Synthesis And Structure-property Relationship Of Phosphonic Acid Funtionalized Carbazole As Hole-transporting Layer

Organic solar cells(OSCs)have many advantages,such as low cost,light-weight,simple preparation process,facile fabrication of large area flexible and transparent/semitransparent devices,and so on.In recent years,with the research and development of new materials and the optimization of device technology,the power conversion efficiency of organic solar cells(OSCs)has been rapidly improved.The power conversion efficiency of OSCs have exceeded 19%.The interface modification layer plays an important role in improving the photovoltaic performance of OSCs.Currently,the commonly used hole-transporting layer materials mainly based on PEDOT: PSS type polymer materials and metal oxide type inorganic materials,but both of them have disadvantages and are not suitable for commercial application.PEDOT: PSS is acidic,which will corrode electrode materials,and has a great influence on acid-sensitive photoactive layer materials.However,metal oxide materials also restrict the improvement of the performance of organic photovoltaic devices due to their complex processing technology and poor conductivity.Based on this,we focus on the surface modification of ITO,that is,self-assembled monolayer(SAM)as hole-transporting layer material on its surface.Thus,a series of phosphonic acid functionalizated carbazole compounds were synthesized,using phosphonic acid group as anchoring group,alkyl chain with different number of carbon atoms as spacer group and carbazole as aromatic tail group.The main achievements and innovations of this paper are as follows:1.The effect of the binding site of phosphonic acid group on the carbazole group on the photovoltaic performance was explored,as well as the optimal binding site of phosphonic acid group and the carbazole core was determined.Through DFT molecular simulation,the reasons of the good photovoltaic performance produced by this binding mode were analyzed.Compared with dicarbazole D(PA-Cz),the alcohol-solubility of monocarbazole PA-Cz is better in device performance,which is beneficial to device processing.2.Based on the molecular structure of PA-CZ,two isomers,namely PACZ-3,6Br and PA-CZ-2,7Br,were synthesized by dibromosubstitution at the 3,6 and 2,7 positions of carbazole,and the effect of substitution sites on the photovoltaic properties was studied.It is clear that the substituents at 3 and 6 positions of carbazole unit can provide a tradeoff between the electron induction effect and steric hindrance effect of substituents,showing that the dipole moment of the SAMs plays a key role in enhancing ITO work function.3.The influence of halogen substituents on the properties of phosphonic carbazole materials was further studied.PA-Cz-3,6Cl,PA-Cz-3,6I and PA-Cz-3I were synthesized by replacing 3 and 6 positions of PA-Cz-3,6Br with different halogens(Cl,Br and I).The design principle of halogen substituent selection is clarified.4.The effect of alkyl chain length of spacer group in the SAMs on the work function of their modified ITO was explored.It is found that the work function tunable ability of the SAMs on ITO surface decreases with the increase of carbon chain length,which is mainly due to two reasons: firstly,the carbon chain increases with the rotational freedom of molecules,making the molecular arrangement irregular and unable to achieve dense arrangement;Secondly,as the carbon chain grows,HOMO electron clouds cannot be evenly distributed at the end of the carbon chain,thus blocking hole transport.