Lecture 2
Introduction to solar energy and
semiconductor physics
Part 1
Introduction to solar energy
Nature’s power plant
The question is not how much energy do we have…
but how to harvest and use this power efficiently!!!
Question: How does the sun (or other stars) generate its huge power? Can we replicate that process on Earth?
The technical potential must not be confused with short-term economic potentials, since price
situations and capital requirements for activating these energy sources on a large scale are not
considered.
The theoretical potential does not take into account land use restrictions, conversion efficiencies,
storage requirements and so on.
23
Solar energy
Note: Light is electromagnetic wave
Light: an electromagnetic wave (carries energy)
Photons
Photons – quanta of light
Quantum theory describes wavelength dependency of
photon energy.
H
DEF = FG = F
I
H = 6.626×10-34 J s is the Planck constant
c = 3×108 m/s is the speed of light in vacuum
λ is the photon's wavelength
1eV = 1.602×10-19 J
Solar energy
• Expanding solar energy utilization is an important
step towards meeting the rising energy demand while
limiting CO2 emissions.
• Solar Energy = Heat + Light
• Solar thermal energy + Photovoltaic
Solar energy = take photon energy and transfer to electrons
3
Thermal energy = vibrating electrons/atoms in a material (or light)
Electric energy = flow of electrons
Chemical energy = electrons confined in chemical bonds
Solar Thermal Power Plant
Solar thermal power generation systems collect and
concentrate sunlight to produce the high temperature heat
needed to generate electricity.
Electromagnetic à Thermal à Electrical energy
Solar Photovoltaic
Photovoltaics (PV) is a term which covers the conversion
of light into electricity using semiconducting materials that
exhibit the *photovoltaic effect.
Alexandre Becquerel first
observed PV effect in 1839
*The photovoltaic effect is closely related to
Photoelectric effect
the photoelectric effect. In either case, light is
absorbed, causing excitation of an electron or
other charge carrier to a higher-energy state. The
main distinction is that the term photoelectric
effect is now usually used when the electron is
ejected out of the material (usually into a vacuum)
and photovoltaic effect used when the excited
charge carrier is still contained within the material.
Einstein won Nobel prize in 1904 1922
Albert Einstein developed
theory for photoelectric effect
Types of Photovoltaic Cells
Monocrystalline
Crystalline Silicon
Polycrystalline
CdTe
CIGS
Thin film
CIS
Amorphous Si
DSSC
Organic
Polymer OPV
Cumulative Photovoltaic Installations
Sustainability 9, 783 (2017)
Worldwide installed photovoltaics
none or unknown
<10 watts per inhabitant
10–100 watts per inhabitant
100–200 watts per inhabitant
200–400 watts per inhabitant
>400 watts per inhabitant
Price of photovoltaic cells
The Market and Cost of Solar Cell
26
25
29
Improvements still needed, but how?
PV system Future: Building integrated PV (BIPV)??
27
BOS system
• For practical applications, a large number of solar cells are
interconnected and encapsulated into units called PV modules, which
is the product usually sold to the customer. They produce DC current
that is typically transformed into the more useful AC current by an
electronic device called an inverter. The inverter, the rechargeable
batteries (when storage is needed), the mechanical structure to
mount and aim (when aiming is necessary) the modules, and any
other elements necessary to build a PV system are called the balance
of the system (BOS).
Thin-film and organic PV cells are suitable
28
How feasible is solar PV energy?
Let’s us assess critically.
1. Photovoltaics will require too much land area to ever meet significant fraction
of world needs.
Sunlight: 4 kWh/m2/day to represent a conservative worldwide average.
Efficiency of PV module is approximately 10%
4 × 10% = 0.4 kWh/m2/day
1000 MW coal or nuclear power plant that operates 24 hours/day (power a city)
This would require
1000 MW X 24 hr = 24000 MWh / day = 24,000,000 kWh / day
24,000,000 (kWh/day) / 0.4 (kWh/m2/day) = 60,000,000 m2 = 60 km2
So, with 60 km2 (or 24 square miles) of photovoltaics we could replace one of last century’s power plants. This is a
square 8 km (or 5 miles) on a side.
31
2. Photovoltaics can meet all of the world’s needs today if we would just pass laws
requiring photovoltaics and halting all fossil and nuclear plants.
