β-alkyl Substituted Metal Phthalocyanines As Hole Transport Layers In Perovskite Solar Cells

Perovskite solar cells have attracted a great deal of attention due to their simple manufacturing process,ideal open circuit voltage,low cost,and high power conversion efficiency（PCE）.As an important component of perovskite solar cells,the hole transport layer plays a key role in improving device efficiency and enhancing hole extraction/transport.Metal phthalocyanines（MPcs）with excellent hole transport properties are well known to be both thermally and chemically stable.They can be easily synthesized and purified with low cost,and have suitable energy levels.In this work,a series of novel metal phthalocyanine derivatives were designed and synthesized.The present research is aimed to explore different phthalocyanine molecular structures and material semiconducting properties by changing central metals and extending the substituted alkyl chains.PSCs with the conventional n-i-p structure and based on these materials were fabricated and characterized,and the relationship between the molecular structures of the MPcs and the device performance was systematically studied.In this study,a series of phthalocyanine derivatives containing different central metals were designed and synthesized:CuEt₂Pc,CuPr₂Pc,NiEt₂Pc,NiPr₂Pc,ZnEt₂Pc and ZnPr₂Pc.The molecular structure of the material was studied by mass spectrometry,elemental analysis,infrared and UV-visible absorption spectroscopy.The influences of the different central metals and alkyl substituents on the electronic energy levels,thermal and chemical stability,and spectral properties of the materials was studied.The results of ionization energy test photoelectron spectroscopy（IPS）showed that the HOMO level of the material is located between-5.1 eV and-5.2 eV,which well matched with the energy level of the perovskite material.The thin films of these materials fabricated by thermal evaporation method were studied by grazing incidence X-ray diffraction（GIXRD）.The results showed that the elongation of the alkyl chain enhances the crystallinity of the film,and thereby increases the charge carrier mobility of the film(from 10^-5 to 10^-4 cm²V^-1S^-1),which can be more favorable for the hole extraction/transport ability of the active layer.The atomic force microscopy（AFM）test results also showed that the elongation of the alkyl chain helps to form a uniform,regular film and improves the interlayer contact between hole transporting and perovskite active layer.So,it can effectively reduce carrier recombination at the interface.In this work,the developed MPc based hole transport layer were employed in perovskite solar cells with the structure of FTO/SnO₂/Perovskite/HTL/Au.The devices based on CuEt₂Pc,CuPr₂Pc,NiEt₂Pc,NiPr₂Pc,ZnEt₂Pc and ZnPr₂Pc were exhibited PCEs of 9.35%and 9.01%.7.97%,12.51%,8.89%,9.87%,respectively.The highest PCE was achieved for the device based on NiPr₂Pc,due to its high carrier mobility and suitable HOMO energy levels.The stability test results confirmed that the newly synthesized MPc based hole transport materials can effectively insulate the intrusion of water and oxygen on the active layer of perovskite,and play a good encapsulation effect on the perovskite layer,and thus effectively improving the stability and life of the device.