**3. Conclusions and outlook**

perature for 30 min. After that, the sintered TiO<sup>2</sup>

lyte, and a counter electrode. For I−

76 Emerging Solar Energy Materials

azolium iodide, 0.3 MLiI, 0.06 M I<sup>2</sup>

short-circuit current density (*J*

in DSSC, the DSSC attains a *Voc* of 0.71 V, *J*

exhibits a *Voc* of 0.49 V, a *J*

Elsevier).

(copyright (2013) Elsevier).

A DSSC device is fabricated by clamping a dye-sensitized TiO<sup>2</sup>

/I3 −

is evaluated under AM 1.5 solar simulator illumination at 100 mW cm−2.

taining dye in ethanol for a long time at room temperature to adsorb dye on TiO2

is used as the electrolyte of DSSCs [35]. Finally, the photovoltaic performance of DSSC device

**Figure 14** compares the photocurrent density-voltage (*J-V*) curves of the DSSCs with NMC-3, Pt, and FTO glass counter electrodes. The photovoltaic parameters of the DSSCs, including

sion efficiency (*η*), are summarized in **Table 5**. The DSSC with FTO glass as counter electrode

efficiency of 0.20%. When NMC-3 porous carbon material is employed as the counter electrode

power conversion efficiency is considerably comparable to 7.26% of the DSSC with Pt counter electrode. The outstanding photovoltaic performances of the DSSCs with NMC-3 counter electrode mostly originate from the vivid electrocatalytic performance of NMC-3 electrode associ-

ated with the nitrogen doping, bimodal mesopore structure, and large surface area.

**Counter electrode** *Voc* **(V)** *Jsc* **(mA cm−2) FF** *η* **(%)** Pt 0.68 16.43 0.65 7.26 NMC-3 0.71 15.46 0.64 7.02 FTO 0.49 4.13 0.10 0.20

**Table 5.** Photovoltaic parameters of the DSSCs using Pt, NMC-3, and FTO glass counter electrode [35] (copyright (2013)

**Figure 14.** Photocurrent density-voltage curves of DSSCs with Pt, NMC-3, and FTO glass counter electrodes [35]

electrodes are immersed into a solution con-

redox shuttle, a mixture of 0.5 M 1-methyl-3-propylimid-

, and 0.4 M 4-tert-butylpyridine in 3-methoxypropionitrile

*sc*),), open-circuit voltage (*Voc*), fill factor (FF), and power conver-

*sc* of 15.46 mA cm−2, FF of 0.64, and *η* of 7.02%. This

*sc* of 4.13 mA cm−2, and a FF of 0.10, leading to a poor power conversion

photoanode, a drop of electro-

photoanode.

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 longterm instability to the I<sup>−</sup> /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 metalfree 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 *J sc* and subsequently PCE of the DSSC. In this chapter, porous carbon materials have been considered as one of the promising candidates for the alternative to PT CE, since they have high surface area and porous nature, chemical corrosion resistance, electrochemical and mechanical stability, low cost, and simple preparation methods compared to Pt counter electrodes. Especially, heteroatom-doped porous carbon materials, such as CTNCs and AnCs, exhibited better electrocatalytic ability, lower charge transfer resistance, and higher PCE than the Pt CEs in Co(bpy)<sup>3</sup> 2+/3+-based electrolyte which make them promising candidates as metal-free CE for DSSCs and open a new research area for porous carbon CEs in Co(bpy)3 2+/3+-based DSSCs. However, they are not sufficiently active in iodide electrolytes, which are more common and desirable electrolytes. In the future, better electrocatalytic ability and electrochemical stability of carbon-based materials toward both redox couples of iodide and cobalt electrolytes still need to be significantly improved for the practical application of DSSCs.
