Title

Molecular Packing in Organic Solar Cell Materials: Insights from the Emission Line Shapes of P3HT/PCBM Polymer Blend Nanoparticles

Authors

Authors

A. M. Crotty; A. N. Gizzi; H. J. Rivera-Jacquez; A. E. Masunov; Z. J. Hu; J. A. Geldmeier;A. J. Gesquiere

Comments

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Abbreviated Journal Title

J. Phys. Chem. C

Keywords

FIELD-EFFECT TRANSISTORS; SINGLE-PARTICLE SPECTROSCOPY; THIN-FILM; TRANSISTORS; CONJUGATED POLYMERS; POLY(SUBSTITUTED THIOPHENE); INTERCHAIN INTERACTIONS; PHOTOCHROMIC COMPOUNDS; OPTICAL-PROPERTIES; ENERGY-TRANSFER; MEH-PPV; Chemistry, Physical; Nanoscience & Nanotechnology; Materials Science, ; Multidisciplinary

Abstract

Semiconducting polymer devices have seen tremendous progress in development of material and device designs, while device efficiencies have made substantial gains. Still, the effect of material morphology on the optoelectronic properties of semiconducting polymers is not completely understood even though these materials make up the active device layer. In this study we use computational methods to simulate different poly(3-hexylthiophene) (P3HT) morphologies, predict their emission spectra, and compare them to experimentally observed emission spectra for P3HT nanoparticles. We use published X-ray diffraction data on P3HT polymorphs to build the molecular models of nanodomains that differ in the side-chain packing. The atomic and electronic structures of both nanodomains are studied with the force field, Hartree-Fock, CIS, and density functional theory methods. The results confirm the coexistence of type I and II nanodomains, where the shift of the backbones in the same stack is determined by the differences in side-chain packing. Upon excitation, the polymer chains in type II domain are free to slide to their optimal arrangement in the stack, whereas in type I domain this sliding is hindered by the steric repulsion of the side chains and the chains are essentially constrained to keep the ground state geometry. These nanodomains, therefore, differ in their emission spectra: type I emission has a single 0-0 vibronic band, while type II demonstrates pronounced vibronic progression. In agreement with Frenkel exciton theory, splitting of the excited state depends on the longitudinal shift of the pi-systems. However, we find that due to the constraints arising from P3HT being confined in nanosized particles, the type I nanodomain increasingly appears as an additional emitter that exhibits J-aggregate character. As a result, a pronounced vibronic structure appears as PCBM blending ratios increase, as opposed to the changes in emission profile due to a different degree of disorder present in weakly coupled H-aggregates. These findings are distinct from those made for bulk P3HT materials.

Journal Title

Journal of Physical Chemistry C

Volume

118

Issue/Number

34

Publication Date

1-1-2014

Document Type

Article

Language

English

First Page

19975

Last Page

19984

WOS Identifier

WOS:000341122200049

ISSN

1932-7447

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