Nucleation and strain-stabilization during organic semiconductor thin film deposition.

Yang Li,Jing Wan,Richard Sun,Randall L Headrick,Detlef-M Smilgies,Nicole Bouffard

doi:10.1038/srep32620

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 °C.

Highlights

We demonstrate the application of real-time optical spectroscopy to study the crystallization process and polymorphic transformation of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin films
We utilize in-situ optical spectroscopy to show that TIPS-pentacene crystallizes in a two-step process, where in the first step the film exists in a supersaturated form with no long range order, as reported previously based on in-situ synchrotron X-ray scattering and Polarized Optical Video Microscopy (POVM) results[3]
We report evidence that TIPS-pentacene molecules aggregate into small clusters in the supersaturated form

Summary

Introduction

We demonstrate the application of real-time optical spectroscopy to study the crystallization process and polymorphic transformation of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin films. We utilize in-situ optical spectroscopy to show that TIPS-pentacene crystallizes in a two-step process, where in the first step the film exists in a supersaturated form with no long range order, as reported previously based on in-situ synchrotron X-ray scattering and Polarized Optical Video Microscopy (POVM) results[3]. Cour et al.[3] have shown that strained small molecule organic semiconductor thin films can be obtained by solution deposition at elevated temperatures (60 °C and 90 °C for TIPS-pentacene). We report that in-situ optical spectroscopy is highly sensitive to changes in molecular packing and to thermal expansion effects We utilize this effect during thermal cycling in combination with X-ray diffraction measurements to verify that Form II TIPS-pentacene films deposited at 135 °C can be stabilized to room temperature if the film thickness is below the critical thickness for cracking. No phase transition occurs even when the sample is heated to 135 °C, which provides confirmation of the strain-stabilization model since no reduction of the transition temperature is observed

Objectives

Methods

Results