Synthesis And Application Of D-π-D Type Organic Small Molecule Hole-Transporting Materials In Perovskite Solar Cells

After more than ten years of rapid development,the photoelectric conversion efficiency(PCE)of perovskite solar cells(PSCs)has been increased from 3.8%to more than 25%,having attracted more and more researchers’ attention.PSCs generally consist of conductive glass,electron transporting layer,perovskite active layer,hole transporting layer and electrodes.Hole transporting layer(HTL)plays a very important role in PSCs.Its main function is to extract and transport the holes generated by the perovskite active layer,and to block the reverse transport of electrons.HTL has great influence on the performance and stability of PSCs.The commonly used hole transporting material(HTM)is spiro-OMeTAD(2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene).Although spiro-OMeTAD molecule exhibits excellent performance as HTM,due to its high cost and inability to passivate perovskite defects effectively,researchers have developed a number of new HTMs to find a substitute for spiro-OMeTAD.Based on this,a series of new HTMs with simple molecular structures and different π cores are designed and synthesized in this paper.By studying the relationship between the structure of HTM and the performance of PSCs,the efficient and feasible design strategy of new HTM is explored.The main work of this paper is as follows:(1)Four D-π-D linear HTMs with 3,4-ethylenedioxythiophene(H101,NY-02)and 3,4-ethylenedithiothiophene(NY-03 and NY-04)as cores were designed and synthesized.NY-02 and NY-04 were gotten from H101 and NY-03 by the extension of introducing double bonds.The effects of extended conjugation and heteroatoms(oxygen and sulfur)on the photophysics,electrochemistry,hydrophobicity,film-forming properties and performance in PSCs of these four molecules were explored.It was found that molecules containing double bonds had better planarity and intermolecular charge transfer,thus improving the molecular hole transport and extraction ability.In addition,molecules containing oxygen(3,4-ethylenedioxythiophene)exhibited better planarity than molecules containing sulfur(3,4-ethylenedithiothiophene)whether they contained double bonds or not.When used in PSCs,it was found that HTM containing double bonds and oxygen showed better photovoltaic performance and stability.Among them,the device based on NY-03 had the best performance,with a PCE of over 20%,outperforming the conventional spiro-OMeTAD under the same conditions.(2)Considering that the perovskite/HTM interface has a large number of defects and that carboxyl group(-COOH)is a common passivating group in many passivators,we introduced the-COOH unit into a thiophen-triphenylamine molecule(named as H-CB)and expected to obtain an efficient multifunctional HTM that can simultaneously passivate the defects.Meanwhile,its non-COOH counterpart(H-S)was also prepared as a reference for investigations.’The results showed that the introduction of-COOH unit could not only lower HOMO levels of the molecule,but also improve charge transport properties.Photoluminescence(PL)illustrated that the HTM containing-COOH could efficiently collect holes from the perovskite.Moreover,theoretical caculation and Fourier transform infrared(FTIR)spectroscopy showed that the HTM based on-COOH could also passivate the defects by interacting with the under-coordinated Pb2+.When used in PSCs,H-CBbased device exhibited higher PCE and lower hysteresis compared with H-S.H-CB-based device showed the PCE of 19.60%,which is even comparable with the conventional spiroOMeTAD.It was worth noting that the introduction of-COOH could effectively enhance the long-term stability and thermal stability of the device.The results provide a new vision for designing multifunctional HTMs.