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Fibril Formation in an Idealised System Adam Hobson 1,2, Tim Richardson 1, Alan Dunbar 2, Stuart Brittle 1 Figure 1: The packing structure of P3HT fibrils.

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Presentation on theme: "Fibril Formation in an Idealised System Adam Hobson 1,2, Tim Richardson 1, Alan Dunbar 2, Stuart Brittle 1 Figure 1: The packing structure of P3HT fibrils."— Presentation transcript:

1 Fibril Formation in an Idealised System Adam Hobson 1,2, Tim Richardson 1, Alan Dunbar 2, Stuart Brittle 1 Figure 1: The packing structure of P3HT fibrils 1, inset: a single P3HT monomer. The formation of fibrillar structure of P3HT from solution is of great interest due to its use in the field of solar cells. P3HT is a polymer of thiophene monomers with attached hexyl groups (See inset, figure 1). When dissolved in solution and allowed to cool it will form fibrillar structures. These structures are of great importance because stable fibrils of P3HT, amorphous mixed areas of P3HT:PCBM & crystalline PCBM are all needed to make efficient organic solar cells. Ideally we would like to control the size, orientation and density of these fibrils in order to tailor them to the required application. Figures 2, 3 and 4 show AFM images of P3HT LS deposited on silicon. These images show that there are no fibrils present at 0 hrs but that they form slowly over a period of 24 to 48 hours (depending on solution used and P3HT chain length). In our case we found fibril creation became saturated at around 48 hours. From these AFM images we managed to extract the typical dimensions of one of our fibrils: Width 60 nm, Height 5 nm. Fibril length is hard to determine due to close packing. These were of a similar height to fibrils reported in literature 1,3, but of a larger width due to our P3HT having a longer chain length (which confirmed the predictions of fibril orientation). Figure 5 shows a UV- Visible spectrum of a solution of P3HT in chlorobenzene. From figure 5 we can make out the typical peaks arising from P3HT fibrils (at 450, 550 and 600 nm). 2 These peaks seem to be broader than others reported in the literature. This is likely due to P3HT dissolved in the chlorobenzene due to more solvent having to be added for spectroscopy. This dissolved P3HT is likely to be the cause of the amorphous areas on the AFM images as we have to dissolve the P3HT in order for it to spread effectively. The next step will be to centrifuge the solution (as suggested in the literature 3 ) in order to separate the dissolved P3HT from the suspended fibrils and thus get a solution of pure fibrils that we can spread on the trough. References 1.Field Effect Transport and Trapping in Regioregular Polythiophene Nanofibers. Jeffrey A. Merlo and C. Daniel Frisbie. Journal of Physical Chemistry. B 2004, 180, 19169 – 19179. 2.A New Method to Improve P3HT Crystalline Behaviour: Decreasing Chains Entanglement to Promote Order-Disorder Transformation in Solution. Kiu Zhao, Longjian Xue, Jiangang Liu et al. Langmuir 2010, 26(1), 471-477. 3.Efficient formation, isolation and characterisation of poly(3-alkylthiophene) Nanofibers: probing order as a function of side chain length. Wibren D. Oosterbaan, Veerle Vrindts, Solenn Berson et al. Journal of materials Chemistry, 2009, 19, 5324-5435. Figure 5: UV-Visible spectroscopy on 10 mg/ml P3HT in chlorobenzene. Solution left at 10mg/ml and diluted to 0.0125 mg/ml before spectroscopy. 1 Department of Physics and Astronomy, University of Sheffield, Sheffield, UK 2 Department of Chemical Engineering, University of Sheffield, Sheffield, UK Figure 2: AFM height image of P3HT LS dipped on silicon. 0 hrs after solution creation. 5  m x 5  m scan. Aged 0 hrs Figure 3: AFM height image of P3HT LS dipped on silicon. 24 hrs after solution creation. 5  m x 5  m scan. Aged 24 hrs Figure 4: AFM height image of P3HT LS dipped on silicon. 48 hrs after solution creation. 2  m x 2  m scan. Aged 48 hrs After centrifugation we hope to be able to get a solution which contains almost no dissolved P3HT and is composed mainly of P3HT fibrils. If we can achieve this then it would then be possible to attempt to align these fibrils on a water substrate. This could be done by either multiple barrier compressions and recompressions or via agitation with a sonic probe. Figure 6 shows a theoretical alignment of fibrils on the water substrate. Hopefully if we could get the P3HT fibrils to align in such a way we could then LS or LB dip these films onto electrodes and make conductivity measurements along both axes. This alignment could help improve the efficiency of organic photovoltaic devices and has numerous other applications for example as a gas sensor. Figure 6: A theoretical arrangement of P3HT fibrils on a water substrate. Hopefully if we could get the fibrils to align in such a way we could then measure the conductivity along and across these structures. Another approach was to spin cast the P3HT directly from the concentrated solution in order to avoid the problem of extra solvent dissolving the P3HT. As we can see from figure 7, the same solution spin cast onto silicon shows no signs of amorphous P3HT areas and shows only fibrils (albeit randomised). The problem with this was that the fibrils are directly cast onto the silicon substrate and the solvent quickly evaporates leaving little chance for the fibrils to become aligned. In order to combat this an experiment where fibrils were originally spin cast onto glass and then “lifted off” onto a water substrate were performed. These fibrils were then agitated via compression and recompression in order to try to align the fibrils. Figure 7: AFM height image of P3HT spit cast on silicon. 48 hrs after solution creation. 10  m x 10  m scan. Figure 8 shows the film which has been compressed and re-compressed 80 times and then again LS dipped onto silicon. As we can see the image is virtually identical to the spin cast image and thus we have determined that either the fibrils are too dense to be aligned (in which case our next step will be to try with a solution that is older but more dilute) or that the compression technique does not provide enough agitation and thus a sonic probe will be used in order to try to further agitate and align the fibrils. Another line of inquiry will be to attempt to mix solvents in order to improve mixing without dissolving P3HT. If we could find the correct solvents and mixing ratios then we could have an ideal world in which we can get only fibrils and no amorphous areas as well as good alignment and spreading. Figure 8: AFM height image of P3HT spit cast onto a glass substrate, floated off onto a water subphase and then compressed and recompressed 100 times before being LS dipped onto hydrophobic silicon. 10  m x 10  m scan.


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