Preparation Of High-performance Perovskite Solar Cells By Interface Engineering

In recent years,perovskite materials have attracted more and more attention due to their excellent photoelectric properties,solution process and low cost.The performance of perovskite solar cells（PSCs）has jumped from an initial 3.8%to 25.5%.However,PSCs still have some problems,such as interface material instability,energy level mismatched and poor photothermal stability of devices,etc.In this thesis,the performance of perovskite devices is improved by modifying the interface materials of PSCs and synthesizing new interface transport materials.The main contents are as follows:（1）The performance of perovskite devices was improved by modifying PEDOT:PSS with metal oxide to change its function and wettability.PEDOT:PSS can be roll-to-roll deposited on the substrate facilely in electronics,but its acidity and mismatched energy level limit the performance and stability.Herein,we incorporate different metal salt into PEDOT:PSS solution to prepare PEDOT:PSS-A_xO_y（VO_x,Mo O_x,WO_x）composite hole transport layer（HTL）and find that the performance of inverted PSCs can be greatly enhanced.PSC using PEDOT:PSS-Mo O_x achieved much higher power conversion efficiency（PCE）（19.64%）than that of pristine PEDOT:PSS（12.19%）.The increased surface free energy（γ_S）of PEDOT:PSS-A_xO_y is beneficial for the formation of large crystal size and pin-hole free film,leading to reduced nonradiative recombination.Meanwhile,the work function of PEDOT:PSS can be tuned to match the energy level of photoactive layer with small amount incorporation,which greatly enhance the photovoltage by a factor of 1.1.Besides,the devices based on PEDOT:PSS-A_xO_yexhibit long-term stability.Unencapsulated PSCs with PEDOT:PSS-Mo O_x retained over 90%and 80%of their initial PCEs in N₂ for 45 days and in ambient air for 20 days,respectively.PEDOT:PSS-A_xO_y overcome the intrinsic imperfection and can be potentially employed for large scale production in the electronic devices.（2）Synthesis of doping-free organic small molecule hole transport material（HTM）to improve the stability of all-inorganic PSCs.As the most commonly used HTL for conventional devices,the presence of dopants in Spiro-OMe TAD greatly reduces the stability of the devices,while the complex synthesis route and purification difficulty increase the manufacturing cost,which are not conducive to large-scale production.In this chapter,an easy to synthesize and low cost free-doped small molecule HTM（BD）was synthesized to replace Spiro-OMe TAD to prepare Cs Pb I₂Br devices.Its BD-based device achieved an efficiency of 12.79%,which was similar to the performance of the Spiro-OMe TAD device（13.31%）.But the cost of the device is greatly reduced and the stability is effectively improved.The efficiency of the device based on BD is basically unchanged when the device is placed in a dry air environment for 60 days.Even at 85 ^oC for 35 days,the device efficiency can be maintained at more than 90%of the initial efficiency.BD with high hole mobility and low cost is expected to replace Spiro-OMe TAD as a promising HTM for PSCs.（3）Interfacial charge adjustment of all-inorganic PSC.Compared to the hybrid perovskite,inorganic perovskite has potential to improve the thermal stability,but the photoelectric performance is limited by the film defect and unbalanced charge transfer.In this work,we pre-treat Sn O₂ layer with fullerene-ethylenediamine derivatives(C₆₀-EDA and C₇₀-EDA)and post-treat the Cs Pb I₂Br perovskite film using 5-aminovaleric acid hydrobromide（5-AVABr）,leading to the high photovoltaic/electroluminescent（PV/EL）performance.Due to the improved film-forming property,Sn O₂:C₆₀-EDA and Sn O₂:C₇₀-EDA induced large-grain Cs Pb I₂Br film with low defect density and superior interface connectivity.The combined pre-post treatments balance the charge transfer at N-I-P interface for injection and extraction,respectively,leading to the efficient inorganic perovskite bifunction device（PBD）.The target Cs Pb I₂Br device achieved a PV efficiency of 16.58%in the PV mode and the EL external quantum efficiency(EQE_EL)of 6.2%in the EL mode,much higher than the control device(PV efficiency of 12.06%and EQE_ELof 1.0%).Besides,the performance can still maintain 93%and 80%of its initial values when stored in dry air atmosphere for 65 days and heated at 85 ^oC for 12 days,respectively.The stable PBD has potential to utilize the solar energy more efficiently for smart power supply in the electronics.（4）The all-inorganic PSC based on doped metal oxides.All-inorganic PSCs cannot deliver high performance due to the poor interface contact.In this paper,we used a simple solution method to prepare Sn doped Ti O₂(Ti_1-xSn_xO₂)ultrathin nanoparticles as electron transport layer（ETL）in conventional n-i-p and inverted p-i-n PSCs.We found that Ti_0.9Sn_0.1O₂ nanocrystals have good conductivity,easy film formation and high work function.The Ti_0.9Sn_0.1O₂/Cs Pb I₂Br/Spiro-OMe TAD n-i-p PSC achieve 15.18%PCE and all-inorganic inverted Ni O_x/Cs Pb I₂Br/Ti_0.9Sn_0.1O₂ p-i-n device also shows a PCE of 14.00%.After stored in85 ^oC for 65 days and continuous light illumination for 600 h,the inverted PSCs have no efficiency decay.Even under continuous illumination with 85 ^oC,the device still retains 85%of the original efficiency after 200 h.We demonstrated that ultrathin binary oxide Ti_1-xSn_xO₂ is suitable to develop high performance and stable PSC.