Spectroscopic ellipsometry characterization of polymer–fullerene blend films

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Thin Solid Films 517 (2008) 1047 – 1052 www.elsevier.com/locate/tsf

Spectroscopic ellipsometry characterization of polymer–fullerene blend films A.M.C. Ng a , K.Y. Cheung a , M.K. Fung a , A.B. Djurišić a,⁎, W.K. Chan b a b

Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Available online 24 June 2008

Abstract In this work, we have used spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) to characterize the properties of blend films commonly used in organic solar cells, namely poly[3-hexylthiophene-2,5-diyl] (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blends. The blend films were prepared using different solvents, and for different ratios of P3HT:PCBM in order to change the surface roughness and phase separation in the blends. The obtained SE results were analyzed with and without the surface roughness correction to examine how the morphology affects the optical properties. © 2008 Elsevier B.V. All rights reserved. Keywords: Spectroscopic ellipsometry; Polymers

1. Introduction Polymer based bulk heterojunction cells have been attracting increasing interest in recent years. Bulk heterojunction cells are promising as an inexpensive alternative to conventional inorganic semiconductor based photovoltaic cells, although the power conversion efficiencies still need to be improved for commercial applications. The best results to date for a single heterojunction cell have been achieved for poly[3-hexylthiophene-2,5-diyl] (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blend cells, where power conversion efficiencies of 5–6% have been obtained for optimized devices [1,2]. Majority of research on bulk heterojunction polymer cells is focused on improving the solar cell efficiency. Some studies have investigated the effect of the film morphology, such as phase separation, nanoscale crystallinity and ordering of the polymer, on the cell performance [3,4]. However, optical modeling of the devices [5] requires knowledge of the optical functions of each layer in the device. While there have been several studies of the optical functions of different polymer blend films [5–10], there have been no detailed studies focusing on the influence of the film morphology on the obtained optical functions from spectroscopic ellipsometry (SE) measurements. Also, it has been shown that in polymer blends the dielectric ⁎ Corresponding author. E-mail address: [email protected] (A.B. Djurišić). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.05.038

function of the blend cannot be expressed in a simple form (addition or using effective medium approximation) of the blend constituents [7]. While in some cases fitting problems due to numerical instabilities were described, and constraints introduced to get reasonable results are specified [5], in majority of studies there was no detailed discussion of the ellipsometry data analysis. In this work, we have studied the properties of P3HT:PCBM blends for different blend compositions and in different solvents. The SE data were analyzed with and without taking into account the surface roughness. Obtained results have been discussed. 2. Experimental details Solutions of pure P3HT (5 mg ml− 1) and P3HT/PCBM blends with ratio 1:1, 2:3 and 1:4 (20 mg ml− 1) were prepared in chlorobenzene and chloroform and were sonicated overnight. Silicon and glass substrates were cleaned by sonication in toluene, acetone, ethanol and deionized water, and dried in a vacuum oven. Glass substrates had a rough back surface. The solutions were passed through a 0.45 µm filter and spin-coated at 1000 rpm for 2 min. The films were then dried in a vacuum oven at room temperature for 30 min and were stored in high vacuum (10− 5–10− 6 torr) overnight. Surface morphologies of the samples were characterized by atomic force microscopy (AFM) using a NT-MDT Solver P47

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in semicontact mode (with a silicon tip coated with high reflective Au, force constant of 11.5 N/m and resonant frequency of 255 kHz). The absorption spectra of the films were measured using a Cary 50 Bio UV–vis spectrophotometer. SE measurements were performed with a J.A. Woollam VVASE spectroscopic ellipsometer. Data were taken from 350 nm to 850 nm with 5 nm step and at incident angles of 65°, 70° and 75°. For each sample, a second set of measurement was taken at different sample orientation to verify the in-plane isotropy of the film. Then the experimental data were fitted to obtain the optical functions of the blends using a Lorentz model. Model parameters were obtained by minimizing the error function which is defined by the following equation [11]: P MSE ¼



jtanðwexp Þtanðwcal Þj rtan ðwÞ

2

 þ

jcosðDexp ÞcosðDcal Þj rcos ðDÞ

N M 1

2 ð1Þ

where MSE is the mean square error, ψexp, Δexp are the measured values and ψcal, Δcal are the calculated values, N is the number of wavelengths at which measurements were performed, and M is the number of parameters used in fitting.

