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Nanostructured graphene-platinum-PEDOT electrode materials for enhanced Schottky performance and power conversion applications
Microelectronic Engineering 216 (2019) 111045
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Nanostructured graphene-platinum-PEDOT electrode materials for enhanced Schottky performance and power conversion applications Shady Abdelnassera, Mahmoud A. Sakrc, Mohamed Serrya,b,
T
⁎
a
Department of Mechanical Engineering, American University in Cairo, New Cairo 11835, Egypt Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA c Graduate Program in Nanotechnology, American University in Cairo, New Cairo 11835, Egypt b
A B S T R A C T
Counter electrode are crucial components in determining the efficiency of dye-sensitized solar cells. An ideal electrode material would have high conductivity (i.e. high electron mobility) and excellent catalytic activity for electrolyte reduction. Various electrode materials have been investigated, however most materials demonstrate either the catalytic activity or the conductivity. In this paper, we investigated combining the catalytic and electrical conductivity for enhanced electrode materials by fabricating of a hybrid structure consisting of PEDOT/graphene/Pt/semiconductor heterojunction. A hybrid structure was successfully fabricated by atomic layer deposition of Pt over the surface of n-silicon followed by plasma-enhanced chemical vapor deposition of graphene sheets and then an immediate spin coating of PEDOT prepared by solution casting polymerization. The hybridstructure showed a noticeable difference in the reflection/absorption measurements where the absorption drastically increased upon PEDOT deposition. The photoluminescence emission spectra showed two main peaks at 525 nm and 700 nm, with an enhancement in the band-to-band PL process attributed to the separation of the photo-generated charges and the excitonic PL signal related to surface defects and states. Moreover, the electrical characterization of the hybrid electrodes showed a response of about 7.15 × 10−6 A, which is nearly 7 times that of the bare graphene, as well as electrical stability for a period of 300 s under dark conditions.
1. Introduction Dye-sensitized solar cells (DSSC) have been regarded as potential substitutes to Si-based solar cells due to their low cost and ease of fabrication. However, they still suffer from lower efficiency. Therefore, intensive research is being conducted on the enhancement of the power conversion efficiency (PCE) of DSSC. Several approaches have been adopted including the utilization of high surface area metal-oxide semiconductor nanoparticles and advanced dye materials which lead to breakthrough in their PCE up to 7% [1–3], However, this is not yet comparable to Si-based solar cells. One of the main approaches to the enhancement of DSSC power conversions is by electronic material optimizations of their counter electrodes (CE) by utilization of new materials instead of the traditionally used platinum CE [4]. The optimization of CE electronic materials for DSSCs means optimizing both their conductivity for electron transfer as well as their catalytic activity for reduction of an electrolyte [5]. The utilization of intrinsic conductive polymers as CEs is a promising approach due to their high conductivity, catalytic activity, high transparency, high flexibility, low cost, and ease of fabrication [6–9]. Poly (3,4–ethylenedioxy thiophene) (PEDOT) is one of the most promising intrinsic conductive polymer materials, it has been utilized as a CE material for a DSSC by several researchers, however, it has been
⁎
demonstrated that a pure PEDOT CE without prior treatment generates lower cell efficiency as compared to platinum-based DSSC [10,11]. Furthermore, carbon-based and 2D materials have been investigated as potential CE materials due to their high specific surface area, high electrical conductivity, high flexibility and transparency. PCEs as high as 6.7% have been reported by utilizing carbon black as CE in DSSC [12,13]. Reduced graphene oxide (rGO) has been utilized as CEs in DSSCs, however, they've exhibited low PCE (2.2%) due to the large resistance of rGO [14]. However, integration of rGO with PEDOT has led to a PCE jump up to 4.5% [15]. Further investigations by Punya et al. intensively studied the behavior of nanocomposite electrodes based on graphene-hybrid conducting polymers (polyaniline, polypyrrole, and polythiophene) in electrochemical energy storage systems (supercapacitors). The investigations revealed fewer diffusion paths and inferior ionic resistance for the diffusion of the counter ions in the fabricated supercapacitors, which in turn accelerated the electrochemical reaction and maintained superior power density. Lee et al. prepared multilayered structuring of graphene/PEDOT films by interchangeable electrochemical deposition of graphene and PEDOT. The complex structure exhibited a specific capacitance of 154 F g−1 and a capacitance retention of 86% after 1000 cycles [16]. Zhu et al. fabricated a bendable free-standing graphene oxide/polypyrrole heterojunction by electrodeposition. In such a
Corresponding author at: Department of Mechanical Engineering, American University in Cairo, New Cairo 11835, Egypt E-mail address: [email protected] (M. Serry).
https://doi.org/10.1016/j.mee.2019.111045 Received 24 March 2019; Received in revised form 28 May 2019; Accepted 11 June 2019 Available online 12 June 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
Microelectronic Engineering 216 (2019) 111045
S. Abdelnasser, et al.
Fig. 1. Schematic of process flow.
CEs can lead to the advancement of better power conversion, more flexible and more environmentally stable DSSC devices. However, the utilization of graphene for enhanced performance is requires the optimization the conductivity and catalytic activity of the graphene layer by controlling various parameters, such as quality of the graphene layer, including number of layers, surface defects, and functional groups [22] affects the process complexity, repeatability and scalability (However, high electrical conductivity and large accessible surface area are contradicting to each other in current electrode materials.). A more reliable and scalable approach to the utilization of graphene in CE and energy conversion devices is by synthesizing a hybrid nanostructure. The structure consists of catalytic and conductive layers with a semiconductor backing. Therefore, in this paper we propose a hybrid nanostructure consisting of PEDOT/graphene/Pt hybrid structure backed by n-Si substrate for energy conversion devices. The combination of PEDOT and graphene promotes electron transport, whereas the Pt layer is highly catalytic leading to the enhancement of energy conversion in DSSC devices. We investigated the influence of poly (3,4–ethylenedioxy thiophene) (PEDOT) on the electrical properties of graphene to be used for energy conversion and storage applications. In comparison with PANI and PPy, PEDOT has been less examined in composites with graphene electrodes for supercapacitor systems. PEDOT has unique properties owing to its low band gap (1.5–1.7 eV), excellent electrochemical stability, moderate electrical conductivity, as well as optical transmittance over the visible wavelength.
