*3.4.3 Cell properties for interfacial modification*

The detailed solar cell research studied a series of five cells, each with a layer of 11.6 μm thick TiO2 NP film. Each of the four samples were treated differently prior

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

to dye and electrolyte infiltration. As a first series of cells, TiO2 film prepared for cell (a) commercial and cell (b) hydrothermal treatment is studied. Next, for understanding the influence on interfacial modified TiO2 surface, post-treated cells with cell (c) with TiCl4 treatment and Tare cell (c) with fluorine etching are investigated. **Figure 12(b)** and **(c)** give, respectively, the *J* � *V* and the impedance measurements of each cells. Using the kinetic model discussed in section and extrapolating the parameters for the best fit to each of the measured impedance curves, summarized the results in **Table 2** for the electrical data for all cells. The model calculation and data fitting provide some physical insight into the differences in the transport properties and effects due to plasma etching of the TiO2 NP films of the cells. Cell (b) can be considered as the control for the other three cells. There is no treatment to the TiO2 NP film in this case. In general, *Jsc* can be approximated by the expression;

$$J\_{sc} = q \eta\_{lh} \eta\_{inj} \eta\_{cc} I\_0 \tag{34}$$

where *q* is the elementary charge, *ηlh* is the light harvesting efficiency of a cell, *ηinj* is the charge-injection efficiency of the excited dye into the TiO2, *ηcc* is charge collection efficiency, and *I0* is the incident photon flux [69]. From this equation, it is clearly that the short-circuit current density (*Jsc*) is directly proportional to the value of *ηlh* related to numerous specific anchoring sites on TiO2 surface for dye absorption and light scattering events for optical absorption as an external property. As we mentioned earlier, the *ηcc* is related to the charge transfer kinetics and this value can be estimated by comparing the charge transport and recombination time constants when both values are measured by EIS measurements.

Cell (c) has a 25.5% increase in the *D*eff, leading to about 19.3% increased electron density (*n*s) compared with the cell (b). With increasing electron density, deeper traps become filled, and trapping/detrapping events occur more frequently in shallower traps, leading to faster transport. The increase in the photocurrent density is well explained by the higher electron density. However, the value of *Rk*/*Rw* related to the recombination related value is decreased by about 10%, leading to a decrease Voc. Cell (d) with the plasma etched device shows the resulting values for about 45.2% lower the charge density value (*n*s,), 31.5% higher the interfacial recombination rate (Rk/Rw) and 33.4% increased *D*eff rather than that of the unetched sample (cell (b)). Although the fitted charge transfer properties on etched sample are substantial, the overall performance of cell is infinitesimal because the efficiency just increased by about 8% with etching.

These changes show that the etching has significant effect on the electron recombination and transport properties of the cell, but the overall effect is not pronounced because the cell efficiency only increased by 8% with etching. As suggested in **Figure 12(a)**, the fluorine etching can help to minimize the electron loss between TiO2 surface and reduced iodide, leading to the highest *V*oc and *FF* value (�0.852 V and 75.1%). The decreased electron transport properties are attributed to the less interconnection between NPs compared to the case of cell (b). However, when a film is treated in the much more time (�30 min), the morphology changed dramatically, and the pin-hole of TiO2 film formed. This result can be confirmed by the dye desorption experiments and the weight loss of TiO2 from etching process. While this situation should improve the dye molecule attachments to the NPs and allow further penetration of the molecules into the TiO2 film, as indicated by a large increased in the values of *D*eff in **Table 2**.

Finally, we believe that combination of TiCl4 and fluorine etching posttreatment with the opposite physical properties make it possible to increase both the *V*oc and *J*sc, giving an efficiency higher than that of cell (b). Therefore, cell (e) is

enfeeble the bond in a titanium and oxygen [68]. Therefore, the TiO2 surface comprised of less surface defects may play important role in a improvement of the

*(a) Schematic illustration for TiO2 surface interfacial mechanism (b) JV curve and (c) AC impedance*

*(a) SEM images of top and side views of pre-etch film and CF4-etched TiO2 film. (inserted in simple illustrations of etched film) (b) film thickness, weight and film morphologies images (top) and relative content*

*of oxygen and fluorine in TiO2 NPs (bottom) as a function of etching time. Reprinted from [51].*

*Solar Cells - Theory, Materials and Recent Advances*

The detailed solar cell research studied a series of five cells, each with a layer of 11.6 μm thick TiO2 NP film. Each of the four samples were treated differently prior

open-circuit potential, *V*oc, of the solar cell.

*measurements of cells with different surface treatments.*

**Figure 12.**

**204**

**Figure 11.**

*3.4.3 Cell properties for interfacial modification*

treated with TiCl4 coating with same treatment as cell (c), followed by fluorine etching with same treatment as cell (c), and cell (f) with the TiO2 NP film etched by fluorine first and followed by TiCl4 treatment and then high-temperature. As we expected, cell (f) has the lowest series resistance (i.e., *R*total = *R*<sup>0</sup> + *R*<sup>1</sup> + *R*<sup>2</sup> + *R*3) and therefore the highest efficiency. The contribution to this increase of cell efficiency comes primarily from the 27.7% increase in *J*sc, no loss in the *Voc* and *FF* value. It also notice from our model that the effective electron diffusion coefficient is a significant improved by as much as 59%, the interfacial recombination (*keff*) is decreased by 52.7%, which leads to large *Rk* compared with the cell (b). These changes are quite significant, and as a result, the cell efficiency was increased nearly 26%.
