Spin coating of conjugated polymers for electronic
and optoelectronic applications
Chao-Ching Chang
a, Chia-Ling Pai
a, Wen-Chang Chen
a,*, Samson A. Jenekhe
b,*
a
Department of Chemical Engineering and Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. b
Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, WA 98195-1750, USA Received 29 June 2004; accepted in revised form 3 December 2004
Available online 15 January 2005
Abstract
Experimental and theoretical study of the spin coating deposition of thin and ultrathin films from dilute solutions of four conjugated polymers, including poly[2-methoxy-5-(2V-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), regioregular poly(3-hexylthiophene), poly(9,9-dioctylfluorenyl-2,7-yleneethynylene), and poly(2,2V-(3,3V-dioctyl-2,2V-bithienylene)-6,6V-bis(4-phenylquinoline)), is reported. Dilute solutions (0.3–2.0 wt.%) of the four conjugated polymers in chloroform were found to be Newtonian fluids with viscosities of 0.7–27.9 cp. The measured film thickness (hf) of the conjugated polymers was found to be well correlated to the initial solution concentration (x1,0) and the spin speed (x) by the simple expression, hf=k x1,0xh. The exponent b is 0.5 for MEH-PPV but is reduced to 0.4 for the other three conjugated polymers. The difference in the b values can be explained by the effect of the accelerative period on the spin coating of less viscous dilute polymer solutions as verified by numerical simulation. A modified Meyerhofer’s model was also found to well correlate the film thickness with the fundamental physical properties of the polymers and solvent. These experimental and theoretical results provide a basis for understanding and optimizing the preparation of thin and ultrathin films of conjugated polymers by spin coating.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Organic semiconductors; Coating; Polymers; Film thickness
1. Introduction
Conjugated polymers are of wide interest for applications in electronics and optoelectronics, including light emitting diodes[1–4], thin film transistors [5–7], photovoltaic cells
[8,9], and electrochromic cells [10]. Spin coating is the primary process for the deposition of uniform thin and ultrathin films of conjugated polymers for these device applications. Fundamental studies of the morphology and physical properties of conjugated polymers, such as physics, charge transport, electrical conductivity, photo-conductivity, third-order optical nonlinearity, are also commonly done on spin coated thin (0.2–1.0 Am) and ultrathin (b0.2 nm) films. It has been well documented that
the film thickness plays a critical role in the morphology and properties of conjugated polymers and the performance of devices based on polymer semiconductors[1–10]. Although there have been many studies of the spin coating process for many other materials [11–31], including photoresists
[12,16,22,23], polyimides [13,24,30], sol-gel materials
[25] and polyimide–silica hybrid materials [31], neither experimental nor theoretical analysis of the spin coating of conjugated polymer solutions has been reported.
Experimental and theoretical analyses of the spin coating of thick films (N1 Am) of photoresists, polyimides, sol-gel materials or polyimide–silica hybrid materials in different solvents have been reported. The dry (final) film thickness, hf, after spin coating of a polymer solution was found to be correlated to the initial polymer weight fraction, x1,0, and the spin speed, x, by Eq.(1)[12,22]:
hf ¼ kx1;0xb ð1Þ
0040-6090/$ - see front matterD 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.12.013
* Corresponding authors. Tel.: +886 223628398; fax: +886 223623040. E-mail addresses: chenwc@ntu.edu.tw (W.-C. Chen)8
where k is a constant that depends on the concentration, solution viscosity, and other properties of the polymer and solvent. In the case of most photoresists or spin-on-glass (SOG) materials, the exponent b of spin speed is close to 0.5 [12,22]. However, for polyimides or hybrid materials, the exponent b shows a wide variation from 0.5 to 1.0[22]. Such a variation of the exponent b has been explained as due to the relative evaporation rate of solvent or achieve-ment of steady state during spin coating[21,22].
