Preparation And Photoelectric Properties Of Cu₂ZnSnS₄ Quantum Dots As Hole Transport Layer In Perovskite Solar Cells

As the fourth generation solar cells,perovskite solar cells can be simply prepared by liquid process and mass-produced through printing or spraying methods.The conversion efficiency of the CH₃NH₃PbI₃ solar cells increased rapidly from 3.8%to over 22%since it was first used as the light absorber in 2009,which attract the attention of scientists around the world.However,the spiro-MeOTAD,which acts as hole transport material,has poor hydrophilic property and complex purification process.This makes the perovskite solar cells have poor stability and high manufacturing cost,which is unfavorable to its commercial promotion.Therefore,searching for a new alternative material has great significance.CZTS quantum dots?QDs?as the novel quaternary direct bandgap semiconductor materials,are often used as light-absorbing materials for solar cells with high carrier mobility.Meanwhile,the preparation process is simple and the raw materials contain no toxic components.CZTS QDs with suitable band gap can be prepared by simply controlling the active temperature,these QDs will improve its photoelectric conversion efficiency and long-term stability as the hole transport layer in perovskite solar cells.In this dissertation,CZTS QDs are used as substitute materials for spiro-MeOTAD to assemble the FTO/TiO₂/Perovskite/CZTS/Au solar cells.Based on the optimization of the original solar cell technological conditions,the high efficiency stable perovskite solar cells are fabricated.In perovskite solar cells,TiO₂ as an electron transport layer is not only serves to block holes transport from perovskite layer,but also affects the morphology of the perovskite layer.In this paper,we study different methods for the preparation of dense TiO₂ layer.By comparing the photoelectric conversion efficiency of the solar cells,we choose the most suitable method to prepare the electron transport layer.At the same time,in order to optimize the TiO₂ prepared by immersion method,the concentration of soaking solution,soaking conditions and the time of anneal process are studied.In this paper,one-step anti-solvent method and two-step method for the preparation of CH₃NH₃PbI₃ light absorption layer are used.The result finds that the perovskite film prepared by one-step anti-solvent method is more smooth and controllable with high repeatability.When the anti-solvent is added at 5th second,a perovskite film with 350 nm thickness is prepared and no voids is found.When Pb?SCN?₂ is used as an additive,the perovskite crystal size increased from 50 nm to about 1.5?m.The suitable concentration of the additive in perovskite precursor solution is 4 wt%.Larger crystal size will reduce the recombination of the carrier pairs,which improves high short circuit current to improve the photoelectric conversion efficiency.In this paper,CZTS QDs were synthesized by thermal injection method,in order to study the influence of the QDs on the power conversion efficiency,different sizes and phases QDs are prepared.The result shows that the hole transport properties of the 6.3 nm hexagonal CZTS QDs are most suitable.With the increasing of synthesis temperature,the QDs increase from 5.3 nm to 8.7 nm,meanwhile the optical band gap decreases from 2.09 eV to 1.57 eV.At the same time,we have explored the influence of different solvents,different spin-coating process and the treatment of quantum dots surface on the photoelectric properties,the study finds that,using hexanethiol as solvent,4000 rpm spin coating speed will prepared a 50 nm flat CZTS quantum dots layer.Simultaneously,the use of PbI₂ solution in the process of cleaning QDs can effectively improve the electron transfer rate of the prepared film,thereby improving the efficiency of the solar cells to 6.5%.Compared to the solar cells based on the spiro-MeOTAD as hole transport material,stability of the solar cells based on CZTS QDs have been greatly improved.All samples storage in the air for 7 days,solar cells based on CZTS QDs maintain 70%of the original power conversion efficiency,while the cells based on spiro-MeOTAD have only remain 40%.