Photovoltaic properties of polythiophene nano-tubule films

Photovoltaic properties of polythiophene nano-tubule films

Materials Chemistry and Physics 82 (2003) 44–48 Photovoltaic properties of polythiophene nano-tubule films Jian Cao a , Jingzhi Sun a , Gaoquan Shi b...

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Materials Chemistry and Physics 82 (2003) 44–48

Photovoltaic properties of polythiophene nano-tubule films Jian Cao a , Jingzhi Sun a , Gaoquan Shi b , Hongzheng Chen a , Qinglin Zhang c , Dejun Wang c , Mang Wang a,∗ a

Institute of Polymer Science and Engineering and State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China b Department of Chemistry, Tsinghua University, Beijing 100084, PR China c Department of Chemistry, Jilin University, Changchun 130023, PR China Received 31 July 2002; received in revised form 11 November 2002; accepted 20 November 2002

Abstract The electronic properties of the polythiophene nano-tubule films were studied with surface photovoltage spectrum (SPS) and field-induced surface photovoltage spectrum (FISPS). The surface photovoltaic responses were resulted from the ␲–␲∗ transition of polythiophene chains. Two extra photovoltaic responses in the near-IR region were observed under the external electric field. Based on the band theory and the principle of FISPS, these responses were ascribed to the charged surface electronic states, which were led by the interaction between polythiophene nano-tubules and the oxygen absorbed on the surface. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface photovoltaic response; Electronic structure; Localized states

1. Introduction Recently, nanoscale materials have attained considerable attention because of their unique opto/electronic properties [1,2]. Since the discovery of carbon nano-tubes in 1991 [3], great research efforts have been focused on the design and preparation of new conducting polymer micro/nano-tubules [4–6]. Micro/nano-tubules of polyanilines and polypyrroles have been reported by several groups [7–10]. With the aid of templates made from microporous alumina membranes, polythiophene nano-tubules have recently been synthesized through a direct electrochemical polymerization method [11]. As a novel organic semiconducting polymeric material, the polythiophene nano-tubules show promising application potential in light-emitting diodes, field effect transistors and electrically pumped lasers [12]. It is crucial important to understand the electronic structures of the polythiophene nano-tubule films because they are fateful factors to the opto/electronic properties. In consideration that surface photovoltage spectrum (SPS) is a highly sensitive tool to study the photophysics of the photogenerated species or excited states without any samplecontamination and destruction [13], this technique has been used to investigate the photoinduced processes such as ∗ Corresponding author. Tel.: +86-571-8795-2557; fax: +86-571-8795-1635. E-mail address: [email protected] (M. Wang).

charge transfer and photocatalysis [14,15]. On the other hand, electric field-induced surface photovoltage spectrum (FISPS) is a modified technique combining the field effect principle with SPS. With an external voltage applied to the two sides of the sample, the mobile direction and the diffusion length of the photogenerated charge carriers can be altered. Moreover, the space charge density and the electronic state of the molecule can be changed. Its superiority in studying the electronic structure and charge behavior of the organic thin films has been showed by our previous work [16]. In this paper, integrating the technique of SPS and FISPS, the electronic structure and the charge behavior of the polythiophene nano-tubule films have been investigated. Besides the intrinsic ␲–␲∗ transition, it is noted that there are two new photovoltage responses which is assigned to the localized electronic states transition appearing under the external electric field. The new responses are attributed to the charged species caused by O2 in the atmosphere. 2. Experimental The preparation process of polythiophene nano-tubules have been reported in detail elsewhere [11]. Here we describe it briefly as follows: as shown in Fig. 1, after a gold layer was deposited onto one side of the alumina membrane template, which had a pore size of 20 nm, an ITO layer contacted closely to the gold layer as a current collector. Then

0254-0584/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00188-3

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Fig. 1. Schematic representation of the methods used to synthesize the polythiophene nano-tubule.

