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Neutral-Color Semitransparent Organic Solar Cells with All-Graphene Electrodes Zhike Liu, Peng You, Shenghua Liu, and Feng Yan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04858 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015
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Neutral-Color Semitransparent Organic Solar Cells with All-Graphene Electrodes Zhike Liu, Peng You, Shenghua Liu and Feng Yan* Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China AUTHOR EMAIL ADDRESS (
[email protected]) RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)
CORRESPONDING AUTHOR FOOTNOTE ( Prof. Feng Yan, Tel: +852 2766 4054, FAX: +852 2333 7629;
[email protected];).
___________________________________________________________________________________ * Address correspondence to: Prof. Feng Yan, Tel: +852 2766 4054, FAX: +852 2333 7629;
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ABSTRACT
Graphene has been considered as a promising material for transparent electrodes due to its advantages including ultrahigh carrier mobilities, high optical transmittance, excellent mechanical flexibility and good stability. Solar cells with all-graphene electrodes are potentially low-cost, high-performance and environmental friendly, which however have not been realized until now. Here, we report the fabrication of semitransparent organic photovoltaics (OPVs) with graphene transparent electrodes as both cathode and anode, which can absorb light from both sides with the power conversion efficiency up to 3.4%. Meanwhile, the OPVs have a neutral color and show the transmittance of ~40% in the visible region, making them suitable for some special applications, such as power-generating windows and building integrated photovoltaics. This work demonstrates the great potential of graphene for the applications in carbon-based optoelectronic devices.
KEYWORDS: (Graphene; Organic Photovoltaic; Semitransparent; Efficiency)
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Organic photovoltaics (OPVs) have attracted much attention in the past decade due to their convenient fabrication, mechanical flexibility and low cost.1-4 Dramatic progress of OPVs has been made recently on device efficiencies and versatile applications especially in the niche areas not applicable for Si-based solar cells.5-9 Besides the organic-bulk-heterojunction active layers, the electrodes especially transparent electrodes in the OPVs are critical to the device performance, which can significantly influence the device efficiency, stability and mechanical flexibility. Indium tin oxide (ITO) is a popularly used transparent electrode material for organic solar cells.9 However, in practical applications, ITO electrodes have some drawbacks, including high cost, limited supply of indium, fragility and instability towards acid or base. Moreover, some disadvantages also exist in metal electrodes used in OPVs, such as the high cost for noble metals (e.g. Au and Ag) and poor chemical stability for Al. Therefore, highperformance and low-cost electrode materials are greatly demanded in OPVs.
Recently, graphene has shown many advantages over conventional electrode materials used in solar cells in terms of transparency, cost, stability, mechanical flexibility and encapsulation ability.10, 11 Many research groups have attempted to use graphene in organic solar cells as transparent anodes.12-14 In 2011, Wang et al. used multilayer stacked graphene prepared by the chemical vapor deposition (CVD) method as anodes of OPVs and obtained the power conversion efficiency (PCE) of 2.5%.15 Hsu et al. reported the application of stacked CVD graphene doped with tetracyanoquinodimethane molecules and showed the maximum PCE of 2.58%.16 In 2013, we prepared package-free flexible OPVs with graphene transparent anodes and showed the highest PCE of 3.2% with excellent air stability since the graphene top electrodes can effectively protect OPVs from air contamination.17 Meanwhile, graphene has been used as cathodes of OPVs although the device efficiencies are relatively low due to some difficulties in device fabrication.18 For example, Shin et al. prepared OPVs based on an active layer of poly(3hexylthiophene):[6,6]-phenyl-C71butyric acid methyl ester (P3HT:PC71BM) and a ZnO-modified graphene transparent cathode with PCEs of 2.27%.19
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Considering the advantages of graphene electrodes, here we develop semitransparent OPVs with allgraphene electrodes (both anode and cathode) for the first time, which can absorb light from each side with the similar PCE. Since the devices are mainly made of carbon, the devices are potentially low cost and
environmental
friendly.
