Random Terpolymers Based On Tetrafluorobenzene Unit For Efficient Non-fullerene Organic Solar Cells

Organic solar cells have the advantages of light weight,low cost,flexibility and can be made into translucent devices.The active layer plays an important role in the photoelectric conversion of organic solar cells.At present,the most widely used donor material in the active layer is the alternating polymer based on electron donor unit(D)and electron withdrawing unit(A).However,with the development of research,it has been found that the donor-acceptor units that can be used to construct D-A alternating copolymers are very limited.Therefore,researchers found that the third unit can be introduced into D-A alternating copolymers to form 1D2A or 1A2D random alternating copolymers.By controlling the amount of the third unit to adjust the energy level,optical absorption range,crystallinity and other physicochemical properties of the copolymer,excellent copolymer donor materials can be obtained.The introduction of the third component in the binary alternating copolymer system is easy to destroy the original planarity,which is not conducive to the charge transfer.In addition,random copolymerization enhances the disorder of polymer structure and reduces the overall crystallinity,which affects the morphology of active layer and device performance.Therefore,the purpose of this study is to accurately design and reasonably introduce the third component unit into the alternating copolymer,which not only retains the advantages of the original system,but also improves the physicochemical properties of the donor,so as to improve the energy conversion efficiency of organic solar cells.In this paper,three random copolymers PM6-L1,PM6-L2 and PM6-L3 were synthesized by random copolymerization using star donor PM6 as matrix and di(3-hexyl-thiophene)tetrafluoro-benzene units(2HTPF4)with different ratios.2HTPF4 unit has good planarity to ensure the planarity of random copolymers.In addition,fluorine atoms with strong electron absorption ability on 2HTPF4 unit can effectively reduce the HOMO energy level of copolymer donor materials,thus improving the open circuit voltage(V_oc)of organic solar cells.In addition,2HTPF4 unit has strong crystallinity.When 10%2HTPF4 unit is introduced into the copolymer,a better active layer morphology is formed.The filling factor(FF)of PM6-L2:Y6 blend device is significantly improved.Finally,the device efficiency based on PM6-L2:Y6 blend reaches 15.53%.The results show that the photovoltaic performance of organic solar cells can be effectively controlled by the ternary random copolymerization strategy.In order to study the effect of introducing the third component with different alkyl side chains on the molecular packing mode of the copolymer,the di(3-(2-ethylhexyl)-thiophene)tetrafluoro-benzene unit(2EHTPF4)was synthesized by substituting 2-ethylhexyl for the hexyl straight chain of thiophene in 2HTPF4 unit.A series of random copolymer alternates PM6-L4,PM6-L5 and PM6-L6 were synthesized by introducing 2EHTPF4 as the third unit into PM6.Through the study of the relationship between molecular structure and photoelectric properties,it is found that the alkyl chain changes the molecular stacking mode,increases the solubility of polymer materials,thus forming a better morphology of active layer and a balanced mobility.As a result,the devices based on PM6-L5:Y6 blends get higher short-circuit current(J_sc),V_oc and FF,which make the energy conversion efficiency reach 16.14%,which is significantly higher than that of PM6:Y6.Therefore,in the strategy of random copolymerization,the alkyl chain has a very obvious influence on the molecular packing,crystallinity and phase separation appearance of the ternary random copolymers,which provides a reference for the selection of the third unit in the strategy of random copolymerization.