Facile fabrication of an organic semiconductor/graphene microribbon heterojunction by self-assembly

Abstract

Herein, we report a facile self-assembly strategy to prepare a novel 1D organic semiconductor/graphene microribbon heterojunction by coating a layer of graphene sheets on the organic semiconductor microribbon. The organic semiconductor microribbon composed of a p-type small molecule 3,7-bis(5-(2-ethylhexyl)thiophen-2-yl)dithieno[2,3-b:2′,3′-e]pyrazine (BEHT-DTP) was prepared by evaporation-induced self-assembly. Subsequently the graphene nanosheets, as an electron acceptor, were self-assembled onto the surface of a BEHT-DTP microribbon in aqueous solution to form a 1D p–n junction. The device based on the single microribbon heterojunction demonstrated enhanced photoconductivity properties. This preliminary work points out a new path to fabricate 1D organic nano/micro-heterojunctions, avoiding complex molecular design and equipment.

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One-dimensional (1D) organic semiconductor nano/micro-wires have demonstrated exciting photoelectric properties and displayed excellent performance in organic field transistors (OFET),^1,2 organic solar cells (OSC),^3,4 organic photoconductors^5,6 sensors,^7,8 etc. However, most of the 1D organic semiconductor nano/micro-materials or devices are fabricated by one single material. Limited by the absence of driving force provided by p–n junction interface for the dissociation of excitons, the homogeneous 1D nano/micro-material is unfavorable for photoelectric conversion devices such as solar cells or photoconductors. Therefore, fabricating appropriate 1D nano/micro-heterostructure material with proper complementary materials is crucial for realizing high-efficient 1D organic nano/micro-photoelectric conversion devices. As so far, the development of organic semiconductor 1D nano/micro-heterojunction is in the initial state due to the lack of appropriate complementary materials and mild fabrication techniques. The techniques for the fabrication of inorganic heterostructure such as oxidation, diffusion and ion implantation are difficult to be applied to organic materials, because they are not mild enough for organic materials. Although several 1D nano/micro-organic heterojunction materials have been fabricated successfully recently by physical vapor transport technique^9,10 or simple self-assembly in solution.^6,11 However, most of these 1D heterojunction nano/micro-materials rely on molecular design and complex preparation procedure. It's necessary to develop a versatile method to fabricate 1D organic semiconductor nano/micro-heterojunction in a facile method.

For fabricating nano/micro-heterogeneous materials, layer-by-layer (LbL) self-assembly technique is a versatile, efficient and facile strategy. It has been widely applied in the assembly of water-soluble polyelectrolyte, nanomaterials or biomolecules.^12–14 Recently, graphene has attracted much attention for its superior structural and electronic properties.^15–20 It can be modified with oxygenated functional groups (–OH, –COOH, epoxide), which will both introduce functionality and facilitate graphene to be dispersed in water,^21–23 providing an accessible platform to self-assemble in water solution. The coupling of graphene with some semiconductors is an efficient way to enhance their photoelectric properties.^24,25 Because of its two-dimensional (2D) morphology and high electron mobility, graphene is an excellent platform to combine with semiconductors to accept and transport electrons in the photovoltaic or photocatalytic processes.^25–27 The planar 2D sp² carbon network of graphene can facilitate the π–π interaction with the π-conjugated organic semiconductors,^25,28 which is beneficial to combine graphene with organic semiconductors based on non-covalent interaction.²⁹ It can be inferred that, the water dispersible graphene nanosheets may be assembled onto 1D organic nano/micro-material via π–π interaction by LbL self-assembly in water, which will keep the 1D organic nano/micro-material from organic solvent. Graphene/inorganic 1D nano/micro-material composite has attracted much attention,³⁰ while its organic counterpart has rarely been reported, especially for 1D organic semiconductor nano/micro-heterojunction.

Here, we have prepared a microribbon composed of an air-stable electron donor material BEHT-DTP by evaporation-induced self-assembly. Thereafter the water-soluble graphene can be assembled onto the surface of the microribbon by a facile self-assembly process, forming a novel heterostructure microribbon. The hybrid microribbon showed efficient photoconductivity.

