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Improving photovoltaic properties by incorporating both SPFGraphene and functionalized multiwalled carbon nanotubes
Solar Energy Materials & Solar Cells 94 (2010) 2148–2153
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improving photovoltaic properties by incorporating both SPFGraphene and functionalized multiwalled carbon nanotubes Zhiyong Liu, Dawei He n, Yongsheng Wang n, Hongpeng Wu, Jigang Wang, Haiteng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 May 2010 Received in revised form 20 June 2010 Accepted 5 July 2010
Solution-processable functionalized graphene (SPFGraphene) and functionalized multiwalled carbon nanotubes(f-MWCNTs) are introduced for heterojunction solar cell. The performance of the device has improved by the incorporation of both SPFGraphene and f-MWCNTs. The open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (Z) were 0.67 V, 4.7 mA/cm2, 32%, and 1.05%, respectively. Here, we expect that SPFGraphene acts as exciton dissociation and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole transportation. SPFGraphene and f-MWCNTs incorporation yields better carrier mobility, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action. & 2010 Elsevier B.V. All rights reserved.
Keywords: SPFGraphene f-MWCNTs Charge
1. Introduction The photovoltaic of inorganic materials based on the ZnO, TiO2, CdSe and CdS has attracted much interest of researcher all over the world [1]. However, the photovoltaic devices based on inorganic materials offer great disadvantage because of their high cost and environment-pollute manufacturing methods. Organic photovoltaics (OPVs) are a promising low-cost alternative to silicon solar cells, thus a great deal of effort has been devoted to increase the power conversion efficiency and to scale up the production processes [2]. An attractive feature of the organic photovoltaics based on conjugated polymers is that they can be fabricated by a coating process (e.g. spin coating or inkjet printing) to cover large areas, and may be formed on flexible plastic substrates [3]. The photovoltaic devices based on organic materials have attracted much interest of researcher including materials, processes and devices [4]. Some lab has reported the manufacture of polymer solar cells using full roll-to-roll processing [5]. It has developed the full manufacture, integration and demonstration of polymer solar cells [6]. Power efficiency of organic photovoltaic devices is still low compared to the traditional inorganic devices [7]. The main factor is structural traps in the form of dead ends, isolated domains and incomplete pathways in the random percolation network [8], which has resulted in inefficient hopping charge transport and electron transport. Therefore, the challenge here is to provide continuous pathways within each component and thus to allow charges to
n
Corresponding authors. E-mail addresses: [email protected] (D. He), [email protected] (Y. Wang).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.07.001
transport efficiently to the electrodes before recombination occurs [9]. So far, the research effort of OPV materials has dominated on the PCBM as the electron acceptor. In addition, the solubility and stability of both donor and acceptor are critically important. The most successful OPV cells are based on soluble poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) as the donor and PCBM as the acceptor [10,11]. Some paper had reported the external quantum efficiency (EQE) of P3HT/PCBM hybrid solar cell to be nearly 80%; the power conversion efficiency (PCE) of organic photovoltaic cells has surpassed 7.4%. The structure of devices is based on ITO/PEDOT:PSS/PTB7: PC71BM/Ca/Al, the PTB7 acted as donor materials and the PC71BM acted as acceptor materials [11,12]. However, the power conversion efficiency of these OPV devices is still low compared to conventional inorganic devices [7]. The commonly accepted mechanism for the light-to-electricity conversion process is light absorption exciton generation, exciton diffusion, exciton dissociation and charge formation and charge transport and charge collection [13]. The main factor of lowpower efficiency compared with conventional inorganic devices is the absorption spectrum of P3HT. Thus, new materials for both donor and acceptor with better HOMO/LUMO matching, stronger light absorption and higher charge mobility with good stability is needed. This has led to studies of other allotropic forms of carbon nanomaterials, including single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWNTs) as acceptors [14]. Functionalized multiwalled carbon nanotubes (f-MWCNTs), SWCNTs and PCBM have shown better power conversion efficiency than pristine samples without CNTs or PCBM [15,16]. In such solar cells, it is suggested that MWCNTs enhance hole transport, whereas SWCNTs enhance electron
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transport. However, practically, the solubility and stability of both donor and acceptor are critically important. Graphene, as a very recent rising star in materials science with two-dimensional (2D) structure consisting of sp2-hybridized carbon, exhibits remarkable electronic and mechanical properties that qualify it for application in future optoelectronic devices [17]. It is a gapless semiconductor with unique electronic properties and its electron mobility reaches 200,000 cm2/V s at room temperature [18]. Its one-atom thickness and large 2D plane lead to a large specific area, and therefore, very large interfaces can form when it was added to a polymer matrix. A conducting film and a transparent anode for PV device applications have also been developed [19,20]. The unique structure and excellent electronic properties, particularly its high mobility, and the ready availability of solutionprocessable functionalized graphene (SPFGraphene), render it a competitive alternative as the electron-accepting material in PV device applications [21]. In this paper, the SPFGraphene not only acts as electron acceptors, but also provide high field at the polymer/SPFGraphene interfaces for exciton dissociation.
