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Organic Photovoltaics
Graphene-Based Integrated Photovoltaic Energy Harvesting/Storage Device Chih-Tao Chien,* Pritesh Hiralal, Di-Yan Wang, I-Sheng Huang, Chia-Chun Chen, Chun-Wei Chen, and Gehan A. J. Amaratunga
Energy scavenging has become a fundamental part of ubiquitous sensor networks. Of all the scavenging technologies, solar has the highest power density available. However, the energy source is erratic. Integrating energy conversion and storage devices is a viable route to obtain self-powered electronic systems which have longterm maintenance-free operation. In this work, we demonstrate an integrated-powersheet, consisting of a string of series connected organic photovoltaic cells (OPCs) and graphene supercapacitors on a single substrate, using graphene as a common platform. This results in lighter and more flexible power packs. Graphene is used in different forms and qualities for different functions. Chemical vapor deposition grown high quality graphene is used as a transparent conductor, while solution exfoliated graphene pastes are used as supercapacitor electrodes. Solution-based coating techniques are used to deposit the separate components onto a single substrate, making the process compatible with roll-to-roll manufacture. Eight series connected OPCs based on poly(3-hexylthiophene)(P3HT):phenyl-C61-butyric acid methyl ester (PC60BM) bulk-heterojunction cells with aluminum electrodes, resulting in a ≈5 V open-circuit voltage, provide the energy harvesting capability. Supercapacitors based on graphene ink with ≈2.5 mF cm−2 capacitance provide the energy storage capability. The integrated-power-sheet with photovoltaic (PV) energy harvesting and storage functions had a mass of 0.35 g plus the substrate.
C.-T. Chien, Dr. P. Hiralal, Prof. G. A. J. Amaratunga Electrical Engineering Division Department of Engineering University of Cambridge Cambridge, CB3 0FA, UK E-mail: [email protected] Dr. D.-Y. Wang, Prof. C.-W. Chen Department of Materials Science and Engineering National Taiwan University Taipei 106, Taiwan Dr. D.-Y. Wang, Prof. C.-C. Chen Institute of Atomic and Molecular Sciences Academia Sinica Taipei 106, Taiwan I.-S. Huang, Prof. C.-C. Chen Department of Chemistry National Taiwan Normal University Taipei 116, Taiwan DOI: 10.1002/smll.201403383 small 2015, DOI: 10.1002/smll.201403383
1. Introduction The number of portable devices and sensors is growing exponentially, and with them, the requirement for long term offgrid power. Energy harvesting from heat, vibration, or light may be employed, however, without a form of energy storage, power generation needs to match power consumption, and often this is not possible. PV generation in particular is attractive due to its power density (10–100 mW cm−2 range) being 1–2 orders of magnitude greater than alternative sources (thermal/vibration/RF), although the times of use are restricted. Adding a rechargeable energy store with the solar cell however comes with additional volume and cost penalties in devices which strive for miniaturization and ubiquity. A solution would be to integrate both these functions onto a single device or substrate, resulting in decreased usage of materials[1] and offering a promising route for providing energy on demand for off-grid applications.
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Nanomaterials have been widely employed in energy harvesting devices, such as polymer organic photovoltaic cells (OPCs), perovskite solar cells, thermoelectrics, and piezoelectric nanogenerators for performance enhancement.[2–4] OPCs in particular are solution-processable, have a low thermal budget, and are hence potentially low cost and suitable for ubiquitous deployment. It is thus a promising option for flexible and portable energy harvesting devices. OPC conversion efficiencies of up to 19% (perovskite cells) have been reported, and much potential remains for further improvement.[5] Nanomaterials have also been used in electrochemical energy storage devices. In supercapacitors in particular to achieve high power density (15 kW kg−1) with rapid charging. Supercapacitors are attractive because they are capable of having very large number of charge–discharge cycles, and relatively low environmental impact.[6] Perhaps more importantly, supercapacitors are inherently safer and do not typically involve any chemical reactions or phase changes. Although energy densities are lower than that of batteries, they are rising as a result of the use of new nanomaterials.[6] Therefore, for systems with small energy requirements such as wireless sensor modes which communicate intermittently, supercapacitors are a suitable form of energy storage. Recently, several attempts have been made to combine energy harvesting and storage into single units for a self-powered system. Wee et al. reported a power system design incorporating organic solar cells and supercapacitors with maximum voltage up to 1 V. They demonstrated a 43% reduction in internal resistance with respect to separate devices connected by metal wires. Furthermore, this type of system could also be made in a flexible fiber format for wearable applications, but photo charging was limited to 0.4 V.[7,8] Most other integration efforts to date have been centered on dye-sensitized solar cell (DSSC) and lithium battery/electrochemical capacitor combination because a synergy in structure is possible through the use of a common electrolyte. Normally, these systems consist of one dye-sensitized photovoltaic device and one storage device integrated on a single substrate, leading to a maximum output below 1 V, limited usually by the Voc of the solar cell.[9–14] Graphene, a 2D hexagonal lattice of carbon atoms, one atom thick, has been found to exhibit excellent thermal and electrical conductivity resulting from its unique energy dispersion. The single atomic layer thickness results in an ultrahigh surface area of 2600 m2 g−1 theoretically (although this is reduced upon electrode formation). This, in combination with its chemical inertness and low cost of exfoliated variants, makes graphene an ideal supercapacitor electrode material.[15,16] In addition, a single layer, high quality graphene sheet produced by chemical vapor deposition (CVD) also exhibits high transparency with 2.4% absorption over the visible range, and high mechanical flexibility, making it a potential alternative indium doped tin oxide (ITO).[17–19] CVD-graphene has been synthesized from a wide range of carbon sources, including food, insects, and waste, potentially providing low cost feed stocks for high quality graphene production.[20]
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In this work, we demonstrate a power system combining series connected OPCs reaching ≈5 V open-circuit voltage and supercapacitors printed on the same substrate having graphene electrodes (GSCs). The complete power-sheet with energy generation and storage has a mass around 350 mg excluding the substrate. An all graphene electrode, ITO free power sheet suitable for flexible application is also reported. The graphene based integrated-power-sheet is made by solution and print coating processes which can in principle be scaled for a large area roll-to-roll production.[34–37]
2. Results and Discussion 2.1. Single Capacitor Integrated-Power-Sheet Characterizations The layout of the integrated-power-sheet is shown in Figure 1. Figure 1a shows a photograph of the overall device, where the different sections can be seen, the supercapacitor is on the left and the photovoltaic cell is on the right of the substrate. Figure 1b shows the schematic of the overall power sheet, whilst Figure 1c,d shows the detailed structure of the supercapacitor and the photovoltaic cell, respectively. The supercapacitor is a symmetric double layer device with electrodes made from bar-coated graphene paste, the photovoltaic cell is based on a standard P3HT:PC60BM organic stack with a 3% conversion efficiency for a single cell (see the Supporting Information, Figure S1). Both devices are connected with a common conducting backplane on a single substrate. The power sheet has three terminals configuration (A, B, and C) which enable continuous output after the supercapacitor is fully charged by the PV cells. The fabrication of the integrated-power-sheet commences with the patterning of ITO glass substrates (Figure 2, step A). For the GSCs, graphene ink is initially bar-coated onto conductive carbon coated aluminum foil, which is subsequently etched and the remaining carbon transferred onto the patterned ITO/glass (Figure 2, step B).[21] After deposition of the bottom supercapacitor electrode, PEDOT:PSS, P3HT:PC60BM active layer and Al electrodes are deposited sequentially to form the OPCs (Figure 2, steps C and D).[38] Eight series connected OPCs are formed on a single ITO/ glass substrate. This allows the development of a sufficient voltage to fully charge the supercapacitor. Next, the GSC is completed by sandwiching an electrolyte wetted separator between the bottom graphene electrode on ITO and a second bar printed graphene electrode on Al foil. The GSC is finally sealed in a laminator using thermoplastic PE/PP sheets. The resulting structure is a sheet with integrated energy generation and storage functions as shown in Figure 2, step E (full fabrication procedures are in the Experimental Section). Before measuring the integrated-power-sheet, OPCs and GSC performance were tested individually through their respective terminals. Current density versus voltage (J–V) characteristic of the 8 series connected OPCs was measured under AM 1.5 illumination (100 mW cm−2). From the J–V profile shown in Figure 3a, the series connected OPCs exhibit a short circuit current density (Jsc) of 5.78 mA cm−2, an open
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Figure 1. Photograph and schematic illustration of a standard graphene based integrated-power-sheet device. a) Photograph of an integratedpower-sheet with light weight (≈350 mg + substrate). Inset is the back view of the device. b) Schematic representation of a power sheet for energy conversion and energy storage based on P3HT:PC60BM series connected organic photovoltaic cells (OPCs) and graphene ink supercapacitor (GSC). c) Cross section view of graphene electrodes supercapacitor with ITO/glass and Al foil current collectors. d) The structure of OPCs device is ITO/ PEDOT:PSS/ P3HT:PC60BM/Al.
