Design And Synthesis Of Asymmetric Phthalocyanine Dyes And Their Dye-sensitized Solar Cell Performance

Dye-sensitized solar cells (DSSC) have attracted much attention due to their relatively low cost and high conversion efficiency. Currently, the most widely used dyes in this field are polypyridyl-ruthenium complexes, however, ruthenium is a rare metal and ruthenium-based dyes with low molar extinction coefficient are usually lack of absorption in the red/near-IR region of the solar spectrum. As a result, the syntheses of novel dyes with efficient absorption covering the relatively whole range of the solar spectrum are imperative for DSSCs in particular the photovoltaic devices with extended near-IR absorption in the area of solar cells. Thus, we designed and synthesized a series of highly asymmetric phthalocyanine derivatives, and carried out investigation into the relationship between molecular structure of phthalocyanine derivatives and the photovoltaic performance of phthalocyanine-sensitized DSSC. The main contents and conclusions obtained are summarized as follows:1. Highly asymmetric phthalocyanine derivatives (Zn-fri-TAPNc-1, Zn-tri-PcNc-1, Zn-tri-PcNc-2and Zn-tri-PcNc-3) based on electronic push-pull effect are designed and synthesized. The effect of benzo-annelation, electron-withdrawing groups (single/double carboxyl) and electron-donating groups (tBu/OBu) on the spectral absorption range and photoelectrochemical properties of zinc phthalocyanine-sensitized DSSC is explored.1) The enlargement of the conjugated π-system from Zn-tri-TAPNc-1to Zn-tri-PcNc-1causes an obviously enhanced molar extinction coefficient and a red-shift of the Q-band, which lead to more effective near-IR light absorption and light harvesting. Moreover, the extended π-system causes a shift of the LUMO toward higher binding energies, and then results in sufficient thermodynamic driving force for the electron injection from the excited dye to TiO2. Thus, The charge recombination and electron transfer resistance are decreased, and the photovoltaic performance of DSSC is improved.2) The introduction of two carboxyl groups into Zn-tri-PcNc-2will decrease the molar extinction coefficient and molecular orbital energy level, which causes the driving force of excited electrons into the conduction band of TiO2insufficient. Thus the probability of recombination is increased, and the photovoltaic DSSC performance is declined.3) When tBu is replaced by OBu in Zn-tri-PcNc-1, the electron-donating ability, the charge density on the phthalocyanine nucleus and the molar extinction coefficient are improved due to the lone pairs of electrons on the O atoms. However, OBu is less sterically hindered than tBu, resulting in the increasing of electron transport resistance and charge recombination rate.Thus the performance of DSSC based on Zn-tri-PcNc-3is unsatisfactory. The above study provides an important theoretical guidance for better understanding the relationship between the molecular structure and the DSSC performance as well as expanding spectral response range and improving the photovoltaic performance of DSSC.2. In order to explore the effect of long-chain alkyl substituents and S atoms in the electron-donating groups on the photoelectrochemical properties of DSSC, thio octane and isooctane are introduced into Zn-tri-TAPNc-2and Zn-tri-TAPNc-3as electron-donating groups.1) The change of long-chain alkyl groups shows little impact on the absorption spectrum and the HOMO/LUMO levels of Zn-tri-TAPNc-2and Zn-tri-TAPNc-3, but they can effectively inhibit the aggregation of dye on the electrode surface. The introduction of long-chain alkyl and S atoms improves the molar extinction coefficient and causes a red-shift of Q-band compared with Zn-tri-TAPNc-1.2) The performance of Zn-tri-TAPNc-3is better than Zn-tri-TAPNc-2in DSSC, because the steric hindrance of thio isooctane is greater than thio octane. As a result, Zn-tri-TAPNc-3can be more effectively suppressed aggregation and reduced recombination. Moreover, the performances of Zn-tri-TAPNc-2and Zn-tri-TAPNc-3are much worse than Zn-tri-TAPNc-1. This is because the thio long-chain alkyl groups cause the interval between the LUMOs of Zn-tri-TAPNc-2and Zn-tri-TAPNc-3and the conduction band of TiO2too small. The driving force of excited state electron