Perylene derivative sensitized multi-walled carbon nanotube thin film

Carbon 43 (2005) 2501–2507 www.elsevier.com/locate/carbon

Perylene derivative sensitized multi-walled carbon nanotube thin film W. Feng

a,*

, A. Fujii b, M. Ozaki b, K. Yoshino

b

a

b

School of Materials Science and Engineering, Department of Polymer Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan Received 12 December 2004; accepted 2 May 2005 Available online 23 June 2005

Abstract Perylene-sensitized multi-walled carbon nanotubes (PV-MWNT) have been prepared by a p-stacking between nanotubes and perylene derivatives, N,N 0 -diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PV). The resultant nanocomposites have been characterized by transmission electron microscope (TEM), UV–vis absorption, photoluminescence (PL) and photocurrent spectra. Long range ordering can be observed in the form of PV-MWNT via p-stacking by TEM. Red-shift in the optical spectra consisting of the UV–vis absorption and PL spectra with the attraction of PV on the surface of the MWNTs were observed. The evident quenching in PL spectra of PV-MWNT was ascribed to the absorption and transfer of recombination energy by MWNT. Photosensitivity spectra demonstrated an increasing photocurrent in the ultraviolet region and a broadening photosensitivity in the red spectral region for solar cells based on PV-MWNT.
2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Transmission electron microscopy; Luminescence; Optical properties; Photoconductivity

1. Introduction Organic planar molecules that present strong p–p interaction have been shown to be an appropriate model for studying the optical and electrical properties of thin solid films. Perylene derivatives N,N 0 -diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PV) exhibits comparably high electron affinity of the large band-gap material, large visible extinction coefficients, photostability and low cost of fabrication [1,2], which make them prime candidates for applications in electronic and optical devices such as field-effect transistors [3], electrophotographic applications [4], and photovoltaic devices [5]. As molecules, it is desirable to organize these chromophores spatially. This can be *

Corresponding author. Tel.: +86 22 278 90372; fax: +86 22 274 04724. E-mail address: [email protected] (W. Feng). 0008-6223/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.014

achieved by non-covalent interactions such as selfassembly via liquid crystals [6] or hydrogen bonding [7], in combination with p–p stacking [8]. Another interesting property is that PV can change their electron– hole-transporting characteristic by collaborating with conjugated polymer or organic molecule in solar cell [9]. Carbon based materials such as organic molecule and nanotubes have the potential to make an impact on a variety of applications ranging from general low cost circuits and displays to power devices, microelectromechanical systems (MEMS) to supercapacitors, sensors, solar cells and displays [10–12]. Carbon nanotubes have demonstrated a wealth of exceptional electrical, mechanical and thermal properties, which have made them useful for potential applications ranging from nanoelectronics to biomedical devices [13]. The formation of nanotube-organic composites has proved to be a promising approach to the effective incorporation of carbon nanotube into devices with possible synergetic

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effects [14]. Designing molecular species that can spontaneously self-assemble into well-defined structures paves the way to material systems with a wide range of applications. For this reason, hybrid molecular systems based on the combination of conjugated organic molecule and carbon nanotubes is currently of interest [15]. Reliable fabrication of such molecular electronic devices is based, to a large extent, on the optimization of molecular selfassembly on solid substrates [16]. It has been shown that by tuning the conditions of thin-film preparation, it is possible to drive the self-assembly towards highly ordered nano- and micro-architectures for deposition both from solution [17] and by sublimation [18]. However, the carbon nanotube itself is insoluble in organic solvent or water, which makes it difficult to form thin films and limits its application in many fields. Recently, considerable effort has been focused on the preparation of soluble carbon nanotubes both in fundamental and applied research areas [19,20]. Metallothionein proteins were trapped inside and placed onto the outer surfaces of open-ended MWNTs [21]. Porphyrin was found to adsorb on SWNTs presumably via Vander-Waals attraction between the nanotubes and porphyrin [22]. Polyaniline covered MWNT via acid–base reaction between emeraldine and MWNT were also realized [23]. Though several successful functionalization reactions for carbon nanotubes have been reported, to our knowledge, no study has been reported on the optical and photoresponse of perylene-MWNT composites. Our interesting has been focused on dye-sensitized multiwalled carbon nanotubes, which would enable further application in solar cells. In this study, we developed a new technique of processing PV-MWNT nanocomposite thin films at the molecular level. Our prepared process is based on the direct p-stacking between PV and MWNTs. PV can act as a good dispersant to absorb onto the sidewall of MWNTs via a p-stacking process (see Fig. 1). PV-MWNT nanocomposites have been characterized by UV–vis absorption, photoluminescence (PL) and photocurrent spectra.