The complex dielectric function according to the Lorentz oscillator model [11–13] is described as: e ¼ einf þ

X m

Fm ; x2m  x2 þ iCm x

ð2Þ

where εinf is a constant, m is the number of oscillators with a frequency ωm, broadening constant Γm, and oscillator strength Fm. Possible anisotropy of the films, which can be significant for some polymers and polymer blends [5,6], has not been taken into account and all the blends were modeled using an isotropic model. The reason for using isotropic model is that the degree of anisotropy is typically affected by the composition of the blend [7], and there is no evidence of ordering for the polymer chains so that attempts to use anisotropic model have not been made. The influence of incorporation of the surface roughness correction was also investigated. Bruggeman effective medium approximation (EMA) was used to perform a surface roughness correction [14], using a two-layer model, where the bottom layer is blend film, and the top layer is an EMA layer composed of air and the bottom layer material. Both the thickness of the rough layer and the fraction of voids were fitted.

Fig. 1. AFM images of the films spin-coated from chloroform solutions a) P3HT, b) 1:1 P3HT:PCBM, c) 2:3 P3HT:PCBM, d) 1:4 P3HT:PCBM. Scan size is 1 μm × 1 μm.

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3. Results and discussion Figs. 1 and 2 show the surface topography of the films with different compositions prepared from chloroform and chlorobenzene solutions, respectively, while absorption of the films is shown in Fig. 3. The root mean square (RMS) roughness values and film thicknesses are summarized in Table 1. The fitted thicknesses of the EMA layers were not necessarily smaller than the RMS roughness value. Different values of RMS roughness value from AFM measurement may be obtained due to different scanning windows sizes [15]. Also, the surface profile, the optical model or inhomogeneity can result in different SE results even with the same RMS roughness [15,16]. Thus, the difference in roughness estimates from AFM and EMA are likely due to these factors [15,16]. The roughness and topography of the films are obviously strongly dependent on the solvent used for spin-coating. Blend films prepared from chlorobenzene solutions, which have slow evaporation time, have a higher roughness than the films prepared from chloroform solutions. This might be due to the different solubility of PCBM in different solvents which results in a different phase separation. In particular, the 1:1 blend films are significantly dependent on solvent since the solubility of

Fig. 3. Absorption spectra of films with different composition.

PCBM decreases with the increase of polymer concentration [17]. On the other hand, pure polymer films prepared from chlorobenzene solution are smoother, as expected, due to slower solvent evaporation rate. It has also been shown that the absorption coefficient of P3HT:PCBM blend films is

Fig. 2. AFM images of the films spin-coated from chlorobenzene solutions a) P3HT, b) 1:1 P3HT:PCBM, c) 2:3 P3HT:PCBM, d) 1:4 P3HT:PCBM. Scan size is 1 μm × 1 μm.

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Table 1 Fitting errors, roughness and thickness of different films Film

MSE MSE RMS d (nm), f Film thickness (EMA) (AFM)/nm (nm)

P3HT, CB 1:1 P3HT:PCBM, CB 2:3 P3HT:PCBM, CB 1:4 P3HT:PCBM, CB P3HT, C 1:1 P3HT:PCBM, C 2:3 P3HT:PCBM, C 1:4 P3HT:PCBM, C

36.6 20.9 12.8 21.2 45.9 52.4 32.4 37.3

35.3 20.6 13.7 22.4 22.3 50.4 24.6 29.7

1.8 8.0 4.5 2.4 3.8 1.4 1.6 1.9

2.8, 0.95 19 4.5, 0.21 57 4.8, 0.21 64 2.8, 0.29 66 1.2, 0.75 36 6.7, 0.13 220 2.4, 0.79 121 1.9, 0.13 234

CB denotes chlorobenzene, C denotes chloroform, d is the thickness of the rough layer, and f is the percentage of voids. Higher thickness of blend films prepared from chloroform is due to required long sonication time for the blend which results in some evaporation of the solvent and higher resulting solution concentration.