Fig. 2. The structure of the photovoltaic cell for measuring the SPS and FISPS.

the polythiophene tubules were grown in the nanoporous of the template by direct oxidation of thiophene in boron trifluoride diethyl etherate (BFEE) solution. Aligned nano-tubule film was obtained by dissolving the template in 1 M KOH solution. SPS and FISPS were measured with a solid junction photovoltaic cell (ITO/sample/ITO) using light source– monochromator–lock-in detection technique. Monochromatic light was obtained by passing light from a 500 W xenon lamp through a double-prism monochromator (Hilger and Watts, D300). A lock-in amplifier (Brookdeal 9503-SC), synchronized with a light chopper, was employed to amplify the photovoltage signal. The principle and its schematic diagrams were discussed elsewhere [16]. In the following discussion, the sign of the applied field is that of the irradiated electrode with respect to the back electrode (Fig. 2). The measurement was performed under the atmospheric pressure and at ambient temperature (about 25 ◦ C). Reflection UV-Vis absorption spectrum was obtained with a Perkin-Elmer Lambda 20 spectrometer.

are illustrated in Fig. 3. One characteristic absorption band assigned to the intrinsic ␲–␲∗ transition appears at 490 nm (2.5 eV). Compared to the value reported in the literature [17], this absorption band has blue-shifted a little, which attributed to the nano-size effect. The profiles of the SPS and the UV-Vis absorption spectrum resemble each other except in the short-wavelength and long-wavelength regions. The spectral resemblance suggests that the band-to-band transition is the major contribution to the SPV [18]. The discriminations are ascribed to the different mechanism of the optical absorption and the SPS. When the absorption is weak, the photons can penetrate deep into the sample, hence it is difficult for the illumination-induced electrons or holes moving to reach the surface. Furthermore, the change of the surface net charges is small and therefore the signal of the SPV response is too weak to be detected duo to the apparatus’ sensitivity. According to the band theory of inorganic semiconductors, the bonding orbital is analogous to the valance band and the anti-bonding orbital to the conduction band in organic semiconductors. In polythiophene nano-tubule film, electron–hole pairs are generated under the illumination.

3. Results and discussion 3.1. Photo-induced charge separation in polythiophene nano-tubule film The reflection UV-Vis absorption spectrum and the surface photovoltage (SPV) response without an external field

Fig. 3. The UV-Vis spectrum (dashed line) and SPS (solid line) of the polythiophene nano-tubule films.

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Fig. 4. SPS and FISPS of polythiophene nano-tubule films under the biases of 0 and ±0.5 V in: (a) UV-Vis region; (b) near-IR region.

Photogenerated holes move in the valance band and the photogenerated electrons in the conduction band. Under the built-in field, the moving of the photogenerated electrons and holes results in the change of the surface net charges and the SPV is produced. From the SPS, it can be derived that polythiophene nano-tubule film has a relatively high efficiency of photo-induced charge separation. 3.2. FISPS of the polythiophene nano-tubule film The changes of the SPV response under an external electric field are shown in Fig. 4(a) and (b), respectively. The SPV responses are distinctly different under a bias of +0.5 and −0.5 V. When a bias of +0.5 V was applied, the SPS intensity was greatly enhanced. While the SPV response decreased and even inversed to the opposite direction when a bias of −0.5 V was applied. As depicted in Fig. 5, a signal on the SPS feature-line is the change in the surface potential (␦Vs = Vs − Vs0 , where Vs0 is the surface potential before illumination, Vs the surface potential after illumination). For a p-type semiconductor, the surface band bend is usually downward. When a positive electric field (the illumination surface is positive) whose direction coincides with the built-in field is applied on the

