1. Introduction

Global energy demand is expected to increase from 18 TW in 2013 to 50 TW in 2050, along with corresponding increase in CO2 emissions due to inevitable increase in population and industrialization in the developing world [1, 2]. So far, most of the energy (~80%) have been derived from fossil fuels, which is not sustainable and detrimental to the environment [2]. Thus,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

sustainable and fossil-free pathways for producing clean energy and fuels such as conversion of sunlight to electricity and molecules in atmosphere, e.g., water, CO2 and nitrogen, to H2, hydrocarbons and ammonia respectively are highly required [3–8]. In this regard, dependence on renewable energy sources such as solar, wind and hydroelectric has been strategically deployed from last few decades with reasonable effect [5, 7]. Among all the renewable energy sources, solar energy is the most potent and exploitable source [9]. However, in order to achieve large scale, cost effective, carbon neutral supply of energy from sun; capture, conversion and storage of energy should be highly efficient and cost effective [6, 9]. In this regard, photovoltaics (PVs) are playing a substantial role in harnessing the sun energy mainly dominated by silicon based solar cells at present. However, manufacturing of silicon PVs require high temperature (>1600�C for silicon melting) and ultra-pure materials, thus adding to the manufacturing complexity and cost [10]. Additionally, the scarcity of silver, a common electrode material greatly limits to meet the future terawatt challenge. This have motivated researchers around the globe to develop strategies for solar energy conversion based on abundant, non-toxic, easy to process, commercially viable and cost effective systems. In this regard solar PVs prepared from mesoscopic metal oxides such as (TiO2, ZnO, SnO2, etc.) and organic light absorbing materials could meet the criteria as a suitable alternative, provided high efficiency can be realized. Metal oxide serves as an electron acceptor and facilitates the transport of electrons, along with being a scaffold for the adsorption of light harvesting constituents in many cases [11, 12]. Out of different metal oxides, mesoscopic (10–50 nm size pores) titanium dioxide (TiO2) by far has been the most widely studied and employed owing to ease process-ability, chemical stability, high surface area, low cost and non-toxic nature. [12–15] Out of the four naturally occurring polymorphs of TiO2, anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (monoclinic), anatase is preferred for PV's applications because of higher conduction band energy and slower recombination rate of charge carriers [16–18].

to 14.3% has been demonstrated by judicious choice of co-sensitizing organic photosensitizers having strong binding to TiO2 and broad absorption, post staining surface capping and tailored solution based redox shuttle and electrolyte additives [25]. However, for further improvement in PCE (1) sensitizers with efficient conversion of absorbed photons to electrons (400–900 nm), particularly beyond 650 nm (2) redox shuttles with photovoltage output greater than 1.2 V (3) minimized recombinations and over potential losses and (4) optimized tandem

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The standard components of a typical DSC embodiment are (1) FTO (fluorine doped tin oxide) deposited on glass substrate (2) mesoscopic TiO2 film (3) sensitizer, organic or metal complex anchored to TiO2 (4) mediator, to regenerate the dye (5) counter electrode with platinum (pt) to reduce the mediator (Figure 1). Upon illumination the sensitizer gets photo-excited and injects an electron in the conduction band (CB) of TiO2 thus generating an electric potential difference. This injected electron then diffuses through the mesoporous TiO2 where it is extracted to outer circuit at photoanode. Meanwhile the oxidized sensitizer is regenerated by the redox mediator, whereas the extracted electron travels through the load to the counter electrode, which then transfers electron to the mediator. At the interface boundary, back electron transfer to the oxidized dye and recombination with the electrolyte has been known the most drastic events which lower the performance along with inefficient light absorption beyond 650 nm [13, 14]. On the same note before printing the mesoporous TiO2 film a compact TiO2 layer (mostly from aqueous TiCl4 solution) is deposited on the FTO glass which prevents the short circuiting of the device, improves adhesion of TiO2 nanoparticles and minimizes the direct contact of electrolyte with FTO [30–32]. In terms of characterization of DSCs, two important measurements are short photocurrent-density (Jsc) and open circuit voltage (Voc) curve also known as

devices are highly required [26–29].

Figure 1. Operational principle of DSCs.

