Carbon materials produced by activation and their precursors with photovoltaic performance.
Counter electrode (CE), as one of the key components of dye-sensitized solar cells (DSSCs), plays a significant role in the overall efficiency and cost of the device. Platinum metal has long been considered one of the most efficient CEs for DSSCs, but its scarcity, high cost, and low stability in I−/I3− redox couple limit its application in the large scale. In this chapter, we provide a broad overview on porous carbon materials as supreme metal-free counter electrode for DSSCs. In the first part, we concisely discuss on the importance and working principle of DSSCs and then the influence of counter electrode on the photovoltaic performance of DSSCs. Afterward, we review different synthetic methods and precursors of porous carbon materials and their efficiency in DSSCs. In the last section, we discuss in detail with example how to characterize and evaluate the device performance using porous carbon materials as counter electrode. Finally, we finish this chapter with a brief summary and outlook of porous carbon materials as counter electrodes in DSSCs.
- porous carbon
- counter electrode
- dye-sensitized solar cells
As the third-generation solar cells, dye-sensitized solar cell (DSSC) is one of the most promising alternatives to the silicon solar cells, due to their simple assembly procedure, good plasticity, transparency, mechanical robustness, ability to work at wider angles, and in low light and environmental friendliness [1, 2]. After a significant breakthrough in the photoelectric conversion efficiency (7.1–7.9%) of DSSCs in 1991, through introducing mesoporous film of TiO2 nanocrystalline to adsorb dye instead of planar semiconductor electrode by O’Regan and Grätzel research group , DSSCs have stimulated a great research interest over the following 25 years and attained an efficiency of ca. 14% [4, 5].
The working principle of DSSCs is not similar to conventional solar cell, but it is similar to natural photosynthesis process where light absorption and charge carrier transportation have different substances, as shown in Figure 1. DSSCs consist of three important components: dye-coated TiO2 film, counter electrode (CE), and electrolyte (or redox shuttle). In the DSSCs, after photoexcitation of sensitized dye, electrons from the LUMO level of dye molecule are injected into the conduction band of semiconductor metal oxide. Then, electrons transport to the anode and flow to the counter electrode (CE) via an external circuit. Finally, mediator electrolyte through reduction and oxidation carries electrons from counter electrode to the HOMO level of dye molecules, and dye is regenerated. This cycle is repeated again and again, and the device generates electric power continuously.
In this overall process of electron transfer, counter electrode plays a significant role on the photovoltaic parameters of DSSCs. The theoretical maximum photovoltage or open-circuit voltage of the DSSCs is higher than the output voltage after loading. This voltage loss is due to the mass transfer overpotential and the kinetic overpotential or charge transfer over potential. The former is mainly attributed to the ionic conductivity of electrolytes and the transportation of mediator species from the CE to the photoanode, whereas the latter is from the electrocatalytic activity of the CE surface toward mediator [6, 7]. The catalytic activity of the CEs can be explained in terms of current density
The overall performance of solar cell is evaluated by the solar-to-electrical energy conversion efficiency,
As low-cost and environmentally friendly materials, porous carbon has high surface area, high catalytic activity, high stability in I−/I3− redox shuttle, and better performance than the Pt CEs. The goal of this chapter is to discuss about the synthesis, characterization, and photovoltaic performance of porous carbon materials as supreme counter electrode for DSSCs.
2. Porous carbon materials as counter electrodes for DSSCs
As counter electrode (CE) is one of the most crucial components regulating the efficiency of DSSCs by catalyzing the reduction of the redox couples used as mediators to regenerate the sensitizer after electron injection, it is important to find a low-cost, high-efficiency, easy scalability, and corrosion-stable counter electrode. Platinum (Pt) has been widely employed as the standard CE in DSSCs due to its high catalytic reduction for redox shuttles, good chemical stability, and high conductivity. However, Pt is an expensive and scarce noble metal, which causes a problem for its large-scale production.
Carbon materials are one of the promising substitutes of Pt CE due to their low cost, environmentally friendly, scale availability, high surface area, high catalytic activity, high electrical conductivity, high thermal stability, good corrosion resistance toward iodine, high reactivity for triiodide reduction, etc. In 1996, Kay and Grätzel first explored graphite-carbon black mixture as CE and achieved a power conversion efficiency of 6.7% . Thereafter, intensive research efforts have been focused on carbonaceous materials, such as carbon black, mesoporous carbon, graphite , graphene [10, 11], carbon nanotubes [12, 13, 14, 15], and carbon nanofibers [16, 17, 18], and they have been successfully employed as counter electrodes.