2. Experimental 2.1. Synthesis of functionalized multiwalled carbon nanotubes (f-MWCNTs) In this work, we aimed to study the role of incorporation of both f-MWCNTs and graphene with conducting polymer to make heterojunction photovoltaic device. Purified MWCNTs was suspended in mixture of concentrated H2SO4/HNO3 (H2SO4:HNO3 is 3:1) and sonicated in a water bath for a few hours. The suspension is diluted by deionized water. A functionalized multiwalled carbon nanotube (0.1 mg) is dispersed in chloroform solvent (1 ml) [14]. 2.2. Synthesis of solution-processable functionalized graphene (SPFGraphene) In this paper, SPFGraphene is been prepared by exfoliated graphene oxide sheets. The first step is the preparation of graphite oxide by the modified hummer method [21]. Five grams of crystalline flake graphite, 30 g KMnO4 and 15 g of NaNO3 (purity 99%) were placed in a flask. Then, 300 ml of H2SO4 (purity 98%) was added, a stirrer chip was placed in the mixture, and the mixture was stirred while being cooled in an ice water bath. The liquid added to 1000 cm3 of deionized water over about 1 h of stirring. Then, 30 ml of H2O2 (30% aqueous solution) was added to the above liquid and the mixture was stirred for 2 h. In order to remove Mn2 + , the resultant liquid purified by repeating the following procedure: centrifugation, removal of the supernatant liquid, addition of a mixed aqueous solution of 0.5% H2O2, and shaking to disperse. The procedure was cycled using aqueous HCl solution (5%) and using H2O, and then drying process in vacuum. The molecular structure of graphite oxide been shown in Fig. 1b. Isocyanate functionalization of graphene oxide: dried graphite oxide (200 mg) was suspended in deionized water (20 ml), and treated with phenyl isocyanate (20 g) for 24 h and the impurities were removed, and finally the isocyanate-treated graphene oxide was obtained [9]. The second step is to exfoliate graphite oxide ultrasonically. Then a phenyl isocyanate treatment resulted in SPFGraphene that can dissolve in organic solvent [22]. 2.3. Fabrication and characterization of optoelectronic devices The organic photovoltaic (OPV) was made using a common fabrication process. The hole-injections buffer layer of (polyethy-
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Fig. 1. (a) Schematic of the devices with P3HT/f-MWCNT-SPFGraphene as the active layer. (b) The chemical structure of SPFGraphene.
lene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) was spin-coated on the indium tin oxide (ITO) coated glass substrate. Then PEDOT:PSS-coated substrate was annealed for 20 min at 120 1C in vacuum. And then spin coating a solution of 15 mg/ml poly(3-hexylthiophene-1,3-diyl) (P3HT) in chlorobenzene with various SPFGraphene contents (0, 1, 2.5, 5, 10, 12.5 and 15 wt%) and 2% f-MWCNTs content onto indium tin oxide (ITO) glass substrate. Then the devices annealed for 10 min at 180 1C in vacuum. LiF and Al were vapor deposited on the active layer. Fig. 1a shows the schematic of the devices with P3HT/SPFGraphene as the active layer. The current–voltage (J–V) was determined using a Keithley 2410 source measure unit. A 150 W xenon lamp acted as a broadband light source and the intensity of incident light is 100 mW/cm2. The photoluminescence been measured using a Fluolog-3fluoresvent spectrometer. The absorption spectra been measured using a Shimadzu UV-3101 PC spectrometer. All measurements were at atmospheric pressure and room temperature.
3. Results and discussion The power conversion efficiency (Z) was calculated according to
Z¼
Voc Isc FF Pin
where Voc, Jsc, Pin and FF are the open-circuit voltage, the shortcircuit current density, the incident light power and the fill factor (FF), respectively. The fill factor (FF) of definition is FF ¼
Vmax Imax Voc Isc
The FF measures the quality of solar cell as a power source and is defined as the ratio between the maximum power delivered to an external circuit and the potential power. Vmax and Imax are, respectively, the values of the voltage and current densities for maximizing for the product of I–V curve in the fourth quadrant, where the device operates as an electrical power source. After functionalization, the SPFGraphene sheet and multiwalled carbon nanotubes introduced many functional groups and the structure been partly isolated by the functional groups. Therefore, the organic functional groups decrease charge transport properties and mobility of the SPFGraphene sheets and f-MWCNTs. This will limit the performance of the above P3HT/ SPFGraphene-f-MWCNTs based device. In view that the functional
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groups can be removed from the SPFGraphene sheet and f-MWCNTs in an elevated temperature under vacuum, the conductivity of the SPFGraphene sheet and f-MWCNTs can be recovered [14,23]. The other affect of introduced functional groups is band gap. Graphene has zero band gap; some paper has reported that the solution-processable functionalized of graphene band gap is 0.4 eV [24]. Clearly, improvement of the overall photovoltaic performance is due to annealing process. Therefore, we will anneal all the optoelectronic devices. For study on absorption spectra of P3HT/SPFGraphene composite film, mixed solution of P3HT/f-MWCNTs, P3HT/f-MWCNTsSPFGraphene (P3HT: 1 mg/ml, SPFGraphene content: 5%) and P3HT dissolved in chlorobenzene was used. Fig. 2 shows the absorption spectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene, as well as the reference solution of P3HT in chlorobenzene. The absorption characteristics of P3HT in the range of 300–800 nm, the original absorption of P3HT centred at 550 nm. However, the absorption spectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene mixed solution is almost the same as the scope and absorption peaks, but the absorption peak of the P3HT/f-MWCNTs-SPFGraphene slightly increases, and enhanced absorption ranging from 340 to 550 nm. This may explain the absorption of P3HT/f-MWCNTs-SPFGraphene composite film. Despite the SPFGraphene content of 5%, the absorption spectra of P3HT/f-MWCNTs-SPFGraphene did not show significant changes. This should be the result of P3HT/f-MWCNTsSPFGraphene mixed in solution, with no significant ground state interaction between the two materials. Therefore, there is no charge transfer in the ground state of P3HT/SPFGraphene composite [21]. SPFGraphene could also exhibit strong donor/acceptor interactions for the conjugated polymers. We will investigate the character of electron acceptor between SPFGraphene and P3HT by photoluminescence (PL). Thus, we will investigate the PL spectra that P3HT/SPFGraphene (P3HT: 5 mg/ml, SPFGraphene content: 0%, 5%, 8% and 10%) mixture solution in chlorobenzene and P3HT (5 mg/ml) solution in chlorobenzene. From Fig. 4 we can see that the pure P3HT solution shows strong photoluminescence between 525 and 750 nm, with excitation at 422 nm. However, introduction of SPFGraphene into the P3HT has remarkably reduced the photoluminescence intensity. It has shown efficient charge/energy transfer along the P3HT/SPFGraphene interface. This efficient quenching of PL emission is due to the efficient electron transfer from P3HT to SPFGraphene. The trend of reduction in PL intensity along with an increase in SPFGraphene content has shown that the efficiency of charge separation has improved in the roughened
1.0
P3HT/f-MWCNT-SPFGraphene P3HT/f-MWCNT P3HT
Absorption (a.u.)
