Comparing Multiple Exciton Generation in Quantum Dots To Impact Ionization in Bulk Semiconductors: Implications for Enhancement of Solar Energy Conversion

Matthew C. Beard, Aaron G. Midgett, Mark C. Hanna, Joseph M. Luther, Barbara K. Hughes, Arthur J. Nozik

Research output: Contribution to journalArticlepeer-review

316 Scopus Citations

Abstract

Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per absorbed photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which requires a further increase in the MEG efficiency. Finally, we discuss the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion since we show the threshold and efficiency are mathematically related.

Original languageAmerican English
Pages (from-to)3019-3027
Number of pages9
JournalNano Letters
Volume10
Issue number8
DOIs
StatePublished - 2010

NREL Publication Number

  • NREL/JA-590-48779

Keywords

  • Carrier multiplication
  • Multiple exciton generation
  • Quantum dots
  • Semiconductor nanocrystals
  • Solar energy conversion

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