Carbon nanotube photocurrent quantum yield greater than 100%*

Carbon nanotubes (CNTs) have been identified as a candidate material to surpass the Shockley-Queisser limit, however, early measurements of photocurrent quantum yield (PCQY) in CNT photodiodes found PCQY < 100%. We studied CNT photodiodes made from individual suspended CNTs with the goal of elucidating the role of nanotube diameter, dielectric environment, axial electric field, and the optical excitation energy. When photons of energy approximately twice the band gap are absorbed in the intrinsic region of a CNT photodiode, it is possible to extract a photocurrent that corresponds to more than one electron-hole pair per absorbed photon. We observed this high-quantum-yield process by increasing CNT diameter to > 2.6 nm and increasing the axial electric field to > 10 V/µm (a previously unstudied regime). Higher energy optical excitation gives even higher PCQY. The observed dependence on diameter and dielectric environment is consistent with theoretical models for exciton binding energy and exciton dissociation rates. Our work suggests there are conditions for which efficient exciton dissociation co-exists with efficient impact ionization.