Improving Stability Of Perovskite Solar Cells Via Modifying ZnO Electron Transport Layer

Recently,perovskite solar cell has attracted increasing attention due to the superb photovoltaic performance thanks to the intrinsic material properties with high absorption coefficient,balanced charge transport behavior and low trap density.These features make them exceptional choices for fabrication of high efficiency photovoltaic devices.Solar-cell devices based on hybrid perovskites of the CH3NH3PbX3(X = I-,Br-,Cl-)type have been largely improved as their power conversion efficiencies(PCEs)have drastically increased from 3.8% to 22.1% in only a few years.The devices based on perovskite materials and titanium oxide(TiO2)also show a bright prospect owing to that they could be fabricated in a simple technology and on flexible substrate.Despite its promising efficiency,commercial use of the perovskite-based solar cell is limited in part by its poor stability with respect to high moisture and temperature.Zinc Oxide(ZnO)is introduced into the perovskite devices to replace the traditional TiO2 because of ZnO is known to have an electron mobility that is substantially higher than that of TiO2.Higher electron mobility can help fabricate better stable perovskite solar cells in moisture environment which makes it an ideal choice for an electron-selective contact.However,instability of ZnO-perovskite based solar cells devices in high temperature condition remains a big problem.Firstly,the fabrication methods of perovskite light-absorption layer(CH3NH3PbI3)had been optimized and a better crystal structure of the CH3NH3PbI3 and compact,good light absorption film were obtained respectively.We got a device exhibiting a power conversion efficiency as high as 12.56% after the package with other interface layer.Secondly,to study the failure of the devices,the perovskite solar cell was set into the environment of 150℃ temperature,then operating parameters were measured.The failure mechanism was pointed out that hydroxide(OH-)in the ZnO film could react with the CH3NH3+ and decompose the CH3NH3PbI3 perovskite material,leading to the failure of devices.Finally,we focused on solving the problem of devices failure and carried two works as follows:(1)We addressed the instability problem of perovskite solar cell by employing stannum-doped zinc oxide(TZO)as the electron transport layer and obtained the film fabrication method of 2% Sn doping annealing at 200℃.CH3NH3PbI3 film deposited on the TZO could be stable as long as 60 minutes in 150℃ temperature environment and the time would be tripled comparing to time of CH3NH3PbI3 filim deposited on the ZnO(20min).FT-IR test could demonstrate that doping of Sn could reduce the amount of hydroxide(OH-)on the surface of ZnO effectively through.Thus,TZO film would indirectly reduce the degration of CH3NH3PbI3 filim and prevent the failure of perovskite solar cells.The devices using TZO as electron transport layer showed a power conversion efficiency of 13.06% and only 20% decline of power conversion efficiency after it was placed in outdoor environment 15 days later.In addition,doping of Sn could increase the coverage of TZO on the ITO and reduce the recombination of electron and hole at the interface between perovskite and TZO,helping improve the electron transport speed and enhance the power conversion efficiency.(2)We also addressed the instability problem of perovskite solar cell by employing ZnO-graphene quantum dots(ZnO-G QDs)as the electron transport layer and obtained extraordinarily thermally stable perovskite layers through 80 mg grapheme oxide(GO)doping annealing at 200℃.CH3NH3PbI3 film deposited on the ZnO-G QDs could be stable as long as 80 minutes in 150℃ temperature environment.It could be concluded that graphene works as a protection layer of perovskite and improves the device stability through FT-IR and GIXRD experiments.The devices using ZnO-G QDs as electron transport layer showed a power conversion efficiency of 13.41% and only 20% decline of power conversion efficiency after it was placed in outdoor environment 15 days later.In addition,the ZnO-G QDs layer in the device structure improves the extraction and transfer of electrons at the interface and as a result.When it comes to fabrication of perovskite solar cells,it is important to choose a suitable electron transport layer,considering the electron transport property,process methods and whether it react with perovskite layer or not.The content of this thesis provides systematic,regular and effective basis for fabricating high performance and stable perovskite solar cells.