**1. Introduction**

The search for grean sources of enegy is considered one of the priorities in today's societies and occupies many policy makers' agendas. Excitonic Solar cell such as organic dye or perovskite -sensitized solar cells (DSSC or PSC) is showing up as a promising solar harvesting technology that has bright future. Unlike conventional silicon p-n junction solar cell, an excitonic solar cell (ESC) can be modeled as a unipolar-junction cell and made from low-cost materials that do not need to be highly purified but still work well using simple manufacturing processes [1]. For example, similar to the photosynthetic process in plants where chlorophyll absorbs photons but does not participate in charge transfer, the photoreceptor and charge carrier are implemented by different components in solar cell. This separation of functions leads to lower purity demands on raw materials and consequently makes exitonic solar cell a low-cost alternative. Nevertheless, to compete with the future PV market, ESC should foucus on the "Golden Triangle" issues, i.e., increasing light-to-electric energy conversion efficiency, enhancing long-term stability, and decreasing device cost [2].

different disciplines: (1) chemists or material scientists worked for developing photosensitizer and charge transport materials; (2) physicists to eluciate photovoltaic properties and working mechanism; and (3) engineers to develop process and device structure for module level production. However, the simultaneous development of all components such as new photosensitier or new classed strucutral perovskite materials with intrinsic stability and beneficial optoelectronic properties, solid state HTM and photoanodes, combined with further investigation of transport

This report the following studies: (i) a brief introduction of the working principle, (ii) the detailed study of the each component materials, mainly including TiO2

The principle of operation of a DSSC is well documented in the literature [7]. The simplified principle of the thin layer DSSC is shown in **Figure 1**. A lightharvesting Ru complex potosensitizers adsorbed on the surface of a porous nanocrystalline film composed of a wide bandgap metal oxide such as TiO2, ZnO or SnO2 absorb incident photon flux. The photosensitizers are exited from the ground state (D) to the excited state (D\*) owing to the metal to ligand charge transfer (MLCT) transition (Eq. (1)). The exited electrons are injected into the conduction band of the TiO2 electrode, resulting in the oxidation of the photosensitizer (Eq. (2)).

D adsorbed on TiO ð <sup>2</sup>Þ þ *hv* ! D ∗ ð Þ adsorbed on TiO2 (1)

D ∗ ðadsorbed on TiO2Þ ! D<sup>þ</sup> ðadsorbed on TiO2Þ þ e� ð Þ injected (2)

) accepts electrons from the I� ion redox mediator, regenerationg the ground

D<sup>þ</sup> ðadsorbed on TiO2Þ þ e� ð Þ! TiO2 D adsorbed on TiO ð Þ<sup>2</sup> (4)

<sup>D</sup><sup>þ</sup> <sup>ð</sup>adsorbed on TiO2Þ þ <sup>3</sup>*=*2 I� ! D adsorbed on TiO <sup>ð</sup> <sup>2</sup>Þ þ ½ I�<sup>3</sup> (3)

The primary energy conversion process in DSSCs is a photoinduced charge

� (Eq. (3)). The injected eletrons

�, diffuses toward the counter electrode and is

�<sup>3</sup> <sup>þ</sup> 2e� ð Þ! TiO2 3I� ð Þ anode (5)

Injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO) and consequently reach the counter electrode through the external load and wiring. The oxidized photosensi-

photoanode under the category of 0D and 3D structures, strategies for cosensitization with porphyrin and organic photosensitizers, and carbon catalytic material via controlled fabrication protocols and fundamental understanding of the working principles of electrochemical photovoltaic cell has been gained by means of electrical and optical modeling and advanced characterization techniques and (iii) new desgined stratages such as the optimization of photon confinement (iv) future prospects and survival stratagies for sensitizer assisted solar cell (especially, DSSC).

dynamics, will lead to ESCs with efficiencies exceeding 30%.

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

**2. Fundamentals of dye-sensitized solar cells**

state (D), and I� is oxidized to the oxidized state, I3

I

separation at the metal oxide/dye/electrolyte interface.

The oxidized redox mediator, I3

rereduced to I� ions.

may recombine either with oxidized sensitier at the TiO2 (Eq. (4)).

**2.1 Operation principle**

tizer (D+

**187**

Since the last third decades, scientists have devoted a great deal of effort on DSSCs' four important components: photosensitizer, photoanodes, electrolytes, and counter electrodes; some significant processes have been achieved. The sensitization of semiconductors using dyes dates back to 19th century. It showed that the photosensitivity can be extended to longer wavelengths by adding a dye to silver halide emulsions [3, 4]. Grätzel has then extended the concept to the DSSC by adsorption of dye molecules on the nanocrystalline TiO2 electrodes [5]. This breakthrough was due to large surface area of the mesoporous TiO2 that allowed anchoring significantly high amount of dye molecules (0.13 mmol/cm<sup>2</sup> ) onto it; thereby increasing the absorption cross-section. Since the first successful demonstration of DSSC over three decades ago, a wealth of DSSC components have been investigated for enhancing energy conversion efficiency. State of the art DSSCs achieve more than 11% energy efficiency allied to good performance under any atmospheric condition and low irradiance. Moreover, the possibilities of transparency and multicolored cells are truly a competitive advantages of DSSCs. However, for stimulating industrial market, performance of DSSCs must be improved close to the efficiency values of the conventioanl silicon solar cells.

