Polymer Fullerene Blend Morphology

This section focuses on the relationship between the microstructure of polymer fullerene blends and the measured properties of OFETs formed from these systems. Despite the experimental data presented within this chapter being restricted to polymer fullerene blends, it will become apparent that the phenomena and techniques discussed here are relevant for other semiconductor composites also. 

The three-terminal structure of OFETs allows the selective injection and transport of carriers into the different components of the polymenfidlerene blend [78]. 

It should be noted that previous studies have shown that when using gold source and drain electrodes, hole injection and transport can take place in [60]PCBM [79]. However, in this case the mobility is significantly lower than that of holes in P3HT [82], for example, and hence one can assume that hole field-effect mobility measurements made on a 1 1 (wt%) P3HT [60]PCBM blend should be representative of the P3HT network only, with a negligible modification due to the [60]PCBM network. The same can also be said of most other p-type polymers. 

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In conclusion, not only the observed larger scale of phase separation but also the difference in the material s phase percolation and thus charge transport properties influence the photovoltaic performance. As such, it becomes evident that the charge carrier mobility measured in these devices must be a function of the blend morphology [139-143]. Furthermore, the electron and hole carrier mobilities depend strongly on the polymer-fullerene blending ratio. Interestingly, the hole mobility of the donor polymer is increased considerably in blends with fullerenes (see Fig. 27) [142,144-147]. 

Park, L.Y., Mrmro, A.M., Ginger, D.S. Controlling film morphology in conjugated polymer fullerene blends with surface patterning. J. Am. Chem. Soc. 130, 15916-15926 (2008)... 

M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P. G. Etchegoin, Y. Kim, T. D. Anthopoulos, P. N. Stavrinou, D. D. C. Bradley, J. Nelson, Morphology Evolution via Self-Organization and Lateral and Vertical Diffusion in Polymer Fullerene Solar Cell Blends. Nat. Mater. 2008, 7, 158-164. 

Fig. 26 Differences in the chlorobenzene (a) and toluene (b) based MDMO-PPV PCBM blend film morphologies are shown schematically. In a both the polymer nanospheres and the fullerene phase offer percolated pathways for the transport of holes and electrons, respectively. In b electrons and holes suffer recombination, as the percolation is not sufficient. (Reprinted from [61], 2005, with permission from Elsevier)...

The solvent in the fabrication of active layer affects the morphology of the active layer and the performance of the device. It was observed that casting the active layer blended from toluene gave the PCE of about 0.9 % with a 1 4 weight ratio of polymer to fullerene. When the solvent was changed to chlorobenzene, the efficiency dramatically improved to 2.5 % [27]. 

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