199 1 year, a Swiss scholar Gratzel published an article in Nature, and proposed a new type of solar cell with dye-sensitized nanocrystalline titanium dioxide film as photoanode, which has the advantages of simple fabrication, low cost, high efficiency and long life. At present, the photoelectric conversion efficiency can reach more than 1 1%, so it has become the main research and development direction of a new generation of solar cells [65438
The improvement of photoelectric conversion efficiency of dye-sensitized solar cells is attributed to its unique nanocrystalline porous film electrode, which can make electrons transport faster in the film and have a large specific surface area, and can absorb a large number of dyes, matching the energy level of dyes. Therefore, due to the complex influence on dye-sensitized solar cells, many scientists are committed to preparing TiO2 nanocrystalline porous thin film electrodes with good functions and properties [5, 6]. Among the three crystal forms of nano-TiO _ 2, anatase has the best photoelectric activity and is the most practical in dye-sensitized solar cells, so rutile phase and brookite phase should be avoided as much as possible when preparing nano-TiO _ 2.
For the growth of TiO _ 2 nanocrystals, many researchers began to prepare TiO _ 2 nanocrystals [7-9] by hydrothermal method using organic base as colloidal solvent. Titanium dioxide nanocrystals with different particle sizes and morphologies were prepared by Yang using three organic bases as colloidal solvents. The results show that the addition of organic base has certain influence on the particle size, morphology and surface area of nanocrystalline [10]. However, there is little discussion on how to prepare the crystal form and morphology that meet the requirements of dye-sensitized solar cells.
In this chapter, based on the hydrothermal method, TiO2 _ 2 nanocrystals were prepared with tetramethylammonium hydroxide (TMOH), tetraethylammonium hydroxide (TEAOH) and tetrabutylammonium hydroxide (TBAOH) as colloidal solvents, and were applied to dye-sensitized solar cells. The effects of different preparation conditions on the morphology, particle size and photoelectric properties of the nanocrystals were studied.
Main drugs and instruments in the experiment
Tetrabutyl titanate, isopropanol, polyethylene glycol 20000, iodine, lithium iodide, 4- tert-butylpyridine (TBP) and OP emulsifier (Triton X- 100)(AR, all purchased from Shanghai Chemical Reagent Company of China Pharmaceutical Group); Sensitizing dye (cis-[(dcbH2)2Ru(SCN)2), Solonix sa. ); Tetramethylammonium hydroxide (TMAH) (25%), tetraethyl ammonium hydroxide (TEAOH)(20%) and tetrabutyl ammonium hydroxide (TBAOH)( 10%) (all purchased from Shanghai Chemical Reagent Company of China Pharmaceutical Group); Magnetic stirrer with controllable temperature (C-MAGHS4 of IKA, Germany); Muffle furnace (Shanghai Experimental Electric Furnace Factory); 100 W xenon lamp (XQ- 100 W, Shanghai electro-optical device co., ltd.); Conductive glass substrate (FTO,15Ω/cm2, Beijing Institute of Building Materials); X-ray powder diffractometer (XRD) D8-advance (Brooke Company); Scanning electron microscope (SEM)S-3500N (Hitachi, Japan); Transmission electron microscope JEM-20 10 (Japan); Infrared spectrum analyzer nicolet impact 4 10 spectrometer; Ultraviolet-visible spectrophotometer UV-VIS3 100 (Shimadzu Company, Japan).
3 experimental part
3. Preparation of1nano-TiO2
According to the preparation method in document [6- 1 1], evenly mix tetrabutyl titanate and isopropanol with equal volume, drop them into distilled water, continue stirring for 30 minutes ([H2O]/[Ti(OBu)4] = 150), filter, and wash 2 with water and ethanol solution.
Under strong stirring, the obtained precipitate was added to the solution containing organic alkali with pH= 13.6, and stirred at 100℃ for 24 hours to obtain translucent colloid. The obtained colloid was put into an autoclave (the filling degree was less than 80%). Hydrothermal treatment was carried out at 200 degrees Celsius for 12 hours. After hydrothermal treatment, a milky white mixture with fishy smell was obtained, indicating that organic bases were decomposed into amine compounds. The TiO _ 2 colloid after autoclave treatment was poured into a beaker together with the precipitate and concentrated to the original 1/5 at 50℃. Then add 20000 polyethylene glycol and a few drops of Triton X- 100, which accounts for 20%-30% of the amount of TiO _ 2, and stir evenly to obtain stable TiO _ 2 nanocrystalline slurry.
