An attempt to measure simultaneously molecular orientation and current–voltage characteristics in thin films

An attempt to measure simultaneously molecular orientation and current–voltage characteristics in thin films

Thin Solid Films 517 (2008) 1358–1361 Contents lists available at ScienceDirect Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i ...

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Thin Solid Films 517 (2008) 1358–1361

Contents lists available at ScienceDirect

Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f

An attempt to measure simultaneously molecular orientation and current–voltage characteristics in thin films Takeshi Komino ⁎, Hiroyuki Tajima, Masaki Matsuda The Institute for Solid State Physics, The University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan

a r t i c l e

i n f o

Available online 13 September 2008 Keywords: Electronics Molecular orientation Polarized absorption Spin coating

a b s t r a c t To investigate the relationship between molecular orientation and current voltage (J–V) characteristics, molecular orientation and J–V characteristics have been simultaneously measured in some indium tin oxide (ITO)/X/Al structures. X were thin films fabricated from poly(3-hexylthiophene) (P3HT), coumarin6 dispersed in poly(N-vinylcarvazole), or biomolecular hemin (Hm). The results have shown that the orientation change of the P3HT chain causes a reproducible loop of the J–V characteristics in P3HT thin film, and that the peak observed in the J–V characteristics in Hm is associated with irreversible molecular orientation change. © 2008 Elsevier B.V. All rights reserved.

1. Introduction In organic electronics, studies related to molecular orientation are very important because the molecular orientation state affects electronic conduction. Generally, these studies have considered that molecular orientation in thin films is fixed. Even in thin films, however, molecular orientation can be changed under an electric field [1–3]. Such changes can improve the unusual current– voltage (J–V) characteristics. We have therefore attempted to simultaneously measure molecular orientation and J–V characteristics. Molecular orientation was proved by polarization dependence of intramolecular electronic transition (polarized absorption) [4–6]. In this paper, we report on the results of this method's application to some organic thin films. We have chosen poly(3-hexylthiophene) (P3HT) [7], coumarin6 (C6) dispersed in poly(N-vinylcalvazole) (PVK), and biomolecular hemin (Hm) as sample materials for three reasons: (i) P3HT reportedly exhibits a correlation between the orientation state and electric conduction [8,9], (ii) C6 possesses a relatively large dipole moment (ca. 7.8 D) [10], and (iii) An unaccountable peak appears in the J–V characteristics in Hm [11]. 2. Experimental P3HT, C6, and Hm were purchased from Sigma-Aldrich Co. (regioregular poly(3-hexylthiophene-2,5-dilyl); coumarin6 98%;

⁎ Corresponding author. Tel./fax: +81 4 7136 3239. E-mail address: [email protected] (T. Komino). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.008

chlorohemin N 80%). PVK was purchased from Tokyo Chemical Industry Co. All of the samples were used without further purification. Fig. 1 shows the molecular structures. The thin films were fabricated from chloroform solutions (in P3HT and C6:PVK) or methanol solution (in Hm) on an indium tin oxide (ITO) electrode by a spin-coating technique. The ratio of PVK:C6 was 5:1 in weight. The thicknesses of the P3HT, PVK:C6, and Hm films were 100 nm, 50 nm, and 30 nm, respectively. After the films were fabricated, Al was deposited on them by vacuum deposition. The evaporation rate and film thickness of Al were 0.1–0.2 nm/s and 15 nm, respectively. The J–V characteristics were measured by applying positive and negative voltages to an ITO and an Al electrodes, respectively. Simultaneously, to prove the orientation state, the differences in the intensities (ΔI = Ip − Is) and in the total intensity (I = Ip + Is) of p- and spolarized transmitted lights were measured at an incident angle (θ) of 45° (polarized absorption). Herein, Ip and Is are the intensities of pand s-polarized transmitted lights, respectively. Before the measurement, ΔI was tuned to zero for initialization (Ip = Is = I0 / 2 in the initial state). The experimental setup and detailed procedure are described elsewhere [7]. The samples were excited at 550 nm (P3HT), 470 nm (C6), and 390 nm (Hm), corresponding to the π − π⁎ transition energy of each molecule (Figs. 1 and 2). All the measurements were conducted in air. 3. Polarized absorption In rod-like molecules, ΔI/I and 1 − I/I0 can be given by Eqs. (1)– (4) [7]. Herein, β, d, θ, and α are the absorption coefficient, film thickness, the angle between the substrate surface and incident light, and the angle between the normal of a substrate and the

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Fig. 1. The molecular structures of (a) C6, (b) PVK, (c) Hm, and (d) P3HT. The arrows shown in (a), (c), and (d) indicate the directions of transition dipoles.