0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
Wavelength (nm) Fig. 2. Absorption spectra of P3HT, P3HT/f-MWCNT and P3HT/f-MWCNTSPFGraphene.
Fig. 3. Energy band diagram of the fabricated device showing band alignment for SPFGraphene.
1.0 SPFGraphene content 12% SPFGraphene content 8% SPFGraphene content 5% SPFGraphene content 0%
0.9 0.8 PL intensity (a.u.)
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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 500
550
600
650
700
750
800
Wavelength (nm) Fig. 4. PL spectra of P3HT and P3HT/SPFGraphene (SPFGraphene contents: 0%, 5%, 8% and 12%) composite films at an excitation wavelength of 422 nm.
P3HT/SPFGraphene configuration. These results show that the quench of fluorophore is due to the electronic interactions at the P3HT/SPFGraphene interfaces. The relative position of donor LUMO and acceptor LUMO is crucial for the aimed charge transfer. Fig. 3 shows that there is a difference between LUMO of P3HT and work function of SPFGraphene. Energy band diagram favored the photoexcited P3HT to transfer electron to SPFGraphene molecule. Therefore, P3HT acted as electron donor and SPFGraphene acted as electron acceptor to prepare donor/acceptor solar cells. The quenching of PL of an appropriate donor polymer by a suitable acceptor gives an indication of an effective donor–acceptor charge transfer from the donor to the acceptor, as described by Sariciftci et al. [25] for composites of p-conducting polymers and SPFGraphene derivatives. The other reason is the increased interfacial areas that facilitate charge separation within the bulk instead of just at the planar interface for the bilayer structure. By referring to previous work with PCBM and carbon nanotubes [26,27], this efficient reduction in PL intensity shows that SPFGraphene expected to be an effective electron acceptor material for organic photovoltaic applications. Fig. 5 shows the current–voltage (J–V) of photovoltaic devices in the dark and AM 1.5 100 mW simulated solar radiation for P3HT/ f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) devices. There is no reaction in the dark of P3HT/ f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) devices. Under simulated 100 mW AM 1.5 G illumination, open-circuit voltage (Voc) of P3HT/f-MWCNTs active layer is 0.65 V, short-circuit current density (Jsc) of P3HT/f-MWCNTs
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active layer is 0.27 mA/cm2, FF of P3HT/f-MWCNTs active layer is 0.27 and power conversion efficiency (Z) of P3HT/f-MWCNTs active layer is 0.65%. In contrast, Voc of P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) has increased to 0.67 V, Jsc has increased to 3.2 mA/cm2, FF has increased to 0.32 and power conversion efficiency (Z) of P3HT/f-MWCNTs-SPFGraphene active layer is 0.9%. Improvement of the overall photovoltaic performance can be attributed to an increase in SPFGraphene. Then we will study the optical and electrical properties of different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%) based on P3HT/f-MWCNTs-SPFGraphene composite, as shown in Fig. 6. The different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%) show different power conversion efficiencies (0.65%, 0.75%, 0.9%, 1.05%, 0.82% and 0.58%), respectively. Fig. 6 shows that along with an increase in SPFGraphene content, the overall performance reached its peak; the best content was 8% and the power efficiency, 1.05%. Voc of the P3HT+f-MWCNTs is 0.65 V and the Voc of composite film P3HT/f-MWCNTs-SPFGraphene is 0.67 V. There are different models describing the Voc of the pure P3HT [25,28]. A single layered organic photovoltaic cell is composed of a pure conjugated
polymer and the Voc principally determined by the work function difference between the two metal electrodes. The configuration of organic photovoltaic devices is the electrode–insulator–metal (MIM) model [29], i.e, ITO–active layer–Al. However, the P3HT/fMWCNTs-SPFGraphene has a BHJ structure, the MIM model is not applicable and Fermi level pinning is the main factor [30]. Therefore, the upper limit of Voc can determine by the difference between the work function of SPFGraphene and f-MWCNTs. Some paper has reported that the work function of as-prepared SPFGraphene is 4.5 eV. The work function of MWCNTs ranges from 4.6 to 5.1 eV. After acid oxidation, carboxylic acid groups were introduced onto the surface of MWCNTs, which produced higher work function (5.1 eV) [14,21]. Energetically favorable charge transportation and band diagram are shown in Fig. 3. Increase in FF can be attributed to the introduction of SPFGraphene in P3HT. Introduced SPFGraphene into P3HT increased the built-in electric field and field-dependent exciton dissociation rate. The SPFGraphene will improve the electron transport and balance the electron-hole pairs transport. Another reason is an improvement of the series and/or the shunt resistance. Introduced SPFGraphene into P3HT will roughen the interface and increase the contact area between the photoactive layer and the Al, and consequently reduce the series resistance [31]. Table 1 shows the J–V curve of different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%). Power conversion efficiencies are 0.65%, 0.75%, 0.9%, 1.05%, 0.82% and 0.58%, respectively. SPFGraphene content of 8% has shown the best results. If the SPFGraphene content is lower than 8%, along with an increase in SPFGraphene content, the power conversion efficiency increases. SPFGraphene content is the main factor improving the power
10
Current Density (mA/cm2)
1 0.1 0.01 1E-3 1E-4
Table 1 Performance details (Voc, Jsc, FF and Z) of the P3HT/f-MWCNT-SPFGraphene based photovoltaic devices.