circuit voltage (Voc) of 4.91 V, and a fill factor (FF) of 44.3%, yielding a photo conversion efficiency of 1.57% (Table 1 in Figure 3 summarizes the device performance, active area is 0.16 cm2). The series connection over long lengths of ITO results in a higher series resistance compared to the single device. Graphene supercapacitor material and device characteristics are shown in Figure 3b,e. Figure 3b shows the Raman spectrum of the graphene electrode, which shows a D band at ≈1328 cm−1, G band at ≈1576 cm−1, and 2D band at 2671 cm−1, respectively. The D band corresponds to the breathing mode of sp2 atoms and is activated by defects in the hexagonal arrangement. The G band corresponds to the E2g phonon at the brillouin zone center. Since the thickness of ink film is in micron scale and the graphene was obtained from exfoliated graphite, the 2D band peak is more graphitelike in the spectrum and D band intensity is high, indicating
the presence or a large number of defects, as expected for chemically exfoliated graphene.[22] Figure 3c shows a scanning electron micrograph of the surface morphology of the film. Parallel lines are observed at low magnifications which result from the bar-coating process. Device characteristics are shown in Figures 3d,e. Cyclic voltammogram (CV) of the GSC shows ≈2.5 mF cm−2 capacitance in the potential range −2 to 2 V (scan rate of 100 mV s−1). Figure 3e shows the galvanostatic charge/discharge test at a constant current of 0.7 mA from which an equivalent series resistance (ESR) of ≈150 Ω is measured. A single GSC device could be charged up to over 2 V by using the organic electrolyte (1 m Et4NBF4 in propylene carbonate). The energy level diagrams in Figure 4a illustrate the electron and hole transport within the integrated-power-sheet. When charging under illumination, photons are absorbed by
Figure 2. Fabrication of an integrated-power-sheet. Patterning the commercial ITO/glass by photo lithography (step A). Printing graphene ink on the conductive carbon/aluminum foil and was transferred to patterned ITO/glass (step B). Spin coating PEDOT:PSS and P3HT:PC60BM active layer and then evaporated Al electrodes (steps C and D). Laminating the supercapacitor with graphene ink electrodes, organic electrolyte, and PE/PP based sealing paper (step E). small 2015, DOI: 10.1002/smll.201403383
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Figure 3. a) J–V curve under AM 1.5 illumination (100 mW cm−2) of the 8 series connected P3HT:PC60BM organic photovoltaic devices (Table 1 shows the characteristics of the cells: 4.91 V for open-circuit voltage, 5.78 mA cm−2 for short-circuit current density, 44.3% for fill factor, and conversion efficiency is 1.57%). Electrochemical characterizations of graphene ink film as electrodes. b) Raman spectra at 633 nm for graphene ink D band is at ≈1328 cm−1, G band at ≈1576 cm−1, and 2D band at 2671 cm−1, respectively. c) SEM image of graphene ink sample for supercapacitor electrodes under low and high magnification (inset figure). d) Cyclovoltammetry (CV) curve for the supercapacitor with graphene electrodes (≈4 cm2) and organic electrolyte (1 M Et4NBF4 in propylene carbonate) measured at 100 mV s−1. e) Galvanostatic charge/discharge curves based on graphene ink electrodes with 0.7 mA.
P3HT and excited excitons are dissociated at the interfaces to produce free electrons and holes. The electrode workfunction difference serves as a driving force for the charge carriers to drift toward the supercapacitor electrodes.[23,24] Figure 4b schematically shows the photo charge/discharge connection and circuit illustration of the integrated-powersheet. The power sheet has a three-terminal design, named A, B, and C in the diagram. Terminal B is the shared backplane between all devices. During photo charging, terminals A and C are connected to provide a charging path for electrons. Upon opening the switch (disconnecting terminals A and C) the LED becomes the only discharge path for the supercapacitor. The dynamic voltage-time plot during photo charging and galvanostatic discharging is shown in Figure 4c. The power sheet was charged up to ≈2.3 V in approximately
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5 min, and then discharged at a constant current of 0.3 mA for 4 min. The nonlinear discharge curve is ascribed to the porosity saturation effect.[39] The voltage of this system is limited by the GSC, and is set by the voltage beyond which the electrolyte reacts with the electrode and breaks down, reducing device lifetime. This voltage limit allows the demonstration of this circuit to power a red LED (turn-on voltage around 1.6 V), as shown in Figure 4d (I–V curve characteristic is shown in the Supporting Information, Figure S2a).
2.2. Extending the Voltage Range – Series Connected GSCs For applications demanding higher voltage power sources, capacitors can be connected in series as well. We demonstrate
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Figure 4. a) Energy level diagrams of the photoactive layer (P3HT:PC60BM), hole transport layer (PEDOT:PSS), and electrodes (ITO, Al, graphene) and the charge transport in the integrated-power-sheet. b) Schematic illustration of power sheet three terminals connection during photo-charging/ discharging process and circuit schematic representation. c) The dynamic voltage-time plot during photo charging and galvanostatic discharging process (0.3 mA). d) Charged power sheet showed in photograph was used to drive a red LED (turn-on voltage is ≈1.6 V).
a 5 V device with 2 series connected GSCs, formed by stacking up layers of graphene electrodes. The resulting device is made up of 8 series connected OPCs and 2 series connected GSCs. To form the series GSC, after the base electrode was transferred, graphene ink was bar-coated onto carbon-Al foil and then another graphene film transferred onto the back aluminum side by the same method used for the base layer. The resulting GSC structure is ITO/conductive carbon (CC)/ graphene ink/organic electrolyte-separator/graphene ink/ (CC)/Al/(CC)/graphene ink/organic electrolyte-separator/ graphene ink/(CC)/Al (detailed schematic representation is shown in the Supporting Information, Figure S2b). The CV of the device is shown in Figure 5a, with ≈2 mF cm−2 capacitance in the potential range −4 to 4 V (scan rate of 100 mV s−1). Figure 5b shows the galvanostatic charge/discharge curve at a 0.7 mA constant current. The ESR of the device is ≈270 Ω. The dynamic voltage-time plot during photo charging and galvanostatic discharging with 0.3 mA is given in Figure 5c. The GSCs were charged by OPCs up to 3.8 V after 10 min under AM 1.5 illumination. While charging, the rate of charge slowed as voltage increased and at 3.8 V, the rate of increase small 2015, DOI: 10.1002/smll.201403383
was ≈0.68 mV s−1. The energy density for whole power sheet is about 0.2 J g−1. As a simple demonstrator, the new circuit was used to drive a red LED with turn-on voltage of about 1.6 V, a green LED with turn-on voltage of about 2 V, and a blue LED turning on around about 2.4 V [Figure 5d, (I–V characteristic of RGB LED is shown in the Supporting Information, Figure S2a)]. The integrated-power-sheet therefore makes a good self-powered source (energy generation and energy storage) for other electronic devices (energy sink).