Among the carbonaceous materials, porous carbon materials have captured extensive attention as CE materials for DSSCs, owing to its exceptional properties, including ultrahigh surface areas, large pore volumes, and tunable pore sizes and shapes, and also exhibit nanoscale effects in their mesopore channels and on their pore walls. High surface areas provide a large number of reaction or interaction sites for surface- or interface-related processes such as adsorption and catalysis. Moreover, mesopore channels facilitate the transport of atoms, ions, and large molecules through the bulk of the material and assist to overcome charge transfer resistance [19, 20].
2.1. Synthetic methods of porous carbon materials and photovoltaic performance in DSSCs
Porous carbon materials are mainly produced by activation [21, 22] and templating [19, 23, 24, 25] methods from different precursors, and the efficiency of porous carbon electrodes depends on synthesis methods and precursors. The synthetic process of porous carbons by activation is simple, low-cost, and environmentally friendly. Also, activated carbons (ACs) have excellent chemical and thermal stability as well as relatively good electrical conductivity. ACs are derived from different types of carbon-rich organic precursors (coffee waste, wood, pitch, coal, polymers, etc.) by carbonization in inert atmosphere with subsequent physical and/or chemical activation or plasma surface treatment. Depending on the activation process and the used carbon precursors, a variety of ACs with different physicochemical properties and well-developed specific surface area (SSA) from 500 to 3000 m2 g−1 have been prepared and used as CEs for DSSCs. The high surface area of activated carbons can considerably contribute to improving the catalytic activity of the CEs in DSSCs.
Imoto et al. firstly reported that porous carbon materials prepared with an activation process were superior to a Pt-sputtered electrode as the DSSC CEs. Moreover, they observed that the photovoltaic performance was strongly influenced by the roughness factor and the apparent charge transfer resistance of DSSC CEs. Generally, the roughness factor of the carbon electrode becomes larger when a porous carbon material with larger surface area was used. Thus, the photovoltaic performance was improved with increasing the roughness factor of the carbon-based CE. Also, the apparent charge transfer resistance for redox reaction of I−/I3− couple on the porous carbon CE decreased with increasing the roughness factor. Therefore, back transfer of the photoinjected electrons to the oxidized dyes and/or I3− ions was inhibited which in turns improved the FF value significantly. Furthermore, the
Nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area were prepared by Wang group through carbonization of polyaniline (PANI) nanorods and consequent chemical activation and explored as the DSSC CEs. The unique combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity for the I3− reduction, which is illuminated by electrochemical measurement. Under simulated AM 1.5 illumination (100 mW cm−2), the DSSC based on N-PCNR counter electrode achieves a conversion efficiency of 7.01%, which is nearly close to that of the cell based on Pt CE (7.25%).
In their experiment, PANI nanorods were firstly prepared by polymerization of aniline in aqueous solution using ammonium persulfate (APS) as the initiator, as shown in Figure 2. Ten grams of aniline and 7 g of citric acid were mixed in 500 mL of distilled water, and then 20 g of ammonium persulfate was dissolved in 150 mL of distilled water. Afterward, ammonium persulfate solution was added into the mixture solution of aniline and citric acid with vigorous stirring. The resulting solution was left standing at about 3°C for 20 h. The obtained dark green sample was filtered and washed with distilled water and then dried under vacuum at 50°C. The as-prepared PANI nanorods were pyrolyzed at 600°C under nitrogen atmosphere for 3 h to obtain nitrogen-doped carbon nanorods (N-CNRs). Finally, N-PCNRs were prepared by activating N-CNRs with KOH .
Recently, H. K. Kim group synthesized anchovy-derived nitrogen and sulfur Co-doped porous carbons (AnCs) by a simple carbonization and alkali activation method for DSSC CEs, as shown in Figure 3. Typically, a three-step strategy was used to prepare the activated carbon. Firstly, a dried anchovy powder was weighed and transferred into a quartz tubular furnace and then pre-carbonized at 300°C for 1 h under atmosphere, yielding a pre-carbonized material. An activation and carbonization process was sequentially carried out as follows: The pre-carbonized material was mixed with various mass ratios of KOH and then directly heated at an elevated temperature (700, 800, 900°C) for 2 h under inert nitrogen atmosphere. Finally, the anchovy-derived activated carbon obtained was washed several times with aqueous 2 M HCl solution and deionized water in order to remove the potassium compounds and any impurities. After filtration, the samples were dried at 70°C in an electric vacuum oven.
