Fluorene functionalized porphyrins as broadband absorbers for TiO<sub>2</sub>nanocrystalline solar cells
作者:Wenhui Li、Liping Si、Zonghao Liu、Zhixin Zhao、Hongshan He、Kai Zhu、Brian Moore、Yi-Bing Cheng
DOI:10.1039/c4ta01954g
日期:——
Three 9,9-dihexyl-9H-fluorene (DHF) functionalized zinc porphyrin dyes (coded as ZZX-N3, ZZX-N4, and ZZX-N5) were designed and synthesized for dye-sensitized solar cells. Then, DHF and benzoic acid were conjugated to the porphyrin ring through triple bonds to act as a spacer to elongate the π-conjugation and as an acceptor for an efficient electron injection, respectively. A bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BFA) and a bis(4-hexylphenyl)amine (BPA) were further linked to DHF to act as electron donors in ZZX-N3 and ZZX-N4, respectively. ZZX-N5 did not have any electron donor and served as a reference. Moreover, ZZX-N3- and ZZX-N4-sensitized cells exhibited broader sunlight absorption than ZZX-N5, and as a result, higher photon-to-electricity efficiency (PCE) (ZZX-N3, 3.83%; ZZX-N4, 4.2%; ZZX-N5, 3.70%) was observed. The results are consistent with well-separated HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in ZZX-N3 and ZZX-N4 than in ZZX-N5. However, the overall conversion efficiency of ZZX-N3- and ZZX-N4-sensitized cells was low, which is due to significant dye aggregation induced by the extra long alkyl-chains on the donor groups. This was evidenced by blue and red shifts of the absorption spectra of dye-coated TiO2 films. In addition, the extra long-chains also did not offer better shielding to prevent electron recombination of injected electrons with I3− in electrolyte as revealed by electrochemical impedance spectroscopy. When a co-sensitizer (coded as PBS) was used, a new peak corresponding to the absorption of PBS at 560 nm was observed on the incident photon to charge carrier efficiency (IPCE) spectra; however, the overall photovoltaic performance was not improved due to the significant decrease of dye-loading density of porphyrin dyes, indicating a need to break off the trade-off between dye-loading and light-harvesting.
设计并合成了三种 9,9- 二己基-9H-芴(DHF)功能化锌卟啉染料(代号分别为 ZZX-N3、ZZX-N4 和 ZZX-N5),用于染料敏化太阳能电池。然后,DHF 和苯甲酸通过三键与卟啉环共轭,分别作为拉长 π 共轭的间隔物和有效注入电子的受体。双(9,9-二己基-9H-芴-7-基)胺(BFA)和双(4-己基苯基)胺(BPA)进一步与 DHF 连接,分别在 ZZX-N3 和 ZZX-N4 中充当电子供体。ZZX-N5 没有任何电子供体,因此作为参照物。此外,与 ZZX-N5 相比,ZZX-N3 和 ZZX-N4 敏化电池对阳光的吸收范围更广,因此光子对电能的效率(PCE)也更高(ZZX-N3,3.83%;ZZX-N4,4.2%;ZZX-N5,3.70%)。与 ZZX-N5 相比,ZZX-N3 和 ZZX-N4 的 HOMO(最高占位分子轨道)和 LUMO(最低未占位分子轨道)分离良好,这与上述结果一致。然而,ZZX-N3 和 ZZX-N4 敏化电池的整体转换效率较低,这是由于供体基团上的超长烷基链诱发了显著的染料聚集。染料涂层二氧化钛薄膜吸收光谱的蓝移和红移证明了这一点。此外,电化学阻抗光谱显示,超长链也不能提供更好的屏蔽,防止注入电子与电解质中的 I3- 发生电子重组。当使用共敏化剂(代号为 PBS)时,入射光子到电荷载流子效率(IPCE)光谱上出现了一个与 PBS 在 560 nm 处的吸收相对应的新峰值;然而,由于卟啉染料的染料负载密度显著降低,整体光伏性能并未得到改善,这表明需要打破染料负载与光收集之间的平衡。