Synthesis of TiO_2 thin films using single molecular precursors by MOCVD method for dye-sensitized solar cells application and study on film growth mechanism

摘要

For dye-sensitized solar cells application, in this study, we have synthesized TiO_2 thin films at deposition temperature in the range of 300-750 ℃ by metalorganic chemical vapor deposition (MOCVD) method. Titanium(IV) isopropoxide, {TIP, Ti(0~iPr)_4} and Bis(dimethylamido)titanium diisopropoxide, {BTDIP, (Me_2N)_2Ti(O~iPr)_2} were used as single source precursors that contain Ti and O atoms in the same molecule, respectively. Crack-free, highly oriented TiO_2 polycrystalline thin films with anatase phase were deposited on Si(l 0 0) with TIP at temperature as low as 450 ℃. XRD and TED data showed that below 500 ℃, the TiO_2 thin films were dominantly grown in the [211] direction on Si(1 0 0), whereas with increasing the deposition temperature to 700 ℃, the main film growth direction was changed to [2 0 0]. Above 700 ℃, however, rutile phase TiO_2 thin films have only been obtained. In the case of BTDIP, on the other hand, only amorphous film was grown on Si(l 0 0) below 450 ℃ while a highly oriented anatase TiO_2 film in the [2 0 0] direction was obtained at 500 ℃. With further increasing deposition temperatures over 600 ℃, the main film growth direction shows a sequential change from rutile [1 0 1 ] to rutile [4 0 0], indicating a possibility of getting single crystalline TiO_2 film with rutile phase. This means that the precursor together with deposition temperature can be one of important parameters to influence film growth direction, crystallinity as well as crystal structure. To investigate the CVD mechanism of both precursors in detail, temperature dependence of growth rate was also carried out, and we then obtained different activation energy of deposition to be 77.9 and 55.4 kJ/mol for TIP and BTDIP, respectively. Also, we are tested some TiO_2 film synthesized with BTDIP precursor to apply dye-sensitized solar cell.