2. Experimental 2.1. Preparation of PV-MWNT solution The raw soot for fabricating MWNTs was produced by the catalytic pyrolysis method and purified by filtration [24]. As a result of filtration treatment, the purity of the MWNT was above 90%. The MWNTs were treated with mixed acid according to a method already described [25]. Poly(3-hexylthiophene) (PAT6) was synthesized and purified as previously reported [26] and PV was purified by sublimating. All other solvents and chemicals were analysis reagent grade. The PV-MWNT nanocomposites were prepared by the following procedure described. 10 mg of PV was added into 40 ml of chloroform and the solution were kept boiling in a reflux system under stirring. The solution changed to a pink color gradually. After boiling them for 30 min, 50 mg of acid treated MWNTs were added, and then sonicated for 2 h at room temperature, followed by centrifugation of the suspension for 2 h to remove insoluble part. 2.2. Fabrication of photovoltaic device PAT6 at appropriate weight ratios are dissolved in PV-MWNT solution. The heterostructure is prepared by spin coating of solution of PAT6–PV-MWNT composite on a patterned ITO coated glass substrate with a sheet resistance of 10 X/h and following deposition of Al contact layer by thermal evaporation through the shadow mask. The active area of the device is 1 · 1 mm2. Typical thickness of PAT6–PV-MWNT film is about 0.1–0.2 lm. 2.3. Characterization Observations of the PV and PV-MWNT films were performed using a Hitachi 8000 transmission electron microscope (TEM). The absorption and photoluminescence spectra were measured using Hitachi 330 UV–vis spectrophotometer and fluorescence spectrophotometer

Fig. 1. Scheme of the PV adsorbing onto the sidewall of a MWNT via p-stacking.

W. Feng et al. / Carbon 43 (2005) 2501–2507

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(F-4500, Hatachi). Photoelectrical measurements were carried out in a vacuum optical cryostat in vacuum of about 10
5 Torr. Current–voltage characteristics were measured by a Keithley 617 picoammeter. A high intensity Xenon lamp (500 W) was used as UV–visible light source. The spectral response of the device was corrected for the response of the lamp-monochromator system by measuring the calibration spectrum with an UV-enhanced Si photodiode placed in the sample position.

3. Results and discussion PV being aromatic molecules with extensive p-system and considerable planarity, exhibit a clear tendency to self-alignment. So it irreversibly adsorbed onto the surfaces of MWNTs in an organic solvent chloroform or dimethylformamide. The perylene group, being highly aromatic in nature, is known to interact strongly with the basal plane of graphite via p-stacking [27], and also found here to strongly interact with the sidewalls of MWNTs in a similar manner. The three distinctly differently colored solutions are shown in Fig. 2. Fig. 2(a)–(c) correspond to the MWNT suspension, PV and PVMWNT solution in chloroform, respectively. MWNT is insoluble and precipitable suspension in chloroform, while PV exhibits a pink transparent solution in chloroform. A purple solution can be obtained upon the PV molecule adsorbing on the sidewall of MWNT. The changes of the solution color suggest that PV can dissolve/disperse MWNT by adsorbing onto the sidewall of MWNT. The PV absorbing onto the sidewall of carbon nanotube via p-stacking can also be confirmed directly by the TEM characterization. The images in Fig. 3 show that stable adsorption layers on the carbon-coated copper net are formed. As TEM photographs has shown (Fig. 3(a)), PV film consisted of vermiform crystalline parti-

Fig. 2. The solutions for MWNT (a), PV (b) and PV-MWNT (c) in chloroform.

