Study on effect of supercritical CO₂ on structural ordering and charge transporting property in thiophene-based block copolymer

Organic materials, which are easy to process and can be used to manufacture large-area devices at low cost, have been developed as an alternative to conventional amorphous silicon. ^1–4) Poly(3-hexylthiophene) (P3HT) is a typical conjugated polymer with a wide range of applications, such as organic thin-film solar cells and organic field transistors. It is known as a functional conductive polymer with relatively high stability, high conductivity, and absorption in the visible light region. ^1,5) The morphology of P3HT thin film consists of crystals with different orientations and sizes, and amorphous regions. ⁶⁾ The crystallites form a lamellar structure with the P3HT chains stacked in the π–π and alkyl side chain directions. ^5,6) In organic thin film solar cells, improving the hole mobility of P3HT, which is responsible for hole transport, is considered to be one of the important factors in achieving improved device performance. It is understood that subtle changes in the packing and orientation of P3HT polymer chain segments, molecular structure, and crystallinity can lead to drastic changes in the carrier mobilities. ^2,6,7) Since electrical charges are favorably transported in the π–π direction, the improvement of π–π stacking and P3HT chain packing is expected to enhance hole mobility. ^4,8,9)

In our previous work, ⁸⁾ we showed a sharp increase in out-of-plane hole mobility in block copolymer of P3HT with electrically inert polystyrene (P3HT-b-PSt). In order to clarify the cause of this phenomenon, structural analyses such as UV–vis absorption, X-ray diffraction, and differential scanning calorimetry (DSC) were performed. DSC measurements revealed that the introduction of the second component (PSt) induces the formation of the rigid amorphous region, which interconnects adjacent crystallites efficiently resulting in the enhancement of mobility. Furthermore, from X-ray diffraction analyses of block copolymer films, suppression of alkyl stacking and extending π–π stacking in the direction normal to the film surface were observed.

Spin coating process has been commonly used to produce smooth and uniform polymer thin films. ¹⁾ However, since in the spin coating process, the solvent evaporates at high speed, it places a lot of stress on the chain structure, and often results in a distorted state far from the equilibrium state. Therefore, post-coating thermal or solvent annealing processes are necessary to form the optimal morphology from non-equilibrated spin-cast thin films of π-conjugated polymers leading to the improvement of the device performance. ^3,5,7) Annealing above the melting temperature of P3HT (T_m ≒ 240 °C) may cause reorientation of P3HT, resulting in degradation of device performance. ^10–12) Annealing at temperatures between T_m and the glass transition temperature of P3HT (T_g ≒ 10 °C) can also reorganize P3HT crystals from a non-equilibrium state to a more thermodynamically stable form with improved crystal order, but the mobility of the chains is limited, so the degree of crystallinity is relatively low and the rate of crystallization is slow. This often affects the optical and electrical properties of P3HT devices. ^12,13) Solvent annealing, in which solvent molecules are incorporated into the thin film as plasticizers, has been shown to improve the charge transporting properties of P3HT chains, ¹⁴⁾ but the use of organic solvents can cause environmental problems. P3HT crystallization in a solvent environment requires detailed optimization of the solvent volume ratio, aging time, and polymer concentration, and the choice of solvent is very limited. ¹⁵⁾ The solvent can dissolve the polymer chains and de-wet them from the substrate, causing undesirable morphology in the spin coated P3HT thin film. Therefore, an alternative post-coating process is desirable to further improve the performance of electronic devices based on conjugated polymers.

Supercritical fluid is a substance that has above a critical value of temperature and pressure. ¹⁶⁾ In general, the supercritical region has been attracting a great deal of technological attention because of its intermediate properties between liquid and gas. ^16,17) However, its potential benefits have not been fully exploited because it can only dissolve a limited polymer. ¹⁸⁾ The supercritical region has the advantage of high compressibility, which allows for a wide range of controllable physical properties such as density, solubility, dielectric constant, viscosity, and diffusivity. ^16,19,20) The solubility of supercritical solvents can be effectively adjusted by pressure. ¹⁶⁾ Supercritical carbon dioxide (scCO₂) has been considered to be one of the "green" plasticizers that can replace harmful organic solvents. ¹⁸⁾ In thin-film polymers, it was discovered that the polymer swells due to excessive sorption of CO₂ molecules along a pressure/temperature line called the "density fluctuation ridge" where the density fluctuation of CO₂ is the greatest. ^18,19,21) It has been reported that the miscibility of the polymer thin film with CO₂ is greatly enhanced in spite of the low bulk miscibility of the polymer along this ridge. ¹⁹⁾ The vapor line below the critical point that extends into the supercritical region due to the scale-effect is called a ridge. It is known that rate constants and equilibrium constants of chemical reactions in supercritical fluids exhibit a maximum, minimum or bifurcation point at the ridge. The plasticizing effect associated with the sorption of excess CO₂ can greatly increase the mobility of chains, and the rate of crystallization, especially near the polymer-air and polymer-solid interfaces, by lowering the bulk T_g and T_m of glassy/semi-crystalline polymers. ²²⁾ In CO₂ annealing, the film irradiated with CO₂ is quenched to atmospheric pressure, allowing it to be frozen without forming micro voids. ^18,20)