system, the graphene oxide acted as an efficient charge-balancing dopant for polypyrrole (PPy) and maintained a specific capacitance of nearly 356 F g−1 at a discharge rate of 0.5 A g−1. This obtained value was reported to be 50% higher than that obtained for pure PPy [17]. In applications related to energy conversion devices (e.g. solar or fuel cells), Liu et al. illustrated that graphene-conjugated polymer could be adopted as a CE in DSSC, providing higher PCE performance in comparison with the unmodified fluorine-doped tin oxide (FTO) or pure polymer [18]. In the same context, Wang et al. synthesized a hybrid of graphene/poly (diallyldimethylammoniumchloride) (PDDA) to be employed as a catalyst in fuel cells. The nanocomposite exhibited an electrocatalytic performance with an onset potential of oxygen reduction reaction (ORR) at −0.15 V (vs. SCE), with respect to an onset potential of −0.25 V for the bare graphene. The electrodes also showed higher fuel selectivity, better tolerance to CO poisoning, and more enhanced stability in comparison with the commonly used Pt/C electrodes [19]. Huang et al. fabricated hybrid field-effect transistors by incorporating both poly (3,3 didodecylquaterthiophene) (PQT-12) and graphene and reported 20 times higher effective mobility for the hybrid structure with respect to the pure organic semiconductor while retaining the on/off ratios at comparable or even better levels [20]. Byoung et al. developed graphene/poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) hybrid electrodes to be used in solar cells and light emitting diodes. The authors explained the doping mechanism on the basis of the electron transfer from graphene to PEDOT:PSS due to the difference in the work function, which leads to a more enhanced fill factor (FF). Byun et al. fabricated a graphene–polymer hybrid nanostructure-based bio-energy storage device that enabled bio motor exchange between on and off states in real time. It was concluded that the doped graphene surface provided better motility for the motor protein motion in comparison with the common glass substrates, which could be desirable for motor protein-based biosensor applications [21]. We can see from the above discussion that the combination of intrinsic conductive polymers and graphene- or 2D- based materials for
2. Materials and methods Ethylene dioxythiophene (EDOT), iron (III) chloride (FeCl3), pyridine, and isopropanol were used as reagents. All chemicals utilized are analytical grade reagents. Prior to the fabrication process, an n-silicon wafer (0.5 mm thick, 99.9% purity, Alfa Aesar) was cut into 2 cm × 2 cm squares as substrates. N-silicon samples with resistivity of 2
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4–11 Ω-cm were degreased by being sonicated in distilled water, acetone, and isopropanol each for 5 min. The Schottky diode was built by the deposition of a thin film of platinum in atomic layer deposition (ALD) via the Cambridge NanoTech Savannah ALD deposition system under a deposition temperature of 275 °C for 50–500 cycles. This was followed by deposition of graphene sheets by PECVD using the Oxford Instruments PlasmaLab 100 PECVD system under a pressure of 1500 mTorr and a temperature of 600 °C in a methane (CH4)-rich atmosphere. As precisely described in Fig. 1, the device is mainly composed of several junctions. The first is a regular metal-semiconductor junction (Pt-Si) where charges transfer from Si to Pt forming a depletion region. The second one is a graphene-Pt junction where electron transfer from graphene to Pt until equilibrium in fermi levels is reached. It is worth noting that the utilized temperature is to promote the graphene growth process, however it is not a high enough temperature (e.g. 16,000 °C) to lead to the formation of other phases (such as platinum silicide), which was further demonstrated by the crystal analysis where no different peaks were observed. PEDOT films were synthesized via the solution-casting polymerization (SCP) technique to enable coverage of the substrate. FeCl3 (0.9 g) was mixed with isopropanol (1.5 g) and stirred for 30 min at room temperature, then 0.03 g of pyridine was added to this mixture and stirred for another 30 min. Finally, EDOT monomer (0.05 g) was added to the solution and stirred for 10 min. The morphology of the fabricated samples was examined using a field emission scanning electron microscope (FESEM-Zeiss SEM Ultra60) after PEDOT deposition. An X-ray diffractometer (XRD; PANalytical X'pert PRO diffractometer with Cu Kα radiations) was utilized for the structural analysis. Raman measurements were carried out on a Raman microscope (ProRaman-L Analyzer) with an excitation laser beam wavelength of 532 nm. The optical characterization of the obtained electrodes was established with a Cary 5000 UV/vis/NIR spectrophotometer with a solid-sample holder for reflectance measurements and an integrated sphere. Cyclic voltammetry was measured using a scanning potentiostat (CH Instruments, model CH 660D) under dark conditions at a scan rate of 20 mVs−1. The electrical properties were evaluated with a 4156 high-precision semiconductor parameter analyzer.
prepared graphene/Pt/PEDOT electrodes at the same spinning speed and PEDOT content (100 μl) while varying the Pt thickness, which varied the deposition of graphene layers. The morphological properties of the surface changed with the variation of Pt thickness, which resulted in enhancing the deposition of PEDOT. The theory beyond this is at the beginning of Pt deposition, an isolated nanocluster is formed and upon increasing the number of cycles, these nanoclusters start to merge to form large covered areas of Pt. This process is called nucleation. The more nucleation occurs on the surface of the n-Si wafer, the greater the growth of the Pt films. In this case, the higher the number of cycles, the thicker the Pt film became [23] as in the example of 50 nm film [24]. At constant PEDOT concentration, we noticed the surface adsorption of an amorphous layer of Pt modifying the surface of the underlying substrate with a higher extent of the adsorbed Pt formed upon increasing the number of cycles. Noteworthy is the inhomogeneity of the adsorbed layer and that there is no complete coverage attained over the surface of the substrate. However, the substrate structure is still very well intact in all cases. Crystallinity of the fabricated structure was characterized using Xray diffraction as shown in Fig. 3 (a). The analysis revealed that the samples were crystalline in the form of a 2D hexagonal honeycomb lattice of carbon atoms which crystallized within the P6/mmm space group (JCPDS No. 85-0423). Note the appearance of the characteristic diffraction peaks of the (101) and (110) facets at 40.44° and 69.5° respectively. Note also the shift in the peak position of the (101) peak towards smaller 2θ values upon polymer deposition (see Fig. 3 (b)). This indicates larger interlayer spacing after deposition compared to the unmodified graphene. This also signifies that the graphene sheets are more loosely stacked in the hybrid films, inhibiting the aggregation of graphene and providing more penetrative channels for the electrolyte, and hence facilitating the ion diffusion and electron transport rate. The change in the d-spacing in (101) planes of the bare and the polymer synthesized samples was calculated using Bragg's law (dbare = 0.1193 nm while dhybrid = 0.1214 nm). To further examine the chemical analysis of the fabricated structures, Raman scattering is another effective technique to investigate the crystallinity and the vibrational properties of materials, as Raman signals are very sensitive to the crystal structure and defects. All the curves in Fig. 4 show the existence of one main strong peak at 1430 cm−1, which is attributed to the existence of PEDOT as previously reported in literature [25]. The graphene samples also exhibited one main peak at
3. Results and discussion Fig. 2 (panels a–d) shows the FESEM top view images of the as-
Fig. 2. FESEM top-view images of as-prepared graphene hybrid PEDOT samples, formed on various depositions of platinum thin films (a) 5 nm (b) 30 nm (c) 40 nm and (d) 50 nm (scale bar is 40 μm). 3
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Fig. 3. XRD patterns of (a) PECVD graphene sheets formed on various Pt thicknesses before and after polymer deposition, and (b) variation of the inter-planar space after polymer deposition.
