Study On Device Structure And Maobi Coating Technique For Large-area Organic Solar Cells

This thesis focuses on printable large-area solution-based organic solar cells.Novel device architecture,solution coating techniques and device yield enhancing strategies are studied.Main results of the thesis are presented below:First,we have achieved high PCE?6.5%?in 10.5 cm² large-area flexible monolithic organic tandem solar cells.Solar cells with this architecture,using bottom metal electrode and top transparent electrode design,exhibit higher PCE and improved fabrication yield than solar cells with a single-junction architecture;thus demonstrating superior properties for area scale-up.Large-area?10.5 cm²?flexible tandem solar cells retain 82%of the PCE of small-area reference cells.We have also shown that undesirable shunt resistance effects are more likely to be mitigated using a tandem architecture approach.The increased number of sequential depositions of various organic layers in a tandem architecture providing that inherent defects in each layer are not registered in the vertical direction,increase the overall shunt resistance of the solar cell,hence yield and performance.This benefit becomes more important as the area of the cell is increased.In the current work,the 10.5 cm²-layers of the cells are prepared by spin coating.The spin coating technique will not be suitable for the fabrication when device area further increases.The advantage of the tandem structure of higher defect tolerance and being less susceptible to parasitic area scaling-up will make it more attractive when large-area cells are fabricated via slot-die coating or other coating techniques.Second,we demonstrate a novel solution-processed technique to fabricate organic solar cells and modules based on the“Maobi”tool?also called“Chinese ink brush”?inspired from old Chinese calligraphy tradition.Film thickness of Maobi-coated active layers and polymer electrodes can be tunable in a broad range from few tens of nanometers to one micrometer that meets the critical requirement on the film thickness for these layers to achieve optimized photovoltaic performance.Organic solar cells with Maobi-coated PBDB-T:ITIC layer exhibit power conversion efficiency of 10.1%,which is comparable to the devices with spin-coated active layer.Furthermore,after the optimization of the polymer electrode and adding the surface modifier into the active layer solution,we realized the organic solar cells via two-step Maobi coating and further solar modules.The demonstrated Maobi-coated solar modules containing 8 sub-cells exhibit an V_OC of 6.30 V and a high FF of 0.71.Large-area solar modules?18 cm²?fabricated via motor-driven,computer-controlled automatic Maobi coating display V_OC up to 11.6 V and efficiency of 6.3%.The coating technique based on the Maobi is low-cost,easy to pattern,large-area scalable and has high utilization rate of solutions.It is promising for the application of organic solar cells as well as other printed electronics.Third,we have reported a physical strategy to patch up the defects on the photo active layer to improve the device yield of organic solar cells for large-area production.Two factors are important to make the strategy effective:one is the patching tool of Maobi that has advantage of patterning to realize selective coating and ink holding;the other thing is the proper selection of patching materials.We have concluded the criteria of selecting the patching materials:electric insulativity,solution processability,solvent orthogonality to the active layer,good wetting on the active layer,chemical inactivity with the active layer and interfacial layer.PMMA and PVA has found to be very effective to act as the patching materials.PEI is effective for donor:fullerene-type active layer,but not for donor:ITIC-type active layer because of the chemical reaction between the PEI and the ITIC.By Maobi-coating a layer of insulating polymer?PVA or PMMA?on the defects area,the efficiency could be fully restored.At the end of the work,we demonstrate 52 cm² monolithic organic solar cells with the assistance of patching strategy.The strategy here is generic and can be applied to other materials as well as other types of devices.