Do we need to revisit the Bohr exciton radius of hot excitons?

Details Do we need to revisit the Bohr exciton radius of hot excitons? Do we need to revisit the Bohr exciton radius of hot excitons?

2011-05-11

arxiv.org/abs/1105.2205v3

Physics

Condensed Matter

Mesoscale and Nanoscale Physics

39 pages including 16 pages of Supplemental Information at the End

Scientific paper

Photo-induced carrier multiplications are predicted to overcome the limiting efficiency of single junction solar cells by generating more than one electron-hole pair (exciton) per absorbed photon. Here, we have studied collisional broadening of excitonic absorption spectra of PbS nano-crystallites at energies much above its fundamental band gap. Observed spectral features necessitate the role of extended band structure of semiconductors to understand the physics of carrier multiplication in nanoparticles. Our analysis also shows that quasi ballistic transport of hot excitons can actually suppress exciton-exciton scattering events required for photo-induced carrier multiplication in very small quantum dots. Measured variations of excitonic broadening clearly indicate that impact ionization of excitons may be efficient only inside an intermediate 'size window'. This can explain the current debates on reported efficiencies of carrier multiplication in semiconductor nanoparticles as compared to bulk materials. Moreover, we will discuss the importance of 'effective Bohr exciton radius' as a direct consequence of significant departure from the usual dielectric screening limits of coulomb interactions of 'hot' excitons in the region of strong dispersion at energies much above the fundamental band gap.

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