Marginal solvents preferentially improve the molecular order of thin polythiophene films

Abstract

The crystalline order within π-conjugated polymer films prepared using solution processing methods determines the electrical properties of the film. A channel's morphology is particularly important to device performance. The molecular order and morphology within a channel region near the underlying active layer have not yet been examined systematically. Here, we characterize the crystal order homogeneity as a function of the solvent penetration depth after applying simple solvent post-treatment. The morphological, optical, and electrical properties of poly(3-hexylthiophene) (P3HT) films could be profoundly improved by casting the films in methylene chloride solutions. The impact of the solvent application was most pronounced in the thin P3HT films, especially in the center of the film. During solvent casting, the central region of the film was exposed to methylene chloride for a longer period of time than the edge region of the film, thereby producing a thinner and more ordered film structure in the central region. Concomitant with the improved order, the charge carrier transport in the resulting field-effect transistors increased.

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1. Introduction

Organic field-effect transistors (OFETs) based on π-delocalized conjugated polymers have progressed tremendously in their development because they may be readily manufactured over large areas at low costs.^1–5 Conjugated polymer film morphologies depend on the inter-chain forces that arise predominantly from weak intermolecular π–π coupling; thus, structural defects are frequently generated within the polymer layer prepared using solution processing techniques.^6,7 The structural defects in poorly organized arrangements can limit charge carrier transport across the disordered segments through weak van der Waals interactions, resulting in poor device performances.⁸ For this reason, the ordered molecular packing of π-conjugated polymers formed through solution processing determines the corresponding electrical properties.⁹

Recently, significant efforts have been directed toward exploring methods for improving the molecular ordering and morphologies of polymer thin films.^10–14 Several processing options, such as the choice of solvent,^15–17 film forming method,^18–20 and post-treatments, such as thermal^21,22 or solvent annealing,^23,24 have successfully improved the morphology and crystalline order of a polymer thin film fabricated using solution processing methods. Several groups have found that device performances could be enhanced simply through solvent treatment.^25–29 Our previous studies revealed that direct contact with a solvent effectively mobilized and reorganized the polymer chains in a short period of time.³⁰ The positive effects of the solvents on device performance are thought to optimize the molecular ordering and orientations among the active layers and are expected to improve the morphologies in the channel region near the interface between the active layer and the dielectric. Systematic studies of the solvent penetration depth or bottom morphology of a thin film prepared using a simple solvent treatment have not yet been conducted, even though the channel morphology is critical to the device performance.^31–34

This report presents a systematic study of the effects of a simple solvent treatment on the morphologies and electrical properties of spin-cast polythiophene thin films of various thickness. We previously examined solvents, such as alcohol or acetone, that poorly solubilize poly(3-hexylthiophene) (P3HT). These solvents did not improve the device performance. The marginal solvent methylene chloride (MC) was chosen here as an assisted solvent to dissolve only the low MW portion of the P3HT at RT. This choice avoided damaging the film. The effects of spin-casting MC onto a P3HT thin film were analyzed using UV-vis and atomic force microscopy (AFM) techniques, which revealed variations in the morphology and crystal order as a function of the film thickness and position across the P3HT film surface.

2. Experimental section

2.1 Preparation of polythiophene thin films and FET devices

P3HT was obtained from Rieke Metals, Inc. (M_w = 37 kDa, RR = 89–92%) and was used without further purification. A highly doped silicon wafer prepared with a 300 nm SiO₂ layer was used as the gate electrode and dielectric, respectively (capacitance = 10.8 nF cm⁻²). All wafers (1.5 × 1.5 cm²) were cleaned in acetone and ethanol for 30 min each using an ultrasonic bath and were dried under nitrogen gas. The dielectric surfaces were modified with hexamethyldisilazane (HMDS). HMDS was spin-coated over the substrate and then annealed at 150 °C for 1 h. The film thickness was controlled by spin-coating P3HT chloroform solutions in various concentration (0.05, 0.1, 0.5, 1.0, and 1.5 wt%) onto the HMDS-treated substrates at 2000 rpm for 60 s. A given volume (100 μL) of MC was subsequently dropped onto the P3HT films (1.5 × 1.5 cm²) precisely and allowed to spread over the film surface. The MC solvent was then spin-coated onto the P3HT film before the P3HT film had solidified. Au source and drain contacts 150 nm thick were then deposited via thermal evaporation through a shadow mask to yield a channel length (L: 100 μm) and width (W: 1000 μm) on the active layer. Glass substrates were used instead of Si substrates for the UV-vis absorption spectra measurements.

