Download Slide 1

Solar Photovoltaic Physics
Basic Physics and
Materials Science of Solar Cells
Original Presentation by J. M. Pearce, 2006
Email: [email protected]
What are Photovoltaics?
• Photovoltaic (PV) systems convert light energy
directly into electricity.
• Commonly known as “solar cells.”
• The simplest systems power the small calculators
we use every day. More complicated systems will
provide a large portion of the electricity in the
near future.
• PV represents one of the most promising means of
maintaining our energy intensive standard of living
while not contributing to global warming and
pollution.
A Brief History
Photovoltaic Technology
•
•
•
•
1839 – Photovoltaic effect discovered by Becquerel.
1870s – Hertz developed solid selenium PV (2%).
1905 – Photoelectric effect explained by A. Einstein.
1930s – Light meters for photography commonly
employed cells of copper oxide or selenium.
• 1954 – Bell Laboratories developed the first crystalline
silicon cell (4%).
• 1958 – PV cells on the space satellite U.S. Vanguard
(better than expected).
Things Start
To Get Interesting...
• mid 1970s – World energy crisis = millions spent in research and
development of cheaper more efficient solar cells.
• 1976 – First amorphous silicon cell developed by Wronski and
Carlson.
• 1980’s - Steady progress towards higher efficiency and many new
types introduced
• 1990’s - Large scale production of solar cells more than 10%
efficient with the following materials:
– Ga-As and other III-V’s
– CuInSe2 and CdTe
– TiO2 Dye-sensitized
– Crystalline, Polycrystalline, and Amorphous Silicon
• Today prices continue to drop and new “3rd generation”
solar cells are researched.
Types of Solar
Photovoltaic Materials
Photovoltaic Materials
Electronic Structure of
Semiconductors
• Silicon
• Group 4 elemental
semiconductor
• Silicon crystal forms the
diamond lattice
• Resulting in the use of
four valence electrons of
each silicon atom.
Crystalline
Silicon
Amorphous Silicon
Solar PV Materials:
Crystalline & Polycrystalline Silicon
• Advantages:
– High Efficiency (14-22%)
– Established technology
(The leader)
– Stable
• Disadvantages:
– Expensive production
– Low absorption coefficient
– Large amount of highly
purified feedstock
Amorphous Silicon
Advantages:
• High absorption (don’t need a lot
of material)
• Established technology
• Ease of integration into buildings
• Excellent ecological balance sheet
• Cheaper than the glass, metal, or
plastic you deposit it on
Disadvantages:
• Only moderate stabilized efficiency 710%
• Instability- It degrades when light hits it
– Now degraded steady state
How do they work?
The physics view
Band Theory
Ef
Eg
Ef
Metal
Insulator
Ef
Semiconductor
• There are 3 types of
materials in Band
Theory, which are
differentiated by their
electronic structure:
– insulators,
– conductors, and
– semiconductors.
Energy Bands in a
Semiconductor
• Conduction
Band – Ec –
empty
• Valence
Band – Ev –
full of
electrons
3 Types of Semiconductors
1. Intrinsic
2. n-type
3. p-type
•
Types 2 and 3 are semiconductors that
conduct electricity - How?
–
–
by alloying semiconductor with an impurity, also
known as doping
carriers placed in conduction band or carriers
removed from valence band.
Note: Color Protocol
Type 1: Intrinsic
• Pure semiconductor
(intrinsic): contains the
right number of electrons
to fill valence band,
therefore, conduction
band is empty.
• Because electrons in full
valence cannot move, the
pure semiconductor acts
like an insulator.
Type 2: n-Type
• n-type: current is carried by
negatively charged electrons
- How?
– group 5 impurity atoms added
to silicon melt from which is
crystal is grown
– 4/5 of outer electrons used to
fill valence band
– 1/5 left is then put into
conduction band. These
impurity atoms are called
donors.
Within conduction
band the electrons
are moving,
therefore, crystal
becomes a
conductor
Type 3: p-Type
• p-Type: current carried by
missing electron holes which
act as positively charged
particles. How?
– group 3 added to silicon melt
– need 4 out of 5 outer electrons
but doping creates lack of
electrons in valence band.
– missing electrons, a.k.a holes,
are used to carry current.
What Carries the Current?
• Prevailing charges are called the majority
carriers
– prevailing charge carrier in n-type: electrons
– prevailing charge carrier in p-type: holes
Creating a Junction
• There are four main types of semiconductor
junctions
–
–
–
–
p-n
p-i-n
Schottcky barrier
Heterojunction
• Each has a built in potential
p-n and p-i-n Junctions
Vbi
Ef
Vbi
Ef
Schottky Barriers and
Heterojunctions
Semiconductor Junctions
• All the junctions contain strong electric field
• How does the electric field occur?
– When two semiconductors come into contact, electrons
near interface from n-type, transfer over to p-type, leaving
a positively charged area
– Holes from p-type by interface transfer over to n-type
leaving a negatively charged area.
– Because electrons and holes are swapped, a middle
potential barrier with no mobile charges, is formed.
– This potential barrier created does not let any more
electrons or holes flow through.
• Electric field pulls electrons and holes in opposite
directions.
Barrier Changes
• Equilibrium means there
is no net current
• Reduced barrier height is
called forward bias
(positive voltage applied to
p-side)
– Result- increases current
through diode
• Increased barrier height is
called reverse bias.
– Result- decreases current to
a very small amount..