Fig. 5. The photoluminescence spectra for the fabricated electrodes with an excitation wavelength of 530 nm.
Fig. 4. Raman spectra of graphene sheets formed on 5–50 nm Pt thicknesses after deposition of PEDOT and the corresponding spectra of pure graphene.
2337 cm−1, attributable to (second order) double phonon scattering either on a single electron, single hole, or electron-hole pair. Fig. 5. shows the room temperature photoluminescence (PL) spectra of the bare graphene samples at different platinum thicknesses before and after polymer deposition under an excitation length of 530 nm. The unsensitized and sensitized samples exhibited similar PL peaks, with no additional peaks observed for the sensitized samples. Two luminescence bands were observed. The first band was located at ~525 nm. The second emission band appeared at 708 nm, and can be assigned to the recombination of generated holes with single ionized charge states such as oxygen vacancies or surface defects, which is a type of deep-level or trap state emission. Note the diminishment of the near band edge emission (NBE) peak in the polymer-sensitized electrodes compared to the unsensitized ones, indicating a lower recombination rate of photoinduced electron–hole pairs. However, the second emission peak was
Fig. 6. The diffuse reflectance spectra of pure graphene as well as polymersensitized graphene.
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Fig. 7. Cyclic voltammogram of (a) graphene-Pt/Si junction (b) PEDOT hybrid graphene-Pt/Si junction (C) pure PEDOT.
Fig. 8. (a) J-V characteristics from −10 to +10 V under dark conditions, inset J-V −1.0 to +1.0 V (b) the corresponding J-t characteristics at a constant bias of +0.3 V under dark conditions.
counter electrode, and a saturated calomel electrode (SCE) as the reference electrode in phosphate-buffered saline (PBS) electrolyte. Fig. 7 (a) displays the cyclic voltammograms of the graphene 30 nm Pt hybrid PEDOT with a potential window from −1.5 to 1.5 V (vs. SCE) at a scan rate of 20 mV s−1. The sensitized and unsensitized states are reflective for peaks in the CV characterization. Two couples of redox peaks were perceived, which describe more like a double capacitor manner related to the synergetic effect of the inclusion of PEDOT in the graphene framework. This signifies the existence of pseudo-capacitive PEDOT and indicates better capacitive characteristics for the hybrid material with
also attenuated, indicating the possibility of surface defects in the network of graphene [26,27]. Fig. 6. shows the absorption spectra of pure and PEDOT-sensitized graphene electrodes. The pure graphene sample showed a continuous wide reflection with a band edge located at 400 nm. The polymersensitized samples showed a similar reflection profile, however, with a tremendous shift towards higher absorption intensities. The cyclic voltammetry of the hybrid electrodes was evaluated at room temperature by a three-electrode configuration composed of the fabricated samples as the working electrode, platinum (Pt) as the 5
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the electrode due to the Pi-Pi interaction at the interface between graphene and PEDOT. This synergetic effect is mainly originated from the presence of residual oxygen containing groups, aromatic containing units and vacancy defects that will act as active sites to enhance the Pdoping level of the graphene-Pt/Si junction leading to acquire functions and properties for the composite material surpassing that of each of the pristine material.
less ohmic resistance [28]. The J-t curve is to assess the stability of the fabricated electrodes over a period of 300 s and also indicates the optimum values of the dark currents. The electrical measurements were performed via semiconductor parameter analyzer where the samples were connected to positive and negative terminals at −0.3 V from the Si side while the current was read from the upper surface. Upon increasing the thickness of the platinum layer above 40 nm and 50 nm, it was shown that the vertical current of the Pt/Si diode started to decrease due to the existence of trapping states. The trap states exist as deep states at the graphene/Pt interface. Due to the large difference between the work function of Pt (6.1 eV) and that of graphene (4.4 eV), a heavily doped structure will lead to detrimental effects and decrease the current [20]. A slight shift in the fermi level towards the valence band, affecting the electrical structure of graphene without changing the band gap which is verified by Fig. 8 (b) illustrating the I-t characteristics of various platinum thicknesses and is consistent with the results obtained from J-V curves, as the 30 nm sample showed its highest current value at approximately −25 μA. Since the work function of Pt is higher than the work function of graphene, a physisorption interaction between the Pt film and graphene will be formed and produce P-doped graphene [29–31]. Further analysis to adequately verify the proof-of-concept is illustrated by describing the role of each interface, a follows: (1) at the Pt/ n-Si interface, a Schottky barrier is formed at this interface where electrons transfer from Si to Pt at low doping levels, when the positive bias is applied from the Pt side, whereas they cannot overcome the barrier when the positive bias is applied from the Si side. At higher doping levels, both the width of the depletion region and the height of the barrier decreases and, as a result, charges can transfer through the barrier in both directions. (2) at the Graphene/Pt interface, graphene has been reported to form junctions with 2D and 3D semiconducting materials where it can improve electric conductivity, improve the interface transport properties, and perform as an ideal Schottky diode. Pdoped graphene is formed because of its lowered fermi level where electrons migrate in both directions through the potential barrier, either when forward or reverse bias is applied. Finally, (3) at the PEDOT/ Graphene interface, PEDOT acts as a sensitizer due to its immense conductivity and thus can facilitate the migration of electrons and achieve excellent stability.
