Design、and Photovoltaic Performance Of Polymerized A-DA’D-A-type Small Molecule Acceptor Materials

Organic solar cell（OSC）is an environmentally-friendly renewable energy device that uses organic semiconductors as active layer materials to convert solar energy into electric energy.Compared with traditional silicon solar cells,it has the advantages of simple preparation,low-cost,low power consumption,solution processing,roll-to-roll printing,light-weight,semitransparent devices,and colorful,etc.Based on the above characteristics,OSCs have broad application prospects,such as integrated photovoltaic chargers for portable electronic products,indoor photovoltaics,agricultural photovoltaics,everything interconnection equipments,building-integrated photovoltaics（BIPV）,and other fields,which have attracted extensive attention and research.Considering that the practical application of OSCs needs to meet the three necessary requirements of high efficiency,high stability,and low-cost,it is necessary to design and synthesize highly efficient and stable organic photovoltaic materials to reduce the energy cost.With the continuous innovation of organic photovoltaic materials and the continuous optimization of processing technology,the power conversion efficiency（PCEs）of organic photovoltaic devices based on conjugated polymer donor（P_D）and non-fullerene small molecule acceptor（NFSMA）systems has exceeded 20%,meeting the requirement of commercial application.However,the poor operating stability in the environment limits its further development.Compared with this kind of system,all-polymer organic solar cells（all-PSCs）is an OSCs device made of P_D and polymer acceptor（P_A）materials,which has the advantages of excellent photostability and thermal stability and good mechanical strength,and is expected to be used in flexible PSCs first.To further reduce the energy cost of photovoltaic technology,development P_A and P_D materials with high efficiency,high stability and low-cost has become one of the research hotspots and difficulties in energy chemistry.Given the current development status of organic photovoltaic materials and devices,this paper focuses on the design and development of high-efficiency P_As with all-PSCs photovoltaic system as the research object,in which A-DA’D-A Y5derivatives is chosen as a conjugate skeleton.We synthesized a series of new near-infrared（NIR）P_As through end-group isomerization,central nuclear skeleton,side-chain engineering,new polymerization site strategy,respectively.In addition,we also systematically studied the effects of material molecular stacking,crystallization and aggregation behavior,and active layer morphology optimization on photovoltaic performance and stability of the device.The research contents are mainly divided into the following four parts.（1）Highly efficient and stable all-polymer solar cells enabled by near-infrared isomerized polymer acceptorsOne of the most promising approaches to realize high-performance all-PSCs is to develop near-infrared P_As with n-type fused-ring electron acceptors（FREAs）as the basic component.However,the effects of regioisomerized structures on the molecular and photovoltaic properties of the P_As have never been reported.In this work,we designed and synthesized three isomeric FREA-based polymer acceptors（namely,PYTT-1,PYTT-2,and PYTT-3）based on different isomeric thiophene-fused ending-groups,and systematically investigated the effects of the isomeric molecular geometry on the optoelectronic properties,charge transport,molecular aggregation packing order,and morphological properties.Matched with P_D PBDB-T,these all-PSCs achieved PCEs of over 12%,while the PBDB-T:PYTT-2 all-PSCs achieved an impressively high PCE of up to 14.32%due to the improved exciton dissociation and charge generation,more balanced charge transport properties,less non-radiative recombination loss,faster charge extraction,and optimized active layer morphology than PYTT-1 and PYTT-3systems.Importantly,the photostability and thermal stability of these three systems were also investigated,with the PYTT-2 system being the most stable one.Our results provide an effective method to develop FREA-based P_As by optimizing thiophene-fused end-groups,which can be used in the next generation of high-performance all-PSCs.