Theoretical Calculation And Design Of Novel Organic Small Molecule Hole Transport Materials

In recent years,perovskite solar cells?PSCs?have always been at the forefront of emerging photovoltaic technologies.The photoelectric conversion efficiency of this new type of solar cell has reached 23.7%.Although PSCs have developed many structures,hole transport materials?HTM?are still indispensable.At present,Spiro-OMeTAD is a hole transport material with higher efficiency,but due to its complex synthesis,low hole mobility and high production cost,it has limited the commercial development.Therefore,the current research hotspot is to find low price and efficient hole transport layer materials.In this paper,density functional theory?DFT?and Marcus theory of electron transfer are used to investigate the effects of atomic substitution,molecular symmetry and?-conjugation on the electronic structure and hole transport properties of hole transport materials.The newly designed hole transport material can provide some clues for the experimenter to design and synthesize new organic HTM.The main research contents include:?1?Based on DFT and Marcus theory of electron transfer,the effects of symmetric TPE,TTE and NTT core units on the electronic structure and charge transport properties of HTM?1⁵?were investigated.Four new hole transport molecules were designed by replacing the core unit and regulating the terminal groups of the model molecule EtheneTTPA?molecule 1?.The results show that the enhancement of the planarity of the symmetric core unit is beneficial to improve the hole mobility,and the HOMO level of the newly designed molecule?2⁵?ranges from-4.96 eV to-5.25 eV.Compared with the model molecule EtheneTTPA,the HOMO level of the new HTM is generally shifted down,which is beneficial to increase the open circuit voltage of the PSCs.The newly designed molecules 3,4 and 5 are considered to be more efficient HTM.?2?Based on the model molecule NTT-4TPA with symmetric core NTT and four triphenylamine?TPA?side arms,the effect of different bridges on optical properties and hole transport properties was systematically studied by modifying the TPA side arm.Studies have shown that the HOMO level of the newly designed molecule can be regulated in the large energy coverage range of-5.08 eV to-5.61 eV,which is beneficial to the interface energy regulation.Inserting a carbon-carbon single bond between adjacent phenyl groups of the TPA unit makes the HOMO level more stable,while the connection of the oxygen bridge makes the HOMO level up-shifted.The quasi-planar molecular architecture and extended?-conjugation properties can promote electron coupling between adjacent molecules,and high hole mobility can be obtained in combination with lower recombination energy.The newly designed T2,T5,T6 and T7and the model NTT-4TPA are considered to be more efficient HTM.?3?Based on DFT and Marcus theory of electron transfer,the effects of multi-ring?-conjugated structures on the electronic structure,optical properties and hole transport properties of D-?-D hole transport materials were investigated.The HOMO levels of the four D-?-D type molecules range from-5.22 eV to-5.39 eV,which is beneficial to increase the open circuit voltage of the solar cell.It is found that the multi-ring?-conjugated core and the delocalized frontier molecular orbital enhance the electronic coupling between adjacent molecules,and a higher hole mobility obtained due to the lower recombination energy(10^-210^-11 cm²v^-1s^-1).The newly design molecules?L1^L4?are considered to be more efficient HTM,and the methods used in this work provide a theoretical basis for the design and synthesis of new high-efficiency small molecule materials.?4?Based on the model HTM X60?S1?,five new hole transport molecules?S2^S6?with different molecular symmetry were designed by adding a bridge in the core unit.We studied the effects of different bridges and molecular symmetry on the properties of HTM from the aspects of electronic structure,light absorption and hole transport properties.The results indicate that the connection of nitrogen atoms?S3 and S6?makes the HOMO level up-shifted,Symmetric molecules show a completely delocalized frontier orbital distribution compared to the asymmetric case?S1 and S2?,Symmetric molecules S4 and S5 exhibit deeper HOMO levels and higher hole mobility.The newly designed S2,S4 and S5 are considered to be more efficient HTM by considering various factors at the same time.Our work can provide some clues for the experimenter to design and synthesize new organic HTM.