Design,Synthesis And Photovoltaic Performance Of N-type Polymer Solar Cell Acceptor Materials

With the increasing global energy demand,organic photovoltaic technology has been favored by the scientific research community as an economical renewable clean energy.In recent years,all-polymer solar cells have attracted widespread attention and great research interest.All-polymer solar cells include p-type polymer donor materials and n-type polymer acceptor materials.It not only has adjustable structural characteristics,optical and electrochemical characteristics,but also has superior characteristics including long-term stability,ease of synthesis,and excellent film forming properties,which has great advantages in achieving large-scale production.In addition,compared to organic solar cells based on non-fullerene small molecules,all-polymer solar cells have better mechanical durability and stretchability,and have good development prospects.According to the latest report,breakthroughs in material design and device processing have successfully pushed the photoelectric conversion efficiency of all-polymer solar cells to more than 11%.However,the photoelectric conversion efficiency of most known all-polymer solar cells is still less than 8%.The main reason is that the research and development of polymer acceptor materials still have bottlenecks.The design of new polymer acceptor materials is the core approach to improve polymer solar photovoltaic efficiency.Based on the advantages of bromine atoms in small molecule acceptor materials,this subject has similar electron-withdrawing capabilities and non-covalent interactions with fluorine and chlorine atoms,and the bromine atom was introduced at the end group.Six new bromine substituted small molecule acceptor materials were designed and synthesized.And through bromofunctionalization,different electron-rich units are selected for coupling and polymerization with these small molecules.A series of D-A conjugated polymer acceptor materials were finally obtained through different polymerization conditions,and the photoelectric and photovoltaic properties were studied.The main research contents of this paper include:（1）Four small molecular materials with bromine atoms substituted,BTIC-2Br-m,BTIC-2Br-γ,BTIC12-2Br-m,and BTIC12-2Br-γwere designed and synthesized.Through core engineering research,the central fused ring structure was selected to be a five-membered ring,and introducing a long alkyl side chain into the molecules to improve the solubility of the materials.Then the thiophene（T）unit was chosen as the electron donor（D）unit,and four N-types polymer acceptor materials PTBT-m,PTBT-γ,PTBT12-m and PTBT12-γwere obtained through Stille polymerization.PBDB-TF:PTBT12-γand PBDB-TF:PTBT12-m device were fabricated with the donor material PBDB-TF,and the optimal device structure was PBDB-TF:PTBT12-γ,then PCE of 4.88%was obtained.（2）Design and synthesis of two fused ring small molecule acceptor materials with the core of trapezoidal electron-deficient units（dithienothiophene[3,2-b]-pyrrolobenzothiadiazole）and different substitution positions of bromine atoms,BTTIC-2Br-m and BTTIC-2Br-γ.The two small molecules were introduced into the polymer acceptors as electron-deficient（A）units.And fluorine-substituted benzodithiophene（BDT）,bidithiophene（TT）and thiophene（T）were introduced into the polymer acceptors as electron-rich（D）units.Through Stille polymerization,six N-type polymer acceptor materials PFBDT-BTTIC-m,PFBDT-BTTIC-γ,PTT-BTTIC-m,PTT-BTTIC-γ,PTBTT-m and PTBTT-γwere obtained.It was found that due to the fused ring structure of the seven-membered ring of the BTTIC unit,when polymerized with the BDT unit and the TT unit,the polymerization products that are too strong aggregation to be soluble in general solvents.So PFBDT-BTTIC-m,PFBDT-BTTIC-γ,PTT-BTTIC-m,and PTT-BTTIC-γcannot be made into effective devices,so only optoelectronic and photovoltaic properties of the two materials PTBTT-m and PTBTT-γare studied.The polymer donor material PBDB-TF,which matches most non-fullerene material with high efficiency,was selected to match the two polymer acceptor materials.Two device structures,PBDB-TF:PTBTT-m and PBDB-TF:PTBTT-γ,were fabricated with the polymer PBDB-TF,then the energy conversion efficiency（PCE）of 3.27%and 4.33%were obtained.The studies have shown that N-type polymer acceptors based on BTTIC series and BTIC series materials have strong light absorption in the near-infrared region,and both of them can obtain a high open-circuit voltage of0.94 V.At the same time,through the comparison of the above materials,the introduction of a monomer with a determined bromine atom position into the polymer will result in a polymer material with relatively better performance than the mixed isomeric monomer.However,due to the low J_sc and FF values of the obtained materials,the performance of the photoelectric conversion efficiency is not as good as expected.Polymerization of non-fullerene receptor small molecules has great development potential as a design strategy for N-type polymer receptor molecules.