In terms of mesoscopic-TiO2 based solar cells, dye-sensitized solar cells (DSCs) are the most widely studied with recent surge in research for perovskite solar cells [12, 19, 20]. The discussion in the remaining chapter will be with the reference to DSCs employing TiO2 as electron accepting and transport layer. The seminal report of 1991 on TiO2 based DSCs by Grätzel and O<sup>0</sup> Regan has garnered more than 26,500 citations (November 2017) highlighting the plethora of knowledge generated and wide spread interest of scientific community [11]. It should be noted that nanocrystalline morphology which goes through necking as the result of sintering and lead to mesoscopic film of TiO2 is essential for the efficient operation of the DSCs, since a monolayer of sensitizer on flat metal oxide surface only absorb small portion of incident light [13]. Realization of this important nanostructure requirement aspect enhanced the adsorption and subsequent light harvesting in DSCs by molecular sensitizers or dyes more than 1000 time [13]. This enabled DSCs only system where charge generation (sensitizer) and transport (semiconductor) is performed by separate components. [14] DSCs are attractive compared to other photovoltaic technologies in terms of economic advantage, tunability of color, can be built on rigid and flexible substrates, made of benign materials such as TiO2 and metal free organic dyes, offer sustained efficiency for indoor applications, and perform independently of the angle of incidence [11, 12, 21–24]. Current, DSC record power conversion efficiency (PCE) up to 14.3% has been demonstrated by judicious choice of co-sensitizing organic photosensitizers having strong binding to TiO2 and broad absorption, post staining surface capping and tailored solution based redox shuttle and electrolyte additives [25]. However, for further improvement in PCE (1) sensitizers with efficient conversion of absorbed photons to electrons (400–900 nm), particularly beyond 650 nm (2) redox shuttles with photovoltage output greater than 1.2 V (3) minimized recombinations and over potential losses and (4) optimized tandem devices are highly required [26–29].

The standard components of a typical DSC embodiment are (1) FTO (fluorine doped tin oxide) deposited on glass substrate (2) mesoscopic TiO2 film (3) sensitizer, organic or metal complex anchored to TiO2 (4) mediator, to regenerate the dye (5) counter electrode with platinum (pt) to reduce the mediator (Figure 1). Upon illumination the sensitizer gets photo-excited and injects an electron in the conduction band (CB) of TiO2 thus generating an electric potential difference. This injected electron then diffuses through the mesoporous TiO2 where it is extracted to outer circuit at photoanode. Meanwhile the oxidized sensitizer is regenerated by the redox mediator, whereas the extracted electron travels through the load to the counter electrode, which then transfers electron to the mediator. At the interface boundary, back electron transfer to the oxidized dye and recombination with the electrolyte has been known the most drastic events which lower the performance along with inefficient light absorption beyond 650 nm [13, 14]. On the same note before printing the mesoporous TiO2 film a compact TiO2 layer (mostly from aqueous TiCl4 solution) is deposited on the FTO glass which prevents the short circuiting of the device, improves adhesion of TiO2 nanoparticles and minimizes the direct contact of electrolyte with FTO [30–32]. In terms of characterization of DSCs, two important measurements are short photocurrent-density (Jsc) and open circuit voltage (Voc) curve also known as

Figure 1. Operational principle of DSCs.

sustainable and fossil-free pathways for producing clean energy and fuels such as conversion of sunlight to electricity and molecules in atmosphere, e.g., water, CO2 and nitrogen, to H2, hydrocarbons and ammonia respectively are highly required [3–8]. In this regard, dependence on renewable energy sources such as solar, wind and hydroelectric has been strategically deployed from last few decades with reasonable effect [5, 7]. Among all the renewable energy sources, solar energy is the most potent and exploitable source [9]. However, in order to achieve large scale, cost effective, carbon neutral supply of energy from sun; capture, conversion and storage of energy should be highly efficient and cost effective [6, 9]. In this regard, photovoltaics (PVs) are playing a substantial role in harnessing the sun energy mainly dominated by silicon based solar cells at present. However, manufacturing of silicon PVs require high temperature (>1600�C for silicon melting) and ultra-pure materials, thus adding to the manufacturing complexity and cost [10]. Additionally, the scarcity of silver, a common electrode material greatly limits to meet the future terawatt challenge. This have motivated researchers around the globe to develop strategies for solar energy conversion based on abundant, non-toxic, easy to process, commercially viable and cost effective systems. In this regard solar PVs prepared from mesoscopic metal oxides such as (TiO2, ZnO, SnO2, etc.) and organic light absorbing materials could meet the criteria as a suitable alternative, provided high efficiency can be realized. Metal oxide serves as an electron acceptor and facilitates the transport of electrons, along with being a scaffold for the adsorption of light harvesting constituents in many cases [11, 12]. Out of different metal oxides, mesoscopic (10–50 nm size pores) titanium dioxide (TiO2) by far has been the most widely studied and employed owing to ease process-ability, chemical stability, high surface area, low cost and non-toxic nature. [12–15] Out of the four naturally occurring polymorphs of TiO2, anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (monoclinic), anatase is preferred for PV's applications because of higher conduction band energy and slower recombination rate of