We
modified
graphene
films
with
poly(3,4-ethylene
dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layers to improve their conductivity, which can be directly used as transparent anode.17 After coating a thin layer of ZnO nanoparticles (ZnO-NPs) as an electron transport layer (ETL) on its surface by solution process, the graphene film can be used as a transparent
cathode
as
thiophene/benzodithiophene
well. (PTB7)
Consequently, and
PC71BM
OPVs with
based the
on
polythieno[3,4-b]-
inverted
structure
of
Graphene/PEDOT:PSS/PTB7:PC71BM/ZnO-NP/PEDOT:PSS/Graphene/Glass are successfully prepared and show PCEs up to 3.4% from either side.2 The semitransparent devices are promising for many emerging applications such as power-generating windows.20
RESULTS AND DISCUSSION
To realize OPVs with all-graphene electrodes, large-area graphene with high conductivity and transparency is needed. Graphene films were synthesized by a CVD method on copper foils and then transferred and stacked onto glass substrates layer by layer with a conventional method.17 Although the sheet resistance (Rs) of the multi-layer graphene decreases with the increase of the layer number, more layers of graphene leads to lower transmittance and higher surface roughness, which will influence the light absorbance and charge transfer in OPVs.15, 21 2-layer stacked graphene (2L-G) was found to be the optimum condition for OPVs in our experiments.17 Therefore, the graphene electrodes used in the following experiments are 2L-G unless otherwise specifically described. The sheet resistance of 2 layers of the intrinsic graphene measured by a four-probe system is ~320Ω/ and its average transmittance is 94±0.5% at λ=550 nm. To increase the conductivity of the 2L-G, doping techniques that can induce more carriers (electrons or holes) in graphene were then introduced in the films. For graphene anode, the
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film was coated with a thin layer of PEDOT:PSS (thickness: ~50nm) and annealed at 125°C for 30 minutes, leading to a much lower sheet resistance of about 170Ω/. In our experiments, to improve the uniformity of the films coated on the surface of graphene, the graphene films were treated by oxygen plasma for 10 seconds to change their surface energy,22, 23 while negligible damage was observed in the graphene conductivity (see Supporting Information, Section 1 and Figure S1-4). PEDOT:PSS has the Fermi energy level of about 5.0eV while intrinsic graphene has the Fermi level of about 4.6eV.17 Therefore, the coated PEDOT:PSS can increase the Fermi level to higher value and induce higher density of holes in graphene, making the graphene film to be more conductive than the intrinsic one.
For graphene cathode, n-type doping can be realized by coating a thin layer of n-type oxide semiconductor, such as ZnO or TiO2, on the surface.19, 21 In this work, the graphene films were modified with ZnO by the following three methods, as shown in Figure 1a. First, a ZnO film was prepared on a graphene film by coating the sol-gel precursor solution of zinc acetate dehydrate dissolved in 2methoxyethanol, followed by thermal annealing at 165°C for 30 minutes. However, the sheet resistance of the graphene film was increased to ~780 Ω/ due to the destructive effect of the precursor solution on graphene during the annealing process.24 Thus we developed the second method to modify the graphene electrode by coating ZnO-NPs on the surface by solution process. ZnO-NPs were synthesized by hydrothermal method and the average size was controlled to be ~5nm, as shown in the TEM image in Figure 1b.25 A uniform ZnO-NP film can be formed on the graphene film (see supporting information, Figure S2d) after a thermal annealing at a lower temperature of 135°C for 30 minutes while the sheet resistance was also increased to ~ 380Ω/. So, the above two methods unfortunately increase the sheet resistances of the graphene films, indicating that the direct coating of ZnO layers on the surfaces may damage the graphene films. To overcome this problem, we coated a thin layer of PEDOT:PSS film (thickness: ~25nm) on graphene, followed by coating a thin layer of ZnO-NPs on the surface. Here, the PEDOT:PSS film can protect the graphene film from a direct contact with ZnO, which leads to the
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decrease of the sheet resistance of graphene down to ~230Ω/. Since the ZnO-NP film on the surface is an ETL, the ZnO-NP/PEDOT:PSS/Graphene film can be used as a transparent cathode in OPVs.
The doping effect in graphene was studied by measuring Raman spectra of the films, as shown in Figure 1c. The pristine CVD graphene film shows G and 2D peaks with the peak height ratio between them of 1:2, indicating that the graphene film is mainly single layer.26 The 2-layer stacked graphene film shows the similar Raman spectrum of the single layer, indicating that the weak interaction between the two layers does not influence their physical properties. We can find that the short-time oxygen plasma treatment cannot change the Raman peaks of the graphene electrodes. After coating a thin layer of PEDOT:PSS on the surface, the 2D peak shows a blue shift, corresponding to a p-type doping effect.[15] The graphene film coated with a ZnO layer or ZnO-NPs shows a red shift of the 2D peak, corresponding to n-type doping of graphene.27 However, the graphene film coated with ZnO by using sol-gel solution shows an additional D peak induced by defects in graphene.28 So the ZnO precursor can damage the graphene film during the deposition process, which is consistent with the observation of the increased sheet resistance of the graphene film after the ZnO deposition. The ZnO-NPs -coated graphene does not show a D peak, indicating that much lower density of defects was introduced in graphene by the ZnONPs.
