All-solution Processed Non-fullerene Organic Solar Cells

Organic solar cells(OSCs)have significant potential for large-scale commercial applications due to their easy processing,mechanical flexibility,and tuning energy levels.The recent emergence of non-fullerene(NF)acceptors has significantly enhanced their power conversion efficiency(PCE).A PCE with 18%has been reported for single-junction NF OSCs.This exciting progress advances OSCs toward practical applications.But these high-performance NF OSCs are obtained with a spin-coated active layer,indium tin oxide(ITO)and vacuum-deposited electrodes in the laboratory.The process can not be compatible with large-scale production(such as roll-to-roll printing).All-solution processes become an ideal choice for large-scale production of OSCs due to their low cost and easy processing.But the introduction of high-efficiency non-fullerene active layers put high requirements in interface engineering.This thesis focuses on the preparation of all-solution processed NF OSCs.The preparation of active layer,solution-processed P-type doped polymer interface layer,and novel device structure are studied.The main results are shown as follows:(1)Deposit NF active layer by using water transfer printing method.Spin-coating is widely used to deposit active layers in the laboratory.But during this process,a large amount of solution will be wasted and some defects will be introduced.Novel fabrication methods to process the NF active layer are highly desirable.We study the fabrication of NF active layer films via a water transfer printing method.This method delivers a uniform film with controllable film thicknesses.NF active layers of PBDB-T:ITIC and PBDB-T-2F:IT-4F were fabricated via the method to validate its effectiveness.Solar cells with the water transfer-printed active layers show comparable performance(up to 11.7%)to the cells with spin-coated active layers.Furthermore,NF solar modules containing 4-sub cells with an active area of 3.2 cm² are also fabricated via the method.The module shows a V_OCof up to3.4 V and a power conversion efficiency of 8.1%with the PBDB-T-2F:IT-4F active layer.(2)Develop a novel doping method to prepare doped polymer films.The solution-processed interface layer plays a critical role in printable OSCs devices.The interface layer should meet the energy level,charge transfer and have good solution processability.Solution-processed hole transport layers(HTL)are more lacking compared with the solution-processed electron transport layers(ETL).We developed a novel doping method(Orthogonal Liquid-liquid Conduct doping,OLLC doping)for producing doped polymer.In this process,dopant and polymer semiconductors are dissolved in water and organic solvent,respectively,and electrical doping occurs during film formation at the orthogonal liquid-liquid(aqueous-organic)interface.A large free volume of polymer and dopant in their solutions enables enough diffusion for effective doping.Thanks to the high surface tension of water,nanometer-thick polymer films form spontaneously on the aqueous surface and simultaneously get doped.The doped thin polymer films on the aqueous surface can be easily transferred to the target substrates.(3)Fabricate all-solution processed flexible NF OSCs.Charge extraction and wetting are the issues persistent at the interface between the NF active layer and the printable top electrode(PEDOT:PSS).How to improve the contact between PEDOT:PSS and NF active layer is the key to achieve high-efficiency devices.So we introduced solution-processed doped polymer film to avoid the direct contact between PEDOT:PSS and NF active layer and improve the performance of the device.In addition,to further improve the PCE of all-solution processed NF OSCs,a higher efficient active layer system(PBDB-T-2F:Y6:IDIC)was adopted.Solution-processed H_xMo O₃ was introduced as HTL to facilitate hole collection in all-solution processed devices.Polyimide films embedded with silver nanowires were used as substrates to improve the yield and performance of the devices.Through the careful design and optimization of the device structure,we fabricated flexible NF OSCs that all of the layers in the devices from the bottom substrate to the top electrode are solution-processed.