Research On The Process Optimization And Micro-Morphology Of Small-molecule Organic Solar Cells

Organic photoelectric materials refer to organic materials that are rich in carbon atoms and have largeπconjugated systems so that they can convert solar energy into electricity through photovoltaic effect.And they can be divided into polymer and small molecular materials according to their molecular weight.Organic small molecule materials have been receiving widespread attention from researchers in the field of photovoltaic cells due to their unique advantages such as low cost of preparation,easy purification and synthesis,easy adjustment of energy levels,and high repeatability of the product.At this stage,the key research direction of organic small molecule solar cells is to greatly improve the consistency of organic small molecule materials in order to make the efficiency of organic small molecule solar cells can have commercial application prospects and then become a new generation of photovoltaic cell devices.At present,the main problems faced by improving the efficiency of organic small-molecule solar cells are:difficulties in controlling the micro-morphology of the active layer,because small molecules are easier to crystallize,resulting in the small molecular forming an over-sized donor region.As a result,both the current density J_SCC and the fill factor FF decrease.In the past 5 years,research on small molecule photovoltaic devices based on solution processing has developed rapidly,especially in this fabrication method,such as the introduction of thermal annealing（TA）,solvent annealing（SVA）,and combination of heat treatment and solvent treatment（TSA）.It can effectively control the micro-morphology of the active layer of organic small molecule solar cells to a large extent.In the work,the above-mentioned advanced technology preparation processes are applied to optimize the morphology of organic photovoltaic devices,and the differences in device performance under different device optimization processes are compared and the micro-morphlogy as the important role of performance improvement is also discussed.The main work of this article is as follows:On the basis of DRCN5T with five thiophene units,a series of isomeric fluorinated thiophene small molecule donor compounds were designed and synthesized,which were named with D5T2F-P,D5T2F-S and D5T2F-T.After thermal annealing,solvent annealing,and a combination of the above four systems were performed,the micromorphology of the active layer was characterized using a variety of characterization techniques in the research,such as atomic force microscope（AFM）,transmission electron microscope（TEM）and two-dimensional grazing incidence wide-angle X-ray scattering（GIWAXS）,etc.The results show that the device based on D5T2F-P exhibits the best performance,and D5T2F-P:IDIC-4F system shows the best surface roughness and the best distribution of the donor and acceptor phases,and the active layer forms a great interpenetrating network structure.In addition,through comparison,it was found that the organic solar cell（ASM-OSC）,a non-fluorinated analog DRCN5T manufactured under the same conditions,was optimized by a series of process optimization,micro-morphology control,and the performance was poor（the efficiency was 8.03%）.The influence of chain fluorination shows a 16%increase in energy conversion efficiency（PCE）by comparing the DRCN5T to the D5T2F-P-based devices.Based on DRCN7T with seven oligothiophene units,three materials substituted with different atoms（-H,-F,-Cl）were designed and synthesized,which were named with DRCN7T,2F7T and 2Cl7T.Solvent annealing and thermal annealing methods were applied to the optimization process of these devices,and they effectively controlled the micro-morphology of the organic active layer,so that the efficiency of the 2Cl7T:Y6 device reached 11.45%.At present,the energy conversion efficiency of11.45%is the highest efficiency of all small molecule non-fullerene organic solar cells based on oligothiophene reported in the literature so far.