As illustrated in Scheme 1, the BEHT-DTP/graphene heterostructure microribbon was prepared by a two-step self-assembly method. Firstly, the BEHT-DTP microribbon was prepared via convenient evaporation-induced self-assembly process by drop-casting BEHT-DTP/THF solution on a Si/SiO₂ substrate. The p-type small molecule BEHT-DTP was synthesized using dithieno[2,3-b:2′,3′-e]pyrazine (DTP) as a building block, as we have reported.⁶ The molecular structure is displayed in Scheme 1. The planar and rigid DTP cores of the molecules may stack on each other by the strong π–π interaction between the molecules, leading to crystalline material. The N atoms in the DTP moiety endows it anti-oxidation ability, which makes it stable in air. Thus, it is appropriate for fabricating organic photoelectric device used under ambient condition. With the evaporation of the solvent, the concentration of the solution increased and crystal seeds formed, then BEHT-DTP grew on the substrate, forming crystalline microribbon. As the evaporation speed of the solvent was quite low, the self-assembly process of the microribbon tended to form a thermodynamically stable structure, resulting in a large-size aligned 1D crystalline (Fig. 1a and S1a†). If we speed up the evaporation by removing the “antisolvent” and the beaker was kept open, the solvent would evaporate in a few minutes, the concentration of the solution would increase dramatically, resulting in numerous crystal seeds, leading to scattering small anomalous kinetically favorable microneedle instead of microribbon (seen in the Fig. S1b†). This result manifests that it’s crucial to control the evaporation rate for fabricating uniform large scale 1D organic nano/micro-materials. The uniform thin ribbon-like 1D material is about 10 μm in width, hundreds of micrometers to several millimeters in length. Selected area electron diffraction (SAED) (JEM-2010) manifested that the microribbon was well crystalline (Fig. 1b). However, it should be noted that the high-power electron beam would damage the crystalline structure, if the microribbon is exposed to electron beam for a long time, the diffraction pattern will distort or disappear.

Scheme 1 The scheme diagram of the self-assembly procedure of BEHT-DTP/graphene heterostructure microribbon.

Fig. 1 (a) The optical microscope image of BEHT-DTP microribbons and (b) TEM image and the SAED pattern of a BEHT-DTP microribbon. Scale: 10 μm.

In the second step, the BEHT-DTP/graphene hybrid microribbon was fabricated in a facile self-assembly process. Similar to the conventional LbL self-assembly process, the microribbon was immersed in the graphene dispersion to assemble the graphene sheets onto the surface of BEHT-DTP microribbon. The critical point for a two-step self-assembly method to fabricate 1D organic semiconductor nano/micro-heterostructure is that the latter solvent should not alter or dissolute the structure of the original nano/micro-structure. Due to the graphene was dispersed in water which is a poor solvent to BEHT-DTP, it can be inferred that such an assembly process is an ideal method for fabricating BEHT-DTP/graphene 1D heterostructure material. To confirm that, we have immersed the BEHT-DTP microribbon into a graphene ethanol solution, as BEHT-DTP is slightly soluble in ethanol, the treatment resulted in the disappearance of microribbon (data is not shown here). In contrast, since the graphene water dispersion is benign to the BEHT-DTP microribbon, the microribbon heterostructure was formed successfully in the graphene water dispersion without altering the morphology of the original microribbon. Consequently, the graphene sheets were adsorbed onto the surface of BEHT-DTP microribbon via π–π interaction.

The formation of BEHT-DTP/graphene hybrid microribbon has been confirmed by atomic force microscope (AFM) (Nanoscope IIIa) measurements. As shown in Fig. 2a, the pristine BEHT-DTP microribbon is smooth in surface without any obvious attachment, the zoom in image in the inset demonstrates that its roughness is about 1.68 nm (RMS). However, for the BEHT-DTP/graphene hybrid microribbon shown in Fig. 2b, we can find that there is a continuous smooth thin film adsorbing on the surface of the microribbon. We observed different regions of a hybrid microribbon in low magnification and found that almost the entire surface of the microribbon was covered by graphene (seen in Fig. S2†). When zoom in (the inset in Fig. 2b), the stacked nanoscale graphene sheets can be observed obviously. The graphene sheets stacked on each other with step heights of about 1.5 nm (see the cross section profile in Fig. S3†), compared to the thickness of the graphene sheets, indicating the graphene sheets are stacked densely. The roughness of the surface is 1.59 nm (RMS), which is compared to that of the pristine BEHT-DTP microribbon, indicating that the covering of graphene sheets on the microribbon would not increase the roughness of its surface obviously. The tensely stacked graphene film would serve as excellent electron acceptor. The strong interaction between the BEHT-DTP microribbon and graphene sheets formed a heterojunction, which would benefit the dissociation of photogenerated excitons and collection of the photogenerated charge carriers. From the AFM measurement we also can determine that the height of the microribbon is about 150 nm.

Fig. 2 The AFM image of the BEHT-DTP microribbon (inset: zoom in image) (a) and the BEHT-DTP/graphene hybrid microribbon (inset: zoom in image) (b).

