# Laser Sintering of TiO\({}_{2}\) Films for Flexible Dye-Sensitized Solar Cells
## 3 Experimental Results and Discussion
### Mechanism of the Laser Sintering  
For the TiO\({}_{2}\) nanoparticles with 80% anatase and 20% rutile phase, since the band gaps of anatase and rutile phases are 3.2 eV and 3.0 eV [15; 34], respectively, and the photon energy of 355 nm UV laser is 3.49 eV, the TiO\({}_{2}\) films can achieve a high absorption at the wavelength of 355 nm. As shown in Figure 6a, when the UV laser irradiates the TiO\({}_{2}\) film, the top layer of the TiO\({}_{2}\) film is sintered first, as the UV scattering of TiO\({}_{2}\) nanoparticles is weak, the heating applied to the lower layer nanoparticles only depends on the heat conduction from the top layer. According to the thermal diffusivity of nanostructured TiO\({}_{2}\) films (0.3-0.5 cm\({}^{2}\)/s), when the UV laser sintering is conducted with the laser fluence of 0.4 J/cm\({}^{2}\), during the duration of a single pulse (40 ns), the thickness of TiO\({}_{2}\) film heated to over 450\({}^{\circ}\)C by the heat conduction is only 54.6-70.6 nm [32]. If the laser influence further increases, the top layer TiO\({}_{2}\) nanoparticles are superheated and lose porosity. Therefore, the 355 nm UV laser cannot achieve uniform sintering for the 13 \(\upmu\)m thick TiO\({}_{2}\) nanoparticle film.  
The photon energy of IR laser (only 1.17 eV) is not only smaller than that of a 355 nm UV laser (3.49 eV), but also smaller than the band gaps of anatase phase (3.2 eV) and rutile phase (3.0 eV), which leads to the sintering mechanism of a IR laser being vastly different to that of a UV laser. TiO\({}_{2}\) is a weak n-type metal oxide semiconductor [41]. In general, there are some oxygen vacancy defects in the crystal lattices, which lead to the TiO\({}_{2}\) existing in the form of TiO\({}_{2\times}\), such as TiO\({}_{1.98}\) or TiO\({}_{1.95}\), \(x\) represents the extent of oxygen vacancy defects. The oxygen vacancy locates between the valence band and conduction band, which enables it to capture electrons with an energy of 0.76 eV. Therefore, the TiO\({}_{2}\) nanoparticles have a certain degree of absorption in the IR region, but the absorption is much smaller than that in the UV region.  
When the TiO\({}_{2}\) nanoparticle film is sintered by the IR laser, as the laser absorptivity is low, and the size of nanoparticles is much smaller than the laser wavelength, only a fraction of laser energy is absorbed by the top-layer TiO\({}_{2}\) nanoparticles, most of the energy is scattered (Figure 6b). Meanwhile, the interspaces between the TiO\({}_{2}\) nanoparticles provide a channel for the propagation of scattered  
Figure 6: Schematic diagrams of the laser sintering with the wavelength of (**a**) 355 nm and (**b**) 1064 nm.  
light, so the IR laser can penetrate through the whole TiO\({}_{2}\) nanoparticle film, which results in the uniform sintering. Moreover, the continuous laser scattering inside the TiO\({}_{2}\) nanoparticle film leads to the multiple absorptions of laser energy by the TiO\({}_{2}\) nanoparticles. During the process of IR laser sintering, the entire nanoparticle layer reaches the phase transition temperature (about 550 \({}^{\circ}\)C) in a short time, and part of the nanoparticles are transformed from the anatase phase to the rutile phase, which leads to the enhancement of natural light scattering. Meanwhile, the absorption of incident light is also improved. During the process of phase transformation, \"neck-type\" connections are formed between the nanoparticles in the thickness direction of TiO\({}_{2}\) films, the enhanced electrification connection results in the improvement of electron collection efficiency, and the performance of the flexible DSC is promoted.