Post on 21-Dec-2015
Solar energy and solar cells
As another renewable energy
Photovoltaic power generation
Solar cells utilizes the photovoltaic effect with the related semiconductors.
Solar energy comes on the surface of the earth and the maximum flux density per 1 m2 is 1.366 kW, which is called the solar constant.
Most of the materials to produce solar cells are abundant on the earth.
Solar energy
Energy reaching the surface of the
earth (100%)
Photosynthesis (0.02%)
Stored in oceans (23%)
Transforming into kinetic energy of waves, winds, and currents
(0.2%)
Surface radiation of sun
(3.86x10 kW)23
Reflection from the surface of the
earth to space (30%)
Energy density1.366 kW/m2
Disadvantages of photovoltaic power
The cost per kilowatt is relatively high.The energy density is low. (It needs a
large space.)It depends on hours of sunlight.It requires an extra system to store
electricity.
Cost of electricity by sources
Nuclear power as the reference to compare
Plant types Cost
Nuclear 1
Coal 1.07
Natural gas 1.20
Hydropower 1.19
Wind turbine 1.11-1.94
Solar (large) 3.38-5.14
Solar (small) 3.75-4.30
Analysis of sunlight
Contents of spectra Visible rays (47%); Infrared rays (46%); and
Ultraviolet rays (7%)
Solar cells and sunlight spectra Crystallized silicon absorbs 0.4 m – 1.2 m. Amorphous silicon absorbs below 0.8 m.
Hours of sunshine
The sunniest places on earth
City Country Hours of sunshine per
year
Hours of sunshine per
day
Yuma, AZ USA 4015 11.0
Phoenix, AZ USA 3872 10.6
Asswan Egypt 3863 10.6
Las Vegas, NV USA 3825 10.5
Dongola Sudan 3814 10.4
Payback time
Energy Pay-Back Time (EPBT) EPT is defined as (entire energy spent through
the life cycle) (the energy produced for one year).
CO2 Pay-Back Time (CO2PBT) CO2PT is defined as (entire CO2 produced
through the life cycle) (the CO2 reduced by introducing a renewable energy system for one year).Most of solar cells have 1.4 to 2.7 years for CO2PBT.
Energy payback time
Renewable energy Payback time (year)
Biomass 2 ~ 6
Hydropower 0.6
Geothermal 1
Wind turbine 0.6 ~ 0.8
Solar cell 1.0 ~ 1.9
Wave ~ 3.8
Solar cells and semiconductors 1
Solar cells are made of semiconductors. N-type semiconductor:
Extra electrons become carriers to make current flows.
P-type semiconductor: Holes are become carriers for current.
I-type semiconductor: There is no extra electrons or holes. It needs certain
heat or light energy to produce extra electrons from the covalent bonds.
Solar cells and semiconductors 2
PN junction (single crystal) A basic structure of solar cells – The efficiency
is about 24.2%.
PIN junction (thin film) I-type is sandwiched by p-type and n-type
semiconductors. – The efficiency is about 10%, but it is inexpensive.
Materials for solar cells (semiconductors)
SiliconBoronPhosphorusTitanium dioxideGallium – arsenateCadmium – tellurium, etc.
The power of solar cells
Open-circuit voltage minimum voltage
Short-circuit current maximum current
Power (max. output) = operating voltage operating current
Current
Voltage
Short-circuitcurrent
Open-circuitvoltage
Voltage-current curve
Maximum power
Operatingvoltage
Operatingcurrent
Why the efficiency cannot be 100%
Light is reflected on the surface of the cell.Some of light transmits through the cell.It cannot absorb all of the wavelengths of
sun light.Part of carriers occur pair annihilation.Inside solar cells contain electric
resistance.
The structure of a single crystal solar cell
Glass substrate
Transparent electrode SnO2
Single silicon crystal (N)
Single silicon crystal (P)
Reflecting electrode (Ag)
150~300m
Higher efficiency 25%Higher cost
The structure of a polycrystalline solar cell
Glass substrate
Transparent electrode SnO2
Polycrystalline (N)
Reflecting electrode (Ag)
100~200m
Polycrystalline (P)
Lower efficiency 18%Lower cost
Amorphous silicon (a-Si) solar cell
Amorphous means non-crystallized. Rate of absorption is large.
This is because of random configuration of atoms. One can use various substrates and produce
thinner solar cells. Photo deterioration of a-Si reduces the output by
10%. However, the output under high temperatures is
better than the others. High temperature improves photodeterioration.
The structure of an amorphous solar cell
Transparent electrode SnO2
Amorphous (N)
Reflecting electrode (Ag)
Amorphous (P)
Less than 1m
Microcrystalline silicon (-Si) cell
Photo deterioration is small.Absorption rate is higher within
wavelengths of sun light.The efficiency is about 10%.The thickness is 2 ~ 3 m.The overall properties of -Si are between
crystallized and amorphous Si.
