In , the same group fabricated and tested a complete carbon-based solar cell device. Although it achieved a PCE of only 5. To date, the PCE is still limited by the low photon absorption of the active layer. New compounds are being used in the active layer or novel carbon nanomaterials, such as fullerene derivatives and SWCNTs, and new architectures have been implemented. Although carbon nanomaterials are strong light absorbers, their percentage in the active layer is optimized to achieve the maximum PCE.
At present, tandem organic solar cells based on the combination of high band gap and low band gap polymers represent a reliable way to achieve improved spectral range for the photoabsorption in the device. However, the challenge of trapping more incoming photons in terms of light-electron conversion  has not been solved. Graphene has also been employed in solar cells other than the organic ones. The efficiency of this type of solar cell was increased to 8. Graphene films in dye-sensitized solar cells are mainly used as counter electrodes, which outperform platinum electrodes in some cases .
The term supercapacitor was introduced by NEC because it was the first company to commercialize a device with the name SuperCapacitor TM in . Supercapacitors have been in development since when Becker  first used carbon flooded with a sulfuric acid electrolyte to develop charge storage at the interface between these two materials. However, it was not until that the company SOHIO  first launched this technology into the market.
The real success of supercapacitors started in the s when government programs in the United States began giving funds for this technology to be incorporated into hybrid vehicles for providing necessary power for acceleration . Supercapacitors can provide a higher power density but a smaller energy density compared to traditional chemical batteries, which make them very attractive for applications where instantaneous power is required.
The other key characteristics of supercapacitors are: ability to charge—discharge within seconds; a long lifetime of more than 10 6 cycles; environmentally friendly; and stable operation at various temperatures. It can be seen that supercapacitors fill the gap between capacitors and batteries . Figure Energy density vs power density Ragone plot for various energy storage devices .
Today, several companies such as Maxwell, FastCap Systems, NEC, Panasonic, Tokin and even car companies such as Volvo are investing further in the development of this technology because of the potential large amount of energy in a small component that can be easily integrated into a device.
Volvo for example is working on reducing the weight and increasing the space in a hybrid vehicle by incorporating supercapacitors in the frame of the car . Supercapacitors are typically divided in two categories: electric double-layer capacitors EDLCs and pseudo-capacitors. Figure Hierarchical classification of supercapacitors and related types . The conventional equation that defines the capacitance is  :. But only in around Gouy  and Chapman  were able to expand the model by considering the thermal motion of the electrolyte ions that lead to a diffuse layer.
The capacitance established at one electrode will be given by the sum of a compact double layer capacitance C H and diffusion region capacitance C diff :. The IHP refers to the distance of closest approach of specifically adsorbed ions generally anions and OHP refers to that of the nonspecifically adsorbed ions. The OHP is also the plane where the diffuse layer begins.
Figure Models of the electrical double layer at a positively charged surface: a the Helmholtz model, b Figure Simple equivalent circuit. In contrast, pseudo-capacitors are devices where the charge is not stored electrostatically but electrochemically, similar to what happens in conventional lithium ion batteries. Pseudocapacitive materials such as conducting polymers e.
Because of these drawbacks, they are usually combined with carbon materials creating hybrid supercapacitors.
The capacitance for a pseudocapacitor is calculated using the following equation  :. Pseudocapacitors are not explored in this work because of the drawbacks previously described and because of the uncertainty of whether they should be categorized in the supercapacitor or in the battery family. In fact, their operating mechanism is more similar to a chemical battery than to a supercapacitor. Figure CV curve of an ideal supercapacitor. Most of the real CV curves for an EDLC show deviations from the ideal shape because of the electrolyte and electrode resistance and unwanted Faradaic reactions.
The capacitance and the internal resistance of the device can be extracted from this measurement technique. The capacitance is calculated from the slope of the charge or discharge curve with the equation  :. The effective series resistance R ESR is calculated from the voltage drop V drop that occurs at the initial portion of the discharge with the equation  :. The smallest V drop can be achieved with the smallest internal resistance.
The longest total charging time is also achieved with the smallest internal resistance. This occurs because the electrode has a smaller effective charge capacity within a specific voltage window . Another useful measurement technique to reveal the properties of a supercapacitor is electrochemical impedance spectroscopy EIS , which measures the impedance Z of a device over a range of frequencies. The data obtained are usually graphed as the real part of the impedance Z real vs the imaginary part of the impedance Z imag , also called a Nyquist plot.
Figure Schematic representation of the Nyquist impedance plot of an ideal capacitor vertical thin line and a supercapacitor with porous electrodes thick line. Figure Schematic representation of the Nyquist impedance plot of an ideal capacitor vertical thin line a To determine the supercapacitor performance two other important factors, apart from the capacitance, need to be considered.
