| Recently,the metal halide perovskite materials have attracted wide attention in photovoltaic field due to its broader absorption spectrum,higher carrier mobility,longer diffusion length and tunable band-gaps.The power conversion efficiency of perovskite solar cells boosts from 3.8%to 22.7%now in just a few years,which pushes perovskite solar cells to be the most promising photovoltaic devices.However,all this breakthrough should be contributed to lead-based perovskite and lead is a kind of poisonous element,which will have a negative impact both on environment and human body during the experimental preparation or for the practical demands in the near future.For this reason,we urgently need to find lead-free perovskite materials.Tin element becomes the best choice to substitute lead for they have similar physical and chemical property.However,tin is significantly instable in its divalent oxidation status when exposed to air,in addition,it is much more challenging for tin-based perovskite to form a homogeneous and full-covered film compared with lead counterpart,thus leading to a poor photoelectric performance.Tin fluoride has been introduced to address the above problems by many researchers,and they tried to explain the role of SnF2 in improving electric property of tin-based perovskite material,but so far the mechanism of SnF2 in improving film quality is still not clear.In this paper,we study the mechanism by spin coating tin-based perovskite films and growing single crystals and fill in gaps in this aspect.To simplify the preparation process,a one-step method was used to prepare the MASnI3-xBrx(x=0,1,2,3)material,and its physical properties were analyzed by means of some regular instruments.At the same time,considering the shorter diffusion length of tin base perovskite,we applied mesoporous structure for solar cell’s structure,namely the FTO/TiO2 compact layer/TiO2 mesoporous layer/Tin-based perovskite/Spiro-OMeTAD/Au,and made a lot of relative tests after preparation of these devices.Here are the main conclusions:1.Bromide doping in MASnI3 can tune band-gaps and improve the film quality of tin-based perovskite material.When doped with two bromide atoms,the photoelectric conversion efficiency reaches the submit in virtue of SnF2.2.The mechanism of SnF2 in improving film quality of tin-based perovskite is as follows:SnF2 will first precipitate from perovskite solution and serve as excess nucleation centers to facilitate heterogeneous nucleation in a faster and more uniform way,and these nucleation sites will grow up and then merge into larger regularly arranged crystalline grains after annealing.It is also confirmed that this mechanism can be applied to the growth of both thin film and single crystal.3.The MASnIBr2 film with 30%SnF2 shows the most compact and uniform morphology and the best photoelectric performance,and the maximum photoelectric conversion efficiency is 3.70%.4.The addition of SnF2 won’t change the crystal and phase form and band-gaps of tin-based perovskite;the grain size of MASnIBr2 crystalline is connected with doping concentration of SnF2:the higher concentration leads to a larger grain size,but it can’t grow up infinitely.5.There is an obvious improvement in electric property of tin-based perovskite after the addition of SnF2 with one order of magnitude decrease in carrier density,one order of magnitude improvement in recombination resistance and a nearly 17-fold improvement in carrier lifetime. |
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