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Fig. 3. Models for molecular orientation in thin films on ITO substrate. n, normal of ITO substrate; d, film thickness; θ, angle between the sample surface and the incident beam. The broken line represents optical length in a thin film. In rod-like molecules (left), μ and α represent the average transition dipole of whole P3HT molecules on the incident beam and the angle between n and μ. On the other hand, in disc-like molecules (right), γ is the angle between n and the normal of the molecular plane.

average transition dipole of whole molecules on the incident beam (Fig. 3).   ΔI βd sinα cos α ¼ 3 cos2 θ Δα þ ð A−BÞΔðβdÞ; I 2 sin θ

1−

 βd sinα cos α  I Δα þ ð A þ BÞΔðβdÞ: ¼ 2 sin2 θ− cos2 θ I0 2 sin θ

like molecules can be calculated as it is in rod-like molecules [7], as given in Eqs. (7)–(10). ð1Þ

ð2Þ

  1 βp ¼ β 1− cos2 θ cos2 γ− sin2 θ sin2 γ 2

ð5Þ

  1 βs ¼ β 1− sin2 γ 2

ð6Þ

ð7Þ



sin2 α ; 4 sinθ

ð3Þ

βd sinγ cos γ  ΔI  2 ¼ sin θ−2 cos2 θ−1 Δγ þ ðC−DÞΔðβdÞ I 2 sin θ



2 cos2 θ cos2 α þ sin2 θ sin2 α : 4 sinθ

ð4Þ

1−

On the other hand, in disc-like molecules, absorption coefficients in polarized light and in non-polarized light are related by Eqs. (5) and (6). Herein, subscripted p/s represents polarization direction, and γ is the angle between the normal of a substrate and normal of the molecular plane (Fig. 3). Using Eqs. (5) and (6), ΔI/I and 1 − I/I0 in disc-

 βd sinγ cos γ  I Δγ þ ðC þ DÞΔðβdÞ ¼ 2 cos2 θ− sin2 θ−1 I0 2 sin θ



2− sin2 γ 4 sinθ



2−2 cos2 θ cos2 γ− sin2 θ sin2 γ 4 sinθ

ð8Þ

ð9Þ

ð10Þ

Eqs. (1)–(4) (or Eqs. (7)–(10)) demonstrate that Δα (or Δγ) and Δ(βd) are capable of causing the changes in ΔI/I and 1−I/I0. From the ratio (ΔI/I)/ (1 −I/I0), however, we can clarify which contribution is dominant. In rodlike molecules, the ratio is given by (3cos2θ) /(2sin2θ −cos2θ) (=3 at θ=45°) when Δ(βd)=0. On the other hand, it is (A −B)/(A +B)b 1 when Δα=0. In disc-like molecules, the ratio is given by (sin2 θ − 2cos2 θ − 1) / (2cos2θ−sin2θ−1) (=3 at θ =45°) when Δ(βd)=0, but is (C−D)/(C+D)b 1 when Δγ =0. 4. Results and discussion 4.1. P3HT

Fig. 2. The absorption spectra of Hm film, C6 solution in chloroform, and P3HT film. PVK used as polymer binder of C6 is transparent in the visible region.

Fig. 4a shows the J–V characteristics in P3HT. A hysteresis loop appeared in the J–V characteristics. Each loop is traversed as indicated by arrows, and can be cycled repeatedly. Similarly, as shown in Fig. 4b and c, hysteresis loops appeared also in the ΔI/I − V and the (1 − I/I0) − V characteristics. The ratio (ΔI/I)/(1 − I/I0) is approximately 5. This

