We are pioneering the exploration of semiconducting carbon nanotubes as the light-absorbing components of polymer-inspired solar cells and photodetectors.[1-2] Carbon nanotubes are conjugated polymer-like materials with built-in long-range crystallinity that gives rise to exceptional charge and energy transport characteristics, strong light absorption tunable throughout the visible and near-infrared spectra, and outstanding stability in air. We have discovered how to efficiently harvest photogenerated charges and excitons from optically excited nanotubes by pairing them in donor / acceptor heterojunctions with more electronegative electron accepting semiconductors. In particular, semiconducting nanotubes form a type-II heterojunction with C60 fullerenes and C60 derivatives with energy offsets sufficient to drive electron transfer from the optically excited nanotubes to C60, with an internal quantum efficiency (QE) for exciton dissociation and charge transfer > 75%, for nanotubes of diameter < 1 nm and gaps > 1 eV. Thus, we have identified the nanotube / C60 materials pair as a promising basis for future nanotube-based light harvesting devices.

 In order to further guide the implementation of nanotubes in devices, we have also characterized exciton transport in nanotube films and shown that excitons can migrate in films by two mechanisms: (i) over short distances of ~ 5 nm via slow inter-nanotube diffusion and (ii) potentially over much longer distances via rapid intra-nanotube diffusion. As a proof-of-principal, we have fabricated both bilayer and blended nanotube / C60 heterojunction devices, which are analogous to polymer solar cells with nanotubes taking on the role of the semiconducting “polymer”. Thus far, we have realized a peak external QE > 20% across 1000 – 1365 nm and a monochromatic power conversion efficiency of 7% at 1050 nm. Our results show that AM1.5G photovoltaic power conversion efficiency > 10% should be possible with future optimization of: (a) the nanotube bandgap (and diameter) distribution and (b) improved control over morphology.

 

[1] D. J. Bindl, M.-Y. Wu, M. S. Arnold, Nano Letters (2011).

[2] D. J. Bindl, A. S. Brewer, M. S. Arnold, Nano Research (2011).