(174c) Evidence for High-Efficiency Exciton Dissociation At Polymer/Single-Walled Carbon Nanotube Interfaces In Planar Nano-Heterojunction Photovoltaics

There is significant interest in combining carbon nanotubes (CNTs) 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 with CNTs incorporated in them have demonstrated relatively poor efficiencies, and little is understood about the polymer/nanotube junction. To investigate this interface, we fabricated a planar heterojunction  (PHJ) comprising well-isolated millimeter-long single-walled carbon nanotubes underneath a poly(3-hexylthiophene) (P3HT) layer. 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.(1) 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. Another PHJ described in literature using P3HT as the donor but PCBM  as the acceptor (rather than SWNTs) also shows 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).  We investigate the photocurrent generation in these two state-of-the-art devices by -for the first time, combining an optical T-matrix model with a Kinetic Monte Carlo Simulation. 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 P3HT/PCBM PHJ solar cells from the literatureThe 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.   Based upon the results of this model it will be possible to more intelligently design polymer hybrid solar cells (both planar and bulk) and optimize them towards higher efficiencies.

Both the experimental as well as the simulation platform are promising for further understanding the potential role of polymer/nanotube interfaces for photovoltaic applications.

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