dependent on the annealing temperature of the films, which affects the crystallization of the P3HT and also affects the film morphology [10]. Therefore, we have examined in detail the optical properties of films with different compositions spin-coated from chloroform and chlorobenzene solutions. Obtained results for the extinction coefficients k and refractive indices n are shown in Figs. 4–7. It can be observed that, in addition to surface roughness correction significantly affecting the obtained results, different values of the refractive index and extinction coefficient are obtained if different solvents are used for film preparation. This can either be a real effect or an experimental artifact as a consequence of existence of multiple solutions in spectroscopic ellipsometry data analysis. The latter can be caused by the use of inadequate optical model of the sample or it can be an inherent feature for that particular film/substrate combination.

Fig. 4. a) Extinction coefficient k and b) refractive index n of the films spincoated from chloroform solution. The values are obtained without roughness correction.

Fig. 5. a) Extinction coefficient k and b) refractive index n of the films spincoated from chloroform solution. The values are obtained with roughness correction using EMA.

Both anisotropic [6] and isotropic [8,18] models for the optical functions have been used for P3HT:PCBM blends. Since we used isotropic model to describe optical functions of the blend, we would expect to obtain similar results as those reported in the literature [8,18]. However, obtained data in Ref. [8] for all blend compositions except 1:1 exhibit high values of extinction coefficient (over 0.5) at 900 nm where P3HT:PCBM blend does not absorb. Also, while extinction coefficient shape for 1:1 blend appears reasonable, the refractive index value exhibits linear increase in near-infrared range [8], which is also unexpected behavior. We have observed during the fitting of the ellipsometry data that good fit can be obtained with extinction coefficient curves showing high values at long

Fig. 6. a) Extinction coefficient and b) refractive index of the films spin-coated from chlorobenzene solution. The values are obtained without roughness correction.

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wavelengths. However, all the solutions which did not exhibit similarity of the extinction coefficient shape with the measured absorption spectra were discarded as unphysical. On the other hand, while the extinction coefficient shape reported in Ref. [18] is reasonable, the obtained value is higher than in our work. It should be noted that measurements in Ref. [8] have been performed on a quartz substrate, while the SE sample preparation, measurements and data analysis details were not given in Ref. [18]. Thus, we have also prepared the films of P3HT:PCBM 1:1 blend on a glass substrate. Comparison of the obtained values of extinction coefficient and refractive index on two types of substrates, with and without effective medium approximation, is shown in Fig. 8. It can be observed that in the case of a glass substrate, significantly higher value of the extinction coefficient is obtained, although obtained value (without EMA correction) is in good agreement with previously reported optical functions of 1:1 P3HT:PCBM blends [18]. While the substrate used may affect roughness and topography of the samples, it is not expected to affect the values of the determined optical functions. Thus, the effect of the substrate on the estimated optical functions using SE requires further study. It is well known that reliable determination of the refractive index of transparent thin films on transparent substrate can lead to large uncertainties if the refractive index values of the film and the substrate are similar [19]. In addition, multiple solutions exist in a case of an absorbing film with a thickness much smaller than the optical penetration depth [20]. This is exactly the case for thin polymer films which are of interest for photovoltaic applications. In such cases, similar quality fit can be obtained for different combinations of thickness and optical functions of the film, and elimination of non-physical solutions does not guarantee that a unique solution would be found [20]. Furthermore, in the

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Fig. 8. a) Extinction coefficient and b) refractive index of the 1:1: P3HT:PCBM films spin-coated from chlorobenzene solution on Si and glass substrates.

case of a transparent substrate, it is necessary to either eliminate or take into account back-surface reflections [21]. Here we have used a common method of roughening the back surface [21,22]. However, refractive index difference up to ~ 0.1 can be obtained if a different method of dealing with back-surface reflections is used [21]. This can introduce additional uncertainties in the determination of the optical functions of films on transparent substrates. Thus, accurate determination of optical constants of polymer films using spectroscopic ellipsometry is a non-trivial and complex problem. Obtained results are strongly affected by the sample model, fitting procedure and the substrate used. In order to improve the results, it may be necessary to carefully consider the film morphology, use more advanced models for the sample structure, as well as consider the anisotropy of the layer. In addition, multiple sample analysis may be necessary to avoid the problem of multiple solutions [20]. 4. Conclusions We have demonstrated that the obtained values of optical functions of P3HT:PCBM polymer blends are strongly dependent on sample preparation, optical model of the sample, as well as the substrate. Multiple solutions yielding similar agreement with the experimentally measured ellipsometric quantities are possible. Further work is needed to establish procedures for obtaining unique and accurate optical functions of thin polymer blend films. Acknowledgements

