Research On Perovskite Solar Cells Based On P-type Cs₂SnI₆ Doped Hole Transport Layers

Power conversion efficiency（PCE）of perovskite solar cells has rapidly risen from the 3.8%in 2009 to the current 26%,in which the hole transport material（HTM）—Spiro-OMe TAD has played a significant role in boosting the device performance.Since Spiro-OMe TAD is an organic semiconductor,its electrical conductivity and carrier extraction ability have considerable room of enhancement.As such,this work focuses on the optimization of Spiro-OMe TAD hole transport layer（HTL）,with the research idea of introducing another kind of HTM—Cs₂SnI₆ into the Spiro-OMe TAD system to synergistically enhance the conductivity and carrier extraction ability,as owing to the excellent carrier properties,lead-free environmental friendliness,and good stability of Cs₂SnI₆ dopant.Nonetheless,there are discrepant reports on the basic physical properties of Cs₂SnI₆,which negatively impacts the optimal designs of materials and PSCs.Therefore,this work aims to explore the physical and chemical properties of Cs₂SnI₆,and then,p-type Cs₂SnI₆ powder was prepared and applied to the doping optimization of Spiro-OMe TAD HTL.Detailed research contents are outlined as below.This work first sets out to investigate the physical and chemical properties of Cs₂SnI₆.By using hydroiodic acid as additive and through composition control,relatively uniform Cs₂SnI₆ thin films are synthesized.It was found that the pure Cs₂SnI₆ is actually a n-type semiconductor,which exhibits stability over 40 days of aging in air with 40%-60%relative humidity.By adopting one-step solution method,Cs₂SnI₆ powder is further prepared.It is found that the optical bandgap of pure Cs₂SnI₆ is 1.26 e V,which has a decomposition temperature of 300℃,thus indicating an excellent thermal stability.Most importantly,it was found that upon the excess of SnI₄ in the precursors,doping properties of the as-prepared Cs₂SnI₆ powder monotonically changed from n-type semiconductor to p-type semiconductor,with its bandgap remaining unchanged at 1.26 e V.In order to interpret the root cause of the variation of p/n characteristics,p-type Cs₂SnI₆ powder was prepared in the I-rich and Sn-rich environments,and it was revealed that the excessive SnI₄ leads to the change in structural defects and is responsible for the n-to-p transformation of doping type in Cs₂SnI₆ powders.The research work also calculates the electronic structure of Cs₂SnI₆ through first-principle theoretical calculations,with the calculated energy band structures being consistent with the experimental results.The calculated results also showed that residual SnI₄ did not affect the electronic structure of Cs₂SnI₆near its band edges,elucidating the reason that excessive SnI₄ did not cause widening of Cs₂SnI₆’s bandgap.Furthermore,p-type Cs₂SnI₆ powder is used to enhance the electrical and carrier properties of Spiro-OMe TAD HTL as a dopant.P-type Cs₂SnI₆ results in increased electrical conductivity,hole extraction,while reducing the trap-state densities within the PSC devices.As a consequence,Cs₂SnI₆-treated PSCs show notable improvements in photovoltaic performance of the target devices as compared to the undoped PSCs,with the highest PCE increasing to 21.4%from the 20%in the control devices.Meanwhile,the introduction of p-type Cs₂SnI₆ powder increases the surface hydrophobicity of HTL,thus improving the air stability of the functionalized PSCs,which retain 93%of the initial PCE after aging in ambient air for 1512 h.