The anchovy-derived activated carbons as a DSSC CE exhibited superior PCE of 12.72% to the Pt CE with 12.23%. It is ascribed to an improved fill factor caused by its better electrocatalytic ability . To the best of our knowledge, this PCE is the highest efficiency value reported for DSSCs based on carbon nanomaterial-based CEs. Table 1 summarizes porous carbon materials produced by activation and their precursors with photovoltaic performance.
|PCE (%)||PCE-Pt (%)||Reference|
|Carbon chunks||Coffee waste||N719-I−/I3−||0.46||72.6||15.09||8.32||8.07|||
Although activated carbon possesses high surface area, but most of the pores are micropore which may increase diffusion impedance of electrolytes into the pore of CEs especially for the bulkier redox couples like Co(bpy)32+/3+. Templating method is used to synthesize mesopore and macropore carbon materials with connected channel. Depending on the template used, templating method can be classified into two classes: hard template and soft template. Typically, silica nanoparticles, zeolites, anodic aluminum oxide (AAO) films, and mesoporous silica have been used as the hard templates. The synthetic process involves the impregnation of carbon precursor into the porous structure of pre-synthesis template following the carbonization and removal of the template. On the other hand, soft templating involves the self-assembly of copolymers and their direct removal through carbonization. Various commercially available triblock copolymers PEO-PPO-PEO (PO, propylene oxide; EO, ethylene oxide), such as F127 (EO106PO70EO106), P123 (EO20PO70EO20), and F108 (EO132PO50EO132), have been extensively used as the soft templates.
Hierarchical nanostructured carbon with a hollow macroporous core of ca. 60 nm in diameter in combination with mesoporous shell of ca. 30 nm in thickness was synthesized and explored as CE in metal-free organic dye-sensitized solar cells exhibited in Figure 4 . The superior structural characteristics, particularly, the unique hierarchical core/shell nanostructure along with 3D large interconnected interstitial volume with large specific surface area and mesoporous volume, were observed compared with other porous carbon counterparts such as activated carbons and ordered mesoporous carbon CMK-3 (fabricated by replication through nano-casting of SBA-15 silica and using phenol as the carbon source) and Pt counter electrode. This hierarchical core/shell nanostructural feature with 3D large interconnected interstitial volume facilitates mass transportation in hollow macroporous core/mesoporous shell carbon (HCMSC), which was synthesized by replicating through nano-casting of solid core/mesoporous shell silica and phenol and paraformaldehyde as precursor, and enables HCMSC to have highly boosted catalytic activity toward the reduction of I3− and so substantially enhanced photovoltaic performance. HCMSC as a DSSC CE displays a
Gokhale et al. introduced a laser photochemical process to synthesize broccoli-type hierarchical morphology for use as a metal-free CE in DSSCs. The method includes pulsed excimer laser irradiation on a thin layer of liquid halo-aromatic organic solvent containing o-dichlorobenzene (DCB). The obtained coating reflects a self-assembled carbon nanoparticle and process-controlled morphology that yields a PCE of 5.1% which was comparable to the PCE of 6.2% with the conventional Pt-based counter electrode .
