Synthesis And Characterization Of High Dielectric Constant Dithienobenzodithiophene-based Polymers

Over the past few years,polymer solar cells（PSCs）have made promising developments,and the power conversion efficiencies（PCEs）of the best single-junction has exceeded 18%.In order to commercialize and give full utilization to the characteristics of light mass and flexibility of PSCs,there is still a need to continuously design and synthesize materials with superior performance and better stability.This work focuses on the study of active layer donor polymers,the derivatives of dithieno[2,3-d:2’,3’-d’]-benzo[1,2-b:4,5-b’]dithiophene（DTBDT）and benzo[1,2-b:4,5-b’]dithiophene（BDT）were selected as the electron donor units,which were combined with benzo[d][1,2,3]triazole（BTA）unit,benzo[c][1,2,5]thiadiazole（BT）based SBT unit and SFBT unit,as well as benzo[1,2-c:4,5-c’]dithiophene-4,8-dione（BDD）and other electron-deficient units were copolymerized to synthesize a series of high dielectric constant conjugated polymers.The dielectric constants of the organic conjugated polymers were improved by introducing special atoms,modifying side chains,and ternary copolymerization.The effects of dielectric constants on the PCEs and charge transport were initially investigated.In the first section of the work,DTBDT-TSn electron-donating unit was prepared by introducing chlorothiophene DTBDT into the side groups,and the electron-deficient SBT and SFBT units were respectively coupling polymerization to obtain two types conjugated polymers,PDBT-SBT,PDBT-SFBT.The highest occupied orbital（HOMO）levels of PDBT-SBT and PDBT-SFBT are-5.37 e V and-5.53 e V,respectively.The introduction of double fluorine atoms in the electron-deficient unit of SFBT makes the HOMO levels of the PDBT-SFBT deepened by 0.16 e V,thereby obtaining a higher open circuit voltage(V_OC)of0.85 e V.The dielectric constants of the two polymers are 3.0 and 3.8,respectively.The PDBT-SFBT exhibited more pronounced aggregations in the film state,which makes the charge separation and transfer readily.Therefore,when the annealing temperature is 120℃,when the ratio of PDBT-SFBT to Y6 acceptor is 1:1.2,the PCE is optimal.Because the higher V_OC is obtained,0.85 V;short-circuit current density(J_SC)is 25.33 m A·cm^-2;fill factor（FF）is58%,the PCE of the devices of PDBT-SFBT is better than PDBT-SBT,which is 12.60%.Side chain engineering is one of the very effective strategies in molecular design for modulating not only the solution processing properties,photoelectric properties and miscibility of polymers,but also the dielectric constants of polymers.In the second section of the work,an organic small molecule with high relative polarity and high dielectric constant was used to modify the side chain end group of the BTA unit.Side chain engineering is one of the most effective strategies in molecular design.The prepared monomer BTA-NO₂ and dialkylthienyl BDT were polymerized by coupling to synthesize two donor polymers,PBTA-C₁₀H₂₁ and PBTA-NO₂.The results show that when the frequency is 1 k Hz,the dielectric constants of PBTA-C₁₀H₂₁ and PBTA-NO₂ are 3.2 and 7.3,respectively.Compared with the alkyl side-chains PBTA-C₁₀H₂₁,the relative dielectric constant of PBTA-NO₂ is increased by about 2.2 times,due to the introduction of p-nitroanisole.The dielectric constant has been improved to a certain extent,but the numerical addition cannot be achieved.The absorption spectrum of the polymer PBTA-NO₂ is slightly blue-shifted.The HOMO level is reduced by 0.1 e V,-5.38 e V to-5.48 e V,and the V_OC of the device is increased 0.02 e V.Although,the device PCE of the two donor polymers is only 1.10%and3.60%,respectively.The polymer PBTA-NO₂ is used as the third component and added to the PM6/Y6 system of the battery device.When the addition content is 0.5%,due to good charge transfer and high J_SC,the device finally exhibits a maximum PCE of 16.50%.In the third section of the work,DTBDT side chain introduced 2-butyl n-octylthiophene side group,C12 alkyl chain to adjust the solubility of polymer.A wide band gap conjugated polymer P20 was obtained by coupling polymerization using DTBDT electron-rich unit and BDD electron-deficient unit.Due to the strong aggregations and high dielectric constant of polymer PBTA-NO₂.Therefore,the BTA-NO₂ unit was incorporated into the polymer P20,and two random polymers P20-5NO₂ and P20-10NO₂ were synthesized through a ternary copolymerization.The absorption spectra of the three polymers are roughly similar,but the aggregation of P20-5NO₂ and P20-10NO₂ in the film state is obviously enhanced.The dielectric constants of P20,P20-5NO₂,and P20-10NO₂ are 3.7,3.8,and 4.1,respectively.Compared with the polymer P20,the dielectric constant of P20-5NO₂ and P20-10NO₂ had been improved.The dielectric constant of the polymer is enhanced by a ternary copolymerization.The HOMO level of polymer P20 is-5.42 e V,and the HOMO levels of three polymers are basically similar.Under the optimal conditions,when the annealing temperature is 110℃,when the weight ratio of polymer to Y6 is 1:1,when 0.5%DIO is added,the device V_OC of the corresponding polymers P20,P20-5NO₂,and P20-10NO₂ are as follows 0.82 V,0.78 V,0.75 V,J_SC is 24.79,25.39,23.14 m A·cm^-2,FF is 67.10%,63.01%,59.10%,PCE is 13.45%,11.68%,10.36%,respectively.The polymer dielectric constant was increased using ternary copolymerization strategy,although the PCE of polymer P20-5NO₂,P20-10NO₂ devices did not improve,and probably the device preparation process needs to be further optimized.