3.2 Preparation of Nanocrystalline Thin Film Electrode
Cover the four sides of the cleaned conductive glass with transparent adhesive tape, control the thickness of the film by controlling the thickness of the adhesive tape and the concentration of colloid [12], leaving a gap of about 1× 1 cm2 in the middle, and evenly spread the nanocrystalline TiO2 _ 2 colloid prepared under acidic conditions in the gap with a glass slide. After natural drying in the air, the temperature rises to 450? C, heat treating for 30 minutes to solidify TiO2 _ 2 and burn off organic substances such as polyethylene glycol, and cooling to 80? By instrumental measurement, the average thickness of the film is about 6 microns.
Soak the obtained nanocrystalline porous film in N3 dye solution for 24 hours to make the dye fully adsorbed on TiO2, take it out and soak it in ethanol for 3-5 minutes, wash off the dye adsorbed on the surface, and naturally dry it in the dark to obtain the dye-sensitized nanocrystalline porous TiO2 film electrode. First, a nanocrystalline porous film was prepared as described above, and the average thickness of the prepared film was about 4.5 microns. Then covered with transparent tape, the large-particle nanocrystalline TiO2 _ 2 slurry prepared with TMAOH as colloidal solvent was evenly spread in the gap with the glass slide. After natural drying in the air, the temperature rises to 450? C heat treatment for 30 minutes, the average thickness of the reflective layer nanocrystalline film is controlled at about 65438 0.5 micron, and the double-layer nanocrystalline film is obtained after heat treatment. Soaking the dye to obtain the double-layer nanocrystalline film electrode.
3.3 assembly of DSSC
Dye-sensitized nanocrystalline porous TiO2 _ 2 film electrode is used as working electrode, and platinum-plated electrode is used as counter cathode [13]. Liquid electrolyte with acetonitrile as solvent and 0.5 mol/l LII+0.05 mol/l I2+0.2 mol/l TBP as solute was dripped into the gap, and the dye-sensitized solar cell was obtained after packaging.
3.4 photoelectric performance measurement
The sunlight simulator adopts 100 W xenon lamp, and its incident light intensity Pin is 100 mW/cm2. The short-circuit current ISC and the open-circuit voltage VOC are recorded at room temperature, and the filling factor ff and photoelectric conversion efficiency η are calculated by formulas.
3.5 Characterization and Analysis
The crystal structure of TiO2 _ 2 was determined by D8-advance X-ray powder diffractometer. The test conditions are: Cu Kα(λ= 1.5405? ), voltage: 40 KV, current: 40 mA. Scanning speed: 6? /min, scanning range: 10? -80? . The infrared spectrum of the sample was measured by KBr tabletting method. The test condition is 400-4000 cm- 1, the software is OMNIC 6.0, and the scanning times are 30 times. The surface morphology and particle size of titanium dioxide nanocrystals were observed by JEM-20 10 (Japan) transmission electron microscope (TEM). Ultraviolet-visible spectrophotometer (UV-3 100) was used to measure the absorbance of dye adsorbed by TiO2 _ 2 nanocrystalline porous membrane electrode with different particle sizes. The heating rate of TG is 10 ℃/min, ranging from room temperature to 1000℃. The test instrument is SDT 2960 synchronous DSC-TGA device (American TA equipment).
4 Results and discussion
4. Effect of1organic base on morphology and particle size of TiO2 _ 2 nanocrystals
Sugimoto and his collaborators have studied some factors affecting the growth of TiO _ 2 nanocrystals, among which pH value, alkyl chain length of organic base, hydrothermal temperature and hydrothermal time have great influence on the size and morphology of TiO _ 2 nanocrystals [14- 17]. It is found that tetraalkyl organic bases are used as templates to control the morphology and size of titanium dioxide nanocrystals.
Therefore, different organic bases can be used to prepare TiO2 nanocrystals with complete crystal form and large specific surface area, which are suitable for photoelectric transmission of dye-sensitized solar cells.