2.2 Characterization

The electrical contacts with the electrodes were prepared using probes, and the electrical characteristics of the transistors were measured using a semiconductor analyzer (Keithley 4200-SCS) at room temperature. The field-effect mobility of each device was calculated from the transfer curves in the saturation regime (V_D = −80 V) using the equation, I_D = μC_i(W/2L)(V_G − V_th)², where V_th is the threshold voltage. AFM images were collected in the tapping mode to observe the top and bottom morphology and the roughness of the P3HT film (Multimode 8, Digital Instruments). The bottoms of the P3HT films were characterized by transferring the films as follows. The P3HT film was spin-coated onto an octadecyltrichlorosilane-treated SiO₂ substrate. An embedding epoxy layer was then dropped onto the top surface of the P3HT film. After curing at 60 °C for 6 h under ambient conditions, the epoxy–P3HT complex was peeled away by dipping in liquid nitrogen.³⁴ The UV-vis spectra of the films formed on pre-cleaned glass slides were acquired using a Thermo Scientific, GENESYS 10S system. The thickness values of the P3HT films were determined using a surface profiler (Bruker, DEKTAK XT-E) and an ellipsometer (J. A. Woollam Co. Inc.). The optical microscopy images were obtained using an Olympus BX51M.

3. Results and discussion

The effects of direct contact between the marginal solvent, MC, and the films were investigated by characterizing the crystalline structures and electrical properties of P3HT films with various thicknesses. As a marginal solvent, MC dissolves only the low-M_W fraction of P3HT molecules at RT.³⁵ Charge transport in the as-spun P3HT films was measured as a function of film thickness prior to studying the effects of direct solvent exposure. The samples were prepared from chloroform solutions in different concentrations by spin-coating onto HMDS-treated substrates. The film thickness varied between 9.7 and 183 nm. Consistent with previous reports, increasing the P3HT layer thickness increased the drain current and field-effect mobility (Fig. 1a). Enhanced electrical characteristics are typically attributed to a reduced number of defects and trap sites as a result of improved molecular ordering in the thicker films.^36–40

Fig. 1 (a) Transfer characteristics (I_D–V_G) of the FETs (V_D = −80 V) fabricated using P3HT films, as function of the P3HT solution concentration. The solution concentration could be used to control the film thickness. (b) Transfer characteristics (I_D–V_G) of the FETs prepared using P3HT films fabricated from a 0.5 wt% solution, before and after MC exposure. (c) Charge carrier mobilities as a function the P3HT semiconductor layer thickness before and after MC exposure. The inset shows the relationship between the solution concentration and the film thickness.

Fig. 1b shows the enhanced transfer characteristics (I_D–V_G) measured in FETs based on a film prepared from a 0.5 wt% P3HT solution and exposed to MC. Also plotted are the I_D–V_G characteristics of the as-spun P3HT FETs. The MC-exposed P3HT film revealed an improved drain current and I_on/I_off relative to the as-spun film, suggesting that the interfacial charge traps decreased upon solvent exposure.⁴¹ Direct exposure of a P3HT film prepared from a 0.5 wt% solution to MC yielded the highest FET mobility, 4.3 times the FET mobility of the as-spun device. By contrast, devices based on P3HT films prepared from a 1.5 wt% solution only increased their FET mobility by a factor of 1.7, as shown in Fig. 1c. These results suggested that solvent exposure changed both the topmost film structure and the buried region near the HMDS-treated SiO₂ dielectric. Effective transport channels in top contact-bottom gate devices are formed within a few nanometers of the semiconductor/dielectric interface. Importantly, the impact of applying an MC solvent was more pronounced in the thin P3HT films. Considering that the MC molecules can diffuse into the polymer matrix only to a small depth, any molecular rearrangements induced by the marginal solvent should be more pronounced in thin P3HT films. Direct MC exposure of very thin films prepared from 0.05 or 0.1 wt% P3HT solutions (9.7 nm or 20 nm thick), however, seriously damaged the films. These films were excluded from further study. Among the various samples examined, representative results were achieved in 48 nm thick films prepared using 0.5 wt% P3HT solutions.