2.3. Prototype of Graphene Electrode Integrated-Power-Sheet Polymer photovoltaic cells are attractive because cells can be manufactured in large-areas at low-cost on mechanically flexible substrates. Although photoactive layers, transport layers, and metal electrode are flexible, one major challenge in fabricating flexible OPCs is the crystalline transparent conductive oxide (TCO) electrode which is brittle and usually deposited on a solid glass substrate. When deposited on a flexible substrate, performance can be degrade after a few
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Figure 5. Electrochemical characterization of 2 series connected graphene ink supercapacitors. a) CV curve for the two stacked-up supercapacitors with graphene electrodes and organic electrolyte (1 m Et4NBF4 in propylene carbonate) measured at 100 mV s−1. b) Galvanostatic charge/discharge curves based on graphene ink electrodes with 0.7 mA. c) The dynamic voltage–time plot during photocharging and galvanostatic discharging process (0.3 mA). d) Charged power sheet was used to drive red LED (turn-on voltage is ≈1.6 V), green LED (≈2 V), and blue LED (≈2.4 V).
cycles of bending due to cracking of the brittle ITO layer.[25] Another disadvantage for this transparent conductive oxide is that indium is a rare element and given the high demand, could be in short supply in the near future. In order to make a portable, flexible, and very light weight power source, graphene, which has ultrahigh carrier mobility, high transparency (2.3% absorption per layer), and excellent mechanical flexibility,[17,26,27] is a promising material. Herein, a prototype of an all graphene integrated-power-sheet using CVD grown graphene as the top electrodes and Cr–Al–Cr as bottom electrodes for the OPCs was evaluated. Since the photovoltaic cell is now inverted, its life span is expected to be longer than the conventional one due to the low work function top electrode (Al) and acidic corrosion by PEDOT:PSS being prevented.[28,29] Furthermore, it is noticeable that the durability of the graphene top electrodes is better than metal one since graphene is impermeable to gas and liquid.[30] The graphene electrode integrated-power-sheet can be flexible, have a lower weight and a longer lifetime. The multilayer graphene top electrodes in the inverted architecture OPCs consist of single layer graphene synthesized on copper foil by CVD.[31] Graphene layers were transferred onto a glass substrate and characterized under Raman spectroscopy at 633 nm (see the Supporting Information, Figure S3a). For mono layer graphene, G band peak was at ≈1587 cm−1, and 2D peak was at ≈2644 cm−1. For bilayer graphene, G band was at ≈1596 cm−1, and 2D band was at
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≈2652 cm−1. It was possible to confirm the number of layers transferred also from the 2D band.[22] On the other hand, there was no obvious D peak in the Raman spectra originating from defects at 1350 cm−1. This indicated that CVDgraphene used is devoid of a significant number of defects and has a high crystalline quality. The detail fabrication procedures of integrated-power-sheet are in the experimental part. Photographic and schematic representations of the all graphene electrodes integrated-power-sheet device are shown in Figure 6a. The structure of inverted type organic photovoltaic cells is Cr(5 nm)–Al(100 nm)–Cr(5 nm)/ZnO/ P3HT:PC60BM/PEDOT:PSS/graphene (8 layers). The layer dependent transmittance spectrum and electrical properties of the transferred graphene are given in the Supporting Information, Figure S3b,c (single layer is about 950 Ω sq−1). The single device exhibited a short-circuit current density of 3.67 mA cm2, an open-circuit voltage of 0.55 V, and a fill factor of 51.2%, yielding a photo conversion efficiency of 1.04% (data are summarized in Table 2 in Figure 6). In this architecture, electrons transport through Cr–Al–Cr side and holes through the graphene electrode. The CV curve in Figure 6c shows GSC has a capacitance of 4.1 mF in the −2 to 2 V range (scan rate of 100 mV s−1). Cr–Al–Cr tenary layers replaced the ITO glass and Al foil as power sheet terminals. During charging, electrodes A and C were connected (the positive terminal with the inverted OPC structure).
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Figure 6. a) Photograph and schematic representation of all graphene electrodes integrated-power-sheet device. The structure of inverted type organic photovoltaic cells device is Cr(5 nm)–Al(100 nm)–Cr(5 nm)/ZnO/P3HT:PC60BM/PEDOT:PSS/graphene (8 layers). b) J–V curve under AM 1.5 illumination (100 mW cm−2) of P3HT: PC60BM organic photovoltaic devices (Table 2 shows the characteristics of the cell: 0.55 V for open-circuit voltage, 3.67 mA cm−2 for short-circuit current density, 51.2% for fill factor, and conversion efficiency is 1.04%). c) CV curve for the supercapacitor with graphene ink electrodes and organic electrolyte (1 M Et4NBF4 in propylene carbonate) measured at 100 mV s−1. d) The dynamic voltage–time plot during photo charging and galvanostatic discharging process (0.02 mA).
Figure 6d shows the galvanostatic charge/discharge test at a constant current of 0.02 mA from which an ESR of ≈150 Ω is extracted. A single OPC (0.18 cm2) can charge a GSC (4 cm2) up to open-circuit voltage (0.55 V) within 40 s. The ESR resulting from with the use of the Cr–Al–Cr similar to that of the ITO based device used previously, indicating a more significant origin elsewhere. However the Cr–Al–Cr additionally brings the advantage of mechanical flexibility over ITO.
3. Conclusion Graphene in different forms and qualities is used to facilitate the energy generation and storage elements for an integrated small 2015, DOI: 10.1002/smll.201403383
power platform. An energy source comprising multiple series connected OPCs and GSCs suitable for self-powered electronics is demonstrated. Importantly, since the photoactive layers and graphene paste are both solution processable, energy harvesting and storage devices can be envisioned to be printed onto single flexible substrates. In the three terminal configuration shown here, open-circuit voltage for the 8 OPCs in series is ≈5 V and the conversion efficiency is ≈1.6%. By the combination of series OPCs and GSCs an overall system voltage of up to ≈4 V was shown, suitable for driving red, green, and blue light emitting diodes. An integrated-power-sheet with graphene electrodes on the OPC is also demonstrated as an ITO-free alternative suitable for roll-to-roll manufacture. Consequently, the integrated-powersheet, which can be printed onto thin sheets, is suitable for
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low-cost, self-powered electronic systems such as wireless sensors which require long-term maintenance free operation.
4. Experimental Section Solar Cell Fabrication: Conventional type organic solar cells were fabricated on pattered ITO glass (Delta Technology, CG-61IN with dimension 24 × 75 × 1.1 mm) substrates with sheet resistance of ≈20 Ω sq−1 in a N2 atmosphere glove box. The ITO substrates were photolithographically patterned to arrange 8 solar cells and a supercapacitor. First, positive photoresist (AZ5214E) was spun at 3000 rpm for 40 s onto an ITO glass and then baked for 1 min at 100 °C. Second, samples were exposed to UV light under a mask for 40 s and immersed into AZ351B developer solution (diluted to a 1:4 volume ratio on DI water) for 20 s, rinsed and then baked for 120 s at 110 °C. The samples with the exposed pattern were placed into hydrochloric acid (37 wt%) for 3 min for ITO etching. After thorough washing, the remaining photoresist was removed in an acetone bath. The substrates were thoroughly cleaned in acetone and isopropanol ultrasonic baths for 15 min in turn and treated in an O2 plasma for 15 s in preparation for active layer deposition. A PEDOT:PSS (Heraeus Clevios P VP AI 4083) solution filtered through a 0.45 µm syringe filter was spun onto the ITO substrate at 5000 rpm (≈40 nm) and baked at 135 °C in air for 15 min. The photoactive layer was then deposited by spinning a 1:1 weight ratio blend consisting of P3HT (Rieke Metal) and PC60BM (Solenne) dissolved in chlorobenzene (32 mg mL−1) at 1300 rpm. Finally, 100 nm thick Al electrodes were deposited via thermal evaporation through a shadow mask (pressure ≈3 × 10−6 Torr). Upon completion of the stack, the devices were post annealed at 150 °C for 5 min in a N2-filled glove box. Alternatively, an inverted type organic solar cell was fabricated on patterned metallic electrode (on the glass slide) consisting of Cr–Al–Cr/ZnO/P3HT:PC60BM/PEDOT:PSS/CVD-graphene layers. Chromium (Cr) 5 nm, aluminum (Al) 100 nm, and chromium (Cr) 5 nm layers were deposited sequentially by thermal evaporation onto a precleaned glass slide which was pretreated by oxygen plasma at 50 W 10 min. The first Cr layer was used as an adhesion layer and the second Cr layer acted as an electron buffer and protection layer. The zinc oxide (ZnO) hole blocking layer was deposited by spinning a 20 × 10−3 M solution of zinc acetate dehydrate (Sigma-Aldrich) prepared in 1-propanol (spectroscopic grade). The procedure was repeated 15 times and air annealed at 150 °C for 30 s after every spin cycle in order to oxidize the precursor. The photoactive layer was deposited on top of the ZnO layer by spinning a 1:1 weight ratio blend solution consisting of P3HT and PC60BM dissolved in chlorobenzene (30 mg mL−1) at 1300 rpm. A PEDOT:PSS solution was mixed with isopropanol in volume ratio 1:0.8 for 15 min, and then with 1-butanol (Sigma-Aldrich) in a 1:0.6 ratio for a further 15 min. The resulting solution was filtered through a 0.45 µm syringe filter and was spun at 3000 rpm onto the photoactive layer. 2 layers of CVD-grown graphene were transferred onto thermal release tapes and these were used to transfer the graphene layer onto the PEDOT:PSS layer by heating the films to 120 °C. The process was repeated up to four times to get an 8 layer thick graphene top electrodes.
