Design,Synthesis And Energy Control Of D-?-D Carbazole-based Hole Transport Materials

With the development of society,sustainable and clean energy sources are eagerly needed.As a new generation of photoelectric conversion device,perovskite solar cells have attracted much attention due to their relatively high efficiency and low cost.The hole transport material plays an important role in perovskite solar cell,which is directly responsible for sucking and transferring the holes generated by perovskite.In this process,the energy level of the hole transport material not only directly determines the thermodynamic possibility of hole transfer,but the energy level difference between hole transport material and perovskite also affects the hole transfer driving force,the level of transport efficiency,and the open circuit photovoltage of the device.Therefore,the energy level of the hole transport material is a key factor restricting the performance of perovskite solar cell devices.Based on the planarity factor,four kinds of D-?-D carbazole-type hole transport materials were designed and synthesized in this thesis,whose molecular planarity was systematically adjusted.By the characterization and evaluation of their various properties,the effects of molecular planarity on the properties of hole transport materials,especially for their energy levels,were investigated.The first chapter briefly introduced the development background,classification,and characteristics of perovskite solar cells.Hole transport materials with carbazole derivatives were also summerized.The research idea of this thesis is proposed:carefully energy control through systematically adjusting the molecular planarity.In chapter 2,the hole transport materials CS-48 and CS-49 with different planarity were synthesized by introducing four and three peripheral groups into the core units,respectively.The molecular structures of all intermediates and target compounds were characterized by H NMR and C NMR.The hole transport material CS-48 has relatively strong steric hindrance effect due to the introduction of four peripheral groups on the core unit,resulting in poor planarity,while CS-49 presented relatively good planarity.The HOMO energy level of CS-48was determined to be-5.41 eV by differential pulse voltammetry?DPV?.Along with the enhancement of the molecular planarity,the HOMO energy level of CS-49 was upshifted by0.02 eV compared with that of CS-48.With the same electron-donating group,we believe that the upshift of the HOMO level here should be attributed to the improvement of the planarity.Considering the perovskite valence band energy level is-5.43 eV,the HOMO energy level of CS-48 is only 0.02 eV higher than the valence band.Such small energy difference may result in insufficient hole transfer driving force,while CS-49 is expected to obtain better hole transfer performance due to its relatively large energy difference.However,CS-49 presented poor film-forming capability,thus resulting in serious short-circuit in the perovskite solar cell device based on CS-49,whose J_SCC was 17.72 mA cm^-2,V_OCC was 0.79 mV,ff was 47.55%,and the PCE was 6.67%.Although the photovoltaic performance of the hole transport material CS-49 is relatively low,its performance is still greatly improved compared with CS-48 based cell devices,which indicates that the performance of hole transport material,especially for the energy level,can be modulated through planarity control.In chapter 3,based on CS-49,CS-50 and CS-51 were further designed and synthesized by introducing thiophene rings into the conjugated bridge chain and replacing twised carbazole donors by planarized ones,respectively,which could further improv the molecular planarity of the hole transporting materials.The HOMO energy levels of CS-50 and CS-51were obtained as-5.36 and-5.29 eV,respectively.It can be found that the HOMO energy levels gradually upshifts from CS-49 to CS-51 as the molecular planarity increases.In addition,with the enhancement of planarity,the hole extraction and transfer capabilities of CS-51 have also been significantly improved,as well as the film-forming capability.Accordingly,perovskite solar cells based on CS-51 achieved the best photovoltaic performance among three hole transport materials with a J_SCC of 20.42 mA cm^-2,a V_OCC of 1.08V,a ff of 69.05%,and a PCE of 15.26%,which may be further enhanced after further optimization.In chapter 4,the conclusions are presented.Based on molecular planarization engineering,hole transport materials CS-48-CS-51 with different planarity were designed and synthesized.Through in-depth research on their important properties,it is found that as the enhancement in molecular planarity,the HOMO energy levels of the hole transport materials gradually upshift,while the hole extraction and transport capabilities have been significantly improved,as well as the film-forming capability.With the best molecular planarity,perovskite solar cells based on CS-51 achieved the best photovoltaic performance among all hole transport materials with a J_SCC of 20.42 mA cm^-2,a V_OCC of 1.08 V,a ff of 69.05%,and a PCE of 15.26%,which may be further enhanced after further optimization.