HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 43
B10 - On the correlation between crystallinity and photophysics for donor polymers
of interest for organic photovoltaic devices
Ying Woan Soon, Safa Shoaee, Iain McCulloch, James R Durrant
Imperial College London, Chemistry department, South Kensington campus, London, SW7 2AZ, GB
Crystalline materials have been widely reported to give good charge carrier mobilities which
can enhance charge transport.(1-2) Hence, in the field of OPV and OLED, highly crystalline
materials have been considered as favourable for obtaining efficient devices. However, the
fluorescence quenching associated with the process of ‘concentration quenching’ has been
observed to be more efficient in crystalline materials.(3-4) This fluorescence quenching
process is thought to be related to faster internal conversion, leading to a potential loss of
device efficiency.
In this study, photophysical properties of a range of conjugated donor-acceptor polymers
with varying crystallinity are investigated using techniques including wide angled x-ray
diffraction, single photon counting, and transient absorption spectroscopy (TAS). The more
crystalline polymers are found to have shorter singlet and triplet lifetimes compared to the
more amorphous polymers. This is consistent with the lack of triplet observation upon
photoexcitation of the more crystalline polymers while high triplet yields are usually found in
the amorphous polymers using microsecond TAS. Although amorphous polymers typically have
lower charge mobilities than the more crystalline polymers, the longer singlet and triplet
lifetimes of the amorphous polymers can potentially aid charge separation. Therefore, both
the highly crystalline and amorphous polymers have different attributes that can compromise
on the performance of OPV devices.
Furthermore, the yield of polaron photogeneration in neat polymer films is found to be
maximum in the semi-crystalline polymers. Recent high performing polymers in the literature
such as poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]
(PTB7) and poly[N-9”-hepta-decanyl-2,7carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)]
(PCDTBT) are found to be semicrystalline
in our context. This finding can have direct implication on the future design of
conjugated polymers for use in organic solar cells.
References
[1] Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A.J. "Thermally Stable, Efficient Polymer Solar Cells with Nanoscale
Control of the Interpenetrating Network Morphology".Adv.Funct.Mater.15, 1617-1622 (2005)
[2] Mihailetchi, V.D.; Xie, H.X.; De Boer, B.; Koster, L.J.A.; Blom, P.W.M. "Charge transport and photocurrent
generation in poly (3-hexylthiophene): Methanofullerene bulk-heterojunction solar cells".Adv. Funct. Mater. 16,
699-708 (2006)
[3] Huignard, A.; Gacoin, T.; Boilot, J-P. "Synthesis and Luminescence Properties of Colloidal YVO4:Eu
Phosphors".Chem. Mater.12,1090-1094 (2000)
[4]Huang, H.; Xu, G.Q.; Chin, W.S.; Gan, L.M.; Chew, C.H."Synthesis and characterisation of Eu:Y2O3 nanoparticles".
Nanotechnology 13, 318-323 (2002)
© SEFIN 2012