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Thursday, August 8, 2013

PPT On Solar Cell Technologies


Solar Cell Technologies Presentation Transcript:
1.Solar Cell Materials & Structures

2.Solar Cells Background
1839 - French physicist A. E. Becquerel first recognized the photovoltaic effect.

Photo+voltaic = convert light to electricity

1883 - first solar cell built, by Charles Fritts, coated semiconductor selenium with an extremely thin layer of gold to form the junctions.

1954  - Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. Daryl Chapin, Calvin Fuller and Gerald Pearson, invented the first practical device for converting sunlight into useful electrical power. Resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6%.

1958 - First spacecraft to use solar panels was US satellite Vanguard 1
3.Solar Cell Technologies
A)    Crystalline Silicon
B)    Thin Film
C)    Group III-IV Cells

4.Crystalline Silicon
Most common for commercial applications
Well known standard processing
Silica is very abundant
Requires expensive highly pure silicon
Competes for silicon with electronics industry

5.Types of Crystalline Silicon
Carefully made Silicon forms crystals. Different levels of crystal structure may exist ranging from single  crystal to totally non-crystalline
Single crystal silicon (Mono crystal silicon)
Multi-crystal silicon
Ribbon silicon
Amorphous silicon
The main difference between each is the crystal grain size and their growth technique

6.Single crystal silicon (Mono crystal silicon)
Monocrystalline silicon or single-crystal Si, or mono-Si is the base material of the electronic industry. It consists of silicon in which the crystal lattice of the entire solid is continuous, unbroken (with no grain boundaries) to its edges. It can be prepared intrinsic, i.e. made of exceedingly pure silicon alone, or doped, containing very small quantities of other elements added to change in a controlled manner its semiconducting properties. Most silicon monocrystals are grown by the Czochralski process, in the shape of cylinders up to 2 m long and 30 cm in diameter (figure shown), which, cut in thin slices, give the wafers onto which the microcircuits will be fabricated.

7.Different Forms of Silicon

8.Single Crystal Growth Techniques
Czochralski Growth (Cz)
Most single crystal silicon made this way
Lower quality silicon than FZ with Carbon and Oxygen present
Cheaper production than FZ
Produces cylinders and circular wafers
Float Zone (FZ)
Better Quality than Cz
More Expensive than Cz
Produces cylinders and circular wafers

9.Czochralski Method
Pure Silicon is melted in a quartz crucible under vacuum or inert gas and a seed crystal is dipped into the melt
The seed crystal is slowly withdrawn and slowly rotated so that the molten silicon crystallizes to the seed (Rock Candy)
The melt temperature, rotation rate and pull rate are controlled to create a ingot of a certain diameter

10.Czocharlski Technique
Spinning rod with “Seed” Crystal lowered into the molten silicon
Slowly pulled up to allow silicon to crystallize on the seed layer
Once to the size desired, the crystal is pulled faster to maintain the needed diameter

11.Czochralski Growth
Entire ingots of silicon produced as one big crystal
Very high quality material with few defects
No boundaries between crystals because it is one crystal in one orientation
Si crystal inevitably contains oxygen impurities dissolved from the quartz crucible holding the molten silicon

12.Float Zone Method
Produced by cylindrical polysilicon rod that already has a seed crystal in its lower end
An encircling inductive heating coil melts the silicon material
The coil heater starts from the bottom and is raised pulling up the molten zone
A solidified single crystal ingot forms below
Impurities prefer to remain in the molten silicon so very few defects and impurities remain in the forming crystal

13.Slicing into Wafers
Ingots are cut into thin wafers for solar cells (200-300 mm)
Two Techniques
Wire sawing
Diamond blade sawing
Both results in loss of silicon from “kerf losses” ? silicon saw dust
Time consuming
Water Cooled, Dirty

14.Single Crystal Silicon
What we are using
Currently supplies a significant but declining solar cell market share
Produced for electronics industry
Allows for higher efficiency solar cells
Requires higher quality of feed stock
More expensive and slower to produce
Circular shape leads to lower packing density in panels or larger waste of silicon

15.Ribbon Silicon
Ribbon silicon is a technique used to grow multi-crystalline silicon
Two graphite filaments are placed in a crucible of molten silicon
The molten silicon is grown horizontally through capillary action along the filaments
Produces a ribbon-like sheet of multi-crystalline silicon which is already a long wafer ? no kerf losses

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