Typically, the film thickness of spin coated photoresists or waveguide materials is in the range of several micro-meters [12,30,31]. However, spin coated conjugated poly-mers for electronic or optoelectronic devices typically have a thickness in the sub-micron range. For example, the film thickness of conjugated polymers in light emitting diodes or photovoltaic cells is generally in the range of 50–200 nm
[1–4,8,9]. For such ultrathin films spin coated from dilute polymer solutions, there are just few prior studies of the spin coating process. Prior studies of the spin coating of ultrathin films of poly(methyl methacrylate) (PMMA) and polystyr-ene [27,28], however, are not applicable to conjugated polymers. k-conjugated polymers are largely rigid-rod polymers, whose solution rheological properties can vary from Newtonian to non-Newtonian and isotropic to liquid crystalline [32–34]. Little is currently known about the rheological properties of solutions of even the most widely used conjugated polymers such as poly(alkythiophene)s, derivatives of poly(p-phenylenevinylene), polyfluorenes, and polyquinolines.
In this paper, we present the first experimental and theoretical study of the spin coating process for the deposition of thin and ultrathin films of several conjugated polymers. We investigated the spin coating of dilute solutions of poly[2-methoxy-5-(2V-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), regioregular poly(3-hexylthiophene) (P3HT), poly(9,9-dioctylfluorenyl-2,7-yleneethynylene) (PDOFE), and poly(2,2V-(3,3V-dioctyl-2,2V-bithienylene)-6,6V-bis(4-phenylquinoline)) (POBTPQ). The molecular structures of
these polymers are shown in Fig. 1; these conjugated polymers are currently used in light-emitting diodes or thin film transistors [1,4,35]. We show that dilute solutions of these conjugated polymers have Newtonian rheological properties. We examined the effects of the polymer solution concentration, solution viscosity, spin speed, and other parameters on the film thickness obtained by spin coating. We show that the expression of Eq. (1) can be used to correlate the film thickness with solution concen-tration and spin speed. The effect of the spin coating condition and the viscosity of solution on the b value was also discussed. A modified Meyerhofer’s model was also found to correlate the film thickness of a spin coated conjugated polymer with the fundamental physical proper-ties of the polymer and solvent.
2. Experimental details
MEH-PV (MPn=86,000, polydispersity index (PDI, MPn/ M
P
n)=1.52), MEH-PPV-2 (MPn=125,000, PDI=1.06), regiore-gular P3HT (98.5% head-to-tail, MPn=27,360, PDI=3.18), and PDOFE (MPn=5300, PDI=2.20) were purchased from Aldrich (Milwaukee, WI, USA) and used without further purification. POBTPQ (MPn=64,900, PDI=2.76) was synthe-sized and characterized according to a previous report [4]. The densities of the conjugated polymers were estimated by the group contribution method[36]and the values are listed inTable 1.
The polymers were dissolved in chloroform to form solutions of different concentrations (weight fractions of polymers). A Brookfield viscometer (DV-II+) and an Ubbelohde viscometer were used to measure the absolute solution viscosity (g0). Each polymer solution was spin coated onto a 2-inch silicon wafer. The spin speeds were 1000, 2000, 3000, 4000, and 5000 rpm, respectively, and the spin time was 60 s. The coated film was then dried on a hot plate at 60 8C for 10 min and at 60 8C in a vacuum furnace for 1 h to remove the solvent completely. The thickness of the polymer thin films was measured using a surface profilometer (Veeco, Dektak 3030 ST).
O O MEH-PPV S C6H13 P3HT n n H17C8 C8H17 n PDOFE S S C8H17 N N H17C8 POBTBQ n
Fig. 1. The molecular structures of the conjugated polymers used in this study.