• Besides the difficulty of convincing the people’s representatives to pass such a law,
the first technical problem faced would be the intermittent nature of the solar
radiation, available only during the day and strongly reduced in overcast skies.
Energy storage would solve this problem but no cheap storage method appears on
the horizon.
• Nevertheless, well-developed electric grids may accept large amounts of PV
electricity by turning off some conventional power plants when PV plants are
delivering power.
• Adequate grid management would allow up to 20 to 30% of the electric produced
by solar energy.
32
3. Photovoltaics cannot meet any significant faction of world needs. It will remain
a small-scale “cottage” industry that will only meet the needs of specialty
markets like remote homes in developing countries or space satellites.
Some used to be considered as specialty markets, for example, the category of “world
off-grid power” which is trying to supply power to the ∼1/3 of the world’s citizens
who lack it. The grid-connected market, whose growth has been meteoric in the past
decade, is by no means a small market.
33
4. Photovoltaics is polluting just like all high-technology or high-energy industries
only with different toxic emissions.
• One of the most valuable characteristics of photovoltaics is its well-deserved image
as an environmentally clean and “green” technology.
• This healthy image obviously results from the cleaner operation of a PV electricity
generator compared to a fossil-fuel fired generator, but this must also extend to the
manufacturing process itself as well as the recycling of discarded modules.
• Manufacturing of PV modules on a large scale requires the handling of large
quantities of hazardous or potentially hazardous materials (e.g. heavy metals,
reactive chemical solutions, toxic gases).
Accounting for the amount of CO2 produced during solar panel
manufacturing, solar panels generate, in effect, around 50g of
CO2 per kilowatt hour during their initial years of operation. This 34
is about 20 times less than the carbon output of coal-powered
electricity sources.
5. PV modules never recover all of the energy required in making them, thus they
represent a net energy loss.
• Among those who envision photovoltaics having an increasingly larger role in
producing the world’s electricity, there is awareness that photovoltaics must
produce much more energy than was required to produce the PV system.
Otherwise, it would be a net energy loss not a net energy source.
• The “energy payback” has been widely studied. It is described in terms of how
many years the PV system must operate to produce the energy required for its
manufacture. After the payback time, all of the energy produced is truly new
energy.
We need to:
1. Reduce PV manufacturing cost/environmental impact
2. Improve storage systems 35
3. Improve solar energy harvesting
• Develop new PV materials – higher efficiency?
• Increase PV area coverage – BIPV?
How solar panels are made
https://www.youtube.com/watch?v=2iRfbWOJtog
In-class discussion
• By 2030, the HKSAR government targets to use solar power to provide about 1% of its total annual
power consumption.
• Estimate the land area that we need to cover with solar PV cells to meet this target.
Part 2
Light-electricity energy conversion in
semiconductors
Energy carriers
• Energy carrier: water, wind, electron, ions, phonon (lattice vibration)... Basically,
“fluid”, i.e., something that can flow
Let’s consider the total energy of one
energy carrier only:
Etotal = P.E. (potential energy) + K.E.
(kinetic energy)
Note that there are many kinds of potential energies:
Gravitational, mechanical (such as a compressed spring), thermal, magnetic, electrochemical...
This is the reason why we can have many different energy storage systems.
Energy carriers in energy conversion cells
http://www.supa.ac.uk/Research/energy/
emerging-power-sources/thermoelectric-materials
1 Solar cell 2 Thermoelectric cell
5!
Energy carriers in these cells
2 Fuel cell 3 Daniell electrochemical cell
http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/
electrochem.html
Energy carriers
• In solar cells and thermoelectric cells: e- and h+
• In fuel cells and other electrochemical cells: e- and ion+ (or ion-)
Therefore, it is important to know energy carriers (in particular electrons) in different materials (metals, semiconductors, 6!
insulators…)
Key questions need to be answered on
free electrons in solids
• How many free electrons: free electron concentration (closely
related to current I)
• Electron energy level (closely related to voltage V)
• How free electrons are generated and transported (closely related to
both I and V, i.e., the power P (= I x V))
• “leakage” problem ! decrease in energy conversion efficiency
……
Remember that a current of electricity is created when electrons move
in a certain direction
Waste heat
Ecarrier
Reservoir #2
Energy carrier
(higher level)
Energy Pump Leakage Load Energy
input output
Energy carrier
(lower level) Reservoir #1
7!
