Contribution of Aggregate States and Energetic Disorder to a Squaraine System Targeted for Organic Photovoltaic Devices.

Squaraine dyes have significant potential for use in organic photovoltaic devices because their chemical and packing structure tunability leads to a broad solid state panchromaticity. Nevertheless, broadening of the spectrum does not always give rise to increasing power conversion efficiencies. Furthermore, the same processing strategy used to make devices from different squaraines does not lead to the same optimized performance. In this work, by varying the environmental conditions of a set of anilinic squaraines, we demonstrate that spin-cast thin films are made up of a complex set of states, with each state contributing differently to the overall device efficiency. We demonstrate crystallochromy in that small changes in the packing structure give rise to dramatically different absorption spectra. Through a remarkable comparison between squaraines in poly(methyl methacrylate) solid solution and squaraine:PC60BM blends, we also show long-range and orientational disorder broadening, which distorts the ability to correlate qualitative spectroscopic assessment with an understanding of the device mechanism. We conclude that a full quantitative assessment of the populations of each excited state must be carried out in order to make progress toward an improved understanding of each state's contribution to charge transfer at the bulk heterojunction interface.

A new model for quantifying the extent of interaction between soluble polyphenylene-vinylenes and single-walled carbon nanotubes in solvent dispersions.

The ability to disperse and, hence, manipulate single-walled carbon nanotubes is critical for their use in organic photovoltaic devices, either as a transparent electrode or as an electron acceptor material. We present data to quantify the physical interaction of single wall carbon nanotubes (SWCNTs) with two soluble phenylene vinylene conjugated polymers, poly[2'-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] and poly(2,5-di(hexyloxy)cyanoterephthalylidene). We provide static quenching constants, associated with polymer-SWCNT complexation, as a weight percent ratio of polymer to nanotubes for different solvent and polymer concentration conditions. Optimization of conditions for nanotube dispersion using a given polymer can now be predicted, and furthermore, we can describe a technique allowing for enhanced relative comparisons of polymer materials for nanotube dispersion.

Spectroscopic evidence for interaction of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] conformers and single-walled carbon nanotubes in solvent dispersions.

Organic photovoltaic devices promise low-cost, flexible options for future renewable energy that will reduce reliance on oil. Single-wall carbon nanotubes (SWCNTs) provide possibilities for increasing the efficiency of organic solar cells through increasing conductivity of composites used in such devices or through use as a charge acceptor in a bulk heterojunction device. We present data to indicate the physical interaction of SWCNTs with a conjugated polymer, poly[2'-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), on the basis of the spectroscopic assignments of various conformational species of different optical signature in N,N-dimethylacetamide (DMA) dispersions. We go on to show that energy transfer from nonaggregated MEH-PPV leads to enhanced SWCNT fluorescence in solutions of poorer solvent quality. Energy transfer from polymer chain lengths that are torsionally restricted is not observed. This would suggest that any electron transfer taking place is occurring through a concerted Dexter mechanism and that use of SWCNTs as an electron acceptor will likely have associated drawbacks.