Exploring copper(i)-based dye-sensitized solar cells: a complementary experimental and TD-DFT investigation
作者:Biljana Bozic-Weber、Valerie Chaurin、Edwin C. Constable、Catherine E. Housecroft、Markus Meuwly、Markus Neuburger、Jennifer A. Rudd、Ewald Schönhofer、Liselotte Siegfried
DOI:10.1039/c2dt31159c
日期:——
The structures and properties of the homoleptic copper(I) complexes [Cu(1)2][PF6] and [Cu(2)2][PF6] (1 = 6,6′-dimethyl-2,2′-bipyridine, 2 = 6,6′-bis2-[4-(N,N′-diphenylamino)phenyl]ethenyl}-2,2′-bipyridine) are compared, and a strategy of ligand exchange in solution has been used to prepare eight TiO2 surface-bound heteroleptic complexes incorporating ligands with bpy metal-binding domains and carboxylate or phosphonate anchoring groups. The presence of the extended π-system in 2 significantly improves dye performance, and the most efficient sensitizers are those with phosphonate or phenyl-4-carboxylate anchoring units; a combination of [Cu(2)2]+ with the phosphonate anchoring ligand gives a very promising performance (η = 2.35% compared to 7.29% for standard dye N719 under the same conditions). The high-energy bands in the electronic absorption spectrum of [Cu(2)2]+ which arise from ligand-based transitions dominate the spectrum, whereas that of [Cu(1)2]+ exhibits both MLCT and ligand π* ← π bands. Both [Cu(1)2][PF6] and [Cu(2)2][PF6] are redox active; while the former exhibits both copper-centred and ligand-based processes, [Cu(2)2][PF6] shows only ligand-based reductions. Results of TD-DFT calculations support these experimental data. They predict an electronic absorption spectrum for [Cu(1)2]+ with an MLCT band and high-energy ligand-based transitions, and a spectrum for [Cu(2)2]+ comprising transitions involving mainly contributions from orbitals with ligand 2 character. We have assessed the effects of the atomic orbital basis set on the calculated absorption spectrum of [Cu(1)2]+ and show that a realistic spectrum is obtained by using a 6-311++G** basis set on all atoms, or 6-311++G** on copper and 6-31G* basis set on all other atoms; a smaller basis set on copper leads to unsatisfactory results. Electronic absorption spectra of six heteroleptic complexes have been predicted using TD-DFT calculations, and the transitions making up the dominant bands analysed in terms of the character of the HOMO–LUMO manifold. The calculational data reveal dominant phosphonate ligand character in the LUMO for the dye found to function most efficiently in practice, and also reveal that the orbital character in the HOMOs of the two most efficient dyes is dominated by the non-anchoring ligand 2, suggesting that ligand 2 enhances the performance of the sensitizer by minimizing back-migration of an electron from the semiconductor to the dye.
比较了[Cu(1)2][PF6]和[Cu(2)2][PF6](1 = 6,6′-二甲基-2,2′-联吡啶,2 = 6,6′-双2-[4-(N,N′-二苯基氨基)苯基]乙烯基}-2,2′-联吡啶)同孔铜(I)配合物的结构和性质、并采用在溶液中交换配体的策略制备了八种二氧化钛表面结合的异极配合物,这些配合物中的配体具有铋金属结合域和羧酸盐或膦酸盐锚定基团。2中扩展π-系统的存在大大提高了染料的性能,最有效的敏化剂是那些具有膦酸盐或苯基-4-羧酸盐锚定单元的配体;[Cu(2)2]+与膦酸盐锚定配体的组合具有非常好的性能(η = 2.35%,而在相同条件下标准染料 N719 的性能为 7.29%)。在[Cu(2)2]+的电子吸收光谱中,由配体跃迁产生的高能带在光谱中占主导地位,而[Cu(1)2]+的电子吸收光谱则同时显示出MLCT和配体π* ← π带。[Cu(1)2][PF6]和[Cu(2)2][PF6]都具有氧化还原活性;前者表现出以铜为中心的还原过程和以配体为基础的还原过程,而[Cu(2)2][PF6]只表现出以配体为基础的还原过程。TD-DFT 计算结果支持这些实验数据。计算结果预测,[Cu(1)2]+ 的电子吸收光谱具有一个 MLCT 带和基于配体的高能转变,而[Cu(2)2]+ 的电子吸收光谱则包括主要来自具有配体 2 特征的轨道的转变。我们评估了原子轨道基集对[Cu(1)2]+ 的计算吸收光谱的影响,结果表明,在所有原子上使用 6-311++G** 基集,或在铜上使用 6-311++G** 基集,在所有其他原子上使用 6-31G* 基集,可以得到逼真的光谱;而在铜上使用较小的基集则会导致不理想的结果。利用 TD-DFT 计算方法预测了六种异谱配合物的电子吸收光谱,并根据 HOMO-LUMO 流形的特性分析了构成主要波段的跃迁。计算数据显示,在实际应用中最有效的染料的 LUMO 中,膦酸配体的特性占主导地位,同时还显示,两种最有效的染料的 HOMO 中的轨道特性由非锚定配体 2 主导,这表明配体 2 通过最大限度地减少电子从半导体向染料的反向迁移,提高了敏化剂的性能。