Fig. 3. TEM images of the PV (a) and PV-MWNT (b) films.

cles with diameters of about 2–4 nm, and PV film appeared very homogeneous, indicating the characteristic of an amorphous film. Interestingly, in PV-MWNT film (Fig. 3(b)), PV-MWNT film appeared PV molecules densely absorbing on the carbon nanotubes, indicating the formation of PV-MWNT via p-stacking and an ordered crystalline film was obtained. Therefore, it could be concluded that in PV-MWNT film most PV molecules packed onto the sidewall of MWNT through p–p interactions to form a highly ordered crystalline film with a well-regulated aggregate structure. Preliminary analysis of the images indicates that PV molecules show a larger tendency to stacking and aggregation in solid film. In the PV-MWNT molecules however, the conjugated planes of the PV are most likely extended on the surface of the carbon nanotubes and are interdigitating

W. Feng et al. / Carbon 43 (2005) 2501–2507

in a manner. Furthermore, the size of PV aggregation turns to more fine and small owing to the p-stacking between carbon nanotubes and PV. From another point of view, the composite film may be more uniform in microcosmic owing to the mutual dispersing effect between PV and MWNT. A comparative study of the solution UV–vis spectra of PV and PV-MWNT shows in Fig. 4. Curve 1 and 2 correspond to the PV-MWNT and PV solution in chloroform, respectively. The UV–vis absorption spectrum for PV solution with the indistinct bands in the ranges 400–600 nm, which is the characteristic vibronic structure associated with the p–p* transition of the perylene moiety with broad maximum at 524 nm with shoulder at 564 nm. The absorption spectra for PV-MWNT in Fig. 4 curve 1 present a red-shifted component and reflected in the increase in the absorbance. The absorption spectra reveal now three peaks with broad maximum at 609 nm, followed by subsequent transitions 564 and 520 nm. The red-shift of the absorption onset observed for PV-MWNT in the ground state is consistent with the formation of p-stacking between the PV molecules and MWNT molecules. The shape and the position of the absorption bands of PV-MWNT are indicative of a dipole–dipole interaction, but no additional peak in the absorption spectrum is observed. As the p-stacking of the PV onto the sidewall of MWNT, the particle size of composite increases, the absorption peak becomes intense and shifts to higher wavelength. The high-wavelength shift is caused by the narrowing of the band gap due to the quantum size effects [28]. The observed absorption increase for higher wavelengths indicates that the transition for a lower vibrational level in the electronic excited state is the most probable. The optical response of molecules or molecular crystals is determined both by the electronic transitions and

1.0 1. PV-MWNT solution 2. PV solution

Absorption (arb.units)

0.8

0.6

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0.2 2 0.0 200

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Wavelength (nm) Fig. 4. UV–vis absorption spectra for PV-MWNT (1) and PV (2) solution.

1.6

1. PV-MWNT film 2. PV film

Absorption (arb.units)

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1.2

0.8 1

0.4 2 0.0 200

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Wavelength (nm) Fig. 5. UV–vis absorption spectra for PV-MWNT (1) and PV (2) films.

by the vibrational degrees of freedom, resulting in a pronounced vibronic structure of the linear absorption of PV-MWNT solution. In the solid phase, these internal dynamics of the molecules are combined with the possibility to transfer the excitation between different sites of the lattice. Furthermore, completely new types of excitons can be formed, involving charge transfer (CT) between adjacent molecules. The absorption spectra of the PV-MWNT and PV thin films were shown in Fig. 5. In the solid films, band broadening can be observed as a result of molecular packing. The PV film absorption spectrum shows p–p* electronic transitions band with main components centred at 537 nm with shoulder at 680 nm. The absorption spectrum of the PV-MWNT film exhibits the p–p* electronic transition band with one red-shifted component, the main peak at 590 and 662 nm with shoulder at 752 nm. The spectra consist in the characteristic vibronic structured absorption bands assigned to the p–p* electronic transitions of PV [29]. Thus, the close proximity of the molecules in the solid enables intermolecular interactions and formation of p-stacking, which plays a major role in the determination of the electronic and optical properties of PV-MWNT solid samples. The fact that electronic absorption bands of the PV-MWNT are markedly shifted to higher wavelength values is in agreement with the absorption of them in solution state. The PV-MWNT exhibits stronger tendencies to self organize via p–p stacking in solution and in solid. This p-stacking results in a change of the relative absorption intensities in the vibronic bands and red-shifts to higher wavelengths. p-stacking of between PV and MWNT not only affects the absorption, but also the photoluminescence spectra. The PL spectra presented in Fig. 6 are the PV-MWNT and the PV in chloroform solution, in which the excitation wavelength is at 480 nm. The spectrum of the PV solution consists in a broad featureless

