N-Type Semiconducting Behavior of Copper Octafluorophthalocyanine in an Organic Field-Effect Transistor

Abstract

Based on the crystal structure analysis, the overlap integral between the frontier molecular orbitals of adjacent F₈CuPcs in the one-dimensional chain is estimated: the overlap integral between the lowest unoccupied molecular orbitals is 5.4 × 10⁻³, which is larger than that in a typical n-type semiconducing material F₁₆CuPc (2.1 × 10⁻³), whereas that between the highest occupied molecular orbitals is 2.9 × 10⁻⁴. Contrary to previous studies in air, we found that an organic field-effect transistor (OFET) composed of F₈CuPc essentially shows clear n-type semiconducting behavior in vacuum.

1. Introduction

Metallophthalocyanines (MPcs) are widely used not only as organic dyes but also as organic semiconductors, owing to their high air and chemical stabilities. They are being actively studied as components of organic electronics, in applications such as organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaic cells [1,2,3]. MPcs are used as hole transport/injection/extraction materials because the energy level of their highest occupied molecular orbital (HOMO), consisting of the π orbital of the phthalocyanine (Pc), matches well with the work function of ordinary hole injection/extraction electrodes such as Au or indium-tin-oxide (ITO) [4,5,6]. They show p-type semiconducting characteristics, with a field effect hole mobility of around 10⁻² cm² V⁻¹ s⁻¹ in thin film transistors and around 1 cm² V⁻¹ s⁻¹ in single crystal transistors [1,7].

Introduction of electron-withdrawing groups onto the peripheral benzene rings of Pc is a valid approach for tuning the electronic properties of MPcs. For instance, F₁₆CuPc, a fully fluorinated CuPc shows n-type characteristics with an electron mobility comparable to its unsubstituted counterpart, CuPc, a p-type material [8,9]. Therefore, F₁₆CuPc has been widely used as an n-type material in organic electronics [10,11,12,13].

For n-type organic semiconductors, in order to obtain a low energy barrier at the interface, the position of the lowest unoccupied molecular orbital (LUMO) should be close to the work function of the electrode. Moreover, a large π–π overlap of the LUMOs between the adjacent molecules in the crystal is preferred [14], because electron transport in the crystalline state occurs through the overlapped LUMOs. As for the LUMO level of F₈CuPc (Figure 1), the reported value, −3.91 eV [15], is similar to that of C₆₀ and its derivatives [16,17]. This implies that the LUMO level of F₈CuPc is positioned optimally to enable it to function as an n-type organic semiconductor, and that F₈CuPc can be potentially used as an electron transport/injection/extraction material in organic electronics, similar to C₆₀. Indeed, the junction between F₈CuPc and p-type MPc has been reported to exhibit a rectification effect [18,19]. In contrast, the highest occupied molecular orbital (HOMO) level of F₈CuPc has been reported to be −6.06 eV [15]. This value is far from the work function of Au or ITO [5,6]. Therefore, we expected F₈CuPc to be an n-type semiconductor. However, recently, thin film or single crystal transistors composed of F₈CuPc have been reported to show ambipolar characteristics in air [20,21].

In this study, we grew a single crystal of F₈CuPc, and determined its crystal structure to estimate the π–π overlap between the frontier orbitals of adjacent molecules by the extended Hückel calculation. We have also examined the charge transport properties of OFETs based on F₈CuPc thin films, and discovered that the transistor is unstable in air, whereas it shows stable n-type transport properties in vacuum.

2. Materials and Methods

2.1. Synthetic and Crystallisation Procedures

All reagents were used as received without further purifications. 5.1 g (30 mmol) of CuCl₂·2H₂O (WAKO) was heated and dried under vacuum, and 5.0 g (30 mmol) of 4,5-difluorophthalonitrile (TCI) and 10 mL of N,N-dimethylformamide (Super Dehydrated grade, WAKO) were added. The mixture was refluxed under Ar for 6 h, and filtered. A bluish-purple powder of F₈CuPc was obtained, which was washed several times with acetone to yield 2.3 g (42%) of the product. Single crystals of F₈CuPc were grown from this, by vacuum sublimation at 380 °C under a pressure less than 40 Pa.