In this study, we performed scCO₂ annealing on the thin films of homo-P3HT, and block copolymers (P3HT-b-PSt and P3HT-b-PEO) with a second block introduced in order to dramatically change the structure of the P3HT phase during the phase separation, and to improve the hole mobility. We report here that the hole mobility was enhanced, and the crystal structure was modulated by scCO₂ annealing. The effects were particularly remarkable for the P3HT-b-PEO, which exhibited the largest increase in mobility. The mechanism of mobility enhancement due to the change in crystallinity and the effect of the type of second block on the properties are discussed.

2.1. Materials

2.2. Device fabrication of space charge limited current (SCLC) measurements

2.3. Experimental procedures

2.3.1. Device electrical measurements

The current–voltage characteristics of the fabricated devices were carried out by using a direct-current–voltage and a current source/monitor (KEITHLEY, 2400, Cleveland, OH USA). The hole mobility was determined using the SCLC equation shown in Eq. (1). J is the hole current density, ${\mu }_{h}$ is the hole mobility, ${\varepsilon }_{r}$ is the relative dielectric constant of the material (3.5), ${\varepsilon }_{0}$ is the dielectric constant of the vacuum, L is the thickness of the active layer, and V is the voltage. Equation (2) is the logarithm of Eq. (1). From the logarithm of the I–V measurement results, log J and log V were plotted to form a log J–log V graph. An approximate line with a slope of 2 was drawn. The hole mobility was calculated from the intercept value obtained by extrapolating the line with slope of 2 according to Eq. (2)

$$\begin{eqnarray}&&J=\displaystyle \frac{9}{8}{\varepsilon }_{r}{\varepsilon }_{0}{\mu }_{h}\displaystyle \frac{{V}^{2}}{{L}^{3}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&\mathrm{log}\,J=2\,\mathrm{log}\,V+\,\mathrm{log}\,\displaystyle \frac{9{\varepsilon }_{r}{\varepsilon }_{0}}{8{L}^{3}}{\mu }_{h}.\end{eqnarray} \tag{ 2 }$$

2.3.2. UV–vis absorption measurements

The UV–vis absorption spectra were obtained with a JASCO V-670 spectrophotometer (Tokyo, Japan). The wavelength range was 300–800 nm. Thin films were prepared by spin coating from chlorobenzene solution of P3HT, P3HT-b-PSt, and P3HT-b-PEO on glass substrates on the same conditions as those of SCLC devices.

2.3.3. XRD measurements

The crystal natures of films on the glass were evaluated by grazing incidence wide angle X-ray diffraction (GIWAXD) (RIGAKU X-ray Diffractometer SmartLab, Tokyo, Japan) (Cu Kα, λ = 1.5418 Å, 45 kV and 200 mA) from 3° to 30° with a step of 0.01° at the scan speed of 1°min⁻¹ in the out-of-plane measurements. Incident angel was fixed to 0.17°.

3.1. SCLC hole mobility

The plots of J–V and log J–log V relationships are shown in Figs. 1(a) and 1(b), respectively. The thickness of the fabricated films and the calculated values of hole mobility are shown in Table I.

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Table I. SCLC hole mobility in films.

Polymer	Film state	Thickness (nm)	SCLC mobility (cm 2 V −1 s −1 )
P3HT	as-spun	150	2.8 × 10−5
P3HT	scCO2	180	4.3 × 10−5
P3HT-b-PSt	as-spun	230	1.4 × 10−4
P3HT-b-PSt	scCO2	220	1.8 × 10−4
P3HT-b-PEO	as-spun	180	1.7 × 10−4
P3HT-b-PEO	scCO2	190	5.1 × 10−4

As shown in the above results, the mobility for all polymers we examined increased in the scCO₂ treated films. Among these three polymers, P3HT-b-PEO showed the largest increase in mobility after scCO₂ annealing.

