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

Abstract

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.

Phase separation, crystallinity and monomer-aggregate population control in solution processed small molecule solar cells

“…The thin film absorbance has two broader peaks at 556 nm and 656 nm. This substantial broadening is typical of analino squaraines (SQs) with linear side chains [22,27,30,31], and has often been attributed to the upper and lower Davydov components in SQ aggregates under the assumption of an oblique transition dipole alignment as part of a herringbone packing geometry [27,31,32]. However, such an interpretation is inconsistent with the single crystal structure of DHSQ(OH) 2 which exhibits a single molecule per unit cell with the conjugated backbones (and transition dipole moments) parallel to one another throughout the crystal [27].…”

“…When the dipole orientation factor of donor and acceptor is large, the Fӧrster Resonance Energy Transfer (FRET) is efficient [40]. The dipole orientation factor is large in a highly ordered crystal material, and is small in an amorphous state with randomized dipole alignments [30]. Hence exciton diffusion rates are enhanced in ordered materials.…”

Confirmation of the Origins of Panchromatic Spectra in Squaraine Thin Films Targeted for Organic Photovoltaic Devices

“…This will allow the opportunity to explain orientational broadening in thin film spectroscopic data. 34 With a full understanding of the nature of the excited states in squaraine aggregates, we provide a foundation to recognize the critical impact of controlling aggregation towards higher efficiencies of OPV devices, which rely on a fast rate of charge transfer at the bulk heterojunction interface. Synthesis.…”

Molecular aspects of squaraine dyes aggregation and its influence on spectroscopic properties

“…Squaraine dyes have attracted attention for their unique properties [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A one-dimensional molecule stacking observed in squaraine solutions is of similar nature as that recognized for cyanine or merocyanine dyes.…”