In terms of mesoscopic-TiO2 based solar cells, dye-sensitized solar cells (DSCs) are the most widely studied with recent surge in research for perovskite solar cells [12, 19, 20]. The discussion in the remaining chapter will be with the reference to DSCs employing TiO2 as electron accepting and transport layer. The seminal report of 1991 on TiO2 based DSCs by Grätzel and O<sup>0</sup> Regan has garnered more than 26,500 citations (November 2017) highlighting the plethora of knowledge generated and wide spread interest of scientific community [11]. It should be noted that nanocrystalline morphology which goes through necking as the result of sintering and lead to mesoscopic film of TiO2 is essential for the efficient operation of the DSCs, since a monolayer of sensitizer on flat metal oxide surface only absorb small portion of incident light [13]. Realization of this important nanostructure requirement aspect enhanced the adsorption and subsequent light harvesting in DSCs by molecular sensitizers or dyes more than 1000 time [13]. This enabled DSCs only system where charge generation (sensitizer) and transport (semiconductor) is performed by separate components. [14] DSCs are attractive compared to other photovoltaic technologies in terms of economic advantage, tunability of color, can be built on rigid and flexible substrates, made of benign materials such as TiO2 and metal free organic dyes, offer sustained efficiency for indoor applications, and perform independently of the angle of incidence [11, 12, 21–24]. Current, DSC record power conversion efficiency (PCE) up

charge carriers [16–18].

388 Titanium Dioxide - Material for a Sustainable Environment

J-V curve and incident photon to current conversion efficiency (IPCE) or EQE (external quantum efficiency). IPCE depicts the photocurrent density response of the device at monochromatic wavelength and depends on the same parameters as Jsc [33].

Jsc can be increased by molecular engineering of the sensitizer, with ideal ground and excited state energetics, high molar absorptivity, and aggregation less anchoring on TiO2 [12]. Voc is the energy difference between the TiO2 fermi level and the redox potential of the mediator and depends on the electron density in TiO2. Higher Voc can be achieved by minimization of the dark current, increase in electron injection, negative (upward) shift in the energy of the conduction band, positive (downward) shift in the energy of the redox shuttle and series connection of devices [35, 44]. Both Voc and FF are hugely related to recombination reactions (dark current) and can be substantially influenced by modification of TiO2 in the presence of additives vide infra. Along with J-V and IPCE measurements, electrochemical impedance spectroscopy (EIS) and small modulation photovoltage transient measurements have been widely employed to fully characterize the devices. Readers are kindly referred to the previously published reviews to learn about these powerful techniques to characterize interface and

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In terms of achieving higher efficiency by modification of TiO2 the main objective is to minimize the recombination losses by blocking the TiO2 surface, increase in electron injection by manipulation of TiO2 CB, aid in better orientation, structure and geometry of the dyes on TiO2 and suppression of dye aggregation and stacking. This enhancement of DSCs devoid of dye and electrolyte designing and arduous manipulations of their molecular structures can be achieved by (1) TiO2 paste additions (2) dye solution co-adsorbing additives (3) post staining surface treatment additives and (4) electrolyte additives (Figure 3). This chapter is now further divided into sections as shown in Figure 3. to integrate and analyze the most successful strategies, their role in enhancing DSCs performance, similarities among different approaches

It is crucial to highlight that just like dyes for DSCs most of the additives will require an anchoring group for immobilization on TiO2, other than plasmonic nanoparticles and composite of TiO2 NPs (Section 1, which become the intrinsic part of TiO2 NPs after sintering also termed as "hard modification"). The most widely used anchoring groups are same as dyes, e.g., carboxylic acid, phosphonic/phosphinic acid, pyridine, and most recently siloxanes. Though multitude of anchoring modes such as covalent attachment, hydrogen bonding, electrostatic interaction, van der Waals interaction and physical entrapment has been proposed [49]. It is important to notice that these anchoring systems should also facilitate the electron transfer. Additionally, due to structural complexity of the interface environment several models are used to elucidate the anchoring. For physical characterization of interface Fourier

charge transfer properties [45–48].

and discussion of proposed mechanisms.