2.2. X-ray Diffraction (XRD) Measurements

Although single-crystal X-ray diffraction (XRD) measurements were performed and the molecular arrangement of F₈CuPc has been revealed in [21], the atomic coordinates are not available. Therefore, XRD data was collected for a single crystal of F₈CuPc, using an automated Rigaku Rapid system with the monochromated Cu-Kα radiation (λ = 1.54187 Å). The structure was solved using a direct method, using SIR2004 [22] and refined by a full-matrix least-squares technique with SHELXL-2014/7 [23] with anisotropic and isotropic thermal parameters for non-hydrogen and hydrogen atoms, respectively. The crystallographic data has been deposited at the Cambridge Crystallographic Data Centre (CCDC) as CCDC-1531473. Crystal data for F₈CuPc: Triclinic, P$\overline{1}$, a = 3.62740(10) Å, b = 12.7357(4) Å, c = 13.4538(4) Å, α = 95.079(9)°, β = 90.529(6)°, γ = 96.441(6)°, and V = 615.07(3) ų, Z = 1, F(000) = 357, d_cal = 1.944 g cm⁻³, μ (CuKα) = 2.231 mm⁻¹ (λ = 1.54187 Å), R₁ = 0.0879, wR₂ = 0.2008, GoF = 0.999.

2.3. Fabrication of OFET and Measurement of the Transport Properties

A thin film organic field effect transistor composed of F₈CuPc was fabricated by the vacuum deposition of F₈CuPc on a SiO₂/n-Si substrate under a pressure less than 1 × 10⁻² Pa, where, n-Si and SiO₂ work as gate electrode and gate insulating layers, respectively. The deposition rate was ca. 0.05 nm s⁻¹, and the resulting thickness of the film was ca. 30 nm. Subsequently, 30 nm thick Al electrodes were deposited on the F₈CuPc film to serve as the source and drain electrodes, under a pressure less than 1 × 10⁻³ Pa. Current–voltage characteristics were measured by an ADCMT 8252 electrometer in air or vacuum (~10² Pa and less than 10⁻² Pa).

3. Results and Discussion

The lattice constant of the F₈CuPc crystal is consistent with those reported in [21]. The crystal structure is shown in Figure 2. F₈CuPc forms a one-dimensional regular chain along the a-axis with an interplanar distance of 3.30 Å, which is smaller than the sum of the van der Waals radii of sp² carbons, suggesting a strong π–π intermolecular interaction. As the charge transport in organic semiconductors occurs via the π–electrons, a strong π–π intermolecular interaction in the crystalline state is desirable, which can be evaluated by the overlap integral in the conduction path consisting of π–orbitals. Using an extended Hückel calculation method (The extended Hückel calculation was performed using CAESAR 2 software developed by PrimeColor Software, Inc. (Raleigh, NC, USA) Default parameters were used for the calculations), the overlap integral between the π–orbitals of adjacent F₈CuPcs in the one-dimensional chain along the a-axis were estimated: the overlap integral between the LUMOs was found to be 5.4 × 10⁻³, which is comparable to that of molecular conductors consisting of MPcs [24], whereas that the between HOMOs was 2.9 × 10⁻⁴. In addition to the optimal position of the LUMO level, the π–π overlap appears to favor electron transfer over hole transfer.

The carrier mobility of an organic thin film depends on the molecular orientation in the film. Figure 3 shows the X-ray diffraction pattern of the F₈CuPc film deposited on a SiO₂/n-Si substrate. A peak at 2θ = 6.56°, corresponding to (001) plane was observed, indicating that the crystallographic c-axis of F₈CuPc in the as-deposited film is perpendicular to the substrate surface. This orientation is favorable for charge transport between the source and drain electrodes of an OFET (vide infra), because the π–π overlap between adjacent F₈CuPc molecules in the one-dimensional chain lies on the substrate.