Wang et al. designed and fabricated a novel bioinspired Pt- and FTO-free integrated pure carbon counter electrode for DSSCs with orderly mesoporous carbon (OMC) as the catalytic layer and a porous carbon sheet as a conducting substrate (see in Figure 5). On a porous conducting substrate, a stiff, crustose lichen-like, integrated carbon-carbon architectured composite as a catalytic layer was made by a sequential process of spin coating, infiltration, and pyrolysis of polymer precursor. To fabricate the integrated pure carbon electrode, porous carbon plates (PCPs) were firstly prepared by a bulk molding-carbonization process using cheap coal powder as precursor. The permeation of electrolyte through the porous substrate has been fairly resolved by impermeable pretreatment through a repeat pitch impregnation-carbonization treatment of paraffin-protected PCPs. A phenol-formaldehyde resin solution including Pluronic F127 (EO106PO70EO106) as a template was spin coated onto the open pore side of the conductive substrate and then carbonized at 800°C under inert nitrogen atmosphere. By an evaporation-induced self-assembly (EISA) mechanism, an identical mesostructured resin film was formed. Successively, a catalytic mesoporous carbon layer that was embedded in the porous carbon plate was produced by carbonization. The architecturally integrated carbon-based CE displays very low charge transfer resistance (
Heteroatom-doped mesoporous carbons have received a great attention because of their enhanced electrocatalytic activity resulted from heteroatoms [35, 36]. The electrocatalytic properties of these systems are typically attributed to high charge polarization arising from the difference in electronegativity between carbon and heteroatom leading to enhanced charge transfer capability and thus increased catalytic activity. Also, to enhance the ordered graphitic layer and stability, porous carbon materials are mixed with polymer, transition metal, and other nanocarbon materials like graphene and CNTs [37, 38, 39, 40, 41].
Wang et al. synthesized activated N-doped porous carbons (a-NCs) by pyrolysis and alkali activation of graphene-incorporated melamine formaldehyde resin (MF). To prepare a-NCs, firstly a homogeneous mixture of activated graphene (a-G) with melamine and formaldehyde was prepared and pre-carbonized in Teflon-lined autoclave at 140°C for 4 h to form graphene-incorporated MF resin. The as-prepared graphene-incorporated MF resin composite then carbonized at 800°C for 2 h in N2 atmosphere followed by activation with KOH, yielding a-NCs. a-NCs with the moderate N-doping level, mesopore-rich porous texture, and incorporation of graphene enable the applications of a-NCs in electrode materials with high specific surface area and good conductivity for dye-sensitized solar cell (DSSCs). At the optimum activation temperature of 700°C, the obtained sample, labeled as a-NC700, possesses a satisfactory graphitization, a specific surface area of 1302 m2 g−1 and an N fraction of 4.5%. A PCE of 6.9% is reached, when as-prepared a-NC700 used as a DSSC CE, which is comparable to that of the Pt CE-based DSSC (7.1%) .
H. K. Kim group prepared copolymer-templated nitrogen-enriched nanocarbons (CTNCs) and observed the better PCEs for both I−/I3− and Co(bpy)32+/3+ redox couples. CTNCs were synthesized by the pyrolysis of PAN-b-PBA copolymer as shown in Figure 6.
The block copolymer was pyrolyzed at 600–1000°C for 0.5 h under N2 gas flow (150 mL min−1) at a heating rate of 10°C min−1, after stabilization through the cross-linking of the PAN block at 280°C for 1 h under air flow (150 mL min−1) at a heating rate of 1°C min−1, purged with N2 gas for 1 h during cooling. Superior performance in catalytic activity toward the reduction of Co(bpy)32+/3+ compared to the conventional Pt CE was observed. The detected remarkable activity of CTNCs is owing to their distinctive electronic properties stemming from the presence of nitrogen heteroatoms placed on the edges of nanographitic domains in combination with high specific surface area delivered by a three-dimensional, hierarchical pore structure. Overall, the application of CTNC CEs enhanced the efficiency and fill factor (FF) of JK-306 dye and Co(bpy)32+/3+ redox couple-based DSSCs at one sun illumination, up to 10.32 and 73.5%, respectively, suggesting the substantial potential of these materials as an striking substitute to expensive Pt-based CEs .
Hasin group prepared Co or Ni species incorporated N-doped mesoporous carbon (Co-N-MC or Ni-N-MC) with high specific surface area and observed brilliant electrocatalytic activity toward the electrochemical reaction of I−/I3− redox couple in DSSC systems as compared with N-doped mesoporous carbon (N-MC) displayed in Figure 7.