Table 1
The solution properties and spin coating parameters of conjugated polymers Polymer q1(g/cm3)a x1,0(wt.%) g0(cp) k (10-3) b MEH-PPV 0.99 0.3 2.45b 521 0.47 0.5 5.74b 881 0.49 1.0 27.86b 1629 0.49 P3HT 1.02 0.5 0.76c 194 0.38 1.0 1.07c 216 0.39 2.0 1.92c 264 0.39 PDOFE 0.94 1.0 0.71c 188 0.37 2.0 0.85c 180 0.37 POBTPQ 1.08 1.0 1.57b 319 0.43 a
Calculated by the group contribution method. b
Measured on a Brookfield viscometer. c
3. Results and discussion
3.1. Rheological properties of polymer solutions
Because there is currently no available information on the rheological properties of solutions of MEH-PPV, P3HT, PDOFE, and POBTPQ, we investigated the rheology of these conjugated polymers in chloroform solutions. Repre-sentative shear stress versus shear rate data is shown inFig. 2 for three solution concentrations of MEH-PPV. Linear relationships indicating Newtonian flow behavior are observed in the three MEH-PPV solutions of Fig. 2. This means that the solutions are Newtonian fluids as-prepared. The solutions of the other three conjugated polymers were also found to be Newtonian fluids. The measured absolute viscosities of the conjugated polymer solutions are listed in
Table 1. The viscosity of MEH-PPV solutions is in the range of 2.45–27.86 cp (centipoise) as the solution concentration is increased from 0.3 to 1.0 wt.% MEH-PPV. The viscosities of the other conjugated polymer solutions are in the range of 0.71–2.37 cp. The polymer solution viscosity, g, can be expressed as a power-law function of the concentration[12]:
g¼ g2þ axb1 ð2Þ
where g2 is the solvent viscosity and x1 is the polymer weight fraction.Fig. 3shows the relationship between the viscosity and the polymer weight fraction of MEH-PPV and P3HT solutions in chloroform. By fitting Eq. (2) to the data, the expressions of g (cp)=0.54+727,300 x12.22 and g (cp)=0.54+255 x11.34 were obtained for solutions of MEH-PPV and P3HT, respectively. Note that the viscosity of the solvent (g2), chloroform, is 0.54 cp.
These results show that the dilute solutions (0.3–2.0 wt.%) of the four conjugated polymers in chloroform are Newtonian fluids with as-prepared absolute viscosities of 0.7–27.9 cp. Compared to the extremely stiff conjugated rigid-rod and ladder polymers that readily form liquid crystalline solutions [32–34], the viscosities and their concentration dependence as measured by the exponent b
in Eq. (2) are substantially lower in the present conjugated polymers (MEH-PPV, P3HT, PDOFE, and POBTPQ). We conclude that prior theoretical models of the spin coating of Newtonian polymer solutions, such as Eq. (1), can be a good starting point for describing the spin coating of solutions of these and other conjugated polymers[37]. 3.2. Spin coating of conjugated polymer solutions
As suggested by Eq. (1), the thickness of a spin coated polymer thin film is proportional to the initial polymer concentration (weight fraction), and the spin speed raised to an exponent b. Fig. 4shows the variation of MEH-PPV film thickness with spin speed and solution concentration. The experimental data are well correlated by the theoretical expression of Eq. (1). The parameter k in Eq. (1) varied from 521,000 to 1,629,000 as the polymer concentration
increased from 0.3 to 1.0 wt.%. However, b remained
constant at 0.47–0.49. This suggests that the dependence of film thickness on the spin speed is the same at the different polymer concentrations. The b value is close to the theoretical value of 0.5 and to those reported for PMMA and polystyrene[27–29]. For the other conjugated polymer thin films, the film thickness dependence on the spin speed
30 20 10 100 200 300 0 0 Shear Rate (s-1)
Shear Stress (dyne/cm
2)
1.0 wt% 0.5 wt% 0.3 wt%
Fig. 2. Variation of the shear stress with the shear rate for MEH-PPV solutions in chloroform. 30 20 10 0 0.00 0.01 0.02 x1,0 (weight fraction) η -η2 (cp) η-η2 = 255 x1,01.34 η-η2 = 727,300 x1,02.22 MEH-PPV P3HT
Fig. 3. Viscosity dependence on the concentration of MEH-PPV and P3HT solutions in chloroform. 0 100 200 300 400 500 0 1000 2000 3000 4000 5000 Spin Speed (rpm) Thickness (nm) 1.0 wt% 0.5 wt% 0.3 wt%
Fig. 4. Variation of the MEH-PPV film thickness with spin speed and polymer concentration.
could also be satisfactorily fitted by Eq. (1). The spin coating parameters, constantk and exponent b of Eq. (1), for all of the conjugated polymer solutions are listed in
Table 1. As shown inTable 1, the value of b is around 0.5 for the high viscosity MEH-PPV solutions but is reduced to around 0.4 for the other polymer solutions which have lower viscosities.
