Preparation Of High-performance Organic Solar Cells Based On Solution Sequential Deposition

Organic solar cells(OSCs)have many unique advantages,such as flexibility,light-weight,high optical transparency,and solution-processed,attracting extensive attention from academic and industrial fields.Due to the improvement of the structure of OSCs and new materials,the power conversion efficiency(PCE)of OSCs has been increased quickly,from the earliest D/A planar heterojunction solar cells with an efficiency of less than 1%to the current single-junction bulk heterojunction(BHJ)solar cells with an efficiency of more than 19%.However,the BHJ structure exit some inevitable shortcomings,such as complex device optimization,difficult morphology control,which is hard to control the interconnection pathways to form efficient hole and electron transport within the donor and acceptor materials to their respective collector electrodes.In this thesis,we focuse on the preparation of Layer-by-Layer(Lb L)OSCs using a solution sequential deposition method that can simplify the device process and obtain the ideal morphology at the same time.Each layer can be independently optimized according to the different properties of the donor and the acceptor,and the two components are ideally distributed in the vertical direction.More importantly,the Lb L structure is not sensitive to the ratio of donor to acceptor,solvent concentration,etc.,which simplifies the optimization process of the device.All works in this thesis can be divided into the following four parts:In Chapter two,we used non-orthogonal solvents to prepare binary Lb L OSCs based on conjugated polymers donor with strong-temperature-dependent aggregation(STDA)property,which are difficult to be dissolved by the common solvents.So,during the depositon process,the solvent did not dissolve the polymer film but provide a swelling effect that creates free volume in the film,which then allows the acceptors to diffuse into the polymer film.Based on the Pff BT4T-2OD:IEICO-4F and PBDB-T-SF:IT-4F binary STDA systems,the performance of Lb L OSCs is better than BHJ OSCs.It can be explained that the Lb L active layer forms an ideal morphology,that is,a vertical interpenetrating network structure in which the donor and the acceptor are enriched at their respective ends,which can form the interconnection pathways to form efficient hole and electron transport within the donor and acceptor materials to their respective collector electrodes.And this structure can maintain the crystallinity of the donor and the acceptor,respectively,so that the free charges can be transferred to the electrode faster,reducing the recombination in the devices.Our work demonstrates that STDA polymer donor is a general strategy for preparing Lb L OSCs with ideal morphologies.In Chapter three,we used non-orthogonal solvents to prepare binary Lb L OSCs based on conjugated polymer donor with weak-temperature-dependent aggregation(WTDA)property,which can be dissolved in common solvents at room temperature.The STDA-polymer need to be prepared at higher temperatures,it is not good for facbricating larger area OSCs,and most of the excellent OSCs are based on WTDA donor.In this chapter,based on the PM6:Y6 binary WTDA system,we uesd low-boiling solvent chloroform to fabricate Lb L OSCs to avoid the lower layer of donor being washed by the upper layer of solvent.Finally,we successfully prepared an excellent Lb L OSC with more than 16%PCE.In addition,we based on the PM6:BTP-e C9 binary WTDA system,we used high-boiling solvent chlorobenzene,also obtained a good Lb L OSC.This work demonstrates a general strategy for the fabrication of Lb L OSCs by sequential deposition:regardless of the type of the underlying donor materials and the upper acceptor solvents,as long as the thickness of the underlying donor layer is sufficient to be washed by the upper solvent,it can be successful to fabricate Lb L OSCs.This general strategy provides a good foundation for blade-coating or module fabrication and industrial production of large-area OSCs.In Chapter four,we combined sequential deposition and ternary blending strategies to prepare ternary Lb L OSCs with controllable morphology.By pre-mixing the two acceptors,the third component is controlled in the acceptor layer,and a new LUMO energy level between the two acceptors is formed through strong electronic coupling inside the acceptor,which is more favorable for charge transfer between the donor and acceptor.Since the position of the third component is controlled,the PCE of the Lb L ternary OSCs is less sensitive to the content of the third component than the BHJ blended ternary OSCs.In addition to better utilization of the advantages of the third component,it also helps to reduce the cost of high-efficiency systems OSCs.Based on the Pff BT4T-2OD:IEICO-4F:FBR and PM6:Y6:FBR ternary systems,efficiencies over 11%and 16%were achieved,respectively.Our work demonstrates that the sequential deposition of ternary Lb L devices is an effective strategy to control the morphology of the ternary active layer and improve the efficiency of OSCs,which can provide guidance for the development of more efficient ternary OSCs.In Chapter five,we developed a versatile method of interfacial modification Ni O_x with a SAM,i.e.,2PACz.The SAM-modification can not only increase the work function of Ni O_xfilm to attain favorable band alignment for efficient hole extraction,but also subdue charge recombination loss by passivating the defect states of Ni Ox film.Finally,the use of the SAM-modification on Ni O_x successfully improve the PCE from 15.3%to 16.7%in the conventional OSCs based on PM6:Y6,which is the highest record for OSCs with Ni O_x as HTLs to date.In the meantime,owing to more robust chemisorption between 2PACz and Ni O_x,the Ni O_x/SAM-based cells exhibit overwhelmingly higher reproducibility than those with 2PACz directly-modified ITO.Importantly,the Ni O_x/SAM-based cells are superior to the PEDOT:PSS-based cells in both efficiency and stabilities due to hydrophobic and nonacid nature as well as enhanced hole-extraction and electron-blocking ability.Our results testify that phosphate-functionalized Ni O_x films can be a good candidate for replacing the conventional HTLs of PEDOT:PSS in OSCs with excellent efficiency and longevity.Finally,we used Ni O_x/SAM as HTL to fabricate Lb L OSCs,we achieved a more stable and better performance OSCs than using PEDOT:PSS as HTL.