two sides of the nano-tubule film, the surface band bending increases downward, so the photogeneration carriers separated more efficiently and a large amount of electrons move to the surface. Consequently, the intensity of SPV response will increase in the original direction. If a negative electric field (whose direction is reverse to the built-in field) is applied, the surface band bending decreases. Thereby the separation efficiency of photogeneration carriers decreases and the intensity of SPS signal reduces, even changes to the reverse direction. For an n-type semiconductor, a reverse phenomenon should be observed. As can be seen in the FISPS of polythiophene film, it is reasonable to conclude that the direction of surface band bending formed between polythiophene film and the irradiated ITO electrode is downward. Hence, polythiophene film has a p-type character. This is in agreement with the assignment by other methods [19]. The alternation of polythiophene’s conducting type can be achieved by introducing different dopants, and controlling over the doping level [20,21]. On the other hand, it is interesting that two new SPV responses are found from the FISPS of the polythiophene nano-tubule film under the external electric field. They are located in the regions of PN1 (620–750 nm) and PN2 (800–1000 nm) and show anti-asymmetrical tendency under the external electric field. No corresponding absorption bands are observed in the UV-Vis absorption spectrum. According to the principal of FISPS [22], these two new bands can be assigned to the electronic transition of localized states. Conducting polymers containing aromatic or heterocycles, such as polypyrrole and polythiophene, have a nondegenerate ground state. Polarons and bipolarons caused by doping are the important excitation species and the dominant charge storage configurations [12]. Their energy levels appear symmetrically with respect to the gap center [23]. In our experiments, we did not observe the sub-bandgap absorption besides the intrinsic ␲–␲∗ transition. Furthermore, the energy of the new photovoltaic response bands does not meet the equation EPN1 + EPN2 = Eg [24]. We also measured the excited Raman spectrum of the polythiophene nano-tubule [11]. It was revealed that the Raman bands with respect to the oxidized (doping) state were very weak and the polythiophene nano-tubules almost might be a neutral species. The main reason is that the nano-tubule skin is rather thin. XPS results demonstrated that the doping level of polythiophene film increased with increasing film thickness if the film was electrochemically synthesized at a constant applied potential [11]. Hence we suggest that the local sub-bandgap photovoltaic responses were introduced by the O2 in the atmosphere. The polythiophene nano-tubules have large surface areas and can easily absorb the O2 in the air. The O2 can react with the neutral molecular and induce charged species [25]. The charged species can form sub-bandgap local energy levels, which are located between those of the CB and VB. Electronic transitions of local states are forbidden, then photogenerated charge carriers are bounded in localized state energy band and

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Fig. 5. Band structure representation of a p-type semiconductor with a Schottky-type barrier: (a) without a bias; (b) with a positive electric field; (c) with a negative electric field. ␦VS , ␦VS , ␦VS : surface potential difference; VS0 : the potential before illumination; VS , VS , VS : the surface potential after illumination; CB: conduction band; VB: valence band; EF : Fermi level.

Fig. 6. The band structure of the localized transition for a p-type semiconductor: (a) without a bias; (b) with a positive electric field; (c) with a negative  : surface potential difference; V 0 : the potential height before illumination; V  , V  : the potential height after illumination. electric field. ␦VSE , ␦VSE SE SE SE

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cannot move freely, so we cannot observe the SPV response or the response is weak without the induction of an external electrical field. When an external electrical field is applied, the localized state energy band tilts along the direction of it. Its optical constant is changed and this results in a larger transition momentum. So the probability of optical transition increases [26]. The lifetime of the photogenerated carriers increases and the odds of the recombination of the photogeneration carriers decreases. As a result, the SPV response can be observed under the effect of an external field. Carriers move under the external electric field and a certain amount of surface net charge is produced. When a positive electric field is applied, the electrons move to the surface and the holes move to the bulk. So the SPV response is positive. If a negative electric field is applied, the photogeneration electrons move to the bulk and the holes move to the surface. Therefore, the SPV response is a negative one (Fig. 6). 4. Conclusions The electronic states of polythiophene nano-tubule films were investigated with SPS and FISPS techniques. The large surface area of electrochemically polymerized polythiophene nano-tubules allowed the tubules to absorb the oxygen atoms in the atmosphere. The interaction between oxygen and polythiophene chains resulted in the formation of charge surface states. These states could be reflected on the photovoltage responses, which were characterized by the response of the local electronic states in the near-infrared region. Acknowledgements This research was supported by the National Natural Science Foundation of China with grant number 90101008.

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