1. Solar Cell Semiconductor Physics
Basic Fundamentals of
Solar Cell Semiconductor Physics

2. Review Topics

3. Wavelength and Frequency
Period (sec)
amplitude
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)

4. 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)

5. 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 eV
Solar Spectrum at Earth Surface
.5 eV eV
2.6 mm (infrared) mm (ultraviolet)
visible
inrfared
ultraviolet
Solar Spectrum
at Earth Surface
(noon time)

6. 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 Joules
h = Plank’s constant (6.625 x Joule-sec or
4.135 x 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]

7. 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)

8. Amount of power over a given area, Watts/m2
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 J-s)(2.99 x 108 m/s)/(0.80 x 10-6 meters)
= 2.48 x J = 1.55 eV
Irradiance = Power/Area = (4 photons/sec)(Energy of 1 photon)/2.00 m2
= 4.96 x W/m2
Typical sunlight irradiance is W/cm2 = 930 W/m2 at l = .55 mm

9. Transmission, Reflection, and Absorption
incident light
reflectance (R)
air
transmittance (T) + absoprtance (A)
material
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.

10. Polarization
Polarizer
Unpolarized light
(e.g. sunlight)
Linearly polarized light
Only one plane of vibration passes

11. Basics of Semiconductor Physics

12. Semiconductor Crystal Lattice
covalent bond
atom
Simple Cubic Structure
Silicon has a more complex lattice structure
but a lattice structure exists nevertheless.

13. Crystalline Silicon Bonds
valance
electrons
Si atom (Group IV) =
covalent bond
(electron sharing)

14. Breaking of Covalent Bond Creating Electron-Hole Pair
free electron moving
through lattice e-
created hole
(missing electron) +
covalent bond
Si atom
Photon (light, heat)
Photon hits valance electron with enough energy to
create a free electron and hole

15. Movement of a Hole in a Semiconductor+ +
Thermal energy causes a valance electron to jump to
an existing hole leaving a hole behind

16. Valance and Conduction Energy Bands
free electron moving in
lattice structure
Conduction
Energy Band e- Ec
Band Gap Energy, Eg = Ec - Ev
Valance
Energy Band Ev +
Hole within valance band
covalent bonds

17. Valance and Conduction Energy Bands
Thermal Equilibrium
free electron combines
with hole
free electron within
lattice structure
Conduction
Energy Band e- e- Ec Eg
Heat enery
given up
Heat energy
absorbed
Valance
Energy Band Ev + +
Hole created within valance band
covalent bonds
Energy absorbed = Energy given up

18. Intrinsic (pure) Silicon Electron-Hole Pairs Thermal Equalibrium
ni = 1.5 x 1010 cm-3
at 300° K
Conduction
Band e- Ec
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
Eg = 1.12 eV
pi = 1.5 x 1010 cm-3
at 300° K
Valance
Band Ev +
covalent bonds
Number of electron-hole pairs increase with increasing temperature

19. Creating a Semiconductor

20. 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

21. Energy Band Diagram of Phospherous Doping
intrinsic free electron
donor free electron
Conduction
Band e- e- Ec
Donor Electron
Energy
n > p (more electrons in conduction band)
A small amount of thermal energy elevates
the donor electron to the conduction band Eg
Valance
Band Ev +
intrinsic hole
covalent bonds
N-type Semiconductor

22. - 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

23. Energy Band Diagram of Boron Doping
intrinsic free electron
Conduction
Band e- Ec
p > n (more holes in valance band)
A small amount of thermal energy elevates
the acceptor electron to the Acceptor band Eg
acceptor electron
Acceptor Electron
Energy e-
Valance
Band Ev + +
created hole
intrinsic hole
covalent bonds
P-type Semicondutor

24. Formation and Basic Physicsof
PN Junctions

25. Intrinsic Silicon Wafer
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

26. PN Junction in Thermal Equilibrium (No Applied Electric Field)
Space Charge Region
metallurgical
junction
metallurgical
junction - +
P-type
N-Type p n
Initial Condition
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.

27. Solar Cell Basic Operation

28. PN Junction Solar Cell Operation
Photon
hn > Eg
Space Charge Region + e-
E field p 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

29. PN Junction Solar Cell Operation
Photon
hn > Eg
Space Charge Region + e-
E field p n
Icell
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

30. Typical Silicon Solar Cell Design
Photons
Protective High Transmission Layer
P-type
Doping
Wires
N-type
Silicon
Wafer
0.6 mm
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

31. Silicon Solar Cell 6 Volt Panel Series-Parallel Design
12 cells in series = 6 volts
p to n connection -
6 volts +

32. 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%