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Exploit the Sun to the Fullest: Silicon Based Solar Cells
•Abundant•Stable •Low impurity concentration•Environmentally friendly
Conversion efficiency Si solar cell ~ 25%Rens LimpensSupervisor: Tuan TrinhProf. dr. Tom Gregorkiewicz
Solar cell
medium energy photonHigh energy photonLow energy photon
Conduction band
Valence band
Bandgap energy = extraction energy
Energy loss Big efficiency killer
Increase the solar cell efficieny by reducing the energy loss
(Electron)
(Electron)
(Hole)
Use high energy photons (bulk)
High energy photon
Conduction band
Valence band
Two excited electrons fromone photon
2nd excited electron is killed, no efficiency increase
Robbins, D. J. Aspects of the Theory of Impact Ionization inSemiconductors 0.1. Phys. Status Solidi B 1980, 97 (1), 9–50.
Use high energy photons (Nanocrystals)Nanocrystals (NCs) are small pieces of semiconductor material which can confine
the electron and holes
Two NCs close together
High energy photon
Energy transfer is possible between the NCs: -Space-separated quantum cutting (SSQC)
Separated excitons live!
Extra energy is used for 2nd exciton and can be extracted
Solar cell efficiency increases!
D. Timmerman et al., Essential enhancement of carrier multiplication in Si nanocrystals, Under submission
excitonWhat is mechanism of SSQC?Important for optimization of
SSQC process!
Possible SSQC mechanismsIndirect SSQC -mobile excitons
Direct SSQC -immobile excitons
Two-step process
One-step process
Not limited by another process, beneficial for the efficiency!
Can limit the efficiencyOf the total processMy question: Direct or indirect SSQC?
-Difference lies in exciton mobility
Distinguish between mobile and immobile excitons
When having two excitons in one NC
Mobile excitons Immobile excitons
2nd exciton stays and is killedBoth excitons live if there is a free NC
2nd exciton is only killed whenthere is no free NC
2nd exciton is always killed
When does the killing start?
Ask the electrons!
Detector
Sample
probe
pump
PC
Vary time delay between pump and probe to measure behaviour in time
Pump-probe technique
The loss in the probe intensity is proportional to the number of excitons
The experiment
2)Measure: -The number of excitons in time
1) Excite the NC sample with some number of photons
t (ps)
A
B
A ∞ # of excitons created by pump pulse
B ∞ # of single excitons left (after killing)Killing ratio = A/B
Will decrease in time when excitons are killed
# of excited NCs
Loss
in p
robe
inte
nsity
The experiment3) Increase the number of incoming photons
4)Measure again: -The number of excitons in time
t [ps]
Loss
in p
robe
inte
nsity A
Bincoming photons
# of excitons (A) will increase
# excited NCs (B) will grow till all NCs are excited and then stay constant
Killing rate (A/B) will increase
When B is constant No free NCs
The experimentWhen does the killing start?
1
Killing will occur after all NCs are excited
Killing rate (A/B)
# NCs excited (B)
Incoming photons
Mobile excitons
# NCs excited (B)
1
Killing rate (A/B)
Incoming photons
Killing starts before all NCs are excited
Immobile excitons
150x10-6
100
50
0
L
(1ns)
5004003002001000
Intensity (mW/cm2)
5 5
4 4
3 3
2 2
1 1
num
ber
of exc
itons
5004003002001000
Intensity (mW/cm2)
Killing rate occurs before all NCs are excited immobile excitons
Results
# NCs excited (B)
1
Killing rate (A/B)
Pump intensity
Immobile excitons
The SSQC process is direct!
Conclusion
• The SSQC process is a direct process– The process should therefore be efficient because it is not
limited by another factor
• Consequence:– SSQC is a perfect candidate for improving solar cell
efficiencies
Acknowledgement
• My colleagues: Prof. T. Gregorkiewicz, W. de Boer, T.M. Trinh, D. Timmerman, N.N. Ha, S. Saeed, K. Dohnalova.
Thank you for your attention