THE PREPARATION OF TITANIA AND QUANTUM DOT NANOPARTICLES AND THEIR APPLICATIONS TO THE PHOTOVOLTAIC CELLS

Abstract

The goal of the present work was the preparation of various kinds of TiO2 electrodes applicable to DSSC as well as QDSSC. The organic dye as a light absorber was replaced by inorganic semiconductor nano particles and studied. In chapter 2, the crystal structure, crystallinity, crystal size, and surface structure of TiO2 was discussed because the overall conversion efficiency is strongly dependent on the phase, size and morphology of TiO2. The TiO2 nanoparticles are prepared by hydrolysis and condensation reaction of two different starting materials such as titanium isopropoxide (TTIP) and titanium tetrachloride (TiCl4). We have prepared various kinds of TiO2 electrodes applicable in photovoltaic cells. First, one of the types of nano TiO2 derived from TiCl4 which is designated as “K” has particle size ca. 30 ~ 40 nm and surface area of 45 m2/g. This size is comparable to that of commercially available powder “P-25”. The overall conversion efficiency of cells made from “K” type TiO2 and P-25 is approximately same. The second, a simple one-step heat-treatment of peroxotitanate complex aqueous solution at around 100oC was resulted in the formation of ellipsoidal anatase TiO2 nanoparticles having a high aspect ratio with no branches without using templates as shape controller. The length of these ellipsoidal TiO2 falls in the range of 150&#8211; 350 nm, depending on mole ratio of Ti4+/H2O2. The third, the morphology and phase of TiO2 particles prepared from different starting material was different. Phase pure nano anatase (long nanorod) was obtained from peroxotitanate gel of pH = 4. A mixture of nano anatase and rutile were obtained from peroxotitanate gel of pH =2 while predominantly rutile (short nanorods) was obtained when its pH was 1. The TiO2 nano particles prepared from TiCl4, on the other hand, were having pure anatase (nanorod) regardless of pH variation (1 < pH < 4). Although the experimental conditions are same, at pH = 1, TTIP is giving rise to rutile (90%) while TiCl4 resulting anatase. In chapter 3, all the different types of lab-prepared nanostructured TiO2 are applied in DSSC devices. An overall electron conversion efficiency of 5.4% at 1 sun illumination has been reached (“K-type”) which is comparable to commercially available P-25 (5.9%). All the prepared electrodes having a thickness of ca. 10㎛ gives optimum overall conversion efficiency. The best overall conversion efficiency value of 6.6% was obtained at L50-4CTS electrode where K layer was introduced to scatter the incident light. This is attributed that lab prepared TiO2 particles which is opaque and small size (30 ~ 40 nm) can scatter the incident light, confine the light into the electrode and absorb large quantity of dye on the surface of K layer leading an increase of overall conversion efficiency, while the particle size of well known scattering layer is ranging from 400 nm to 1000 nm which make it decrease the overall conversion efficiency due to the low surface area. In chapter 4, the size and morphology of PbSe QDs synthesized by a hot chemical solution method were controlled by reaction temperature, time, concentration and chemistry of surfactant. The particle size of PbSe QDs increased with the decrease of chain length of surfactant carboxylic acid as the steric hindrance decreased in order of OA (oleic acid) < HA (hexanoic aicd) < AA (acetic aicd). It is observed that the presence of acetic acid in Pb-oleate solution used as a Pb source played important role in the morphology of PbSe QDs. The yield of PbSe QDs is ca. 1.4%. But, comparatively high yield (9.5%) of PbSe QDs was obtained using Pb-oleate and TBPSe as lead and selenium source respectively. This yield is ca. 5-6 times compared to the synthesis of PbSe QDs using TOPSe where the lead source is same. In chapter 5, the prepared QDs and TiO2 electrode were utilized in QDSSC (QDs-sensitized solar cell). First, the size tunable colloidal quantum dots are bonded to the mesoporous TiO2 film using bifunctional molecular ligand and used as an inorganic sensitizer. The maximum electron conversion efficiency of 0.23 % with Voc of 600.2 mV, Isc of 0.57 mA/cm2, and FF of 0.65 was observed for film thickness at thickness of 12.2㎛. This is attributed to the fact that colloidal quantum dots cannot cover the whole surface of mesoporous TiO2 film unlike the organic dye sensitized solar cell. To increase the QDs coverage of QDs onto the mesoporous TiO2, two methods are combined. The size tunable colloidal quantum dots are self-assembled onto the mesoporous TiO2 film surface using bifunctional molecules. CBD method is employed to deposit the QDs on the TiO2 surface. The maximum conversion efficiency of 0.38 % with the Voc and Jsc value of 673.2 mV and 0.68 mA/cm2 was achieved when 1 cycle of CBD was introduced. Further increase in the electron conversion efficiency was achieved by introducing a TiCl4 post treatment (45% increase of Jsc) and compact layer along with TiCl4 post treatment (20% increase of Voc). The cell performance was carried out by depositing PbS and PbSe QDs using CBD along with CQDs onto the mesoporous TiO2 in alcohol medium due its better wettability and penetration ability than water. The cell performance was carried out by depositing PbS and PbSe QDs using CBD along with CQDs onto the mesoporous TiO2 film in alcohol medium due its better wettability and penetration ability than water. However, overall conversion efficiency in alcohol medium was not increased compared to those in aqueous medium. Finally, /I- redox couple was replaced by Co-complex due to the dissolution of QDs in /I- redox couple. The overall efficiency of the cell with Co-complex as electrolyte is half of the cell with /I- as electrolyte, while polysulfide as electrolyte is quarter of the cell with /I- as electrolyte due to the diffusion problem.