Design And Synthesis Of The Organic Hole-transport Materials With Dual Functions

After a rapid development in about one decade,perovskite solar cells（PSCs）have reached a power conversion efficiency（PCE）of 25.7%,which is comparable to the traditional silicon solar cells.However,for commercialization,two key issues need to be overcame,manufacturing costs and device stability.The hole transport layer（HTL）can optimize the perovskite interface,adjust the energy level between the electrode and the perovskite active layer,effectively transmit holes and block electrons,thus playing a key role in improving the performance of PSCs.Spiro-OMe TAD and PTAA are benchmark hole transport materials used in laboratories,but have been criticized for their negative effects on device stability and high synthesis costs.Therefore,the development of simple and inexpensive dopant-free small molecule hole-transport materials is a major research direction in perovskite solar cells.In this paper,two kinds of hole-transport materials are designed and synthesized,which can not only transport the hole,but also passivate the defects in the perovskite,thus increasing the efficiency and improving the stability.The main research contents and results are as follows:（1）A novel dopant-free organic small molecule hole-transport material DBT-MPC was synthesized by Buchwald-Hartwig reaction with dibenzo[b,d]thiophene（DBT）as the central unit and 3,6-bis（6-（methylthio）pyridin-3-yl）-9H-carbazole（MPC）as the terminal group.Due to MPC is a new type of terminal group,two synthetic routes have been designed to maximize the yield.The thermal decomposition temperature of DBT-MPC is 384℃ at 5%weight loss.The glass transition temperature of the DBT-MPC is 229℃,and there is no obvious crystallization peak.The absorption edge of DBT-MPC is 387 nm and the optical bandgaps is3.20 eV.In the solution state,the HOMO level of DBT-MPC tested by CV is-5.57 eV,and according to the band gap,the LUMO level is-2.37 eV.The DMF/water contact angles of DBT-MPC films are 10.68±0.30°and76.70±0.70°,respectively,indicating that the material has good wettability to DMF and hydrophobicity to water.The hole mobility of DBT-MPC is1.71×10^-4 cm² V^-1 s^-1 tested by SCLC（the film thickness of DBT-MPC is approximately 10 nm）.XRD and SEM show that perovskite films on top of DBT-MPC are well crystallized.The appearance of C-S and C=N peaks in FTIR,and Pb 4f peaks in XPS illustrates that DBT-MPC can interact with the defects in perovskites(Pb²⁺).The p-i-n PSCs based on DBT-MPC as the hole-transport layer can reach the PCE of 13.07%,with the J_SCof19.58 m A cm^-2,theV_OC of 1.00 V,and the FF of 66.76%.（2）DCBT-BMTP based on 2,2’-dithiophene with the chlorine atom substituted at the 3,3’position as the central group,and the 4-（methylthio）-N-（4-（methylthio）phenyl）-N-phenylaniline as the end group,has been designed and synthesized,which was applied in p-i-n PSCs.The thermal decomposition temperature of DCBT-BMTP at 5%weight loss is383℃.The absorption edge of DCBT-BMTP film is 523 nm and its band gap is 2.37 eV.The HOMO level of DCBT-BMTP is-5.32 eV in the solution state and-5.30 eV in the film state,respectively.The contact angle of DMF on DCBT-BMTP film is 13.08±0.65°,and the contact angle of water is 77°.The C-Cl peak at 908 cm^-1 and the C-S peak at 1465 cm^-1 in FTIR were significantly offset,and the peaks of Pb and I in XPS shifted to the lower binding energy,indicating the strongest interaction between DCBT-BMTP and the perovskite.AFM shows that DCBT-BMTP films have good film morphology.The perovskites deposited on top of DCBT-BMTP films has good crystallinity.The defective density of DCBT-BMTP-based PSCs is 2.34×10¹⁶ cm^-3characterized by SCLC.DCBT-BMTP-based p-i-n devices have an efficiency up to 21.23%,with the J_SC of 22.96 m A cm^-2,theV_OC of 1.13 V,and the FF of 81.14%.In the N₂ atmosphere,the devices based on DCBT-BMTP can maintain more than 90%of the original efficiency after 40 days and 400 h at 70℃,which manifests the PSCs based on DCBT-BMTP show good stability.