Figure 3. Summary layout for TiO2 modification approaches.

J-V curve is measured by scanning the voltage across the device from 0 to higher voltage (forward bias) or higher to zero voltage (reverse bias) either with a source meter or a potentiostat. Power conversion efficiency (PCE) of the cells, the main performance metric is then calculated according to the equation, PCE = (JscVoc FF)/I0, where FF is the fill factor which is simply the measure of the squareness of the J-V curve, and depicts the electrochemical losses in the device with value between 0 and 1 (normally between 0.65 and 0.75 for DSCs) [34]. I0 is the power input for the incident irradiation which is normally 1 sun (100 mW/cm2 ). A high performing DSC should behave an ideal diode with infinite shunt resistance and minimum series resistance which will lead to higher FF and PCE ultimately [13]. Briefly, Jsc α LHE.φinj.φreg.ηcoll, where LHE is light harvesting efficiency of sensitizer on given thickness of TiO2, φinj and φreg are the quantum yield of electron injection and dye regeneration and ηcoll is the charge collection efficiency [12].

In Figure 2, thermodynamic requirements for electron injection (up to 300 mV) and dye regeneration (100–500 mV) overpotential has been shown. Dotted lines highlight the unwanted recombination reactions with redox shuttle (10<sup>2</sup> sec) and oxidized sensitizer (10<sup>4</sup> sec) also known as "dark current" [12, 35, 36]. Kinetically, electron injection happens in 100 s of pico seconds and vary with sensitizer, with usual ms to μs range of recombination with electrolyte and oxidized dye, respectively. [24] Tuning of electrochemical properties of sensitizers (Ru (II), organic, and porphyrin) with optimized geometry offering higher light absorption and minimum aggregation and redox shuttles (iodide/triiodide, cobalt and copper) has been widely studied for DSCs with the aim of minimum overpotential loss, broad absorption and higher PCE [37–40]. Interested readers are encouraged to consult the more detailed reviews on design principles of sensitizers and redox shuttles for DSCs [21, 33, 40–43].

Figure 2. Energy level diagram, overpotential requirements and typical time constants.

Jsc can be increased by molecular engineering of the sensitizer, with ideal ground and excited state energetics, high molar absorptivity, and aggregation less anchoring on TiO2 [12]. Voc is the energy difference between the TiO2 fermi level and the redox potential of the mediator and depends on the electron density in TiO2. Higher Voc can be achieved by minimization of the dark current, increase in electron injection, negative (upward) shift in the energy of the conduction band, positive (downward) shift in the energy of the redox shuttle and series connection of devices [35, 44]. Both Voc and FF are hugely related to recombination reactions (dark current) and can be substantially influenced by modification of TiO2 in the presence of additives vide infra. Along with J-V and IPCE measurements, electrochemical impedance spectroscopy (EIS) and small modulation photovoltage transient measurements have been widely employed to fully characterize the devices. Readers are kindly referred to the previously published reviews to learn about these powerful techniques to characterize interface and charge transfer properties [45–48].

In terms of achieving higher efficiency by modification of TiO2 the main objective is to minimize the recombination losses by blocking the TiO2 surface, increase in electron injection by manipulation of TiO2 CB, aid in better orientation, structure and geometry of the dyes on TiO2 and suppression of dye aggregation and stacking. This enhancement of DSCs devoid of dye and electrolyte designing and arduous manipulations of their molecular structures can be achieved by (1) TiO2 paste additions (2) dye solution co-adsorbing additives (3) post staining surface treatment additives and (4) electrolyte additives (Figure 3). This chapter is now further divided into sections as shown in Figure 3. to integrate and analyze the most successful strategies, their role in enhancing DSCs performance, similarities among different approaches and discussion of proposed mechanisms.

