Nucleation and strain-stabilization during organic semiconductor thin film deposition

TL;DR

This study investigates the nucleation, growth, and strain effects in organic semiconductor thin films, revealing how supersaturation, temperature, and thickness influence crystalline phases and stability during solution deposition.

Contribution

It introduces in-situ optical spectromicroscopy to analyze nucleation and strain stabilization in organic semiconductor thin films, highlighting the effects of supersaturation and film thickness on phase behavior.

Findings

Films form via supersaturation before solidification.

Crystalline phases are strain-stabilized up to 135°C.

Thinner films are constrained to lattice constants at deposition temperature.

Abstract

The nucleation mechanisms during solution deposition of organic semiconductor thin films determine the grain morphology and may influence the crystalline packing in some cases. Here, in-situ optical spectromicroscopy in reflection mode is used to study the growth mechanisms and thermal stability of 6,13-bis(trisopropylsilylethynyl)-pentacene thin films. The results show that the films form in a supersaturated state before transforming to a solid film. Molecular aggregates corresponding to subcritical nuclei in the crystallization process are inferred from optical spectroscopy measurements of the supersaturated region. Strain-free solid films exhibit a temperature-dependent blue shift of optical absorption peaks due to a continuous thermally driven change of the crystalline packing. As crystalline films are cooled to ambient temperature they become strained although cracking of thicker films is observed, which allows the strain to partially relax. Below a critical thickness, cracking is not observed and grazing incidence X-ray diffraction measurements confirm that the thinnest films are constrained to the lattice constants corresponding to the temperature at which they were deposited. Optical spectroscopy results show that the transition temperature between Form I (room temperature phase) and Form II (high temperature phase) depends on the film thickness, and that Form I can also be strain-stabilized up to 135$^\circ$C.