Synthesis And Properties Of High Dielectric Constant Wide-Band Gap Conjugated Polymers With Alkyl-Sulfur Side Groups

After 30 years of development of polymer solar cells（PSCs）,especially the emergence of small molecule receptors in recent years,the current device efficiency has reached 20%.Recent studies have shown that current PSCs materials show higher exciton binding energy,carrier recombination rate and lower dielectric constant than inorganic or inorganic/organic hybrid semiconductors,and the dielectric constant is considered to be the decisive factor.With the increasing in the dielectric constant of the active layer,the binding energy of excitons,the energy of uniliteral-triplet states,the recombination energy,the bimolecular recombination and trap recombination,and the space charge effect could be reduced.Many methods have employed to improve the dielectric constant of copolymers,such as fluorine atom,cyanogroup,alkyl-sulfur chain,alkyl-oxygen chain,etc.,can effectively improve the dielectric constant.In this paper,the alkyl-sulfur chain modified benzo[1,2-c:4,5-c’]dithiophen-4,8-dione（SBDD）as acceptor and fluorinated or different alkyl-sulfur chain modified benzo[1,2-c:4,5-c’]dithiophene（BDT）as donor to construct a series of high dielectric constant wide band gap conjugated polymers,to study the effects of alkyl-sulfur chain and fluorine atoms on absorption,electrochemistry,dielectric constant and photovoltaic performance.The main contents are as follows:Two polymers PSBDB-TF and PSBDB-T are obtained by using the BDT which flanked the fluorinated and unfluorinated thiophene side chains as the donor unit and the acceptor SBDD.With the addition of fluorine,polymer PSBDB-TF shows lower HOMO energy level and better aggregation.At the same time,it is found that the polymer containing alkyl sulfur chain PSBDB-T can show higher dielectric constant,and the introduction of fluorine atom can further improve the dielectric constant of PSBDB-TF.The increase in dielectric constant also enhances exciton dissociation and reduce recombination of PSBDB-TF based device,which results in higher PCE.The PCE of PSBDB-TF based device can reach 8.11%,mainly due to the high V_OC and FF.while,the PCE of the unfluorinated polymer PSBDB-T is just 6.12%.The BDT units containing 0,2 and 4 substituted thiophene side chains were polymerized with SBDD to obtain three conjugated polymers PBDT-T-SBDD,PBDT-TS-SBDD and PBDT-TDS-SBDD containing 1,3 and 5 alkyl-sulfur chains on each repeating unit.With the addition of alkyl-sulfur chain,the absorption band edge show a blue-shifted,and the band gaps of the three polymers are 1.8 e V,1.82 e V and 1.85 e V,respectively.The HOMO energy level and the dielectric constant gradually increases.The optimal device based on PBDT-SBDD has the highest PCE of 7.45%,while the polymers PBDT-TS-SBDD and PBDT-TDS-SBDD only have the PCE of 5.63%and 2.80%.Although V_OC increases gradually with the increase of alkyl-sulfur chain,PBDT-T-SBDD-based devices show the best J_SC and FF.It mainly benefits from higher mobility and charge hole equilibrium transport,higher saturation current density and exciton dissociation,lower recombination.The SBDD electron-absorbing unit was introduced into the polymer PM6,and three ternary polymers PBDB-TF-S5,PBDB-TF-S10 and PBDB-TF-S20 were synthesized.The optical,electrochemical and dielectric constant based photovoltaic properties of the polymers were precisely adjusted to achieve better performance.With the increase of the ratio of SBDD units,the absorption of polymer gradually blue shift,HOMO energy level gradually decreases,dielectric constant gradually increases.The increase of dielectric constant can reduce the exciton binding energy and increase the V_OC of the device.All three polymer devices exhibit high V_OC of 0.862 V,0.867 V and 0.893 V,respectively.In addition,5%SBDD added to the polymer has little effect on mobility,dissociation and recombination.PBDB-TF-S5 showed the best performance in ternary polymer,with PCE of 14.64%,J_SC of 23.68 m A·cm^-2 and FF of71.74%.