Part V. Solar Cells
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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 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:
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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 Radiation
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Solar radiation is primarily as electromagnetic radiation in the ultraviolet to
infrared region (0.2 ~ 3 m), from a nuclear
fusion reaction in the sun.
Solar constant
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Air mass (AM)
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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
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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
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 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 Principles
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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)
Voc 
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I 
kT
ln  L 
q
 IS 
Short-circuit current (Isc)
Isc = IL
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Maximum Output Power and The Fill Factor
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The output power P
P = Is V( eqV/kT – 1) – ILV
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Pm is obtained when dP/dV = 0
Vm  Voc 
kT  qVm 
ln 1 
q 
kT 

1 
I m  I L 1 
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 qVm kT 
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kT  qVm  kT 
Pm  I mVm  I L Voc 
1 

q 
kT  q 

The Fill Factor (FF)
FF 
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I mVm
I LVoc
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.
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Conversion Efficiency
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The power conversion efficiency of a
solar cell is
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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.
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FF  I L Voc
Pin
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).
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Degradation 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
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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.
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For Si solar cells at 300 K, the
recombination current can cause 25%
reduction in efficiency.
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Materials of Solar Cells
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Material requirement:
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Silicon
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GaAs, GaP, InP, etc.
heterojunction structures are used to enhance
the conversion efficiency.
II-VI compound semiconductors
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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
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A bandgap matching the solar spectrum
Having high carrier mobility
Having long carrier lifetime
CdS, CdSe, CdTe, etc.
Organic materials and others
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Design Considerations of the Device Structures
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Design considerations:
An ultra-thin (500-1000Å) window layer
to minimize surface recombination and
optical absorption in this layer
Broadband antireflection coating on top to
minimize reflection losses.
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4)
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)
The top finger stripes of contacts have to
be properly designed to keep the cell series
resistance to a low value.
Use of solar concentrator systems for
obtaining more power per solar cell.
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Various Device Configurations of Solar Cells
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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
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