Transcript Solar Cell Applications - Northwestern University

Quantum Dots:
Photon Interaction Applications
Brad Gussin
John Romankiewicz
12/1/04
Semiconductor Structures
Bulk Crystal (3D) 
3 Degrees of Freedom (x-, y-, and z-axis)
Quantum Well (2D) 
2 Degrees of Freedom (x-, and y-axis)
Quantum Wire (1D) 
1 Degree of Freedom (x-axis)
Quantum Dot (0D) 
0 Degrees of Freedom
(electron is confined in all directions)
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Structure vs. Energy
Quantum Dots are sometimes called “artificial atoms”
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Infrared Photodetection QWIP
Quantum Well IP
Bulk Crystal
Wavelength
Photons
+
-
CB
VB
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QWIP Drawbacks
• High Intensity / Low Temp
• Polarization scatter grating
• Detector only works when
photon hits semiconductor
perpendicularly to the two
unconfined axis
Grating
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Quantum Dots
(Structure and Formation)
Self-Assembly (a.k.a StranskiKrastanow Method): Mismatched
lattice constants cause surface tension
which results in Qdot formation with
surprisingly uniform characteristics.
GaAs  5.6533 Å InAs  6.0584 Å
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QDIPs
• 3D e- Confinement: Sharper wavelength discrimination
E = n / R2  E controlled by dimensional parameter R
• No need for Polarization
• “Photon Bottleneck” : e- stays excited for a longer time
(i.e. less recombination), resulting in a more efficient
detector and resistance to temperature.
• Higher temperatures and lower intensity.
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The Future of QDIPs
Self-assembly techniques still unstable: tune photodetection
properties by manipulating the shape and size of Qdots
Possible Solutions:
• Different Material Combinations
• More precise control of parameters (T, pressure, physical setup)
• Combine self-assembly with lithography and etching techniques
• For example: create crevices or pre-etched holes to
encourage qdot growth in specified positions.
* Dr. Bijan Movaghar estimates 5-10 years before
commercially practical QDIPs are in use.
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QDIP Applications
Increase detectivity  Increase number of applications
Medical
(Thermal Imaging)
Weather
Military
Astronomy:
Infrared Image
of the Milky Way
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Solar Cell Applications
Currently, quantum wells and quantum
dots are being researched for use in solar
cells.
 Factors that affect solar cell efficiency:

– Wavelength of light
– Recombination
– Temperature
– Reflection
– Electrical Resistance
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
On an I-V curve
characterizing the output
of a solar cell, the ratio
of maximum power to
the product of the opencircuit voltage and the
short circuit current is the
fill factor. The higher the
fill factor, the “squarer”
the shape of the I-V
curve.
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Quantum Well Application

Photocurrent and output voltage can be
individually optimized
– Absorption edge and spectral characteristics can
be tailored by the width and depth of QWs.

In GaAs/AlxGa1-xAs p – i – n structure with
inserted QWs, researchers have observed
enhancement in the short-circuit current and
thus efficiency in comparison with control
samples that are identical except without the
QWs.
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Quantum Dot Application

Have potential to improve efficiency
– Reducing recombination
 Channeling the electrons and holes through the coupling
between aligned QD’s
 Photon bottleneck
– Increasing the amount of usable incident light
– Can be used at higher temperatures

Drawbacks
– Size is harder to control
 Thus harder to control the light absorption spectrum

Solutions for quantum dot solar cells are similar
to QDIP solutions
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Structure and Energy Diagram
P-type, intrinsic, ntype structure
 Based on a selforganized
InAs/GaAs system
 Quantum dots
used in active
regions

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Conclusion

Qdot technology will help optimize the photon
detection and photovoltaic industries by making
devices more efficient and functionally effective.

Possible other areas for commercial development
include use in the automobile or robotics industries
 Detect object or humans in the vicinity of the device.
 Power generation for device.
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Sources





Interview with Dr. Bijan Movaghar, visiting professor
(November 19, 2004).
Aroutounian, V. et al. Journal of Applied Physics. Vol.
89, No. 4. February 15, 2001.
Razeghi, Manijeh. Fundamentals of Solid State
Engineering. Kluwer: Boston. 2002.
Center For Quantum Devices.
<http://cqd.ece.northwestern.edu/>
Quantum Dots Introduction
<http://vortex.tn.tudelft.nl/grkouwen/qdotsite.html>
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