Preparation And Properties Of Green Solvent Processable D-A Conjugated Polymers

Because of narrow-band gap,wide-range spectral absorption,superior light-harvesting capabilities and high carrier mobility,donor-acceptor(D-A)conjugated polymers have attracted much attention in the fields of organic photovoltaics including organic solar cells,electrochromic devices,organic electrochemical transistors,and organic field effect transistors.However,the highly conjugated backbone structure often leads to poor solubility of D-A conjugated polymers.Thus,these polymers are not easily available for basic research and application development by solvent processing methods(spin coating,scraping,spraying,etc.).In this context,"side chain engineering" of conjugated polymers with the purpose of increasing solubility is an effective way to solve this problem.By adopting this method,A large number of high-performance D-A conjugated polymers have been explored and applied,especially in the field of organic solar cells.Currently,the side chain of the high-performance D-A conjugated polymers reported are generally chosen to be non-polar alkyl chain.These conjugated polymers are usually soluble in high-boiling point halogenated solvents or non-halogenated aromatic solvents,thus their corresponding organic photovoltaic devices with high photoelectric conversion efficiency are obtained by solvent processing in halogenated solvents or non-halogenated aromatic solvents.In the past few years,along with the continuous deepening,the photovoltaic conversion efficiency of organic solar cells based on D-A conjugated polymers has been improving,with an increasing demand for large-scale industrial fabrication of organic solar cells.Using halogenated solvents or non-halogenated aromatic solvents for large-scale industrial fabrication of organic solar cells is harmful to human health and also a burden to the ecological environment.Moreover,the production and management of these hazardous solvents requires large cost,and the use of these solvents raises the manufacturing cost of the device.To address this challenge,scientists propose to enhance the solubility of polymers in greener polar or non-halogenated non-aromatic solvents by attaching high-polar functional groups or chain segment in the side chains of D-A conjugated polymers,thus realizing the ideal industrial manufacture for device fabrication with green solvents(water,alcohol,etc.).Currently,research work in this direction has been undertaken and has established as one of the critical research directions in this field.In this paper,the oligoethylene glycol(OEG)side chains with high polar and hydrophilicity are selected to functionalize high-performance donor-acceptor(benzodithiophene-isoindigo structured and naphthalenediimide-bithiophene structured)conjugated polymers.And a series of OEG-attached narrow-band-gap P type conjugated polymers(PBDT-ISO)and narrow-band-gap N type conjugated polymer(PNDI2020)is successfully prepared.After successful introduction of OEG side chains,P type PBDT-ISO could dissolve in green solvents,such as 2-methltetrahydrofuran,cyclopentyl methyl ether and limonene.Moreover,N type PNDI2020 could not only dissolve in the above green solvents,but also show satisfactory solubility properties in aqueous-alcoholic solvents.Further characterization tests revealed that both polymers show promising potential for green solvent processing.The research work of this paper mainly includes the following two parts:1.A novel series of benzodithiophene-isoindigo(BDT-ISO)structured P type conjugated polymers(PBDT-ISOs)containing different lengths of OEG side chains were designed and synthesized.All polymers were subjected to sufficient structural characterization and fundamental properties studies.In solubility study,OEG side chains were employed to impart sufficient solubility for PBDT-ISO in non-halogenated nonaromatic(2-methyltetrahydrofuran,limonene,cyclopentyl methyl ether,etc.)green solvents.Moreover,adjusting OEG side chain length could change the thermal,optical(spectral absorption range,absorption intensity,optical band gap,etc.)and electrochemical properties(HOMO/LUMO energy level,onset oxidation/reduction potential,etc.)of PBDT-ISO.It is particularly important for the application of PBDT-ISO in the field of organic photovoltaics,especially in organic solar cells.2.A novel naphthalenediimide-bithiophene(NDI-T2)structured N type conjugated polymer(PNDI2020)containing branched OEG side chains was designed and synthesized.The polymer was subjected to sufficient structural characterization and fundamental properties studies.The solubility test results showed that PNDI2020 exhibits satisfactory solubility in water/ethanol mixture,alongside with outstanding solubility in green solvents such as tetrahydrofuran,methyl tetrahydrofuran,cyclopentyl methyl ether and limonene.Basic optical experiments show that the steric effects of branched OEG side chains led to the change of structural flatness and intra-/inter-chain stacking state for PNDI2020 in the thin-film state.Therefore,PNDI2020 has a wide optical band gap and a deep HOMO/LUMO energy level.In summary,the work of this paper not only expands the range of "green solventprocessable OPV conjugated polymer materials",but also provides highly informative synthetic experiences and studies in the field of "material design and synthesis".