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Title: Enhancing Charge Separation and Transport in Polymer Solar Cells
Author: Barkhouse, D. A. R.
ISNI:       0000 0001 3445 4260
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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This work investigates means of overcoming the problems of Short exciton diffusion length and low charge mobility in polymer solar cells. The first issue is addressed by using a novel polyphenylenevinylene (PPY) derivative, which has a permanent dipole on the repeat unit of the polymer, to dissociate excitons in the polymer bulk, rather than requiring exciton diffusion to a dissociating interface. The second is dealt with by adding a lithium salt to polymer/Ti02 devices to improve hole mobility. The photophysical characteristics and photovoltaic device performance of a PPY derivative with a fluorene sidegroup, with and without an electron withdrawing nitro moiety which bestows a larger dipole moment across the repeat unit of the polymer, are reported. The photoluminescence of the polymer with the nitro group (nitrofluoro-PPY) is lower than that of its non-nitro (fluoro-PPY) analogue by a factor of eight. Lightinduced electron spin resonance experiments show that the nitrofluoro-PPY has more photoinduced spins, indicating that the luminescence quenching is due to intramolecular charge separation. Photovoltaic devices made with the nitrofluoro-PPY are three times as efficient as those with the fluoro-PPY, mainly due to an increase in current. Both types of device are inefficient, similar to previously reported all-polymer devices, and their efficiency is greatly improved by the incorporation of a solubilized C60 electron transporting phase. The addition of a lithium salt (Li[CF3S02hN) to MEH-PPY/Ti02 solar cells drastically improves device performance by increasing both the fill-factor and shortcircuit current by up to 40%. The efficiency of Li[CF3S02hN modified devices is 1.05% under 80 mWIcm2 simulated solar illumination, twice that of control devices without the salt and the highest reported to date for any polymer/Ti02 solar cell. The improved performance is attributed to a large increase in the hole mobility in the polymer, as measured by space-charge-limited current measurements, in the presence of the lithium salt. Modification of devices with the salt LiCI04 is not as effective at improving device performance. Secondary ion mass spectroscopy studies suggest that this is due to poorer diffusion of lithium into the polymer layer for LiCI04 relative to Li[CF3S02hN. The novel charge separation polymers studied here may serve to inform the development of all-organic dyes, and lithium salt treatment of bulk heterojunction devices may lead to significant improvements in power conversion efficiency.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available