The constant k could be correlated to the viscosity g0by the expression k=kVg0
a
. Hence, the effect of solution viscosity on the film thickness was further investigated.
Fig. 5shows the variation of the ratio of the film thickness to initial concentration (hf/x1,0) with solution viscosity g0 and spin speed for MEH-PPV. The film thickness is seen to increase with increasing g0and is well-fitted by Eq. (1) if k=kVg0a. Similar results were also found for P3HT. By substituting the various constants into Eq. (1), the film thickness of spin coated MEH-PPV and P3HT can be predicted by Eqs.(3a) and (3b), respectively:
MEH PPV : hfðnmÞ ¼ 2:89 106x1;0g0:40 x
0:5 ð3aÞ
P3HT : hfðnmÞ ¼ 1:47 106x1;0g0:40 x
0:4 ð3bÞ
The average deviation between the experimental data and the theoretical prediction from Eqs. (3a) and (3b) was only 2–3%. Hence, we conclude that Eqs. (3a) and (3b) can be used to accurately predict the film thickness of MEH-PPV and P3HT spin coated from solutions of known concen-tration (x1,0) and viscosity (go).
The effect of molecular weight on the derived spin-coating model was studied by investigating two different MEH-PPV (MPn=86,000) and MEH-PPV-2 (MPn=125,000) at the same solution concentration of 0.3 wt.% in CHCl3. The values of k and b of the MEH-PPV-2 solution fitted by Eq. (1) are 1140103, and 0.47, respectively. The goof the 0.3 wt.% MEH-PPV-2 is 23.06 cp, which is higher than that of the 0.3 wt.% MEH-PPV solution with 2.45 cp. The same b for MEH-PPV and MEH-PPV-2 suggests that the dependence of film thickness with spin speed is the same for two different molecular weights of polymer. The difference of k for MEH-PPV and MEH-PPV-2 comes from the different solution viscosity. If we fit the film thickness of the MEH-PPV-2 by Eq.(3a), there is only 6% difference
between the theoretical and experimental results. This
indicates that Eq.(3a) could be a general equation for
predicting the MEH-PPV thin film for electronic and optoelectronic applications.
The finding that the value of b is less than 0.5 for the cases of P3HT, PDOFE, and POBTPQ, as shown inTable 1
requires an explanation. This may be due to the effect of the accelerative period during the early stage of a spin coating process involving very dilute and less viscous solutions
[12,20,25]. To understand the deviation of b from the theoretically expected value of 0.5, we used a numerical method to calculate the variation of the film thickness with spin time. For a Newtonian fluid, an expression for the rate of convective thinning of a uniform film on a spinning disk has been reported[11]:
dh
dt ¼
2qx2h3
3g ð4Þ
where h, q, and g are the film thickness, the initial solution density, and the solution viscosity, respectively. As shown previously the solution film thickness h consists of the thickness of solid polymer (S) and the thickness of solvent (L), and thus, the rate of solution thinning due to convective flow and evaporation can be decomposed into Eq.(5a) and (5b)[12]: dS dt ¼ S Sþ L 2qx2ðSþ LÞ3 3g ð5aÞ dL dt ¼ L Sþ L 2qx2ðSþ LÞ3 3g K ð5bÞ
where K is the solvent evaporation term described by Eq.(5c)[20]: K¼ cDg m1=2g q ! P02M2 RT x1=2 x 2 x2l ð Þ ð5cÞ
where x2, x2l, c, Dg, mg, P20, T , M2, and R are, respectively, the solvent mass fraction, the solvent mass fraction in the solution that would be in equilibrium with the solvent mass fraction in the gas phase, a constant that depends on the Schmidt number of the overhead gas, the binary diffusivity of the solvent in the overhead gas, the kinematic viscosity of the overhead gas, the solvent vapor pressure, the coating temperature, the solvent molecular weight, and the ideal gas constant. These parameters used in this paper are given in Tables 1 and 2. For a given initial film thickness, Eqs. (5a) (5b) (5c) can be solved numerically. Fig. 6 shows the experimental final film thickness as a function of the spin speed for 1.0 wt.% MEH-PPV and P3HT solutions. Also shown inFig. 6are the numerical solution results. The theoretical results are in a satisfactory agreement with the experimental results, sug-gesting that the numerical method is accurate.Fig. 7shows the thickness versus spin time results for 1.0 wt.%
MEH-0 0.1 0.2 0.3 0.4 h h/x 0 (cp) 1,0 1000 rpm 2000 rpm 3000 rpm 4000 rpm 5000 rpm
Fig. 5. Variation of the ratio of the film thickness to the initial concentration (hf/x1,0) with the solution viscosity and spin speed for MEH-PPV.
