Green-Solvent-Processed Conjugated Polymers Based Solar Cells

Over the past decade, bulk-heterojunction polymer based solar cells have attracted great attention because of their advantages and potential to enable fabrication of light-weight, low-cost and flexible devices through solution process. These years, through device optimization, materials design and interface modification, significant improvements have been made and the power conversion efficiency of polymer based solar cells has achieved over 10%. With the rapid progress of this field, the technology of polymer based solar cells is nearing the critical point of industrialization. From the perspective of making efficient polymer based solar cells in a safe and environmentally friendly way, the device fabrication processes involving harmful solvents should be avoid. Up to now, the most widely used and successful solvents in device fabrication of polymer based solar cells are aromatic, chlorinated solvents such as chloroform, chlorobenzene, and 1,2-dichlorobenzene. These solvents are unsafe and harmful to human health and the environment. Therefore, the industrialization of polymer based solar cells requires the design of new materials and the use of greener solvents in the device fabrication process. In this thesis, we designed and developed new polymer materials: conjugated polyelectrolytes and polymer nanoparticles which could be processed with nonaromatic and nonchlorinated solvents. Then we used these polymer materials as interfacial modification materials and donor materials in the polymer solar cells and polymer: inorganic nanoparticles hybrid solar cells. The introduction of these polymer materials suggests simple and universal methods towards using safe and environmentally friendly solvents in device fabrication. We foresee a bright future of green-solvent-processed polymer based solar cells.1. In chapter II, a new quaternized ammonium polyfluorene polyelectrolyte PFBTBr100 is designed and synthesized. This conjugated polyelectrolyte could be well dissolved in the green solvent methanol. We used PFBTBr100 as the cathode interfacial material of a polymer solar cell based on poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl C61-butyric acid methyl ester(PC61BM). Electrostatic force microscopy(EFM) measurements demonstrated that the PFBTBr100 formed a interfacial dipole layer between the active layer and the cathode. Atomic force microscopy(AFM) measurements of PFBTBr100 layers with varied concentrations showed that the morphology of the PFBTBr100 layer plays a direct, important role in the contact quality between the active layer and the PFBTBr100 interfacial layer, which can strongly affect the performance of devices. X-ray photoelectron spectroscopy measurements(XPS) indicated that PFBTBr100 prevented the diffusion of hot aluminum atoms into the active layer and served as a protective agent for the active layer. Under the illumination of AM 1.5G, 100 m W cm2, the power conversion efficiency(PCE) of the PSCs with the PFBTBr100 layer reached 3.9%, which is 1.6 times higher than that(2.4%) of the device without the PFBTBr100 layer.2. In chapter III, we designed and synthesized a new quaternized ammonium polyfluorene polyelectrolyte PFBTBr80. This conjugated polyelectrolyte is soluble in N,N-Dimethylformamide(DMF) which is a green solvent. We used PFBTBr80 as donor and aqueous Cd Te nanoparticles as acceptor, and fabricated the green-solvent-processed hybrid solar cells. Through the device optimization, we got the best device fabrication conditions. And then we investigated the influence of Cl activation on the device performance. As a result, we found that the Cl activation should be processed before annealing process. Comparing with the dipping and dropcasting method, Cl activation through spincoating and vapour method could reach higher performance. At last, we demonstrated that the traditional Cl source Cd Cl2 could replaced by a safer and cheaper Cl source Mg Cl2. The PFBTBr80:Cd Te nanoparticles based solar cells achieved a PCE of 6.01%.3. In chapter IV, we fabricated a novel aqueous polymer material P3 HT dots through a reprecipitation method. From the transmission electron microscopy(TEM), the P3 HT dots present an average diameter of 2.09 nm. We incorporated P3 HT dots as donor and aqueous Cd Te nanoparticles as acceptor, and fabricated the green-solvent-processed hybrid solar cells. We investigated the dependence of the device performance on the donor-acceptor ratio, and the optimized ratio is 1:24. The investigation of the thermal annealing conditions‘ influences on device performance showed the optimized annealing temperature is 265 °C, and the incorporation of P3 HT dots as donor materials successfully reduced the annealing time from 1 h to 10 min. In addition, the TEM and AFM measurements demonstrated that as the annealing time increased the size of the Cd Te NCs increased, and facilitated the formation of a smoother interpenetrating network in the active layer. Therefore, the more efficient charge separation and transport helped the P3 HT dots:Cd Te NCs solar cells achieved 4.32% PCE. The polymer dots and Cd Te NCs based aqueous-solution-processed HSCs provide an effective way to avoid a long-time thermal annealing process of the P3 HT dots:Cd Te NCs layer and largely broaden the donor materials for aqueous HSCs.In summary, we have designed and developed novel green solvent processable polymer materials: conjugated polyelectrolytes and conjugated polymer nanoparticles. We used these materials as interfacial materials and donor materials for polymer based solar cells. The green solvent processable polymer materials will bring the polymer based solar cells advantages of safe and environmentally friendly device fabrication process, and prospectively be the hot issue and trend in the polymer based solar cells research and industrialization.