Synthesis And Characterization Of Low-band Gap Conjugated Polymers For Photovoltaic Cells

In recent years, the conjugated polymer solar cells have attracted much attention to academia and industry due to the advantages such as: low cost, light weight, easy fabrication by solution processing, suitable for making large area flexible devices. Over the past few years of constant efforts, research in the field of polymer solar cells have been made great progress, but if it would be widely used in the market, the lifetime, stability and power conversion efficiency of the polymer solar cells still to be considerably improved. As electron donor materials of conjugated polymer solar cells, the goal is to have wide spectrum and strong absorption in the visible region, high hole mobility, good solubility and easy processing performance, good film and thermal stability. Therefore, the synthesis of new narrow band gap conjugated polymers materials, which have narrower band gap, spectral response and solar radiation more match, higher charge carriers mobility, would have important significance to improve the power conversion efficiency of polymer solar cells.In this dissertation, three series of narrow band gap conjugated polymers were synthesized, and their structures and optical and electrical performance were characterized and researched, respectively.In chapter 3, the narrow band gap conjugated polymers PIF-DTP, PIF-DTTP, PIF-DPTP and PIF-DOTP were synthesized by Suzuki coupling reaction, which derived from 2,8-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2-yl)-6,6′,12,12′-tetraoctylindenofluorene (IFB), 5,7-bis(5-bromothien-2-yl)thieno[3,4-b]pyrazine (DBrDTP), 2,3-dimethyl-5,7-bis (5-bromothien-2-yl)thieno[3,4-b]pyrazine (DBrDTTP), 2,3-diphenyl-5,7-bis(5-bromothi-en-2 -yl)thieno[3,4-b]pyrazine (DBrDPTP) and 2,3-dioctyl-5,7-bis(5-bromothien-2-yl)thieno [3,4-b]pyrazine (DBrDOTP). The optical band gap (Eg) between the 1.61-1.78 eV in the copolymers, which showed the properties of high molecular weight, good solubility and thermal stability, soluble in common organic solvent such as chloroform, toluene, THF etc, with good processing film. Through the light under UV irradiation stability testing showed that no introducing functional groups or introducing alkyl in the 2,3-position of thieno[3,4-b]pyrazine will make its light stability down even was completely quenches, but by introducing the aromatic groups in the 2,3-position of thieno[3,4-b]pyrazine could increase the photochemical stability of such polymers. It indicated that copolymer PIF-DPTP can realize electron donor materials of D-A narrow band gap conjugated polymer in the organic photovoltaic solar cells.In chapter 4, the narrow band gap conjugated polymers named as PF-DDTT,PCZ-DDTT,PIF-DDTT and PICZ-DDTT were synthesized by Suzuki coupling reaction in the mono-microwave reaction system, which derived from 5,7-bis(4-octylthiophene-2-yl)thie- no[3,4-b]thiadiazole (DDTT) unit and 9,9-dioctyl-2,7-dibromofluorene (F), N-9′-heptadecanyl-2,7-dibromocarbazole (CZ), 6,6′,12,12′-tetraoctylindenofluorene (IF), 5,11-bis(9-heptadecanyl) indolo[3,2-b]carbazole (ICZ) unit. The copolymers present good solubility, soluble in common organic solvent such as chloroform, toluene, THF etc, with good processing film. On-set band gap of copolymer are extending to 880 nm, have better matching with near-earth solar spectrum. It has enormous development prospects of the application of electron donor materials in the organic photovoltaic solar cells. The performances of photovoltaic devices based on these materials are testing.In chapter 5, the narrow band gap conjugated polymers PICZ-DSeBT and PICZ-DTSeBT were synthesized by Suzuki coupling reaction in the mono-microwave reaction system, which derived from 3,9-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2-yl)-5,11-bis(9-hept-adecanyl)indolo[3,2-b]carbazole (ICZB), 4,7-bis(5-bromo-thien-2-yl)-2,1,3-benzoselenathiazole (DBrDSeBT) and 4,7-bis(5-bromo-thieno[3,2-b]thien-2-yl)-2,1,3-benzoselenadiazole (DBrDTSeBT). The optical band gap (Eg) are 1.66 eV and 1.59 eV in the solid film of copolymers PICZ-DSeBT and PICZ-DTSeBT. The photoluminescence (PL) maximum emission are 712 nm and 760 nm in the 1,2-dichlorobenzene solution for PICZ-DSeBT and PICZ-DTSeBT, the PL maximum emission red-shift to 755 nm and 790 nm in the solid film. Under an AM1.5 simulator (100 mW/cm~2), the photovoltaic devices based on the blends of PICZ-DSeBT/PC61BM (1:2) and PICZ-DTSeBT/PC61BM (1:2), show the power conversion efficiencies (PCEs) of 1.06% and 1.52%, with the open circuit voltage (Voc) of 0.75 V and 0.70 V, short circuit current densities (Jsc) of 3.45 mA/cm~2 and 5.30 mA/cm~2, fill factors (FF) of 0.41 and 0.41, and the photocurrent response on-set wavelength extending up to 760 nm and 800 nm, respectively. It indicates that PICZ-DSeBT and PICZ-DTSeBT are viable electron donor materials for photovoltaic solar cells.