Feb 10, 2012
Anomalous thickness-dependence of photocurrent explained for planar nano-heterojunction organic solar cells
When researchers from the Strano Research Group at MIT tried to figure out why using single-walled carbon nanotubes (SWNTs) in organic photovoltaics did not result in higher efficiencies, despite the material's obvious advantages, the group found that bundling of SWNTs was detrimental to the solar cell's performance. As a proof of concept, the scientists designed a planar heterojunction (PHJ) solar cell with long, parallel SWNTs as the acceptor and P3HT as the donor. The efficiency per SWNT increased by a factor of ~50 (ACS Nano 2010 4 6251). By adjusting the thickness of the P3HT layer covering the SWNTs, the researchers found that a maximum photocurrent was reached at a value of ~60 nm. This observation served as the motivation for the team's current work, which has just been published in Nanotechnology.
Typically, a maximum efficiency in PHJs is expected for ~10 nm of active material – the value of the exciton diffusion length in most conjugated polymers. The idea behind this statement is that if an exciton is generated at a distance greater than its diffusion length from the hetero-interface of the PHJ, it would be very unlikely to be able to contribute to the photocurrent.
Searching through the literature, the researchers noted that their device was not the only P3HT-based PHJ device to display anomalous behaviour. Another design, this time with PCBM molecules functioning as the acceptor material, was also reported to show a similar trend in photocurrent versus P3HT thickness.
Predicting device behaviour
The Strano team decided to explore these results in more detail by combining both an optical T-matrix and a Kinetic Monte Carlo (KMC) model. The optical model determines where photons are being absorbed in the device and shows that interference plays a much larger role in the P3HT/PCBM device than in the P3HT/SWNT device because the reflecting electrode is positioned directly beneath the active layer. Once it is known where photons are absorbed (and hence excitons are generated), the KMC simulation focuses on tracking the path of each exciton.
By taking into account the recent experimental observation that excitons can also dissociate in the bulk of the active layer, not just at the hetero-interface, the model accurately predicts the trend for the P3HT/SWNT device. The bulk exciton dissociation is more pronounced for smaller thicknesses of P3HT (due to a corresponding stronger electric field), which shifts the maximum photocurrent to larger values of the P3HT thickness.
In the P3HT/PCBM case this effect is offset by the increased positive optical interference at lower thicknesses. Here the model teaches us that the shift of the maximum is caused by PCBM molecules interdiffusing into the P3HT upon annealing.
Based on the results of this model it will be possible to improve the design of nanostructured photovoltaics.
Further information can be found in the journal Nanotechnology.
About the author
This study was conducted in the Strano Research Group at MIT's Department of Chemical Engineering and was financially supported by Eni, S.p.A (Italy) through the MIT Energy Initiative Program. Geraldine L C Paulus, lead author of this work, is a fourth-year graduate student in the Strano group with a strong interest in studying and manipulating excitons in nanostructured devices used for energy applications. She developed the simulation presented in this work. Moon-Ho Ham is an assistant professor of Materials Science and Engineering at Gwangju Institute of Science and Technology in Korea and used to be a postdoc in the Strano Research Group. He is grateful for support from the Korea Research Foundation Grant funded by the Korean Government (MOEHRD; KRF-2007-357-D00133). Michael S Strano is the Charles and Hilda Roddey associate professor of Chemical Engineering at MIT and the P.I. for this project.