Synthesis, Structures And Photovoltaic Properties Study Of Malononitrile Substituted Alkyl-quinacridone Compounds

Solar power has attracted much attention due to its pollution free, widespread and non-expensive characters. It may become one of the major ways for people to get energy in the near future. Nowadays, the commercialized solar cells are based on silicon material. Although such solar cell has the power conversion efficiency as high as 20%, the high cost and heavy work in producing silicon solar cell has made this technology difficult to popularize. Thus, the organic solar cell, which is cheap, less complicated and easy to carry with, has aroused great interest of the scientists. The most common and promising organic solar cell is a bulk-heterojunction type with P3HT as the donor and fullerene derivatives as the acceptor. Although fullerene and its derivatives are proved to be the most efficient acceptor for organic solar cell, their weak absorption in the solar spectrum has seriously limited the improvement of power conversion to solar energy. Then, it will be more meaningful to develop novel organic none-fullerene small molecule acceptors since they are more flexible in molecule modulation, synthesis and more possible to achieve proper energy level and improved absorption. Up to now, there have been some novel acceptor materials reported, however, the efficient one with power conversion efficiency more than 1% is still scarce. Thus, in this thesis, I have synthesized a series of novel organic non-fullerene acceptors by modulation to the typical dye material alkyl-quinacridone and characterized its photovoltaic properties. A careful study of the open circuit voltage was also carried out.1. In Chapter Two, we had designed and synthesized 8 malononitrile substituted alkyl-quinacridone compounds and studied their photophysical, electro-chemical and thermo-dynamical properties. A theoretical calculation to investigate their electron structure was also included. By introducing the electron drawing malononitrile groups, the absorption of alkyl-quinacridone has significantly red-shifted, which has testified the feasibility of obtaining narrow band gap material by enhancing the intramolecular charge transfer transition effect. In addition, these malonitrile substituted alkyl-quinacridone compounds have relatively good solvability, high thermo-stability, stable electro-chemical properties and proper energy level match to P3HT donor showing that they are good candidates for organic bulk-heteroj unction solar cell.2. In Chapter Three, the structure, packing styles and crystallization properties of malononitrile substituted alkyl-quinacridone were closely studied. It was found that the substitution by malononitrile has caused a large bend to the quinacridone molecule, which has significantly weakened the intermolecularπ...πinteractions and decreased the crystallization. Besides, the competition of alkyl-alkyl and other weak interactions are responsible for the 2-dimentional growth character and the amorphous cooled aggregates of some malononitrile substituted quinacridone compounds.3. In Chapter Four, the bulk hetero-junction solar cells using P3HT as the donor and molononitrile substituted alkyl-quinacridone as the acceptor were prepared and characterized. By using an acceptor with intense absorption in the visible spectrum, the solar spectrum response range of the former P3HT:PCBM device has been expanded from 650 nm to about 700 nm. One of the device using DCN-8CQA as the acceptor has achieved the short circuit current density of 5.73 mA/cm2 and the power conversion efficiency of 1.57% with external quantum efficiency more than 15% at wavelength region more than 650 nm. This has proved the feasibility of improving the performance of device by using acceptors that has intense absorption in visible spectrum. Moreover, the impact of alkyl chain length on the molecule aggregating behaviors, the morphology of the active layer and the corresponding performance of the device are also discussed. The conclusion we have is that a good-performed solar cell should have a moderate molecule aggregation in the active layer.4. In Chapter Five, we have synthesized a series of mono-malononitrile substituted alkyl-quinacridone compounds. Their thermo-dynamic, photophysical and photovoltaic properties were studied. By using mono-malononitrile substituted alkyl-quinacridone compounds, all solar cells fabricated have a prominent improved opencircuit voltage than the device using corresponding double-malononitrile substituted alkyl-quinacridone compounds. Notably, the device using SCN-8CQA has achieved the open circuit voltage as high as 0.66 V. By electro-chemical analysis and XPS method, we found that the large open circuit voltage of these devices not only originates from the higher LUMO level of the mono-malononitrile substituted alkyl-quinacridone compounds, but also originates from their stronger interaction with cathode Al and the existence of Al2O3 thin layer at the organic/electrode interface. This work has provided valuable support to the research and improvement of open circuit voltage of the organic solar cell.In conclusion, we have obtained a series of novel non-fullerene small molecule acceptor materials with narrow band gap, good thermo-stability, proper energy level matching and high power conversion efficiency. These materials are powerful proof for the strategy of using donor and acceptor co-absorption to improve the performance of the device. As a novel type of non-fullerene small molecule acceptor materials and semiconductors, they also have promising potential in application.