Structure Design And Interface Modification Of Electron Transport Layer For High-efficiency Perovskite Solar Cells

As a promising next-generation photovoltaic device type in current photovoltaic field,perovskite solar cells have achieved rapid development in recent years.Due to the excellent optical and electrical properties of perovskite materials,the efficiency of laboratory-scale prototype cells has been increased from the initial 3.8%to 25.7%.This achievement is comparable to commercial solar cells on market such as silicon,gallium arsenide,cadmium telluride and copper indium gallium selenium.However,perovskite solar cells still suffer from difficulties such as poor stability and obvious hysteresis effect,which restrict the industrial development of perovskite solar cells.Electron transport layer（ETL）,as the critical component of the perovskite solar cells,plays an important role in extracting or transporting electrons and blocking holes.However,the current mainstream electron transport layer architectures still have problems such as low electron extraction efficiency,poor chemical stability and unfavorable interfacial band arrangement.Therefore,this dissertation will focus on structural design and interface modification of novel high-efficiency ETL,and analyze the characteristics of ETL and its impact mechanism on performance of perovskite solar cells from the perspective of materials and devices;while improving the power conversion efficiency and long-term stability of perovskite solar cells,suppressing hysteresis.The main contents are summarized as follows:（1）The properties of perovskite films and device performance based on planar and mesoporous ETL were constructed and compared,systematically revealing the positive roles of TiO₂ mesoporous framework in CH₃NH₃Pb I₃ perovskite cells.At present,both of planar and mesoporous architectures prevail for perovskite solar cells.However,how the ETL architectures affects the performance of the device is still a problem worthy of further analyzed,especially the specific role of mesoporous framework still needs to be further clarified.The systematic characterization of material architectures,film morphology,optical and electrical properties reveals that the existence of mesoporous TiO₂ layer helps to increase the perovskite grain size,enhance light harvesting,accelerate the effective extraction of electrons and inhibit charge recombination.Finally,compared with planar device,the power conversion efficiency of mesoporous device is significantly increased to 18.18%and hysteresis phenomenon is suppressed.This research can provide guidance for designing and optimizing the architectures of high-performance devices.（2）The application of mesoporous TiO₂ ETL modified with colloidal SnO₂nanoparticles in CH₃NH₃Pb I₃ and CH（NH₂）₂Pb I₃ perovskite solar cells was investigated.From previous work that perovskite solar cells based on the mesoporous TiO₂ have been widely applied due to their high efficiency.However,the surface of TiO₂ has a high density of trap states,and its electron mobility is relatively low.In addition,under the excitation of ultraviolet light,the oxygen vacancies in the TiO₂ film are easily converted into active trap states,which will lead to the loss of photogenerated carriers and degradation of perovskite films.This work utilizes SnO₂ nanoparticles to modify mesoporous TiO₂,which effectively improves the conductivity and inhibits the photocatalytic activity of TiO₂.At the same time,the modification of SnO₂nanoparticles helps to improve the wettability of mesoporous TiO₂ to perovskite precursor solution,enhance perovskite crystallinity and increase perovskite grain size.Therefore,the introduction of SnO₂ can passivate the trap states existing in perovskite film and inhibit interface charge recombination,which is beneficial to further improve the environmental stability of devices.Utilizing SnO₂ nanoparticles to modify mesoporous TiO₂ layer,the efficiency of CH₃NH₃Pb I₃ mesoporous perovskite solar cells has been increased from 17.87%to 19.09%,and the efficiency of CH（NH₂）₂Pb I₃mesoporous perovskite solar cells has reached 20.09%（3）Based on CdS nanorod ETL,a bulk-heterojunction structure perovskite solar cell with efficient electron transmission channel was constructed.The inherent electron mobility of traditional mesoporous TiO₂ is extremely low,which leads to the problem of carrier diffusion imbalance in the mesoporous-type perovskite solar cells,and it is also difficult to fill the mesoporous pores.To solve these problems,CdS with higher electron mobility and lower photocatalytic activity is used as an electron transport material,and a highly ordered one-dimensional nano-array structure is designed to achieve efficient charge transport to replace the mesoporous framework.One-dimensional CdS nanorods are prepared by hydrothermal method.The open load space between the nanorods avoids the difficulty of pore filling and promotes the full penetration of perovskite precursor solution to form a tightly coupled ETL-perovskite bulk heterojunction structure.At the same time,the nanorod array provides a direct transmission channel for electrons,which improves the efficiency of charge collection.Furthermore,the introduction of TiO₂ underlayer improves the vertical growth characteristics of CdS nanorods,and a well-matched gradient band arrangement is formed between TiO₂ and CdS,which improves the built-in electric field of the device and reduces the leakage current.Finally,the perovskite solar cells based on the TiO₂/CdS nanorod composite nanostructure achieved an efficiency of 16.30%,which is the highest efficiency of the current CdS-based CH₃NH₃Pb I₃ perovskite solar cells.（4）Research and propose an effective strategy to solve the parasitic absorption phenomenon in CdS-based ETL,and reveal the effect of colloidal SnO₂ nanoparticle interface modification on the performance of CdS-based perovskite solar cells.It can be seen from the previous work that there has serious parasitic light absorption effect in perovskite solar cells based on CdS.Depositing an ultra-thin CdS layer is an effective method to reduce parasitic absorption.However,a potential problem of ultra-thin ETL is that it may cause direct contact between the conductive substrate and the perovskite layer,resulting in undesirable shunt current leakage.In this work,a planar-type CH₃NH₃Pb I₃ perovskite solar cell based on colloidal SnO₂ nanoparticles-assisted CdS ETL prepared under fully low-temperature was constructed.The systematic characterization of morphology,optics and energy level structure reveals that the interface modification of colloidal SnO₂ provides the ameliorated continuity,reduces the surface roughness,improves the wettability of film,and forms a favorable gradient band structure with CdS,thereby promoting effective charge transfer.The research of charge transfer mechanism shows that the introduction of colloidal SnO₂ interface layer can effectively reduce shunt leakage and inhibit interface charge recombination.Finally,the perovskite cell based on colloidal SnO₂-assisted CdS ETL achieved an efficiency of16.26%,which is twice as high as that of traditional CdS-based devices.In addition,the flexible device based on designed ETL obtained an efficiency of 11.24%.