Design Of Dithieno[3,2-b:2',3'-d]pyrrol Cored Hole Transpoting Materials For Dopant-free Perovskite Solar Cells

Recently,perovskite solar cells(PVSCs)have gained explosive development with a highest certified power conversion efficiency(PCE)of 25.2% for n-i-p structured device.Hole transporting materials(HTMs)play a vital role in PVSCs.At present,the most popular hole transporting material is 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene(Spiro-OMeTAD).However,the pursuit of high PCE PVSCs with Spiro-OMeTAD as HTMs demands for the use of dopants,which would affect the device stability.In addition,the complicated synthesis and difficult purification make Spiro-OMeTAD impossible for wide exploration.Therefore,it is urgent to design and develop new dopant-free HTMs to replace Spiro-OMeTAD with simple synthesis,low cost and high hole mobility.In this dissertation,three HTMs based on dithieno[3,2-b:2',3'-d]pyrrol(DTP)were designed and synthesized.The correlation between molecular structure and optoelectronic properties was systematically studied.In chapter 2,a butterfly-type HTM(DTPC8-ThDTPA)was developed through the conjugation extention using thiophene bridged triphenylamine towards octyl DTP fused core.The lab cost of this material was calculated to be only 1/28 of Spiro-OMeTAD.Compared with Spiro-OMeTAD,DTPC8-ThDTP film exhibited an ionization potential of 5.13 e V,which is more compatible with the perovskite layer.DTPC8-ThDTP showed a high hole mobility of6.50×10^-5 cm²V^-1s^-1in film with good film forming capability.The n-i-p type device of FTO/SnO₂/C₆₀-SAM/MA_0.7FA_0.3PbI_2.85Br_0.15/PMMA/HTM/Au achieved a PCE up to 19.42%(V_OC is 1.139V,Jsc is 23.02mAcm^-2,FF is 0.741),among the highest values for dopant-free molecular HTMs based PVSCs in literature.In chapter 3,a carbonyl bond was introduced at the N-position of DTP to further tune the HOMO energy level of the HTM material,whilehe overall conjugated structure remained as DTPC8-ThDTP in chapter 2.The introduced carbon-oxygen double bond can form weak coordination with lead ions,thereby passivating the perovskite surface defects to improve the device performance.Theoretical results show that the strong electron withdrawing effect of carbonyl bond can greatly turn the electron cloud.Its experimental cost is only 1/17 of Spiro-OMeTAD,which reduces the cost.The highest occupied molecule orbital(HOMO)energy level is-4.85eV.After HTM concentration optimization,the n-i-p type device of ITO/SnO₂/MA_0.7FA_0.3Pb I_2.7Br_0.3/HTM/MoO₃/Au can achieve a maximum PCE of 16.05%.In chapter 4,on the basis of the previous two chapters,we desined a new star type HTM(DTP-Th TTPA)by connecting thiophene bridged TPA side arms to the 2,6 and N-position of DTP.It has a high hole mobility(1.20×10^-3cm²V^-1s^-1),with a HOMO energy level of-5.06eV.The theoretical analysis shows that there is no molecular tension similar to Spiro-OMeTAD helical core.The glass transition temperature is as high as 120.3°C.The HTM with high hole mobility,good thermal stability and matching energy level was successfully obtained through molecule structure design.The PCE of n-i-p type device structure of ITO/SnO₂/MAPb I₃/HTM/MoO₃/Au increased from 7.1% to 13.16% through HTM concentration optimization.