Monitoring of Crystallization Process in Solution-Processed Pentacene Thin Films by Chemical Conversion Reactions

Abstract

Solution-processable organic semiconductors having bulky substituent groups on the π-conjugated skeleton are rapidly gaining attention for their potential applications to large-area electronics. While the substituent groups contribute to the good solubility in organic solvents, they give rise to hopping sites in a thin film, affecting adversely the charge-carrier transport. As an alternative material, a solvent-soluble precursor compound with thermally cleavable functional groups is promising, which can be converted by heat treatment into a thin film to generate the desired material consisting solely of conjugated systems. This precursor approach is practically applied to various thin-film-based devices. The overall process of the thin film growth, however, remains unrevealed. In the present study, solution-processed pentacene thin films are prepared from a thermally convertible precursor, and the structural evolution during the chemical conversion reaction has been revealed by a combination of cutting-edge analytical tools of two-dimensional X-ray diffraction (2D-GIXD) and p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). The highlight is that pentacene is crystallized in a stepwise manner in the thermally converted films, which is substantially different from a typical growth process. In addition, influences of the oxidation reaction of pentacene on the molecular arrangement are also discussed quantitatively. This study provides a fundamental schematic of thin films grown by the precursor method.

Quantitative Analysis of Photochemical Reactions in Pentacene Precursor Films

“…No Bragg reflections are observed in the diffraction pattern of the as-spun films (Figure a,b), indicating that the precursor films have an amorphous structure for both solvents, which agrees with previous reports. , Since the precursor compound can form crystalline structures at room temperature, ,, this result implies that the amorphous structure in the spin-coated films is kinetically limited. ,, The amorphous structure is indeed confirmed by the featureless topographic images obtained from the AFM measurements shown in Figure a,b.…”

“…This ambiguously weak intensity would suggest that the crystallites in the bulk phase are randomly oriented due to the low crystallinity. According to our previous studies on the precursor method for PEN, , unreacted precursor molecules disturb molecular aggregation of the product compound, resulting in a less ordered film structure. Therefore, the low crystallinity of PEN in the photoconverted film without the additive would be due to the low conversion rate, as discussed earlier.…”

“…One of the most representative precursors of PEN is 13,6- N -sulfinylacetamidopentacene (SAP), and it is thermally converted to the target material, , as shown in Figure . This precursor compound is fully converted to PEN by thermal annealing in thin films, and the converted molecules form a highly ordered polycrystalline structure. , The precursor compound has an advantage in that the desired film is easily obtained, whereas it has a problem in that a high annealing temperature of ∼420 K or higher is required to complete the thermal conversion . In general, such a high-temperature process poses risks of degradation, sublimation, and oxidation for organic semiconductors.…”

“…In addition, an optically isotropic (OI) spectrum, which is a molecular-orientation free spectrum, is also obtained by simply averaging the IP and OP spectra . Since the band intensity of the OI spectrum is proportional only to the quantity of chemical species, the quantitative analysis of chemical reactions can be accurately performed. ,, IR spectroscopy has another powerful benefit for detecting trace molecules irrespective of the crystallinity, , such as impurities, including residual solvents.…”

Stereoisomer-dependent conversion of dinaphthothienothiophene precursor films

“…In the case of a vapor-deposited pentacene film, immediately after deposition on the substrate, a polycrystalline film is formed with the molecular axis perpendicular to the film surface 14 – 17 . The pentacene molecules that can thermally be converted from a precursor compound, on the other hand, exhibit stepwise crystal growth beginning with a random orientation 18 , 19 . At the early stage of the reaction, the residual unreacted molecules make the self-aggregation of the reaction product poor to produce a randomly oriented film.…”