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

Solar cell medium energy photon High energy photon Low 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) (Hole)

Use high energy photons (bulk) High energy photon Conduction band Valence band Two excited electrons from one photon 2nd excited electron is killed, no efficiency increase Robbins, D. J. Aspects of the Theory of Impact Ionization in Semiconductors 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 exciton What is mechanism of SSQC? Important for optimization of SSQC process!

Possible SSQC mechanisms Indirect 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 efficiency Of the total process My question: Direct or indirect SSQC? -Difference lies in exciton mobility

Distinguish between mobile and immobile excitons When having two excitons in one NC Mobile excitonsImmobile excitons 2nd exciton stays and is killed Both excitons live if there is a free NC 2nd exciton is only killed when there 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 pulseB ∞ # of single excitons left (after killing) Killing ratio = A/B Will decrease in time when excitons are killed # of excited NCs Loss in probe intensity

The experiment 3) Increase the number of incoming photons 4)Measure again: -The number of excitons in time t [ps] Loss in probe intensity A B incoming 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 experiment When 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

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!

Possible SSQC mechanisms Indirect SSQC Direct SSQC

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

Solar spectrum losses

Efficiency increase

PL SSQC proof Emission and non-absorbed excitation light QE = N emphotons /N abphotons

The experiment Highest and lowest intensity transients: sample Prof. Fujii, Kobe University SiO 2