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PCowith the symmetrical structure were designed, synthesized, and characterized, which would be used as D-(A-π-A) 2 motif dye sensitizers. These complexes adopt Cd(II), Zn(II), Cu(II), and Co(II) complexes as auxiliary electron acceptors (A), thienylbenzo[1,2-b:4,5-b 0 ]dithiophene (BDTT) as electron donor (D), and the 8-quinolinol derivative as the π bridge and the acceptor (A) of D-(A-π-A) 2 motif dye sensitizers. The photophysical, electrochemical, and photovoltaic properties of these complexes have been investigated. The four complexes-based dye-sensitized solar cells (DSSCs) device exhibited a shortcircuit photocurrent density (J sc ) of 17.51, 17.02, 15.07, and 14.38 mA cm −2 and an attractive power conversion efficiency (η) of 8.82%, 8.32%, 8.03%, and 7.55%, respectively, under the AM 1.5 irradiation (100 mW cm −2 ). The photovoltaic conversion efficiency (PCE) and the short-circuit photocurrent density (J sc ) of BDTT-PCd, BDTT-PZn, BDTT-PCu, and BDTT-PCo are sequentially reduced; it attributed to the difference of the electron-withdrawing ability of the auxiliary acceptor resulted by the difference of coordination abilities of metal ions Cd(II), Zn(II), Cu(II), and Co(II) with ligand. These results provide strong evidence that D-(A-π-A) 2 motif complexes has great potential as promising photosensitizers in the applications of DSSCs. K E Y W O R D Sauxiliary acceptor, coordination metal (Cd, Zn, Cu, and Co), D-(A-π-A) 2 motif dye sensitizer, dye-sensitized solar cells | INTRODUCTIONSince O'Regan and Gratzel first proposed dye-sensitized solar cells (DSSCs) in 1991, [1] they have attracted extensive attention from researchers in recent years [2,3] due to their simple manufacturing, flexible structure, low cost, and high conversion efficiency compared with the traditional silicon-based solar cells. [4,5] Dye sensitizers, as the core part of DSSCs to complete the photoelectric conversion and the breakthrough point for the greatest

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Four novel polymeric metal complexes with a D-A-π-A motif, BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni, were designed, synthesized and characterized. These polymeric metal complexes were made up with Cd( ), Zn( ), Cu( ), Ni( ) complexes, thienylbenzo-[1,2-b:4,5-b'] dithiophene (BDTT) and the 8quinolinol derivative, which were used severally as dye sensitzers' auxiliary electron acceptors (A), electron donor (D) and π bridges as well as the acceptors (A). Under AM 1.5 irradiation (100 mW cm -2 ), the devices of dye sensitized solar cells (DSSC) based on four polymer complexes exhibited short-circuit photocurrent densities (J sc ) of 17.45 mA cm -2 , 14.75 mA cm -2 , 13.94 mA cm -2 , and 12.00 mA cm -2 , as well as attractive power conversion e ciencies (PCE) of were 9.73 %, 8.02 %, 6.82 % and 6.12 %, respectively. The photovoltaic conversion e ciency (PCE) and short-circuit photocurrent density (J sc ) of BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni decrease in order because the radius and charge number of the metal ion affect the strength of the coordination bond between the metal ion and the ligand. These results provides a new way of development for e cient and stable dye sensitizers in the future.

In the study to improve the light absorption range and intensity of dye sensitizers in the visible region and promote their photovoltaic performance, five novel polymeric metal complexes with sulfur coordination (BDTT-VBT-Ni, BDTT-VBT-Cu, BDTT-VBT-Zn, BDTT-VBT-Cd, and BDTT-VBT-Hg) to be used as D–A−π–A motif dye sensitizers were designed, synthesized, and characterized. In these polymeric metal complexes with sulfur coordination, the metal complexes with sulfur coordination of benzodithiophene derivatives are used as auxiliary electron acceptors, 8-quinolinol derivatives are used as π-bridge and electron acceptors, and thienylbenzene-[1,2-b:4,5-b′] dithiophene (BDTT) are used as electron donors. The effect of different metal complexes with sulfur coordination on the photovoltaic performance of dye sensitizers has been systematically studied. Under AM 1.5 irradiation (100 mW cm–2), the devices of dye-sensitized solar cells (DSSCs) based on five polymeric metal complexes with sulfur coordination exhibited a short-circuit current density (J sc) of 13.43, 15.07, 18.00, 18.99, and 20.78 mA cm–2, respectively, and their power conversion efficiencies (PCEs) were 7.10, 8.59, 10.68, 11.23, and 12.89%, respectively, and their thermal decomposition temperatures (T d) were 251, 257, 265, 276, and 277 °C, respectively. The result shows that the J sc and PCE of five polymeric metal complexes increase by degrees, and the PCE of BDTT-VBT-Hg is up to 12.89%, which is because of the strength of the coordination bonds between Ni­(II), Cu­(II), Zn­(II), Cd­(II), and Hg­(II) and sulfur increases in turn so that the electron-withdrawing ability and electron-transfer ability of auxiliary electron acceptors is enhanced. These results provide a new way to develop stable and efficient metal complexes with sulfur coordination dye sensitizers in the future.