Rational Materials Design,Device Fabrication And Morphology Optimization For Conjugated Polymer Based Solar Cells

In this thesis,a series of organic semiconducting conjugated polymer were designed and synthesized. We further investigate the correlation between molecular structure and optoelectronic properties. The applications of these novel conjugated polymers on organic photovoltaic(OPV) devices and organic field-effect transistors(OFETs) were investigated and expended, with the emphasis on bulk hetero-junction(BHJ) blend morphology. These results are summarized as follows:1. A series of donor-acceptor(D-A) conjugated polymer containing 2-D conjugated side-chain modified benzo[1,2-b:4,5-b′]dithiophene(BDT) have been synthesized and characterized. We revealed the power of molecular structure on D-A conjugated polymer to enhance light-harvesting, improve charge transport, match polymer-fullerene energy levels, optimize BHJ blend morphology and ultimately boost solar cell device performance. As a result of improved morphology and fine-tuned energy levels, enhanced power conversion efficiencies(PCEs) from 5.0% to 6.20% of 1.0 V are achieved using these new D-A copolymers, with a highest Voc of 1.00 V, which exhibits great potential for further application in highperformance polymer tandem solar cells.2. By adopting alkyl aromatic side-chains to both D/A monomers on the polymer backbone,a new D-A polymer PTP8 with enhanced packing order was synthesized and demonstrated a high PCE(6.18%) at a very low blend ratio of 1:0.5. The increased side-chain rigidity and bulkiness prevent the intercalation of fullerenes at a molecular level,further promote the polymer:fullerene demixing, leading to a large D/A ratio and highly ordered fullerene clusters. We further investigate polymer molecular weight, crystallinity and fullerene side-chain bulkiness on D/A blend ratio. Our findings provide opportunities for further development of stable and cost-effective polymer solar cells.3. We develop a series of efficient polymer-polymer BHJ blends. By introducing different numbers of alkyl aromatic side chains to the donor polymers, the performance of all-polymer solar cells was greatly improved and PCE of 4.35% was obtained for PTP8/N2200 blend. By modifying the crystallinity and intermolecular contacts of polymer acceptors,the FF and PCE of PTP8 based all-polymer solar cells can be further improved to over 6 % with a high Voc of 1.0 V. Through synergetic morphological investigation,our findings are particularly important when further considering correlates between molecular structure and property, device processing and long-term non-fullerene organic solar cells durability under various conditions.4. New generation D-A copolymer with a novel D1-A-D2-Aand D1-D2-A-D2 structure were designed and synthesized for exploring high performance organic optoelectronic devices(PCE=7.0-8.0%). The new structure with multiple building blocks can simultaneously meet the optical band gap, energy level, crystallinity, intermolecular packing and charge transport requirements for high-efficiency. The applications of these new generation materials on low energy loss solar cells,high stable OFETs and Sustainable-Processed organic solar cells were investigated and expended, which we hope will spark future materials design and device engineering to achieve higher efficiency by incorporating cost-effective and sustainably-processed materials and solvents into organic solar cells.5. We specifically designed a series of new polymers with the desired HOMO energy levels to further boost the efficiency of polymer/nanocrystal hybrid solar cells(HSCs). Optimized PCEs of 4.23% and 5.50% were achieved for polymer/PbS and polymer/PbSxSe1-x, respectively, which is the highest one up to date. More importantly,we systematically investigated the effect of polymer structures and quantum dot size on the device performance. The energy alignment between polymers and PbS QDs is considered critical for their charge transfer. The device PCE is also largely dependent on the polymer/NC blend morphology. Our findings can be beneficial to the future molecular design towards highly efficient polymer/nanocrystal HSCs.