*3.3.2 Study for the optimal TiO2 film thickness*

To find the optimized condition for hydrothermal treated TiO2 film, the cell efficiency as a function of the TiO2 film thicknesses is studied. In **Figure 6** (top), I give plots of *V*oc, *J*sc, *FF*, and *η* as functions of film thicknesses. The efficiency per NP increases almost linear with decreasing film thickness until about 2 μm. From these measurements, the optimal TiO2 film thickness is around 11.5 μm with our cell architect and fabrication procedures. It should also be noted that the highest open circuit voltage and fill factor are achieved at the thinnest active layer thickness, while the short circuit current density increases with increasing film thickness (for thickness below 15 μm). From this thickness study, we then varied other cell parameters to optimize the overall cell efficiency. In **Figure 6** (bottom) we plot cell efficiency times the number TiO2 NP as a function of NP film thickness. it is shown that the efficiency per TiO2 NP increases dramatically as the active NP layer decreases. This shows the large inherent loss of charges in these DSSCs.

In **Figure 7**, AC impedance measurements with best-fit model curves of the cell is used for analyzing function on the different TiO2 NP film thicknesses. Form the Bode phase plots, the negative shift of the frequencies of the main peaks with an

All cases of samples were equal in the total cell thicknesses (*L*). Therefore, there is a tradeoff relation between cell gaps of dye coated TiO2 film and electrolyte, for example, for the thicker thin films of TiO2 film, the electrolyte spacing is smaller. The resulting data from our model shows specific characteristics worth noting; (1) In the case of ultra-thin TiO2 film, two distinctive circles can be found and the impedance **Z3** is dominated rather than that of Z1 and Z2 in the low-frequency region. (2) For thicker TiO2 layers, the contribution from Z2 is more pronounced (3) The estimated the diffusion coefficient in the electrolyte (*D1*) is the highest for the thinnest TiO2 layer [26]. From this model, low *k*eff, high *R*k/*R*w, high *D*eff and high *n*<sup>s</sup> are necessary condition to attain highly efficient DSSC. As seen in **Figure 7 (c,d)**, over 10 μm thick film exhibit the improved electron density (*ns*) and high electron diffusion coefficient (*D*eff), but the recombination time is shorter than the time for diffusion across the TiO2 layer (Rk < <Rw, wd < <wk). As a result, we conclude about 11.5 μm is the optimized film thickness in my system. The detailed

There are numerous efforts to improve cell performance from different modifications techniques such as different semiconductors, dyes, or ionic conductors or on changing its nanostructures [53, 54]. Most of modification works show a tradeoff relation between the short-circuit current and open-circuit voltage. This can be explained the the modification of components affect sensibility of charge-transport and recombination dynamics [55]. Therefore, this has been a big challenge in the field of DSSC design. In this book, effective surface passivation and treatment method provide on how it possible to contribute on the cell performance.

The best-known techiqnue to improve the performance of the solar cells is a post-treatment of the TiO2 film with a solution in TiCl4 is grown onto an extra layer of TiO2 nanoparticles constituting the film. The TiCl4 treatment results in an improvement in photocurrent, normally between 10% and 30%. Depending on the quality of the TiO2 used to make the initial film, the extrema of the improvement can be from <5% to >200% [35, 56]. The largest improvements come when using the poorest quality TiO2 films. **Figure 8(a)** show the SEM images and XRD patterns for TiCl4 treated TiO2 film. When the TiCl4 exposure condition is increased, the

2.4 2.10 31.6 46.2 0.368 19 1.41 42.0 0.932 4.851 72.1 3.26 4.6 0.56 14.2 6.79 0.122 11.7 4.36 7.2 0.919 7.252 73.9 4.93 6.2 0.64 10 4.04 0.084 8.1 5.94 4.5 0.910 8.029 76.8 5.61 10.4 2.79 20 2.76 0.048 7.1 10.5 3.2 0.802 11.61 73.0 6.80 12.4 2.58 10 2.38 0.055 8.3 9.39 0.09 0.808 12.54 72.2 7.32 15.0 3.29 10 2.14 0.093 6.9 6.06 0.56 0.762 12.46 68.6 6.51 17.1 9.05 10 3.13 0.133 3.6 4.36 0.21 0.764 11.19 65.7 5.62

*Parameters for the best fit of the impedance data for the different thickness. Measured in Figure 6.*

*ns* **(10<sup>18</sup> cm<sup>3</sup> )**

*D***<sup>1</sup> (10<sup>6</sup> cm2 s 1 )** *V***OC (V)**

*J***sc (mA/ cm2 )**

*FF* **(%)** **EFF (%)**

physical values are summarized in **Table 1**.

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

**3.4 Interfacial modification**

*3.4.1 TiCl4 treatment*

**Film Thickness (μm)**

**Table 1.**

**199**

*D***eff (10<sup>5</sup> cm2 s 1 )**

*k***eff (Hz)** **Rk/ Rw**

*Con* **(Ωcms<sup>1</sup> )** *R***<sup>d</sup> (Ω)**

#### **Figure 6.**

*Relationship of DSSC device performance (top) and efficiency per TiO2 NP (bottom) depending on film thickness (top). Reprinted from [26, 52].*

#### **Figure 7.**

*AC impedance measurement (a) bode, (b) Nyquist plots, (c) electron density (ns) and (d) the relation between the electron recombination resistance (Rk) and the electron diffusion resistance (Rw) of cells with different TiO2 NP film thicknesses. The solid curves are from a best fit model.*

increase in the film thickness. In a simple equation, since *ωmax* is inversely associated with the life time of electron τ =1/(2πf), the decrease in *ωmax* indicated a reduced rate for the charge-recombination process in DSSC. Hence, electrons with longer *τ* values were prevented from recombining, characterized by a larger charge transfer resistance. In the aspect of *ωmax*, about 6 μm thick film show the longest electron lifetime and relatively small total series resistance, leading to high *Voc* and *FF*. However, in the case of N719 dye, the dye absorption at that thickness is not enough to reach the maximum performance. The more detailed phenomenon can be understood by the further consolidated impedance model we suggested.

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

All cases of samples were equal in the total cell thicknesses (*L*). Therefore, there is a tradeoff relation between cell gaps of dye coated TiO2 film and electrolyte, for example, for the thicker thin films of TiO2 film, the electrolyte spacing is smaller. The resulting data from our model shows specific characteristics worth noting; (1) In the case of ultra-thin TiO2 film, two distinctive circles can be found and the impedance **Z3** is dominated rather than that of Z1 and Z2 in the low-frequency region. (2) For thicker TiO2 layers, the contribution from Z2 is more pronounced (3) The estimated the diffusion coefficient in the electrolyte (*D1*) is the highest for the thinnest TiO2 layer [26]. From this model, low *k*eff, high *R*k/*R*w, high *D*eff and high *n*<sup>s</sup> are necessary condition to attain highly efficient DSSC. As seen in **Figure 7 (c,d)**, over 10 μm thick film exhibit the improved electron density (*ns*) and high electron diffusion coefficient (*D*eff), but the recombination time is shorter than the time for diffusion across the TiO2 layer (Rk < <Rw, wd < <wk). As a result, we conclude about 11.5 μm is the optimized film thickness in my system. The detailed physical values are summarized in **Table 1**.