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Supercapacitor Fabrication for Standard Power Sheet: The graphene supercapacitor (GSCs) device consists of the following stack: ITO/conductive carbon/graphene ink/separator-organic electrolyte/graphene ink/conductive carbon coated Al foil. Graphene ink was commercially purchased from Vorbeck Materials. Both electrodes of the capacitor were bar-coated. For the ITO/glass side, graphene ink was first printed on the conductive carbon coated aluminum foil (60 µm wet deposit thickness) with a spun poly(methyl methacrylate) (PMMA) layer (3000 rpm, 1 min) on the ink film. The Al foil was etched away by 2 M hydrochloric acid and the ink film was transferred onto the patterned ITO/glass using the PMMA as the support. The other electrode is bar-coated graphene on Al foil. A Whatman qualitative filter paper (Grade 1) soaked in 1 M tetraethylammonium tetrafluoroborate (Et4NBF4) in propylene carbonate was used as separator, and was sandwiched in between the two graphene electrodes. The stack was finally sealed by laminating the device between PET based laminator sheets. Preparation of CVD Graphene: Graphene monolayer was synthesized on polycrystalline copper foil (from Nilaco Inc.) by chemical vapor deposition (CVD). Foil was placed in a wall furnace with a one inch fused silica tube and then heated up to 1000 °C. First, a reduction reaction was conducted by H2 flow for about 40 min before introducing CH4. Then, the graphene growth process was carried out with a H2/CH4 flow rate of 2/35 sccm for 10 min. After growth for 30 min, CH4 was turned off and the furnace tube cooled down to room temperature in H2 or Ar gas flow. The growth step process was conducted at low pressure ≈500 mTorr. Fabrication for Graphene Electrodes Integrated-Power-Sheet: There are 6 steps for fabricating the graphene electrodes integrated-power-sheet, the detailed procedure is shown in the Supporting Information, Figure S4. After CVD growth, graphene with copper foil was first attached to thermal release tape (Nitto Denko) and then immersed in 0.2 M iron nitrate solution (Fe(NO3)3.9H2O, Sigma-Aldrich) for 2 h to etch away the copper foil (step A). The thermal release tape with single layer graphene was attached to another piece of copper foil with graphene to get bilayer graphene samples (step B).[29] In parallel, thoroughly cleaned and plasma treated glass substrate was put into a thermal evaporator for bottom metal electrode deposition consisting of three different layers chromium (5 nm)–aluminum (100 nm)–chromium (5 nm)[32,33] (step C). This ternary layer forms a common electrode platform for the integrated-power-sheet. Graphene ink film was transferred to form supercapacitor electrodes by the method given in Figure 2 (step D). The photovoltaic device layers ZnO, P3HT:PC60BM blend, and PEDOT:PSS are deposited sequentially by spin coating (step E). After the OSC active later is coated, the top graphene electrode of the supercapacitor is deposited and sealed. Finally, the CVD graphene collected onto the thermal release tapes was transferred to form the top contact of the OSC. This was repeated 4 times to render 8 layers (step F).[34–39] Materials and Device Characterizations: UV–visible absorption spectra of layer dependent graphene samples were obtained by using a Jasco V570 UV/Vis/NIR spectrophotometer. Photovoltaic performance measurement was done with a computer controlled sourcemeter (HP 4140B pA meter/DC voltage source) with cells exposed to a Newport 150 W lamp with an AM1.5 filter. Supercapacitor (GSCs) performance was measured with a potentiostat/ galvanostat (Autolab PGSTAT 302N). Raman measurements were
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conducted by using a 633 nm He–Ne laser as an excitation source and detected by a HORIBA iHR 550 monochrometer equipped and a Symphony CCD detector. For the demonstrator, different color light emitting diodes (RGB) were purchased from Philips Lumileds.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partially supported by the Nokia-Cambridge University Strategic Research Alliance in Nanoscience and Nanotechnology. The authors would like to thank Dilek Özgit for the scanning electron microscope image of the graphene ink samples, Wen-Peng Deng for the light emitting diode experiments, and Dr. Li Shao-Shian for Raman spectra measurements.
[1] a) F. Boico, B. Lehman, Solar Energy 2012, 86, 463; b) Z. L. Wang, Adv. Mater. 2012, 24, 280. [2] G. Li, R. Zhu, Y. Yang, Nat. Photon. 2012, 6, 153. [3] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [4] G. Zhu, R. Yang, S. Wang, Z. L. Wang, Nano Lett. 2010, 10, 3151. [5] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446. [6] a) B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Springer, US 1999; b) P. Hiralal, H. Wang, H. E. Unalan, Y. Liu, M. Rouvala, D. Wei, P. Andrew, G. A. J. Amaratunga, J. Mater. Chem. 2011, 21, 17810. [7] G. Wee, T. Salim, Y. M. Lam, S. G. Mhaisalkarab, M. Srinivasan, Energy Environ. Sci. 2011, 4, 413. [8] Z. Zhang, X. Chen, P. Chen, G. Guan, L. Qiu, H. Lin, Z. Yang, W. Bai, Y. Luo, H. Peng, Adv. Mater. 2014, 3, 466. [9] W. Guo, X. Xue, S. Wang, C. Lin, Z. L. Wang, Nano Lett. 2012, 12, 2520. [10] Y. Fu, H. Wu, S. Ye, X. Cai, X. Yu, S. Hou, H. Kafafya, D. Zou, Energy Environ. Sci. 2013, 6, 805. [11] X. Chen, H. Sun, Z. Yang, G. Guan, Z. Zhang, L. Qiu, H. Peng, J. Mater. Chem. A 2014, 2, 1897. [12] J. Xu, H. Wu, L. Lu, S. F. Leung, D. Chen, X. Chen, Z. Fan, G. Shen, D. Li, Adv. Funct. Mater. 2013, 13, 1840. [13] Z. Yang, L. Li, Y. Luo, R. He, L. Qiu, H. Lin, H. Peng, J. Mater. Chem. A 2013, 1, 954. [14] H. W. Chen, C. Y. Hsu, J. G. Chen, K. M. Lee, C. C. Wang, K. C. Huang, K. C. Ho, J. Power Sources 2010, 195, 6225. [15] X. Han, Y. Chen, H. Zhu, C. Preston, J. Wan, Z. Fang, L. Hu, Nanotechnology 2013, 24, 205304.
