Influencing Optical and Charge Transport Properties by Controlling the Molecular Interactions of Merocyanine Thin Films

TL;DR

This study demonstrates how controlling molecular aggregation and orientation in merocyanine thin films significantly enhances optical and charge transport properties, with a three-order magnitude increase in mobility through thermal annealing and substrate templating.

Contribution

It reveals the relationship between molecular aggregation, orientation, and charge mobility, and introduces methods to control these factors for improved organic semiconductor performance.

Findings

Charge mobility increases by about 1000 times due to aggregation.

Upright, co-facial molecular orientation enhances lateral charge transport.

Template growth on graphite improves morphological order and domain size.

Abstract

In organic semiconductors charge transport typically takes place via slow hopping processes. Molecular aggregation can lead to enhanced exciton and charge transport through coupling of the transition dipole moments. In this work, we investigate the optical, morphological, and electronic properties of thin films of a merocyanine dye, that aggregates due to its high ground-state dipole moment. The degree of aggregation of spin-coated thin films can be easily tuned by thermal annealing. We demonstrate the relationship between charge carrier mobility and the degree of aggregation. The mobility is increased by approximately three orders of magnitude due to aggregation. We combine variable angle spectroscopic ellipsometry and polarization-resolved absorption spectroscopy with density functional theory to demonstrate that the aggregated molecules are oriented in an upright, standing configuration relative to the substrate surface. This arrangement involves a co-facial orientation of the molecular pi-systems which is advantageous for lateral charge transport. By utilizing highly oriented pyrolytic graphite as an ordered substrate, we are able to template the growth of the merocyanine layer in vapor phase deposition, and to improve the in-plane morphological order drastically. By correlating atomic force microscopy and photoluminescence microspectroscopy we observe oriented domains of 100s of {\mu}m^2 in size, emitting linearly polarized light, whereby maintaining the edge-on molecular arrangement. This promises a further significant enhancement of lateral charge carrier mobility.