Acknowledgments This work has been funded by an American University in Cairo Faculty Support Grants. References [1] B. Oregan, M. Gratzel, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films, Nature 353 (6346) (1991) 737–740. [2] X.M. Fang, T.L. Ma, G.Q. Guan, M. Akiyama, T. Kida, E. Abe, Effect of the, thickness of the Pt film coated on a counter electrode on the performance of a dye-sensitized solar cell, J. Electroanal. Chem. 570 (2) (2004) 257–263. [3] M.M. Byranvand, Recent development of carbon nanotubes materials as counter electrode for dye-sensitized solar cells, J. Nanostruct. 6 (1) (2016) 1–16. [4] H. Wang, Y.H. Hu, Graphene as a counter electrode material for dyesensitized solar cells, Energy Environ. Sci. 5 (8) (2012) 8182–8188. [5] H. Wang, K. Sun, F. Tao, D.J. Stacchiola, Y.H. Hu, 3D honeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dye-sensitized solar cells, Angew. Chem. Int. Ed. 52 (35) (2013) 9210–9214. [6] G.R. Li, J. Song, G.L. Pan, X.P. Gao, Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells, Energy Environ. Sci. 4 (5) (2011) 1680–1683. [7] T. Muto, M. Ikegami, T. Miyasaka, Polythiophene-based mesoporous counter electrodes for plastic dye-sensitized solar cells, J. Electrochem. Soc. 157 (8) (2010) B1195–B1200. [8] B. Ballarin, S. Masiero, R. Seeber, D. Tonelli, Modification of electrodes with porphyrin-functionalised conductive polymers, J. Electroanal. Chem. 449 (1–2) (1998) 173–180. [9] J.H. Wu, Q.H. Li, L.Q. Fan, Z. Lan, P.J. Li, J.M. Lin, S.C. Hao, Highperformance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Sources 181 (1) (2008) 172–176. [10] T. Muto, M. Ikegami, K. Kobayashi, T. Miyasaka, Conductive polymer-based mesoscopic counterelectrodes for plastic dye-sensitized solar cells, Chem. Lett. 36 (6) (2007) 804–805. [11] B. Fan, X. Mei, K. Sun, J. Ouyang, Conducting polymer/carbon nanotube composite as counter electrode of dye-sensitized solar cells, Appl. Phys. Lett. 93 (14) (2008). [12] A. Kay, M. Grätzel, Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder, Sol. Energy Mater. Sol. Cells 44 (1) (1996) 99–117. [13] H. Irie, Y. Watanabe, K. Hashimoto, Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst, Chem. Lett. 32 (8) (2003) 772–773. [14] W. Hong, Y. Xu, G. Lu, C. Li, G. Shi, Transparent graphene/PEDOT–PSS composite films as counter electrodes of dye-sensitized solar cells, Electrochem. Commun. 10 (10) (2008) 1555–1558. [15] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, Flexible graphene films via the filtration of watersoluble noncovalent functionalized graphene sheets, J. Am. Chem. Soc. 130 (18) (2008) 5856–5857. [16] J.M. Chem, Doping, Coating and Synergistic Effect for Energy Storage, (2012), pp. 6300–6306. [17] J. Huang, et al., Polymeric semiconductor/graphene hybrid field-effect transistors, Org. Electron. 12 (9) (2011) 1471–1476. [18] A.S. Aricò, et al., Nanostructured Materials for Advanced Energy Conversion and Storage Devices, vol. 4, (2005) no. May. [19] P. Blake, E.W. Hill, Making Graphene Visible, (2007), pp. 2007–2009. [20] A. Kumar, C.H. Lee, Synthesis and Biomedical Applications of Graphene: Present and Future Trends, (2013). [21] L. Baker, et al., Nucleation and growth of Pt atomic layer deposition on Al2O3 substrates using (methylcyclopentadienyl)-trimethyl platinum and O2 plasma, 84333 (2011) (2015). [22] B. Pang, L. Dong, S. Ma, H. Dong, L. Yu, Performance of FTO-free conductive graphene-based counter electrodes for dye-sensitized solar cells, RSC Adv. 6 (47) (2016) 41287–41293. [23] P.A. Khomyakov, G. Giovannetti, P.C. Rusu, G. Brocks, J. Van Den Brink, P.J. Kelly, First-Principles Study of the Interaction and Charge Transfer Between Graphene and Metals, (2009), pp. 1–12. [24] A.A. Farah, et al., Poly (styrenesulfonate) Films Post-Spincasting, vol. 113709, (2012). [25] J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, Review of Photoluminescence Performance of Nano-Sized Semiconductor Materials and its Relationships with Photocatalytic Activity, 90 (2006), pp. 1773–1787. [26] Y. Chang, C. Liu, C. Chen, H. Cheng, The Effect of Geometric Structure on Photoluminescence Characteristics of 1-D TiO2 Nanotubes and 2-D TiO2 Films, vol. 159, (2012), pp. 401–405 no. 7.
4. Conclusion This work represents the development of enhanced performance Graphene-Pt/Si Schottky junction by incorporating conductive polymer, PEDOT as a supporting material via simple, low-cost and effective spin coating technique utilized to modify the structure and properties of Graphene-Pt/Si Schottky junction. In summary, we have successfully developed graphene PEDOT hybrid electrode structure via PECVD of graphene sheets followed by an immediate spin coating of PEDOT. Due to the synergetic effect of the constituents of the hybrid electrodes, the fabricated samples exhibited better absorption properties, a more enhanced current response, as well as superior capacitive performance. This opens a novel pathway to significantly enhance the performance of graphene-based energy conversion devices by introducing a repeatable and scalable process to enhance the performance of graphene-integrated electrodes. In this configuration, the hybrid structure will exhibit the unique properties derived from the presence of graphene due to its large specific surface area, high electrical conductivity, superior electron transfer rate and substantial mechanical strength overcoming the unexistence of a band gap which doesn't enable the device to be switched off by the presence of PEDOT layer with its redox properties having the ability to change its electrochemical properties with oxidation state due to the loss of electrons (oxidation) or the gain of electrons (reduction). Besides, PEDOT will also provide elevated chemical and electrochemical stability, low ohmic drop across 6
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multimodal energy conversion, Sensors and Actuators A: Physical, 245 2016, pp. 169–179 Jul 1. [30] M. Serry, M.A. Sakr, Study of flexoelectricity in graphene composite structures, MRS Adv. 1 (39) (2016) 2723–2729 Jan. [31] M. Serry, A. Sharaf, A. Emira, A. Abdul-Wahed, A. Gamal, Nanostructured graphene–Schottky junction low-bias radiation sensors, Sensors Actuators A Phys. 232 (2015) 329–340.
