Design, Sythesis And Photovoltaic Properties Of Novel Donor-acceptor Conjugated Polymers

Conjugated polymers (CPs) are crucial to the photovoltaic properties of polymer solarcells (PSCs). Design and synthesis of new monomers may lead to polymers withtailor-made photo-physical and semiconducting properties for high performance solar cellapplications. In order to formulate the design rules for donor CPs in fullerene basedbulk-heterojunction devices and improve the performance of PSCs, it is needed to explorenovel conjugated backbone of CPs. In this paper, the novel conjugated polymers weredesigned and synthesized via the donor-acceptor (D-A) approach, and its photovoltaicproperties were also investigated. My work focus on the repeating unit of D A polymers.The isoindigo and its derivatives were used for electrondeficient A moieties,naphthodifurans (NDFs) were used for electron-rich D moieties. The works can be dividedinto four constituting components:1. The new N-acylated isoindigo units and its polymers were synthesized andcharacterized. The new polymers show broad absorption from450nm to900nm in thevisible and near infrared range. Due to the strong electron withdrawing characteristic ofN-acylated isoindigo, the polymers exhibit narrow optical bandgaps (1.31-1.38eV) anddeep HOMO levels compared with the polymers based on N-alkylated isoindigo. However,the LUMO level of the polymers reduced to-4.10eV or less, indicated that the N-acylatedisoindigo-based polymers were a kind of promising conjugated polymers for applying inn-type acceptors of PSCs.2. An isoindigo derivative (IBTI) was designed and synthesized, and the stericrepulsion in the structure of isoindigo unit was avoided. Due to extended conjugation ofIBTI, the IBTI spectrum was significant red-shifted and had much large extinctioncoefficient at its peak compare with isoindigo spectrum. The polymer PBDT-IBTI andPBDTT-IBTI based on the new building block showed absorption from470nm to700 nm, suitable HOMO/LUMO levels, and ideal optical bandgap. PSCs based onPBDTT-IBTI:(6,6)-phenyl-C61butyric acid methyl ester (PC61BM) blends achieved aPCE of3.94%. Bulk-heterojunction solar cells fabricated with PBDT-IBTI and PC60BMdisplayed power conversion efficiencies (PCE) of6.41%. This work has demonstrated thatthe IBTI is promising building block for organic photovoltaic polymers. This finding hasalso revealed the versatility of oxindole as basic building element for expanded isoindigolike conjugated monomer.3. Two different angular-shaped naphthodifurans (NDFs) and its polymers weresynthesized and characterized. The effect of molecular shapes of NDF building blocks onthe physicochemical properties of polymers and the relationship between molecularstructures and device performances were studied. The solution absorption spectrumPIDNDF1of showed relatively high molar extinction coefficients compared to thespectrum of PIDNDF2, the UV-vis absorption spectra of polymer PIDNDF1recorded indilute solution and as thin-film were both red-shifted compared with those of polymerPIDNDF2. The dihedral angle between naphthodifuran and isoindigo was26.89ofor themodel compound of PIDNDF2, which was almost twice that of the model compound ofPIDNDF1(14.61o). The HOMO levels of PIDNDF1and PIDNDF2were similar at-5.40eV. There was only slight difference in the LUMO levels of these two polymers. Therelatively high open circuit voltages were obtained for devices based on both PIDNDF1and PIDNDF2. The maximum PCE of PIDNDF1was1.83%which was more than twicethat of PIDNDF2.The large difference on the devices performance likely resulted from theshape of NDF repeating units and backbone geometry of polymers.4. A new naphthodifuran (NDFT) building block has been designed and synthesized.The distinguishing feature of the monomer is two dialkyl substituted thieno groups whichwere placed in the middle of the ring-fused structure as solubilizing groups. The stericinteractions between repeating units in polymer can be reduced by placing the solubilisinggroups in the middle of ring-fused structure. Conjugated polymers based on the new building blocks have been synthesized and characterized. Due to inherent characteristic ofacceptor moieties in D-A polyners, Polymers showed various optical properties andbandgap (1.32-2.10eV). LUMO levels of polymers vary with electrondeficient propertiesof acceptor moieties. All polymers have a deep HOMO energy level. The dihedral anglebetween NDFT and acceptors was＜1ofor the model compound of PNDFT-ID orPNDFT-DPP, indicated that the conjugated backbone reduce steric repulsion comparedwith those of polymer PIDNDF1(14.61o). A PSC based on PNDFT-ID showed apromising PCE of2.48%with a rather high open circuit voltage of0.87V, which washigher than that of PNDFT-DPP/PC71BM-based PSC (2.19%); however, PNDFT-DPPshowed rather high FF of60.9%. The inverted PSCs devices bsed on the PNDFT-DTBTas donor and PC61BM as acceptor achieved a PCE of5.22%. This is the highestperformance in solar cell devices among all polymers based on NDF building blocks.