Co-N-MC and Ni-N-MC were synthesized on the basis of replication through nano-casting of SBA-15 mesoporous silica. The polymerization of aniline was conducted in the pore of SBA-15 mesoporous silica functionalized with alkyldiamine group. Cobalt(II) or nickel(II) ion was doped in the polyaniline (PANI) within the pores of SBA-15 followed by pyrolyzing in quartz tube. The considerable photoelectric conversion efficiency of Co-N-MC- and Ni-N-MC-based DSSCs is ascribed to the good electrical conductivity in their frameworks and the dominating contents of pyridinic and quaternary N species over pyrrolic N in their structural configuration. Furthermore, significantly lower charge transfer resistance (
|PCE (%)||PCE-Pt (%)||Reference|
|PCE (%)||PCE-Pt (%)||Reference|
2.2. Morphological and structural characterization of porous carbon materials
The morphology of porous carbon materials is usually characterized by using scanning electron microscopy (SEM) or field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Figure 8(a)–(c) shows the FE-SEM images for the surface morphology and microstructures of honeycomb-like activated porous carbons (HPCs) . As shown in Figure 8(a), the micro-sized particles of the as-prepared sample revealed a honeycomb-like morphology containing a huge number of minute holes over its entire frameworks. Magnified view of FE-SEM image of the sample confirmed the interconnected macroporous structures with sizes of ~100–600 nm (in Figure 8(b)). Moreover, a close inspection reveals that the pore and surface wall of HPC sample comprised of many mesopores and micropores as shown in Figure 8(c). In addition, the TEM image in Figure 8(d) also confirmed the honeycomb-like morphology with the porous property.
X-ray diffraction (XRD) and Raman spectroscopy are used to ascertain the extent of crystallinity in the structure of porous carbon samples. Porous carbon materials display diffraction peaks at about 24 and 44°, which are the equivalent of hexagonal graphitic 002 (2θ = 24°) and 101 (2θ = 44°) crystal planes as shown in Figure 9(a) . To identify the degree of graphitization, Raman spectra are used as follows: Two peaks at 1340 and 1590 cm−1 that is assigned to the D band and G band are observed as shown in Figure 9(b).
The breathing mode vibration of A1g, associated with the disordered carbon or defective graphitic carbon appeared at around 1353 cm−1 corresponding to the D band. Another peak related to the G band appeared at around 1590 cm−1, which specifies the in-plane stretching vibration mode of E2g in sp2C–C bond vibrations, as in graphitic phase .
Additionally, X-ray photoelectron spectroscopy (XPS) analysis gives the information about the presence of surface functional groups in the porous carbon. In Figure 9(c), the high-resolution XPS spectrum of C1s in the binding energy range between 280 and 288 eV revealed several carbon-based functional groups, which are assigned to the C–OH, C–C/C–H, C–O, and C=O bonds, respectively. Meanwhile, as shown in Figure 9(d), the deconvoluted XPS spectra of the O 1 s exhibited the three distinct peaks at the binding energies of 526–538 eV, which are attributed to the C–OH, C=O groups and chemisorbed water molecules, respectively .
The surface area and pore volume of porous carbon materials are measured through the experiments of adsorption-desorption isotherms, as shown inset in Figures 9(a) and 10(b). Porous carbon materials typically exhibit III/IV isotherms with pronounced hysteresis loops, indicating the larger surface area.
The specific surface area is calculated from the Brunauer-Emmett-Teller (BET) equation, and the pore size distribution is derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method (Figure 10(b)) .
2.3. Electrochemical characterization of porous carbon material-based counter electrodes
The catalytic properties of counter electrodes in DSSCs are usually characterized by electrochemical impedance spectroscopy (EIS) [53, 54]. EIS measurements are performed in a symmetric thin-layer cell (called dummy cell) comprising two identical electrodes and same electrolyte used in the full DSSCs. The Nyquist plots for porous carbon CE consist of three semicircles, which are assigned in the order of increasing frequency to the Nernst diffusion impedance in the bulk electrolyte (˂ 10 Hz), the charge transfer processes at the electrode/electrolyte interface (2500–25 Hz), and the Nernst diffusion impedance in the pores of electrode materials (100–2.5 KHz) in Figure 11(a).
The equivalent circuit elements for fitting the EIS data, which is well known as a Randles-type equivalent circuit, comprise of the ohmic serial resistance (
where is the angular frequency, B is the CPE parameter, and is the CPE exponent (0≤𝛽≤1, for ideal capacitance 𝛽 = 1).
