# Laser Sintering of TiO\({}_{2}\) Films for Flexible Dye-Sensitized Solar Cells
## 3 Experimental Results and Discussion
### IR Laser Sintering  
Since the spectral absorption of TiO\({}_{2}\) films at 1064 nm is much smaller than that at 355 nm, and the incident beam with a wavelength of 1064 nm can transmit through the films (Figure 1d), the whole film can be modified, so the 1064 nm fiber laser may be more suitable for sintering the TiO\({}_{2}\) films. The diameter of the laser spot is about 0.4 mm with a defocusing distance of 8 mm, and the scanning speed is fixed at 30 mm/s. Figure 3 shows the micrographs of the 1064 nm fiber laser sintered TiO\({}_{2}\) films. The main ingredients of the paste for preparing the TiO\({}_{2}\) films are absolute ethanol and TiO\({}_{2}\) nanoparticles, after the doctor-blade coating procedure, as the absolute ethanol volatilizes to air, the TiO\({}_{2}\) films shrink horizontally to form some microcracks (Figure 3a). When the laser power density is 4.5 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), there is no obvious change on the film surface (Figure 3b). As the power density increases to 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), the number of the microcracks is reduced (Figure 3c), but the size of the microcracks increases obviously, and the enlarged microcracks are arranged in parallel with each other. This indicates that the laser sintering promotes the further shrinking of the TiO\({}_{2}\) films. When the power density increases to 6.53 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\), the excessive heat transmitting to the PEN substrate will lead to a thermal deformation (Figure 3d), and one part of the TiO\({}_{2}\) nanoparticles break off from the flexible substrate. The mechanical adhesion strength of the TiO\({}_{2}\) nanoparticle film can be measured by the ISO/DIS 15184 method [15; 33], which determines the film hardness by scratching with a pencil. The non-sintered TiO\({}_{2}\) nanoparticle film is easily scratched with a 1B pencil. Also, the TiO\({}_{2}\) film sintered with a power density of 5.88\(\times\)10\({}^{3}\) W/cm\({}^{2}\) cannot be scratched using a pencil with the hardness up to HB, which indicates that the IR laser sintering with a suitable laser power density can improve the connection between TiO\({}_{2}\) nanoparticles. However, the field emission SEM images show that the IR laser induced morphologic changes are not obvious (Figure 3e,f). Figure 2: Scanning electron microscope (SEM) images of (**a**) original TiO\({}_{2}\) film, ultraviolet (UV) laser sintered TiO\({}_{2}\) films with the laser fluence of (**b**) 0.22 J/cm\({}^{2}\), (**c**) 0.28 J/cm\({}^{2}\), and (**d**) 0.4 J/cm\({}^{2}\), (**e**) section image of laser sintered TiO\({}_{2}\) film with the laser fluence of 0.4 J/cm\({}^{2}\). As the interspaces between nanoparticles are increased by the volatilization of absolute ethanol after doctor-blade coating procedure, the IPCE of the original sample is low (less than 0.7%), even if for the IR laser treated TiO\({}_{2}\) film, the IPCE is still less than 1%. The CIP procedure can effectively eliminate the increased interspace between TiO\({}_{2}\) nanoparticles; with this technology, the ICPE of the original sample is increased to 4.5%. For the laser sintering of TiO\({}_{2}\) nanoparticle films, power density is a key parameter, as shown in Figure 4a. With the increasing power density, the IPCE of flexible DSC increases linearly, and the maximum IPCE is achieved at the power density of 5.88 \(\times\) 10\({}^{3}\) W/cm\({}^{2}\). When the power density is further increased to 6.18 \(\times\) 10\({}^{3}\) W / cm\({}^{2}\), due to the thermal deformation of the flexible substrate, one part of the TiO\({}_{2}\) nanoparticles falls off, which results in a sharp decline in IPCE.