Title: Solid-State Solvation and Enhanced Exciton Diffusion in Doped Organic Thin Films under Mechanical Pressure

Abstract

Direct modification of exciton energy has been previously used to optimize the operation of organic optoelectronic devices. One demonstrated method for exciton energy modification is through the use of the solvent dielectric effects in doped molecular films. To gain a deeper appreciation of the underlying physical mechanisms, we test the solid-state solvation effect in molecular thin films under applied external pressure. We observe that external mechanical pressure increases dipole–dipole interactions, leading to shifts in the Frenkel exciton energy and enhancement of the time-resolved spectral red shift associated with the energy-transfer-mediated exciton diffusion. Measurements are performed on host:dopant molecular thin films, which show bathochromic shifts in photoluminescence (PL) under increasing pressure. This is in agreement with a simple solvation theory model of exciton energetics with a fitting parameter based on the mechanical properties of the host matrix material. We measure no significant change in exciton lifetime with increasing pressure, consistent with unchanged aggregation in molecular films under compression. However, we do observe an increase in exciton spectral thermalization rate for compressed molecular films, indicating enhanced exciton diffusion for increased dipole–dipole interactions under pressure. The results highlight the contrast between molecular energy landscapes obtained when dipole–dipole interactions are increased by the pressuremore » technique versus the conventional dopant concentration variation methods, which can lead to extraneous effects such as aggregation at higher doping concentrations. The present work demonstrates the use of pressure-probing techniques in studying energy disorder and exciton dynamics in amorphous molecular thin films.« less

Authors:

Chang, Wendi ^[1]; Akselrod, Gleb M. ^[2]; Bulović, Vladimir ^[1]

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+ Show Author Affiliations

1. Massachusetts Inst. of Technology, Cambridge, MA (United States). Dept. of Electrical Engineering and Computer Science
2. Univ. of Duke, Durham, NC (United States). Dept. of Electrical and Computer Engineering

Publication Date:

Thu Apr 02 00:00:00 EDT 2015

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Center for Excitonics (CE)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1370966

Grant/Contract Number:  

SC0001088

Resource Type:

Accepted Manuscript

Journal Name:

ACS Nano

Additional Journal Information:

Journal Volume: 9; Journal Issue: 4; Related Information: CE partners with Massachusetts Institute of Technology (lead); Brookhaven National Laboratory; Harvard University; Journal ID: ISSN 1936-0851

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; solar (photovoltaic); solid state lighting; photosynthesis (natural and artificial); charge transport; optics; synthesis (novel materials); synthesis (self-assembly); synthesis (scalable processing); organic semiconductor; solid-state solvation effect; Onsager dielectric theory; pressure probing; Förster radiative energy transfer