(Valence) electrons in solid materials (insulators,
semiconductors and conductors)
For example, a typical solid-state p-n junction solar
cell consists of all three kinds of solid materials.
Ref.: M.S. Sze, Semiconductor Devices
Classification of materials in terms of their conductivity or 9!
resistivity
Periodic table of semiconductor materials
Si
All materials listed in this periodic table are of interest for electronic applications. However,
silicon (Si) and gallium arsenide (GaAs) are the most most important materials. Germanium
(Ge) is only of interest for niche applications.
GaAs is a compound semiconductor, meaning it is an alloy of gallium and arsenic. GaAs
is non-toxic in its solid state phase. GaAs is a III/V semiconductor, because it is
composed of material out of column III and column V of the periodic table. GaAs can be
seen as a alloy of gallium and arsenic. Other important materials out of the group of III/V
semiconductors are Indium Phosphide (InP), and Gallium Nitride (GaN). The electrical
and the optical properties of III/V compound materials are different from the properties of
silicon. The materials are of main interest for high speed electronics, photonics, optical
communication and high-end solar cells. 10!
Energy Bands
Two story parking bldg
six allowed
states at same
energy
two allowed
states at same
energy
The 4 remaining valence band electrons are bound weakly and can be involved in
chemical reactions. Therefore, we can concentrate on the outer shell (n=3 level). The
n=3 level consists of a 3s (n=3 and l=0) and a 3p (n=3 and l=1) subshells. The subshell
3s has two allowed quantum states per atom and both states are filled with an electron
(at 0 Kelvin). The subshell 3p has 6 allowed states and 2 of the states are filled with the
19!
remaining electrons.
Energy Bands
20!
3D
2D
21!
Band gaps
The “bandgap energy” can most simply be understood, as
the finite amount of energy needed to excite a highly
localized electron into a delocalized, excited state.
Bonds: Why material is tough Excited electrons: Why material conduct
• An atom in isolation has discrete electron energy levels.
• As atoms move closer together, as in a crystal, electron wave functions overlap.
• Electrons are Fermions, meaning two particles cannot occupy the same state.
• Discrete atomic electron energy levels split, forming bands.
• The gap between bands, denoting an energy range in which no stable orbitals exist,
is the “bandgap”.
Fermi-Dirac Distribution
The Fermi-Dirac probability density
function provides the probability
that an energy level is occupied by
a Fermion which is in thermal
equilibrium with a large reservoir. 7C
1
6 7 =
9 (;<;= )/@A + 1
(7C : Fermi Level)
Quantum mechanics: Pauli Exclusion principle states that no two identical fermions can share the same quantum
state simultaneously. (In contrast, Bosons can occupy same quantum states at same time, e.g. photons).
Materials classification
(conduction band)
(valence band)
Fermi level: an energy level of an electron, such that at thermodynamic
equilibrium this energy level would have a 50% probability of being occupied
at any given time.
Besides+“Energy”,+we+also+need+to+consider+“Momentum”:+
+Energy+Momentum+Diagram!
Dr.!F.!Liu@@@HKU:!MECH!6043! 6!
Dr.!F.!Liu@@@HKU:!MECH!6043! 9!
Dr.!F.!Liu@@@HKU:!MECH!6043! 10!
Light+absorp)on+and+emission+in+directSband+and+indirectSband+
semiconductors++
• Phonons are vibrational energy that arises from
oscillating atoms within molecules and crystals
• Energy carriers of thermal energy (heat)
• Needed in order for indirect-gap materials to
emit light (photons have negligible momentum)
• That’s why crystalline silicon are not good light
emitters (LED and lasers), AND also not the best
PV material
• i.e. a good PV material should ALSO be a good
Dr.!F.!Liu@@@HKU:!MECH!6043!
light
13!
emitting material (we will cover this later)
hGp://ned.ipac.caltech.edu/level5/Sept03/Li/Li4.html!
Just as in case of LED and lasers…
Being+“direct+semiconductor”+or+“indirect+semiconductor”+maTers!+for PV!!
Dr.!F.!Liu@@@HKU:!MECH!6043! 14!
