| As the global energy demand continues to increase every year, the limiting supply of today's main energy sources (i.e. oil, coal, natural gas) and their detrimental long-term effects on the natural balance on our planet, force people develop some renewable energy sources. Harvesting energy directly from the sunlight using photovoltaic (PV) technology is being widely recognized as an essential component of future global energy production. Although the photovoltaic cells based on inorganic materials have been proved to convert sunlight to electricity efficiently, the high cost for manufacture limits them to be widely used. Polymer solar cells, based on polymer materials as active layer, possess the advantage of plenty of choices for active layer, low cost, flexibility and easily being large-scale etc., which attracts great attention in recent years.The photovoltaic performance of devices is strongly dependent on the band gap, energy levels, light harvesting, film forming ability, molecular weight, charge carrier mobility of donor and acceptor materials. In addition, the surface morphology of active composite layer and the interface modification between active layer and electrodes could also influence the power conversion efficiency of the solar cell.The thesis focus on how the above factors such as the band gap of materials, the surface morphology of active layer, the interface modificaton between active layer and electrodes affect the performance of solar cells. We used a low band gap donor-acceptor (D-A) conjugated polymer as donors and PC61BM as acceptor, meanwhile, we investigated the relation between the molecular structure and their device performance; we optimize the solar cell by interface modification, different acceptor material, solvents and blend ratios; We used the plasmonic silver nanodisks modificate the indium tin oxide (ITO) and fabricate the polymer solar cells. More details are now listed below.1. First of all, the important steps during the development history of polymer bulk-heterojunction (BHJ) solar cells, work principle, the device structure, characterization and related measure instruments are described, and we also reviewed the recent work focusing on synthesis of materials, device physics and related processing and so on.2. A new low band gap copolymer containing dialkylfluorene and 4,7-dithienyl-2,1,3-benzothiadiazole (TBT), poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PF-TBT) was synthesized by Heck cross-coupling polymerization. The copolymer exhibited broad absorption band with an absorption edge close to 700 nm and optical band gap of 1.82 eV. Cyclic voltammetric study indicated that the relatively low HOMO energy level assured a higher open circuit voltage (Voc) when PF-TBT is used as donor material in photovoltaic cell. The Bulk heterojunction (BHJ) solar cell by using PF-TBT as donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as acceptor with the structure of ITO/ [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)](PEDOT:PSS)/copolymer: PC61BM/LiF/Al exhibited a Voc of 0.86 V, short-circuit current (Jsc) of 3.97 mA/cm2, fill factor (FF) of 0.35, and power conversion efficiency (PCE) of 1.18% under one sun of AM 1.5 solar simulator illumination (100mW/cm2).3. Bulk heterojunction organic photovoltaic cells using blends of [poly(phenylenevinylene-alt-4,7-diphenyl-2,1,3-benzothiadiazole)] and [6.6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron donor and acceptor were fabricated and characterized. By optimizing the donor/acceptor ratio and the thickness of the active layer, the choice of solvent (1,2-dichlorobenzene, chloroform and chlorobenzene) and using the interface modification of ZnO nanoparticles, the solar cells with the structure of ITO/PEDOT:PSS/PP-DBT:PC71BM/ZnO/Al exhibited a Voc of 0.96 V, short-circuit current (Jsc) of 7.46 mA/cm2, fill factor (FF) of 0.50, and power conversion efficiency (PCE) of 3.36% under one sun of AM 1.5 solar simulator illumination (100mW/cm2).4. We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks self-assembled layer which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (silver nanodisks)/PEDOT:PSS/ poly(3-hexylthiophene)(P3HT):PC6iBM)/LiF/Al exhibited an open circuit voltage (Voc) of 0.61±0.01V, short-circuit current density (Jsc) of 9.24±0.09mAcm-2, fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM 1.5 G) irradiation (100mW/cm2). The PCE was increased from 2.72±0.08% to 3.46±0.07% as compared to the devices without silver nanodisks self-assembled layer, mainly resulting from the improved photocurrent density as a result of the excited localized surface plasmon resonance induced by the silver nanodisks.
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