Design,Synthesis And Application Of Organic Small Molecule Hole Transport Materials In Perovskite Solar Cells

As a rapidly developing third-generation solar cell technology,the power conversion efficiency(PCE)of perovskite solar cells(PSCs)has grown from an initial 3.8% to the current 25.8%(certified efficiency)in the last decade or so.In conventional PSC devices,the hole transport materials(HTMs),as an important component,can not only effectively extract holes from the perovskite active layer and transport holes to the metal electrodes,but also inhibit charge recombination at perovskite/HTM interfaces.Currently,the most widely used organic small molecule HTMs are 2,2’,7,7’-tetra[N,N-bis(4-methoxyphenyl)amino]-9,9’-spirobifluorene(Spiro-OMe TAD).However,they still have the disadvantages of high synthesis cost,complicated preparation process,low hole mobility and intrinsic conductivity,and insufficient energy level matching with perovskite.Therefore,it is of great importance to design and prepare new HTMs with easy synthesis,matching energy level with perovskite and high hole mobility.In this thesis,we have designed and synthesized a series of novel small molecule HTMs by modulating the properties of hole transport materials at the molecular level,and successfully applied them to perovskite solar cells.The specific research contents and results are as follows.1)Two asymmetric small molecules,3,6-BOC and 2,7-BOC,were designed and synthesized as hole transporting materials for perovskite solar cells.Both compounds have an asymmetric N-methyl-3-benzoylcarbazole as the core structure,and the peripheral carbazole diphenylamine groups are connected to the asymmetric core through different sites.The asymmetric structures confer good thermal stability to the materials.The geometrically optimized structures and reorganization energies of both were calculated by density functional theory(DFT).And compared to 2,7-BOC,3,6-BOC showed a more spacious molecular conformation and lower recombination energy,which facilitated stronger intermolecular interactions and higher hole mobility.In addition,the carbonyl group of the asymmetric core can interact with perovskite,which was verified by Fourier-transform infrared spectroscopy(FT-IR),X-ray photoelectron spectroscopy(XPS)and other tools.In summary,the 3,6-BOC-based device obtained a power conversion efficiency of 21.52%,and the device also exhibited good long-term and thermal stability.2)In this section,we prepare doped-free organic small molecule hole transporting materials by introducing intramolecular noncovalent interactions(INIs).The small molecule DCT with non-covalent interactions and the small molecule DTT without INIs were synthesized as control molecules.The physicochemical properties,film morphology and their corresponding device properties of the two new materials were systematically analyzed.The single-crystal structure of DCT was analyzed by X-ray single-crystal diffraction,which confirmed the existence of S···O intramolecular noncovalent interactions and that DCT showed ordered molecular stacking.The hole mobilities of both materials were tested by the space-charge-limited current(SCLC)method,and the results showed that INIs conferred higher hole mobilities to DCT.We attempted to prepare doped-free DCT-based devices without the addition of dopants.The doped-free DCT-based PSCs obtained a photovoltaic conversion efficiency of 22.50%,which is higher than that of the doped Spiro-OMe TAD standard device(22.06%),and have good long-term stability.