P3HT/f-MWCNT dark P3HT/f-MWCNT-SPFGraphene dark
1E-5
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P3HT/f-MWCNT light
SPFGraphene content (%)
Voc (V)
Jsc (mA/ cm2)
FF
Z (%)
0 1 5 8 10 12
0.65 0.66 0.67 0.67 0.65 0.54
3.7 3.9 4.4 4.7 4.2 3.5
0.27 0.29 0.31 0.32 0.3 0.31
0.65 0.75 0.9 1.05 0.82 0.58
P3HT/f-MWCNT-SPFGraphene light
1E-6 -0.5
0.0 0.5 Voltage (V)
1.0
1.5
Fig. 5. J–V characteristics of PV devices based in P3HT/f-MWCNT, P3HT/f-MWCNT-SPFGraphene (SPFGraphene content is 5%) in the dark and light.
1.0
4.6
Jsc
4.4 0.8
4.2 4.0 3.8 Jsc PCE
3.6
0.6
3.4
0.68 0.32
0.66 0.64 0.62 Voc
4.8
Power conversion efficiency (PCE)
5.0
0.30 FF
-1.0
0.60 0.58 0.28
0.56 Voc FF
0.54 0.52
0
2
4
6
8
10
12
Weight fraction of SPFGraphene on P3HT/f-MWCNTs solution (%)
0.26 0
2
4
6
8
10
12
Weight fraction of SPFGraphene on P3HT/f-MWCNTs solution (%)
Fig. 6. (a) Dependence of the short-circuit current density and the power conversion efficiency on different SPFGraphene concentrations. (b) Dependence of the open-circuit voltage and the FF on the different SPFGraphene concentrations.
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conversion efficiency. While SPFGraphene concentrations are lower, such as 1%, the SPFGraphene film is too small to form a continuous donor/acceptor interface and the transport pathway for the active layer P3HT matrix. Therefore, the electron cannot effectively meet the donor/acceptor interface and transported smoothly through the active layer. However, SPFGraphene concentration further increased to 8%, the SPFGraphene film can form a continuous donor/acceptor interface and produce a better way to transport smoothly through P3HT matrix. This will improve the electronic transport to form the transport pathway of LUMO–SPFGraphene–Al. The work functions of SPFGraphene are closer to the work functions of Al; this will decrease the barrier of Al/LUMO to form the transport pathway of LUMO–SPFGraphene–Al. In this phase, the SPFGraphene acted as the percolation paths of electron. The work functions of f-MWCNTs are closer to the work functions of ITO; this will decrease the barrier of ITO/HOMO to form the transport pathway of HOMO– f-MWCNTs–ITO; the hole will be transported from HOMO of P3HT to f-MWCNTs, and then transported from f-MWCNTs to ITO. In this phase, the f-MWCNTs acted as the percolation paths of hole. The other reason is that the SPFGraphene acted as an electron acceptor. In the pure conjugated polymer, the excitons can dissociate at the interface of polymer. As can be seen in the band diagram of Fig. 3, while introducing SPFGraphene and f-MWCNTs into the polymers, the excitons can dissociate at the polymer/SPFGraphene and the polymer/f-MWCNTs interfaces. The electrons were captured by the SPFGraphene and transferred to the Al, and the hole was captured by the f-MWCNTs and transferred to the ITO, which is energetically favoured. This results in a faster electron transport than could be achieved in the pristine device by hopping only through the polymer molecule. If there is a further increase in the concentration of SPFGraphene, such as 10% and 12%, then the aggregation of SPFGraphene may occur; the average distance between individual SPFGraphene has decreased and the photogeneration rate has reduced. For a high photocurrent value, we require sufficient interfaces to ensure efficient exciton dissociation and continuous conducting paths for electrons and holes to the appropriate electrodes [32]. In the active layer, the exciton generation takes place only in the polymer. However, SPFGraphene concentration beyond 8%, the average distance between individual SPFGraphene has decreased and the photogeneration rate has reduced. The maximum intensity of the solar spectrum is at a wavelength of about 550 nm within the green band. Otherwise, the inevitable presence of SPFGraphene enhances recombination. SPFGraphene has no band gap and acts as trapping and recombination centres in the band gap of the composite semiconductor medium. On increasing the SPFGraphene concentration, it is likely that the SPFGraphene will align parallel to each other and pack into crystalline ropes due to strong van der Waals attraction. The percentage of SPFGraphene will significantly increase, since only one SPFGraphene is sufficient to convert an entire bundle to a quasi-metallic state. In this way, the negative impact of the SPFGraphene is boosted, since in a given bundle only one SPFGraphene is adequate for transformation of the entire bundle to a quasi-metallic state. It will reduce hole mobility due to increased trapping observed and suppressed carrier extraction. As a result, SPFGraphene content increases beyond 8%, the photocurrent decreases, confirming that the number of extracted carriers decreases. Nevertheless, more charge transport experiments are required to clarify this argument.
4. Conclusion In this paper, SPFGraphene acted as the acceptor material in the organic photovoltaic cells. In the photovoltaic device based on
ITO/PEDOT:PSS/P3HT-f-MWCNTs-SPFGraphene/LiF/Al, P3HT acts as the photoexcited electron donors; SPFGraphene act as electron acceptor and provide percolation paths of electron; f-MWCNTs provide percolation paths of hole. When the SPFGraphene content is 8 wt%, the best Jsc has reach 4.7 mA/cm2, the best Voc has reach 0.67 V, the best FF has reach 0.32 and the power conversion efficiency is 1.05% compared to the other devices.
