Transcript H.S. Semiconductor Physics of Solar Cells I

Basic Fundamentals
of
Solar Cell Semiconductor Physics
Review Topics
Wavelength and Frequency
amplitude
Period (sec)
time
Frequency (n) = 1/Period [cycles/sec or Hertz]
Wavelength (l) = length of one Period [meters]
For an electromagnetic wave c = nl, where c is the speed
of light (2.998 x 108 m/sec)
Spectrum
Intensity
Frequency (n)
Range of frequency (or wavelength, c/n) responses or source emissions.
The human eye has a response spectrum ranging from a wavelength of
0.4 microns (0.4 x 10-6 meters) (purple) to 0.8 microns (red)
Solar Spectrum at Earth Surface (noon time)
925 W/m2
E (eV) = hc/l
l = hc/E
Visable range
.75 mm (red) - .4 mm (purple)
1.6 eV - 3.1 eV
Solar Spectrum at Earth Surface
.5 eV - 3.6 eV
2.6 mm (infrared) - 0.34 mm (ultraviolet)
visible
inrfared
ultraviolet
Solar Spectrum
at Earth Surface
(noon time)
Energy and Power
Electromagnetic waves (light, x-rays, heat) transport
energy.
E = hn or hc/l [Joules or eV (electron-volts)]
1 eV = 1.6 x 10-19 Joules
h = Plank’s constant (6.625 x 10-34 Joule-sec or
4.135 x 10-15 eV-sec)
n = frequency
c = speed of light
l = wavelength
Power is the amount of energy delivered per unit time.
P = E/t [Joules/sec or Watts]
Photons
A light particle having energy. Sunlight is a spectrum of
photons. X-rays and heat are photons also.
Photon Energy
E = hn or hc/l [Joules or eV (electron-volts)]
(higher frequency = higher energy)
(lower energy)
Irradiance
Amount of power over a given area, Watts/m2
4 red photons every second
Area = 2.00 m2
Energy of 1 red photon = hc/l = (6.63 x 10-34 J-s)(2.99 x 108 m/s)/(0.80 x 10-6 meters)
= 2.48 x 10-19 J = 1.55 eV
Irradiance = Power/Area = (4 photons/sec)(Energy of 1 photon)/2.00 m2
= 4.96 x 10-19 W/m2
Typical sunlight irradiance is 0.093 W/cm2 = 930 W/m2 at l = .55 mm
Transmission, Reflection, and Absorption
incident light
air
material
reflectance (R)
transmittance (T) + absoprtance (A)
• Incident light = T + R + A = 100%
• Non-transparent materials have either very high
reflection or very high absorption.
• Absorption decreases transmission intensity with
increasing depth into material.
Polarization
Polarizer
Unpolarized light
(e.g. sunlight)
Linearly polarized light
Only one plane of vibration passes
Basics of Semiconductor Physics
Semiconductor Crystal Lattice
covalent bond
atom
Simple Cubic Structure
Silicon has a more complex lattice structure
but a lattice structure exists nevertheless.
