1. Applications of Photovoltaic Technologies

2. Junction under illumination
Generation of voltage in P-N junction
radiation - +
P-type
N-type - +
P-type
N-type Ln Lp W
Direction of current flow under illumination
P-N behaves like a forward bias P-N junction under illumination

3. Solar Cell Structure
The basic steps in the operation of a solar cell are:
the generation of light-generated carriers;
the collection of the light-generated carries to generate a current;
the generation of a large voltage across the solar cell; and
the dissipation of power in the load and in parasitic resistances.

4. No Resistive Losses Solar Cell model The I-V relation is given as: I VID V IL
Io －dark saturation current ,
IL －light generated current. ,
n －ideality factor .

5. Solar Cell I-V Curve V I
I (diffu.) I0
A P-N junction in the dark consumes power, as it can be operated in 1st or 3rd quadrant
Under illumination solar cell can be operated in the fourth quadrant corresponding to delivering power to the external circuit
Effect of solar radiation on the I-V curve
Current in the illuminated solar cell is negative, flows against the conventional direction of a forward diode

6. Solar Cell I-V Curve Solar cell parameters I Isc Pm Im V
Voc - open circuit voltage,
Isc - short circuit current,
Pm - maximum power point
Im, Vm – current and voltage
at maximum power point
FF – fill factor
η – efficiency
Rs – series resistance
Rsh – shunt resistance
Isc I Vm Im
Voc Pm V
Usual I-V plot of solar cell – Current is shown on positive y -axis

7. Short-Circuit Current, IscVm Im
Voc Pm X
The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited).
The short-circuit current is due to the generation and collection of light-generated carriers.
The short-circuit current is the largest current which may be drawn from the solar cell.
At V=0  I = -IL= Isc

8. Open Circuit Voltage: Voc
Isc I Vm Im Pm X
Voc
The open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and this occurs at zero current.
The open-circuit voltage corresponds to the amount of forward bias on the solar cell junction due to illumination.
At I=0  V = Voc

9. Maximum power: Pm I Isc Pm Im Pm = Im x Vm
Power out of a solar cell increases with voltage, reaches a maximum (Pm) and then decreases again. Im
Power Vm
Voc
Pm = Im x Vm
Remember we get DC power from a solar cell

10. Fill Factor: FF I Im Ideal diode curve Isc
The FF is defined as the ratio of the maximum power from the actual solar cell to the maximum power from a ideal solar cell Pm Im Vm
Voc
Graphically, the FF is a measure of the "squareness" of the solar cell

11. Efficiency: η I Isc Pm ImVm Im Pm X
Voc
Power
Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.
The efficiency is the most commonly used parameter to compare the performance of one solar cell to another.
Efficiency of a cell also depends on the solar spectrum, intensity of sunlight and the temperature of the solar cell.

12. Solar cell structure How a solar cell should look like ?
 It depends on the function it should perform, it should convert light into electricity, with high efficiency
It should be a P-N junction
It should absorb all light falling on it
It should reflect less light
Most of the light should go in
N-type
P-type
There should be ohmic contact at both side
It should convert all absorb light into electricity

13. Solar Cell-structure A solar cell is a P-N junction device
Light shining on the solar cell produces both a current and a voltage to generate electric power.
Busbar
Antireflection coating
Fingers
Emitter
Antireflection texturing
(grid pattern)
Base
Rear contact

14. Minimizing optical losses
There are a number of ways to reduce the optical losses: .
Anti-reflection coatings can be used on the top surface of the cell.
Reflection can be reduced by surface texturing
The solar cell can be made thicker to increase absorption
The optical path length in the solar cell may be increased by a combination of surface texturing and light trapping.
Top contact coverage of the cell surface can be minimized

15. Optical properties of surface
Photons in the spectrum can generate EHP, ideally all the sun light
falling on the cell should be absorbed
Short circuit current (ISC) is usually reduced due to optical losses
What are optical losses:
 Reflection
 Shadowing due to metal contact
 Partial absorption
Design criteria for small optical losses :
Mminimize optical loss

16. Choice of ARC n2 > n1 > n0
Air, n0
Semiconductor, n2
ARC, n1
The thickness of a ARC is chosen such that the reflected wave have destructive interference  this results in zero reflected energy
n2 > n1 > n0
The thickness of the ARC is chosen so that the wavelength in the dielectric material is one quarter the wavelength of the incoming wave (destructive interference).

