Synthesis And Photovoltaic Properties Of Non-fullerene Acceptors With Drawing Electron Ending Groups Modification

Recently,the photoelectric conversion efficiency of solar cells based on NFAs has been increasing rapidly,depending on the design and synthesis of molecules.The best example is the system with D18 as donor and Y6 as acceptor,which was published by Ding Liming’s research group in Science Bulletin in January 2020.Its photoelectric conversion efficiency（PCE）reaches 18.22%.In order to further improve PCE of this donor-acceptor system,people are committed to develop more new donor and acceptor materials.As a device to convert solar light energy into electrical energy,its material should have the characteristics of wide absorption of solar spectrum,which is one of the key methods to improve the PCE of such solar cells.The relatively successful monocrystalline silicon solar cell has a wide spectrum absorption band,which can respond to 1100nm,making the photoelectric conversion efficiency more than 25%.Therefore,in the field of OSCs,the design,synthesis and application of narrow band gap optoelectronic materials are very important to further improve the efficiency.Although the efficiency is close to the maximum value of Shockley-Queisser theory,the energy loss of solar cell based on nonfullerene acceptor is huge（0.7-1.0eV）,which results in the actual open circuit voltage of the device is less than half of the optical band gap of the photoelectric material.In addition,people are committed to the synthesis of narrow band gap materials because the spectral absorption of such materials will be wider,which is conducive to improving the efficiency of devices.Therefore,the trend between molecular energy level and absorption spectrum makes it impossible for devices to obtain high open circuit voltage and short circuit current density at the same time.Therefore,people think of the medium band gap materials.On the one hand,the raised LUMO energy level by such materials will increase the open circuit voltage of devices.On the other hand,if the polymer donor materials are properly matched,the devices may also obtain high short circuit current density.Therefore,in this paper,we take this as the idea to carry out research.In the third chapter,we introduce the terminal group N-Ethyl thiobarbituric acid（TBA）,which has weak electron absorption ability,into the nonfullerene acceptors.One is to make the molecular absorption blue shift to match the narrow band gap polymer donor material.The other is to improve the molecular energy level to reduce the energy loss in the solar cell.In order to systematically verify the effect of structural changes on the performance of nonfullerene acceptors with TBA as the end group,we used TBA as the end group and connected different conjugate aromatic nucleus to obtain three mid-band nonfullerene electronic acceptors,named IDT-T,C₆IDT-T and IDTT-Th-T respectively.Due to the high molar extinction coefficient and exciton dissociation rate of C₆IDT-T,the device efficiency based on this material reaches 7.72%,which is higher than that of other two materials.It also shows that the change of side chain structure is very important for the improvement of the photoelectric performance of this kind of nonfullerene acceptor with TBA as the terminal group.In Chapter 4,i-CDT-CHO was synthesized by palladium catalyzed Stille-coupling reaction.And the three new non fullerene small molecule receptors i-CDT-IC,i-CDT-4F and i-CDT-CPT were designed and synthesized with IC,2F-IC,and CPT as end-capping groups respectively.The differences in non-fullerene acceptors based on different end-capping groups in terms of spectral absorption and energy levels were studied,the optimal ratio of donor to acceptors and the optimal annealing temperature for preparing devices were also systematically investigated.We have illustrated the availability of non-fullerene acceptors based on i-CDT-CHO.Due to the red spectral absorption and the highest exciton dissociation rate of i-CDT-4F,the optimized device based on PBDB-T:i-CDT-4F has a VOC of 0.75 and a JSC of 17.03mA/cm-2.FF is 49.8%,which gives the highest device efficiency of 6.36%of the three materials,which indicates that the use of 2FIC as a non-fullerene small molecule end-capping group is of great significance for improving its photovoltaic performance.