Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?

TL;DR

This paper proposes that impact excitation can explain the high efficiency of carrier multiplication observed in carbon nanotube photodiodes, contrasting with previous understanding from other nanomaterials.

Contribution

It introduces a detailed impact excitation mechanism accounting for efficient carrier multiplication in carbon nanotubes, considering time, energy scales, temperature, and diode length.

Findings

Impact excitation explains observed carrier multiplication efficiency.

Temperature and diode length influence multiplication process.

Analysis aligns with recent experimental results.

Abstract

We address recent experiments (Science 325, 1367 (2009)) reporting on highly efficient multiplication of electron-hole pairs in carbon nanotube photodiodes at photon energies near the carrier multiplication threshold (twice the quasi-particle band gap). This result is surprising in light of recent experimental and theoretical work on multiexciton generation in other confined materials, such as semiconducting nanocrystals. We propose a detailed mechanism based on carrier dynamics and impact excitation resulting in highly efficient multiplication of electron-hole pairs. We discuss the important time and energy scales of the problem and provide analysis of the role of temperature and the length of the diode.