Research On Structure Regulation And Photovoltaic Performance Of Furanbased Polymers

Bulk heterojunction organic solar cells（OSCs）have advantages of solution processability,flexibility,large-area fabrication,which are considered as one of important next-generation photovoltaic technologies.Up to date,the efficiencies of single junction organic solar cells have been up to 19%.However,organic solar cells still have some deficiencies.The highest power conversion efficiency is still lower than that of silicon-based and perovskite photovoltaic technologies.Moreover,high-performance materials are insufficient and highly efficient devices usually required complicated processes.Moreover,devices stability is insufficient.Organic photovoltaic materials are the critical factors of organic solar cells.Promoting synergetic development of photovoltaic donor and electron accepor materials is expected to fundamentally solve the problems of organic solar cells.Currently,high-performance polymer donors and electron acceptors mainly rely on thiophene derivatives such as benzo[1,2-b:4,5-b’]dithiophene（BDT）polymer donors and indaceno[1,2-b:5,6-b’]dithiophene（IDT）-based non-fullerene electron acceptors.However,thiophene reserves are insufficient,which is not benifical for mass production.In contrast,furan has enriched sources and its derivatives can be extracted from plants and some benzo[1,2-b:4,5-b’]difuran（BDF）polymer donors have displayed better photovoltaic performances.However,compared with thiophene derivatives,the development of furan-based photovoltaic materials largely lags behind.To promote the development of furan-based photovoltaic materials,this thesis use side-chain and main-chain engineering to design and synthesize a series of two-dimensional wide band-gap BDF polymer donors,and also designed and synthesized two donor（D）-acceptor（A）-donor（D）type indaceno[1,2-b:5,6-b’]difuran（IDF）non-fullerene electron acceptors through Knoevenagel reaction at room temperature.Their relationship between molecular structure and photovoltaic performances was explored.Fluorinated or chlorinated thiophene side-chain conjugated BDF as donor unit and bisfluorobenzotriazole as acceptor unit were used to design and synthesize two-dimensional D-A type wide band-gap BDF polymer donors PFTBDF-TBz（F10）and PCl TBDF-TBz（F11）.Compared with side-chain non-halogenated BDF polymer donor PTBDF-TBz,F10 and F11 obtained down-shifted energy level and improved molecular stacking.Compared with fluorine atom,chlorine atom is more effective in reducing energy levels and enhancing aggregation of BDF polymer due to heavy atom effect,empty 3d orbitals and larger atomic radius.When m-ITIC is used as electron acceptor,no solvent additives and solvent annealing are used,organic solar cells based on F10achieved higher open circuit voltage（0.908 V）and power conversion efficiency（10.5%）in comparison with PTBDF-TBz device,and the open circuit voltage and power conversion efficiency of F11 device further increased to 0.921 V and 11.37%,respectively.Among reported BDF polymer organic solar cells at that time,11.37%was among the highest values of power conversion efficiency.These results demonstrate that halogenation strategy can effectively down-shift the energy level of BDF polymer donors and improve power conversion efficiency,and also provide a reference for the choice of halogenation on BDF polymers.Based on the research on F10 and F11,we adopted well structured and stronger electronegative difluorobenzene side-chain conjugated BDF as donor unit and bisfluorobenzotriazole as acceptor unit to synthesize a two-dimensional D-A type wide band-gap BDF-based polymer donor PDi FPBDF-TBz（F13）.Benefiting from strong electronegativity of difluorobenzene side-chain,F13 obtained lower energy level and stronger aggregation.With Y6 as electron acceptor,the pristine unencapsulated F13device achieved a power conversion efficiency（12.71%）,open circuit voltage is 0.80 V and short-circuit current density is 22.61 m A/cm² and the fill factor is 70.28%.Benifical from stable morphology of F13:Y6 active layer and its strong interface effect with Mo O₃,the power conversion efficiency of unencapsulated F13 device after being aged in nitrogen interestingly increases to 13.34%,open circuit voltage increases to 0.815 V and short-circuit current density increases to 23.27 m A/cm² and fill factor is 70.37%.After aging in ambient air and nitrogen for about 1150 h and 1370 h,respectively,F13 device still maintain 92%of initial efficiency.Among the reported BDF polymer organic solar cells at that time,13.34%was among the highest values of power conversion efficiency and its corresponding energy loss（0.525 e V）was smong the smallest values.These results demonstrate that rational design on molecular can be efficient in developing low-cost,high-performance,improved stability of BDF polymer organic solar cells.Based on the research on F13,a two-dimensional wide band-gap BDF terpolymer donor F13-R was synthesized by introducing ethyl formate thiophene as a copolymer acceptor unit into F13 main-chain.Compared with F13,F13-R obtains slightly blue-shifted absorption spectra,down-shifted energy level and improved molecular stacking.When IT-4F is used as electron acceptor,the as-cast F13-R device achieved a power conversion efficiency of 12.2%,open circuit voltage is 0.904 V,short-circuit current density is 17.61 m A/cm²,fill factor is up to 76.60%.In contrast,the power conversion efficiency of F13 device fabricated from same process is only 4.02%,fill factor is only53.86%,and the corresponding open circuit voltage is 0.820 V,short-circuit current density is 9.11 m A/cm².The higher open circuit voltage of F13-R device is attributed to lower F13-R energy level,and its excellent power conversion efficiency and fill factor are mainly attributed to improved molecular stacking and more appropriate active layer morphology.The performance of as-cast F13-R device was among the most effecient reported organic solar cells and even outperform the BDT polymers analog as-cast organic solar cells at that time.These results demonstrate that ternary copolymerization can effectively adjust molecular structure and aggregation behavior to develop low-cost and high-performance BDF polymer as-cast organic solar cells.In order to promote the synergistic development of furan-based donors and electron acceptors,we designed and synthesized indaceno[1,2-b:5,6-b’]difuran（IDF）donor unit for the first time.Compared with IDT,IDF has stronger electron donating capablity,better planar conjugated structure and low-cost advantages.Its was successfully used to synthesize two novel A-D-A type IDF non-fullerene electron acceptors IDF-IC and IDF-4F with strong electron-deficient 1,1-dicyanomethylene-3-indanone（IC）and difluorinated IC（2F-IC）as end groups through Knoevenagel reaction at room temperature,respectively.Compared with IDT non-fullerene electron acceptor（IDT-IC）,IDF-IC and IDF-4F have stronger intramolecular charge transfer,narrower optical band gap and red-shifted absorption spectra.When PM6 is used as polymer donor and no solvent additives and solvent annealing are used,IDF-IC device achieves a power conversion efficiency of 7.80%,an open-circuit voltage of 0.905 V,and short-circuit current density of 14.55 m A/cm².Similarly,the power conversion efficiency IDF-4F device is 7.81%with decreased open circuit voltage（0.736 V）and increased short circuit current density（17.49 m A/cm²）.These results demonstrate that indaceno[1,2-b:5,6-b’]difuran has great potential in developing low-cost and high-performance non-fullerene acceptor organic solar cells.