Regulation Of Photovoltaic Performance Of D-A Alternating Copolymer Donor By Random Copolymerization

In recent years,organic solar cells(OSCs),because of their advantages of flexibility,light weight,low cost,and environmental friendliness,are more conducive to future commercialization and have become an area of widespread concern for researchers.After decades of development,the photoelectric conversion efficiency(PCE)of bulk heterojunction(BHJ)organic solar cells has exceeded 18%.The rapid development of the photoelectric conversion efficiency of organic solar cells is largely due to the continuous innovation and improvement of active layer materials.Therefore,designing and synthesizing new and efficient active layer materials is the core way to improve the photoelectric conversion efficiency of OSCs.For the design of polymer donor molecules,A common strategy to develop a new polymer donor is accomplished by construct D-A copolymer via alternating copolymerization of donor unit(D)and acceptor unit(A).In fact,the existing high-efficiency electron-donating units and electron-withdrawing units are very limited.Most of the time,only the side chains can be modified on existing high-efficiency D-A type alternating copolymers.The synthetic route is complicated and higher cost.random copolymer donors are achieved by introducing the third unit(D2 or A2)into the D-A binary copolymer.Therefore,by adjusting the structure and quantity of the third unit,the electronic energy levels,light absorption of copolymer donors can be easily modulated.Unfortunately,the device performance of the random copolymer donor is still lacking compared to the D-A alternating copolymer.This is mainly because the introduction of the third unit in the random copolymerization brings about the improvement of the photoelectric performance,and also has a lot of damage to the planarity of the polymer molecule,which is not conducive to the transfer of charge.How to balance the relationship between the improvement of the photoelectric performance brought by the introduction of the third unit in the random copolymerization and the destruction of the molecular structure becomes the key.The main content of this study is to rationally introduce a third unit in random copolymerization to improve the photovoltaic performance and molecular structure of alternating polymer donors.In the first work,We selected the highly crystalline and highly efficient Alkylsilyl functionalized copolymer J71 as the matrix,and introduced the benzo[1,2-c:4,5-c]dithiophene-4,8-dione(BDD)block with strong self-assembly and large absorption coefficient as the third unit,and synthesized two random copolymers with different proportions.The absorption coefficient of random copolymers becomes larger,and the periodic sequence of alternating copolymers is slightly reduced by random copolymerization,which promotes compatibility with receptors and reduces the phase separation scale reasonably.At the same time,the introduction of a small number of large conjugate plane BDD units increases the face-on orientation of the polymer molecules.As a result,the Jsc and FF of both random polymers have improved.Among them,PBDT-TZ-BDD-1/19:ITIC blended device efficiency reached 11.2%.In the second work,the open circuit voltage(Voc)of organic solar cells based on non-fullerene receptors IT-4F and Y6 is generally low.Through random copolymerization,different levels of chlorination with strong electron pull-on Benzo[1,2-c:4,5-c]dithiophene-4,8-dione(BDD-Cl)was introduced into the D-A type alternating copolymer PM6 as the third unit to synthesize two random ratios of random Copolymers PBDT-T-Cl-1/4 and PBDT-T-Cl-1/2.The BDD-Cl unit has a stronger electron affinity,which lowers the molecular energy level of the random copolymer and better matches the Y6 energy level of the acceptor.Moreover,the chlorine(Cl)atoms in the BDD-Cl unit help to enhance the light absorption of the random copolymer.At the same time,the Cl atom on the BDD-Cl unit and the(S)atom on the benzo[1,2-b:4,5-b]dithiophene(BDT)unit can form noncovalent interactions,the torsion angle between the two conjugated units is very small,thereby reducing the damage to the polymer molecule planarity caused by the introduction of the third unit.Among them,the introduction of 20%BDD-Cl unit random copolymer PBDT-T-Cl-1/4 and Y6 blended device Voc and Jsc improved,the device efficiency also increased to 15.63%.