Fig. 7. a) Extinction coefficient and b) refractive index of the films spin-coated from chlorobenzene solution. The values are obtained with roughness correction using EMA.

This work is partly supported by the Strategic Research Theme, University Development Fund, Seed Funding Grant and Outstanding Young Researcher Award (administrated by The University of Hong Kong). The authors would like to thank Dr. A. H. W. Ngan and T. K. Liu for AFM measurements.

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References [1] K. Kim, J. Liu, M.A.G. Namboothiry, D.L. Carroll, Appl. Phys. Lett. 90 (2007) 163511. [2] M. Reyes-Reyes, K. Kim, D.L. Carroll, Appl. Phys. Lett. 87 (2005) 083506. [3] G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang, Y. Yang, Adv. Funct. Mater. 17 (2007) 1636. [4] C.W. Chu, H. Yang, W.J. Hou, J. Huang, G. Li, Y. Yang, Appl. Phys. Lett. 92 (2008) 103306. [5] H.K. Persson, M. Schubert, O. Inganas, Sol. Energy Mater. Sol. Cells 83 (2004) 169. [6] U. Zhokhavets, T. Erb, H. Hoppe, G. Gobsch, N.S. Sariciftci, Thin Solid Films 496 (2006) 679. [7] H.K. Persson, H. Arwin, O. Inganas, J. Appl. Phys. 97 (2005) 034503. [8] E. Lioudakis, A. Othonos, I. Alexandrou, Y. Hayashi, J. Appl. Phys. 102 (2007) 083104. [9] H. Hoppe, N.S. Sariciftci, D. Meissner, Mol. Cryst. Liq. Cryst. 385 (2002) 233. [10] T. Erb, U. Zhokhavets, H. Hoppe, G. Gobsch, M. Al-Ibrahim, O. Ambacher, Thin Solid Films 511 (2006) 483.

[11] G.E. Jellison, Thin Solid Films 234 (1993) 416. [12] A.B. Djurišić, C.Y. Kwong, T.W. Lau, E.H. Li, Z.T. Liu, H.S. Kwok, L.S. M. Lam, W.K. Chan, Appl. Phys. A 76 (2003) 219. [13] Z.T. Liu, C.Y. Kwong, C.H. Cheung, A.B. Djurišić, Y. Chan, P.C. Chui, Synth. Met. 150 (2005) 159. [14] S. Logothetidis, J. Appl. Phys. 65 (1989) 2416. [15] P. Petrik, L.P. Biro, M. Fried, T. Lohner, R. Berger, C. Schneider, J. Gyulai, H. Ryssel, Thin Solids Films 315 (1998) 186. [16] S.J. Fang, W. Chen, T. Yamanaka, C.R. Helms, Appl. Phys. Lett. 68 (1996) 2837. [17] S. Nilsson, A. Bernasik, A. Budkowski, E. Moons, Macromolecules 40 (2007) 8291. [18] F. Monesteir, J.J. Simon, P. Torchio, L. Escoubas, F. Dlory, S. Bailly, R. de Bettignies, S. Guillerez, C. Defranoux, Sol. Energy Mater. Sol. Cells 91 (2007) 405. [19] K.M. Gustin, Appl. Opt. 26 (1987) 3796. [20] K. Järrendahl, H. Arwin, Thin Solid Films 313–314 (1998) 114. [21] D.J. Hayton, T.E. Jenkins, Meas. Sci. Technol. 15 (2004) N17. [22] K.O. Sylvester-Hvid, T. Ziegler, M.K. Riede, N. Keegan, M. Niggemann, A. Gombert, J. Appl. Phys. 102 (2007) 054502.