[27] T. Kuilla, S. Bhadra, D. Yao, N. Hoon, S. Bose, J. Hee, Progress in polymer science: recent advances in graphene based polymer composites, Prog. Polym. Sci. 35 (11) (2010) 1350–1375. [28] L. Mao, M. Li, J. Xue, J. Wang, RSC advances: bendable graphene/conducting polymer hybrid films for freestanding electrodes with high volumetric capacitances †, RSC Adv. 6 (2015) 2951–2957. [29] M. Serry, M.A. Sakr, Graphene-metal-semiconductor composite structure for
7
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Nanostructured graphene-platinum-PEDOT electrode materials for enhanced Schottky performance and power conversion applications Shady Abdelnassera, Mahmoud A. Sakrc, Mohamed Serrya,b,
T
⁎
a
Department of Mechanical Engineering, American University in Cairo, New Cairo 11835, Egypt Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA c Graduate Program in Nanotechnology, American University in Cairo, New Cairo 11835, Egypt b
A B S T R A C T
Counter electrode are crucial components in determining the efficiency of dye-sensitized solar cells. An ideal electrode material would have high conductivity (i.e. high electron mobility) and excellent catalytic activity for electrolyte reduction. Various electrode materials have been investigated, however most materials demonstrate either the catalytic activity or the conductivity. In this paper, we investigated combining the catalytic and electrical conductivity for enhanced electrode materials by fabricating of a hybrid structure consisting of PEDOT/graphene/Pt/semiconductor heterojunction. A hybrid structure was successfully fabricated by atomic layer deposition of Pt over the surface of n-silicon followed by plasma-enhanced chemical vapor deposition of graphene sheets and then an immediate spin coating of PEDOT prepared by solution casting polymerization. The hybridstructure showed a noticeable difference in the reflection/absorption measurements where the absorption drastically increased upon PEDOT deposition. The photoluminescence emission spectra showed two main peaks at 525 nm and 700 nm, with an enhancement in the band-to-band PL process attributed to the separation of the photo-generated charges and the excitonic PL signal related to surface defects and states. Moreover, the electrical characterization of the hybrid electrodes showed a response of about 7.15 × 10−6 A, which is nearly 7 times that of the bare graphene, as well as electrical stability for a period of 300 s under dark conditions.
1. Introduction Dye-sensitized solar cells (DSSC) have been regarded as potential substitutes to Si-based solar cells due to their low cost and ease of fabrication. However, they still suffer from lower efficiency. Therefore, intensive research is being conducted on the enhancement of the power conversion efficiency (PCE) of DSSC. Several approaches have been adopted including the utilization of high surface area metal-oxide semiconductor nanoparticles and advanced dye materials which lead to breakthrough in their PCE up to 7% [1–3], However, this is not yet comparable to Si-based solar cells. One of the main approaches to the enhancement of DSSC power conversions is by electronic material optimizations of their counter electrodes (CE) by utilization of new materials instead of the traditionally used platinum CE [4]. The optimization of CE electronic materials for DSSCs means optimizing both their conductivity for electron transfer as well as their catalytic activity for reduction of an electrolyte [5]. The utilization of intrinsic conductive polymers as CEs is a promising approach due to their high conductivity, catalytic activity, high transparency, high flexibility, low cost, and ease of fabrication [6–9]. Poly (3,4–ethylenedioxy thiophene) (PEDOT) is one of the most promising intrinsic conductive polymer materials, it has been utilized as a CE material for a DSSC by several researchers, however, it has been
⁎
demonstrated that a pure PEDOT CE without prior treatment generates lower cell efficiency as compared to platinum-based DSSC [10,11]. Furthermore, carbon-based and 2D materials have been investigated as potential CE materials due to their high specific surface area, high electrical conductivity, high flexibility and transparency. PCEs as high as 6.7% have been reported by utilizing carbon black as CE in DSSC [12,13]. Reduced graphene oxide (rGO) has been utilized as CEs in DSSCs, however, they've exhibited low PCE (2.2%) due to the large resistance of rGO [14]. However, integration of rGO with PEDOT has led to a PCE jump up to 4.5% [15]. Further investigations by Punya et al. intensively studied the behavior of nanocomposite electrodes based on graphene-hybrid conducting polymers (polyaniline, polypyrrole, and polythiophene) in electrochemical energy storage systems (supercapacitors). The investigations revealed fewer diffusion paths and inferior ionic resistance for the diffusion of the counter ions in the fabricated supercapacitors, which in turn accelerated the electrochemical reaction and maintained superior power density. Lee et al. prepared multilayered structuring of graphene/PEDOT films by interchangeable electrochemical deposition of graphene and PEDOT. The complex structure exhibited a specific capacitance of 154 F g−1 and a capacitance retention of 86% after 1000 cycles [16]. Zhu et al. fabricated a bendable free-standing graphene oxide/polypyrrole heterojunction by electrodeposition. In such a
Corresponding author at: Department of Mechanical Engineering, American University in Cairo, New Cairo 11835, Egypt E-mail address: [email protected] (M. Serry).
https://doi.org/10.1016/j.mee.2019.111045 Received 24 March 2019; Received in revised form 28 May 2019; Accepted 11 June 2019 Available online 12 June 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
Microelectronic Engineering 216 (2019) 111045
S. Abdelnasser, et al.
Fig. 1. Schematic of process flow.
CEs can lead to the advancement of better power conversion, more flexible and more environmentally stable DSSC devices. However, the utilization of graphene for enhanced performance is requires the optimization the conductivity and catalytic activity of the graphene layer by controlling various parameters, such as quality of the graphene layer, including number of layers, surface defects, and functional groups [22] affects the process complexity, repeatability and scalability (However, high electrical conductivity and large accessible surface area are contradicting to each other in current electrode materials.). A more reliable and scalable approach to the utilization of graphene in CE and energy conversion devices is by synthesizing a hybrid nanostructure. The structure consists of catalytic and conductive layers with a semiconductor backing. Therefore, in this paper we propose a hybrid nanostructure consisting of PEDOT/graphene/Pt hybrid structure backed by n-Si substrate for energy conversion devices. The combination of PEDOT and graphene promotes electron transport, whereas the Pt layer is highly catalytic leading to the enhancement of energy conversion in DSSC devices. We investigated the influence of poly (3,4–ethylenedioxy thiophene) (PEDOT) on the electrical properties of graphene to be used for energy conversion and storage applications. In comparison with PANI and PPy, PEDOT has been less examined in composites with graphene electrodes for supercapacitor systems. PEDOT has unique properties owing to its low band gap (1.5–1.7 eV), excellent electrochemical stability, moderate electrical conductivity, as well as optical transmittance over the visible wavelength.
