Applications of Photovoltaic Technologies

2

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

P-type

N-type

• There should be ohmic contact at both side

• It should absorb all light falling on itIt should reflect less light Most of the light should go in

• It should convert all absorb light into electricity

3

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

Fingers

Emitter

Base

Rear contact

Antireflection coating

Antireflection texturing

(grid pattern)

4

Minimizing optical losses

• 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

• 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

•There are a number of ways to reduce the optical losses: .

5

Optical properties of surface

What are optical losses:

Reflection

Shadowing due to metal contact

Partial absorption

• 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

• Design criteria for small optical losses :

Mminimize optical loss

6

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

• 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).

1

01 4nd

110 n

n2 > n1 > n0

Choice of ARC

7

Reflection from various combination

• Multilayer structure reduces the reflection losses

• Index of refraction is also a function of wavelength, minimum reflection is obtained for one wavelength

• More than one ARC can be used, but expensive

Source: PV CDROM - UNSW

8

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

9

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

10

Light trapping

2211 sinsin nn

• Rear side reflector or rear side texturing is used to increase the optical path length in solar cell

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

(1 for Si is 36 degree)

)(sin1

211 n

n For TIR

11

Lambertian Rear Reflectors

• Increases the path length by 4n2, very good in light trapping, path ;length increases by about 50

Random reflector from the rear side

TIR

• Lambertian reflector is one which reflects the lights in a random direction this together with the front texturing increases the optical path length

12

P-N junction

Current loss due to recombination

• Recombination areas

Surface recombination

Bulk recombination

Depletion region recombination

• Recombination of carriers reduces both short circuit current as well as open circuit voltage

Bulk semiconductor rear surface

Front surface

•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

13

ww

hhd

Emitter

finger and busbar spacing, the metal height-to-width, aspect ratio, the minimum metal line width and the resistivity of the metal

Top contact

• One example of top metal contact design

• Design criteria: minimize losses (resistive, shadow)

14

Resistive Losses: Series resistance, Rs

1. the movement of current through the emitter and base of the solar cell

3. resistance of the top and rear metal contacts

2. the contact resistance between the metal contact and the silicon

Contributing factors to Rs :

Bus bar

Fingers

N-layer

p-layer

Baseemitter

M-S contact

15

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.

• A high doping creates a "dead layer“.

• Ohmic contact,

• High doping, tunneling contact

16

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

LRs

W

L

t

A

LR

• 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/□

W

L

t

17

Emitter resistance: Power loss

t

•P

•Nd

L

xdx

d/2

• Zero current flow exactly at midpoint of fingers

• Maximum current density at the finger edge 2max

dJLI

• Resistance dR in infinitesimally thin layer of dx tL

dxdR