Preparation And Photovoltaic Properties Of Fused-Ring Electron Acceptors Based On Fluorene

In recent years,non-fullerene small molecular acceptors（SMA）have achieved good results for polymer solar cells（PSCs）due to wide absorption range,clear structure,strong preparation controllability and tunable energy level.Therefore,it is favored in the field of organic solar cells.Non-fullerene（NF）acceptors can be arbitrarily divided into three constituting components of the conjugated backbone,the side chains and substituents,which are used to be engineered towards better performance.It is of great value to introduce heteroatoms into non-fullerene small molecules to tune both energetical and morphological issues of NF SMA.So far,the introduction of heteroatoms is mostly achieved by forming aromatic heterocycles（thiophenes,furans,etc）,or heteroatom-containing alkyl side chains.However,it has rarely been reported that introducing methoxy or methylthio groups on the conjugated backbone by using the differences on electronegativity and polarizability of O and S atom to study its influence on the photoelectric properties.In addition,constructing an A-D-A conjugated backbone,where A and D represent the electron-withdrawing and electron-donating units respectively,has been recognized as one of the most efficient strategies for the synthesis of SMAs.The conjugated plane with fluorene as central unit could be expanded by means of a multiple fused rings,which can enhance the planarity of resulted small molecules.The increase ofπ-πinteraction between ending groups is beneficial to the intermolecular charge transport,the effective absorption of the near-infrared（NIR）region of the SMAs,and the improvement of the molecular stacking state.The performance of the PSCs could thereafter be improved.Based on these ideas,the main research contents of this master thesis are as follows:（1）.Two novel non-fullerene small molecule acceptors,DICTF-OMe and DICTF-SMe,were prepared by introducing methoxy and methylthio substituents at the 3 and 6 positions of fluorene,respectively.When the polymer PBDB-T is used as the electron donor,the PBDB-T:DICTF-OMe devices with the maximum PCE of3.36%showed a Jsc of 8.73 mA cm^-2.The PBDB-T:DICTF-SMe devices had a maximum PCE of 4.68%with the Jsc of 9.26 mA cm^-2.We revealed the reasons for the low short-circuit current of the device from the view point of the efficiency of exciton dissociation,carrier transport and recombination.It was shown that the leakage currents of the two devices were serious,resulting in poor generation efficiency of photo-generated charge carriers.The charge carrier mobilities were rather low.Obvious mone-molecule and bimolecular recombination behaviors were observed for the two devices,which greatly reduced the carrier transport and collection efficiency,and ultimately led to low device performance.（2）.A ladder-type nonacyclic arene（bis（thieno[3,2-b]thieno）cyclopentafluorene（BTTF））has been designed and synthesized through fusing thienothiophenes with the fluorene core from the synthon of dimethyl 9,9-dioctyl-2,7-bis（thieno[3,2-b]thiophen-2-yl）fluorene-3,6-dicarboxylate.With BTTF as the central donor unit,a novel acceptor-donor-acceptor（A-D-A）type non-fullerene small molecule acceptor（BTTFIC）was prepared with 1,1-dicyanomethylene-3-indanones（IC）as the peripheral acceptor units.BTTFIC exhibits broad and strong light absorption,and presents a narrow optical band gap of 1.58 eV.Encouragingly,polymer solar cells based on the blends of BTTFIC with the representative wide and low band gap polymer donors（PBDB-T,1.82 eV.PTB7-Th,1.58 eV）offer power conversion efficiencies over 8%（8.78±0.18%for PBDB-T:BTTFIC and 8.18±0.29%for PTB7-Th:BTTFIC）,respectively.The tests show higher carrier mobility,more balanced electron and hole transport.The results show the advantages of ladder-type BTTF units in the preparation of high-performance non-fullerene acceptors.