Quantum dots are semiconductor nanoparticles between 2-10 nm in diameter that exhibit quantum confinement effects. Their bandgap can be tuned by varying their size, making them useful for photovoltaics and lasers. For photovoltaics, quantum dots can improve solar cell efficiency by generating multiple electron-hole pairs from a single photon and enabling intermediate bandgaps. For lasers, quantum dots require a lower threshold current for lasing due to their density of states, resulting in improved temperature stability and higher operation speeds compared to quantum well lasers. Recent research has demonstrated lasing with solution-processed quantum dots and threshold currents approaching zero.

1. Quantum Dots for Photovoltaics
and Lasers
Bob Kyeyune
Lappeenranta University of Technology
Technical Physics
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 1 / 10

2. What are Quantum Dots (QD) ?
Semiconductor nano-particle with a
2 − 10 nm diameter (10-50 atoms).
Obey quantum mechanical principles of
quantum confinement-(occurs when an
exciton is confined to dimension smaller
than it’s Bohr radius)-approximation to
a particle in a box.
An electromagnetic radiation emitter
with tunable band gap.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 2 / 10

3. TEM Images of Quantum Dot
Can be synthesised
as a solution.
How is this possible ?
In between the QDs are
Organic Ligands allow
QDs to be stable
in solution .
In cartoon, green
-Ligand and red-QD
We can tune the properties of the QD by changing its size.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 3 / 10

4. Size and band gap
Band gap energy ,
E(QD) = Ebulk +
h2
8r2
1
m∗
e
+
1
m∗
h
−
1.8e2
4π r
≈
hc
λ
The radius r corresponds to the wavelength λ.
Band gap increases with size of the dot.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 3 / 10

5. Photovoltaics
The atmosphere limits efficiency of
photovoltaics cells
Traditional solar cells have a
limited theoretical efficiency of
about 33 % (Shockley-Queisser
limit) for single junctions. ( so
far upto 22.08% efficiency has
been achieved)
Theoretically, an infinite number
of junctions would have a
limiting efficiency of 86.8%
under highly concentrated
sunlight (so far 44% efficiency
has been achieved).
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 4 / 10

6. Why not atleast the Shockley-Queisser limt been achieved?
Photons with lower energy than the band gap are not absorbed.
Photons with greater energy than the band gap are absorbed but the
excess energy is lost as heat.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 5 / 10

7. How can quantum dots improve solar cell efficiency ?
The ability to tune the band gap of QD due to their size, intermediate
band gaps can be created ( With this, theoretically an efficiency as
high as 63.2% can be achieved ).
QDs can generate multiple electron-hole
pairs with a single incident photon.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 5 / 10

8. Quantum Dot Lasers
A quantum dot laser is a semiconductor laser that uses quantum
dots as the active medium.
Figure: A three level system laser
basic working principle
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 6 / 10

9. Why quantum dots for lasers ?
The confinement and nature of the
density of states results in,
small population inversion
necessary for lasing.
lower threshold current.
small line width.
reduced power consumption.
Temperature stability of threshold
current.
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 7 / 10

10. Breakthrough
QD with great temperature stability
High-speed (10Gbps) laser was
realised at temperatures upto
700c (2004- world’s fastest
10Gbps QD laser )
High-speed operations at 25
Gbps (Fujitsu Limited,University
of Tokyo-world’s fastest 25Gbps
QD laser-2010 ).
Graphs tell us that,
QD lasers have narrow line
width compared quantum well
lasers
Observations by Fujitsu limited
(2004)
Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 8 / 10

11. Recent activities
Los Alamos national laboratory, 2010 has demonstrated that by
adding extra electrons in a QD, they can reduce the threshold
currents virtually to zero.
LOS ALAMOS, N.M., March 20, 2017, has demonstrated steady
state lasing with solution-processed nanoparticles called “colloidal
quantum dots,”
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12. Bob Kyeyune ( Lappeenranta University of Technology Technical Physics)Quantum Dots for Photovoltaics and Lasers 10 / 10