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improving photovoltaic properties by incorporating both SPFGraphene and functionalized multiwalled carbon nanotubes Zhiyong Liu, Dawei He n, Yongsheng Wang n, Hongpeng Wu, Jigang Wang, Haiteng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 May 2010 Received in revised form 20 June 2010 Accepted 5 July 2010
Solution-processable functionalized graphene (SPFGraphene) and functionalized multiwalled carbon nanotubes(f-MWCNTs) are introduced for heterojunction solar cell. The performance of the device has improved by the incorporation of both SPFGraphene and f-MWCNTs. The open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and power conversion efficiency (Z) were 0.67 V, 4.7 mA/cm2, 32%, and 1.05%, respectively. Here, we expect that SPFGraphene acts as exciton dissociation and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole transportation. SPFGraphene and f-MWCNTs incorporation yields better carrier mobility, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action. & 2010 Elsevier B.V. All rights reserved.
Keywords: SPFGraphene f-MWCNTs Charge
1. Introduction The photovoltaic of inorganic materials based on the ZnO, TiO2, CdSe and CdS has attracted much interest of researcher all over the world [1]. However, the photovoltaic devices based on inorganic materials offer great disadvantage because of their high cost and environment-pollute manufacturing methods. Organic photovoltaics (OPVs) are a promising low-cost alternative to silicon solar cells, thus a great deal of effort has been devoted to increase the power conversion efficiency and to scale up the production processes [2]. An attractive feature of the organic photovoltaics based on conjugated polymers is that they can be fabricated by a coating process (e.g. spin coating or inkjet printing) to cover large areas, and may be formed on flexible plastic substrates [3]. The photovoltaic devices based on organic materials have attracted much interest of researcher including materials, processes and devices [4]. Some lab has reported the manufacture of polymer solar cells using full roll-to-roll processing [5]. It has developed the full manufacture, integration and demonstration of polymer solar cells [6]. Power efficiency of organic photovoltaic devices is still low compared to the traditional inorganic devices [7]. The main factor is structural traps in the form of dead ends, isolated domains and incomplete pathways in the random percolation network [8], which has resulted in inefficient hopping charge transport and electron transport. Therefore, the challenge here is to provide continuous pathways within each component and thus to allow charges to
n
Corresponding authors. E-mail addresses: [email protected] (D. He), [email protected] (Y. Wang).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.07.001
transport efficiently to the electrodes before recombination occurs [9]. So far, the research effort of OPV materials has dominated on the PCBM as the electron acceptor. In addition, the solubility and stability of both donor and acceptor are critically important. The most successful OPV cells are based on soluble poly(3-hexylthiophene) (P3HT) and poly(3-octylthiophene) (P3OT) as the donor and PCBM as the acceptor [10,11]. Some paper had reported the external quantum efficiency (EQE) of P3HT/PCBM hybrid solar cell to be nearly 80%; the power conversion efficiency (PCE) of organic photovoltaic cells has surpassed 7.4%. The structure of devices is based on ITO/PEDOT:PSS/PTB7: PC71BM/Ca/Al, the PTB7 acted as donor materials and the PC71BM acted as acceptor materials [11,12]. However, the power conversion efficiency of these OPV devices is still low compared to conventional inorganic devices [7]. The commonly accepted mechanism for the light-to-electricity conversion process is light absorption exciton generation, exciton diffusion, exciton dissociation and charge formation and charge transport and charge collection [13]. The main factor of lowpower efficiency compared with conventional inorganic devices is the absorption spectrum of P3HT. Thus, new materials for both donor and acceptor with better HOMO/LUMO matching, stronger light absorption and higher charge mobility with good stability is needed. This has led to studies of other allotropic forms of carbon nanomaterials, including single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWNTs) as acceptors [14]. Functionalized multiwalled carbon nanotubes (f-MWCNTs), SWCNTs and PCBM have shown better power conversion efficiency than pristine samples without CNTs or PCBM [15,16]. In such solar cells, it is suggested that MWCNTs enhance hole transport, whereas SWCNTs enhance electron
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transport. However, practically, the solubility and stability of both donor and acceptor are critically important. Graphene, as a very recent rising star in materials science with two-dimensional (2D) structure consisting of sp2-hybridized carbon, exhibits remarkable electronic and mechanical properties that qualify it for application in future optoelectronic devices [17]. It is a gapless semiconductor with unique electronic properties and its electron mobility reaches 200,000 cm2/V s at room temperature [18]. Its one-atom thickness and large 2D plane lead to a large specific area, and therefore, very large interfaces can form when it was added to a polymer matrix. A conducting film and a transparent anode for PV device applications have also been developed [19,20]. The unique structure and excellent electronic properties, particularly its high mobility, and the ready availability of solutionprocessable functionalized graphene (SPFGraphene), render it a competitive alternative as the electron-accepting material in PV device applications [21]. In this paper, the SPFGraphene not only acts as electron acceptors, but also provide high field at the polymer/SPFGraphene interfaces for exciton dissociation.
