Third-order optical susceptibility in polythiophene thin films prepared by spin-coating from high-boiling-point solvents

Highlights

• Enhancements in nonlinear optical properties of a conjugated polymer were examined.

• Thin films were fabricated by spin-coating using a solvent with a high boiling point.

• The third-order optical susceptibility increased with increasing boiling point.

• An additional enhancement was obtained by the vapor-treatment technique.

• These thin films were sufficiently homogeneous for use in nonlinear optical devices.

Abstract

We examined the enhancements in the third-order optical susceptibility (χ⁽³⁾) of spin-coated thin films of poly(3-hexylthiophene) using an anhydrous solvent with a high boiling point. The χ⁽³⁾ value was found to be enhanced as the boiling point of the solvent increased. In this study, the largest value of χ⁽³⁾ was obtained for thin films that were spin-coated in an inert atmosphere using anhydrous dichlorobenzene and then was subsequently exposed to its vapor for 1 h. The maximum value of the imaginary part of χ⁽³⁾ was determined to be 1.8 × 10^- 9 esu, which is more than three times greater than that of thin films spin-coated in an ambient atmosphere using a solvent with a low boiling point, such as chloroform.

Introduction

π -conjugated polymers have been extensively studied as materials for nonlinear optical devices because of their large third-order optical susceptibility (χ⁽³⁾), ultrafast response, [1] and solution processability [2]. Many theoretical and experimental studies [3], [4], [5], [6], [7] have revealed that the following relationship exists between the achievable largest χ⁽³⁾ value of a π -conjugated polymer and its optical gap energy (E_g) : χ⁽³⁾ ∝ E_g^− 6. This -6 power law suggests that π -conjugated polymers with lower E_g's, such as polythiophene and its derivatives (PTs), have the potential to exhibit larger χ⁽³⁾ values. However, in literature, [8] the reported χ⁽³⁾ values of PTs were significantly lower than the χ⁽³⁾ values expected on the basis of their E_g's. These lower-than-expected χ⁽³⁾ values are caused primarily by torsions and bends of the main chains, [9], [10] which occur in disordered regions of thin films and reduce the coherence length of excitons (i.e., the effective conjugation length). The χ⁽³⁾ values may also be degraded by oxygen and water in a solvent or in an ambient atmosphere in the case of π -conjugated polymers with low air stability, such as PTs [8].

Recently, we demonstrated that a χ⁽³⁾ value expected from the E_g of PTs could be achieved with poly(3-hexylthiophene) (P3HT) when its thin films were fabricated in an inert atmosphere using an anhydrous solvent onto quartz substrates; adsorbed water was preliminarily removed from the substrates by baking them at 100 °C in a vacuum chamber [8]. In the experiments, we used the drop-casting technique to prepare thin films instead of the widely used spin-coating technique. In the drop-casting technique, a solution is naturally dried on substrates. Consequently, the P3HT chains have more time to self-organize into a well-ordered π − π stacking structure, where P3HT chains adopt more planar conformations; [11], [12], [13] thus, the effective conjugation length is elongated. Therefore, a larger χ⁽³⁾ value is obtained more easily with the drop-casting technique than with the spin-coating technique. However, the baking process reduces the surface energy of the quartz substrates, and the resultant poor wettability makes the fabrication of flat, homogeneous thin films with the drop-casting technique difficult (see Fig. 1). For the use of P3HT thin films for nonlinear optical devices, the homogeneity of film is also essential and must be achieved.

On the basis of our experience, homogeneous thin films are difficult to deposit onto baked quartz substrates using the drop-casting technique. Thus, in this study, we employed the spin-coating technique and examined how to enhance the χ⁽³⁾ values of spin-coated thin films. In the research field of organic field-effect transistors, a solvent with a high boiling point is often used to obtain ordered spin-coated thin films [14]. Because high-boiling-point solvents evaporate slowly even in the spin-coating technique, an effect similar to that observed with the drop-casting technique can be expected. Vapor treatment is also known to improve the crystallinity of small molecules [15] and to change the polymer conformations into more planar ones [16], [17]. The advantage of the latter technique is that it is applicable to fabricated thin films. Thus, the vapor treatment can be performed on thin films that have been spin-coated using a high-boiling-point solvent. Here we report that a combination of these techniques indeed allowed us to prepare homogeneous thin films with a χ⁽³⁾ value close to the largest value expected based on its E_g.