Molten salt assisted self-assembly (MASA) : synthesis, characterization and solar cell performances of mesoporous silica-CdSe and titania-CdSe thin films

Abstract

A series of solutions of a salt ([Cd(H2O)4](NO3)2), a polymerizing agent (Si(OCH3)4, TMOS or Ti(OC4H9), TTBO), two surfactants (cetyltrimethylammonium bromide, CTAB and 10-lauryl ether, C12H25(OCH2CH2)10OH, C12EO10) have been prepared and used as the molten salt assisted self-assembly precursors by only changing the Cd(II)/surfactant mole ratios for the preparation of mesoporous films. The thin films were prepared in two steps: in the first step, the titania particles (titania particles are typically 20-25 nm (P25)), dispersed in ethanol, have been spin coated over various substrates and annealed at 450oC and in the second step, the clear solutions given above (salt, CTAB, C12E10, silica or titania source and ethanol) were drop casted or spin coated over the titania (P25) films and calcined at 450oC. Slow calcination of the films (with an increment of 1 oC/min), starting from the melting point of the salt (around 65oC) to 450 oC has produced the mesoporous silica/titania-cadmium oxide (CdO) thin films denoted as meso-CdO-SiO2-P25 and meso-CdTiO3-P25. The clear solutions were also used to make the thin films without P25 and denoted as meso-CdTiO3. The films were then exposed to a H2Se atmosphere at 100oC for 30 min, and the samples were denoted as meso-CdSe-SiO2-P25, meso-CdSe-TiO2-P25 and meso-CdSe-TiO2. The silica sample was further treated with a dilute HF solution (etching process) that results silica free meso-CdSe-P25. The characterization of the materials produced in this thesis was made by using XRD, FT-IR spectroscopy, UV-VIS, Raman, EDX and Solar Measurement techniques. Silica samples have greater amount of CdSe than titania samples according to the Raman and EDX data which implies that silica samples are more reactive. Also in both silica and titania, the samples with a Cd/surfactant mole ratio of 6 have the greatest amount of CdSe. Fluorine doped SnO2 (FTO) has been used as a transparent conductive substrate for the preparation of the anode electrode for the solar measurements. According to the solar measurements, the silica and titania samples on P25 show greater efficiency than the titania samples without P25 and generally have similar efficiencies (the efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight). The most efficient samples are generally the samples prepared using Cd/surfactant mole ratios equal to 6 and 8. In order to increase the efficiency, 3rd row transition metal cations such as Mn(II), Fe(III), Cu(II) and Co(II) are doped into above samples. The efficiency has been increased by 10 % in the silica and 30 % in the titania samples upon doping with 15% Mn(II). The solar cell characteristics of the electrodes were tested using the following parameters; by changing the Cd(II)/surfactant mole ratio (many samples were prepared and used for this purpose), by changing the aging of the electrolyte (Na2S/S8), by using multiple coating the MASA solution in the preparation stage of the electrodes, by doping the samples with various transition metal ions, and finally by modifying the surface of the electrode by coating with ZnS or CTAB. Each of these parameters has an effect on the cell performance. The effect of each parameter is monitored by measuring the I-V characteristics of the cells and found out that the best results were obtained from the electrodes made using 6 or 8 Cd/surfactant mole ratio in MASA system, freshly prepared Na2S/S8 electrolyte, coating of the MASA solution twice on electrode, doping Mn(II) cations with 0.15 Mn(II)/Cd(II) ratio and using CTAB for coating anode electrode.