2. Experimental 2.1. Synthesis of functionalized multiwalled carbon nanotubes (f-MWCNTs) In this work, we aimed to study the role of incorporation of both f-MWCNTs and graphene with conducting polymer to make heterojunction photovoltaic device. Purified MWCNTs was suspended in mixture of concentrated H2SO4/HNO3 (H2SO4:HNO3 is 3:1) and sonicated in a water bath for a few hours. The suspension is diluted by deionized water. A functionalized multiwalled carbon nanotube (0.1 mg) is dispersed in chloroform solvent (1 ml) [14]. 2.2. Synthesis of solution-processable functionalized graphene (SPFGraphene) In this paper, SPFGraphene is been prepared by exfoliated graphene oxide sheets. The first step is the preparation of graphite oxide by the modified hummer method [21]. Five grams of crystalline flake graphite, 30 g KMnO4 and 15 g of NaNO3 (purity 99%) were placed in a flask. Then, 300 ml of H2SO4 (purity 98%) was added, a stirrer chip was placed in the mixture, and the mixture was stirred while being cooled in an ice water bath. The liquid added to 1000 cm3 of deionized water over about 1 h of stirring. Then, 30 ml of H2O2 (30% aqueous solution) was added to the above liquid and the mixture was stirred for 2 h. In order to remove Mn2 + , the resultant liquid purified by repeating the following procedure: centrifugation, removal of the supernatant liquid, addition of a mixed aqueous solution of 0.5% H2O2, and shaking to disperse. The procedure was cycled using aqueous HCl solution (5%) and using H2O, and then drying process in vacuum. The molecular structure of graphite oxide been shown in Fig. 1b. Isocyanate functionalization of graphene oxide: dried graphite oxide (200 mg) was suspended in deionized water (20 ml), and treated with phenyl isocyanate (20 g) for 24 h and the impurities were removed, and finally the isocyanate-treated graphene oxide was obtained [9]. The second step is to exfoliate graphite oxide ultrasonically. Then a phenyl isocyanate treatment resulted in SPFGraphene that can dissolve in organic solvent [22]. 2.3. Fabrication and characterization of optoelectronic devices The organic photovoltaic (OPV) was made using a common fabrication process. The hole-injections buffer layer of (polyethy-
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Fig. 1. (a) Schematic of the devices with P3HT/f-MWCNT-SPFGraphene as the active layer. (b) The chemical structure of SPFGraphene.
lene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS) was spin-coated on the indium tin oxide (ITO) coated glass substrate. Then PEDOT:PSS-coated substrate was annealed for 20 min at 120 1C in vacuum. And then spin coating a solution of 15 mg/ml poly(3-hexylthiophene-1,3-diyl) (P3HT) in chlorobenzene with various SPFGraphene contents (0, 1, 2.5, 5, 10, 12.5 and 15 wt%) and 2% f-MWCNTs content onto indium tin oxide (ITO) glass substrate. Then the devices annealed for 10 min at 180 1C in vacuum. LiF and Al were vapor deposited on the active layer. Fig. 1a shows the schematic of the devices with P3HT/SPFGraphene as the active layer. The current–voltage (J–V) was determined using a Keithley 2410 source measure unit. A 150 W xenon lamp acted as a broadband light source and the intensity of incident light is 100 mW/cm2. The photoluminescence been measured using a Fluolog-3fluoresvent spectrometer. The absorption spectra been measured using a Shimadzu UV-3101 PC spectrometer. All measurements were at atmospheric pressure and room temperature.
3. Results and discussion The power conversion efficiency (Z) was calculated according to
Z¼
Voc Isc FF Pin
where Voc, Jsc, Pin and FF are the open-circuit voltage, the shortcircuit current density, the incident light power and the fill factor (FF), respectively. The fill factor (FF) of definition is FF ¼
Vmax Imax Voc Isc
The FF measures the quality of solar cell as a power source and is defined as the ratio between the maximum power delivered to an external circuit and the potential power. Vmax and Imax are, respectively, the values of the voltage and current densities for maximizing for the product of I–V curve in the fourth quadrant, where the device operates as an electrical power source. After functionalization, the SPFGraphene sheet and multiwalled carbon nanotubes introduced many functional groups and the structure been partly isolated by the functional groups. Therefore, the organic functional groups decrease charge transport properties and mobility of the SPFGraphene sheets and f-MWCNTs. This will limit the performance of the above P3HT/ SPFGraphene-f-MWCNTs based device. In view that the functional
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groups can be removed from the SPFGraphene sheet and f-MWCNTs in an elevated temperature under vacuum, the conductivity of the SPFGraphene sheet and f-MWCNTs can be recovered [14,23]. The other affect of introduced functional groups is band gap. Graphene has zero band gap; some paper has reported that the solution-processable functionalized of graphene band gap is 0.4 eV [24]. Clearly, improvement of the overall photovoltaic performance is due to annealing process. Therefore, we will anneal all the optoelectronic devices. For study on absorption spectra of P3HT/SPFGraphene composite film, mixed solution of P3HT/f-MWCNTs, P3HT/f-MWCNTsSPFGraphene (P3HT: 1 mg/ml, SPFGraphene content: 5%) and P3HT dissolved in chlorobenzene was used. Fig. 2 shows the absorption spectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene, as well as the reference solution of P3HT in chlorobenzene. The absorption characteristics of P3HT in the range of 300–800 nm, the original absorption of P3HT centred at 550 nm. However, the absorption spectra of P3HT/f-MWCNTs, P3HT/f-MWCNTs-SPFGraphene mixed solution is almost the same as the scope and absorption peaks, but the absorption peak of the P3HT/f-MWCNTs-SPFGraphene slightly increases, and enhanced absorption ranging from 340 to 550 nm. This may explain the absorption of P3HT/f-MWCNTs-SPFGraphene composite film. Despite the SPFGraphene content of 5%, the absorption spectra of P3HT/f-MWCNTs-SPFGraphene did not show significant changes. This should be the result of P3HT/f-MWCNTsSPFGraphene mixed in solution, with no significant ground state interaction between the two materials. Therefore, there is no charge transfer in the ground state of P3HT/SPFGraphene composite [21]. SPFGraphene could also exhibit strong donor/acceptor interactions for the conjugated polymers. We will investigate the character of electron acceptor between SPFGraphene and P3HT by photoluminescence (PL). Thus, we will investigate the PL spectra that P3HT/SPFGraphene (P3HT: 5 mg/ml, SPFGraphene content: 0%, 5%, 8% and 10%) mixture solution in chlorobenzene and P3HT (5 mg/ml) solution in chlorobenzene. From Fig. 4 we can see that the pure P3HT solution shows strong photoluminescence between 525 and 750 nm, with excitation at 422 nm. However, introduction of SPFGraphene into the P3HT has remarkably reduced the photoluminescence intensity. It has shown efficient charge/energy transfer along the P3HT/SPFGraphene interface. This efficient quenching of PL emission is due to the efficient electron transfer from P3HT to SPFGraphene. The trend of reduction in PL intensity along with an increase in SPFGraphene content has shown that the efficiency of charge separation has improved in the roughened
1.0
P3HT/f-MWCNT-SPFGraphene P3HT/f-MWCNT P3HT
Absorption (a.u.)
