| The hybrid organic-inorganic perovskites have become the most remarkable photovoltaic materials due to their exceptional properties such as high light absorption coefficient,long carrier diffusion length and adjustable band gap.In the past few years,the photoelectric conversion efficiency(PCE)of organic-inorganic hybrid perovskite solar cells(PSCs)has increased rapidly from 3.8%to 23.7%.This unprecedented rise in efficiency,with the added advantage of low-cost processing has made perovskite solar cells a promising photovoltaic technology for commercialization.However,PSCs suffer from poor stability,which is the major obstacle of bringing PSCs to the commercial market.Therefore,the most challenging issue in PSCs today is how to further improve the long-term device stability without sacrificing the efficiency.In order to prepare more stable PSCs with higher efficiency we have systematically studied the intrinsic stability and properties of the perovskite materials by growing Cs-doped ternary cation and mixed halogen perovskite single crystals for optimizing a more stable new component.Then,we explored and optimized the preparation process of PSCs based on this new component by improving the morphology and crystal quality of perovskite film to increase device performance.At last,we made effort to further improve the performance of PSCs through interface passivation with ionic liquids.The main contents and results are as follows:1.We systematically studied the intrinsic stability and properties of perovskite single crystals for optimizing a more stable new component.In this work,series of ternary cation and mixed halide perovskite single crystal alloys(FAPbI3)1-x-y(MAPbBr3)y(CsPbBr3)x were successfully prepared by inverse temperature crystallization method(ITC),which enabled systematic investigations on the intrinsic structural stability against light,heat,water and oxygen.The experimental results show that incorporation of inorganic cesium ions into FA-based perovskites can effectively inhibit the formation of non-perovskite phase(δ-FAPbI3),and improve the structural stability.With Cs content of 5%,the perovskite materials exhibit excellent water-oxygen stability and light stability.However,when the Cs content reaches 10%,phase separation occurs and δ-CsPbI3 is detected after being in contact with water and oxygen.Eventually,we optimized a highly stable new composition-(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05.The band gap of the single crystal is 1.52 eV,and its carrier lifetime is up to 16μs.Moreover,it does not undergo any decomposition or phase change during the 10,000 h water-oxygen and 1,000 h light stability tests.Besides,this composition also exhibits excellent thermal stability and thermal decomposition is observed until 311 ℃.These findings indicate that the new composition is particularly suitable for the preparation of promising PSCs with high efficiency as well as long-term stability.2.We optimized the device fabrication process for PSCs based on the new stable composition.The performance of PSCs is closely related to the quality of(FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 perovskite film and its preparation process.Since the solubility of the cesium species in the perovskite precursor solution is small,the crystal nucleation growth process is fast,resulting in a small crystal grain size,poor crystallinity of the perovskite film formed via one-step solution processing.We demonstrated that replacing the CsBr with CsI/DMSO solution to prepare the perovskite precursor solution can increase the solubility of the cesium species and thus slow down the nucleation process effectively.Moreover,we found that employing MACl as an additive can further improve the crystal quality and increase the grain size of the perovskite film through changing the intermediate.Finally,a high-quality perovskite film with good crystallinity and large crystal grains was obtained,and PCE of 20.3%of relevant device was achieved.3.We h ave tried to passivate charge defects at the surfaces and grain boundaries of perovskite film by ionic liquid interfacial modification to further improve the performance of PSCs.Ionic liquids are composed of anions and cations,which can simultaneously passivate the positive and negative charged defects of the perovskite film.We initially selected two kinds of different ionic liquids to explore the effects of hydrophilicity and steric hindrance on device stability and efficiency.The results show that the hydrophilic ionic liquids cause degradation of the solar cell,while hydrophobic ionic liquids cannot spread evenly on the surface of perovskite film.We therefore used isopropyl alcohol to reduce the concentration of the ionic liquid,and achieved uniform spreading of the hydrophobic ionic liquid.We demonstrated that hydrophobic choline hexafluorophosphate can effectively improve the performance of PSCs in both the efficiency and stability.Moreover,[EMI]PF6 and[BMI]PF6 with different carbon chain lengths were used to investigate the effect of steric hindrance on device efficiency.The results reveal that[EMI]PF6 is superior to[BMI]PF6 in defect passivation because[BMI]PF6 with longer chain causes a decrease in short-circuit current of PSCs.Therefore,the structure of the ionic liquid must conform to the structure of the perovskite for efficiently passivation. |
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