PPV and P3HT solutions, respectively, obtained by numer-ical simulations. As seen inFig. 7, the conjugated polymer films reach their final thicknesses within a very short time because of the high convection and evaporation rates when the solution viscosity is low and the solvent evaporation (chloroform) is high. For spin coated thick films from highly viscous photoresists [12,16] or polyimide precursor solu-tions [30], these usually require a spin time of 30–60 s. Besides, the P3HT solution reaches its final thickness much faster than the MEH-PPV solution at the same concen-tration. This is due to the lower viscosity of the P3HT solution than that of MEH-PPV.
As noted above, the deviation of exponent b from 0.5 for three of the conjugated polymers (P3HT, PDOFE, POBTPQ) may be due to the accelerative period at the beginning of spin coating. We now further examine this hypothesis. In the accelerative period, the spin speed is increased from zero to a desired constant speed (xset, set speed). An exponential equation x=xset (1e10t) was assumed to account for the accelerative period of the spin speed and substituted into Eqs. (5a) and (5b). By testing an example of xset=1000 rpm, the exponent b obtained by numerical simulation using x=xset(1e10t) was 0.42 for the 1.0 wt.% P3HT solution, which is very close to the experimental 0.39 (Table 1). However, if a constant spin speed x=xset(=1000 rpm) was employed in the simulation, the b value obtained was 0.5, which is the theoretically predicted value and what we also observed for MEH-PPV. These simulation results confirm our hypothesis that the
accelerative period significantly affects the film thickness and reduces the exponent b for some of the conjugated polymer solutions. Because the time of arriving at the final thickness is mainly inversely proportional to the solution concentration, viscosity, and spin speed, the effect of accelerative time becomes apparent in a very dilute and less viscous solution or at high spin speeds, such as the cases of P3HT, PDOFE, and POBTPQ solutions. Clearly because the MEH-PPV solutions have a higher viscosity than the other conjugated polymers in the present study, the effect of the accelerative period on the film thickness or the exponent b is not significant.
The fact that the exponent b of Eq. (1) deviated from the theoretically expected 0.5 in the case of solutions of P3HT, PDOFE, and POBTPQ due to their low viscosities means that one can expect higher molecular weight samples of these polymers to result in higher viscosities and thus a b value of 0.5. There is clearly a need to further explore the spin coating of higher molecular weight samples of these conjugated polymers. We suggest that a b value of 0.5 be used for these and other conjugated polymers if their solution viscosities are comparable to those of MEH-PPV (Table 1).
