The Design, Synthesis And Applications Of Novel Low Band-gap Donor Materials For Polymers Solar Cells

Since the 20 th century, human beings has experienced an unprecedented rapid development in many aspects, such as economy, science and technology, culture and so on. Behind each of these fields energy industry is the strong background. However, as the global petrochemical resources are limited, it is difficult to solve such a contradiction between supply and demand of human developing. So looking for new, non-polluting, renewable energy resources have become a worldwide issue. In many new types of renewable energy, solar energy with its inexhaustible feature has been considered to be the most promising new energy to solve the problem of energy crisis. Polymer solar cells(PSC) have attracted much attention, due to its low cost, roll-to-roll printing process, the easy fabrication on flexible, thin and large area substrates compared to silicon-based solar cells. Power conversion efficiency(PCE) of the PSC can exceed 10%, but there is still a long way to go for the commercial applications.Since naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole)(naphthothiadiazole or NT) was used as an electron acceptor in PSC, it has been attracted extensive attention due to its excellent photovoltaic property. The main task of this thesis is to expand the NT-based polymer system by means of chemical synthesis and then characterize the photovoltaic properties of the system in detail. The work could be divided into three sections, which include the modification of backbone structure, side chain structure and NT itself.In the second chapter, four donor-acceptor-type low-bandgap conjugated polymers based on a NT acceptor and different donors bridged by a bithiophene spacer have been designed and synthesized. Bulk-heterojunction-type polymer solar cells based on the new polymers were investigated and all of the devices exhibited good photovoltaic performance. The best device performance was achieved by PFC12 NT with a PCE of 6.51%, which was as we know the highest PCEs that contain a fluorene segment in the donor polymer. The results demonstrate that bithiophene spacer and NT acceptor are a promising combination, and provide a good reference to the polymer design.In the third chapter, we synthesized a series of random conjugated polymers based on benzo[1,2-b:4,5-b’]dithiophene(BDT), 2,1,3-benzothiadiazole(BT) and NT. We change the ratio of acceptor segments to investigate how NT segment will influence the power conversion efficiency of the resulting polymers when in different ratio. We find that as NT segment increased in polymer chains, the PCE of polymers firstly raise then decrease finally raise again, and reach the peak value when NT content is at 30%. Considering NT is a kind of high cost building block in PSC, it is of great importance for industrial production to reduce the amount of this material while maintaining high PCE.In the forth chapter, we synthesized a series of polymers with the same main chain structure and different alkyl side chain structure with the purpose of finding out the relationship between side chain and PCE. We find that the fine-tuning of alkyl structure could significantly influence the spectrum behavior and intermolecular aggregation ability of conjugated polymers, which ultimately reflects in the huge difference in PCE. These results provide valuable experience in how to choose alkyl side chain in polymer systems based on BDT and NT.In the fifth chapter, we change the side chain aromatic group of the BDT unit to decrease the HOMO energy level of the conjugated polymers. We find that HOMO energy level could be effectively lowered as the electron donating ability of side chain aromatic group decrease, thus leading to a higher Voc value of the resulting PSC.In the sixth chapter, a new acceptor unit of 2,3,8,9-tetrakis(3-alkoxy)phenyl)-6,12-di(thiophen-2-yl)-2,3,8,9-tetrahydroquinoxalino[6,5-f]quinoxaline(DTNQx) were designed and synthesized for constructing well-performing polymer solar cells. The quinoxalino[6,5-f]quinoxaline core is center symmetric structure with four fused aromatic ring just like NT unit, thus possess good molecular stacking ability. However, due to more reaction site on the unit, the solubility of DTNQx could be tuned readily. We copolymerized DTNQx with BDT unit with different side chains and characterized their photovoltaic properties. We find that these DTNQx based polymers show strong packing ability and good power conversion efficiency. This results demonstrate the potential of DTNQx-based copolymers in the PSCs.