17. Reflection from various combination
Index of refraction is also a function of wavelength, minimum reflection is obtained for one wavelength
Multilayer structure reduces the reflection losses
More than one ARC can be used, but expensive
Source: PV CDROM - UNSW

18. Surface texturing
Any rough surface decreases the reflection by increasing the chances of the reflected rays bouncing back on the surface
Surface texturing can be obtained by selective etching  a process by which material is removed by chemical reaction
Selective etching is based on the concept of different material property in different direction in crystals,
Etching rate are different in <100> dirn than in <111> dirn

19. Surface texturing
Chemical etching in KOH results in pyramid formation on the Si surface  etching is faster in <100> direction than in <111> direction
Using photolithography, inverted pyramids can be obtained, which are more effective
<111> surface

20. Light trapping Snell’s law
Rear side reflector or rear side texturing is used to increase the optical path length in solar cell
(1 for Si is 36 degree)
 Increased optical path is required for thin solar cell (thin solar cell have higher Voc. It saves expensive Si)
Total internal reflection (TIR) condition are used to increase the optical path length
Snell’s law
For TIR

21. Lambertian Rear Reflectors
Lambertian reflector is one which reflects the lights in a random direction  this together with the front texturing increases the optical path length
Increases the path length by 4n2, very good in light trapping, path ;length increases by about 50
TIR
Random reflector from the rear side

22. Current loss due to recombination
Recombination of carriers reduces both short circuit current as well as open circuit voltage
Bulk semiconductor
rear surface
Front surface
Recombination areas
Surface recombination
Bulk recombination
Depletion region recombination
P-N junction
Design criteria: The carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining

23. Top contact Design criteria: minimize losses (resistive, shadow) d
One example of top metal contact design w h d
Emitter
finger and busbar spacing,
the metal height-to-width, aspect ratio,
the minimum metal line width and
the resistivity of the metal

24. Resistive Losses
Resistive effects (series and shunt resistance) in solar cells reduce the efficiency of the solar cell by dissipating power in the resistances.
Solar Cell model V I Rs IL If
Rsh
Both the magnitude and impact of series and shunt resistance depend on the geometry of the solar cell and solar cell area
Resistance are given in Ω-cm2
The key impact of parasitic resistance is to reduce fill factor.

25. Resistive Losses: Series resistance, Rs
Fingers
Contributing factors to Rs :
Bus bar
1. the movement of current through the emitter and base of the solar cell
M-S contact
N-layer
p-layer
emitter
Base
2. the contact resistance between the metal contact and the silicon
3. resistance of the top and rear metal contacts

26. Contact resistance N
Heavy doping under contact to minimize contact resistance
Metal to semiconductor contact
Contact resistance losses occur at the interface between the silicon solar cell and the metal contact. To keep top contact losses low, the top N+ layer must be as heavily doped as possible.
Ohmic contact,
High doping, tunneling contact
A high doping creates a "dead layer“.

27. Sheet resistance
In diffused semiconductor layers, resistivity is a strong function of depth. It is convenient to a parameter called the "sheet resistance" (Rs). W L t
Rs is called sheet resistance with unit of ohms/square or Ω/□ (actual unit is Ohms)
The L/W ratio can be thought of as the number of unit squares (of any size)
Sheet resistance of a solar cell emitter is in the range of 30 to 100 Ω/□

28. Emitter resistance: Power lossd L x dx
d/2 t P N
Zero current flow exactly at midpoint of fingers
Maximum current density at the finger edge
Resistance dR in infinitesimally thin layer of dx

29. Characteristic resistance, Rch
Effect of Rs and FF
Isc
Voc V I
Characteristic resistance, Rch
Medium Rs
Large Rs
Normalized series resistance, rs
Slope of the I-V curve near Voc gives indication about Rs
Effect of series resistance on the FF and maximum power

30. Effect of Rsh on FF Isc Normalized shunt resistance, rsh Medium Rsh I
Voc V I
Normalized shunt resistance, rsh
Medium Rsh
Slope of the I-V curve near Isc gives indication about Rsh
Effect of series resistance on the FF and maximum power