0.8 0.6 0.4 0.2 0.0 300
400
500
600
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Wavelength (nm) Fig. 2. Absorption spectra of P3HT, P3HT/f-MWCNT and P3HT/f-MWCNTSPFGraphene.
Fig. 3. Energy band diagram of the fabricated device showing band alignment for SPFGraphene.
1.0 SPFGraphene content 12% SPFGraphene content 8% SPFGraphene content 5% SPFGraphene content 0%
0.9 0.8 PL intensity (a.u.)
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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 500
550
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650
700
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Wavelength (nm) Fig. 4. PL spectra of P3HT and P3HT/SPFGraphene (SPFGraphene contents: 0%, 5%, 8% and 12%) composite films at an excitation wavelength of 422 nm.
P3HT/SPFGraphene configuration. These results show that the quench of fluorophore is due to the electronic interactions at the P3HT/SPFGraphene interfaces. The relative position of donor LUMO and acceptor LUMO is crucial for the aimed charge transfer. Fig. 3 shows that there is a difference between LUMO of P3HT and work function of SPFGraphene. Energy band diagram favored the photoexcited P3HT to transfer electron to SPFGraphene molecule. Therefore, P3HT acted as electron donor and SPFGraphene acted as electron acceptor to prepare donor/acceptor solar cells. The quenching of PL of an appropriate donor polymer by a suitable acceptor gives an indication of an effective donor–acceptor charge transfer from the donor to the acceptor, as described by Sariciftci et al. [25] for composites of p-conducting polymers and SPFGraphene derivatives. The other reason is the increased interfacial areas that facilitate charge separation within the bulk instead of just at the planar interface for the bilayer structure. By referring to previous work with PCBM and carbon nanotubes [26,27], this efficient reduction in PL intensity shows that SPFGraphene expected to be an effective electron acceptor material for organic photovoltaic applications. Fig. 5 shows the current–voltage (J–V) of photovoltaic devices in the dark and AM 1.5 100 mW simulated solar radiation for P3HT/ f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) devices. There is no reaction in the dark of P3HT/ f-MWCNTs and P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) devices. Under simulated 100 mW AM 1.5 G illumination, open-circuit voltage (Voc) of P3HT/f-MWCNTs active layer is 0.65 V, short-circuit current density (Jsc) of P3HT/f-MWCNTs
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active layer is 0.27 mA/cm2, FF of P3HT/f-MWCNTs active layer is 0.27 and power conversion efficiency (Z) of P3HT/f-MWCNTs active layer is 0.65%. In contrast, Voc of P3HT/f-MWCNTs-SPFGraphene (SPFGraphene content is 5%) has increased to 0.67 V, Jsc has increased to 3.2 mA/cm2, FF has increased to 0.32 and power conversion efficiency (Z) of P3HT/f-MWCNTs-SPFGraphene active layer is 0.9%. Improvement of the overall photovoltaic performance can be attributed to an increase in SPFGraphene. Then we will study the optical and electrical properties of different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%) based on P3HT/f-MWCNTs-SPFGraphene composite, as shown in Fig. 6. The different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%) show different power conversion efficiencies (0.65%, 0.75%, 0.9%, 1.05%, 0.82% and 0.58%), respectively. Fig. 6 shows that along with an increase in SPFGraphene content, the overall performance reached its peak; the best content was 8% and the power efficiency, 1.05%. Voc of the P3HT+f-MWCNTs is 0.65 V and the Voc of composite film P3HT/f-MWCNTs-SPFGraphene is 0.67 V. There are different models describing the Voc of the pure P3HT [25,28]. A single layered organic photovoltaic cell is composed of a pure conjugated
polymer and the Voc principally determined by the work function difference between the two metal electrodes. The configuration of organic photovoltaic devices is the electrode–insulator–metal (MIM) model [29], i.e, ITO–active layer–Al. However, the P3HT/fMWCNTs-SPFGraphene has a BHJ structure, the MIM model is not applicable and Fermi level pinning is the main factor [30]. Therefore, the upper limit of Voc can determine by the difference between the work function of SPFGraphene and f-MWCNTs. Some paper has reported that the work function of as-prepared SPFGraphene is 4.5 eV. The work function of MWCNTs ranges from 4.6 to 5.1 eV. After acid oxidation, carboxylic acid groups were introduced onto the surface of MWCNTs, which produced higher work function (5.1 eV) [14,21]. Energetically favorable charge transportation and band diagram are shown in Fig. 3. Increase in FF can be attributed to the introduction of SPFGraphene in P3HT. Introduced SPFGraphene into P3HT increased the built-in electric field and field-dependent exciton dissociation rate. The SPFGraphene will improve the electron transport and balance the electron-hole pairs transport. Another reason is an improvement of the series and/or the shunt resistance. Introduced SPFGraphene into P3HT will roughen the interface and increase the contact area between the photoactive layer and the Al, and consequently reduce the series resistance [31]. Table 1 shows the J–V curve of different SPFGraphene contents (0%, 1%, 5%, 8%, 10% and 12%). Power conversion efficiencies are 0.65%, 0.75%, 0.9%, 1.05%, 0.82% and 0.58%, respectively. SPFGraphene content of 8% has shown the best results. If the SPFGraphene content is lower than 8%, along with an increase in SPFGraphene content, the power conversion efficiency increases. SPFGraphene content is the main factor improving the power
10
Current Density (mA/cm2)
1 0.1 0.01 1E-3 1E-4
Table 1 Performance details (Voc, Jsc, FF and Z) of the P3HT/f-MWCNT-SPFGraphene based photovoltaic devices.