system, the graphene oxide acted as an efficient charge-balancing dopant for polypyrrole (PPy) and maintained a specific capacitance of nearly 356 F g−1 at a discharge rate of 0.5 A g−1. This obtained value was reported to be 50% higher than that obtained for pure PPy [17]. In applications related to energy conversion devices (e.g. solar or fuel cells), Liu et al. illustrated that graphene-conjugated polymer could be adopted as a CE in DSSC, providing higher PCE performance in comparison with the unmodified fluorine-doped tin oxide (FTO) or pure polymer [18]. In the same context, Wang et al. synthesized a hybrid of graphene/poly (diallyldimethylammoniumchloride) (PDDA) to be employed as a catalyst in fuel cells. The nanocomposite exhibited an electrocatalytic performance with an onset potential of oxygen reduction reaction (ORR) at −0.15 V (vs. SCE), with respect to an onset potential of −0.25 V for the bare graphene. The electrodes also showed higher fuel selectivity, better tolerance to CO poisoning, and more enhanced stability in comparison with the commonly used Pt/C electrodes [19]. Huang et al. fabricated hybrid field-effect transistors by incorporating both poly (3,3 didodecylquaterthiophene) (PQT-12) and graphene and reported 20 times higher effective mobility for the hybrid structure with respect to the pure organic semiconductor while retaining the on/off ratios at comparable or even better levels [20]. Byoung et al. developed graphene/poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) hybrid electrodes to be used in solar cells and light emitting diodes. The authors explained the doping mechanism on the basis of the electron transfer from graphene to PEDOT:PSS due to the difference in the work function, which leads to a more enhanced fill factor (FF). Byun et al. fabricated a graphene–polymer hybrid nanostructure-based bio-energy storage device that enabled bio motor exchange between on and off states in real time. It was concluded that the doped graphene surface provided better motility for the motor protein motion in comparison with the common glass substrates, which could be desirable for motor protein-based biosensor applications [21]. We can see from the above discussion that the combination of intrinsic conductive polymers and graphene- or 2D- based materials for
2. Materials and methods Ethylene dioxythiophene (EDOT), iron (III) chloride (FeCl3), pyridine, and isopropanol were used as reagents. All chemicals utilized are analytical grade reagents. Prior to the fabrication process, an n-silicon wafer (0.5 mm thick, 99.9% purity, Alfa Aesar) was cut into 2 cm × 2 cm squares as substrates. N-silicon samples with resistivity of 2
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4–11 Ω-cm were degreased by being sonicated in distilled water, acetone, and isopropanol each for 5 min. The Schottky diode was built by the deposition of a thin film of platinum in atomic layer deposition (ALD) via the Cambridge NanoTech Savannah ALD deposition system under a deposition temperature of 275 °C for 50–500 cycles. This was followed by deposition of graphene sheets by PECVD using the Oxford Instruments PlasmaLab 100 PECVD system under a pressure of 1500 mTorr and a temperature of 600 °C in a methane (CH4)-rich atmosphere. As precisely described in Fig. 1, the device is mainly composed of several junctions. The first is a regular metal-semiconductor junction (Pt-Si) where charges transfer from Si to Pt forming a depletion region. The second one is a graphene-Pt junction where electron transfer from graphene to Pt until equilibrium in fermi levels is reached. It is worth noting that the utilized temperature is to promote the graphene growth process, however it is not a high enough temperature (e.g. 16,000 °C) to lead to the formation of other phases (such as platinum silicide), which was further demonstrated by the crystal analysis where no different peaks were observed. PEDOT films were synthesized via the solution-casting polymerization (SCP) technique to enable coverage of the substrate. FeCl3 (0.9 g) was mixed with isopropanol (1.5 g) and stirred for 30 min at room temperature, then 0.03 g of pyridine was added to this mixture and stirred for another 30 min. Finally, EDOT monomer (0.05 g) was added to the solution and stirred for 10 min. The morphology of the fabricated samples was examined using a field emission scanning electron microscope (FESEM-Zeiss SEM Ultra60) after PEDOT deposition. An X-ray diffractometer (XRD; PANalytical X'pert PRO diffractometer with Cu Kα radiations) was utilized for the structural analysis. Raman measurements were carried out on a Raman microscope (ProRaman-L Analyzer) with an excitation laser beam wavelength of 532 nm. The optical characterization of the obtained electrodes was established with a Cary 5000 UV/vis/NIR spectrophotometer with a solid-sample holder for reflectance measurements and an integrated sphere. Cyclic voltammetry was measured using a scanning potentiostat (CH Instruments, model CH 660D) under dark conditions at a scan rate of 20 mVs−1. The electrical properties were evaluated with a 4156 high-precision semiconductor parameter analyzer.
prepared graphene/Pt/PEDOT electrodes at the same spinning speed and PEDOT content (100 μl) while varying the Pt thickness, which varied the deposition of graphene layers. The morphological properties of the surface changed with the variation of Pt thickness, which resulted in enhancing the deposition of PEDOT. The theory beyond this is at the beginning of Pt deposition, an isolated nanocluster is formed and upon increasing the number of cycles, these nanoclusters start to merge to form large covered areas of Pt. This process is called nucleation. The more nucleation occurs on the surface of the n-Si wafer, the greater the growth of the Pt films. In this case, the higher the number of cycles, the thicker the Pt film became [23] as in the example of 50 nm film [24]. At constant PEDOT concentration, we noticed the surface adsorption of an amorphous layer of Pt modifying the surface of the underlying substrate with a higher extent of the adsorbed Pt formed upon increasing the number of cycles. Noteworthy is the inhomogeneity of the adsorbed layer and that there is no complete coverage attained over the surface of the substrate. However, the substrate structure is still very well intact in all cases. Crystallinity of the fabricated structure was characterized using Xray diffraction as shown in Fig. 3 (a). The analysis revealed that the samples were crystalline in the form of a 2D hexagonal honeycomb lattice of carbon atoms which crystallized within the P6/mmm space group (JCPDS No. 85-0423). Note the appearance of the characteristic diffraction peaks of the (101) and (110) facets at 40.44° and 69.5° respectively. Note also the shift in the peak position of the (101) peak towards smaller 2θ values upon polymer deposition (see Fig. 3 (b)). This indicates larger interlayer spacing after deposition compared to the unmodified graphene. This also signifies that the graphene sheets are more loosely stacked in the hybrid films, inhibiting the aggregation of graphene and providing more penetrative channels for the electrolyte, and hence facilitating the ion diffusion and electron transport rate. The change in the d-spacing in (101) planes of the bare and the polymer synthesized samples was calculated using Bragg's law (dbare = 0.1193 nm while dhybrid = 0.1214 nm). To further examine the chemical analysis of the fabricated structures, Raman scattering is another effective technique to investigate the crystallinity and the vibrational properties of materials, as Raman signals are very sensitive to the crystal structure and defects. All the curves in Fig. 4 show the existence of one main strong peak at 1430 cm−1, which is attributed to the existence of PEDOT as previously reported in literature [25]. The graphene samples also exhibited one main peak at
3. Results and discussion Fig. 2 (panels a–d) shows the FESEM top view images of the as-
Fig. 2. FESEM top-view images of as-prepared graphene hybrid PEDOT samples, formed on various depositions of platinum thin films (a) 5 nm (b) 30 nm (c) 40 nm and (d) 50 nm (scale bar is 40 μm). 3
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Fig. 3. XRD patterns of (a) PECVD graphene sheets formed on various Pt thicknesses before and after polymer deposition, and (b) variation of the inter-planar space after polymer deposition.