To investigate the interaction between BEHT-DTP microribbon and the attached graphene sheets, we have measured the fluorescence spectra of the microribbon with and without graphene. The spectra were shown in Fig. 3, it can be seen that the pristine BEHT-DTP microribbon shows strong fluorescence between 450 and 550 nm with excitation at 365 nm. While the fluorescence intensity is dramatically reduced after the introduction of graphene sheets, indicating that efficient energy transfer occurs at the BEHT-DTP/graphene interface where the two materials interact strongly. The measurement manifests that the excited BEHT-DTP molecule was quenched by the electronic interaction at the interface. This efficient quenching of fluorescence has also been observed in a polymer/graphene organic solar cell where the graphene acted as an electron acceptor,²⁷ it is very likely that our hybrid microribbon can also be a heterojunction and demonstrate photoconductive property.

Fig. 3 The fluorescence spectra of pristine BEHT-DTP and BEHT-DTP/graphene microribbon.

Raman spectroscopy was also used to characterize the hybrid microribbon. As shown in Fig. 4a and b, the main characteristic peaks of the Raman spectrum of pristine BEHT-DTP microribbon lie in 1180, 1374 and 1510 cm⁻¹, and the D, and G bands of graphene lie in 1360 and 1589 cm⁻¹ respectively (the full spectrum of graphene can be seen in Fig. S4†). The sharp peaks of the Raman spectrum of BEHT-DTP microribbon demonstrate its excellent crystallinity. It's noteworthy that, because of the interference of the strong fluorescence of BEHT-DTP, the Raman spectra of BEHT-DTP and hybrid microribbon were only recorded the wavenumber below 1800 cm⁻¹. The Raman spectrum in Fig. 4c reveals that when the graphene sheets were self-assembled onto BEHT-DTP microribbon, only the G band can be observed (2D band has not been detected), the D band of graphene is swallowed up by the strong peak of BEHT-DTP. Interestingly, the G band of graphene sheets has shifted from 1589 to 1566 cm⁻¹ after assembly on the BEHT-DTP. This downshift of G-band frequency may originate from the interaction with the electron donor material BEHT-DTP, manifesting the charge transfer process between BEHT-DTP and graphene,^31,32 showing promising application in photoconductive devices.

Fig. 4 The Raman spectra of (a) pristine BEHT-DTP mciroribbon (b) graphene and (c) BEHT-DTP/graphene hybrid microribbon.

To discover the photoconductivity of the microribbon and reveal the effect of the graphene on the photoconductivity property of the microribbon, we have fabricated bottom contact devices based on one single microribbon. The BEHT-DTP microribbon was self-assemble on the Si/SiO₂ substrate with photolithography-defined gold electrodes array (shown in Fig. 5b). The thiophene moiety in the BEHT-DTP would facilitate the interaction between the microribbon and the gold electrodes,^33,34 leading to good electrode contacts of the microribbon on the bottom electrodes. Firstly, we have measured the photoconductivity property of the device of pristine BEHT-DTP microribbon. As shown in Fig. 5a, although the microribbon is well crystalline and the microribbon and electrodes are good in contact, the conductivity of the pristine BEHT-DTP microribbon in dark condition is poor. When illuminated by white light (∼79.4 mW cm⁻²), the homogeneous BEHT-DTP microribbon device demonstrated a quite low photoresponse, the on/off ratio is less than 2. It can be inferred that, the homogeneous BEHT-DTP microribbon lacks donor/acceptor interface and electron acceptor, which results in the difficulty in the dissociation of the photogenerated excitons in BEHT-DTP and the collection of photogenerated electrons. Thus, the dissociation efficiency of the photogenerated excitons in pristine BEHT-DTP microribbon is quite low.

Fig. 5 The photoconductive performance of the device based on pristine BEHT-DTP (a) and BEHT-DTP/graphene (inset: the optical microscope image of BEHT-DTP/graphene microribbon device, scale: 20 μm) (b) microribbon; the switch property of the photoconductivity (c) and the air-stability (d) of BEHT-DTP/graphene.