Fig. 2 presents AFM top and bottom images of the P3HT film morphologies prepared from a 0.5 wt% solution before and after MC exposure. Prior to solvent treatment, the AFM top and bottom images of the as-spun films exhibited featureless morphologies and were very smooth, with root mean square surface roughness (R_q) values of 0.637 and 0.402 nm respectively. The value of R_q for the top and bottom films increased slightly with the concentration due to the improved ordering in the thicker film.⁴² After exposure to the MC solvent, the top surface of the P3HT films tended to display a percolating network of 20 nm nanoscale aggregate structures that increased R_q to 3.24 nm. This behavior was further investigated by examining the bottom morphologies of the MC-exposed films. The bottom morphology of the MC-exposed P3HT films prepared from 0.5 wt% solutions tended to include much more ordering, with larger aggregates and a high R_q of 0.665 nm, a factor of 1.65 greater than the value obtained from the as-spun films. These bottom morphology results support the observation that a few seconds of exposure to the solvent were sufficient for rearranging the polymer chains at both the top and bottom film surfaces. Thick P3HT layers, however, displayed less pronounced changes in the bottom film morphologies upon direct MC exposure due to the limited diffusion length of MC into the polymer matrix. After direct exposure to MC, the bottom surface R_q increased by a factor 1.19 in P3HT films prepared from a 1.0 wt% solution, compared to the factor of 1.03 observed in the P3HT film prepared from a 1.5 wt% solution, as shown in Fig. 2c.

Fig. 2 AFM phase images of the top (up) and bottom (down) sides of the P3HT films fabricated from a 0.5 wt% solution (a) before and (b) after MC exposure. (c) Top (up) and bottom (down) RMS roughness values of the P3HT films as a function of solution concentration, before and after MC treatment.

Fig. 3 plots the thickness of a P3HT film prepared from a 0.5 wt% solution as a function of position across the film surface after MC exposure. Fig. 3a presents the UV-vis absorption spectra at the center and edge regions of the spin-cast P3HT films before and after direct exposure to MC. The spectrum of the as-spun P3HT thin film revealed a dominant peak at λ = 534 nm (0-2), corresponding to the intrachain π–π* transition of P3HT, with a weak minor shoulder (0-0) at lower energies (λ = 603 nm) in both the center and edge regions of the film.⁴³ The thickness values of the as-spun P3HT films, determined using the surface profiler, remained constant, 48 nm, regardless of the position across the film surface (Fig. 3b). After exposing the P3HT thin films to MC, the UV-vis absorbance decreased dramatically to 60–80%, depending on the position across the film surface, due to film thinning. These results revealed that the relatively small and mobile polymer chains in the films could be washed away by MC spin-casting prior to complete film solidification. When the spinning process is finished, the solvent molecules remains in the film, interrupting solidification of films. Therefore mobile polymers can be washed easily by MC. Surface profile measurements revealed that the P3HT films were 12 nm thick in the center and 15 nm thick at the edges (Fig. 3c).

Fig. 3 (a) UV-vis absorption spectra in the center and edge regions of the P3HT thin films cast from a 0.5 wt% solution before and after MC exposure. The thickness values at the center and edge regions in (b) the as-spun and (c) MC-treated P3HT films prepared from a 0.5 wt% solution.

The molecular ordering in the P3HT films after direct solvent exposure was characterized by collecting the UV-vis absorption spectra of the P3HT films at each position across the film surface (Fig. 4). The as-spun P3HT films displayed unchanging UV-vis absorbance intensities and features in the center and edge regions of the film. After solvent exposure, low-energy bands at 554 nm (0-1) and 603 nm (0-0) appeared. These features were correlated with an increase in the effective π-conjugation length, namely, an increase in the number of ordered P3HT aggregates formed by interchain π–π stacking due to the self-assembly of the mobile polymer chains in the presence of MC.^44,45 These features were more pronounced at the center of the film than at the edges after MC treatment. For example, the intensity ratios of the (0-1) and (0-0) bands relative to the (0-2) band at 520 nm differed and were, respectively, 1.08 and 0.74 in the center, and 1.02 and 0.68 at the edges after solvent exposure. These values were indistinguishable, 0.94 and 0.55, in the center and edges prior to solvent exposure.

