The Investigation Of P-type Dopants For Organic Hole-transporting Materials In Perovskite Solar Cells

At present,the world needs more than ever renewable energy sources to save the planet from the greenhouse effect as the result of carbon dioxide emission into the atmosphere.Solar energy is one of the possible renewable energy sources,which has a vast potential to satisfy the ever-increasing energy demand of our society.Recently,perovskites have emerged as an efficient light absorber,which has attracted the attention of the researchers in the past few years.The general chemical formula of a perovskite is given by ABX3[A=CH3NH3+(MA),NH=CHNH3+(FA)or Cs+;B=Pb2+or Sn2+;X=Cl-,Br-,I-],which has shown outstanding photovoltaic properties,such as,high diffusion length,high charge carrier mobility,high absorption coefficient,and abroad spectral absorption range,etc.Power conversion efficiency(PCE)of perovskite solar cells(PSCs)has rapidly increased from 3.8%in 2009 to over 25.0%at present.There are five main components of PSCs,including transparent conducting oxide(TCO),electron-transporting layer(ETL),active layer,hole-transporting layer(HTL),and a metal electrode.The use of hole-transporting materials(HTMs)in PSCs has been important in addressing the issues of poor device stability and improving the power conversion efficiency.HTMs play important roles in the extraction and transportation of photo-generated holes from the perovskite to the selective contact electrode,but most of the HTMs suffer from lower conductivity.Some p-type dopants are used to enhance the conductivity of the HTMs.The conducting polymer,poly(3-hexy]thiophene-2,5-diyl)(P3HT),has been widely used as HTMs in PSCs.However,due to its relatively low conductivity in pristine form,the P3HT-based devices typically exhibited modest overall performance.Herein,as an efficient p-type dopant for P3HT in mesoscopic PSCs,tetrafluoro-tetracyano-quinodimethane(F4TCNQ)has been successfully introduced.The introduction of F4TCNQ into P3HT significantly enhanced the overall performance.Under optimal doping conditions(1.0%,w/w),the efficiency from 10.3%for the pristine P3HT to 14.4%for doped one has been achieved.The improvement in PCE has been assigned to the increase of the photocurrent density and fill factor,which is attributed to the increased bulk conductivity of the F4TCNQ,doped P3HT.The conductivity of P3HT has increased by more than 50 times after doping with 1.0%F4TCNQ.The bulk conductivity has increased for the polymer that is demonstrated by the UV-Vis and FTIR.It is evident that the p-doping occurs via the electron transfer from the highest occupied molecular orbital(HOMO)level of P3HT to the lowest unoccupied molecular orbital(LUMO)level of F4TCNQ.Furthermore,under ambient conditions,and with a humidity of?40%,the PSCs-based on P3HT:F4TCNQ composite as an HTM also showed excellent long-term stability.For achieving efficient and stable PSCs,the F4TCNQ as p-dopant for P3HT has proven to be an effective strategy to improve the electrical property.Poly(9,9-dioctylfluorene-co-bis-N,N-(-4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenedia mine)(PFB)conjugated polymer with F4TCNQ as a p-dopant has been demonstrated as a HTM in mesoscopic PSCs.To optimize and control the electrical properties of the material for the best function in the solar cell,various concentrations of p-dopant with PFB have been investigated.The oxidation in a p-type doped PFB HTM has been indicated by the UV-Vis absorption measurements.The transfer of electrons from the HOMO of PFB to the LUMO level of F4TCNQ has assigned.Raman spectra have shown that the oxidation has occurred in the doped HTM.The PCE has increased from 9.09%for the pristine form to 14.04%for the doped one,which is 57%higher than that of pristine PFB.Furthermore,under ambient conditions,and with a humidity of-40%,the PSCs-based on PFB:F4TCNQ composite as an HTM also showed excellent long-term stability.F4TCNQ as a p-dopant for PFB has been found to be an effective way to improve the electrical properties,and thus to achieve efficient and stable PSCs.To substitute 2,2',7,7'-Tetrakis[N,N-di-(4-methoxyphenyl)amino]-9,9'-Spirobifluorene(Spiro-OMeTAD)in mesoscopic PSCs,N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)spiro[fluorene-9,9'-xanthene]-2,2',7,7'-tetraamine(X60)as an alternative HTM has been developed.The ionic liquid N-butyl-N'-(4-pyridylheptyl)imidazolium bis(trifluoromethane)sulfonimide(BuPylm-TFSI)has been used as a p-dopant to increase the conductivity of the X60-based PSCs.With different concentrations of BuPyIm-TFSI,4.85 mM is the optimal value according to the device performance.A PCE of 14.65%has been achieved,which is higher than that without BuPylm-TFSI,owing to its much-increased conductivity.UV-vis measurements showed that the oxidation process happened by adding BuPyIm-TFSI.Moreover,the devices based on X60 with BuPyIm-TFSI composite as an HTM showed superior long-term stability under ambient conditions,and with a humidity of?40%.The BuPylm-TFSI as p-dopant for X60 is very useful and has improved the electrical property as well as the overall performance for PSCs.