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[16] a) D. Wei, P. Andrew, H. Yang, Y. Jiang, F. Li, C. Shan, W. Ruan, D. Han, L. Niu, C. Bower, T. Ryhänen, M. Rouvala, G. A. J. Amaratunga, A. Ivaska, J. Mater. Chem. 2011, 21, 9762; b) Z. Cui, C. X. Guo, W. Yuanb, C. M. Li, Phys. Chem. Chem. Phys. 2012, 14, 12823; c) J. L. Xie, C. X. Guo, C. M. Li. Energy Environ. Sci. 2014, 7, 2559. [17] A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183. [18] R. R. Nair, P. Blake1, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 2008, 320, 1308. [19] Z. Yin, J. Zhu, Q. He, X. Cao, C. Tan, H. Chen, Q. Yan, H. Zhang, Adv. Energy Mater. 2014, 4, 1. [20] G. Ruan, Z. Sun, Z. Peng, J. M. Tour, ACS Nano 2011, 5, 7601. [21] X. Li, C. W. Magnuson, A. Venugopal, J. An, J. W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, L. Fu, E. M. Vogel, E. Voelkl, L. Colombo, R. S. Ruoff, Nano Lett. 2010, 10, 4328. [22] a) A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, A. K. Geim, Phys. Rev. Lett. 2006, 97, 187401; b) H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H. Adamson, R. K. Prud’homme, R. Car, D. A. Saville, I. A. Aksay, J. Phys. Chem. B 2006, 110, 8535. [23] A. J. Heeger, Adv. Mater. 2014, 26, 10. [24] H. Kim, M. Shin, Y. Kim, EPL 2008, 84, 58002. [25] H. Chang, G. Wang, A. Yang, X. Tao, X. Liu, Y. Shen, Z. Zheng, Adv. Funct. Mater. 2010, 20, 2893. [26] a) L. G. D. Arco, Y. Zhang, C. W. Schlenker, K. Ryu, M. E. Thompson, C. Zhou, ACS Nano 2010, 4, 2865; b) T. Chen, W. Hu, J. Song, G. H. Guai, C. M. Li, Adv. Funct. Mater. 2012, 22, 5245. [27] S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Özyilmaz, J. H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574. [28] Z. Liu, J. Li, Z. H. Sun, G. Tai, S. P. Lau, F. Yan, ACS Nano 2011, 6, 810. [29] Y. Y. Lee, K. H. Tu, C. C. Yu, S. S. Li, J. Y. Hwang, C. C. Lin, K. H. Chen, L. C. Chen, H. L. Chen, C. W. Chen, ACS Nano 2011, 5, 6564. [30] Z. Liu, J. Li, F. Yan, Adv. Mater. 2013, 25, 4296. [31] S. Y. Kwon, C. V. Ciobanu, V. Petrova, V. B. Shenoy, J. Bareno, V. Gambin, I. Petrov, S. Kodambaka, Nano Lett. 2009, 9, 3985. [32] B. Zimmermann, H. F. Schleiermacher, M. Niggemann, U. Würfel, Solar Energy Mater. Solar Cells 2011, 95, 1587. [33] B. Zimmermann, U. Würfel, M. Niggemann, Solar Energy Mater. Solar Cells 2009, 93, 491. [34] F. C. Krebs, S. A. Gevorgyan, J. Alstrup, J. Mater. Chem. 2009, 19, 5442. [35] F. C. Krebs, T. Tromholt, M. Jørgensen, Nanoscale 2010, 2, 873. [36] J. Yeoa, G. Kim, S. Hong, M. S. Kim, D. Kim, J. Lee, H. B. Lee, J. Kwon, Y. D. Suh, H. W. Kang, H. J. Sung, J. H. Choi, W. H. Hong, J. M. Koe, S. H. Lee, S. H. Choa, S. H. Ko, J. Power Sources 2014, 246, 562. [37] F. C. Krebs, Org. Electron. 2009, 10, 761. [38] J. Wan, G. Fang, P. Qin, Q. Zheng, N. Liu, N. Sun, Y. Tu, X. Fan, F. Cheng, X. Zhao, Solar Energy Mater. Solar Cells 2012, 101, 289. [39] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Adv. Mater. 2014, 26, 2219.
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Received: November 13, 2014 Revised: January 16, 2015 Published online:
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9
Organic Photovoltaics
Graphene-Based Integrated Photovoltaic Energy Harvesting/Storage Device Chih-Tao Chien,* Pritesh Hiralal, Di-Yan Wang, I-Sheng Huang, Chia-Chun Chen, Chun-Wei Chen, and Gehan A. J. Amaratunga
Energy scavenging has become a fundamental part of ubiquitous sensor networks. Of all the scavenging technologies, solar has the highest power density available. However, the energy source is erratic. Integrating energy conversion and storage devices is a viable route to obtain self-powered electronic systems which have longterm maintenance-free operation. In this work, we demonstrate an integrated-powersheet, consisting of a string of series connected organic photovoltaic cells (OPCs) and graphene supercapacitors on a single substrate, using graphene as a common platform. This results in lighter and more flexible power packs. Graphene is used in different forms and qualities for different functions. Chemical vapor deposition grown high quality graphene is used as a transparent conductor, while solution exfoliated graphene pastes are used as supercapacitor electrodes. Solution-based coating techniques are used to deposit the separate components onto a single substrate, making the process compatible with roll-to-roll manufacture. Eight series connected OPCs based on poly(3-hexylthiophene)(P3HT):phenyl-C61-butyric acid methyl ester (PC60BM) bulk-heterojunction cells with aluminum electrodes, resulting in a ≈5 V open-circuit voltage, provide the energy harvesting capability. Supercapacitors based on graphene ink with ≈2.5 mF cm−2 capacitance provide the energy storage capability. The integrated-power-sheet with photovoltaic (PV) energy harvesting and storage functions had a mass of 0.35 g plus the substrate.
C.-T. Chien, Dr. P. Hiralal, Prof. G. A. J. Amaratunga Electrical Engineering Division Department of Engineering University of Cambridge Cambridge, CB3 0FA, UK E-mail: [email protected] Dr. D.-Y. Wang, Prof. C.-W. Chen Department of Materials Science and Engineering National Taiwan University Taipei 106, Taiwan Dr. D.-Y. Wang, Prof. C.-C. Chen Institute of Atomic and Molecular Sciences Academia Sinica Taipei 106, Taiwan I.-S. Huang, Prof. C.-C. Chen Department of Chemistry National Taiwan Normal University Taipei 116, Taiwan DOI: 10.1002/smll.201403383 small 2015, DOI: 10.1002/smll.201403383
1. Introduction The number of portable devices and sensors is growing exponentially, and with them, the requirement for long term offgrid power. Energy harvesting from heat, vibration, or light may be employed, however, without a form of energy storage, power generation needs to match power consumption, and often this is not possible. PV generation in particular is attractive due to its power density (10–100 mW cm−2 range) being 1–2 orders of magnitude greater than alternative sources (thermal/vibration/RF), although the times of use are restricted. Adding a rechargeable energy store with the solar cell however comes with additional volume and cost penalties in devices which strive for miniaturization and ubiquity. A solution would be to integrate both these functions onto a single device or substrate, resulting in decreased usage of materials[1] and offering a promising route for providing energy on demand for off-grid applications.