Fig. 4 (a) UV-vis absorption bands, normalized at 520 nm, collected from the P3HT films fabricated from a 0.5 wt% solution. (b) Magnified view of the normalized UV-vis absorption bands collected from various positions across the P3HT film surface.

A phase contrast AFM study was performed to verify whether the changes in the molecular ordering were related to the morphological changes in the films. Fig. 5 shows the top and bottom morphologies in the P3HT film spin-coated from a 0.5 wt% solution at the center and edge positions. Fig. 5b shows the AFM bottom morphology of the center and edge positions in the 0.5 wt% P3HT film, after MC treatment. The center morphology changes upon MC solvent exposure were more pronounced, and R_q increased to 0.665. By contrast, R_q increased to 0.500 at the edge of the film. The MC droplets were dropped onto the center of the P3HT film and then spread across the film during spin-coating (Fig. 5a). Most of the MC solvent was thrown off the film during the spin-coating process; therefore, only a small amount of residual solvent remained on the P3HT film for an extended period of time. During this process, the center region of the film was exposed to MC for a longer time than the edge region of the film, thereby thinning the film and improving the molecular ordering at the center region.

Fig. 5 Morphologies of the films before and after MC exposure. (a) Optical images (top) of the film surfaces of the as-spun and MC-exposed films. (b) AFM bottom phase images of the P3HT films fabricated from a 0.5 wt% solution, in the center (left) and edge regions (right) after MC exposure.

The relationships between the structural and morphological variations and the electronic properties of the P3HT films were characterized in each film position by comparing the uniformity of the charge carrier mobility in the P3HT film before and after MC exposure (Fig. 6). The performances of twenty-five transistor units fabricated on a 1.5 × 1.5 cm² square wafer were measured. Direct exposure of the 48 nm film prepared by 0.5 wt% to MC yielded the highest FET mobility increase of a factor of 4.3 due to improved self-organization among the P3HT chains, despite a reduction in the film thickness to 12 nm. The coefficient of variation of the field-effect mobility in the as-spun film was small, 9.4%. The coefficient of variation in the MC-exposed film, on the other hand, was larger, 32.2%. The value in the center region was a factor of 1.5 larger than the value in the edge region. The uniformity of the charge carrier mobility agreed well with the structural variations characterized by the UV-vis absorbance and AFM measurements obtained at each position on the film. Overall, the UV-vis and AFM measurements suggested that exposing the P3HT films to MC, which moderately dissolved the polymer, improved the crystallinity of the P3HT films due to self-assembly among the mobile polymer chains.

Fig. 6 Field-effect mobility distribution achieved from a 0.5 wt% P3HT thin film. (a) The field-effect mobilities of the as-spun film were uniform across the film area. (b) The MC-exposed film displayed its highest mobility at the center of the film.

The molecular ordering in the bottom region of the resulting P3HT films increased as the film thickness decreased. These notable changes as a result of solvent exposure may result from the limited film penetration depth of the marginal solvent. The enhancement in the field-effect mobility of the MC-treated P3HT film was attributed to the increased structural ordering.

4. Conclusions

In summary, we investigated the effects of direct exposure to MC on the molecular structural changes in P3HT thin films across the film surface. Exposing P3HT films to MC dissolved the disordered polymer layer and promoted self-assembly among the mobile polymer chains introduced by the solvent. This approach provides a facile route to systematically controlling the morphology and crystallinity of a film, as supported by the UV-vis absorption and AFM imaging measurements. The impact the MC solvent was more pronounced in thin P3HT films, especially in the center region of the film. The changes in the molecular ordering and morphology within the P3HT film translated into higher charge carrier mobilities. This systematic study of solvent exposure provides important guidelines for the fabrication of practical and useful high-performance polymer devices.

Acknowledgements

This research was supported by the Incheon National University Research Grant in 2013 and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057015).

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Footnote

† S. K. and W. H. L. contributed equally to this work.

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