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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. Rens Limpens Supervisor: Tuan Trinh Prof. dr. Tom Gregorkiewicz. Conversion efficiency Si solar cell ~ 25%. Solar cell. Conduction band.

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Exploit the Sun to the Fullest: Silicon Based Solar Cells

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  1. Exploit the Sun to the Fullest: Silicon Based Solar Cells • Abundant • Stable • Low impurity concentration • Environmentally friendly Rens Limpens Supervisor: TuanTrinh Prof. dr. Tom Gregorkiewicz Conversion efficiency Si solar cell ~ 25%

  2. Solar cell Conduction band High energy photon Energy loss Big efficiency killer (Electron) medium energy photon Bandgap energy = extraction energy (Electron) (Hole) Valence band Low energy photon Increase the solar cell efficieny by reducing the energy loss

  3. Use high energy photons (bulk) Conduction band High energy photon Twoexcitedelectronsfrom one photon 2nd excitedelectron is killed, no efficiency increase Valence band Robbins, D. J. Aspects of the Theory of Impact Ionization in Semiconductors 0.1. Phys. Status Solidi B 1980, 97 (1), 9–50.

  4. Use high energy photons (Nanocrystals) Nanocrystals (NCs) are small pieces of semiconductor material which can confine the electron and holes High energy photon Energy transfer is possible between the NCs: -Space-separated quantum cutting (SSQC) What is mechanism of SSQC? Important for optimization of SSQC process! exciton Separated excitons live! Extra energy is used for 2nd exciton and can be extracted Two NCs close together Solar cell efficiency increases! D. Timmerman et al.,Essentialenhancement of carrier multiplication in Si nanocrystals, Undersubmission

  5. Possible SSQC mechanisms Indirect SSQC -mobile excitons Two-stepprocess Can limit the efficiency Of thetotalprocess My question: Direct or indirect SSQC? -Difference lies in exciton mobility Direct SSQC -immobile excitons One-stepprocess Notlimitedbyanotherprocess, beneficialfor the efficiency!

  6. Distinguish between mobile and immobile excitons When having two excitons in one NC Mobile excitons Immobile excitons When does the killing start? 2nd exciton stays and is killed Both excitons live if there is a free NC Ask the electrons! 2nd exciton is only killed when there is no free NC 2nd exciton is always killed

  7. Pump-probe technique probe PC Detector pump Sample The loss in the probe intensity is proportional to the number of excitons Vary time delaybetween pump and probe to measurebehaviour in time

  8. The experiment 1) Excite the NC sample with some number of photons A 2)Measure: -The number of excitons in time Will decrease in time when excitons are killed B A ∞ # of excitons created by pump pulse t (ps) B ∞ # of single excitonsleft (afterkilling) # of excited NCs Killing ratio = A/B Loss in probe intensity

  9. The experiment 3) Increase the number of incoming photons 4)Measure again: -The number of excitons in time # of excitons (A) will increase Killing rate (A/B) will increase A # excitedNCs (B) will grow till all NCs are excited and then stay constant Loss in probe intensity incoming photons B WhenB is constant  No free NCs t [ps]

  10. The experiment When does the killing start? Killing starts before all NCs are excited Killing will occur after all NCs are excited Killing rate (A/B) Killing rate (A/B) # NCs excited (B) # NCs excited (B) 1 1 Incoming photons Incoming photons Immobile excitons Mobile excitons

  11. Results Immobile excitons Killing rate (A/B) # NCs excited (B) 1 Pump intensity Killing rate occurs before all NCs are excited  immobile excitons The SSQC process is direct!

  12. Possible SSQC mechanisms Indirect SSQC Direct SSQC

  13. Conclusion • The SSQC process is a direct process • The processshouldtherefore be efficient because it is not limited by another factor • Consequence: • SSQC is a perfect candidate for improving solar cell efficiencies

  14. 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

  15. Solar spectrum losses

  16. Efficiency increase

  17. PL SSQC proof Emission and non-absorbed excitation light QE = Nemphotons/Nabphotons

  18. The experiment • Highest and lowest intensity transients: sample SiO2 Prof. Fujii, Kobe University

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