3.2. Structural analyses

Figure 2 shows the UV–vis absorption spectra of P3HT, P3HT-b-PSt, and P3HT-b-PEO films, where the peaks at 552 and 602 nm correspond to the 0–1 and 0–0 transition, respectively. From the ratio of these two peaks, the effective conjugate length W can be calculated by Eq. (3), ²⁸⁾

$$\begin{eqnarray}&&\displaystyle \frac{{A}_{0-0}}{{A}_{0-1}}={\left(\displaystyle \frac{1-0.24W/{E}_{p}}{1+0.073W/{E}_{p}}\right)}^{2},\end{eqnarray} \tag{ 3 }$$

where E_p represents the phonon energy of the main oscillator coupled to the electronic transition (0.18 eV), and it is known that the smaller the value of W, the larger the effective conjugation length. ²⁸⁾

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The values of the effective conjugate length W are listed in Table II. W values for the scCO₂ treated film decreased by comparing with those of the as-spun films for all polymers indicating the development of higher conjugation. This can be attributed to the fact that CO₂ molecules entered the amorphous regions of P3HT, PSt, and PEO by scCO₂ treatment resulting in plasticizing the local regions, which assisted to improve the ordering of the P3HT chains to lengthen the effective conjugation length.

Table II. Structural features of as-spun and scCO₂ treated films.

Polymer	Film state	W (meV)	Intensity a)	Crystal size (Å)
P3HT	as-spun	85	1.00	71
P3HT	scCO2	79	1.01	78
P3HT-b-PSt	as-spun	103	0.54	63
P3HT-b-PSt	scCO2	99	0.64	64
P3HT-b-PEO	as-spun	113	0.98	62
P3HT-b-PEO	scCO2	110	1.19	73

^a)Intensity is estimated from the diffraction intensity of peak (100), and is normalized by the intensity of as-spun P3HT.

Figure 3 shows the results of out-of-plane wide-angle X-ray diffraction (GIWAXD) measurements. Diffraction (100) (2θ ≒ 5.5) is attributed to an alkyl stack in edge-on orientation with P3HT aligned perpendicular to the substrate. The crystallite size was calculated using the Scherrer Eq. (4). D is the crystallite size, λ is the Cu Kα wavelength of the X-rays (1.5418 Å), β is the half width, θ is the diffraction angle, and k is the Scherrer constant (0.94). The intensity was also estimated by the peak area of (100) and film thickness

$$\begin{eqnarray}&&D=\displaystyle \frac{k\lambda }{\beta \,\cos \,\theta }.\end{eqnarray} \tag{ 4 }$$

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From the results shown in Table II, it can be seen that the crystallite size in the film treated with scCO₂ increased. In addition, the scCO₂ treatment increased the intensity, which indicates that the crystallinity of the film was improved. In other words, P3HT chain packing was promoted by scCO₂ treatment, leading to the increase in crystallite size and crystallinity. This might increase the hole mobility in the scCO₂ treated film as mentioned in SCLC mobility session.

Figure 4 shows the schematic representation for scCO₂ annealing. Table II shows that both the increase in crystallite size and the improvement in crystallinity were greatest for P3HT-b-PEO, since CO₂ molecules and PEO are known to have higher affinity. ²⁹⁾ This affinity promotes the more efficient intake of CO₂ in PEO domains, which plasticize not only PEO itself but neighboring P3HT mobile and/or rigid amorphous P3HT domain. This may be the origin why P3HT-b-PEO showed the largest increase in hole mobility after scCO₂ treatment compared to other polymers.

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We evaluated the effect of scCO₂ treatment on the structure and hole mobility of the block copolymers. From the SCLC analyses, the hole mobility was improved by scCO₂ annealing. In particular, P3HT-b-PEO showed the largest increase in mobility. From UV–vis measurements, the effective conjugate length of P3HT was found to be improved by scCO₂ annealing, and XRD measurements revealed that scCO₂ increased the crystal size and crystallinity. It was attributed to the fact that the crystal structure of P3HT, which was aligned by the dissolution of CO₂ molecules into the amorphous regions of PSt, PEO, and P3HT, which allowed the plasticization and reorganization of P3HT chains, leading to the improvement in mobility. A higher affinity between CO₂ and PEO, more CO₂ molecules dissolved in P3HT-b-PEO, especially in microphase separated PEO domain, assisted to improve the packing of P3HT and led to the increase in mobility.