencyConversion Efficiency

• The power conversion efficiency of a solar cell is

• To maximize the efficiency, we should maximize all three items of FF, IL and Voc.

• The efficiency has a broad maximum and does not depend critically on Eg.

– Therefore, semiconductors with bandgaps between 1 ~ 2 eV can all be considered solar cell materials.

• The efficiency can be largely enhanced at an optical concentration of 1000 suns (C = 1000)

• A well-made Si cell can have about 10% efficiency (~ 100 W/m2 of electrical power under full illumination).

in

oc

PVIFF L

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Degradation Effects of the Conversion EfficiencyDegradation Effects of the Conversion Efficiency

The series resistance Rs from the ohmic loss in the front surface and the recombination current in the depletion region are two of the major factors that degrade the ideal efficiency.

The series resistance depends on– the junction depth– the impurity concentration of p-type

and n-type regions– the arrangement of the front surface.

The efficiency for the recombination current case is found to be much less less than the ideal current case due to the degradation of both Voc and the fill factor.

– For Si solar cells at 300 K, the recombination current can cause 25% reduction in efficiency.

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Materials of Solar CellsMaterials of Solar Cells

Material requirement:– A bandgap matching the solar spectrum– Having high carrier mobility– Having long carrier lifetime

• Silicon– Single-crystalline, poly-crystalline and

amorphous Si.– Although the a-Si solar cells (with an effective

bandgap of 1.5 eV) has lower efficiency than the single-crystal Si cells, their production costs are considerably lower. Therefore, a-Si solar cell is one of the major candidates for large-scale use of solar energy.

• III-V compound semiconductors– GaAs, GaP, InP, etc. – heterojunction structures are used to enhance

the conversion efficiency.• II-VI compound semiconductors

– CdS, CdSe, CdTe, etc. • Organic materials and others

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Design Considerations of the Device Design Considerations of the Device StructuresStructures

Design considerations:1) An ultra-thin (500-1000Å) window layer

to minimize surface recombination and optical absorption in this layer

2) Broadband antireflection coating on top to minimize reflection losses.

– The refraction index of the AR coating must be near or higher than 1.87

– SiO2 (n = 1.5), Si3N4 (n = 2.0), Al2O3 (n = 1.86), Ta2O5 (n = 2.25), TiO2 (n = 2.2)

3) The top finger stripes of contacts have to be properly designed to keep the cell series resistance to a low value.

4) Use of solar concentrator systems for obtaining more power per solar cell.

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Various Device Configurations of Solar CellsVarious Device Configurations of Solar Cells

• The “back surface field” (BSF) solar cell• The “textured” solar cell• The V-groove multi-junction solar cell• The tandem-junction solar cell• The vertical-junction solar cell• Heterojunction solar cell• Schottky-Barrier solar cell• MIS solar cell• Thin-Film solar cell• Amorphous solar cell