Since 2012, organic–inorganic perovskites solar have attracted considerable research attention for photovoltaic device due to its superb light-harvesting characteristics and electrical properties. Within relatively a short time, this organic– inorganic lead halide perovskite has yielded photovoltaic efficiencies of 25.5%, being the highest-performing solution-processed solar cell on record and displacing technologies such as DSSCs and organic photovoltaics (OPVs) [6]. In addition, the use of perovskite materials as light absorbers and hole transporting mateiral (HTM) in solid-state sensitized solar cells has opened up a new direction for the highefficiency thin-film solar cells. Despite the best candidate at satisfying the need for high efficiencies, concerns surrounding the long-term stability in an ambient atmosphere as well as the water-soluble toxic lead components in the archetypal perovskite, APbX3 (A = methylammonium, formamidium, cesium, X = I, Br, Cl), have the potential risk for the environment issue. After around 10 years of intense PSC researches, several physical chemistry still have questions for the basic mechanism. In this aspect, the physical and chemical operation of DSSC has becom more clear and a good example of a ESC system where the function of the overall device is better than prodicted from the sum of the properties of its components. Therefore, this book address imporant breakthough's our laboratory research for DSSC with taking optimized processing recipe as the standard cell fabrication procedure.

In order to meet this requiremnt, it will be necessary to alter at least two of the three major components simultaneously. In the majority of cases, however, researchers who come from the different background could engage on certain aspects of the components to improve the photovoltaic performances from

### *A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

such as organic dye or perovskite -sensitized solar cells (DSSC or PSC) is showing up as a promising solar harvesting technology that has bright future. Unlike conventional silicon p-n junction solar cell, an excitonic solar cell (ESC) can be modeled as a unipolar-junction cell and made from low-cost materials that do not need to be highly purified but still work well using simple manufacturing processes [1]. For example, similar to the photosynthetic process in plants where chlorophyll absorbs photons but does not participate in charge transfer, the photoreceptor and charge carrier are implemented by different components in solar cell. This separation of functions leads to lower purity demands on raw materials and consequently makes exitonic solar cell a low-cost alternative. Nevertheless, to compete with the future PV market, ESC should foucus on the "Golden Triangle" issues, i.e., increasing light-to-electric energy conversion efficiency, enhancing long-term stability, and

Since the last third decades, scientists have devoted a great deal of effort on DSSCs' four important components: photosensitizer, photoanodes, electrolytes, and counter electrodes; some significant processes have been achieved. The sensitization of semiconductors using dyes dates back to 19th century. It showed that the photosensitivity can be extended to longer wavelengths by adding a dye to silver halide emulsions [3, 4]. Grätzel has then extended the concept to the DSSC by adsorption of dye molecules on the nanocrystalline TiO2 electrodes [5]. This breakthrough was due to large surface area of the mesoporous TiO2 that allowed anchor-

increasing the absorption cross-section. Since the first successful demonstration of DSSC over three decades ago, a wealth of DSSC components have been investigated for enhancing energy conversion efficiency. State of the art DSSCs achieve more than 11% energy efficiency allied to good performance under any atmospheric condition and low irradiance. Moreover, the possibilities of transparency and multicolored cells are truly a competitive advantages of DSSCs. However, for stimulating industrial market, performance of DSSCs must be improved close to the efficiency

Since 2012, organic–inorganic perovskites solar have attracted considerable research attention for photovoltaic device due to its superb light-harvesting characteristics and electrical properties. Within relatively a short time, this organic– inorganic lead halide perovskite has yielded photovoltaic efficiencies of 25.5%, being the highest-performing solution-processed solar cell on record and displacing technologies such as DSSCs and organic photovoltaics (OPVs) [6]. In addition, the use of perovskite materials as light absorbers and hole transporting mateiral (HTM) in solid-state sensitized solar cells has opened up a new direction for the highefficiency thin-film solar cells. Despite the best candidate at satisfying the need for high efficiencies, concerns surrounding the long-term stability in an ambient atmosphere as well as the water-soluble toxic lead components in the archetypal perovskite, APbX3 (A = methylammonium, formamidium, cesium, X = I, Br, Cl), have the potential risk for the environment issue. After around 10 years of intense PSC researches, several physical chemistry still have questions for the basic mechanism. In this aspect, the physical and chemical operation of DSSC has becom more clear and a good example of a ESC system where the function of the overall device is better than prodicted from the sum of the properties of its components. Therefore, this book address imporant breakthough's our laboratory research for DSSC with taking optimized processing recipe as the standard cell fabrication procedure.

In order to meet this requiremnt, it will be necessary to alter at least two of the

three major components simultaneously. In the majority of cases, however, researchers who come from the different background could engage on certain aspects of the components to improve the photovoltaic performances from

) onto it; thereby

ing significantly high amount of dye molecules (0.13 mmol/cm<sup>2</sup>

values of the conventioanl silicon solar cells.

**186**

decreasing device cost [2].

*Solar Cells - Theory, Materials and Recent Advances*

different disciplines: (1) chemists or material scientists worked for developing photosensitizer and charge transport materials; (2) physicists to eluciate photovoltaic properties and working mechanism; and (3) engineers to develop process and device structure for module level production. However, the simultaneous development of all components such as new photosensitier or new classed strucutral perovskite materials with intrinsic stability and beneficial optoelectronic properties, solid state HTM and photoanodes, combined with further investigation of transport dynamics, will lead to ESCs with efficiencies exceeding 30%.

This report the following studies: (i) a brief introduction of the working principle, (ii) the detailed study of the each component materials, mainly including TiO2 photoanode under the category of 0D and 3D structures, strategies for cosensitization with porphyrin and organic photosensitizers, and carbon catalytic material via controlled fabrication protocols and fundamental understanding of the working principles of electrochemical photovoltaic cell has been gained by means of electrical and optical modeling and advanced characterization techniques and (iii) new desgined stratages such as the optimization of photon confinement (iv) future prospects and survival stratagies for sensitizer assisted solar cell (especially, DSSC).
