Novel Polythiophene Derivatives with Terthiophene-vinylene (TTV) Conjugated Side Chains: Synthesis, Properties and Their Application in Photovoltaics

• Main Authors: Chuen-Yo Hsiow, 蕭全佑
• Other Authors: 邱文英
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/t3yw4v

博士 === 國立臺灣大學 === 高分子科學與工程學研究所 === 103 === In this thesis, a series of novel polythiophenes with terthiophene-vinylene moieties as conjugated side-chains were designed and synthesized by Stille polycondensation. The correlations between the chemical structure and properties of such polymers and their effect on the performance of solar cells were intensively investigated. In the first part, two isomeric polythiophenes with terthiophene-vinylene (TTV) as conjugated side-chains, PBTTTV-h and PBTTTV-v, were designed, synthesized, and characterized. Although both polymers show low-lying HOMO energy levels and good solubility, the architecture of conjugated side chain significantly affects the optical, electrochemical, molecular self-assembling and photovoltaic properties. The PBTTTV-h displays two distinct absorption peaks in UV-Vis spectrum and a crystal polymer as verified by XRD; on the other hand, the PBTTTV-v displays a single broad absorption peak and is an amorphous material. Moreover, the BHJ solar cell fabricated from PBTTTV-h/PC71BM exhibited a higher power conversion efficiency (PCE) of 4.75 % under AM1.5G illumination at 100 mWcm-2 than PBTTTV-v/PC71BM of 4.00 %. Based on the results of the previous part that the two-dimensional polythiophene with parallel TTV conjugated side chains has good crystallinity, we further varied the anchoring site of alkyl groups on the conjugated side chain and substitute fused thiophenes in different sizes, TT and DTT, for BT as a spacer to control the spatial distance between bulky terthiophene-bearing monomers. The results clearly indicate the length of π-spacer is a crucial factor to the crystallinity of these 2D-polythiophenes as well as the anchoring site of alkyl chains and the coplanirity of main chain apparently influence the degree of molecular packing and the d-spacing of crystallites. Under AM1.5G illumination at 100 mWcm-2, the cell based on PBTTTV-h2/PC61BM exhibits the best PCE of 4.62%. In the third part, the 2D conjugated copolymers, PBTTTV-h-r-PBTDBT, containing 12% and 23% of 2, 1, 3-benzothiadiazole (BTD) were synthesized by Stille random-copolymerization and their optical, electrochemical, charge transport and photovoltaic properties were investigated. Both copolymers show a broader absorption spectrum, covering the spectral range from 300 nm up to 750 nm, and a reduced optical bandgap of 1.68 eV. The low content of BTD unit in PBTTTV-h-r-PBTDBT allowed preservation of many important properties of PBTTTV-h, including low-lying HOMO and high crystallinity. Under AM1.5G illumination at 100 mWcm-2, the polymer solar cell based on PBTTTV-h-r-PBTDBT-12%/PC71BM exhibits the best PCE of 5.31% with an open-circuit voltage (Voc) of 780 mV, a short-circuit current density (Jsc ) of 10.82 mAcm2, and a fill factor (FF) of 62.92 %. The IPCE measurements revealed a strong photo response from the random copolymers up to 750 nm, with IPCE values above 40% from 300 nm to 650 nm for PBTTTV-h-r-PBTDBT. These results indicate that the incorporation of acceptor units into a 2D-polymer by random copolymerization provide a simple and effective route toward polymers with a broad photocurrent response in PSCs. In last part, two new copolymers were designed, synthesized, characterized and applied in polymer solar cells (PSCs) as donor materials. Copolymer PBTTTVDK attaching two electron-withdrawing heptanoyl groups at TTV conjugated side chains showed a deeper HOMO energy level, better solubility, and red-shifted absorption of conjugated side chains. By introduction of strong electron-withdrawing BTD unit, PBTTTVDK-r-BTD possessed further red-shifted absorption band with absorption onset at 743 nm and effectively improved the coplanity of polymer backbone and ability of self-assembling to enhance carrier mobility. The bulk-heterojunction PSCs based on PBTTTVDK/PC61BM and PBTTTVDK-r-BTD/PC61BM displayed PCE of 1.52% and 4.17%, respectively, under the illumination of AM1.5G, 100 mW cm-2. We have successfully designed, synthesized and characterized a family of polythiophenes with a terthiophene-vinylene (TTV) as conjugated side-chains and applied them in PSC device. The achievement of structure-property correlations will be benefit of developing high performance 2D-conjugated polymeric semiconductor materials.