The molecular exciton model has received its most extensive development and application in the field of molecular crystals1'2. More recently, numerous applications to non-crystalline molecular composite systems have been made, including van der Waals and hydrogen-bonded dimers, trimers, and higher order aggregates. Another type of composite system has also been investigated, namely the composite molecule consisting of covalently bonded molecular units, with intrinsic individual unsaturated electronic systems so isolated by single bonds that but little or insignificant electronic overlap between units may occur. It is now well established that in molecular aggregates and in composite molecules, exciton effects may be observed if sufficiently strong electronic transitions exist in the component sub-units. The result of exciton splitting of excited states in the composite molecule may be the appearance of strong spectral shifts or splittings (which may be of the order of 2000 cm—1) of the absorption bands for the component molecules. At the same time, as a consequence of the exciton splitting of the excited state manifold, an enhancement of triplet state excitation may result. The purpose of this paper is to present a summary of the various type cases for molecular dimers, trimers and double and triple molecules in the description of the molecular exciton strong-coupling model. Then it will be shown by new experimental examples that, even in those cases where no significant exciton effect is observable in the singlet—singlet absorption spectrum for the composite molecule (intermediate and weak coupling cases), the enhancement of lowest triplet state excitation may still be conspicuous and significant. The ideas which are summarized in this paper have a curious history. Long ago, Kautsky and Merkel3 demonstrated experimentally that aggregation of dyes facilitated their action as photophysical sensitizers in photochemical reactions, at the same time diminishing their fluorescence efficiency. Kautsky attributed these easily demonstrated effects to enhancement of metastable state excitation in the aggregate dye. There is no doubt today that the metastable state he described is the lowest triplet state of the molecules studied. However, he did not distinguish between intrinsic and enhanced metastable (triplet) state excitation, so his interpretations were largely overlooked. Forster in l946 used the quasi-classical vector model to

/pdf/the-exciton-model-in-molecular-spectroscopy-2vwkowivjh.pdf

The exciton model in molecular spectroscopy

Recently, squaraine (SQ) dyes used as donor materials in organic photovoltaic (OPV) cells have received increasing attention because of their simple synthetic routes, high absorption coefficients with tunable bandgaps and bandwidths in the visible-near infrared region, as well as high photochemical and thermal stabilities. Using fullerenes or their derivatives as electron accepting materials, SQ:fullerene-based OPV devices have shown power conversion efficiencies as high as over 8%, which indicates the potential of SQ dyes for future solar cell applications. In this review, we briefly introduce the history of this group of materials, and present recent advances in SQ dye materials and their applications to OPV cells, including the major methodologies employed to exploit the full potential of these molecules as light harvesting materials in OPV devices. From the perspective of the current status of this field, we provide an overview of the primary issues affecting future development of OPV, as well as possible solutions.

Squaraine dyes for organic photovoltaic cells

Dye-dye interactions affect the optical and electronic properties in organic semiconductor films of light harvesting and detecting optoelectronic applications. This review elaborates how to tailor these properties of organic semiconductors for organic solar cells (OSCs) and organic photodiodes (OPDs). While these devices rely on similar materials, the demands for their optical properties are rather different, the former requiring a broad absorption spectrum spanning from the UV over visible up to the near-infrared region and the latter an ultra-narrow absorption spectrum at a specific, targeted wavelength. In order to design organic semiconductors satisfying these demands, fundamental insights on the relationship of optical properties are provided depending on molecular packing arrangement and the resultant electronic coupling thereof. Based on recent advancements in the theoretical understanding of intermolecular interactions between slip-stacked dyes, distinguishing classical J-aggregates with predominant long-range Coulomb coupling from charge transfer (CT)-mediated or -coupled J-aggregates, whose red-shifts are primarily governed by short-range orbital interactions, is suggested. Within this framework, the relationship between aggregate structure and functional properties of representative classes of dye aggregates is analyzed for the most advanced OSCs and wavelength-selective OPDs, providing important insights into the rational design of thin-film optoelectronic materials.

Slip-Stacked J-Aggregate Materials for Organic Solar Cells and Photodetectors

Due to their amenability to highly scalable fabrication and steadily improving efficiencies, organic photovoltaics (OPVs) offer great potential as an alternative to carbon-based fuel sources. With recently reported power conversion efficiencies of 11–12%, OPVs are on the brink of economic viability. However, to push these technologies forward into widespread use, further optimizations of efficiency are needed. The process of exciton dissociation and charge separation at donor/acceptor interfaces is a major factor influencing the power conversion efficiency of these devices, with loss of useful energy occurring if the charges fail to separate. However, this process of exciton dissociation and separation at OPV heterojunctions is not fully understood, and experimental efforts to optimize these processes via trial and error are costly. Thus, theoretical modeling plays a key role in understanding and improving the rate of charge generation in OPVs. Here we review key theoretical approaches to modeling the process of exciton dissociation and charge separation and their contributions to the challenge of optimizing OPV technology.

Charge generation in organic photovoltaics: a review of theory and computation

Squaraines offer significant potential for organic photovoltaics because of their broad absorbance and high extinction coefficients as well as their expected use as mechanistic probes in such devices. In this work steps are taken to develop a comprehensive understanding of the excited-state properties of squaraines on the basis of their molecular structure and the resulting solid-state packing. Accurate assignments of the absorption spectral peaks are made on the basis of an essential states model, expanded to include intermolecular charge transfer (ICT). A comparison of simulated spectra with spectra for two symmetric squaraine derivatives confirms that ICT has a major influence on the optical and electronic properties of squaraine aggregates, fully accounting for the origin of the strong panchromaticity of these systems.

Confirmation of the Origins of Panchromatic Spectra in Squaraine Thin Films 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 abi...

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

The effect of controllable thin film crystal growth on the aggregation of a novel high panchromaticity squaraine viable for organic solar cells

/pdf/characterization-of-organic-solar-cell-morphology-4jtcubuqje.pdf

Characterization of organic solar cell morphology

Cage opening and rearrangement of 1-iodocubane-4-carboxaldehyde

http://absimage.aps.org/image/MAR13/MWS_MAR13-2012-007140.pdf

Nanoscale Thermal Analysis of Organic Solar Cells

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