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4.3. Hm Fig. 6a shows the J–V characteristics in Hm. A peak in those characteristics appeared at 4.5 V. As reported previously [11], the peak appeared only in the initial sweep of the voltage. As can be seen in Fig. 6b and c, ΔI/I and 1 −I/I0 were changed from 4.5 V when we increased the bias voltage, and did not reverse to zero when we decreased the voltage. The ratio (ΔI/I)/(1 −I/I0) is approximately 3. This corresponds to θ = 45°, which is the same as the incident angle of θ = 45°. This indicates that Δ(βd) is negligible in this case, and that the changes in ΔI/I and 1 −I/I0 are caused by the molecular orientation change. Because the coefficient of Δγ is negative at θ = 45° in Eq. (7), ΔI/I is negatively proportional to Δγ. Therefore, the result shown in Fig. 6b suggests that the molecular plane of Hm tends to be upright on the substrate when we apply a bias voltage above 4.5 V. This indicates that the peak observed in the J–V characteristics is associated with irreversible molecular orientation change. (Although we could not confirm the external quantum efficiency of electroluminescence and electroluminescence spectra in this measurement, irreversible transitions of external quantum efficiency and electroluminescence spectra are also expected at 4.5 V [11].) One may consider that the electric field does not cause the change of molecular orientation for both of Hm and P3HT, since these molecules do not possess a permanent dipole. In the present case, however, electrons and holes are injected from electrodes, and the excited molecules are formed. Since molecules in this state have excited dipole, its orientation should be changed by the electric field. The point of the above discussion is that the charge injection is a requirement for the molecular orientation change. This is consistent

Fig. 4. (a) The J–V, (b) ΔI/I − V, and (c) (1 − I/I0) − V characteristics in P3HT. The arrows represent the sequence of the voltage sweep.

corresponds to θ = 42°. This value is roughly consistent with the incident angle of θ= 45°. This indicates that Δ(βd) is negligible in the present case, and that the changes in ΔI/I and 1 − I/I0 are caused by the molecular orientation change. Because Eq. (1) indicates that ΔI/I is proportional to Δα, the result shown in Fig. 4b suggests that the main chain of P3HT tends to be upright (or horizontal) on the substrate when we increase (or decrease) the bias voltage. Since the molecular orientation affects the electric characteristics [8,9], we speculate that the hysteresis of J– V characteristics was caused by the molecular orientation change. Although its underlying mechanism in detail is not yet known, this orientation change may be associated with the increase in the electric field or joule heating. 4.2. PVK:C6 In PVK:C6, clear molecular orientation change was not observed at room temperature. To confirm whether or not molecular orientation change has occurred, the changes in ΔI/I and 1 − I/I0 at higher film temperature (ca. 40 °C) have been investigated using a heater attached to the substrate, since molecular orientation change is dependent on temperature. The film temperature was raised under applying a bias voltage of 9.5 V. Fig. 5a, b, and c show ΔI/I, 1 − I/I0, and voltage as a function of time, respectively. As shown in Fig. 5a and b, ΔI/I and 1 − I/I0 were changed when we raised the film temperature. However, these changes were not caused by molecular orientation change, since (ΔI/I)/(1 − I/I0) b 1. We consider that the changes in ΔI/I and 1 − I/I0 were caused by thermally assisted spectral change. Since molecular orientation did not change at ca. 40 °C, nor should it change at room temperature.

Fig. 5. Transient measurement of (a) ΔI/I, (b) 1 − I/I0, and (c) voltage. The film was heated for 150 s as indicated by the arrows.

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such a change of carrier mobility is the origin of the hysteretic loop of the J–V characteristics. 5. Summary Our measurement of J–V, ΔI/I, and 1 − I/I0 characteristics in P3HT, PVK:C6, and Hm thin films revealed that the orientation change of the P3HT chain causes a reproducible loop of the J–V characteristics in P3HT thin film, and that the peak observed in the J–V characteristics in Hm is associated with irreversible molecular orientation change. These results indicate that the proposed measurement method can be useful in investigating the relationships between molecular motion and unusual electric characteristics in a thin film. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas of Molecular Conductors (No. 15073207) from the ministry of Education, Culture, Sports, Science and Technology, Grantin-Aid for Scientific Research (B: No. 18350070) from the Japan Society for the Promotion of Science, and Global COE Program for Chemistry Innovation. References [1] [2] [3] [4] [5] [6] Fig. 6. (a) The J–V, (b) ΔI/I − V, and (c) (1 − I/I0) − V characteristics in Hm. [7] [8]

with the experimental results, i.e., molecular orientation change does not occur when the current flow is negligible. Another important point for this phenomenon is that the change of molecular orientation should cause the change of molecular overlap, resulting in the change of carrier mobility. We are considering that

[9] [10] [11]

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