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Nanomaterials have been widely employed in energy harvesting devices, such as polymer organic photovoltaic cells (OPCs), perovskite solar cells, thermoelectrics, and piezoelectric nanogenerators for performance enhancement.[2–4] OPCs in particular are solution-processable, have a low thermal budget, and are hence potentially low cost and suitable for ubiquitous deployment. It is thus a promising option for flexible and portable energy harvesting devices. OPC conversion efficiencies of up to 19% (perovskite cells) have been reported, and much potential remains for further improvement.[5] Nanomaterials have also been used in electrochemical energy storage devices. In supercapacitors in particular to achieve high power density (15 kW kg−1) with rapid charging. Supercapacitors are attractive because they are capable of having very large number of charge–discharge cycles, and relatively low environmental impact.[6] Perhaps more importantly, supercapacitors are inherently safer and do not typically involve any chemical reactions or phase changes. Although energy densities are lower than that of batteries, they are rising as a result of the use of new nanomaterials.[6] Therefore, for systems with small energy requirements such as wireless sensor modes which communicate intermittently, supercapacitors are a suitable form of energy storage. Recently, several attempts have been made to combine energy harvesting and storage into single units for a self-powered system. Wee et al. reported a power system design incorporating organic solar cells and supercapacitors with maximum voltage up to 1 V. They demonstrated a 43% reduction in internal resistance with respect to separate devices connected by metal wires. Furthermore, this type of system could also be made in a flexible fiber format for wearable applications, but photo charging was limited to 0.4 V.[7,8] Most other integration efforts to date have been centered on dye-sensitized solar cell (DSSC) and lithium battery/electrochemical capacitor combination because a synergy in structure is possible through the use of a common electrolyte. Normally, these systems consist of one dye-sensitized photovoltaic device and one storage device integrated on a single substrate, leading to a maximum output below 1 V, limited usually by the Voc of the solar cell.[9–14] Graphene, a 2D hexagonal lattice of carbon atoms, one atom thick, has been found to exhibit excellent thermal and electrical conductivity resulting from its unique energy dispersion. The single atomic layer thickness results in an ultrahigh surface area of 2600 m2 g−1 theoretically (although this is reduced upon electrode formation). This, in combination with its chemical inertness and low cost of exfoliated variants, makes graphene an ideal supercapacitor electrode material.[15,16] In addition, a single layer, high quality graphene sheet produced by chemical vapor deposition (CVD) also exhibits high transparency with 2.4% absorption over the visible range, and high mechanical flexibility, making it a potential alternative indium doped tin oxide (ITO).[17–19] CVD-graphene has been synthesized from a wide range of carbon sources, including food, insects, and waste, potentially providing low cost feed stocks for high quality graphene production.[20]
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In this work, we demonstrate a power system combining series connected OPCs reaching ≈5 V open-circuit voltage and supercapacitors printed on the same substrate having graphene electrodes (GSCs). The complete power-sheet with energy generation and storage has a mass around 350 mg excluding the substrate. An all graphene electrode, ITO free power sheet suitable for flexible application is also reported. The graphene based integrated-power-sheet is made by solution and print coating processes which can in principle be scaled for a large area roll-to-roll production.[34–37]
2. Results and Discussion 2.1. Single Capacitor Integrated-Power-Sheet Characterizations The layout of the integrated-power-sheet is shown in Figure 1. Figure 1a shows a photograph of the overall device, where the different sections can be seen, the supercapacitor is on the left and the photovoltaic cell is on the right of the substrate. Figure 1b shows the schematic of the overall power sheet, whilst Figure 1c,d shows the detailed structure of the supercapacitor and the photovoltaic cell, respectively. The supercapacitor is a symmetric double layer device with electrodes made from bar-coated graphene paste, the photovoltaic cell is based on a standard P3HT:PC60BM organic stack with a 3% conversion efficiency for a single cell (see the Supporting Information, Figure S1). Both devices are connected with a common conducting backplane on a single substrate. The power sheet has three terminals configuration (A, B, and C) which enable continuous output after the supercapacitor is fully charged by the PV cells. The fabrication of the integrated-power-sheet commences with the patterning of ITO glass substrates (Figure 2, step A). For the GSCs, graphene ink is initially bar-coated onto conductive carbon coated aluminum foil, which is subsequently etched and the remaining carbon transferred onto the patterned ITO/glass (Figure 2, step B).[21] After deposition of the bottom supercapacitor electrode, PEDOT:PSS, P3HT:PC60BM active layer and Al electrodes are deposited sequentially to form the OPCs (Figure 2, steps C and D).[38] Eight series connected OPCs are formed on a single ITO/ glass substrate. This allows the development of a sufficient voltage to fully charge the supercapacitor. Next, the GSC is completed by sandwiching an electrolyte wetted separator between the bottom graphene electrode on ITO and a second bar printed graphene electrode on Al foil. The GSC is finally sealed in a laminator using thermoplastic PE/PP sheets. The resulting structure is a sheet with integrated energy generation and storage functions as shown in Figure 2, step E (full fabrication procedures are in the Experimental Section). Before measuring the integrated-power-sheet, OPCs and GSC performance were tested individually through their respective terminals. Current density versus voltage (J–V) characteristic of the 8 series connected OPCs was measured under AM 1.5 illumination (100 mW cm−2). From the J–V profile shown in Figure 3a, the series connected OPCs exhibit a short circuit current density (Jsc) of 5.78 mA cm−2, an open
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Figure 1. Photograph and schematic illustration of a standard graphene based integrated-power-sheet device. a) Photograph of an integratedpower-sheet with light weight (≈350 mg + substrate). Inset is the back view of the device. b) Schematic representation of a power sheet for energy conversion and energy storage based on P3HT:PC60BM series connected organic photovoltaic cells (OPCs) and graphene ink supercapacitor (GSC). c) Cross section view of graphene electrodes supercapacitor with ITO/glass and Al foil current collectors. d) The structure of OPCs device is ITO/ PEDOT:PSS/ P3HT:PC60BM/Al.
circuit voltage (Voc) of 4.91 V, and a fill factor (FF) of 44.3%, yielding a photo conversion efficiency of 1.57% (Table 1 in Figure 3 summarizes the device performance, active area is 0.16 cm2). The series connection over long lengths of ITO results in a higher series resistance compared to the single device. Graphene supercapacitor material and device characteristics are shown in Figure 3b,e. Figure 3b shows the Raman spectrum of the graphene electrode, which shows a D band at ≈1328 cm−1, G band at ≈1576 cm−1, and 2D band at 2671 cm−1, respectively. The D band corresponds to the breathing mode of sp2 atoms and is activated by defects in the hexagonal arrangement. The G band corresponds to the E2g phonon at the brillouin zone center. Since the thickness of ink film is in micron scale and the graphene was obtained from exfoliated graphite, the 2D band peak is more graphitelike in the spectrum and D band intensity is high, indicating
the presence or a large number of defects, as expected for chemically exfoliated graphene.[22] Figure 3c shows a scanning electron micrograph of the surface morphology of the film. Parallel lines are observed at low magnifications which result from the bar-coating process. Device characteristics are shown in Figures 3d,e. Cyclic voltammogram (CV) of the GSC shows ≈2.5 mF cm−2 capacitance in the potential range −2 to 2 V (scan rate of 100 mV s−1). Figure 3e shows the galvanostatic charge/discharge test at a constant current of 0.7 mA from which an equivalent series resistance (ESR) of ≈150 Ω is measured. A single GSC device could be charged up to over 2 V by using the organic electrolyte (1 m Et4NBF4 in propylene carbonate). The energy level diagrams in Figure 4a illustrate the electron and hole transport within the integrated-power-sheet. When charging under illumination, photons are absorbed by
Figure 2. Fabrication of an integrated-power-sheet. Patterning the commercial ITO/glass by photo lithography (step A). Printing graphene ink on the conductive carbon/aluminum foil and was transferred to patterned ITO/glass (step B). Spin coating PEDOT:PSS and P3HT:PC60BM active layer and then evaporated Al electrodes (steps C and D). Laminating the supercapacitor with graphene ink electrodes, organic electrolyte, and PE/PP based sealing paper (step E). small 2015, DOI: 10.1002/smll.201403383
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Figure 3. a) J–V curve under AM 1.5 illumination (100 mW cm−2) of the 8 series connected P3HT:PC60BM organic photovoltaic devices (Table 1 shows the characteristics of the cells: 4.91 V for open-circuit voltage, 5.78 mA cm−2 for short-circuit current density, 44.3% for fill factor, and conversion efficiency is 1.57%). Electrochemical characterizations of graphene ink film as electrodes. b) Raman spectra at 633 nm for graphene ink D band is at ≈1328 cm−1, G band at ≈1576 cm−1, and 2D band at 2671 cm−1, respectively. c) SEM image of graphene ink sample for supercapacitor electrodes under low and high magnification (inset figure). d) Cyclovoltammetry (CV) curve for the supercapacitor with graphene electrodes (≈4 cm2) and organic electrolyte (1 M Et4NBF4 in propylene carbonate) measured at 100 mV s−1. e) Galvanostatic charge/discharge curves based on graphene ink electrodes with 0.7 mA.
P3HT and excited excitons are dissociated at the interfaces to produce free electrons and holes. The electrode workfunction difference serves as a driving force for the charge carriers to drift toward the supercapacitor electrodes.[23,24] Figure 4b schematically shows the photo charge/discharge connection and circuit illustration of the integrated-powersheet. The power sheet has a three-terminal design, named A, B, and C in the diagram. Terminal B is the shared backplane between all devices. During photo charging, terminals A and C are connected to provide a charging path for electrons. Upon opening the switch (disconnecting terminals A and C) the LED becomes the only discharge path for the supercapacitor. The dynamic voltage-time plot during photo charging and galvanostatic discharging is shown in Figure 4c. The power sheet was charged up to ≈2.3 V in approximately
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5 min, and then discharged at a constant current of 0.3 mA for 4 min. The nonlinear discharge curve is ascribed to the porosity saturation effect.[39] The voltage of this system is limited by the GSC, and is set by the voltage beyond which the electrolyte reacts with the electrode and breaks down, reducing device lifetime. This voltage limit allows the demonstration of this circuit to power a red LED (turn-on voltage around 1.6 V), as shown in Figure 4d (I–V curve characteristic is shown in the Supporting Information, Figure S2a).
