**10. Future prospects of ESC**

ITO NW. For an electro-spraying, aqueous solvent in the hydrothermal treated TiO2 nanoparticle solution is replaced by ethanol and then alpha-terpineol is added into the ethanol solution of TiO2 particles. The replaced solution was loaded into a syringe equipped with a 27-gauge stainless steel needle. The spaying rate (25 μL/ min) was controlled using a syringe pump. The electric field (12–15 kV) was applied between a metal orifice and the aluminum foil at a distance of 10 cm using a power supply and was electrosprayed onto ITO NWs substrate. The TiO2 coated electrode was gradually calcined under an air flow at 150°C for 15 min, at 320°C for 10 min, at 500°C for 30 min. As seen in **Figure 32(a)**, TiO2 sphere solution prepared from pure EtOH formed the film on the top of ITO NWs, which make it difficult to penetrate into the ITO NWs. Therefore, adding alpha-terpineol solvent with viscous and low evaporation rate into TiO2 solution, TiO2 solution is directly dropped into highly charged ITO NWs in between and spreading through x-y-z moving robot system. As a result, completely filled and crack free TiO2 film in ITO NWs can be

0.75.2% are derived from the *JV* curve with purified N719 dye and liquid electrolyte, thus giving an overall power conversion efficiency (*η*) of 10.9% under illumination

3D PhC, the efficiency is further improved to 12.45% (*J*sc = 19.75 mAcm<sup>2</sup>

*V*oc = 0.838 V, *FF* = 75.2%). The dramatically improved PCE can be achieved by using cosensitization system with a TTAR/ YD2-oC8/YDD6 and 3D PhC designed

maximum power (*P*max) = 1.99 mW. In conclusion, our concept and designed material and concept lead to significant promotion of the overall performance of a

*I-V and P-V curve for the best performance DSSC with the cosensitization and 3D PhC.*

**Figure 33(a)** presents the solar performance for the best cell. Form an effective approach to enhance the light-harvesting ability and to retard the charge recombination, the maximum PCE is about 13.26%, with short-circuit current density (*J*sc)

, open circuit voltage (*V*oc) = 0.811 V, fill factor (*FF*) = 73.3% and

, a *V*oc of 0.846 V, and a *FF* of

). While attached with a

,

obtainable (see **Figure 32(b)**). A *J*sc of 17.2 mAcm<sup>2</sup>

*Solar Cells - Theory, Materials and Recent Advances*

structure.

DSSC.

**Figure 33.**

**238**

=22.3 mAcm<sup>2</sup>

with standard AM 1.5G simulated sunlight (1000 mWcm<sup>2</sup>

Compared with traditional silicon solar cells, the DSSC device promises to be less expensive (the ease of fabrication and cost effectiveness of materials, which do not need to be highly purified process), thinner, more flexible, and amenable to a wide range of lighting conditions, all of which makes it viable and efficient solar cells for the future. Therefore, they said that ESC along with its good performance will definitely replace c-Si dominated photovoltaic markets very sooner. However, although each material is extremely inexpensive, the cost of silicon continues to fall as well. Silicon photovoltaics module are a mature technology and their costs have continued to reduce from US \$1.52 W<sup>1</sup> in 2010 to just US\$ 0.39 W<sup>1</sup> in 2018, with module efficiencies ranging from 15–20% and lifetime guaranteed to 25 years [182]. Unfortunately, the ESC technology is not so trivial that the highest module efficiency, the fastest production methods, or the lowest materials cost necessarily provides the best module solution. In addition, long term stability is the big challenge. Therefore, for surviving the future photovoltaic market, we believe ESCs have to develop the system of transparency with esthetics ability.

Building integrated photovoltaic (BIPV) technology has become an emerging research hotspot of solar PV technology because it intends to achieve "zero energy building (ZEB)" consumption through transformations of buildings from energy consumers to sustainable energy producers [183]. A sustainable building can minimize energy consumption, while at the same time supplying its own energy demand through self-generation. PV was generally installed on building's roofs, but it's very harsh to meet strict requirements of ZEB regulation. The potential power of rooftop in the United States is approximately estimated as 1400 TWh/yr. (0.16TW) at about 16% of module efficiency cell, nearly 40% of the total electricity generation of the US [184]. In buildings rooftop system with conventional PV application, the available area for PV installation is limited and it cannot fulfill the building's energy needs. On the other hands, transparent or semi-transparent PV windows have a tremendous potential to increase harvesting area as well as reduce the annual electricity consumption for cooling and heating. Therefore, we believe transparent PV is the most promising future energy system and research efforts for minimizing the tradeoff relation between the average visible transparency (AVT) and power conversion efficiency (PCE) can decide ESC's fate [185].