The modified graphene was firstly used as transparent cathodes in OPVs. Devices with the inverted structure of Au/MoO3/PTB7:PC71BM/buffer layer/graphene/glass were fabricated at optimized conditions. Energy diagram of the device is shown in Figure 1d. Three types of graphene cathodes modified with different buffer layers, including (I) ZnO/graphene (II) ZnO-NP/graphene and (III) ZnONP/PEDOT:PSS/graphene electrodes, were prepared and used in the devices. The current densityvoltage (J-V) characteristics and external quantum efficiencies (EQEs) of the OPVs with different graphene cathodes are shown in Figure 1e and 1f, respectively. The photovoltaic parameters of a group of devices are summarized in Table 1. As expected, the device based on ZnO-NP/PEDOT:PSS/graphene
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cathode shows the highest PCE (5.78%) compared to the devices with only ZnO (PCE: 4.64%) or ZnONP (PCE: 4.89%) as the buffer layers, which is mainly attributed to the highest fill factor (FF) of the former because of the lowest sheet resistance of the ZnO-NP/PEDOT:PSS -modified graphene cathode (type III).
Next, we prepared transparent graphene anodes and laminated them on inverted OPVs as top transparent electrodes. A graphene/PMMA film was stacked on a thin polydimethylsiloxane (PDMS) film (< 1 mm), making it convenient to be transferred and laminated on a conformal surface of a device, as shown in Figure 2a and b. Then the graphene/PMMA/PDMS film was coated with a thin PEDOT:PSS film to have p-type doping in graphene. To improve the adhesion of the top electrode on the organic active layer during lamination, an effective electronic glue (D-sorbitol) for organic optoelectronic devices was added in the PEDOT:PSS film with the weight ratio of ~5%,29 which can greatly improve the stability of the laminated devices. As shown in Figure S3 in the supporting information, PEDOT:PSS can be uniformly covered on the graphene film in large area and the area enclosed
by
purple
lines
denotes
the
position
of
graphene.
Then,
the
PEDOT:PSS/graphene/PMMA/PDMS film was laminated on a PTB7:PC71BM/ZnO-NPs/ITO device. In the end, the device was encapsulated with a piece of glass and epoxy resin on the top. It is obvious that the device is semitransparent and can absorb light from both ITO and graphene sides. The corresponding J-V curves of the semitransparent device illuminated from ITO or graphene side are shown in Figure 2c. The photovoltaic parameters of the devices are summarized in Table 2. The devices show average PCEs of 4.20% and 3.75% when illuminated from graphene and ITO side, respectively, which are much higher than the PCEs of the previously reported semitransparent solar cells with graphene electrodes.14, 19 It is interesting to find that the device shows higher FF and efficiency from graphene than from ITO side, which is presumably due to the increased conductivity of the graphene electrode under direct light illumination.30 Figure 2d shows the EQE spectra of the device illuminated from either side. It can be found that, in the wavelength region longer than 550 nm, the device shows higher EQE from ITO side ACS Paragon Plus Environment
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than from graphene side, while in the wavelength range from 370 to 550 nm, higher EQE can be observed from graphene side, which can be attributed to the different transmittance spectra of the two electrodes (see supporting information, Figure S5). A sharp decrease of EQE in the wavelength region shorter than 370nm can be found when the device is illuminated from graphene side, which is due to the light absorption of the epoxy resin in the UV region (see supporting information, Figure S6).
Based on the above graphene cathode and anode, semitransparent OPVs with all-graphene electrodes can be conveniently prepared with the procedure shown in Figure 3. Inverted OPVs with the three types of graphene cathodes and the same graphene anodes (PEDOT:PSS/graphene) were prepared. The fabrication details are described in the experimental section. Figure 4a shows the J-V characteristics of the three types of OPVs illuminated from each side. The related photovoltaic parameters are presented in Table 3. It is reasonable to find that the FFs and PCEs of the devices increase with the decrease of the sheet resistance of the graphene cathode. The device with the ZnO-NP/PEDOT:PSS/graphene cathode (type III) shows the best performance due to the lowest sheet resistance of the graphene cathode and exhibits Voc of 0.67 (0.65) V, Jsc of 12.10 (11.87) mA/cm2, FF of 41.4 (39.7) % and η of 3.35 (3.06) % when it is illuminated from the top (bottom) graphene electrode. The data in the bracket is for the device illuminated from the bottom graphene electrodes.
