Building Up The Conjugated Backbones Of Active Materials For Organic Solar Cells And Their Applications

Organic solar cells（OSCs）are a kind of green and clean photovoltaic energy source.In contrast to silican based photovoltaic technique,OSCs possessing many outstanding advantages including low cost,light weight,solution processing,flexibility and translucency,have received global research attention.The research of OSCs can be roughly divided into two stages.First is focusing on donor materials（small molecular donors and polymer donors）design and synthesis when fullerene derivatives dominate the acceptor materials.In the second stage,the research interest moves to the design and development of non-fullerene acceptors because of a large pool of donor materials available.The success in donor/acceptor active materials combined with device fabrication techniques and the use of carrier transport interface layers between the electrodes and active layers,drive the photon-conversion efficiency（PCE）of OSCs to exceed 17%.Although the PCEs can be further improved by optimizing bulk heterojunction（BHJ）blends of active layers by thermal and solvents annealing and the use of additives,the active layers obtained in such a manner are in their metastable state.The devices performance shall be greatly affected by the operating environments,which significantly affects the working stability of the device.Therefore,from the perspective of molecular engineering,the development of stable and efficient new donor/acceptor materials to achieve stable blended films is an important strategy for materials and devices research.The results of high performance as-casting BHJ films directly formated through non-additives and no post-treatment to obtain high-efficiency devices will benefit the reseach in other optoelectronic devices based on organic semiconductor materials.The continuous development and application of organic semiconductor materials still has a broad research space and organic solar cells are expected to achieve higher achievements in the improvement of PCE values.Besides the first chapter,major works described in this thesis include three parts:In the second part,this designed dibrominated a-IDT monomer was copolymerized with（DTBT）4,7-bis（4-hexylthiophen-2-yl）-2,1,3-benzothiadiazole as the acceptor unit through Stille coupling reaction to furnish an alternating D-A conjugated polymer Pa-IDTDTBT.Similarly,l-IDT unit and its D-A conjugated polymer Pl-DTDTBT were also synthesized for comparison.The geometric shape of l-IDT and a-IDT subunits have tremendous influence on the conjugated backbone curvature for Pl-IDTDTBT and Pa-IDTDTBT,mainly reflections in changes observed in their photoelectric properties,hole mobilities,film morphology and photovoltaic performance.BHJ PSCs device based on Pl-IDTDTBT/PC₇₁BM（1:3,w/w）yielded a PCE of 5.34%with a higher J_SCC of 10.82 mA cm^-2 and FF of 57.4%as a result of wider absorption,better hole mobility and the film-forming properties.With the same device fabrication condition,Pa-IDTDTBT/PC71BM（1:3,w/w）exhibited an inferior PCE of 3.64%with a high V_OCC of 1.02 V relative to those of Pa-IDTDTBT,which is one of the highest V_OCC values of IDT-based polymers.In the third part,using thiophene-benzene-thiophene（TBT）as a key parent structure,we designed and synthesized a class of polymer donor materials,where D-A units are connected by a single bond showing more flexibility.Three polymeric analogues,PTBT-C,PTBT-C8,and PTBT-C16 with different side chains and symmetry were synthesized and compared.The optical properties and thermal properties of the materials were characterized by UV-vis absorption,fluorescence,electrochemistry and TGA/DSC as well as theoretic simulations.The polymers were applied as donor materials with PC₇₁BM as the electron acceptor for photovoltaic devices.The polymer donor PTBT-C8 with a symmetrical C8 branch chain displayed a J_SC of 6.27 mA/cm²,a V_OC of 0.89 V,a fill factor FF of 61.77%,and a photoelectric conversion efficiency of 3.55%,priliminarily.In the fourth part,the polymer acceptor material PNDI20-TBT using alkyl-naphthalene diimide as electron accepting units and the TBTC8-C8 donor unit described in chapter 3 was synthesized.The physical and chemical properties of PNDI20-TBT was compared with the reported PNDI20-2T.The optical properties,electrochemical properties,thermodynamic properties and molecular configuration of the polymer were tested and characterized.PNDI20-TBT shows superoior strong and broad absorption in the film and has a very narrow optical band gap of 1.29eV.