**3.1 Basic principles of DSSCs**

**3. Dye-sensitized solar cells**

*Ion Beam Techniques and Applications*

**Figure 3.**

*[22–25].*

**42**

The basic idea of dye-sensitization technique was given by Vogel and Berlin in 1873 [7] and the well understood sensitization was perfect in early 1960s and 1970s, with the pervasive photoelectrochemical examination of dye-sensitized singlecrystal electrodes. However, as compared to silicon based photovoltaic devices, the performance of these early DSSCs was poor (efficiency <1%). The major obstacle was the low light harvesting efficiencies of these single-crystal cells by a dye monolayer adsorbed onto a planar TiO2 surface. Some improvements in efficiency were achieved by coating a thick layer of dye onto the planar TiO2 surface; the efficiency was still limited to <2% due to less proficiency in charge-collection from the faraway dye molecules [14]. A breakthrough in DSSC performance was achieved in early 1990s by the research group of Grätzel who creatively demonstrated that a practical DSSC which consists of ruthenium sensitizer dye-adsorbed mesoporous titanium dioxide (thin 10 μm) layer on fluorine doped tin oxide glass substrate serving as a photoanode (PA), a platinum-coated counter electrode (CE) and a redox couple liquid electrolyte introduced in between the two electrodes [3]. The main parts of DSSC are shown in **Figure 3** [22–25]. Considerable developments in DSSC efficiency have been reached since then, and the record A.M. 1.5 conversion efficiency for a DSSC presently touched at 14.3%, making it comparable to the conventional *p-n* junction silicon solar cells in terms of efficiency and costeffectiveness [7]. Despite intense study of DSSCs over the past two decades, the

*(a) Semiconductor (photoanode, PA), (b) sensitizer dye, (c) redox couple/electrolyte, and (d) counter electrode*

**Figure 4** depicts typical structure of a DSSC and its operational principle [22–25]. In DSSC the photo-excitation of electrons from lowest unoccupied molecular orbital (LUMO) of dye molecules takes place with the external light irradiance, by choosing sufficient energy, electron reaches highest occupied molecular orbital

### **Figure 4.**

*(a) Schematic of a typical DSSC. (b) The basic sequence of events in a DSSC. (c) Electron transfer in dye sensitized solar cell. (d) Electrical losses in DSSCs [22–25].*

(HOMO) of dye molecule, which is further injected into the conduction band (CB) of the TiO2 and fast transferred to the external circuit through the CE, generating an electrical current. The inserted electrolyte here plays a vital role in donating an electron back to the unusual state of dye molecule and serves as a redox couple. The CE proceeds charge from the external circuit back to the cycling circuit in the cell [6–8]. Nevertheless, the device performance depends on the photoelectrode material as well. The voltage produced under irradiance shows resemblance to the potential difference between the electrolyte redox couple (I�/I3�) and the quasi-Fermi level of the electron in TiO2. The net outcome is the conversion from light to electricity without any permanent chemical transformation. DSSC is thus a regenerative type photo-electrochemical cell [9, 10]. The sequence of the charge transfer processes responsible for the operation of a dye sensitized solar cell is given in the following Eqs. (1)–(7) [26]: The charge transfer processes and the unwanted loss mechanisms presented below are well discussed in details in Ref. [8].

$$\mathbb{S} + h\nu \to \mathbb{S}^\* \tag{1}$$

boundaries. The morphology and structure of the ion implanted TiO2 photoanode depends on the absorption of dye, electron injection, transportation and the

*Modification of Physical and Chemical Properties of Titanium Dioxide (TiO2) by Ion…*

The basic information of nucleation and growth mechanisms of TiO2 thin film is essential to understand microstructure and properties for DSSC [4, 16]. The TiO2 nanoparticle with anatase and rutile phases has been synthesized by ion implantation technique, where the degree of surface modification is significantly influenced by various implantation parameters, e.g., current density of ion beam, substrate temperature, and energy of an implant ions [17], etc. In ion implantation technique, ion dose (denoted by F0) is the key factor to determine the total implant ions. TiO2 implantation can be categorized as low-dose and high-dose under conditional surface modification. Low-dose irradiation, e.g., F0 <sup>≤</sup> 5.0 <sup>10</sup><sup>14</sup> ion/cm<sup>2</sup> (energy of the implant is transported to the matrix by electron shell excitation and nuclear collisions [27]), causes the radiation-induced defects, which may in turn become reversible or irreversible modification of the material structure. Several types of defects are present in TiO2 crystal such as point defects, line defects, staking faults defects, local crystallization, etc. The range of high-dose implantation can be clas-

.

sified into two sub-ranges, e.g., 10<sup>15</sup> <sup>≤</sup> F0 <sup>≤</sup> <sup>10</sup><sup>17</sup> ion/cm<sup>2</sup>

DSSCs with ion implantation [15–18].

**Figure 5.**

**45**

**Figure 5** depicts the cross-section view of nucleation and growth of nanoparticles. Once the concentration of doped ions over does the solubility limit of TiO2 atoms in matrices and the system relaxes [19]. The threshold dose values which nucleate the TiO2 nanoparticles are dependent on the sort of the implant and dielectric matrix. Therefore, the ion implantation method delivers a suitable way to alter physical and chemical properties of materials. Still there is rare information or literature available on improvement of photovoltaic properties for

*Basic physical stages of TiO2 nanoparticle synthesis by ion implantation in dependence on ion dose [27].*

recombination.

**4. Ion implantation**

*DOI: http://dx.doi.org/10.5772/intechopen.83566*

$$\mathcal{S}^\* \to \mathcal{S}^+ + \mathfrak{e}^-\_{\text{CB}} \tag{2}$$

$$\text{S}^{+} + \frac{\text{3}}{\text{2}}\text{I}^{-} \rightarrow \frac{\text{1}}{\text{2}}\text{I}\_{3}^{-} + \text{S} \tag{3}$$

$$\rm I\_3^- + 2e\_{\rm CE}^- \to 3I^- \tag{4}$$

In addition to the forward electron transfer and ionic transport processes, several electron loss pathways could be analysed as shown in **Figure 4(d)**, which gives the electron transfer losses occurred at the dye-sensitized heterojunction. The earlier studies clearly indicate that the main losses in DSSC were due to the potential drop in the dye regeneration and recombination losses between electrons in the TiO2 and acceptor species in the electrolyte [26].

• Fall off the electron from dye excited state to the ground state

$$\mathcal{S}^\* \to \mathcal{S} \tag{5}$$

• Recombination of the vaccinated electron with the dye cations

$$\mathcal{S}^{+} + \mathfrak{e}\_{\mathcal{C}\mathcal{B}}^{-} \to \mathcal{S} \tag{6}$$

• Recombination of the vaccinated electron with the (3*I* �) redox mediator

$$I\_3^- + 2\mathfrak{e}\_{\text{CB}}^- \to \mathfrak{H}^- \tag{7}$$


Hence, the challenge is to effectively regenerate the oxidized dye molecules with efficient charge transport through the ion implanted TiO2 matrix followed by the decrease in recombination which can happen at the TiO2/dye/electrolyte

boundaries. The morphology and structure of the ion implanted TiO2 photoanode depends on the absorption of dye, electron injection, transportation and the recombination.