2.2. Extending the Voltage Range – Series Connected GSCs For applications demanding higher voltage power sources, capacitors can be connected in series as well. We demonstrate
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Figure 4. a) Energy level diagrams of the photoactive layer (P3HT:PC60BM), hole transport layer (PEDOT:PSS), and electrodes (ITO, Al, graphene) and the charge transport in the integrated-power-sheet. b) Schematic illustration of power sheet three terminals connection during photo-charging/ discharging process and circuit schematic representation. c) The dynamic voltage-time plot during photo charging and galvanostatic discharging process (0.3 mA). d) Charged power sheet showed in photograph was used to drive a red LED (turn-on voltage is ≈1.6 V).
a 5 V device with 2 series connected GSCs, formed by stacking up layers of graphene electrodes. The resulting device is made up of 8 series connected OPCs and 2 series connected GSCs. To form the series GSC, after the base electrode was transferred, graphene ink was bar-coated onto carbon-Al foil and then another graphene film transferred onto the back aluminum side by the same method used for the base layer. The resulting GSC structure is ITO/conductive carbon (CC)/ graphene ink/organic electrolyte-separator/graphene ink/ (CC)/Al/(CC)/graphene ink/organic electrolyte-separator/ graphene ink/(CC)/Al (detailed schematic representation is shown in the Supporting Information, Figure S2b). The CV of the device is shown in Figure 5a, with ≈2 mF cm−2 capacitance in the potential range −4 to 4 V (scan rate of 100 mV s−1). Figure 5b shows the galvanostatic charge/discharge curve at a 0.7 mA constant current. The ESR of the device is ≈270 Ω. The dynamic voltage-time plot during photo charging and galvanostatic discharging with 0.3 mA is given in Figure 5c. The GSCs were charged by OPCs up to 3.8 V after 10 min under AM 1.5 illumination. While charging, the rate of charge slowed as voltage increased and at 3.8 V, the rate of increase small 2015, DOI: 10.1002/smll.201403383
was ≈0.68 mV s−1. The energy density for whole power sheet is about 0.2 J g−1. As a simple demonstrator, the new circuit was used to drive a red LED with turn-on voltage of about 1.6 V, a green LED with turn-on voltage of about 2 V, and a blue LED turning on around about 2.4 V [Figure 5d, (I–V characteristic of RGB LED is shown in the Supporting Information, Figure S2a)]. The integrated-power-sheet therefore makes a good self-powered source (energy generation and energy storage) for other electronic devices (energy sink).
2.3. Prototype of Graphene Electrode Integrated-Power-Sheet Polymer photovoltaic cells are attractive because cells can be manufactured in large-areas at low-cost on mechanically flexible substrates. Although photoactive layers, transport layers, and metal electrode are flexible, one major challenge in fabricating flexible OPCs is the crystalline transparent conductive oxide (TCO) electrode which is brittle and usually deposited on a solid glass substrate. When deposited on a flexible substrate, performance can be degrade after a few
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Figure 5. Electrochemical characterization of 2 series connected graphene ink supercapacitors. a) CV curve for the two stacked-up supercapacitors with graphene electrodes and organic electrolyte (1 m Et4NBF4 in propylene carbonate) measured at 100 mV s−1. b) Galvanostatic charge/discharge curves based on graphene ink electrodes with 0.7 mA. c) The dynamic voltage–time plot during photocharging and galvanostatic discharging process (0.3 mA). d) Charged power sheet was used to drive red LED (turn-on voltage is ≈1.6 V), green LED (≈2 V), and blue LED (≈2.4 V).
cycles of bending due to cracking of the brittle ITO layer.[25] Another disadvantage for this transparent conductive oxide is that indium is a rare element and given the high demand, could be in short supply in the near future. In order to make a portable, flexible, and very light weight power source, graphene, which has ultrahigh carrier mobility, high transparency (2.3% absorption per layer), and excellent mechanical flexibility,[17,26,27] is a promising material. Herein, a prototype of an all graphene integrated-power-sheet using CVD grown graphene as the top electrodes and Cr–Al–Cr as bottom electrodes for the OPCs was evaluated. Since the photovoltaic cell is now inverted, its life span is expected to be longer than the conventional one due to the low work function top electrode (Al) and acidic corrosion by PEDOT:PSS being prevented.[28,29] Furthermore, it is noticeable that the durability of the graphene top electrodes is better than metal one since graphene is impermeable to gas and liquid.[30] The graphene electrode integrated-power-sheet can be flexible, have a lower weight and a longer lifetime. The multilayer graphene top electrodes in the inverted architecture OPCs consist of single layer graphene synthesized on copper foil by CVD.[31] Graphene layers were transferred onto a glass substrate and characterized under Raman spectroscopy at 633 nm (see the Supporting Information, Figure S3a). For mono layer graphene, G band peak was at ≈1587 cm−1, and 2D peak was at ≈2644 cm−1. For bilayer graphene, G band was at ≈1596 cm−1, and 2D band was at
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≈2652 cm−1. It was possible to confirm the number of layers transferred also from the 2D band.[22] On the other hand, there was no obvious D peak in the Raman spectra originating from defects at 1350 cm−1. This indicated that CVDgraphene used is devoid of a significant number of defects and has a high crystalline quality. The detail fabrication procedures of integrated-power-sheet are in the experimental part. Photographic and schematic representations of the all graphene electrodes integrated-power-sheet device are shown in Figure 6a. The structure of inverted type organic photovoltaic cells is Cr(5 nm)–Al(100 nm)–Cr(5 nm)/ZnO/ P3HT:PC60BM/PEDOT:PSS/graphene (8 layers). The layer dependent transmittance spectrum and electrical properties of the transferred graphene are given in the Supporting Information, Figure S3b,c (single layer is about 950 Ω sq−1). The single device exhibited a short-circuit current density of 3.67 mA cm2, an open-circuit voltage of 0.55 V, and a fill factor of 51.2%, yielding a photo conversion efficiency of 1.04% (data are summarized in Table 2 in Figure 6). In this architecture, electrons transport through Cr–Al–Cr side and holes through the graphene electrode. The CV curve in Figure 6c shows GSC has a capacitance of 4.1 mF in the −2 to 2 V range (scan rate of 100 mV s−1). Cr–Al–Cr tenary layers replaced the ITO glass and Al foil as power sheet terminals. During charging, electrodes A and C were connected (the positive terminal with the inverted OPC structure).
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Figure 6. a) Photograph and schematic representation of all graphene electrodes integrated-power-sheet device. The structure of inverted type organic photovoltaic cells device is Cr(5 nm)–Al(100 nm)–Cr(5 nm)/ZnO/P3HT:PC60BM/PEDOT:PSS/graphene (8 layers). b) J–V curve under AM 1.5 illumination (100 mW cm−2) of P3HT: PC60BM organic photovoltaic devices (Table 2 shows the characteristics of the cell: 0.55 V for open-circuit voltage, 3.67 mA cm−2 for short-circuit current density, 51.2% for fill factor, and conversion efficiency is 1.04%). c) CV curve for the supercapacitor with graphene ink electrodes and organic electrolyte (1 M Et4NBF4 in propylene carbonate) measured at 100 mV s−1. d) The dynamic voltage–time plot during photo charging and galvanostatic discharging process (0.02 mA).
Figure 6d shows the galvanostatic charge/discharge test at a constant current of 0.02 mA from which an ESR of ≈150 Ω is extracted. A single OPC (0.18 cm2) can charge a GSC (4 cm2) up to open-circuit voltage (0.55 V) within 40 s. The ESR resulting from with the use of the Cr–Al–Cr similar to that of the ITO based device used previously, indicating a more significant origin elsewhere. However the Cr–Al–Cr additionally brings the advantage of mechanical flexibility over ITO.
