Exploit the Sun to the Fullest: Silicon Based Solar Cells

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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 Limpens Supervisor: Tuan Trinh Prof. dr. Tom Gregorkiewicz

description

Exploit the Sun to the Fullest: Silicon Based Solar Cells. Abundant Stable Low impurity concentration Environmentally friendly. Rens Limpens Supervisor: Tuan Trinh Prof. dr. Tom Gregorkiewicz. Conversion efficiency Si solar cell ~ 25%. Solar cell. Conduction band. - PowerPoint PPT Presentation

Transcript of Exploit the Sun to the Fullest: Silicon Based Solar Cells

Page 1: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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

Page 2: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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)

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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.

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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!

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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

Page 6: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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!

Page 7: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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

Page 8: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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

Page 9: Exploit the Sun to the Fullest: Silicon Based Solar Cells

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

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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

Page 11: Exploit the Sun to the Fullest: Silicon Based Solar Cells

150x10-6

100

50

0

L(

1ns)

5004003002001000

Intensity (mW/cm2)

5 5

4 4

3 3

2 2

1 1

num

ber o

f 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!

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Possible SSQC mechanisms

Indirect SSQC

Direct SSQC

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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

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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

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Page 16: Exploit the Sun to the Fullest: Silicon Based Solar Cells

Solar spectrum losses

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Efficiency increase

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PL SSQC proofEmission and non-absorbed excitation light

QE = Nemphotons/Nabphotons

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The experiment

• Highest and lowest intensity transients:

400x10-6

300

200

100

0

L

10008006004002000

Delay time (ps)

Pump at 3.1 eVProbe at 0.95 eV

2 mW 6 mW

sample

Prof. Fujii, Kobe University

SiO2