An OFET composed of F₈CuPc was fabricated using a SiO₂/n-Si substrate and Al electrodes. Figure 4 shows a schematic of the fabricated transistor and its current-voltage characteristics measured in air. F₈CuPc shows an n-type semiconducting behavior. However, parabolic current-voltage curves exhibiting a large hysteresis were obtained, although the first half of each measurement was consistent with the current–voltage characteristics reported previously [20]. Furthermore, the results could not be reproduced in repeated measurements. Figure 5 shows the current–voltage characteristics measured in vacuum (~10² Pa). Compared to the measurement in air, it is obvious that the instability is rather suppressed. These features indicate that the F₈CuPc transistor is unstable in air, and therefore, it is difficult to evaluate the transistor characteristics under air. In fact, we could not observe the reported ambipolar characteristics under air [20,21] even when Au was used as the source and drain electrodes. On the other hand, Figure 6 shows the current–voltage characteristics measured under high vacuum (under a pressure less than 10⁻² Pa). F₈CuPc shows a typical, stable n-type semiconducting behavior in vacuum, and the field effect electron mobility in the thin film transistor was calculated to be 7.9 × 10⁻⁴ cm² V⁻¹ s⁻¹ (on/off ratio being 3.6 × 10³). Obviously, the p-type characteristics could not be found in high vacuum, irrespective of the source and drain electrodes.

As for an n-channel F₁₆CuPc transistor, it has been previously shown that the F₁₆CuPc film could be rendered air-stable by the introduction of F atoms [8]; fluorination renders the organic molecules resistant to oxidation, because the energy levels of their frontier orbitals are lowered. It has also been suggested that the conformational orientation of the F₁₆CuPc molecules in the film with the F-atoms being exposed to air prevents the penetration of moisture into the film [8]. Photoemission spectroscopies revealed that the energy levels of the frontier orbitals of F₈MPc are positioned between those of MPc and F₁₆MPc [15,25]. Therefore, they are clearly lower than that of MPc, and higher than that of F₁₆MPc. Consequently, F₈CuPc could be more readily oxidized compared to F₁₆CuPc. Furthermore, the un-substituted H-atoms in the benzene ring of F₈CuPc and the attendant changes in the molecular arrangement in the crystal structure might permit moisture to penetrate through the thin film. Apparently, these factors are responsible for the air instability of the fabricated n-channel F₈CuPc film transistor.

The field effect electron mobility of 7.9 × 10⁻⁴ cm² V⁻¹ s⁻¹ is almost 10 times lower than that of the F₁₆CuPc film transistor; Bao et al. reported a field effect electron mobility of 5 × 10⁻³ cm² V⁻¹ s⁻¹ for the F₁₆CuPc thin film transistor [8], and we too obtained a similar value with our measurement system. As for the electron transport, the overlap of the LUMOs is responsible for the transport. In the case of F₁₆CuPc, the overlap integral between the LUMOs of adjacent F₁₆CuPcs in the one-dimensional chain could be estimated to be 2.1 × 10⁻³, according to the crystal data in [26]. Despite the larger overlap integral between the LUMOs, the F₈CuPc film transistor shows smaller electron mobility compared to that of the F₁₆CuPc film transistor. The lower electron mobility in the F₈CuPc film transistor could be attributed to the relatively low crystallinity in the as-deposited film, as indicated by the broad diffraction peak observed in Figure 3, whereas sharp diffraction peaks suggesting high crystallinity were observed in the F₁₆CuPc film [8]. When a single crystal of F₁₆CuPc was used, the field effect electron mobility of the F₁₆CuPc transistor increased by two orders of magnitude [9]. This implies that higher field effect electron mobility could be obtained for F₈CuPc, if a single crystal is used. Indeed, a recent study on a single-crystal transistor of F₈CuPc demonstrated that the field effect electron mobility of F₈CuPc is comparable to that of F₁₆CuPc [21], even though the measurements were performed in air.

4. Conclusions

Based on the crystal structure analysis, we have estimated the overlap integral between the LUMOs of adjacent molecules along the a-axis of the one-dimensional regular chain of F₈CuPc, and discovered that it is larger than that in a typical n-type semiconducting material F₁₆CuPc. The X-ray diffraction pattern shows a peak attributable to (001) plane, implying that the one-dimensional chain in the as-deposited F₈CuPc film lies on the substrate. A field-effect transistor composed of an F₈CuPc film on a SiO₂/n-Si substrate clearly shows n-type semiconducting behavior with the field effect electron mobility of 7.9 × 10⁻⁴ cm² V⁻¹ s⁻¹ in vacuum, while being unstable in air. The larger overlap integral between the LUMOs in F₈CuPc compared to that in F₁₆CuPc implies that the electron mobility of F₈CuPc could potentially exceed that of F₁₆CuPc. Considering the LUMO level and the electron transport properties, F₈MPcs are good candidates for n-type semiconduction in organic electronics.