On the contrary, as shown in Scheme 2, when the graphene layer is introduced into the microribbon to form a heterostructure hybrid microribbon, the interface between the BEHT-DTP microribbon and the graphene layer will serve as a heterojunction to promote the dissociation of the photogenerated excitons. Meanwhile, the graphene will serve as an excellent electron acceptor, which will collect and shuttle the photogenerated electrons efficiently.^25,26 Then the electrons within graphene sheets will be caught by the oxidants (such as oxygen molecules)^35,36 or oxidative group attached on graphene.²⁵ The collection of the electrons will facilitate to reduce the density of the electrons dissociated from excitons located in the heterojunction, which is beneficial to the further dissociation process of the excitons and prevents the recombination of charge carriers. As shown in Fig. 5b, the introduction of the nanoscale graphene sheets hasn't increased the conductivity of the microribbon in the dark. For the stacking state of graphene between each other, the electrons transport between the graphene sheets should pass through the graphene sheets planes and interfaces. Through-plane conductivity is quite low.³⁷ It will reduce the conductivity of the stacked graphene sheets. Moreover, the graphene sheets possibly don't contact the electrodes directly because they are adsorbed on the BEHT-DTP microribbon which contacts the electrodes directly. Therefore, the adsorption of graphene sheets on BEHT-DTP microribbon will not increase its conductivity significantly in the dark. However, when illuminated by white light, the conductivity increased drastically, at the bias of 30 V, the on/off ratio of the current in light and dark is more than 50. Furthermore, the conductivity can be switched promptly by switching the light on and off (Fig. 5c), showing an excellent reversibility. However, limited by the sensitivity of our measurement instrument for weak current, it is hard to determine the exact response time. The conductivity gain originates from the photogenerated holes in BEHT-DTP near the heterojunction. When the excitons dissociate into free holes and electrons, the electrons are collected by the graphene sheets, while the holes leave in BEHT-DTP side at the heterostructure interface. The accumulated holes in BEHT-DTP will form a conductive path when a bias is applied to the two ends of the microribbon, thus, the conductivity will be tremendously increase. As we have reported previously,⁶ the device fabricated by BEHT-DTP is air-stable, to explore that, we have tested the photocurrent of the hybrid microribbon for different time that the device was stored in the air (Fig. 5d). The high air-stability of BEHT-DTP endowed the device excellent stability, after being stored in air for 24 hours, the photoconductivity performance (defined as the ratio of the measured photocurrent to the original photocurrent at bias of 30 V) still remained about 93%, and the decay of the photocurrent tended to slow down with the time increase. The good air-stability is beneficial to research and application.

Scheme 2 The mechanism diagram of the photoconductivity of the BEHT-DTP/graphene hybrid microribbon.

Although the hybrid microribbon demonstrated a significant enhanced photoconductivity, however the on/off ratio was not quite high, it may due to the weak absorption of visible light for BEHT-DTP. To confirm that, we have investigated the UV-vis absorption spectrum of the BEHT-DTP microribbon (Fig. 6a). The absorption spectrum of the microribbon concentrates in short wavelength, especially in the UV region bellow 350 nm. As it is commonly known, absence of absorption in long wavelength is adverse to highly efficient photovoltaic devices. To investigate the wavelength dependence of the photoconductivity, we have measured the external quantum efficiency (EQE) spectrum of the device (Fig. 6b), the calculation was according to the method we took before.³⁸ The UV-vis spectrum of the microribbon has barely changed after the assembly of graphene (Fig. 6a), indicating that the graphene layer will not contribute to the light absorption. Therefore, the EQE spectrum of the device is highly consistent with the absorption spectrum of BEHT-DTP which contributes to most of the light absorption and generation of excitons. The results demonstrate that the device has better photoconductivity when illuminated by the light of short wavelength, especially for UV light. From another perspective, our device may be a good UV light photodetector.

Fig. 6 (a) The UV-vis absorption spectrum of the BEHT-DTP and BEHT-DTP/graphene microribbon; (b) the EQE spectrum of the hybrid microribbon device.

In summary, we have developed a facile method to fabricate 1D organic nano/micro-heterojunction by self-assembly. With the formation of BEHT-DTP/graphene heterostructure microribbon, the photoconductivity performance has improved by more than 25 folds. Interestingly, the air-stable donor material BEHT-DTP we synthesized gave rise to a stable photoconductivity performance of the heterostructure microribbon in the air. This research shows that the water dispersion of graphene can be assembled onto organic semiconductor material to form a 1D nano/micro-heterojunction efficiently by a facile self-assembly method. Although we concentrate on BEHT-DTP microribbon only here, since the readily accessible graphene sheets can interact with various organic semiconductors via π–π interaction, this method can be available for many other organic semiconductors. This research may show promising applications in nano/micro-flexible photovoltaic devices, photoconductors and sensors.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51273020, 51373022) and Research Fund for the Doctoral Program of Higher Education of China (20130006110007).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and some data. See DOI: 10.1039/c6ra09053b

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