3. Conclusion Graphene in different forms and qualities is used to facilitate the energy generation and storage elements for an integrated small 2015, DOI: 10.1002/smll.201403383
power platform. An energy source comprising multiple series connected OPCs and GSCs suitable for self-powered electronics is demonstrated. Importantly, since the photoactive layers and graphene paste are both solution processable, energy harvesting and storage devices can be envisioned to be printed onto single flexible substrates. In the three terminal configuration shown here, open-circuit voltage for the 8 OPCs in series is ≈5 V and the conversion efficiency is ≈1.6%. By the combination of series OPCs and GSCs an overall system voltage of up to ≈4 V was shown, suitable for driving red, green, and blue light emitting diodes. An integrated-power-sheet with graphene electrodes on the OPC is also demonstrated as an ITO-free alternative suitable for roll-to-roll manufacture. Consequently, the integrated-powersheet, which can be printed onto thin sheets, is suitable for
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low-cost, self-powered electronic systems such as wireless sensors which require long-term maintenance free operation.
4. Experimental Section Solar Cell Fabrication: Conventional type organic solar cells were fabricated on pattered ITO glass (Delta Technology, CG-61IN with dimension 24 × 75 × 1.1 mm) substrates with sheet resistance of ≈20 Ω sq−1 in a N2 atmosphere glove box. The ITO substrates were photolithographically patterned to arrange 8 solar cells and a supercapacitor. First, positive photoresist (AZ5214E) was spun at 3000 rpm for 40 s onto an ITO glass and then baked for 1 min at 100 °C. Second, samples were exposed to UV light under a mask for 40 s and immersed into AZ351B developer solution (diluted to a 1:4 volume ratio on DI water) for 20 s, rinsed and then baked for 120 s at 110 °C. The samples with the exposed pattern were placed into hydrochloric acid (37 wt%) for 3 min for ITO etching. After thorough washing, the remaining photoresist was removed in an acetone bath. The substrates were thoroughly cleaned in acetone and isopropanol ultrasonic baths for 15 min in turn and treated in an O2 plasma for 15 s in preparation for active layer deposition. A PEDOT:PSS (Heraeus Clevios P VP AI 4083) solution filtered through a 0.45 µm syringe filter was spun onto the ITO substrate at 5000 rpm (≈40 nm) and baked at 135 °C in air for 15 min. The photoactive layer was then deposited by spinning a 1:1 weight ratio blend consisting of P3HT (Rieke Metal) and PC60BM (Solenne) dissolved in chlorobenzene (32 mg mL−1) at 1300 rpm. Finally, 100 nm thick Al electrodes were deposited via thermal evaporation through a shadow mask (pressure ≈3 × 10−6 Torr). Upon completion of the stack, the devices were post annealed at 150 °C for 5 min in a N2-filled glove box. Alternatively, an inverted type organic solar cell was fabricated on patterned metallic electrode (on the glass slide) consisting of Cr–Al–Cr/ZnO/P3HT:PC60BM/PEDOT:PSS/CVD-graphene layers. Chromium (Cr) 5 nm, aluminum (Al) 100 nm, and chromium (Cr) 5 nm layers were deposited sequentially by thermal evaporation onto a precleaned glass slide which was pretreated by oxygen plasma at 50 W 10 min. The first Cr layer was used as an adhesion layer and the second Cr layer acted as an electron buffer and protection layer. The zinc oxide (ZnO) hole blocking layer was deposited by spinning a 20 × 10−3 M solution of zinc acetate dehydrate (Sigma-Aldrich) prepared in 1-propanol (spectroscopic grade). The procedure was repeated 15 times and air annealed at 150 °C for 30 s after every spin cycle in order to oxidize the precursor. The photoactive layer was deposited on top of the ZnO layer by spinning a 1:1 weight ratio blend solution consisting of P3HT and PC60BM dissolved in chlorobenzene (30 mg mL−1) at 1300 rpm. A PEDOT:PSS solution was mixed with isopropanol in volume ratio 1:0.8 for 15 min, and then with 1-butanol (Sigma-Aldrich) in a 1:0.6 ratio for a further 15 min. The resulting solution was filtered through a 0.45 µm syringe filter and was spun at 3000 rpm onto the photoactive layer. 2 layers of CVD-grown graphene were transferred onto thermal release tapes and these were used to transfer the graphene layer onto the PEDOT:PSS layer by heating the films to 120 °C. The process was repeated up to four times to get an 8 layer thick graphene top electrodes.
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Supercapacitor Fabrication for Standard Power Sheet: The graphene supercapacitor (GSCs) device consists of the following stack: ITO/conductive carbon/graphene ink/separator-organic electrolyte/graphene ink/conductive carbon coated Al foil. Graphene ink was commercially purchased from Vorbeck Materials. Both electrodes of the capacitor were bar-coated. For the ITO/glass side, graphene ink was first printed on the conductive carbon coated aluminum foil (60 µm wet deposit thickness) with a spun poly(methyl methacrylate) (PMMA) layer (3000 rpm, 1 min) on the ink film. The Al foil was etched away by 2 M hydrochloric acid and the ink film was transferred onto the patterned ITO/glass using the PMMA as the support. The other electrode is bar-coated graphene on Al foil. A Whatman qualitative filter paper (Grade 1) soaked in 1 M tetraethylammonium tetrafluoroborate (Et4NBF4) in propylene carbonate was used as separator, and was sandwiched in between the two graphene electrodes. The stack was finally sealed by laminating the device between PET based laminator sheets. Preparation of CVD Graphene: Graphene monolayer was synthesized on polycrystalline copper foil (from Nilaco Inc.) by chemical vapor deposition (CVD). Foil was placed in a wall furnace with a one inch fused silica tube and then heated up to 1000 °C. First, a reduction reaction was conducted by H2 flow for about 40 min before introducing CH4. Then, the graphene growth process was carried out with a H2/CH4 flow rate of 2/35 sccm for 10 min. After growth for 30 min, CH4 was turned off and the furnace tube cooled down to room temperature in H2 or Ar gas flow. The growth step process was conducted at low pressure ≈500 mTorr. Fabrication for Graphene Electrodes Integrated-Power-Sheet: There are 6 steps for fabricating the graphene electrodes integrated-power-sheet, the detailed procedure is shown in the Supporting Information, Figure S4. After CVD growth, graphene with copper foil was first attached to thermal release tape (Nitto Denko) and then immersed in 0.2 M iron nitrate solution (Fe(NO3)3.9H2O, Sigma-Aldrich) for 2 h to etch away the copper foil (step A). The thermal release tape with single layer graphene was attached to another piece of copper foil with graphene to get bilayer graphene samples (step B).[29] In parallel, thoroughly cleaned and plasma treated glass substrate was put into a thermal evaporator for bottom metal electrode deposition consisting of three different layers chromium (5 nm)–aluminum (100 nm)–chromium (5 nm)[32,33] (step C). This ternary layer forms a common electrode platform for the integrated-power-sheet. Graphene ink film was transferred to form supercapacitor electrodes by the method given in Figure 2 (step D). The photovoltaic device layers ZnO, P3HT:PC60BM blend, and PEDOT:PSS are deposited sequentially by spin coating (step E). After the OSC active later is coated, the top graphene electrode of the supercapacitor is deposited and sealed. Finally, the CVD graphene collected onto the thermal release tapes was transferred to form the top contact of the OSC. This was repeated 4 times to render 8 layers (step F).[34–39] Materials and Device Characterizations: UV–visible absorption spectra of layer dependent graphene samples were obtained by using a Jasco V570 UV/Vis/NIR spectrophotometer. Photovoltaic performance measurement was done with a computer controlled sourcemeter (HP 4140B pA meter/DC voltage source) with cells exposed to a Newport 150 W lamp with an AM1.5 filter. Supercapacitor (GSCs) performance was measured with a potentiostat/ galvanostat (Autolab PGSTAT 302N). Raman measurements were
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conducted by using a 633 nm He–Ne laser as an excitation source and detected by a HORIBA iHR 550 monochrometer equipped and a Symphony CCD detector. For the demonstrator, different color light emitting diodes (RGB) were purchased from Philips Lumileds.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partially supported by the Nokia-Cambridge University Strategic Research Alliance in Nanoscience and Nanotechnology. The authors would like to thank Dilek Özgit for the scanning electron microscope image of the graphene ink samples, Wen-Peng Deng for the light emitting diode experiments, and Dr. Li Shao-Shian for Raman spectra measurements.
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Received: November 13, 2014 Revised: January 16, 2015 Published online:
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