Chuong 4 solar cell

Chương 4. SOLAR CELL



THUẬN LỢI VÀ KHÓ KHĂN
 Thuận lợi
+ Nguồn năng lượng sạch, không ô nhiễm.
+ Dồi dào. Theo một tính toán, năng lượng mặt trời
chiếu trên trái đát trong vòng 30 ngày bằng tổng năng
lượng nhiên liệu hóa thạch trên hành tinh kể cả đã sử
dụng và chưa sử dụng!
 Khó khăn
+ Ánh sáng mặt trời chiếu sáng khác nhau theo thời
gian và không gian.
+ Năng lượng mặt trời phân tán, không tập trung.
+ Để giải quyết vấn đề này:
1) collection, 2) conversion, 3) storage.


Solar Source
• The average solar power incident on Continental US is
1600 x 1012 W
This is 500x the power consumption in US (3.3 x
1012 W)
• If we cover 2% of the continental US with 10%
efficient PV systems, we would make all the energy we
need.
• FYI:
• 1.5% of Continental US is covered by roads

Solar Energy to Heat Living Spaces



Proper design of a building is for it to act as a solar
collector and storage unit. This is achieved through
three elements: insulation, collection, and storage.


Photovoltaics (PV)
Photo + voltaic = convert light to electricity
Solar cell = convert sun to electricity


Solar Cells Background

 1839 - French physicist A. E. Becquerel first recognized the
photovoltaic effect.
 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.

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Driven by Space
Applications in Early Days


Worldwide PV Growth

Source: European Commission meeting Dec. 2004




The first generation photovoltaic, consists of a
large-area, single layer p-n junction diode, which is
capable of generating usable electrical energy from
light sources with the wavelengths of sunlight.
These cells are typically made using a silicon wafer


Inorganic pn Junction Solar Cell
(First Generation)

Silicon

Shared electrons

Si

Si

Si

Si

Si

Si

Si

Si

-

Si

 Silicon is group IV element – with 4 electrons in their valence shell.
 When silicon atoms are brought together, each atom forms covalent
bond with 4 silicon atoms in a tetrahedron geometry.


Extrinsic Semiconductor, n-type Doping
Conducting band, Ec
Si

Si

Si

As

Si

Si

Si

Extra
Electron

Ed ~ 0.05 eV
Eg = 1.1 eV

Si

-

Si

Valence band, Ev
 Doping silicon lattice with group V elements can creates extra electrons in the
conduction band — negative charge carriers (n-type), As- donor.
 Doping concentration #/cm3 (1016/cm3 ~ 1/million).


Extrinsic Semiconductor, p-type doping
Conducting band, Ec
Si

Si

Si

Hole
Si

Eg = 1.1 eV

B

Si

Ea ~ 0.05 eV
Si

Si

-

Si

Electron

Valence band, Ev

 Doping silicon with group III elements can creates empty holes
in the conduction band — positive charge carriers (p-type), B(acceptor).

THE PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS
How the device works?

The p-n junction under illumination (on the right). A photon induced hole-electron
pair is separated by the local field of the junction. Taken from: F. C. TREBLE
(Editor); Generating Electricity from the Sun; Pergamon Press, Inc.;New York; 1991
.


The second generation of photovoltaic materials
is based on the use of thin-film deposits of
semiconductors. These devices were initially
designed to be high-efficiency, multiple junction
photovoltaic cells.


Inorganic Thin Film Solar Cells

(Second Generation)




Third generation photovoltaics are very different from
the previous semiconductor devices as they do not rely on
a traditional p-n junction to separate photogenerated
charge carriers. These new devices include
photoelectrochemical cells, polymer solar cells, and
nanocrystal solar cells. Dye-sensitized solar cells are now

in production. Examples include Amorphous silicon,
Polycrystalline
silicon,
micro-crystalline
silicon,
Cadmium telluride, copper indium selenide/sulfide.


Dye-sensitized solar cells: Operation
1.

Dye electrons are excited by solar
energy absorption.

2.

They are injected into the
conduction band of TiO2.

3.

Get to counter-electrode (cathode)
through the external circuit.

4. I3-  2e  3I- : Redox regeneration at
the counter-electrode (oxidation).

5. 3I  I3  2e
: Dye regeneration
reaction (reduction).

6.

Potential used for external work:

Vext  EF  Vredox

Red=IOx =I3-

Ionic Liquid
[bmim]+ I -


Fourth
generation
Composite
photovoltaic
technology with the use of polymers with nano
particles can be mixed together to make a single
multispectrum layer. Then the thin multi spectrum
layers can be stacked to make multispectrum solar
cells more efficient and cheaper based on polymer
solar cell and multi junction technology used by
NASA on Mars missions