TEM images of TiO2 _ 2 nanocrystals prepared with different organic bases as peptizing agents are shown in figs. a, b and c, respectively, in which TMAH as peptizing agent, TEAOH as peptizing agent and TBAOH as peptizing agent. It can be seen from the figure that at the same pH value, when different organic bases are used as peptizing agents, the prepared nanocrystals are obviously different, which shows that peptizing agents have great influence on the particle size and morphology of TiO2 nanocrystals, and with the extension of alkyl chain of organic alkali peptizing agents, the particle size of TiO2 nanocrystals decreases and the particles are polyhedral. When TMAOH is used as colloidal solvent, the particles of TiO2 nanocrystals are mostly tetragonal, with a width of 12-20 nm and a length of 20-40 nm, as shown in figure1a. When TEAOH is used as colloidal solvent, TiO2 _ 2 nanocrystals have uneven particles and irregular morphology, including polyhedron and tetrahedron. The particle width is 8- 10 nm and the length is 10-25 nm, as shown in figure1b. However, when the alkyl chain length of organic base is increased from two carbon atoms to four carbon atoms, that is, when TBAOH is used as colloid solvent, the prepared nanocrystalline particles have uniform particle size, regular morphology, mostly cubic, and the particle size is generally about 5nm, as shown in figure1c. In the hydrothermal growth process of TiO _ 2 nanocrystals, organic bases are first adsorbed on the crystal nucleus of TiO _ 2, but the adsorption amount varies with the length of alkyl chain. The larger the adsorption amount, the more it will hinder the growth of nanocrystals. It is found that [6], the longer the alkyl chain, the greater the adsorption force of organic base on the crystal nucleus, which will hinder the growth of crystal, so with the increase of alkyl chain length of organic base, the number of nanocrystalline particles is decreasing; It is also found that the concentration of peptizing agent should not be too high, and the prepared TiO2 nanocrystals will have serious agglomeration [10].
4.2 Effect of Organic Alkali on the Crystal Form of TiO2 _ 2 Nanocrystals
It is the XRD pattern of TiO _ 2 nanocrystals prepared by using three kinds of organic bases as colloidal solvents. A is the XRD pattern of TiO _ 2 nanocrystals after natural air drying, and B is the XRD pattern of three kinds of TiO _ 2 nanocrystals after heat treatment at 50℃ for 30 minutes.
As can be seen from fig. 2a, 2θ = 25.3 is the characteristic peak of TiO2 _ 2 nanocrystalline anatase, but there are some other miscellaneous peaks, which prove to be the peak of organic amine. When the prepared nanocrystals were heat-treated at 450℃ for 30 minutes, the impurity peaks in Figure A disappeared, and the D values of the diffraction peaks of TiO _ 2 at 2q =25.3, 37.55, 47.85, 53.75, 55.05 and 62.35 were all consistent with the diffraction peaks of anatase TiO _ 2 in standard PDF cards, which indicated that the prepared TiO _ 2 was the same. In the traditional hydrothermal method, nanocrystalline TiO2 prepared with nitric acid as colloidal solvent contains a small amount of rutile phase and brookite phase, and its photoelectric properties are poor, which affects the photoelectric conversion efficiency of dye-sensitized solar cells. However, TiO2 nanocrystals prepared with organic alkali as colloidal solvent can meet the requirements of anatase phase in dye-sensitized solar cells. With the increase of alkyl chain of organic base, the characteristic diffraction peak width of the sample gradually increases and the diffraction peak gradually decreases, indicating that the prepared nanocrystalline particles are decreasing, which is consistent with the TEM results.
4.3 Thermal Stability Analysis of Titanium Dioxide Nanocrystals
This is the infrared spectrum of titanium dioxide nanocrystals prepared with three organic bases. (a) drying the prepared nanocrystalline powder at 80℃ for 24 hours, and (b) heat treating the prepared nanocrystalline powder at 450℃ for 65,438+0 hours, with the spectral range of 400-4,000 cm-65,438+0. According to the infrared spectra, the infrared spectra of the three nanocrystals are similar. In fig. 3(a), some bonds of organic compounds, such as C-H, N-H and O-H, appear, but these bonds disappear after heat treatment at 450°C 1 hour, and the infrared spectrum of TiO2 _ 2 thin film mainly shows the stretching vibration peak of Ti-O-Ti bond near 500cm- 1. This shows that TiO2 _ 2 nanocrystals prepared under the condition of organic alkali are stable anatase phase after 450℃, and the organic matter adsorbed on its surface is completely decomposed. From the XRD results (Figure 3b), it can also be concluded that all organic compounds disappear completely after heat treatment at 450℃, which indicates that titanium dioxide compounds can be crystallized into stable anatase phase TiO2 nanocrystals after heat treatment above 450℃.
