合成含 Bithiazole 低能隙高分子及與氫鍵交聯體形成超分子網狀結構在太陽能高分子電池之應用

Abstract

Prime aim of this dissertation is to bring together the areas of low band-gap conjugated (LBG) polymers and their supramolecular networks complexed with π-conjugated cross-linkers for the applications of organic solar cells. These D-A conjugated polymers/their supramolecular networks possessed broad absorption sensitization in the region of 300-750 nm, having the lowest optical band gaps as low as 1.68 eV. Both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the LBG polymers/their supramolecular networks were within the desirable range of ideal energy levels. Hole and electron mobilities of these polymers/their supramolecular networks are in the range of 10 -6 -10 -8 cm 2 /Vs, which were calculated from the space-charge limited current experiments. Because of these properties, these were applied to polymer solar cell (PSC) as electron donors with (6,6)- phenyl-C 61 -butyric acid methyl ester (PC 61 BM) or (6,6)-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as an acceptor. First, a series of LBG donor-acceptor conjugated main-chain copolymers (P1-P4) containing planar 2,7-carbazole as electron donors and bithiazole units (4,4'-dihexyl-2,2'- bithiazole and 4,4'-dihexyl-5,5'-di(thiophen-2-yl)-2,2'-bithiazole) as electron acceptors were synthesized and studied for the applications in PSC. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic properties of these LBG copolymers were investigated. The photovoltaic device bearing an active layer of polymer blend P4:PC 71 BM (1:1.5 w/w) showed the best power conversion efficiency (PCE) value of 1.01% with a short circuit current density (J sc ) of 4.83 mA/cm 2 , a fill factor (FF) of 35%, and V oc = 0.60 V under 100 mW/cm 2 of AM 1.5 white-light illumination. Again, five LBG conjugated polymers (P1-P5) consisting of one dithieno[3,2-b:2’,3’-d]pyrroles (DTP) unit as an electron donor and various bithiazole units as electron acceptors were designed. The PSC device containing an active layer of P5:PCBM=1:1 exhibited a best PCE of 0.69%, with a V oc of 0.40 V, a J sc of 4.0 mA/cm 2 , and a FF of 43% under the illumination of AM 1.5, 100 mW/cm 2 . Furthermore, a conjugated main-chain copolymer (PBT) consisting of bithiazole, DTP, and pendent melamine units was synthesized by Stille polymerization, which can be hydrogern- bonded (H-bonded) with proper molar amounts of bi-functional π-conjugated cross-linker F (i.e., two uracil motifs covalently attached to a fluorene core through triple bonds symmetrically) to develop a novel supramolecular polymer network (PBT/F). The effects of multiple H-bonds on light harvesting capabilities, HOMO levels, and photovoltaic properties of polymer PBT and H- bonded polymer network PBT/F are investigated. The preliminary results show that the solar cell device containing 1:1 wt. ratio of PBT/F and PC 71 BM offers the best power conversion efficiency (PCE) value of 0.86% with a J sc of 4.97 mA/cm 2 , an V oc of 0.55 V, and FF of 31.5%. Besides, Stille polymerization was employed to synthesize another LBG conjugated main-chain polymer PBTH consisting of bithiazole, DTP, and pendent melamine derivatives. Novel supramolecular polymer networks PBTH/C and PBTH/F were developed by mixing proper molar amounts of polymer PBTH (containing melamine pendants) to be hydrogen-bonded (H- bonded) with complementary uracil-based conjugated cross-linkers C and F (i.e., containing two symmetrical uracil moieties connected with carbazole and fluorene units through triple bonds). The formation of multiple H-bonds between polymer PBTH and cross-linkers C or F was confirmed by FT-IR measurements. In contrast to polymer PBTH, the supramolecular design with multiple H-bonds can enhance the photovoltaic properties of PSC devices containing H- bonded polymer networks PBTH/C and PBTH/F by tuning their light harvesting capabilities, HOMO energy levels, and crystallinities. The PCE values of PSC devices containing supramolecular polymer networks PBTH/C and PBTH/F (as polymer:PC 71 BM=1:1 w/w) are found to be 0.97 and 0.68%, respectively, in contrast to 0.52% for polymer PBTH. The highest PCE value of 1.56% with J sc value of 7.16 mA/cm 2 , a V oc value of 0.60 V, and a FF of 0.36 was obtained in the PSC device containing supramolecular polymer networks PBTH/C as polymer:PC 71 BM=1:2 w/w.