Small-molecular Hole Transporting Materials:Design,Synthesis,and Applications In Perovskite Solar Cells

Hole transporting materials(HTMs)play an important role in perovskite solar cells(PSCs),such as hole transporting,suppressing charge carriers recombination,promoting perovskite crystallizting and protecting perovskite layer et al.The properties of HTMs show great influence on the photovoltaic performance of PSCs.At present,the most commonly used HTM is spiro-OMeTAD,however,the high price and relatively poor stability limit its application.Therefore,developing novel,inexpensive,efficient and stable substitutes for spiro-OMeTAD plays an important role in the commercial application of PSCs.Organic small molecules own a definite structure,which is easy to modify its energy levels,solubility,and mobility through molecular engineering.Therefore,searching for new,inexpensive and stable small-molecular substitutes for spiro-OMeTAD have become a focus of the research in PSCs.Exploring the influence of HTMs structure on the performance of PSCs and obtaining a guidance for the development of novel HTM are also the starting points for this paper.In this thesis,a series of HTMs have been designed and synthesized for the application in PSCs.The structures of the HTMs are characterized by proton nuclear magnetic resonance(1H NMR and 13C NMR)technology and mass spectra(MS).The photophysical,electrochemical,thermal stability,hole mobility and film-forming properties of the HTMs were studied.The detailed relationship between dye structures and solar cell performances has been extensively investigated.Firstly,two facile thiophene-arylamine HTMs,obtained by varying the substitution position of arylamine moieties on the thiophene ?-linker,are developed(H-3,4 and H-2,5).Compared with H-3,4,the HOMO of H-2,5 is delocalized over the whole molecule with a clear redshift of the absorption spectrum,and higher hole mobility.Therefore,H-2,5 has a higher hole-extraction ability,which shows better performance in PSC than H-3,4.We further developed two HTMs(DTh101 and TTh101)by increasing fused thiophene on ?-linker in the parallel direction of molecules from the above H-2,5(Th101).It is noted that the spectrum is red-shifted,HOMO levels decrease,thermal stability and hole mobility increase along with the increasing fused thiophene rings of ?-linker in HTM.Therefore,the hole-extraction ability gradually increases,which lead to better performance in PSCs along with the increasing fused thiophene rings in HTMs.To further study the influence of ?-linker in HTMs on the performance of PSCs,we develop three simple HTMs by gradually increase the vertical benzene rings of?-linker(B101,DB101,and TB101).It is a remarkable fact that the trend of hole mobility,stokes shift and reorganization energy along with the extending vertical?-conjugation is opposite to that of conventional extending parallel ?-conjugation.Furthermore,due to the similar HOMO level,low hole mobility and conductivity of pristine HTMs,the increased reorganization energy along with extending vertical?-conjugation reduce the charge separation efficiency at the perovskite/(pristine HTM)interface,leading to a progressively worse photovoltaic response in PSCs.Interestingly,when the HTMs are doped,the devices show a peculiar behavior compared to that of the pristine HTMs.After doped,the hole mobility is promoted and HOMO level is reduced along with extending vertical ?-conjugation,which enhance the charge separation efficiency at perovskite/(doped HTM)interface,leading to a progressively better photovoltaic response in the PSCs.In addition to the ?-linker,we have developed several HTMs with different peripheral substituent groups.Firstly,three benzene-arylamine HTMs with different numbers of terminal groups were prepared(H-Di,H-Tri,and H-Tetra).The spectrum is red-shifted along with the increasing arms,which may be resulted from gradually decreasing planarity.Therefore,the electron cloud is delocalized over the whole molecule.The hole mobility gradually increases along with the increasing arms due to the increasing charge transmission channel.The HOMO level of H-Tri is lower than that of H-Di and H-Tetra,and H-Tri shows the best film-forming ability.Therefore,H-Tri shows the best hole-extraction ability,leading to the best photovoltaic responses in PSCs.Secondly,we designed three HTMs(Ph-TPM,DPA-TPM and TPA-TPM)with tetraphenylmethane as core and arylamine moieties as peripheral substituent groups.Among them,the film-forming ability of Ph-TPM and DPA-TPM is poor due to bad solubility,and hole mobility is also lower than TPA-TPM,leading to poor photovoltaic responses in PSCs.Further research shows that TPA-TPM own relatively high glass-transition temperature,high water contact angle,and higher hole mobility.The performance of PSCs with TPA-TPM is comparable to that of spiro-OMeTAD with good stability.