Research On Bandgap Regulation Of Tin-Based Perovskite And Optimization Of Photovoltaic Cell Performance

Due to the simple preparation method,low cost and excellent optoelectronic properties of organic-inorganic hybrid perovskite,the perovskite solar cell efficiency has increased from the initial 3.8%to the current 25.8%.The efficiency leaps bring commercialization prospects of perovskite cells one step closer.However,traditional lead-based organic-inorganic hybrid perovskite cells contain toxic element lead（Pb）,which hinders the pace of their large-scale industrialization.Therefore,finding low-toxic or even non-toxic elements to partially or completely replace Pb has become an urgent problem to be solved.Since tin（Sn）has low toxicity and the radius of Sn²⁺is similar to that of Pb²⁺,it is guaranteed that partial or complete replace Pb with Sn will not bring about large lattice distortion.The bandgap width of Sn-based perovskites is narrower than that of Pb-based perovskites.According to the Shockley-Queisser theory,it can be deduced that the theoretical limit efficiency of Sn-based perovskites is 33%,which is more advantageous than the theoretical limit efficiency of Pb-based perovskites,and also provides a huge space for improving the efficiency of Sn-based perovskite cells.However,the bandgap of Sn-based perovskite is different from that of Pb-based organic-inorganic hybrid perovskite,and the current preparation of Sn-based perovskite solar cell still uses a charge transport layer suitable for the bandgap of Pb-based perovskite solar cell.Therefore,the efficiency improvement of Sn-based perovskite cells is greatly limited.Aiming at the problem of bandgap alignment between Sn-based perovskite and charge transport layer,this dissertation takes the bandgap alignment problem as the main research direction.By regulating the bandgap of Sn-based perovskite,the bandgap between the perovskite and the charge transport layer can be optimally matched,thereby improving the efficiency of the perovskite device.The main work of this dissertation is divided into the following four parts:1.The Pb-based organic-inorganic hybrid perovskite with a simple preparation method was selected as the research basis,and the solution-grown TiO₂ was used as the electron transport layer.By adjusting the growth time of TiO₂,the content of Cl and OH^-on the surface of the TiO₂ film was controlled.Modeling and analysis of the effects of Cl and OH^-on the surface of TiO₂ thin films on defects between TiO₂ and perovskite interfaces by DFT simulation.The simulation results showed that Cl increases the interfacial defect formation energy,while OH^-decreases the interfacial defect formation energy.The reduction of interfacial defects inhibits the recombination of carriers,which is beneficial to the improvement of device efficiency.While adjusting the growth time of TiO₂,the bandgap of TiO₂ is also adjusted.The experimental results show that the bandgap alignment between the 1+0.5 h TiO₂ film and the perovskite is the best.Based on the optimal bandgap alignment structure,the highest efficiency of 24.31%is achieved.2.Learn from the experience in the previous section on regulating the bandgap of the electron transport layer and realizing high-efficiency perovskite devices based on bandgap alignment.The triple cation perovskite with excellent stability was selected as the research basis,in this perovskite composition,Sn I₂ is used to replace Pb I₂.In order to reduce the content of Pb in perovskite and reduce the impact of toxic element Pb on the environment.By optimizing the mixing ratio of Pb-Sn in the perovskite,the regulation of the perovskite bandgap is realized.It is found that when the mixing ratio of Pb-Sn is x=0.336,the bandgap of the perovskite and the charge transport layer are optimally matched.The simulation analysis of different Pb-Sn mixed perovskites shows that the bandgap change trend caused by the change of the Pb-Sn mixing ratio during the simulation process is consistent with the bandgap change trend during the experiment.It is further verified that adjusting the mixing ratio of Pb-Sn can realize the regulation of the perovskite bandgap.Based on the excellent bandgap alignment between the Pb-Sn hybrid perovskite and the charge transport layer at x=0.336,the perovskite solar cell achieves the highest efficiency of 16.10%with the smallest hysteresis.Further,two explanations are given for the cause of the hysteresis phenomenon:（1）The change of the Pb-Sn mixing ratio brings about the change of the perovskite grain size distribution.Under the optimal perovskite grain size distribution,the perovskite hysteresis is the smallest.（2）The more alignment of the bandgap between the Pb-Sn mixed perovskite and the charge transport layer,the smaller the band offset（△E）formed between the two layers,and the smaller the band offset,the more conducive to the transport of carriers.Thereby the less the accumulation of carriers between the two energy bands,the ionic capacitance formed between the two layers is also smaller.The smaller the ionic capacitance,the smaller the hysteresis of perovskite.3.In order to completely eliminate the influence of the toxic element Pb in perovskite,based on MASn I₃,the composition of perovskite was regulated by introducing ethylenediamine iodide（EAI）,and it was found that the introduction of organic large cation EA⁺improved the morphology of MASn I₃ perovskite,slows down the crystallization rate of the film,and promotes the formation of the MASn I₃ perovskite structure perpendicular to the substrate orientation,which is conducive to the carriers transport.At the same time,the introduction of EAI also plays a role in regulating the bandgap of MASn I₃ perovskite.Based on the optimal introduction ratio of 40%EAI,the MASn I₃ perovskite solar cell achieves the highest efficiency of 9.24%。4.Considering that EA⁺has a regulating effect on the morphology,crystallization rate,bandgap and crystal structure orientation growth of MASn I₃ perovskite,therefore,it is envisaged to introduce Br^-into the MASn I₃ while retaining EA⁺.Based on the synergistic effect of EA⁺and Br^-,the oriented crystallization of MASn I₃ perovskite is regulated,and the perovskite is induced to grow perpendicular to the substrate.The different proportion of EABr introduced also plays a role in regulating the bandgap of the MASn I₃ perovskite.Under the optimal addition of 30%EABr,the bandgap of the MASn I₃ perovskite and the charge transport layer achieves the best alignment.Based on the optimal bandgap alignment,the perovskite device achieves a maximum efficiency of9.60%.In summary,in order to achieve high-efficiency Sn-based perovskite solar cells,the optimal matching of the perovskite bandgap and the energy band of the charge transport layer is a prerequisite.The bandgap alignment enables the inter-interface carriers to be effectively extracted,suppresses the inter-interface carrier recombination,and reduces the solar cell hysteresis.This dissertation provides a reference for Sn-based perovskite bandgap regulation and optimization of Sn-based perovskite photovoltaic cell performance.