Synthesis, Structural Modifications And Photovoltaic Performance Of D-π-A Organic Sensitizers

With the energy crisis and environment pollution becoming more and more serious, the development and utilization of new energy has become one of the immediate tasks for human. Solar energy has great potential for exploitation, the characteristics of rich resources, convenient collection and clean pollution-free completely meets the requirements of sustainable development of human society. Dye-sensitized solar cells (DSSCs) have received widespread attention due to their low costs, easy of fabrication and high power conversion efficiency (PCE).A complete DSSC is made of photoanode, sensitizer, electrolyte and counter electrode. The sensitizer is the most important core component of the device, thus, the development of novel and efficient sensitizers is one of the most direct and effective ways to improve the performance of DSSC. We designed and synthesized a series of D-π-A zinc porphyrin and phenothiazine dyes. We hope to broaden the spectral response, suppress dye aggregation, reduce charge recombination and improve the electron injection efficiency by molecular engineering and optimization different donors (D),π-bridges (it) and anchors. In this thesis, the optical properties, electrochemical properties, electron injection dynamics, interfacial charge recombination process and photovoltaic performance of the dyes were carefully studied. We expect to find out the general regularity of the molecular structures of the dyes effect the performance of the devices. The main contents are as follows:1. We synthesized YD2-O-C8 according to the literature, and prepared three D-π-A zinc porphyrin dyes JP1, JP2 and JP3 by modifying the molecular structure of YD2-O-C8. Their electron donors and acceptors have been further improved. 2-Hexylthiophene chromophores were introduced to the donor groups, which extended the π-conjugation system effectively, then broadened the range of spectral response and improved molar extinction coefficient. Moreover, we used triphenylamine replaced diphenylamine as the electron donors, and introduced alkynyl between the triphenylamine and porphyrin ring, this method can effectively extend the π-conjugation system and improve the electron transfer properties. Dye JP3 extended the spectral response to 750 nm, which was red shifted 10 nm at the Soret band and 20 nm at the Q band with respect to dye YD2-O-C8, and the molar extinction coefficient of JP3 was double that of YD2-O-C8 at the Q band. We used thiophene-2-carboxylic acid instead of the traditional benzoic acid as an anchor group, which can make the molecules arrange to tilted orientation when adsorbed on the TiO2 surface, and this may effectively suppress the dye aggregation and prevent charge recombination by preventing the I3 of the electrolyte penetrating into the TiO2 surface, thus, the Voc of the devices were improved. The PCE of JP1, JP2 and JP3 were 5.09%,5.62% and 6.40% under AM 1.5G irradiation, respectively. Under the same conditions, the PCE of YD2-O-C8 was 6.83%.2. The π-bridge plays a crucial role in the D-π-A framework, it can extend the length of the π-conjugated system, and ensure that the electrons successfully move from the donor to acceptor. Heterocycles and their derivatives are often used as π-bridges in the design of sensitizers due to their excellent conjugate structures, and thiophene and furan is often widely used. In this thesis, two heterocycles (thiophene and furan) were used as the π-bridges in DSSC. With the purpose of comparing the two different π-bridges, two dyes (JP-S and JP-O) differing only in n-bridge were prepared. We found that the two dyes had clearly different optical properties and photovoltaic performances. The optical properties indicated that JP-S was red-shifted compared to JP-O, and the adsorption of dye on TiO2 surface was also improved. It has a great influence on their IPCE, led to the Jsc of JP-S (12.63 mA cm-2) was much higher than that of JP-O (10.32 mA cm-2). In addition, the EIS analysis suggested that the dye with the thiophene π-bridge had a lower charge recombination rate compared to the dye with the furan π-bridge. Thus, the Voc of JP-S was 676 mV, in contrast, the Voc of JP-0 was 661 mV. Based on these differences, the DSSC based on JP-S showed a higher PCE of 5.84%, and the PCE of the DSSC based on JP-0 was only 4.68%.3. The anchoring groups determine the electron injection rate and the binding energy of the dye on the semiconductor surface. Picolinic acid was first used as an anchoring group in DSSC, the interfacial stability and electron injection rate were both improved. We synthesized two D-π-A zinc porphyrin dyes JA1 (carboxylate anchor) and JA2 (picolinic acid anchor). The optical properties indicated that JA2 was red-shifted compared to JA1, and the adsorption of dye on TiO2 surface was also improved. The furier-transform infrared spectroscopy (FTIR) indicated that picolinic acid can anchor dye on TiO2 surface via tridentate mode. The anchoring group can make the dye choose the Lewis acid sites and Bronsted acid sites of TiO2 synchronously. Thus, the interfacial stability and Jsc of the device were both improved. The PCE of the DSSC based on JA1 was 5.76%, and the DSSC based on JA2 showed a higher PCE of 7.20%. The triphenylamine dye TTR2 was used as a cosensitizer, the picolinic acid anchor was more suitable for the cosensitization system than the carboxylate anchor, for there was almost no competitive adsorption between JA2 and TTR2. The JA2+TTR2 based-device showed the highest PCE of 8.98% under AM 1.5G irradiation.4. Two D-π-(A)2 phenothiazine-based dyes ZJA2 (two carboxylate anchors) and ZJA3 (two pyridine anchors) were synthesized. The single D-π-A phenothiazine-based dye (ZJA1) with mono-anchoring group was also prepared for comparison. The steady-state emission spectra and time-resolved fluorescence experiments all indicated that the electron injection efficiency of dye ZJA2 was improved by the increased electron extraction channels. The EIS analysis suggested that this way is in favour of reducing electron recombination, thus, the Voc can be improved. As a result, the PCE of the DSSC based on ZJA2 (4.55%) was superior to the DSSC based on ZJA1 (2.72%). The DSSC based on ZJA3 showed the poorest PCE (0.46%) due to the low dye loading.