Hussain Maqbool, Islam Ashraful, Bedja Idriss, Gupta Ravindra Kumar, Han Liyuan, El-Shafei Ahmed
Polymer and Color Chemistry Program, North Carolina State University, Raleigh, NC 27695, USA.
Phys Chem Chem Phys. 2014 Jul 28;16(28):14874-81. doi: 10.1039/c4cp00907j.
Four novel Ru(II) bipyridyl complexes MH12–15 were synthesized and characterized for dye-sensitized solar cells (DSSCs). Their photovoltaic performance including incident photon-to-current conversion efficiency (IPCE), total solar-to-power conversion efficiency (η%) and ground and excited state oxidation potentials and photoelectrochemical properties were evaluated on mesoporous nanocrystalline TiO2 and compared with the benchmark N719-dye under the same experimental conditions. MH12–15 showed stronger MLCT with significantly higher molar extinction coefficient for the lower energy absorption bands at 553 nm (27,500 M(−1) cm(−1)), 554 nm (34,605 M(−1) cm(−1)), 577 nm (23,300 M(−1) cm(−1)), and 582 nm (39,000 M(−1) cm(−1)), respectively, than that of N719 (14,200 M(−1) cm(−1)). The introduction of a cyclometallated ligand in dyes MH14 and 15 improved the optical properties and red-shifts of 24 nm and 28 nm, respectively, compared to the non-cyclometallated analogs MH12 and 13. The red shift in the UV-Vis spectra of MH14 and 15 can be attributed to the destabilization of the HOMO t2g of Ru(II). However, the destabilization of the HOMO furnished an upward shift of the ground state oxidation potentials (GSOPs) of MH14 and 15 at −5.44 eV and −5.36 eV against vacuum, respectively, which resulted in a driving force of only 0.22 and 0.16 eV for regeneration of dyes MH14 and 15, respectively. In the case of NCS analogs, MH12 and 13, the GSOPs, however, were −5.56 and −5.51 eV, respectively, which produced a driving force of more than 0.25 eV for dye regeneration. The nanosecond transient absorbance measurements showed that the time needed for the oxidized forms of MH12–MH15 to regenerate the neutral dye is 6 μs, 4 μs, 13 μs and 18 μs, respectively, compared to N719 (2.3 μs). These kinetic data confirmed that the weak thermodynamic force, small negative free energy (−ΔG), for regeneration of MH14 and 15 neutral dyes makes the dye regeneration process kinetically sluggish, which contributed significantly to the loss of both photocurrent and photovoltage. This study clearly elucidated that although cyclometallation may produce significantly better light harvesting, the driving force of less than 0.25 eV is not sufficiently enough for effective dye regeneration.
合成了四种新型钌（II）联吡啶配合物MH12 - 15，并对其用于染料敏化太阳能电池（DSSC）进行了表征。在介孔纳米晶二氧化钛上评估了它们的光伏性能，包括入射光子到电流转换效率（IPCE）、总太阳能到电能转换效率（η%）以及基态和激发态氧化电位和光电化学性质，并在相同实验条件下与基准N719染料进行了比较。MH12 - 15表现出更强的金属到配体电荷转移（MLCT），在553 nm（27,500 M⁻¹ cm⁻¹）、554 nm（34,605 M⁻¹ cm⁻¹）、577 nm（23,300 M⁻¹ cm⁻¹）和582 nm（39,000 M⁻¹ cm⁻¹）处的低能量吸收带的摩尔消光系数分别显著高于N719（14,200 M⁻¹ cm⁻¹）。与非环金属化类似物MH12和13相比，染料MH14和15中环金属化配体的引入分别改善了光学性质并产生了24 nm和28 nm的红移。MH14和15的紫外 - 可见光谱中的红移可归因于Ru（II）的最高占据分子轨道（HOMO）t2g的不稳定。然而，HOMO的不稳定分别使MH14和15的基态氧化电位（GSOPs）相对于真空向上移动至 - 5.44 eV和 - 5.36 eV，这导致染料MH14和15再生的驱动力分别仅为0.22 eV和0.16 eV。对于NCS类似物MH12和13，GSOPs分别为 - 5.56 eV和 - 5.51 eV，这产生了超过0.25 eV的染料再生驱动力。纳秒瞬态吸收测量表明，与N719（2.3 μs）相比，MH12 - MH15的氧化形式再生中性染料所需的时间分别为6 μs、4 μs、13 μs和18 μs。这些动力学数据证实，MH14和15中性染料再生的弱热力学力、小的负自由能（ - ΔG）使染料再生过程在动力学上缓慢，这对光电流和光电压的损失有显著贡献。这项研究清楚地表明，尽管环金属化可能产生明显更好的光捕获，但小于0.25 eV的驱动力不足以实现有效的染料再生。
