Tin(Ⅱ)-induced Defect Passivation Mechanism And Regulation In Tin Halide Perovskites

Attention has been drawn to Perovskite solar cells（PSCs）due to their remarkable photovoltaic characteristics.However,the high toxicity of lead has severely hindered the commercial applications of this technology.Tin（Sn）,a non-toxic metallic element with similar electronic properties to lead（Pb）,is considered as the most suitable element to replace Pb.However,tin-based PSCs also face a number of serious challenges,for instance,the rapid crystallization process,low crystallinity,poor film quality and low stability,resulting in a large number of defects in tin-based perovskite films.These defects induce severe carrier recombination,leading to the poor device performance.In order to enhance the performance and stability of tin-based PSCs,it is necessary to find strategies to reduce the density of trap states in the tin-based perovskite layers.In this thesis,the role of density of trap states in the tin-based PSCs is investigated in depth by using additive engineering to improve the quality of the hole transfer layers and perovskite films.A series of characterisations of the films as well as the devices are performed.The main studies are as follows:（1）Hydroxyurea（HYU）was first introduced into poly（3,4-ethylenedioxythiphene）:poly（styrenesulfonate）（PEDOT:PSS）layer to tune its conductivity and hole mobility.HYU doping significantly reduced the defect density in the perovskite films from 1.21×10¹⁶ cm^-3 in the control films to 7.24×10¹⁵ cm^-3,which enhanced the carrier transport capacity and accelerated the carrier transfer,thus extending the carrier transport lifetime.Hence,the PCE of modified devices was elevated from 8.10%to 10.17%.（2）4,4-Biphenyldisulfonic Acid（4,4-BA）was doped into the perovskite precursors.It was shown that the interaction of 4,4-BA with Sn I₂ was effective in released the lattice residual microstain in the perovskite films,leading to the directional growth of tin-based perovskite films,resulting in high-quality perovskite films and decreasing the trap concentration of the light-absorbing layer from 1.20×10¹⁶ cm^-3 to 8.92×10¹⁵ cm^-3.A PCE of 9.45%was obtained for the 4,4-BA optimized devices.（3）Ammonium tartrate（Ammonium L-（+）-tartrate,AT）was used to modify the perovskite films.The mechanism of action between the functional groups and the defect density of states was further investigated.It was shown that AT effectively reduced the defect density of states in the perovskite films,increased the carrier collection efficiency of the devices and improved the carriers transfer rate.Finally,the modified devices showed a 23%increase in PCE compared to the control devices.Therfore,additive engineering was applied in this thesis to effectively reduce the defect concentration,and the carrier transfer mechanism was discussed to prepare highly efficient and stable tin-based PSCs.