Abstract
Six new heteroleptic ruthenium(II) complexes (JM1–JM6), each bearing a highly π-conjugated bipyridine ancillary ligand (a methoxy-substituted analog (L1) and a phenanthroline-type anchoring ligand (L2) (dcphen or dcvphen; [Ru(L)2(NCS)2][TBA]2; L1 = 4,4′-bis{2-(3,4-dimethoxyphenyl)ethenyl}-2,2′-bipyridine (dmpbpy), 4,4′-bis{2-(1,1′-biphenyl)-4-ylethenyl}-2,2′-bipyridine (bpbpy), or 4,4′-bis{2-(4′-methoxy-[1,1′-biphenyl]-4-ylethenyl}-2,2′-bipyridine (mbpbpy); L2 = 4,7-dicarboxy-1,10-phenanthroline (dcphen) or 4,7-bis(E-carboxyvinyl)-1,10-phenanthroline (dcvphen)) were synthesized, and their physical and photovoltaic properties were investigated. Various dye-sensitized solar cells (DSSCs) were fabricated using heteroleptic ruthenium(II) complexes. Ruthenium(II) complex JM1, ligated to dmpbpy (ancillary) and dcphen (anchoring) ligands, exhibited the maximum power conversion efficiency (PCE) value of 3.40%, which was approximately 71% of the efficiency exhibited by the commercially available N719-sensitized solar cells. Ruthenium(II) complex JM5, ligated to mbpbpy (ancillary) and dcphen (anchoring) ligands, exhibited the second-best PCE value (2.52%), and ruthenium(II) complex JM3, ligated to bpbpy (ancillary) and dcphen (anchoring) ligands, exhibited a PCE value of 1.45%. It was observed that the PCE values of the DSSCs could be significantly improved by introducing the electron-donating methoxy group at proper positions of the ancillary ligands present in the heteroleptic ruthenium(II) complexes (such as JM1 and JM5).
Highlights
Dye-sensitized solar cells (DSSCs) that operate in the presence of sunlight are interesting next-generation energy devices as they are cost-effective and exhibit high-power efficiency [1]. e principles of green energy technology are followed to fabricate and operate DSSCs
Two phenanthroline-based anchoring ligands functionalized with carboxylic acid groups and three highly π-conjugated and/or methoxy-incorporated conjugated bipyridine ancillary ligands were prepared for the synthesis of the new heteroleptic ruthenium(II) complex sensitizers for the fabrication of DSSC materials (Scheme 1). e anchoring ligands, which would enable the efficient chemisorption of the sensitizers onto the mesoporous oxide, were synthesized following reported protocols (Scheme 1(a)) [16, 18]
4,4′-dimethylbipyridine was converted to the phosphonate-functionalized bipyridine intermediate 4,4′-bis-2,2′bipyridine (7). e compound could be used as a precursor of the ancillary ligands. e prepared compound was used as a substrate in the Wittig cross-coupling reaction and was reacted with the appropriate benzaldehyde derivative (8, 9, or 10) to synthesize the three essential ancillary ligands needed for our studies (Scheme 1(b))
Summary
Dye-sensitized solar cells (DSSCs) that operate in the presence of sunlight are interesting next-generation energy devices as they are cost-effective and exhibit high-power efficiency [1]. e principles of green energy technology are followed to fabricate and operate DSSCs. Dye-sensitized solar cells (DSSCs) that operate in the presence of sunlight are interesting next-generation energy devices as they are cost-effective and exhibit high-power efficiency [1]. Among the various metal-based systems used, systems containing ruthenium(II) complexes are studied extensively as they exhibit interesting electrochemical and photophysical properties. E most important component in a DSSC is the photosensitizer that helps improve the power conversion efficiency (PCE, η). It is important to have knowledge on molecular engineering to enhance the efficiency of the DSSCs fabricated using ruthenium(II) based dye-sensitizers. In the case of known ruthenium(II) complex sensitizers, carboxylic acid group-containing bipyridine N,Nligand, 4,4′-dicarboxy-2,2′-bipyridine (dcbpy) was mostly used as the anchoring ligand. E discovery of BD led to studies on various terpyridine-type anchoring ligands containing ruthenium(II) for the development of complex sensitizers. Several representative examples of terpyridinetype ligands studied previously, such as 4′-carboxy-2,2′: 6′,2′′-terpyridine (cterpy, 1) [8], 4′-(4-carboxyphenyl)-2,2′: 6′,2′′-terpyridine (cpterpy, 2) [9], 4′-(4-carboxyphenyleneethylene)-v-terpyridine (cpeterpy, 3) [10], 4′-(4-carboxyphenylene-ethylene-phenylene-ethylene)-v-terpyridine (cp epeterpy, 4) [10], 4′-thiophene-4,4′-dicarboxy-2,2′:6′,2′′terpyridine or 4′-(3,4-ethylenedeoxythiophene)-4,4′-dicar boxy-2,2′:6′,2′′-terpyridine (R-dcterpy, 5) [11, 12], and 2,6bis(4-carboxyquinolin-2-yl) terpyridine (cqterpy, 6) [13] are shown in Figure 1(b). e power conversion efficiencies of DSSCs fabricated using the reported systems were in the range of 2.2%–10%. e difficulty in derivatizing the ligand limits the synthetic applications of the BD-type sensitizers
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