Low-temperature Synthesis And Modification Of Charge Transport Layers In Perovskite Solar Cells

Due to the advantages such as high absorption coefficient,long carrier diffusion distance and adjustable energy band gap of the organic-inorganic hybrid perovskite materials,perovskite solar cells(PSCs)using the organic-inorganic hybrid perovskite materials as the light absorbing layer have attracted extensive attentions in recent years.Since their first discovery in 2009,PSCs have achieved rapid development,and the power conversion efficiency(PCE)has been continually increasing with an incredible rate.At present,PSCs have achieved a certificated efficiency over 25%,exhibiting great commercial application prospect.However,the inferior stability of PSCs is still a major bottleneck restricting their commercial development.As important components of PSCs,the charge(electron and hole)transport layers play key roles in both the photovoltaic performance and the stability of PSCs.In order to fabracatite highly efficient and stable PSCs,in this dissertation,we concentrate our research on the optimization of charge transport layers.We not only develop novel and efficient electron and hole transport materials via low-temperature solution-combustion method but also modify the new synthesized and the commonly used hole transport materials.The results show that these strategies improve the photovoltaic performance and the stability of PSCs simultaneously.The main research contents and results are summarized as follows:(1)The commonly used TiO₂ eletron transport layer(ETL)not only needs high-temperature(>500?)sintering process but also induces the instability of PSCs by its photocatalytic effects.In this regard,we adopted the solution-combustion method to prepare Nb₂O₅ electron transport layer(ETL)at a low temperature of 200? and employed Nb₂O₅to replace high-temperature(>500?)sintered mesoporous TiO₂(m-TiO₂)as ETL in a regular PSC.The concentration of Nb₂O₅ precursor solution has a great effect on the device performance and the optimized device with Nb₂O₅ ETL prepared with 0.075 M precursor solution delivers a PCE of 16.40%,which is comparable to that of the device based on m-TiO₂ ETL(16.77%).In order to further improve the photovoltaic performance of the device,we doped Nb₂O₅ ETL with Zn element.After optimizing the doping content of Zn,an obviously improved PCE of 17.70%has been achieved.The results show that Zn doping can enhance the conductivity and the charge transfer ability of Nb₂O₅ ETL,which reduces the charge recombination loss and thus enhances the device efficiency.Furthermore,it indicates that the perovskite films deposited on Nb₂O₅ and Zn:Nb₂O₅ possess higher quality with less trap density than that deposited on m-TiO₂,which endows the Nb₂O₅ and Zn:Nb₂O₅ based devices exhibit superior stability in air as compared to the device with m-TiO₂ ETL.(2)The traditional PEDOT:PSS hole transport layer(HTL)is acidic and hygroscopic,which not only causes poor device performance but also reduces the device stability.Aiming at this problem,we prepared a novel CuNbO_x HTL by the solution-combustion method at250? for an inverted PSC.The results indicate that CuNbO_x HTL has high optical transparency in visible light,well-matched energy level with the perovskite layer,and can also extract and transport the photogenerated holes from the perovskite layer effectively.Then,we systematicly studied the influence of the concentration of CuNbO_x precursor solution on the device performance.It is found that the optimized device with CuNbO_x HTL prepared with 0.01 M precursor solution obtains a champion PCE of 16.01%,which is obviously higher than that of the device based on PEDOT:PSS HTL(10.00%).More importantly,the CuNbO_x based devices manifest outstanding stability due to the good chemical stability and the hydrophobic nature of the inorganic CuNbO_x,which can maintain77%of their initial efficiencies after being stored in air for 1000 h.(3)To further improve the photovoltaic performance of the CuNbO_x based PSCs,we doped CuNbO_x HTL with Zn element.It indicates that the conductivity and hole mobility of CuNbO_x is obvioysly improved by Zn doping.Furthermore,the valence band maximum(VBM)of CuNbO_x is shifted downward,thus the energy barrier between the HTL and the perovskite layer is effectively reduced.Additionally,Zn doping significantly improves the film quality of both the CuNbO_x HTL and the upper perovskite layer.All these improvements lead to faster charge transport and reduced charge recombination.After optimizing the doping content of Zn,the PCE of the Zn doped CuNbO_x(Zn:CuNbO_x)based device is enhanced to 16.71%as compared to that of the pristine CuNbO_x based device(16.01%).Meanwhile,the stability of the Zn:CuNbO_x based devices in air is also improved.(4)The regular PSCs based on Spiro-OMe TAD HTL usually exhibit high efficiency,however,the commonly used Li-TFSI dopant in Spiro-OMe TAD HTL is hygroscopic that induces the instability of PSCs.To mitigate this issue,we adopted hydrophobic endohedral metallofullerene named Er@C₈₂ for the first time to modify the Spiro-OMe TAD HTL.The results show that Er@C₈₂ can improve the quality of Spiro-OMe TAD film and facilitate a more uniform distribution of Li-TFSI in the bulk film,which promotes the oxidation of Spiro-OMe TAD and consequently enhances its conductivity and hole mobility.Moreover,Er@C₈₂ optimizes the energy level alignment between the Spiro-OMe TAD HTL and the perovskite layer.Under the optimized concentration of Er@C₈₂(0.09 mg/m L),the device exhibits a significantly enhanced PCE of 19.22%as compared to that of the control device without Er@C₈₂(17.53%).More importantly,the Er@C₈₂-modified device presents obviously better stability than the control device due to the hydrophobic nature of Er@C₈₂and the improvement in film quality of Spiro-OMe TAD HTL by Er@C₈₂,which can maintain 70%of its initial efficiency after 400 h exposure in air and can even retain 80%of the initial efficiency after 2000 h storage in an argon-filled glovebox.