Electric Currents in p-n
Junction Under External Bias
Diode I-V Characteristics
Current in a Solar Cell
• Output current = I = Il-Io [ exp(qV/kT)-1]
–
–
–
–
Il=light generated current
q = electric charge
V = voltage
k = Boltzman’s constant = 1.3807 × 10-23 J/K
• When in open circuit (I=0) all light generated current passes
through diode
• When in short circuit (V=0) all current passes through
external load
2 Important points:
1) During open circuit the voltage of open circuit,
Voc = (kT/q) ln( Il/Io +1)
2) No power is generated under short and open circuit - but
Pmax = VmIm=FFVocIsc
I-V Curve for Solar Cells
Fourth quadrant (i.e., power quadrant) of the illuminated I-V
characteristic defining fill factor (FF) and identifying Jsc and Voc
Light Absorption by a
Semiconductor
•
•
•
Photovoltaic energy relies on light.
Light → stream of photons → carries energy
Example: On a clear day 4.4x1017 photons hit 1 m2
of Earth’s surface every second.
• Eph()=hc/ =hf
– h = plank’s constant = 6.625 x 10-34 J-s
–  = wavelength
– c = speed of light =3 x 108 m/s
– f = frequency
• However, only photons with energy in excess of
bandgap can be converted into electricity by solar
cells.
The Solar Spectrum
The entire spectrum is
not available to single
junction solar cell
Generation of Electron
Hole Pairs with Light
• Photon enters, is absorbed,
and lets electron from VB
get sent up to CB
• Therefore a hole is left
behind in VB, creating
absorption process:
electron-hole pairs.
• Because of this, only part
of solar spectrum can be
converted.
• The photon flux converted
by a solar cell is about 2/3
of total flux.
Generation Current
• Generation Current = light induced electrons across bandgap
as electron current
• Electron current:= Ip=qNA
– N = # of photons in highlighted area of spectrum
– A = surface area of semiconductor that’s exposed to light
• Because there is current from light, voltage can also occur.
• Electric power can occur by separating the electrons and holes
to the terminals of device.
• Electrostatic energy of charges occurs after separation only if
its energy is less than the energy of the electron-hole pair in
semiconductor
• Therefore Vmax=Eg/q
• Vmax= bandgap of semiconductor is in EV’s, therefore this
equation shows that wide bandgap semiconductors produce
higher voltage.
Direct vs Indirect Bandgap
• Everything just talked about, where all energy
in excess of bandgap of photons are
absorbed, are called direct-bandgap
semiconductors.
• More complicated absorption process is the
indirect-gap series
– quantum of lattice vibrations, of crystalline
silicon, are used in the conversion of a photon
into electron-hole pair to conserve momentum
there hindering the process and decreasing the
absorption of light by semiconductor.
The Solar Cell
•
Electric current generated in semiconductor is extracted by
contacts to the front and rear of cell.
Widely spaced thin strips (fingers) are created so that light is
allowed through.
•
–
these fingers supply current to the larger bus bar.
• Antireflection
coating (ARC) is
used to cover the
cell to minimize
light reflection
from top surface.
• ARC is made
with thin layer of
dielectric material.
Different Types of
Photovoltaic Solar Cells
Diffusion
Drift
Excitonic
Diffusion
• n-type and p-type are
aligned by the Fermilevel
• When a photon comes in
n-type, it takes the place
of a hole, the hole acts
like an air bubble and
“floats” up to the p-type
• When the photon comes
to the p-type, it takes
place of an electron, the
electron acts like a steel
ball and “rolls” down to
the n-type
Diagram of p-n Junction and
Resultant Band Structure
Drift
• There is an intrinsic
gap where the
photon is absorbed
in and causes the
electron hole pair to
form.
• The electron rises
up to the top and
drifts downwards
(to n-type)
• The hole drifts
upwards (to p-type)
Excitonic Solar Cell
• Dye molecule
– electron hole pair
splits because it
hits the dye
– the electron shifts
over to the electric
conductor and the
hole shifts to the
hole conductor
Power
Losses in
Solar Cells
Recombination
• Opposite of carrier generation, where
electron-hole pair is annihilated
• Most common at:
– impurities
– defects of crystal structure
– surface of semiconductor
• Reducing both voltage and current
Series Resistance
• Losses of resistance caused by
transmission of electric current
produced by the solar cell.
• I-V characteristic of device:
• I = Il-I0 [exp(qV+IRs / mkT) – 1]
• m= nonideality factor
Other Losses
• Current losses- called collection efficiency,
ratio b/w number of carriers generated by
light by number that reaches the junction.
• Temperature dependence of voltage
– V decreases as T increases
• Other losses
– light reflection from top surface
– shading of cell by top contacts
– incomplete absorption of light
Minimize Recombination Losses
by Adapting the Device
Tandem Cells
Silver Grid
Indium Tin Oxide
p-a-Si:H
Blue Cell
i-a-Si:H
n-a-Si:H
p
Green Cell
i-a-SiGe:H (~15%)
n
p
Red Cell
i-a-SiGe:H (~50%)
n
Textured Zinc Oxide
Silver
Stainless Steel Substrate
Schematic diagram of state-of-theart a-Si:H based substrate n-i-p
triple junction cell structure.
• Tandem cellseveral cells,
– Top cell has
large bandgap
– Middle cell mid
eV bandgap
– Bottom cell
small bandgap.
Solar Photovoltaics
is the Future
Acknowledgements
• This is the first in a series of presentations created
for the solar energy community to assist in the
dissemination of information about solar
photovoltaics.
• This work was supported from a grant from the
Pennsylvania State System of Higher Education.
• The author would like to acknowledge assistance in
creation of this presentation from Heather Zielonka,
Scott Horengic and Jennifer Rockage.