It is crucial to highlight that just like dyes for DSCs most of the additives will require an anchoring group for immobilization on TiO2, other than plasmonic nanoparticles and composite of TiO2 NPs (Section 1, which become the intrinsic part of TiO2 NPs after sintering also termed as "hard modification"). The most widely used anchoring groups are same as dyes, e.g., carboxylic acid, phosphonic/phosphinic acid, pyridine, and most recently siloxanes. Though multitude of anchoring modes such as covalent attachment, hydrogen bonding, electrostatic interaction, van der Waals interaction and physical entrapment has been proposed [49]. It is important to notice that these anchoring systems should also facilitate the electron transfer. Additionally, due to structural complexity of the interface environment several models are used to elucidate the anchoring. For physical characterization of interface Fourier

Figure 3. Summary layout for TiO2 modification approaches.

J-V curve and incident photon to current conversion efficiency (IPCE) or EQE (external quantum efficiency). IPCE depicts the photocurrent density response of the device at monochro-

J-V curve is measured by scanning the voltage across the device from 0 to higher voltage (forward bias) or higher to zero voltage (reverse bias) either with a source meter or a potentiostat. Power conversion efficiency (PCE) of the cells, the main performance metric is then calculated according to the equation, PCE = (JscVoc FF)/I0, where FF is the fill factor which is simply the measure of the squareness of the J-V curve, and depicts the electrochemical losses in the device with value between 0 and 1 (normally between 0.65 and 0.75 for DSCs) [34]. I0 is

performing DSC should behave an ideal diode with infinite shunt resistance and minimum series resistance which will lead to higher FF and PCE ultimately [13]. Briefly, Jsc α LHE.φinj.φreg.ηcoll, where LHE is light harvesting efficiency of sensitizer on given thickness of TiO2, φinj and φreg are the quantum yield of electron injection and dye regeneration and ηcoll is

In Figure 2, thermodynamic requirements for electron injection (up to 300 mV) and dye regeneration (100–500 mV) overpotential has been shown. Dotted lines highlight the unwanted recombination reactions with redox shuttle (10<sup>2</sup> sec) and oxidized sensitizer (10<sup>4</sup> sec) also known as "dark current" [12, 35, 36]. Kinetically, electron injection happens in 100 s of pico seconds and vary with sensitizer, with usual ms to μs range of recombination with electrolyte and oxidized dye, respectively. [24] Tuning of electrochemical properties of sensitizers (Ru (II), organic, and porphyrin) with optimized geometry offering higher light absorption and minimum aggregation and redox shuttles (iodide/triiodide, cobalt and copper) has been widely studied for DSCs with the aim of minimum overpotential loss, broad absorption and higher PCE [37–40]. Interested readers are encouraged to consult the more detailed

reviews on design principles of sensitizers and redox shuttles for DSCs [21, 33, 40–43].

Figure 2. Energy level diagram, overpotential requirements and typical time constants.

). A high

the power input for the incident irradiation which is normally 1 sun (100 mW/cm2

matic wavelength and depends on the same parameters as Jsc [33].

the charge collection efficiency [12].

390 Titanium Dioxide - Material for a Sustainable Environment

transform infrared spectroscopy (FT-IR) and photoelectron spectroscopy (PES) are mainly employed [50]. However for anchoring on TiO2 for the well-known carboxylic acid, (similarly anchoring phosphonic/phosphinic acid, siloxane, etc.) (Figure 4) covalent interaction can only offer the strongest coupling for stable anchoring with ester type bonds or metal complexation for pyridine additives [51]. For an in depth analysis of anchoring mode and surface adsorption for different anchoring groups, readers are kindly referred to reviews published previously [49, 51].

and gold nanoparticles (Section 1). Additionally, different types of TiO2 geometries such as nanotubes (NTs) and hollow spheres can be mixed with nanoparticles (NPs) to achieve higher loading (Section 1.2 and 1.3). An important class of additives to modify and enhance the interface properties of TiO2 is the addition of electronically insulating molecules with anchoring groups (Section 2). Quite recently, simple surface treatment by chemical bath method on dye anchored TiO2 (stained) films has been explored with impressive enhancements, such strategies are discussed in Section 3. Historically, most widely studied approach in regard to enhancement of DSCs and modification of TiO2 is the introduction of new electrolyte additives including solvent, surface and recombination blocking pyridines and different anchoring groups, which are discussed in Section 4. At the end an overall perspective on the state of

