Photoconductance of bulk heterojunctions with tunable nanomorphology consisting of P3HT and naphthalene diimide siloxane oligomers

The relation between the morphology, optical, and photoconductive properties of thin-film bulk heterojunctions of poly(3-hexylthiophene) (P3HT) with a series of electron-accepting siloxanes with a different number (x = 2, 4, 5) of pendant naphthalene diimide (NDIS) moieties is reported. All NDIS siloxanes show good electronaccepting properties, when blended with P3HT. Interestingly, the film (nano)morphology can be controlled by relatively small changes in the molecular structure of the NDIS siloxanes. Spin-coating from orthodichlorobenzene (ODCB) yields complex film (nano)morphologies, being correlated with the weight ratios of P3HT and NDIS siloxanes, and the molecular structure of the latter. On the other hand, all blends spincoated from chloroform (CHCl3) show good mixing of the components at the molecular level. It is inferred that the nanomorphology of the blends can greatly influence their photoconductive properties: samples spincoated from ODCB invariably display a higher photoconductance than corresponding samples spin-coated from CHCl3. This is explained in terms of a higher mobility of holes in samples spin-coated from ODCB, as measured by time-resolved microwave conductivity measurements. These data are useful to delineate the conditions for the research in strives for efficient organic photovoltaics. © 2009 American Chemical Society