# Laser Sintering of TiO\({}_{2}\) Films for Flexible Dye-Sensitized Solar Cells
## 3 Experimental Results and Discussion
### IR Laser Sintering
Figure 3: Microscope photographs of IR laser sintered TiO\({}_{2}\) films with the laser power density of (**a**) 0, (**b**) 4.5 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), (**c**) 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), (**d**) 6.53 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), SEM images of (**e**) original TiO\({}_{2}\) film, (**f**) IR laser sintered TiO\({}_{2}\) films with the laser power density of 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\). For a continuous-wave laser, when the power density and scanning interval are fixed, the laser energy radiating to a unit area depends on the scanning speed, which can be expressed as the following equations:  
\[\Phi=\frac{Pt}{d_{0}Vt}=\frac{P}{d_{0}V} \tag{1}\]  
where \(t\) is the time (s) of laser scanning, \(P\) is the laser power, \(d_{0}\) is the spot diameter (\(\upmu\)m), and \(V\) is the scanning speed. It can be seen that \(\Phi\) is inversely proportional to the scanning speed. With a power density of 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), a spot diameter of 0.4 mm, and a scanning pitch of 0.25 mm, Figure 4b shows the effect of scanning speed on the IPCE of laser sintered flexible DSC. When the scanning speed is less than 20 mm/s, the energy accumulated by the low scanning speed is too high, so the IPCE of the laser sintered flexible DSC is even less than that of the original sample (4.5%). As the scanning speed increases, the IPCE increases. However, when the scanning speed is higher than 40 mm/s, the IPCE constantly decreases, and with a scanning speed of 60 mm/s, the IPCE of laser sintered flexible DSC is equal to that of the original sample. So in this paper, the optimal parameter combination for IR laser sintering is a power density of 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\) and a scanning speed of 30 mm/s. As shown in Equation 1, by increasing the power density and scanning speed in equal proportions, \(\Phi\) can be kept constant. Therefore, the laser sintering efficiency can be improved by increasing the scanning speed with a higher power density. Besides, with an optimal \(\Phi\), splitting the laser beam or shaping the point laser source into a line source can further improve the sintering efficiency. To eliminate the accidental factors in the fabrication process of flexible DSCs, hundreds of flexible DSCs are prepared based on the laser sintered and unsintered TiO\({}_{2}\) films. The result (Figure 5a) shows that the IPCE of the IR treated DSCs is mainly concentrated in the range of 5.2%-5.7%, which is higher than that of the original sample (3.9-4.5%). As shown in Table 1, after the IR laser sintering, the maximal IPCE of the flexible DSCs is improved from 4.5% to 5.7%, the increase rate reaches 23.9%. Meanwhile, the short circuit current is increased from 9.2 mA/cm\({}^{2}\) to 10.4 mA/cm\({}^{2}\) (Figure 5b), and the fill factor is also significantly improved from 0.71 to 0.77. Those results reveal that the IR laser sintering plays a positive role in improving the performance of the flexible DSCs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Samples** & _Voc (mV)_ & _Jsc (mA/cm\({}^{2}\))_ & _Fill Factor_ & _IPCE_ \\ \hline Original & 720 & 9.2 & 0.71 & 4.6\% \\ Laser sintered & 716 & 10.4 & 0.77 & 5.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance of the flexible dye-sensitized solar cells (DSCs) based on the laser sintered and unsintered TiO\({}_{2}\) films. IPCE: incident photon-to-electron conversion efficiency. Figure 4: Effect of (**a**) laser power density and (**b**) scanning speed on the IPCE of the flexible DSCs based on laser sintered TiO\({}_{2}\) films. The photoelectric performance of DSCs depends mainly on two aspects, one is the absorption rate of the incident light, and the other is the collection efficiency of electrons. To improve the collection efficiency, the electrons which do not go through the external circuit and directly combine with the electrolyte should be inhibited as far as possible.