One is the energy density that corresponds to the amount of energy stored per unit volume or mass and the other one is the power density that combines energy density with the speed that energy can be drawn out of a device. The maximum energy and power densities are achieved at the maximum voltage applied to the device, which is usually determined by the maximum voltage that the electrolyte can tolerate before decomposition and breakdown of the electrode material.
Organic electrolytes can achieve higher voltages compared to aqueous electrolytes, allowing for a dramatic increase in the energy density. In summary, several important characteristics of an EDLC have to be considered to maximize the performance of the device  :. Activated carbons ACs are the most commonly used materials for commercial electrodes in supercapacitors because of their stable electrical properties, large surface area and low cost.
This difference indicates that not all of the pores are contributing to the charge storage mechanism and that the specific surface area is not the only parameter to be considered in a supercapacitor . Pore shape and structure, pore size distribution, electrical conductivity and wettability of the electrode are other important parameters that contribute to the performance of the device as previously discussed.
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The pore size distribution in ACs is still a problem to be addressed . Carbon nanomaterials such as CNTs and graphene are excellent candidates to replace ACs as electrode materials in supercapacitors because of their remarkable chemical stability, large specific surface area, and high electric conductivity [,].
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Commercial supercapacitors contain metal foils such as Al, Cu, and stainless steel, as current collectors which require special techniques to passivate the metal surface to avoid corrosion effects due to the use of alkali or acidic electrolytes . Because of the high conductivity of CNTs and graphene, they can function as the capacitor electrode and the current collector, leading to a more simple and lightweight device.
CNTs in the form of either arrays grown on a substrate [,] or network films processed from a suspension  have been employed in supercapacitors. Figure Schematic illustration of the space in a carbon nanotube bundle available for the storage of electrolyte ions.
Figure Schematic illustration of the space in a carbon nanotube bundle available for the storage of electr CNTs grown on a substrate can be very useful for high power applications when compared to ACs. Figure Comparison of conducting paths for electron and electrolyte ions in aligned carbon nanotubes and granular activated carbon. Figure Comparison of conducting paths for electron and electrolyte ions in aligned carbon nanotubes and gr However, apart from their high volumetric capacitance values  obtained by optimizing the growth process, SWNTs are preferred because they exhibit a higher surface area and consequently better overall performance .
Many strategies have been proposed to increase the surface area of SWNTs such as oxidizing methods, pyrolysis methods or by the use of liquid zipping effects [,,]. The electrochemical oxidation in KOH can increase the capacitance by three times because it can facilitate the opening of some tubes, which increases the surface area. Vertical SWNTs with high purity and high density have also been grown by CVD and then removed from the substrate as a single unit, uniformly densified and engineered into different shapes by the zipping effects of liquid. The surface tension of the liquids and the strong van der Waals interactions can connect the SWNTs together in near-ideal graphitic spacing.
In fact, for supercapacitor applications, methods like the chemical exfoliation of graphite or the thermal reduction of GO are probably the most used because of the straightforward, large production quantity of quality materials for electrodes. The addition of certain functional groups can also help to disperse the material in different solvents .
Stoller et al. Reprinted with permission from . In order to maximize the performance by increasing the surface area, Wang et al. Figure Morphology of graphene oxide and graphene-based materials. Others, like Chen et al. In particular, Lv et al. Instead, Zhu et al. Certainly, these are not the highest values reported but the ability to use a simple microwave creates a pathway for a scalable and inexpensive process to fabricate electrodes for supercapacitors.
A similar concept was proposed in by El-Kady et al. Figure a—d Schematic illustration of the process to make laser-scribed graphene-based electrochemical capacitors. The low power, infrared laser changes the stacked GO sheets immediately into a well-exfoliated few-layered laser-scribed film, as shown in the cross-sectional SEM images.
The inset is a digital photograph showing the flexibility of the device.
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The inset image shows the glow of an LED light by the four devices in series. Figure a—d Schematic illustration of the process to make laser-scribed graphene-based electrochemical ca In , El-Kady et al. Figure a—c Schematic diagram showing the fabrication process for a laser-scribed graphene micro-supercapacitor. The disc is inserted into a LightScribe DVD drive and a computer-designed circuit is etched onto the film. The laser inside the drive converts the golden brown GO into black, laser-scribed graphene at precise locations to produce interdigitated graphene circuits a.
Copper tape is applied along the edges to improve the electrical contacts, and the interdigitated area is defined by polyimide Kapton tape b. An electrolyte overcoat is then added to create a planar micro-supercapacitor c.
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This technique has the potential for the direct writing of microdevices with a high areal density d,e. More than microdevices can be produced on a single run. The microdevices are flexible and can be produced on virtually any substrate. Figure a—c Schematic diagram showing the fabrication process for a laser-scribed graphene micro-supercap Planar structures have limited capacitance due to the restacking of graphene sheets, which reduces the surface area. Therefore, other graphene structures have been proposed to further boost the performance of EDLCs.