Excitonic processes in molecular crystalline materials
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troduction Many of the technological applications envisioned for organic electronic materials, ranging from photovoltaics to sensors to light-emitting diodes, involve the absorption and emission of light. The low-lying bound electronic states (excitons) that determine the absorption and emission properties of these materials can be probed using optical spectroscopy, but the characterization of excitons in organic materials such as polymers is often complicated by the disordered nature of the sample. In principle, single crystals provide the best opportunity for drawing predictive structure–function relationships thanks to their high degree of structural order and chemical purity. Furthermore, the outstanding performance of small molecule, crystalline materials in applications such as light-emitting diodes, solar cells, and field-effect transistors suggests that this class of materials will play an important role in practical applications of organic electronic materials. Advances in sample preparation and characterization, spectroscopy, and theory are leading to new insights that will provide guidance for the design of new generations of crystalline molecular electronic materials. The purpose of this review is to provide a basic overview of exciton structure and dynamics using the molecule tetracene as a prototype system to demonstrate some of the relevant phenomena.
Overview of electronic states in molecular crystals The term “exciton” has often been used as a generic label for an excited state in an organic material, and several reviews discuss their structure.1–7 Here we present a very basic picture based on concepts developed almost 50 years ago.8,9 Figure 1 provides a schematic outline of how the electronic eigenstates evolve as individual molecules self-assemble into a crystal. At large separations, when two molecules, A and B, have no measurable Coulomb interaction, absorption of a photon leads to either molecule A or molecule B being in its excited state (denoted with an asterisk), with the two possible product wave functions (Ψ∗A ΨB and ΨA Ψ∗B ) of the system given in the figure. Similarly, if we consider ionized states that are usually higher in energy than excited neutral states, we also have two possible system wave functions that are degenerate. As molecules A and B are brought closer to each other, the intermolecular interaction becomes significant, and the “localized” excited states Ψ∗A ΨB and ΨA Ψ∗B give way to their symmetric and anti-symmetric linear combinations, where the excitation now resides on both A and B simultaneously. A similar change occurs for the ionized states. As additional molecules are added, they interact and create N different states that are now delocalized over N molecules. The collection of N states forms a “band.” If the band
Christopher J. Bardeen, Department of Chemistry, University of California, Riverside; [email protected] DOI: 10.1557/mrs.2012.312
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • JANUARY 2013 • www.mrs.org/bulletin
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EXCITONIC PR
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