Processing of Polymeric Semiconductors: Blending with Conventional Polymers

The importance of melt processing properties for conjugated polymers in de­vice construction was demonstrated recently [34]. However, most conjugated polymers show no glass transitions or only side-chain glass transitions. The embedding of the photoactive conjugated polymer-fullerene blend into a con­ventional polymer matrix (guest-host approach) presented in recent studies [29,171] is a sound and promising method in this respect, with the possibility of improving the photoactive sample quality. The reasons are as follows: [20]

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Fig. 5.56. (a) Isc of various PS/MDMO-PPV/PCBM devices under different exci­tation intensities vs. PS percentage. The inset shows the dependence of Isc on the electroactive component concentrations (100% PS wt. %) in a log-log plot. Lines are power law fits according to 7SC ~ [wt. %]“. Best fits are obtained with a ~ 3. (b) Power efficiency rjefj of various PS/MDMO-PPV/PCBM cells vs. excitation intensity. Lines are drawn as a guide to the eye. Excitation is provided by Ar+ laser at 488 nm

the intermolecular distances through morphology control or the overall dielectric constant of the system.

For large scale production of plastic solar cells, the rheological properties will also become relevant. To this end, and for the reasons listed above, we stud­ied the behavior of highly efficient conjugated polymer/methanofullerene cells (ITO/MDMO-PPV:PCBM/Al) blended into a conventional polymer matrix [29,171]. Figures 5.56a and b show the dependence of Isc and the efficiency r]es on the content of an inert, photoinactive conventional polymer (polystyrene, PS) under various illumination intensities. Introducing small amounts of PS (10 wt. %) does not significantly change the efficiency of the cells. Further increase in the PS concentration results in a significant decrease in Isc. The percolation threshold for the interpenetrating network of the conjugated poly­mer/methanofullerene mixture in the host matrix determined the onset of a strongly enhanced photovoltaic response. The intensity dependence of the spectrally resolved photocurrent and the short-circuit current indicates that, at higher light intensities, enhanced annihilation of mobile charge carriers occurs. This is also a side-effect of the increased thickness when blended with a further polymeric component. However, these results clearly demonstrate that it is possible to enhance the rheological and/or mechanical properties of the bulk heterojunction solar cells by addition of smaller (over 10 wt. %) amounts of photoinactive ‘additives’.

Updated: August 20, 2015 — 6:39 am