Fused Ring Electron Acceptors With Spiro-Side Chains For Organic Solar Cells:Design,Synthesis And Properties

Bulk heterojunction（BHJ）organic solar cells,as a device that converts solar energy into electric energy,have emerged as a hot research field in photovoltaic techniques because of their low cost,semi-transparent,and solution processibility on flexible substrates.Photo-active materials are the most important components in organic solar cells.Active layer materials can principally be divided into donor materials and acceptor materials.Numerous donor materials have become developed based on a D-A intermolecular charge transfer（ICT）concept to achieve narrow bandgap absorption during the past decades.Before 2015,fullerene-based acceptors dominated the reseach of acceptors to match high performance donor materials.However,the development of fullerene based acceptor lags behind because of intrinsic drawback,i.e.,limited energy level adjustment and unstable morphology,while the non-fullerene small molecule acceptors received extensive attentions because of their tunable energy structures,ease of synthesize and high performance reproducibility,etc.The current photoelectric conversion efficiency of organic solar cells based on non-fullerene receptors has exceeded 18%,which has huge commercial application prospects,and the development of new and efficient non-fullerene receptors materials are the driving force to promote the development of solar cells.Based on this,this article designed and synthesized three non-fullerene small molecule receptors,and studied the relationship between their structures and properties.The specific content is as follows:In the second chapter,the Spiro[4H-cyclopenta[2,1-b:3,4-b’]dithiophene-4,9’-[9H]fluorene]is used as the core of the three-dimensional spiro structure.This three-dimensional core can effectively inhibit the self-aggregation of small molecule receptors,and BT was used the bridging unit,PM158 as the end-capping group,and then bisalkoxybenzene was introduced to increase the solubility,and designs non-fullerene small molecules Acceptor Spiro-BTCN,this material is simple to separate and purify,has high thermal stability,and the thermal decomposition temperature reaches 369°C.Then the obtained product is combined with the commercial polymer donor material PM6 to fabricate a photovoltaic device（ITO/Zn O/PM6:Spiro-BTCN/Mo O₃/Ag）the photo-electric conversion efficiency can reach 5.87%after annealing,in which the open circuit voltage(V_oc)was0.82 V,the short-circuit current density(J_sc)was 15.64 m A/cm²,and the fill factor（FF）was 0.46.In Chapter 3,the Spiro[fluorene-9,9’-xanthene],which can be synthesized by the"one-pot method",can be synthesized in one step,avoiding the complex synthesis steps in the structure of the spiro core in Chapter 2.It also uses BT as the bridging unit,PM158 as the end-capping group,and the designed molecule is named as SFX-BTCN non-fullerene small Molecular acceptor,this material is simple to synthesize,and the photovoltaic device fabricated with PM6（ITO/Zn O/PM6:SFX-BTCN/Mo O₃/Ag）has a photoelectric conversion efficiency of 7.60%when the mass ratio of the donor/acceptor is 1:2,where the open circuit voltage(V_oc)is 1.00V,the short-circuit current density(J_sc)is 14.43 m A/cm²,and the fill factor（FF）is 0.53.Based on the designed molecule in chapter 3,we introduced a alkylthiophene（act as a bridging unit）between the spiro ring core and the bridging unit BT,and designed the non-fullerene small molecule receptor SFX-S-BTCN,it shows high thermal stability,with a high thermal decomposition temperature around 370℃.Notably,the introduction of alkylthiophene extended conjugation,which effectively adjusts the absorption properties,From UV-vis spectrum analysis,we observed the maximum absorption peak show a red shift～41 nm compared to SFX-BTCN,which is 570 nm,also the band gap of the molecule was reduced.The designed photovoltaic device（ITO/Zn O/PM6:SFX-S-BTCN/Mo O₃/Ag）achieve a 8.1%photoelectric conversion efficiency when the donor/acceptor mass ratio at 1:1.5,also it shows a open circuit voltage(V_oc)around 1.00 V,short-circuit current density(J_sc)around 14.5 m A/cm²,and fill factor（FF）0.56.