Extremely Efficient Multiple Electron-hole Pair Generation in Carbon Nanotube Photodiodes

The efficient generation of multiple electron-hole (e-h) pairs from a single photon could improve the efficiency of photovoltaic solar cells beyond standard thermodynamic limits [1] and has been the focus of much recent work in semiconductor nanomaterials [2,3]. In single walled carbon nanotubes (SWNTs), the small Fermi velocity and low dielectric constant suggests that electron-electron interactions are very strong and that high-energy carriers should efficiently generate e-h pairs. Here, I will discuss observations of highly efficient generation of e-h pairs due to impact excitation in SWNT p-n junction photodiodes [4]. To investigate optoelectronic transport properties of individual SWNT photodiodes, we focus a laser beam over the device while monitoring the electronic characteristics. Optical excitation into the second electronic subband $E_{22}$\textit{ $\sim $ 2 E}$_{GAP}$ leads to striking photocurrent steps in the device $I-V_{SD}$ characteristics that occur at voltage intervals of the band gap energy $E_{GAP} / e$. Spatially and spectrally resolved photocurrent combined with temperature-dependent studies suggest that these steps result from efficient generation of multiple e-h pairs from a single hot $E_{22}$ carrier. We conclude that in the SWNT photodiode, a single photon with energy greater than$ 2E_{GAP}$ is converted into multiple e-h pairs, leading to enhanced photocurrent and increased photo-conversion efficiency. [1] W. Shockley, and H. J. Queisser, \textit{Journal of Applied Physics} \textbf{32}, 510 (1961). [2] R. D. Schaller, and V. I. Klimov, \textit{Physical Review Letters} \textbf{92} (18), 186601 (2004). [3] R. J. Ellingson, et al, \textit{Nano Letters}, \textbf{5} (5), 865-871 (2005). [4] Nathaniel M. Gabor, Zhaohui Zhong, Ken Bosnick, Jiwoong Park, and Paul McEuen, \textit{Science}, \textbf{325}, 1367 (2009).