P3HT/f-MWCNT dark P3HT/f-MWCNT-SPFGraphene dark
1E-5
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P3HT/f-MWCNT light
SPFGraphene content (%)
Voc (V)
Jsc (mA/ cm2)
FF
Z (%)
0 1 5 8 10 12
0.65 0.66 0.67 0.67 0.65 0.54
3.7 3.9 4.4 4.7 4.2 3.5
0.27 0.29 0.31 0.32 0.3 0.31
0.65 0.75 0.9 1.05 0.82 0.58
P3HT/f-MWCNT-SPFGraphene light
1E-6 -0.5
0.0 0.5 Voltage (V)
1.0
1.5
Fig. 5. J–V characteristics of PV devices based in P3HT/f-MWCNT, P3HT/f-MWCNT-SPFGraphene (SPFGraphene content is 5%) in the dark and light.
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Jsc
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0.66 0.64 0.62 Voc
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0.56 Voc FF
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0.26 0
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Fig. 6. (a) Dependence of the short-circuit current density and the power conversion efficiency on different SPFGraphene concentrations. (b) Dependence of the open-circuit voltage and the FF on the different SPFGraphene concentrations.
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conversion efficiency. While SPFGraphene concentrations are lower, such as 1%, the SPFGraphene film is too small to form a continuous donor/acceptor interface and the transport pathway for the active layer P3HT matrix. Therefore, the electron cannot effectively meet the donor/acceptor interface and transported smoothly through the active layer. However, SPFGraphene concentration further increased to 8%, the SPFGraphene film can form a continuous donor/acceptor interface and produce a better way to transport smoothly through P3HT matrix. This will improve the electronic transport to form the transport pathway of LUMO–SPFGraphene–Al. The work functions of SPFGraphene are closer to the work functions of Al; this will decrease the barrier of Al/LUMO to form the transport pathway of LUMO–SPFGraphene–Al. In this phase, the SPFGraphene acted as the percolation paths of electron. The work functions of f-MWCNTs are closer to the work functions of ITO; this will decrease the barrier of ITO/HOMO to form the transport pathway of HOMO– f-MWCNTs–ITO; the hole will be transported from HOMO of P3HT to f-MWCNTs, and then transported from f-MWCNTs to ITO. In this phase, the f-MWCNTs acted as the percolation paths of hole. The other reason is that the SPFGraphene acted as an electron acceptor. In the pure conjugated polymer, the excitons can dissociate at the interface of polymer. As can be seen in the band diagram of Fig. 3, while introducing SPFGraphene and f-MWCNTs into the polymers, the excitons can dissociate at the polymer/SPFGraphene and the polymer/f-MWCNTs interfaces. The electrons were captured by the SPFGraphene and transferred to the Al, and the hole was captured by the f-MWCNTs and transferred to the ITO, which is energetically favoured. This results in a faster electron transport than could be achieved in the pristine device by hopping only through the polymer molecule. If there is a further increase in the concentration of SPFGraphene, such as 10% and 12%, then the aggregation of SPFGraphene may occur; the average distance between individual SPFGraphene has decreased and the photogeneration rate has reduced. For a high photocurrent value, we require sufficient interfaces to ensure efficient exciton dissociation and continuous conducting paths for electrons and holes to the appropriate electrodes [32]. In the active layer, the exciton generation takes place only in the polymer. However, SPFGraphene concentration beyond 8%, the average distance between individual SPFGraphene has decreased and the photogeneration rate has reduced. The maximum intensity of the solar spectrum is at a wavelength of about 550 nm within the green band. Otherwise, the inevitable presence of SPFGraphene enhances recombination. SPFGraphene has no band gap and acts as trapping and recombination centres in the band gap of the composite semiconductor medium. On increasing the SPFGraphene concentration, it is likely that the SPFGraphene will align parallel to each other and pack into crystalline ropes due to strong van der Waals attraction. The percentage of SPFGraphene will significantly increase, since only one SPFGraphene is sufficient to convert an entire bundle to a quasi-metallic state. In this way, the negative impact of the SPFGraphene is boosted, since in a given bundle only one SPFGraphene is adequate for transformation of the entire bundle to a quasi-metallic state. It will reduce hole mobility due to increased trapping observed and suppressed carrier extraction. As a result, SPFGraphene content increases beyond 8%, the photocurrent decreases, confirming that the number of extracted carriers decreases. Nevertheless, more charge transport experiments are required to clarify this argument.
4. Conclusion In this paper, SPFGraphene acted as the acceptor material in the organic photovoltaic cells. In the photovoltaic device based on
ITO/PEDOT:PSS/P3HT-f-MWCNTs-SPFGraphene/LiF/Al, P3HT acts as the photoexcited electron donors; SPFGraphene act as electron acceptor and provide percolation paths of electron; f-MWCNTs provide percolation paths of hole. When the SPFGraphene content is 8 wt%, the best Jsc has reach 4.7 mA/cm2, the best Voc has reach 0.67 V, the best FF has reach 0.32 and the power conversion efficiency is 1.05% compared to the other devices.
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