Figure 4b shows the EQE spectra of the device with the ZnO-NP/PEDOT:PSS/graphene cathode (type III). The device shows higher EQE in the visible region from the graphene anode (top) than from the graphene cathode (bottom) due to the different transmittance spectra of the two graphene electrodes. The sharp decrease of EQE illuminated from the top graphene electrode in the UV region (wavelength ≤ 370nm) is due to the absorption of epoxy resin, which is similar to the case in Figure 2d. The transmittance of the two types of semitransparent OPVs, including the device with a graphene anode and an ITO cathode and the device with all-graphene electrodes, was characterized and shown in Figure 4c. The two OPVs exhibit desirable average transmittance of ~40% in the visible region, making it suitable ACS Paragon Plus Environment
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for some special applications. The high transmittance of the device is due to the high transparency of the graphene electrodes and the thin thickness of the PTB7:PC71BM active layer, which was characterized to be about 90nm. Obviously, the transmittance can be further tuned by changing the thickness of the active layer. The image in Figure 4d shows that the semitransparent solar cell with all-graphene electrodes induces little colour change across it. Figure 4e shows the International Commission on Illumination (CIE) color perception diagram of the AM1.5 solar simulator and the light pass through the two semitransparent OPVs. The spectrum of the AM1.5 solar simulator shows the color coordinate of (0.3202, 0.3324), which are very close to that of white light (0.33, 0.33). It is interesting to find that lights pass through the two OPVs with graphene cathode or ITO cathode have the color coordinates of (0.3098, 0.3306) and (0.3109, 0.3357), respectively, representing excellent color-neutrality.31 So the devices we obtained are neutral-color semitransparent OPVs.
In comparison with the standard OPVs based on PTB7:PC71BM,2 our devices show acceptable quantum efficiencies and short circuit current density (Jsc) since they only absorb light for about 60% and the internal quantum efficiencies are estimated to be as high as 90%. However, the FFs of the devices with all-graphene electrodes ( FF: ~40% ) are relatively low due to the high sheet resistances (~200Ω/) of the graphene electrodes. So the key issue in improving the efficiencies of the devices is to decrease the sheet resistances and keep the high transmittance of the graphene electrodes. One feasible approach is to develop more effective doping strategies on the graphene electrodes, which will be investigated in the future. Moreover, since large-area CVD graphene layers can be transferred by roll-toroll processes on various substrates,11 the device fabrication techniques reported here can be further developed for the mass production of large-area OPVs.
It is notable that carbon-based solar cells have aroused much research interests in recent years since carbon materials are inexpensive, readily available and environmental friendly.32-39 However, as shown in Figure 5, the efficiencies of the currently available carbon-based devices are very low in comparison
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with those of OPVs and the recently reported perovskite solar cells.7-9, 40-42 Therefore, there is a great challenge in realizing high-efficiency carbon-based solar cells due to the lack of suitable carbon materials for both active layers and electrodes. For devices based on carbon active layers, such as carbon nanotubes and C60, the highest PCE is only about 1.3%.32 The reported all-carbon solar cells with the reduced graphene oxide and single-wall carbon nanotubes as anode and cathode, respectively, show a PCE of only 5.7×10-3 %, which is too low to be used in practical applications.36 On the other hand, for solar cells with all-carbon electrodes, the reported efficiencies are normally less than 2%.38, 39 Therefore, the OPVs with all-graphene electrodes presented in this paper exhibit the highest PCE in this field, as shown in Figure 5 and Table S1 in the supporting information, and may provide a feasible approach to realizing high-performance all-carbon solar cells in the future.
SUMMARY AND CONCLUSION
In summary, we developed a novel method for preparing OPVs with all-graphene electrodes by replacing the conventional cathode (ITO) and anode (Au or Ag) with n-type (ZnO) and p-type (PEDOT:PSS) semiconductors –modified CVD graphene films, respectively. The semitransparent OPVs show an average transmittance of ~40% with neutral colour and PCEs up to ~3.4% from either side, which can find many potential applications such as power-generating windows and building integrated photovoltaics. The devices can be conveniently prepared by low-temperature processes, including film transfer, spin coating and laminating, indicating that the technique is compatible with the fabrication of many other optoelectronic devices, such as other types of solar cells and light emitting diodes. This work paves a way for realizing high-performance carbon-based optoelectronic devices in the future.
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METHODS Preparation of graphene cathode and anode: Single-layer graphene was synthesized on copper foils by CVD method.26 A layer of poly(methyl methacrylate) (PMMA) (300 nm) was spin-coated on graphene and then annealed at 90oC for about 30 min. After that, it was immersed in an aqueous solution of iron chloride for several hours to etch the Cu foil. The graphene/PMMA film was washed by deionized (DI) water and then stacked on another Cu foil with a single-layer graphene, the 2L-G/PMMA film was annealed at 90oC for about 30 min, and Cu foil was removed away in the iron chloride solution. For 2Layer graphene cathodes (bottom electrode), graphene/PMMA film was transferred onto glass substrate. PMMA layer was removed by acetone for 3 times. Then the graphene electrodes were modified with three methods, including (I) ZnO sol gel solution,43 (II) ZnO-NPs and (III) ZnO-NPs/PEDOT:PSS films. For the graphene anode (top electrode), graphene/PMMA film was attached on a PDMS film (
