Synthesis and Characterization of Isoindigo Based Copolymers and their Application in Bulk Heterojunction Polymer Solar Cells

• Main Authors: Song-Fu Liao, 廖崧甫
• Other Authors: Chi-Yang Chao
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/4h7y5d

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === Isoindigo is a lactam derivative with a large plane and excellent coplanarity. Due to its strong electron-deficient properties, isoindigo is used as an electron-acceptor unit (A) which were usually incorperated with electron-donors (D) such as thiophene derivatives to form donor-acceptor (D-A) type conjugated copolymers. Since this kind of isoindigo based D-A copolymers generally feature wide absorption ranges, low energy gaps and deeper highest occupied molecular orbital (HOMO) energy levels, they were used as polymer donor (p-type) materials and blended with suitable acceptor (n-type) materials such as fullerene derivatives to form photoactive layers, which were widely applied in the research of bulk-heterojunction polymer solar cells (BHJ PSCSs). This doctoral dissertation consists of four chapters. In the first chapter, we will introduce the working principle and application advantages of BHJ PSCs as well as the review and development status of isoindigo based copolymers for BHJ PSCSs in recent years. Subsequently, we will also demonstrate the required chemicals and solvents as well as the experimental instruments, measurement and analysis methods as well as the fabrication details of organic electronic devices such as polymer solar cell devices、hole-only SCLC devices and polymer thin film transistors (TFTs) in this chapter. In the second chapter, we synthesize P4TI-series copolymers based on isoindigo and different quaterthiophene derivatives. The central bithiophene part of the quaterthiophene moieties were modified with electron-withdrawing substituents such as fluorine atoms or cyano- groups, in which P4TIF has two fluorine atoms in the core bithiophene (3,3''-difluoro-2,2''-bithiophene) of the quaterthiophene block; P4TIN has two cyano groups in the core bithiophene (3,3''-dicyano-2,2''-bithiophene) of the quaterthiophene block (dicyanodithiophene, 3,3''-dicyano-2) 2''-bithiophene). P4TIH was used as a control, which core bithiophene of the quaterthiophene segment was not modified by any electron-withdrawing substituent. The results show that P4TIF has the best PSC performance. When the thickness of the P4TIF:PC61BM active layer reaches 250 nm, the power conversion efficiency (PCE) can reach 9.54%. It is worth mentioning that after introducing the strong cyano group, although the P4TIN:PC61BM only shows the best PCE= 8.36% (active layer thickness is 180 nm), but it exhibits a very good open circuit voltage (VOC) of 1.06 V. The design concept of P4TIN can be considered as a promising strategy for the development of PSCs with high VOC values. In the third chapter, we successfully synthesized PnTIF series copolymers which are containing the same isoindigo moiety as the acceptor unit and different length of oligothiophene donor units along their respective conjugated backbones. They are P2TIF, P4TIF*, and P6TIF copolymers. P2TIF was obtained by polymerization of isoindigo with 3,3''-difluoro-2,2''-bithiophene directly; unlike P2TIF, the chemical structure of P4TIF* and P6TIF features one and two 3-dodecyl thiophene spacers insreted between the isoindigo acceptor unit and the 3,3''-difluoro-2,2''-bithiophene donor unit, respectively, resulting that P4TIF* has an isoindole-quaterthiophene and P6TIF has an isoindigo-sexithiophene repeat unit along their copolymer backbones. When the introduced 3-dodecyl thiophene spacer is increased, physical properties such as light absorption, electrochemistry, thermal stability, crystallinity, and crystal orientation of the copolymers change a lot. The difference in physical properties actually affects the performance of the PSCs. As the results, P4TIF* with the moderate length of the isoindigo-quaterthiophene repeat unit, leading to its P4TIF*:PC61BM active layer shows the best PCE of 10.69% (the thickness of active layer is 300 nm). All of P4TI-series and P4TIF* and P6TIF copolymers can be considered as potential polymer donor materials for the large scale, industrial producing for the PSCs due to their relatively easy photoactive layer fabrication process features since the common complicate active layer fabrication method such as pre-heating of the blend solution, adding the solvent additives, and post-thermal annealing treatment were not required during their active layers fabrication processes. In the last chapter, we reviewed the opportunities and challenges encountered in this doctoral dissertation and presented the outlook for the future research works. The appendix is the identification data of the compounds, monomers and polymers synthesized in this dissertation, including 1H NMR, 13C NMR spectrums, and HR-MASS results.