injection into the conduction band of TiO2is serious shortage, resulting in an increase in recombination reactions and shortened electronic life expectancy. These results further confirmed the conclusions obtained in the second chapter:azaporphyrin derivatives and TiO2conduction band energy levels do not match, so they are not suitable for TiO2based DSSC. Phthalocyanine with18π electronic structure is more suitable as a DSSC dye, which provides important basis data for seeking new and efficient phthalocyanine dyes.3. To further enhance the photovoltaic performance of Zn-tri-PcNc-1sensitized DSSC, the effect of CDCA and the adsorbent temperature on the DSSC photoelectrochemical properties is investigated. Co-grafted CDCA on TiO2electrode efficiently diminishs the aggregation of the sensitizer on the TiO2electrode although it also reduces the loading amount of Zn-tri-PcNc-1, which is proved to be limited effect on the photocurrent. Advantageously, the addition of CDCA can enhance the electron injection efficiency and retards the charge recombination, and therefore resulting in the improved cell efficiency.2) When the adsorbent temperature increases from5℃to25℃, dyes adsorption capacity and the degree of aggregation on TiO2film surface increase, resulting in the increase of charge recombination, decreased electron injection efficiency and DSSC photovoltaic properties. When the temperature is reduced from5℃to-15℃, dye adsorption rate slows on TiO2film surface and then the adsorption amount reduces, so that the dye can not reach the saturation monolayer adsorption on the TiO2film electrode, also resulting in reduction in DSSC photovoltaic performance.3) The TiO2electrode immersing into5.0×l0-5M Zn-tri-PcNc-1ethanol solutions containing7.5mM CDCA at5℃for12h is the optimal dye-sensitization condition, and the maximum conversion efficiency (n) of the fabricated solar cell achieve2.89%, improved by47%as compared to the solar cell fabricated with TiO2electrode sensitized by the Zn-tri-PcNc-1solution in absence of CDCA. Although CDCA reduces the loading amount of dye, can significantly inhibit the aggregation, improve the electron injection efficiency, inhibit recombination and significantly improve the final DSSC photovoltaic performance. These results show that adding a physical adsorbent and optimizing sensitized temperature are also important ways to improve the phthalocyanine sensitized DSSC performance.4. To further suppress molecular aggregates, asymmetric phthalocyanine derivatives Zn-tri-PcNc-4and Zn-tri-PcNc-5containing2,6-diphenyl phenol and2,6-diphenyl thiophenol as electron donating groups are synthesized, and coadsorbent and the effect of O, S atoms in electron donating groups on the cell performance is explored. When O atom is replaced by S atom in Zn-tri-PcNc-4, Zn-tri-PcNc-5shows higher molar extinction coefficient and a red-shift of absorption spectrum due to the lone pairs of electrons on the S atoms. However, the C-S band is unstable and HOMO-LUMO energy level moves down, reducing the difference between LUMO and TiO2conduction band levels which causes the driving force of excited electrons into the conduction band of TiO2insufficient. Thus the electron transfer resistance and recombination are increased, and the photovoltaic DSSC performance is declined from3.22%to1.30%.2) By chemical modification, the introduction of large steric handrance substituents can completely hinder molecular aggregation. The addition of CDCA in Zn-tri-PcNc-4will significantly reduce light-capturing efficiency and electron injection life, resulting in the DSSC performance decreasing to2.75%. However, the addition of CDCA in Zn-tri-PcNc-5have little effect on light-capturing efficiency, but will cause a change of TiO2conduction band energy level, which causes reduced recombination and increases the photoelectric conversion efficiency to2.10%. The results provides a new way for better understanding the relationship between the molecular structure and the DSSC performance as well as expanding spectral response range. Meanwhile, molecular aggregation can be completely hindered and the molecular level can be adjusted by chemical modification means. It provides an effective solution for hindering molecular aggregation and the application of the level mismatch phthalocyanines in DSSC.