**3. Summary and future prospects**

*Nanostructures*

TMCs could replace the Pt CE.

electrocatalyst for the reduction of I3

JSC of 18.03 mA cm<sup>−</sup><sup>2</sup>

**88**

**Figure 9.**

*The scheme of vertical aligned structures of (a and b) nanowall and (c) nanosphere with nanorod [64, 77].*

CoSe2/C-NCW CE reaches the highest efficiency of 8.92%, with a VOC of 0.73 V, a

the cell with Pt (8.25%). The CZTS nanowall electrodes (NWD) on Mo substrate show nanowalls with a width of ~500 nm, a thickness of nearly 15 nm, and a height of ~1.5 μm, which were adequately aligned in a densely packed array, which was nearly perpendicular to the surface of the Mo substrate, as shown in **Figure 8(l)** [72]. In this case, CZTS-NWD demonstrates a concept of "nano-geogrid"-reinforced CZTS nanowall electrode by synthesizing a thin layer of a porous CZTS nanostructure mimicking a geogrid on a substrate and then fabricating a CZTS nanowall on top of the nanostructure, as shown in **Figure 9(b)**. The *η* of the NWD device is 7.44%, which is comparable to the Pt device (7.21%). **Figure 8(m)** shows the film of

(110) planes of orthorhombic CoSe2 [53]. Impressively, the single-crystal CoSe2 CE

The 2D structure of TMCs also has a specific electron pathway and it could be vertical to the substrate to offer sufficient electrons on active sites. Moreover, the hierarchical structure has both the advantages of a large reaction area and vertical electron pathway. For example, the direction of the fractured NbSe2 sheet shows a structure with the [001] crystallographic orientation and revealed a very thick (>100 mm), disordered network arrangement of 2D sheets, as shown in **Figure 8(i)**; in comparison, the ground materials were very thin, separated nanosheets [55]. The NbSe2 sheet CE has an *η* of 7.73%, which reveals the potential to replace Pt CE (7.01%). The WSe2 is composed of several interlaced nanosheets with an average thickness of approximately 15 nm and a width between 60 and 100 nm, as shown in **Figure 8(j)** [66]. The WSe2 CE shows good electrical conductivity, subsequent energy band calculation results, and large reaction area that exhibits an *η* of 7.48%. Vertically-aligned structures of electrocatalysts were reported to facilitate faster charge transport from the substrate through the electrocatalysts to the electrolyte [64, 72, 77], as shown in **Figure 9**. This structure is expected to have better electrocatalytic ability. The nanowall and the hierarchical nanorod are used with TMCs. The CoSe2 nanoclimbing wall (CoSe2/C-NCW) reveals arrays of vertically-aligned nanowalls with sharp edges, as shown in **Figure 8(k)** [64]. In addition, the nanowalls are covered with dot-matrix-like projections; these projections are expected to provide a large surface area to the film. On account of direct electron transfer and large surface area, the CoSe2/C-NCW film, on the whole, could be a better

<sup>−</sup> to I<sup>−</sup>, as shown in **Figure 9(a)**. The cell with

, and an FF of 0.67; this efficiency is even higher than that of

0.73, which is better than the Pt CE (8.17%). The Ni3S4-PtFe heteronanorods are highly monodispersed with an average length of ∼34.0 nm and an average diameter of 9.0 nm, as shown in **Figure 8(h)** [75]. The DSSCs using Ni3S4-PtFe produce an *η* of 8.79%, which is higher than that of the Pt CE (7.83%). The 1-D structure is obviously promoting the electrocatalytic ability of TMCs and most of the 1-D structures for TMCs exhibit a better *η* than Pt CE. It can be claimed that the 1-D structures of

, and a FF of

produces an *η* of 10.20% with a VOC of 0.753 V, a JSC of 18.55 mA/cm<sup>−</sup><sup>2</sup>

The counter electrode is a paramount part of DSSCS and has a significant influence on both the photovoltaic performance and the device cost of DSSCs. As a counter electrode, it must possess high conductivity and good catalytic activity toward electrolyte regeneration, as well as good stability. The DSSC devices employing CEs of different materials including carbon materials, conductive polymers, and transition metal composites have been summarized and discussed. One key point is that the CE performance can be optimized by combining special nanostructures into CE films to promote the industrialization of Pt-free CE catalysts. The nanostructure can briefly be classified into 0D, 1D, and 2D, which have different properties. The different materials with various nanostructures can overcome the problem of the material.

The carbon materials have numerous advantages including low cost, plasticity, simple fabrication procedures, high electrical conductivity, high thermal stability, and good corrosion resistance. The *η* of carbon materials has been improved by the hierarchal structures (nanotube with nanosheet and nanotube with nanoribbon) and most of the carbon materials with hierarchal structure CE have a better value of *η* than the traditional Pt CE. However, most of the performances of the DSSCs with carbon material CEs are still slightly lower than those DSSCs with Pt CEs. This mostly results from various resistances associated with the structurally complex carbon electrodes, such as bulk resistance through the comparatively thick carbon CE, contact resistance to the TCO substrate, the diffusion resistance in the pores of the CE, etc.

The conductive polymer materials possess outstanding electron conductivity, good adhesion, and easy fabrication. According to the literature above, it can be concluded that the 1D structure conductive polymer material-based CE can provide better *η* than the particles, nanosphere, and nanosheet structures. Although conductive polymer materials have larger reaction area and specific electron pathway, most of the conductive polymer material-based CEs still have a lower *η* than the Pt-based CEs. Only a few examples show better performance than Pt CE. It means that the conductive polymer materials need a hybrid with other electrocatalysts to obtain better electrocatalytic ability.

The TMCs exhibit great electrocatalytic ability, easy preparation, and modification. However, the poor conductivity needs to be solved in order to replace Pt CE. By synthesizing nanostructures, including nanoparticle, double-shelled ballin-ball hollow sphere, hollow spherical particle, acicular nanorod array, nanorod, nanosheet, nanoclimbing wall, hierarchical nanorod, etc., TMCs reveal better performances than the Pt CE. It can be said that the TMCs with nanostructure successfully replace Pt CE.

#### *Nanostructures*

Moreover, changing DSSC electrolyte toward the Cu, Co, Fe, etc. redox couples is another important research topic. Furthermore, the dim light condition application is another prospect of DSSCs. Among them, the matching material of CE is a key point to promise the *η* of DSSC. To get the point, it is dependent on the nanostructure and the hybrid materials, such as carbon materials with TMCs, carbon materials with conductive polymer materials, conductive polymer materials with TMCs, and TMCs with both conductive polymer materials and carbon material. Further development should focus on these main requirements: conductivity, catalytic activity, stability, efficiency, cost, and environmental friendliness. Also, the regulation mechanism for photo-induced charge carrier generation, evolution, and transportation should be of concern.
