(609a) Exciton Engineering with Carbon Nanotubes and Graphene for Solar Energy Conversion: From Exciton Antennae to Nano-Heterojunctions

Michael S. Strano

Charles and Hilda Roddey Associate Professor of Chemical Engineering

66-566 Department of Chemical Engineering

77 Massachusetts Avenue

Cambridge, MA 02139-4307

Email: strano@MIT.EDU

phone: (617) 324-4323

fax: (617) 258-8224

http://web.mit.edu/stranogroup/

                Our laboratory has been interested in how low dimensional materials, such as single walled carbon nanotubes or graphene, can augment and enhance solar conversion efficiencies and demonstrate new photonic concepts.  We will discuss two recent examples of our work in this space.  

                In the first example, there is significant interest in combining carbon nanotubes with semiconducting polymers for photovoltaic applications because of potential advantages from smaller exciton transport lengths and enhanced charge separation. However, to date, bulk heterojunction (BHJ) devices have demonstrated relatively poor efficiencies, and little is understood about the polymer/nanotube junction. To investigate this interface, we fabricate a planar nano-heterojunction comprising well-isolated millimeter-long single-walled carbon nanotubes underneath a poly(3-hexylthiophene) (P3HT) layer (1). The resulting junctions display photovoltaic efficiencies per nanotube ranging from 3% to 3.82%, which exceed those of polymer/nanotube BHJs by a factor of 50-100. The increase is attributed to the absence of aggregate formation in this planar device geometry. It is shown that the polymer/nanotube interface itself is responsible for exciton dissociation. Typical open-circuit voltages are near 0.5 V with fill factors of 0.25-0.3, which are largely invariant with the number of nanotubes per device and P3HT thickness. A maximum efficiency is obtained for a 60 nm-thick P3HT layer, which is predicted by a Monte Carlo simulation that takes into account exciton generation, transport, recombination, and dissociation. We combine for the first time both optical T-matrix and kinetic Monte Carlo models to investigate the photocurrent generation in two state-of-the-art PHJ photovoltaics.  The combined model takes into account the rates of exciton generation, transport, recombination and dissociation using literature values.  By including the optical, electronic and structural properties of the different materials, we are able to predict the short-circuit current of recently reported P3HT/SWNT PHJ and also P3HT/PCBM PHJ solar cells from the literature.   The experimental data for each of these devices show a maximum photocurrent output at a P3HT thickness of 60-65 nm, in contradiction to the expected value equal to the diffusion length of excitons in P3HT (8.5nm).  The model demonstrates how a bulk exciton sink can explain this shifted maximum in the P3HT/SWNT case, whereas the maximum is mainly determined by PCBM interdiffusing in P3HT in the P3HT/PCBM case.   This platform is promising for further understanding the potential role of polymer/nanotube interfaces for photovoltaic applications.

                In the second example, there has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. We dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core–shell structures with radial gradients in the optical bandgap (2). We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-orderAuger process. Core–shell structures consisting of annular shells of mostly (6, 5) SWNTs (Eg = 1.21 eV) and cores with bandgaps smaller than those of the shell (Eg = 1.17 eV (7,5)–0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet–near-infrared wavelength regime provides quasi-singular photoemission at the (8, 7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

1. Ham MH, Paulus GLC, Lee CY, Song C, Kalantar-zadeh K, Choi W, Han JH and Strano MS: Evidence for High-Efficiency Exciton Dissociation at Polymer/Single-Walled Carbon Nanotube Interfaces in Planar Nano-heterojunction Photovoltaics. ACS NANO, 4 (2010) 6251-6259

2. Han JH, Paulus GLC, Maruyama R, Jeng ES, Heller DA, Kim WJ, Barone PW, Lee CY, Choi JH, Ham MH, Song C, Fantini C, Strano MS: Exciton Antennas and Concentrators from Core-Shell and Corrugated Carbon Nanotube Filaments of Homogeneous Composition. NATURE MATERIALS, 9, 833 - 839 (2010).