Fig. 5. The photoluminescence spectra for the fabricated electrodes with an excitation wavelength of 530 nm.
Fig. 4. Raman spectra of graphene sheets formed on 5–50 nm Pt thicknesses after deposition of PEDOT and the corresponding spectra of pure graphene.
2337 cm−1, attributable to (second order) double phonon scattering either on a single electron, single hole, or electron-hole pair. Fig. 5. shows the room temperature photoluminescence (PL) spectra of the bare graphene samples at different platinum thicknesses before and after polymer deposition under an excitation length of 530 nm. The unsensitized and sensitized samples exhibited similar PL peaks, with no additional peaks observed for the sensitized samples. Two luminescence bands were observed. The first band was located at ~525 nm. The second emission band appeared at 708 nm, and can be assigned to the recombination of generated holes with single ionized charge states such as oxygen vacancies or surface defects, which is a type of deep-level or trap state emission. Note the diminishment of the near band edge emission (NBE) peak in the polymer-sensitized electrodes compared to the unsensitized ones, indicating a lower recombination rate of photoinduced electron–hole pairs. However, the second emission peak was
Fig. 6. The diffuse reflectance spectra of pure graphene as well as polymersensitized graphene.
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Fig. 7. Cyclic voltammogram of (a) graphene-Pt/Si junction (b) PEDOT hybrid graphene-Pt/Si junction (C) pure PEDOT.
Fig. 8. (a) J-V characteristics from −10 to +10 V under dark conditions, inset J-V −1.0 to +1.0 V (b) the corresponding J-t characteristics at a constant bias of +0.3 V under dark conditions.
counter electrode, and a saturated calomel electrode (SCE) as the reference electrode in phosphate-buffered saline (PBS) electrolyte. Fig. 7 (a) displays the cyclic voltammograms of the graphene 30 nm Pt hybrid PEDOT with a potential window from −1.5 to 1.5 V (vs. SCE) at a scan rate of 20 mV s−1. The sensitized and unsensitized states are reflective for peaks in the CV characterization. Two couples of redox peaks were perceived, which describe more like a double capacitor manner related to the synergetic effect of the inclusion of PEDOT in the graphene framework. This signifies the existence of pseudo-capacitive PEDOT and indicates better capacitive characteristics for the hybrid material with
also attenuated, indicating the possibility of surface defects in the network of graphene [26,27]. Fig. 6. shows the absorption spectra of pure and PEDOT-sensitized graphene electrodes. The pure graphene sample showed a continuous wide reflection with a band edge located at 400 nm. The polymersensitized samples showed a similar reflection profile, however, with a tremendous shift towards higher absorption intensities. The cyclic voltammetry of the hybrid electrodes was evaluated at room temperature by a three-electrode configuration composed of the fabricated samples as the working electrode, platinum (Pt) as the 5
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the electrode due to the Pi-Pi interaction at the interface between graphene and PEDOT. This synergetic effect is mainly originated from the presence of residual oxygen containing groups, aromatic containing units and vacancy defects that will act as active sites to enhance the Pdoping level of the graphene-Pt/Si junction leading to acquire functions and properties for the composite material surpassing that of each of the pristine material.
less ohmic resistance [28]. The J-t curve is to assess the stability of the fabricated electrodes over a period of 300 s and also indicates the optimum values of the dark currents. The electrical measurements were performed via semiconductor parameter analyzer where the samples were connected to positive and negative terminals at −0.3 V from the Si side while the current was read from the upper surface. Upon increasing the thickness of the platinum layer above 40 nm and 50 nm, it was shown that the vertical current of the Pt/Si diode started to decrease due to the existence of trapping states. The trap states exist as deep states at the graphene/Pt interface. Due to the large difference between the work function of Pt (6.1 eV) and that of graphene (4.4 eV), a heavily doped structure will lead to detrimental effects and decrease the current [20]. A slight shift in the fermi level towards the valence band, affecting the electrical structure of graphene without changing the band gap which is verified by Fig. 8 (b) illustrating the I-t characteristics of various platinum thicknesses and is consistent with the results obtained from J-V curves, as the 30 nm sample showed its highest current value at approximately −25 μA. Since the work function of Pt is higher than the work function of graphene, a physisorption interaction between the Pt film and graphene will be formed and produce P-doped graphene [29–31]. Further analysis to adequately verify the proof-of-concept is illustrated by describing the role of each interface, a follows: (1) at the Pt/ n-Si interface, a Schottky barrier is formed at this interface where electrons transfer from Si to Pt at low doping levels, when the positive bias is applied from the Pt side, whereas they cannot overcome the barrier when the positive bias is applied from the Si side. At higher doping levels, both the width of the depletion region and the height of the barrier decreases and, as a result, charges can transfer through the barrier in both directions. (2) at the Graphene/Pt interface, graphene has been reported to form junctions with 2D and 3D semiconducting materials where it can improve electric conductivity, improve the interface transport properties, and perform as an ideal Schottky diode. Pdoped graphene is formed because of its lowered fermi level where electrons migrate in both directions through the potential barrier, either when forward or reverse bias is applied. Finally, (3) at the PEDOT/ Graphene interface, PEDOT acts as a sensitizer due to its immense conductivity and thus can facilitate the migration of electrons and achieve excellent stability.