TG analysis of thermal stability of titanium dioxide nano-powder prepared by organic base as colloidal solvent. These nanocrystalline powders were dried at 65438 005℃ for 24 hours without any heat treatment. As can be seen from the figure, there are two weightless processes.
The first process is the obvious weight loss between100 and 250 C, which can be considered as the loss of water molecules and some alcohols adsorbed on the surface of nanocrystalline powder. The second process is weightlessness between 250 ~ 400°C, which is due to the loss of organic components adsorbed in the powder. There are strong bonds and interactions between organic compounds and prepared oxides, and these organic compounds encapsulate oxides. When the temperature reaches 400℃, these bonds and interactions will disappear, and the organic compounds will be completely decomposed, which indicates that the force combination between organic compounds and nanocrystalline particles is not too great and will not affect the crystallization of nanocrystals. In addition, it was found that the weight loss of nanocrystalline powders prepared in different organic base colloidal solvents was obviously different, and the weight loss when TBAOH was used as colloidal solvent was obviously higher than that when TMOH was used as colloidal solvent, indicating that more organic substances were adsorbed on the surface of the former. The difference of organic matter adsorption shows that the morphology and particle size of the prepared nanocrystalline powder are obviously different [14], which is consistent with the results of TEM. When TBAOH is used as colloidal solvent, TiO2 _ 2 nanocrystalline particles have a small surface area, which makes the organic matter adsorbed on the surface of nanocrystals increase, so they lose more weight during thermal decomposition. However, when TMAOH is used as colloidal solvent, the prepared TiO2 _ 2 nanocrystals have much larger particles and smaller surface area, so the adsorbed organic matter will be reduced, so the weight loss during thermal decomposition is less. From the weightlessness, we can also simply analyze the similarities and differences of the prepared nanocrystalline particles and morphology.
Using organic base as colloidal solvent to prepare titanium dioxide nanocrystals will have a certain influence on its crystal form and stability. Fig. 5 is the XRD spectrum of TiO2 _ 2 nanocrystals prepared with organic alkali TEAOH as colloidal solvent and samples sintered at 300℃, 500℃, 700℃, 800℃ and 900℃ for 65,438 0 hours respectively. In the crystal form of TiO2 nanocrystals, the peak at 2θ = 25.3 is the characteristic diffraction peak of anatase phase, and the peak at 2θ = 27.4 is the characteristic diffraction peak of rutile phase. It can be seen from the figure that the crystal form of TiO2 _ 2 nanocrystals did not change before sintering at 800 C, and rutile phase crystals appeared after sintering at 800 C, which is consistent with the research results of Young et al. [18]. It is reported that when the sintering temperature reaches 600°C, anatase crystals begin to transform into rutile crystals [19]. However, the transition temperature of TiO _ 2 nanocrystals prepared with organic alkali TEA OH as colloidal solvent has increased, which indicates that the thermal stability of TiO _ 2 nanocrystals prepared with organic alkali TEA OH as colloidal solvent has improved. This stability shows that anatase TiO 2 nanocrystals can be sintered at higher temperature without changing their crystal forms, that is, no rutile nanocrystals appear.
Study on 4.4 BET and Dye Adsorption Capacity
The specific surface area of TiO 2 nanocrystalline powder prepared with different organic bases as colloid solvent was analyzed. The experimental results show that the specific surface area of TiO _ 2 nanocrystalline powder prepared with organic alkali TMOH as colloid solvent is 66 m2 g-1,while the specific surface areas of TiO _ 2 nanocrystalline powder prepared with TEAOH and TBAOH as colloid solvent are 78 m2 g-1and 82 m2·g respectively, which is consistent with the result that the larger the particle size, the smaller the specific surface area. The particle size is shown in figure 1, which shows that the smaller the particle, the larger the specific surface area.