W. Feng et al. / Carbon 43 (2005) 2501–2507

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Fig. 6. Photoluminescence spectra for PV-MWNT (1) and PV (2) solution.

Fig. 7. Photoluminescence spectra for PV-MWNT (2) and PV (1) films.

band with the maximum centred at 592 nm, which could be due to the excited state excimer-like interactions among neighboring peryleneimides [30–32]. In general, interactions between two or more chromophores among the PV molecules affect the luminescence properties. One option is excimer formation by the collision of an electronically excited molecule with a molecule in the ground state. The excimer is stabilized by excitonic interactions and charge transfer [33]. Excimers often exhibit a characteristic broad and unstructured, redshifted emission. On the other hand, the PV-MWNT films spectrum shows a broad band centred at 619 nm with two shoulders at 588 and 674 nm. The PV-MWNT showed pronounced progression in the red spectral region and more quenching of the photoluminescence. This luminescence quenching indicates that singlet excitons generated in PV are diminished before radiative recombination, due to the presence of the MWNT in addition to absorption and scattering by the MWNT. Furthermore, in this case, the MWNT acts as a nanometric heat sink, which dissipates the heat generated from the incident laser beam. Therefore, it should be noted that the observed PL quenching arises not only from absorption and multiple scattering by MWNT but also from energy transfer. In addition to the change in intensity, the maximal luminescence peak exhibits a red-shifted component from 592 nm of PV to 619 nm of PV-MWNT. This evidence suggests a greater extended molecular packing between PV and MWNT. Excimer-type emission has also been observed in the solid state. The PL spectra of the PV and PV-MWNT films are exhibited in Fig. 7. The PL spectra of PVMWNT and PV in films are quite different from the solution spectra. In the solid state, and in general in aggregated chromophores, coupling of transition dipole moments may occur, resulting in a splitting of electronic levels in the excited state. The photoluminescence prop-

erties of these exciton-coupled chromophores, strongly depends on their relative orientation and may show the characteristics of an excimer. The observation of a strong excimer emission for PV and PV-MWNT is attributed to the formation of organized structures or molecular stacking in the solid films, which requires the chromophore to be arranged with parallel and overlapping ring systems. A shifting of the PL maximum toward longer wavelengths is observed in the PV-MWNT film comparing with that in PV film. The excitation light is absorbed mainly by PV and the electron–hole pairs are generated in the PV-MWNT. A part of the recombination energy of the electron–hole pairs is transferred to MWNT. The quenching and red-shift effect is depended on the formation of a packing film with better morphology. UV/visible absorption and photoluminescence measurements indicate that PV-MWNT can be used as the photoactivation layer in photovoltaic cells. In our previous work, higher photovoltaic conversion efficiency was realized by blending soluble conjugated polymer with PV [9,34]. It has been shown that in these organic systems exciton dissociation and photoexcited electrons transfer onto the perylene occurs efficiently at interfaces between the conjugated polymer and perylene. Here, the photovoltaic effects in PAT6–PV and PAT6–PVMWNT are studied by fabricating single-layer devices, ITO/PAT6–PV/Al and ITO/PAT6–PV-MWNT/Al. Fig. 8 shows the photocurrent spectra of the devices under illumination by a 500 W Xenon lamp. A comparison of the results shows that the device fabricated from the PAT–PVMWNT exhibits the better performance with regards to photocurrent than fabricated that from the PAT–PV film, throughout nearly all of the visible wavelength range. The difference in photoresponse between the PV-MWNT cell can be explained as follows.

