Design,Synthesis And Application Of Electron-deficient Core-based Small Molecule Hole Transport Materials

The organic-inorganic hybrid perovskite solar cells(PSCs)have attracted wide interests due to the rapidly increased efficiency and facile device fabrication.Hole transport materials(HTMs)play an important role in transport holes,blocking carrier composite and protecting perovskite layer as an important part of PSCs.And its performance has a great influence on the device efficiency and stability of PSCs.To date,the most widely used HTMs in PSCs is 2,2?,7,7?-tetrakis(N,N?-di-p-methoxyphenylamine)-9,9?-spirobifluorene(Spiro-OMe TAD),however,the complex synthetic routes and high cost remain a major obstacle to commercialization and practical applications of PSCs.Therefore,it is still necessary to develop low-cost and efficient new HTMs.When designing HTMs,electron-donating groups such as triphenylamine,biphenyl and carbazole are commonly used as core units,while HTMs with electron-withdrawing groups as the centre are rare.The electron-deficient group can increase the polarity of the molecule and enhance the interaction between the molecules,thereby promoting the charge transfer between the molecules and improving the hole mobility.When designing HTMs,electron withdrawing groups can be used to increase molecular polarity and improve hole mobility.We note that at present,most of the reported work focused on exploiting new materials for HTMs,yet,a single component might be di ficult to meet various requirements.To obtain HTMs with comprehensive functions,in addition to continually designing new structures,the development of blended HTMs might be a promising strategy.Blending different materials as HTMs may play the effect of“one plus one greater than two”,and the two materials could be complementary of each other with regard to their functions.Therefore,in this thesis,we have designed and synthesized a series of electron-deficient group-based organic small molecule HTMs.The molecular structure was characterized by ¹H NMR,¹³C NMR spectrum,and the effects of its photophysical properties,electrochemistry,thermal stability,hole mobility,and film-forming ability were studied.In addition,the effect of mixed HTMs on its performance was further studied.The main content of this paper includes the following two aspects:1.In this work,a unique 2,2'-diphenyl-5,5'-bipyrimidine building block is used to synthesize two new hole transport materials(HTMs),named MD-T and MD-C,which are applied in conventional perovskite solar cells(PSCs).Carbazole derivatives and dimethoxydiphenylamine moieties are included in MD-C and MD-T as periphery groups,respectively.The introduction of the electron-withdrawing bipyrimidine group increases molecular dipole moment,thus leading to an enhanced dipole-dipole interaction,which promots charge transfer and affords a high hole mobility.Notably,MD-C with carbazole periphery exhibits more suitable energy level with perovskite material,and shows a higher hole mobility along with better film morphology.Eventually,the devices containing MD-C HTM achieve a superior power conversion efficiency(PCE)of 19.93%with a high fill factor(FF)of 80.02%,overwhelming that of the MD-T based devices(PCE=15.55%).2.The two molecules(denoted as 3,6-SFY and 2,7-SFY)were composed of the fluorine-substituted sulfonyldibenzene as the core structure and the carbazole-diphenylamine periphery moieties.The two compounds show good compatibility when they are mixed due to their highly similar conjugated structures.Further characterizations reveal that the mixed HTM exhibits suitable energy level,uniform surface morphology,and enhanced hole mobility and hole extraction ability in comparison with the single-component HTM.As a result,conventional perovskite solar cells using the mixed isomers as HTM deliver a promising power conversion efficiency of 21.41%,with a short circuit current density(J_sc)of 23.98 m A/cm²,a open circuit voltage(V_oc)of 1.10 V and a high FF of 0.81.Notably,the efficiency of the blended HTM-based PSC is superior to that obtained in 3,6-SFY-based device(19.02%)and 2,7-SFY-based device(20.29%).