It is found that the adsorption capacity of dye (RuL2(SCN)2) does not necessarily increase with the increase of specific surface area. In order to study the adsorption capacity of TiO2 _ 2 nanocrystalline porous membrane for dye-sensitized solar cells, the sensitized electrode was desorbed in 5 mL 0.05 mol/L NaOH solution, and then the absorbance of the dye alkaline solution was analyzed. The results of UV-Vis absorption spectrum are shown in Figure 5. In the figure, curves A, B and C are TiO2 nanocrystals prepared by using TMOH, TEAOH and TBAOH as colloidal solvents, respectively. According to Lambert-Beer law, the absorbance increases with the increase of concentration. The results show that TiO2 _ 2 nanocrystals prepared with TMAOH as colloidal solvent have the least adsorption capacity for dyes, which is consistent with the small specific surface area, but the adsorption capacity is much smaller than the other two nanocrystals. Although the specific surface area of TiO _ 2 nanocrystals prepared with TBAOH as colloidal solvent is larger than that prepared with TEAOH as colloidal solvent, the latter absorbs more dyes than the former. The possible explanation here is that the TiO _ 2 nanocrystalline particles prepared by using TBAOH as colloidal solvent are too small to reach 10nm, so the nanocrystalline porous membrane prepared by TBAOH is too dense, which reduces the absorbed dyes.
4.5 Study on photoelectric properties of dye-sensitized solar cells
Three kinds of TiO2 _ 2 nanocrystals with different morphologies and particle sizes were prepared by using organic alkali, and were used as sensitized electrodes to study the photoelectric properties of dye-sensitized solar cells, as shown in Figure 6. Table 1 gives the values of short-circuit current, open-circuit voltage, filling factor and photoelectric conversion efficiency of assembled batteries with three different electrodes. Under the illumination of 100 mW/cm2, the short-circuit currents of the three batteries were 10.7, 13. 1, 10.4 mA/cm2, and the open-circuit voltages were 0.779, 0.700 and 0.698V, respectively. 0.62? 0.60, and the photoelectric conversion efficiency reaches 4.4% respectively? 5.67%? 4.4%。 From the experimental results, it can be seen that the photoelectric conversion efficiency of the battery assembled with TiO2 nanocrystals prepared by organic alkali TEAOH is higher than that of the other two batteries.
It can be seen that the open-circuit voltage of the battery prepared by organic alkali TEAOH is lower than that prepared by organic alkali TMAOH, but the short-circuit current and filling factor of the battery are higher than that of the battery assembled by TiO2 prepared by other two organic bases. This may be because the (1) TiO _ 2 nanoparticles prepared by organic alkali TEAOH are mild and the particles in the porous membrane are closely combined, thus improving the electron propagation speed in the membrane; (2) Compared with the other two porous membranes, the research shows that the amount of adsorbed dye is directly proportional to the photocurrent generated. The more dye absorbed, the greater the photocurrent generated. TiO2 _ 2 porous membrane prepared with organic alkali TEAOH as colloidal solvent absorbs the most dyes, and the dye-sensitized solar cell assembled with it has the highest short-circuit current and the best photoelectric conversion efficiency.
5 conclusion
In this chapter, titanium dioxide nanocrystals were prepared from tetrabutyl titanate, three organic bases were used as colloidal solvents, and three kinds of sensitized nanocrystalline porous films were used as electrodes TiO2 assemble dye-sensitized solar cells, and their photoelectric properties were tested. The effects of these three organic solvents on the growth of TiO2 nanocrystals were studied. The morphology and size of nanocrystals prepared with three kinds of organic bases with different alkyl chains are very different. It was found that with the extension of alkyl chain, the morphology of nanocrystals began to become regular and the particle size became smaller. However, the concentration of organic base should not be too high, otherwise it will lead to the agglomeration of nanocrystals. Therefore, when organic base is used as colloidal solvent, pH = 60 is adopted. Through thermal stability analysis, it was found that the organic base adsorbed on the surface of TiO _ 2 nanocrystals was completely decomposed after heat treatment at 450°C, which indicated that the organic matter had been completely decomposed when preparing the nanocrystalline porous membrane, and the porous membrane was pure TiO _ 2 nanocrystals. Due to the different morphologies and sizes of three kinds of TiO2 _ 2 nanocrystals, the adsorption capacity of the prepared porous membrane for dyes is also different. It was found that the TiO _ 2 sensitized electrode prepared with organic alkali TEAOH as colloid solvent had the largest adsorption capacity for dyes, and the photoelectric performance test of the battery also showed that the open-circuit current of the battery prepared with this TiO _ 2 nanocrystal reached13.1Ma cm-2, and the photoelectric conversion efficiency reached 5.67%, which was higher than that of the other two batteries, indicating that TiO prepared with organic alkali TEAOH as colloid solvent. For more graduation thesis, please go to.