（2）A near-infrared polymer acceptor enables over 15%efficiency for all-polymer solar cellsFinding effective molecular design strategies to achieve efficient charge generation,high charge transport,and low energy loss is a long-term challenge to develop high-performance all-PSCs.Here,we designed and synthesized a fused-aromatic-ring-constructed NIR polymer acceptor PYT-Tz with fused-ring benzotriazole（BTz）-based A-DA’D-A structure as electron-deficient-core,n-nonane as alkyl-side-chain,and thiophene asπ-bridge,and achieved a PCE of 15.10%for the all-PSCs with PYT-Tz as P_A and a medium-bandgap PBDB-T as the P_D.A control P_A PYT reported by our lab recently was introduced for investigating the synergistic effect of the electron-deficient-core and alkyl-side-chain on the optoelectronic properties and photovoltaic performance of the n-type P_As.Compared with PYT,the designed PYT-Tz exhibits intense and red-shifted absorption,upshifted energy levels,higher₀),and ordered molecular packing in the active layers,and,blended with PBDB-T,yields the efficient hole injection,ultrafast charge generation,and decreased non-radiative recombination loss of 0.17 e V.Of note is that the PCE of 15.10%is one of the highest PCE for an all-PSC reported to date.Importantly,the photostability,thermal stability,and mechanical property of these two systems were also investigated,with the PYT-Tz system being the more stable one.These results suggest that the PBDB-T:PYT-Tz photoactive layer not only improves photovoltaic performance but also has a synergistic benefit on solar cell stability.Our results indicated BTz based fused-aromatic-ring-constructed P_As are promising NIR acceptor in the all-PSCs.（3）Constructing a double-cable low-bandgap polymerized small molecule acceptors for high-performance all-polymer solar cellsFinding effective molecular design strategies to enable narrow-bandgap（NBG）n-type polymerized small molecular acceptors（PSMAs）with high electron mobility is a longstanding challenge for developing high-performance all-PSCs.Herein,we demonstrate a promising strategy to synthesize a graft-structural double-cable polythiophene-based conjugated P_A PT-YTz that employs a fused-ring benzotriazole（BTz）-based A-DA’D-A structure（YTz）as a pendant side unit.PT-YTz possesses an ultra-NBG of 1.31 e V and a high absorption coefficient(ε_max)of 1.39×10⁵ cm^-1.When blended with medium-bandgap polymer donor PM6（1:1.5,w:w）,annealed at 100℃for10 minutes and added with 1.5 vol%1-chloronaphthalene（CN）,the efficiency of the photovoltaic device reached 14.30%and a low energy loss of only 0.46 e V,which is the highest value of all-PSCs which based on double-cable polymer acceptor.Impressively,the PM6:PT-TYz all-polymer system also demonstrated slower light-induced degradation compared to the abovementioned control solar cells.Considering that the photophysical and electrical properties of the double-cable ultra-NBG polymer acceptors can be further tailored through molecular engineering on the D or A units of the NFSMA pendant as well as theπ-conjugated polymer backbones（including D and/or A units）,this new double-cable polymerization strategy can open the great possibility to develop a new family of efficient PSMA materials with ultra-narrow bandgaps and extend the current success of NFSMAs into other application areas.（4）A dimeric non-fullerene acceptor containing twin Y-series moiety and flexible aliphatic linker for high-performance and stable polymer solar cellsHigh photovoltaic performance and long-term operational stability are both crucial for meeting the requirements in practical applications of PSCs.Thus,effective strategies of molecular structure need to be developed to overcome the degradation of morphology while maintaining high performance.Herein,we designed and synthesized a novel double-decker geometry dimeric Y-series acceptor,DT19,featuring twin traditional Y-series molecule derivate as the conjugated building block and flexible aliphatic chain as linkers.Benefiting from increasing the molecular weight and enlarging the conjugated area by alkyl flexible linker,DT19 showed more ordered packing structure,and regulated molecular crystalline compared to its counterpart T19.By using a popular medium-bandgap polymer PM6,the corresponding device afforded an impressively high PCE up to 16.35%,with an open circuit voltage(V_OC)of 0.878 V,a short-circuit current(J_SC)of 25.03 m A cm^-2,and a high fill factor（FF）of 74.41%,which is superior to the device based on monomer T19（PCE=13.86%）due to the improved exciton dissociation and charge generation,more balanced charge-transport properties,less charge recombination,faster charge extraction,and optimized active layer morphology.Importantly,thermal degradation,as well as a slow light-induced degradation,were found for two systems,and the dimer DT19 system exhibited more stability.Meanwhile,the improved morphological features of the PM6:DT19 blend enabled higher mechanical stretchability and robustness with a COS of 7.21%and toughness of 0.69 J m^-3.Our results highlighted the promising design of the Y-series acceptor by the flexible linker strategy.