| Effectively harnessing solar energy is a promising way to solve the energy crisis. Dye-sensitized solar cells (DSSCs), as a promising light-harvesting device, have attracted more and more attention because of their easy fabrication process and low cost compared with traditional silicon-based solar cells. Sensitizers are considered as one of the key materials in DSSCs, which play an important role in determining the performance of the solar cells. To achieve higher solar power conversion efficiency (PCE), great efforts have been focused on designing and synthesizing new sensitizers: zinc porphyrin complexes, ruthenium complexes, and organic dyes. However, organic dyes have recently received more interest due to their unique advantages such as facile modification and purification, readily available raw materials, low cost, and high molar extinction coefficients.Recently, our group reported cyclic thiourea functionalized triphenylamine as donor to construct dye molecules. Compared with corresponding triphenylamine-based dyes, the dyes containing such donor displayed a higher Jsc combined with a higher Voc due to their broad light absorption band and suppression of dye aggregations. Among these thiourea functionalized dyes, dye AZ6 with bithiophene as the π-linker exhibits high JSC and limiting VOC. The high JSC might be explained by the prominent electron-transport properties of the bithiophene and the broad absorption band, while the low VOC may be caused by the shorter electron lifetimes from the DSSCs. Based on the previous research, the π-linkers’structural modification was systematically investigated in this article, in order to get some dye sensitizers with more excellent performances. The detailed research contents are described below.(1) A series of novel cyclic thiourea functionalized dyes containing nine thiourea functionlized triphenylamine based dyes and one simple-sructure dye were designed and synthesized. First, the thiourea functionlized donor and various π-linkers were joined together by Suzuki coupling reaction or Heck reaction. Then the obtained intermediates were converted to corresponding target dyes via Knoevenagel condensation reaction. All the intermediates and target dyes were characterized by 1H NMR,13C NMR, and MS measurements, and were found to be consistent with the proposed structures.(2) The modification of π-linkers and photoanode selection were performed for investigating their effects on the short-circuit current density (Jsc) and open-circuit voltage (Voc). The results showed that AZ261 bearing a n-hexyl chain on the π-linker generates higher JSC and VOC than those of AZ6 without substituent on the bithiophene. Further insertion of a double-bond into the π-linker (AZ263) yielded the highest JSC with the somewhat lower VOC, and delivered the highest PCE up to 8.24%. In addition, the function of mesoporous spherical TiO2 on the photoanodes were also systematically examined and proved to be beneficial to increase both dye-loading capacity and light-harvesting.(3) Election-rich thienothiophene (TT) was contained as the π-linkers to construct dye molecules. The prominent electron-transport properties of the TT significantly enhance the electron injection efficiency. However, the introduction of TT results in a narrow energy gap, which may increase intermolecular interaction, resulting in the intermolecular aggregation and the decrease of VOC. The absorption band was broadened along with the extension of π-conjugation linker, which is beneficial to improve the light-harvesting abilities of photoanode, resulting in the increase of JSC.(4) Electron-rich section with steric effect,3,4-ethylenedioxythiophene (EDOT), was used to construct the dye molecules. The broad absorption spectra indicate that the light-harvesting abilities were enhanced by the introduction of the electron-rich EDOT. In addition, the bulky character of EDOT unit can also contribute to the suppression of the intermolecular π-π aggregation and charge recombination. Such dyes exhibit the best photovoltaic performance.(5) The structure-property relationships of several thiourea functionalized dyes containing binary π-linkers were investigated. The only difference in these dyes is that one segment of the binary π-linkers is respectively thiophene, n-hexylthiophene, thienothiophene,3,4-ethylenedioxythiophene and benzene, and the other one is thiophene. Among them, the dyes incorporating electron-rich thienothiophene or 3,4-ethylenedioxythiophene exhibit the broader absorption bands and higher molar extinction coefficients, which can enhance the light-harvesting ability and improve the JSC; on the other hand, the dyes bearing bulky n-hexylthiophene or 3,4-ethylenedioxythiophene can effectively suppress the intermolecular aggregation and electron recombination owing to larger steric hindrance. Additionally, density functional theory (DFT) calculations show that the π-linker with an arc-shaped coplanarity exhibits the best photophysical and photovoltaic performances.(6) One simple-structure dye and two bulky dyes have been designed for exploring a new co-sensitization system utilizing synergy between simple-structure and bulky organic dyes. This small-sized dye molecule can fill up the space defects between bulky dyes, and its strong absorption band can also complement the absorption valleys of the bulky dyes. Finally, the photovoltaic performances in co-sensitized solar cells improve significantly through the complementary advantages. Additionally, the dye loading of titanium dioxide surface was investigated by the combined method of Thermogravimetric Analysis (TGA) and High Performance Liquid Chromatography (HPLC), and the loading mechanism between two co-sensitizers was also discussed. These results exhibit that the simple molecule can be used not only as a co-adsorbent to inhibit charge recombination and intermolecular aggregation between the bulky dyes but also as a co-sensitizer to improve light-harvesting ability. |
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