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Integration of subwavelength plasmonic nanostructures and morphologically varied mesoporous films of TiO2 have been widely explored for enhancing DSCs performance. Hard modification of TiO2, such as sintering step is required at high temperature (500 0C) to activate the functionality

Plasmonic enhancement or light entrapment in DSCs by means of the plasmonic resonance of metal nanostructures has been a topic of intense research in the last decade. Since the first report in 2000 of metal nanostructured mediated enhancements in DSCs, many successful studies has been published outlining the role of size, shape and composition of metal nanoparticles on DSCs performance and working mechanisms [59–61]. Metal nanostructures capable of surface plasmon such as Au and Ag has been systematically introduced with TiO2 NPs. Such as these nanoparticles can be designed and integrated in TiO2 NPs in a way to offer

In plasmonic materials the coupling of incident photons to conduction band electrons upon excitation give rise to collective oscillations of electrons defined as localized surface plasmon resonances (LSPR) [63, 64]. By the engineering of plasmonic nanostructure's geometry, dimensions and composition LSPR's radiative (hot electron transfer, plasmon resonant energy transfer) and non-radiative (far-field scattering >50 nm size, near field coupling 3–50 nm size) processes can be tailored (Figure 5) [55, 56]. Out of four processes summarized in Figure 5 far-field scattering and near field coupling are easily observed for DSCs such as by improvement in IPCE, whereas role of hot electron transfer and PRET to improve DSCs is thus far poorly explored [62]. Detailed discussion of each process and its implications for DSCs are

In radiative effects metal nanostructure acts as a secondary light source and in non-radiative effects absorbed energy is subsequently transferred to neighboring semiconductor NPs (Figure 5).

DSCs and the role of learnings to other fields is analyzed.

2. TiO2 paste modifications

2.1. Plasmonic enhancement of DSCs

light entrapment from visible to NIR region [62].

beyond the scope of this chapter [62, 65].

are discussed below.

For efficient light harvesting different kinds of TiO2 pastes (active layer for dye anchoring with scattering or reflective layer on top of it) are used for achieving specific features such as iodide/ triiodide systems mainly employ 18–20 nm size NPs based formulations, whereas for larger size redox shuttles such as cobalt and copper based systems 28–31 nm size NPs are employed [52, 53]. This selectivity comes from the mass-transport related limitations of outer sphere based redox shuttles (cobalt and copper) which is mitigated by the larger pore size of bigger NP size based TiO2 films [54]. On top of active layer, 4–5 μm thick scattering or reflective layer is printing with NPs size of >100 nm, to back scatter light into the cell.

## 1.1. Scope

Though synthesis and preparation of TiO2 paste for film formation has historical importance, however, at this stage more than 95% of the studies employ a commercially available TiO2 paste which is developed after years of research and employ patented methods [53, 55]. However, design of morphologically new structures, and development of efficient synthesis routes for anatase TiO2 is an active area to achieve higher loading, better charge transport, and minimum recombinations losses [18]. For this chapter please be referred to commercially available TiO2 (transparent 18–20 nm from Dyesol or Solaronix, or 30–31 from Dyesol or Dyenamo nm particle size for active layer and > 100 nm size for scattering layer from Dyesol, Solaronix and Dyenamo) for improvement [56–58]. With ready to use TiO2 paste in hand, its light absorption properties can be enhanced by simple mixing in systematic way with silver

Figure 4. Depiction of anchoring mode of general additives for TiO2 modifications.

and gold nanoparticles (Section 1). Additionally, different types of TiO2 geometries such as nanotubes (NTs) and hollow spheres can be mixed with nanoparticles (NPs) to achieve higher loading (Section 1.2 and 1.3). An important class of additives to modify and enhance the interface properties of TiO2 is the addition of electronically insulating molecules with anchoring groups (Section 2). Quite recently, simple surface treatment by chemical bath method on dye anchored TiO2 (stained) films has been explored with impressive enhancements, such strategies are discussed in Section 3. Historically, most widely studied approach in regard to enhancement of DSCs and modification of TiO2 is the introduction of new electrolyte additives including solvent, surface and recombination blocking pyridines and different anchoring groups, which are discussed in Section 4. At the end an overall perspective on the state of DSCs and the role of learnings to other fields is analyzed.