The parameter Npore can be omitted for Pt CE where the catalytic reaction occurs on virtually nonporous surface (the equivalent circuit for the symmetric cell comprising of Pt CEs is shown in the inset of Figure 11(a)). As a result, the Nyquist plot for Pt electrode exhibits two semicircles, allocating in the order of decreasing frequency to the charge transfer process at Pt electrode/electrolyte interface and Nernst diffusion of redox couple in electrolyte. From the diameter of the high-frequency semicircle,
|Thickness of NMC layer (m)|
Tafel polarization measurements are carried out in a symmetric cell similar to the one used in EIS experiment. Usually, the Tafel curve comprises of three distinguishable zones, the polarization zone at low potential (voltage range of −120 to 120 mV), the Tafel zone at middle potential (with a sharp decrease), and the diffusion zone at high potential (horizontal part). The exchange current density (
Furthermore, cyclic voltammetry of dummy cell is used to verify the electrocatalytic activity of porous carbon. The peak-to-peak separation (
EIS is also used to check the stability of porous carbon CEs. Figure 13(a) and (b) shows the electrochemical stability of the dummy cells with CTNC-800 and Pt CEs under cumulative potential cycling with Co(bpy)32+/3+ in acetonitrile. After 100 cycles, although only a very modest increase of
The greater slopes of the linear dependence of the reduction and oxidation currents against the square root of potential and higher peak current of CTNC-800 electrodes at all scan rate confirm the better catalytic activity of CTNC-800 toward the reduction of Co(bpy)32+/3+ than the Pt CE.
2.4. Photovoltaic performance evaluation of DSSCs with porous carbon CEs
To evaluate the photovoltaic performance of DSSCs employing porous carbon CEs, generally a prescribed amount of carbon materials is grounded in a mortar with 0.1 ml tetrabutyl titanate and 8 ml n-butanol to obtain the carbon paste. Then, the electrode is prepared by coating carbon paste on FTO glass using doctor-blade method and heated at a certain temperature for 30 min. On the other hand, photoanode is prepared by depositing TiO2 film with the thickness of 10 μm on FTO glass substrate by doctor-blade method and then sintered at a certain temperature for 30 min. After that, the sintered TiO2 electrodes are immersed into a solution containing dye in ethanol for a long time at room temperature to adsorb dye on TiO2 photoanode.
A DSSC device is fabricated by clamping a dye-sensitized TiO2 photoanode, a drop of electrolyte, and a counter electrode. For I−/I3− redox shuttle, a mixture of 0.5 M 1-methyl-3-propylimidazolium iodide, 0.3 MLiI, 0.06 M I2, and 0.4 M 4-tert-butylpyridine in 3-methoxypropionitrile is used as the electrolyte of DSSCs . Finally, the photovoltaic performance of DSSC device is evaluated under AM 1.5 solar simulator illumination at 100 mW cm−2.
Figure 14 compares the photocurrent density-voltage (
3. Conclusions and outlook
Dye-sensitized solar cells (DSSCs) have aroused intense interest and been regarded as one of the most prospective solar cells, due to low-cost, flexibility, simple device fabrication, and high conversion efficiency under weak light, in comparison to the conventional photovoltaic devices. Very recently, G2E in Swiss and G24i in the UK including Korean and Japan companies have demonstrated commercial and prototyped components based on DSSC technology with liquid electrolytes. However, the unit costs, long-term device stability, and power conversion efficiency must be further improved for real-life applications. For this purpose, considerable efforts have been devoted to the search for low-cost Pt-free catalysts that exhibit high electrochemical activity and fast electron transfer kinetics, while a platinum (Pt) metal is still known as the highly efficient and extensively used counter electrode (CE) in DSSCs; however, it has more or less problems that make it improper for the real-life application in DSSCs, such as its high manufacturing cost owing to its natural scarcity and insufficient long-term instability to the I−/I3− redox couple in DSSCs. As a result, significant efforts have been devoted to finding possible alternatives to Pt, including carbon blacks, carbon nanotubes, functionalized graphene, and heteroatom-doped graphene nanoplatelets as efficient metal-free electrocatalysts. An ideal counter electrode in DSSCs must possess the following properties: high electrocatalytic ability and high conductivity, optimum thickness, high surface area and porous nature, low charge transfer resistance, high electrochemical and mechanical stability, energy level that matches the potential of the redox couple electrolyte, high reflectivity, and good adhesivity with TCO. High electrocatalytic ability and low charge transfer resistance of CEs increase FF and
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) through the Mid-career Researcher Program (2017R1A2A1A17069374), the Functional Districts of the Science Belt Support Program (2017K000494), and the Korea Research Institute of Chemical Technology (KRICT & Project No. KK1702-A00).