Multi-junction silicon solar cell
These are more efficient. Silicon-type: 20% or more; Compound-type:
35% or moreThese can absorb more wavelengths due
to multiple materials.Multiple structure makes it complicated
and increases internal losses of various properties.
Multiple junction makes it bulky and heavy.
Spherical silicon solar cells
The amount of silicon used is 1/5 to 1/30 compared with the other cells.
The spherical silicon is made by free fall and its surface tension.
The efficiency is 11% ~ 14%.
Reflector
Spherical silicon
P-type
N-type
Positive electrode
Negative electrode
Compound solar cells 1
Periodic tableGroup
number1 11 12 13 14 15 16
Groups for semiconduc
tors
I II III IV V VI
Period 1 H(Hydrogen)
Period 2 ___ ___ ___ B
(Boron)
C
(Carbon)
N(Nitrogen)
O(Oxygen)
Period 3 ___ ___ ___ Al(Aluminum)
Si
(Silicon)
P(Phosphorus)
S
(Sulfur)
Period 4 ___ Cu
(Copper)
Zn
(Zinc)
Ga
(Gallium)
Ge(Germanium)
As(Arsenic)
Se(Selenium)
Period 5 ___ Ag
(Silver)
Cd(Cadmium)
In
(Indium)
Sn
(Tin)
Sb(Antimony)
Te(Tellurium)
Compound solar cells 2
If the total of the number of electrons in the valence band is multiples of 8, it becomes a stable crystal.
Si-Si has: 4 + 4 = 8. Ga-As has: 3 + 5 = 8. Cu(In,Ga)Se2 has: 1+3+62=16. Cu2ZnSnS4 has: 12+2+4+64=32
I II III IV V VIGroup
The number of electrons in the valence band
1 2 3 4 5 6
Compound solar cells 3 (Classification)
Silicon-diamond structureIII-V: Zincblende structure (In, Ga, As, P)II-VI: Zincblende structure (Cd, Te, S)I-III-VI: Chalcopyrite structure CIS (Cu, In,
Se) and CIGS (Cu. In, Ga, Se)I-II-IV-VI: (Cu, Zn, Sn, S, Se)
Compound solar cells 4 (Example)
Examples of compound semiconductors
III-V group II-VI group I-III-VI group
Binary compound
AlAs GaAs InP InAs AlSb GaSb
GaP AlP
ZnS ZSe CdTe CdS ZnTe
Ternary compound
AlGaAs GaAsP GaInP AlGaSb
ZnSSe CdZnTe CIS: CuInS2
Quaternary compound
InGaAsP InGaAlP InAlGaAs InGaAsSb
CIGS: Cu(In,Ga)Se2
Compound solar cells 5
The most efficient ones are GaAs and InGaAs solar cells. (35.8%)
The theoretical efficiency of compound multi-junction solar cells: one layer = 37%; two layers = 50%; three layers = 56%; and 36 layers = 72%
Concentrator Photo Voltaic System
When the collection efficiency is n, the area of solar cell required is 1/n.
This system is used for more expensive semiconductors.
The current efficiency is about 40%.
lens
Solar cell
Cadmium-tellurium solar cell
Lower cost
This type of solar cell reached grid parity in 2009. (Namely, the cost is equal to the electric power generation.)
The amount of cadmium is very small and it will not harm the environment and human.
Organic-type solar cell 1
OSC (Organic Solar Cell) Conductive polymer and fullerene are used for
the surface layer.
DSC (Dye-sensitized Solar Cell) The mechanism is based on Grätzel cell. The pigments in electrolyte are positively
ionized to absorb electrons with light. Fluorine-doped tin oxide (FTO) and Indium tin
oxide (ITO) are used.
Organic-type solar cell 2
This type can be printed (OSC) Evaporation method Print-on method
The efficiency is about 7%.
DSC has achieved about 10% of efficiency.
Quantum dot solar cell 1
Confine electrons 3 dimensionally with the diameter of dozens of nanometers.
When photons hit the quantum dots, excitons are generated by the energy absorption.
Therefore, electrons and holes are emerged.
Quantum dot solar cell 2
The small quantum dots absorb light with shorter wavelengths.
The large quantum dots absorb light with longer wavelengths.
Small quantum dots
N-type Substrate
Reflecting reducing film
Transparent electrode
Surface electrode
Large quantum dots
Visible light Near infrared light
Quantum dot solar cell 3
This solar cell can utilizes wide range of light.
This absorbs various wavelengths by using different sizes of quantum dots to laminate the cell.
In theory, the efficiency can go up as the technique improves.