Crystalline Silicon Bonds
valance
electrons
Si atom (Group IV)
=
covalent bond
(electron sharing)
Breaking of Covalent Bond Creating
Electron-Hole Pair
free electron moving
e- through lattice
+
covalent bond
created hole
(missing electron)
Si atom
Photon (light, heat)
Photon hits valance electron with enough energy to
create a free electron and hole
Movement of a Hole in a Semiconductor
+
+
Thermal energy causes a valance electron to jump to
an existing hole leaving a hole behind
Valance and Conduction Energy Bands
free electron moving in
lattice structure
Conduction
eEnergy Band
Ec
Band Gap Energy, Eg = Ec - Ev
Valance
Energy Band
covalent bonds
+
Ev
Hole within valance band
Valance and Conduction Energy Bands
Thermal Equilibrium
free electron combines
free electron within
with hole
lattice structure
Conduction
eeEnergy Band
Ec
Eg
Heat enery
given up
Valance
Energy Band
covalent bonds
Heat energy
absorbed
+
+
Ev
Hole created within valance band
Energy absorbed = Energy given up
Intrinsic (pure) Silicon Electron-Hole Pairs
Thermal Equalibrium ni = 1.5 x 1010 cm-3
at 300° K
Conduction
eBand
Ec
Eg = 1.12 eV
hole density = electron density
number of holes per cubic centimeter =
number of free electrons per cubic centimeter
pi = ni = 1.5 x 1010 cm-3
pi = 1.5 x 1010 cm-3
at 300° K
Valance
Band
+
Ev
covalent bonds
•Number of electron-hole pairs increase with increasing temperature
Creating a Semiconductor
Doping or Substitutional Impurities
Group V Atom (Donor or N-type Doping)
Phospherous (Group V)
P atom
e-
covalent bond
Si atom (Group IV)
The donor electron is not part of a covalent bond so
less energy is required to create a free electron
Energy Band Diagram of Phospherous Doping
intrinsic free electron
Conduction
Band
donor free electron
e-
eEc
Donor Electron
Energy
n > p (more electrons in conduction band)
Eg
Valance
Band
covalent bonds
A small amount of thermal energy elevates
the donor electron to the conduction band
+
intrinsic hole
N-type Semiconductor
Ev
Doping or Substitutional Impurities
Group III Atom (Acceptor or P-type Doping)
Boron (Group III)
+
B atom
covalent bond
created hole
covalent bond
Si atom
Boron atom attacts a momentarily free valance
electron creating a hole in the Valance Band
Energy Band Diagram of Boron Doping
intrinsic free electron
Conduction
Band
eEc
p > n (more holes in valance band)
Eg
A small amount of thermal energy elevates
the acceptor electron to the Acceptor band
acceptor electron
Acceptor Electron
Energy
Valance
Band
e+
+
Ev
created hole
covalent bonds
intrinsic hole
P-type Semicondutor
Formation and Basic Physics
of
PN Junctions
PN Junction Formation
Masking Barrier
Boron Atom
Doping
Phophorous Atom
Doping
Intrinsic Silicon Wafer
• Doping Atoms are accelerated towards Silicon Wafer
• Doping Atoms are implanted into Silicon Wafer
• Wafer is heated to provide necessary energy for Doping Atoms to become
part of Silicon lattice structure
PN Junction in Thermal Equilibrium
(No Applied Electric Field)
metallurgical
junction
P-type
Space Charge Region
metallurgical
junction
N-Type
Initial Condition
p
-
+
+
+
+
n
E field
Equilibrium Condition
• Free electrons from n-region migrate to p-region leaving donor atoms behind.
• Holes from p-region migrate to n-region leaving acceptor atoms behind.
• Internal Electric Field is created within Space Charge Region.
Solar Cell Basic Operation
PN Junction Solar Cell Operation
Photon
hn > Eg
p
Space Charge Region
+
+
+
+
+
E field
eeeee-
n
• Photons create hole-electron pairs in space charge region
• Created hole-electron pairs swepted out by internal E field
• Excess holes in p-region
• Excess electrons in n-region
PN Junction Solar Cell Operation
Photon
hn > Eg
p
Space Charge Region
+
+
+
+
+
E field
eeeee-
Icell
n
Resistor
+
Vcell
• Attaching a resistive load with wires to the PN Junction creates
current flow from p to n regions
• Electrons flow from n-region to combine with holes in p-region
• Photons create new hole-electron pairs to replace combined pairs
Typical Silicon Solar Cell Design
Photons
Protective High
Transmission Layer
P-type
Doping
Wires
N-type
Silicon
Wafer
4-6 inches
To load
• Photons transmit through thin protective layer and
thin P-type doped layer and create hole-electron
pairs in space charge region
• Typical Silicon Single Cell Voltage Output = ~ 0.5 volts
0.6 mm
Silicon Solar Cell 6 Volt Panel Series-Parallel Design
12 cells in series = 6 volts
6 volts
+
p to n connection
External Factors Influencing Solar Cell Effeciency
• Photon transmission, reflection, and absorption of protective layer
• Maximum transmission desired
• Minimum reflection and absorption desired
• Polarization of protective layer
• Minimum polarized transmission desired
• Photon Intensity
• Increased intensity (more photons) increases cell current, Icell
• Cell voltage, Vcell, increases only slightly
• Larger cell area produces larger current (more incident photons)
• Theoretical Silicon Solar Cell Maximum Efficiency = 28%
• Typical Silicon Solar Cell Efficiency = 10-15%