Although Eq. (1) has been shown above as an excellent expression for estimating the film thickness of spin coated conjugated polymers, it does not correlate the film thickness with the fundamental physical properties of the polymer and solvent. Bornside et al.[20]. and Hall et al.[27], attempted to calculate the film thickness of spin coated PMMA and polystyrene thin films based on Meyerhofer’s model[12]by taking solvent evaporation into account[20]. However, their predicted results were not in good agreement with the experimental data for some polymer/solvent systems. We believe that this was because they did not consider the difference between the polymer solution density and the solvent density. Here, a modified model based on the equations of Meyerhofer [12]and Bornside[20]is used to predict the film thickness of spin coated conjugated Table 2
Parameters of air and solvent (CHCl3) for predicting thin film thickness at T=298 Ka R=82.06 atm cm3/mol K M 2=119.38 c=0.5474b U2=1.50 g/cm3 vg=0.1553 cm2/s g2=0.54 cp Dg=0.106 cm2/s P20=0.26 atm x2,l=0 aReference[38]. b Reference[20]. 0 100 200 300 400 500 600 700 800 0 2000 4000 6000 8000 Spin Speed (rpm) Thickness (nm) MEH-PPV (1.0 wt.%) P3HT (1.0 wt.%)
Fig. 6. The predicted final film thickness of the 1.0 wt.% MEH-PPV and P3HT solutions with spin speed and their comparison with the experimental data. Solid symbols: measured data; solid lines: calculated from numerical method. 50 40 30 20 10 0.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Spin Time (s) Thickness ( µ m) T hickness ( µ m) (a) MEH-PPV (1.0 wt%) (b) P3Ht (1.0 wt%) 1000 rpm 2000 rpm 3000 rpm 4000 rpm 5000 rpm 1000 rpm 2000 rpm 3000 rpm 4000 rpm 5000 rpm 50 40 30 20 10 0 0.0 0.2 0.4 0.60.8 1.0 1.2 1.4 1.6 Spin Time (s)
Fig. 7. Numerical simulation of the variation of film thickness with spin time for the (a) 1.0 wt.% MEH-PPV solution. Insert figure is the variation of film thickness with spin time for the (b) 1.0 wt.% P3HT solution.
polymers. The wet film thickness, hw, after spin coating but before drying can be expressed as[26]:
hw¼ 3g0 2q0x2 cDg m1=2g q0 ! P0 2M2 RT x1=2x2;0 x2l " #1=3 ð6aÞ where x2,0is the initial solvent mass fraction. The final film thickness, hf, taken to be the dry thickness of the conjugated polymer film can be expressed by the following equation: hf¼ v1;0 3g0 2q0x2 cDg m1=2g q0 ! P0 2M2 RT x1=2 x 2;0 x2l " #1=3 ð6bÞ where v1,0 is the initial polymer volume fraction in the solution. The definitions of other parameters have previ-ously been explained in Eq. (5c). The predicted film thicknesses of spin coated MEH-PPV, P3HT and PDOFE, using Eq. (6b), and their comparison with experimental data are shown in Fig. 8, respectively. The predicted film thickness shows a very good agreement with the exper-imental film thickness, especially at low polymer concen-trations. However, the predicted film thickness is slightly higher than the experimental data at the higher spin speeds. This is due to the accelerative period in the spin coating process, as discussed above. Therefore, Eqs. (6a) (6b) can be used to predict the film thickness of spin coated conjugated polymers if the physical properties of the polymer and solvent are known.
4. Conclusions
Our experimental results for several representative conjugated polymers, including MEH-PPV, P3HT, PDOFE, and POBTPQ, show that their dilute solutions (b3 wt.%) in chloroform have Newtonian rheological properties with
viscosity in the 0.7–27.9 cp range. Theoretical models for the spin coating of Newtonian polymer solutions were thus found to be suitable starting points for the analysis of thin and ultrathin films of spin coated conjugated polymers. The measured film thickness of the four conjugated polymers was described well by the simple expression hf=k x1,0xb of Eq. (1), where b was 0.5 for MEH-PPV and 0.4 for P3HT, PDOFE, and POBTPQ. Since the deviation of the exponent b from the theoretical value of 0.5 is due to the less viscous nature of solutions of the latter three polymers, we expect that higher molecular weight samples of the polymers will show the theoretically expected b value. Eq. (6b) was found to correlate the film thickness of the spin coated conjugated polymers with the fundamental physical properties of the polymer and solvent. The present results provide simple methodologies for controlling the spin coating of thin and ultrathin films of conjugated polymers.
Acknowledgement
Financial support came from the National Science Council, the Ministry of Education, and the Ministry of Economic Affairs of Taiwan, R.O.C. Work at University of Washington was supported by the Air Force Office of Scientific Research (Grant F49620-03-1-0162).
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