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

主要的研究方向是利用共軛性高的分子，及超分子的作用力，連結低能隙的高分子 (LBG)，形成交聯的結構後，在有機太陽電池上的應用。這些 D-A 共軛高分子和其交聯的 結構，皆具有寬廣的吸收帶(300~750 nm)，其中最低光學能隙為 1.68 eV，且它們的 HOMO 和 LUMO 皆位於理想的能階，電子與空穴的遷移率在 10 -6 -10 -8 cm 2 /Vs，由於以上 之理想的條件，將之運用於有機太陽能電池，並以 PC 61 BM 作為電子受體。 首先，合成以 LBG 為骨幹的高分子，平面的 carbazole 為電子予體，bithiazole 為電 子受體的共軛高分子(P1-P4)，做光伏元件的研究。其中 bithiazole 對耐熱性、光學、電化 學、光伏性質隊主體結構的影響皆被研究。其光伏元件以 P4:PC 71 BM (1:1.5 w/w)得到最 佳效率(PCE) 為 1.01% 、短路電流(J sc ) 為 4.83 mA/cm 2 , 曲線因子(FF) 35%, 和開路電壓 ( V oc ) = 0.60 V。同樣地，包含 dithieno[3,2-b:2’,3’-d]pyrroles (DTP)為電子予體的高分子(P1- P5)，最佳光電轉換效率為 P4:PCBM=1:1，PCE=0.69% 、短路電流(J sc ) 為 4.0 mA/cm 2 , 曲 線因子(FF) 43%, 和開路電壓( V oc ) = 0.40 V。 另外，也合成包含 bithiazole, DTP, 和 pendent melamine 為主幹的共軛高分子(PBT)， 此系列高分子可以與具可配對氫鍵官能基的分子 F，進一步形成超分子的交聯結構 (PBT/F)。研究發現，多重氫鍵可以影響吸收光譜的範圍，HOMO 能階，和光伏的性質。 最佳光電轉換效率為 PBT/F : PC 71 BM =1:1，PCE=0.86% 、短路電流(J sc ) 為 4.97 mA/cm 2 , 曲線因子(FF) 31.5%, 和開路電壓( V oc )= 0.55 V。此外，進一步合成以 LBG 為主幹，包含 bithiazole、DTP、和 pendent melamine 的新穎高分子 PBTH，並以具可配對氫鍵官能基的 分子 F、C 形成交聯結構 PBTH/C 和 PBTH/F(C 和 F 分別為 carbazole and fluorene 並含 uracil 官能基)。其多重氫鍵的結構利用 FT-IR 去證明，實驗結果證明，以超分子作用力行 程共軛的交聯結構，可以增加可見光，改變 HOMO 和晶體結構，進而提升光伏效應。以 重量比 polymer:PC 71 BM=1:1，其 PBTH/C、PBTH/F 和 PBTH 的光電轉換效率分別為 0.97、0.68 和 0.52%，以重量比 PBTH/C:PC 71 BM=1:2，最佳光電轉換效率為 1.56% 、短 路電流(J sc ) 為 7.16 mA/cm 2 , 曲線因子(FF) 36%, 和開路電壓( V oc ) = 0.60 V。

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.