Acknowledgments This work has been funded by an American University in Cairo Faculty Support Grants. References [1] B. Oregan, M. Gratzel, A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 films, Nature 353 (6346) (1991) 737–740. [2] X.M. Fang, T.L. Ma, G.Q. Guan, M. Akiyama, T. Kida, E. Abe, Effect of the, thickness of the Pt film coated on a counter electrode on the performance of a dye-sensitized solar cell, J. Electroanal. Chem. 570 (2) (2004) 257–263. [3] M.M. Byranvand, Recent development of carbon nanotubes materials as counter electrode for dye-sensitized solar cells, J. Nanostruct. 6 (1) (2016) 1–16. [4] H. Wang, Y.H. Hu, Graphene as a counter electrode material for dyesensitized solar cells, Energy Environ. Sci. 5 (8) (2012) 8182–8188. [5] H. Wang, K. Sun, F. Tao, D.J. Stacchiola, Y.H. Hu, 3D honeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dye-sensitized solar cells, Angew. Chem. Int. Ed. 52 (35) (2013) 9210–9214. [6] G.R. Li, J. Song, G.L. Pan, X.P. Gao, Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells, Energy Environ. Sci. 4 (5) (2011) 1680–1683. [7] T. Muto, M. Ikegami, T. Miyasaka, Polythiophene-based mesoporous counter electrodes for plastic dye-sensitized solar cells, J. Electrochem. Soc. 157 (8) (2010) B1195–B1200. [8] B. Ballarin, S. Masiero, R. Seeber, D. Tonelli, Modification of electrodes with porphyrin-functionalised conductive polymers, J. Electroanal. Chem. 449 (1–2) (1998) 173–180. [9] J.H. Wu, Q.H. Li, L.Q. Fan, Z. Lan, P.J. Li, J.M. Lin, S.C. Hao, Highperformance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Sources 181 (1) (2008) 172–176. [10] T. Muto, M. Ikegami, K. Kobayashi, T. Miyasaka, Conductive polymer-based mesoscopic counterelectrodes for plastic dye-sensitized solar cells, Chem. Lett. 36 (6) (2007) 804–805. [11] B. Fan, X. Mei, K. Sun, J. Ouyang, Conducting polymer/carbon nanotube composite as counter electrode of dye-sensitized solar cells, Appl. Phys. Lett. 93 (14) (2008). [12] A. Kay, M. Grätzel, Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder, Sol. Energy Mater. Sol. Cells 44 (1) (1996) 99–117. [13] H. Irie, Y. Watanabe, K. Hashimoto, Carbon-doped anatase TiO2 powders as a visible-light sensitive photocatalyst, Chem. Lett. 32 (8) (2003) 772–773. [14] W. Hong, Y. Xu, G. Lu, C. Li, G. Shi, Transparent graphene/PEDOT–PSS composite films as counter electrodes of dye-sensitized solar cells, Electrochem. Commun. 10 (10) (2008) 1555–1558. [15] Y. Xu, H. Bai, G. Lu, C. Li, G. Shi, Flexible graphene films via the filtration of watersoluble noncovalent functionalized graphene sheets, J. Am. Chem. Soc. 130 (18) (2008) 5856–5857. [16] J.M. Chem, Doping, Coating and Synergistic Effect for Energy Storage, (2012), pp. 6300–6306. [17] J. Huang, et al., Polymeric semiconductor/graphene hybrid field-effect transistors, Org. Electron. 12 (9) (2011) 1471–1476. [18] A.S. Aricò, et al., Nanostructured Materials for Advanced Energy Conversion and Storage Devices, vol. 4, (2005) no. May. [19] P. Blake, E.W. Hill, Making Graphene Visible, (2007), pp. 2007–2009. [20] A. Kumar, C.H. Lee, Synthesis and Biomedical Applications of Graphene: Present and Future Trends, (2013). [21] L. Baker, et al., Nucleation and growth of Pt atomic layer deposition on Al2O3 substrates using (methylcyclopentadienyl)-trimethyl platinum and O2 plasma, 84333 (2011) (2015). [22] B. Pang, L. Dong, S. Ma, H. Dong, L. Yu, Performance of FTO-free conductive graphene-based counter electrodes for dye-sensitized solar cells, RSC Adv. 6 (47) (2016) 41287–41293. [23] P.A. Khomyakov, G. Giovannetti, P.C. Rusu, G. Brocks, J. Van Den Brink, P.J. Kelly, First-Principles Study of the Interaction and Charge Transfer Between Graphene and Metals, (2009), pp. 1–12. [24] A.A. Farah, et al., Poly (styrenesulfonate) Films Post-Spincasting, vol. 113709, (2012). [25] J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, Review of Photoluminescence Performance of Nano-Sized Semiconductor Materials and its Relationships with Photocatalytic Activity, 90 (2006), pp. 1773–1787. [26] Y. Chang, C. Liu, C. Chen, H. Cheng, The Effect of Geometric Structure on Photoluminescence Characteristics of 1-D TiO2 Nanotubes and 2-D TiO2 Films, vol. 159, (2012), pp. 401–405 no. 7.
4. Conclusion This work represents the development of enhanced performance Graphene-Pt/Si Schottky junction by incorporating conductive polymer, PEDOT as a supporting material via simple, low-cost and effective spin coating technique utilized to modify the structure and properties of Graphene-Pt/Si Schottky junction. In summary, we have successfully developed graphene PEDOT hybrid electrode structure via PECVD of graphene sheets followed by an immediate spin coating of PEDOT. Due to the synergetic effect of the constituents of the hybrid electrodes, the fabricated samples exhibited better absorption properties, a more enhanced current response, as well as superior capacitive performance. This opens a novel pathway to significantly enhance the performance of graphene-based energy conversion devices by introducing a repeatable and scalable process to enhance the performance of graphene-integrated electrodes. In this configuration, the hybrid structure will exhibit the unique properties derived from the presence of graphene due to its large specific surface area, high electrical conductivity, superior electron transfer rate and substantial mechanical strength overcoming the unexistence of a band gap which doesn't enable the device to be switched off by the presence of PEDOT layer with its redox properties having the ability to change its electrochemical properties with oxidation state due to the loss of electrons (oxidation) or the gain of electrons (reduction). Besides, PEDOT will also provide elevated chemical and electrochemical stability, low ohmic drop across 6
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multimodal energy conversion, Sensors and Actuators A: Physical, 245 2016, pp. 169–179 Jul 1. [30] M. Serry, M.A. Sakr, Study of flexoelectricity in graphene composite structures, MRS Adv. 1 (39) (2016) 2723–2729 Jan. [31] M. Serry, A. Sharaf, A. Emira, A. Abdul-Wahed, A. Gamal, Nanostructured graphene–Schottky junction low-bias radiation sensors, Sensors Actuators A Phys. 232 (2015) 329–340.
[27] T. Kuilla, S. Bhadra, D. Yao, N. Hoon, S. Bose, J. Hee, Progress in polymer science: recent advances in graphene based polymer composites, Prog. Polym. Sci. 35 (11) (2010) 1350–1375. [28] L. Mao, M. Li, J. Xue, J. Wang, RSC advances: bendable graphene/conducting polymer hybrid films for freestanding electrodes with high volumetric capacitances †, RSC Adv. 6 (2015) 2951–2957. [29] M. Serry, M.A. Sakr, Graphene-metal-semiconductor composite structure for
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