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W. Feng et al. / Carbon 43 (2005) 2501–2507 Table 1 Photovoltaic characteristics of devices under illumination

1. ITO/PAT6-PV-MWNT/Al

1.2

Photocurrent (µΑ cm2)

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0.8

Structure of device

Isc (lA/cm2)

Voc (mV)

FF (%)

g (%)

ITO/PAT6–PV-MWNT/Al ITO/PAT6–PV/Al

0.42 0.24

228 164

28 26

0.060 0.023

2 0.4

0.0 300

400

500

600

700

800

Wavelength (nm) Fig. 8. Photocurrent spectra of PAT6–PV-MWNT (1) and PAT6–PV (2).

Usually, in a dispersed heterojunction device, both the photocurrent generation and charge transport are functions of the morphology. Photocurrent generation requires uniform blending on the scale of the exciton diffusion length, while charge transport requires continuous paths from interfaces to contacts. The charge separation of the excitons occurs only at the interface between PAT6 and PV-MWNT (or PV) in the singlelayer devices. Since the PAT–PV-MWNT film possesses plentiful interfaces at not only PAT–PV but also PVMWNT, together with the favorable electron transport conductor of MWNT, the excitons separation and thecharge transport are easier and more efficient in the PAT–PV-MWNT film than in the PAT–PV film. The effective sensitized area is larger in PAT–PVMWNT interface where generate a higher current density than in PAT–PV interface. Moreover, the decrease of aggregate or ‘‘isolated island’’ of carbon nanotube owing to the p-stacking between PV and carbon nanotube in PAT–PV-MWNT film would improve the morphology of activated layer. The large interface and better morphology between donor and acceptor mean that the magnitude of the photogenerated charge carriers must be large. Thus, the photocurrent of ITO/ PAT–PV-MWNT/Al is larger than that of ITO/PAT– PV/Al. Apart from the increasing in intensity, the new photocurrent peak appears in the range from 640 to 690 nm in the PAT–PV-MWNT cell. This evidence suggests PV absorbing on the sidewall of carbon nanotubes via p stacking, which is accordant with the absorption spectrum of PV-MWNT film. In this case, the photoresponse is enhanced in longer wavelength due to extended molecular packing is notable. The photovoltaic characteristics of the devices under illumination are listed in Table 1. Both devices are irradiated with light at 500 nm and 45 l W/cm2 in intensity from the ITO electrode side. The fill factor (FF) is de-

fined as FF = ImaxVmax/IscVoc. Conversion efficiency (g) is defined g = Isc Æ Voc Æ FF/Popt(k), where Imax, VmaxIsc, Voc and Popt(k) are the current and voltage values for the maximum power point in the I–V curve under illumination, the short circuit, the open circuit voltage and the incident light flux in W cm
2, respectively. Comparing the conversion efficiencies between the ITO/PAT6–PV/ Al and the ITO/PAT6–PV-MWNT/Al device, we observed that the conversion efficiency in the ITO/PAT6– PV-MWNT/Al device was enhanced to more twofold that of ITO/PAT6–PV/Al owing to the increase in the effective interface area. It is indicated that a photoinduced electron transfer not only from the PAT6 to PV but also from PAT6 to MWNT contributes to the enhancement of charge separation and collection.

4. Conclusions MWNTs were functionalized and solubilized via p-stacking of PV onto the sidewall. In this specific complex nanocomposite, the underlying change revealed itself in the improvement of optical and photovoltaic properties of the resulting PV-MWNT. Long range ordering can be observed in the form of PV-MWNT via p-stacking by TEM. The red-shift in absorption spectra and the quenching in photoluminescence were discussed in terms of the p–p stacking between PV and MWNT. Photovoltaic device fabricated with the composite with PV-MWNT were also found to be photosensitive. The enhanced photoconductivity and strongly quenched photoluminescence are explained tentatively by an efficient photoinduced charge separation at the interface of PAT–PV and PAT–MWNT and the separated charge transfer by MWNT and PV.

Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 60307001) and the Natural Science Foundation of Tianjin City.

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