1. Multi-Exciton Generation and Solar Cell Physics
Advanced Photons and Science Evolution 2010
2010 Jun 14-18, Osaka
Multi-Exciton Generation and Solar Cell Physics
[Short Presentation 5min.]
Good afternoon. My name is Takeshi Tayagaki and I’m from Kyoto University.
Today, I’m going to talk about the solar cell physics and the multi-exciton generation.
The multi-exciton generation is one of the hot topics to realize highly efficient solar cells.
Takeshi Tayagaki
ICR, Kyoto University, Kyoto, Japan
PRESTO-JST, Saitama, Japan

2. What is Physical Limit of Solar Cells?
Conduction
Band
Valence Eg
hn>>Eg
hn>Eg
hn<Eg
Heat
Photon Energy (eV)
Light Intensity
We know that the solar spectrum contains a broad energy range. So, photons with energies above the band gap create electron hole pair with an excess kinetic energy. A major factor limiting the conversion efficiency is that the absorbed photon energy above the band gap is lost as heat through electron–phonon interaction. In conventional solar cells using a single threshold absorber, the maximum efficiency is limited to be about 30%, which is called as Shockley-Queisser limit.
Shockley-Queisser limit: ~30 %

3. Present and Future Solar Cell Technologies
Production Cost
Efficiency(%) 20 40 60 80
100 I II
III
LOW
Cost per
Energy
HIGH
Shockley-
Queisser limit
At present, solar cell technologies face cost and scalability problems. So, to realize much lower cost per energy and to use the solar cells as a primary energy source, we need to increase the efficiency beyond the Shockley Queisser limit. One of the approaches toward third generation solar cells is to use the many-body effect of photoexcited carriers in semiconductors, which is called as multi-exciton generations.
c-SI
a-SI
organic
M. A. Green, Third Generation Photovoltaics: Advanced
Solar Energy Conversion (Springer-Verlag, Berlin, 2004).

4. Multi-Exciton Generation ?
Time (ps)
Electron Density t0 A B
h=A/B t1
hn > 2~3Eg Eg
So, what is multi-exciton generation? The multi-exciton generation is the process where the absorption of a single high-energy photon generates more than one electron-hole pairs, using the excess kinetic energy. The multi-exciton generation was originally proposed more than twenty years ago. Since the highly efficient multi-exciton generation was claimed in the semiconductor nanocrystals in 2004, the many-body effect in semiconductor nanocrystals has attracted much attention again. However, at the moment, the mechanism of the multi-exciton generation is not clearly understood, and the highly efficient conversion in nanocrystals is still being debated.
Highly Efficient MEG in Nanocrystals ?

5. Multi-Exciton in Si & Ge Based Nanostructures
Auger Recombination
10 K, Ex: 1.5eV < 2 Eg
Sample:
Ge QDs on Si substrate Eg Si Ge Si
High density
Low density
In my research, we used the germanium quantum dots because the fabrication method of the silicon-based nanostructures are well established. At First, we studied the carrier dynamics under low-energy photoexcitation, using the time-resolved spectroscopy. We found that under the high-density photoexcitation, the rapid decay component appears just after the pulsed-laser excitation. This rapid decay reflects the appearance of the another recombination process, which is Auger recombination due to the three-carrier collisions. The appearance of the rapid decay indicates that the high-density electron-hole pairs are generated in the quantum dots.
The Auger recombination is known as the inverse process of the multi-exciton generation. Eg

6. Multi-Exciton Generation in Ge QDs ?
Ex: 3.0eV ~ Eg
Multi-Exciton
Generation?
Auger
Recombination Eg
hn > 2~3Eg Eg
To observe the multi-exciton generation, we study the carrier dynamics under the high-energy photoexcitation. Under the photoexcitation with the energy of 4 times larger than the band gap energy, we observed the rapid decay component even under weak photoexcitation condition. As mentioned in the previous slide, such a rapid decay indicates the appearance of the Auger recombination and the high-density electron-hole pairs are generated in the quantum dots. So, this implies that the multi-exciton generation occurs via an absorption of the single high-energy photon. In my poster, I’m going to discuss the mechanism of the multi-exciton generation in nanocrystals. Thank you for your attention.
1 Photon
>2 e-h pairs ?
< 100 fs
< ns
Thank you for your attention and welcome to my poster !