The Study Of Conjugated Small Molecule Hole Transport Materials For Perovskite Solar Cells

Perovskite solar cells（PSCs）have been developed rapidly in recent years due to their highly power conversion efficiencies（PCEs）,simpleness of preparation and low cost.At present,the best researched efficiency of PSCs has reached 25.2%,which reported by the NREL.It is critical to optimize the functional materials of perovskite solar cells to improve the efficiency and stability for the devices.This study designing and synthesing four series of organic small molecule as hole transport materials（HTMs）in aim to paving a way for developing efficiency and stability PSCs.（1）Two kinds of small molecule hole transport materials（named D101 and D102）were designed and synthesized by using p-benzaldehyde or m-benzenedicaldehyde as raw materials.The aldehyde group was transformed into dibromoethylene by Corey Fuchs reaction,and then four triphenylamine groups were connected by Suzuki-Miyaara reaction.The HOMO energy levels of the HTMs were-5.34 e V for D101 and-5.37 e V for D102 are positive than that of perovskite,which were derived from cyclic voltammetry.The PCE of devices based on D101 and D102 were 18.47%and 17.33%,respectively,and that of spiro-OMe TAD is 17.20%.（2）Three dopant-free hole transport materials（named D104,D105 and D106）were designed and synthesized by using benzaldehyde,ammonium acetate and acetophenone derivatives as raw materials.The intermediate of pyridine was formed by pericyclic reaction,and then different amount of diphenylamine groups were connected by Buchwald-Hartwig reaction.With the increase of the number of substituted methoxy groups with diphenylamine,the electrochemistry and photophysical properties of the compounds change regularly.The hole mobility for different HTMs were 7.1×10^-5,1.65×10^-4 and 2.41×10^-4 cm² V^-1 s^-1,respectively.The PCE of devices based on D104,D105 and D106 were 16.24%,17.40%and 18.24%,respectively,and that for PEDOT:PSS as reference was 14.37%.The PCE of D106-based device still remain over 75%initial PEC after being stored for 275 hours in the room temperature without encapsulation,while the PEDOT:PSS-based device retained only 50%.（3）Three X-shaped organic dopant-free hole transport materials（named D31,D32and D33）were designed and synthesized by a benzene core unit with four arms attached.Two of them are triphenylamines and two are azomethine bridges connected to functionalized phenyl rings.The effects of functional groups with different electronic properties on the properties of compounds and devices were investigated by changing the substituent groups on the side chain benzene ring to methoxy group as the electron donor and cyano group as the electron acceptor.The HOMO energy levels of the compounds were-5.35 e V,-5.36 e V and-5.38 e V,respectively,which were derived from cyclic voltammetry.The hole mobility for different HTMs were 1.02×10^-4,1.39×10^-4 and 2.41×10^-4 cm² V^-1s^-1,respectively.The PCE of devices based on D31,D32and D33 were 13.47%,15.83%and 17.85%,respectively.The device based on D31and D33 cells were stored in environment condition for 400 hours,and the PCE of the devices can still be maintained above 70%of the initial value.（4）Four dopant-free organic HTMs（named D41,D42,D43 and D44）based on pyrrolo[3,2-b]pyrrole core with four aryl arms are prepared for fabrication of inverted planar perovskite solar cells.The HTMs were accessible for one-pot synthesis from the reaction of aromatic aldehydes with p-anisidine and butane-2,3-dione.By change of substituents（methyl,cyanyl,fluorine,trifluoromethyl）on aromatic aldehydes,significantly affected their molecular surface charge distribution and thin-film morphology.The hydrophobic angles of D41-D44 were 74.0°,62.4°,70.6°,and 73.0°,respectively,which were much higher than that of the PEDOT:PSS（19.5°）and beneficial for the stability of perovskite.The surface roughness of the hole transport films was measured by AFM,and it was found that the surface of D42 show the best surface morphology.The PCE of devices based on D41,D42,D33,D44 and PEDOT:PSS were 11.21%,17.30%,12.13%,12.56%and 12.51%,respectively.The devices based on the D43 and D44 were stored in environment condition for 22 days,and PCE of the devices can still be maintained above 70%of the initial values.