Optimization Of Carrier Transport To Improve The Device Performance Of The Perovskite Solar Cells Based On Different Perovskites

In perovskite solar cells（PSCs）,the extraction and transport of photogenerated carriers is crucial for obtaining high-performance devices.Three-dimensional（3D）perovskites have a good carrier transport ability,one of the factors that limits the device performance is the transport layer characteristics,including its microstructure,transmittance,conductivity,surface energy,etc.The transport layer under the perovskite not only determines the extraction and transport of photogenerated carriers,but also affects the growth and crystallization of perovskite films,which ultimately affects the device performance.For quasi-two-dimensional（2D）perovskites,the carrier transport of the perovskite film itself is limited due to the existence of the quantum well structure and the multiple phase distribution characteristics.To improve the extraction and transport of carriers in perovskite solar cells of different systems and reduce the defect states that affect the carriers transport,the following research are carried out in this thesis:（1）Study on the influence of hole transport layer characteristics of PSCs on device performance:In view of the shortcomings of poly（3,4-ethylenedioxythiophene）polystyrene sulfonate（PEDOT:PSS）,which is commonly used in p-i-n 3D PSCs,such as acidity,hygroscopicity,and carrier transport capacity needs to be improved,we propose to replace it with the organic small molecule material N1,N1＿-（[1,1＿-Biphenyl]-4,4＿-diyl）bis（N4-（naphthalen-1-yl）-N1,N4-diphenylbenzene-1,4-diamine）（NBNDD）.The NBNDD film has a good optical transmittance（>80%）in the visible light range（400-800 nm）,and its HOMO energy level（-4.85 e V）matches the valence band of MAPb I₃perovskite（-5.40 e V）.In addition,the more hydrophobic surface of NBNDD reduces the drag force onγ-GBL,the primary perovskite precursor solvent,promoting the growth of perovskite grains and enhancing the perovskite crystallization quality.Using NBNDD as HTM can also effectively passivate the perovskite/NBNDD interlayer,improving the open-circuit voltage(V_OC)and fill factor（FF）of the corresponding device.Compared with PEDOT:PSS,the electrical conductivity of NBNDD is higher and closer to that of the electron transport material PC₆₁BM,which can balance the transport of electron and hole carriers and improves the short-current density(J_SC)of the device.The device power conversion efficiency（PCE）using NBNDD as HTM increased from 12.15%to 16.09%.The disappearance of the Pb I₂ diffraction peak in the X-ray diffraction results also proved the enhancement of the MAPb I₃ film stability based on NBNDD.（2）Effect of tin dioxide（Sn O₂）electron transport layer（ETL）modification on the performance of 3D PSCs:Sn O₂ thin films prepared by colloidal aqueous solution are commonly used ETL in n-i-p structure 3D PSCs,however,Sn O₂ nanoparticles are prone to aggregate in aqueous solution,and the presence of aggregations will affect the morphology and electrical properties of Sn O₂ ETL.To solve this problem,a simple heat treatment method（thermal solution method）was used to improve the size distribution of Sn O₂ in aqueous solution,and to improve the film morphology and lateral conductivity of Sn O₂ ETL.With the increase of the treatment temperature to 70°C,the size of Sn O₂nanoparticles in aqueous solution changed from bimodal to unimodal,indicating that the size distribution is more uniform,which allows the distribution of Sn O₂ nuclei more uniform during film formation,resulting in the formation of compact Sn O₂ films and decreased grain gaps,resulting in a threefold increase in the lateral conductivity of Sn O₂ETL.At the same time,the compact Sn O₂ ETL can promote the growth of FA_0.1MA_0.9Pb I₃perovskite grains.Compared with Sn O2 ETL prepared at room temperature,the average grain size of FA_0.1MA_0.9Pb I₃ perovskite grown on Sn O₂ ETL prepared by thermal solution method increases from 273 nm to 565 nm.The larger grain size promotes the absorption of FA_0.1MA_0.9Pb I₃,resulting in an increase in the device J_SC from 23.34 m A/cm² to 24.99m A/cm².In addition,the Sn O₂ ETL prepared by the thermal solution method can extract photogenerated electrons more efficiently,reduce the FA_0.1MA_0.9Pb I₃ interface recombination,improve the V_OC and FF,and increase the PCE from 17.63%to 20.67%.After 100 h of maximum power point tracking,the efficiency of the device maintains at85%of the initial efficiency.（3）Binary organic spacer cation regulates phase formation and morphological properties to improve hole transport in quasi-2D PSCs:The incorporation of hydrophobic organic spacer cations enables quasi-2D perovskites to exhibit better humidity stability than their 3D counterparts.However,the quasi-2D perovskite films show poor morphology,and the multiple phase distribution in the films can hinder the carrier transport.In response to this problem,we propose a binary organic spacer cation synergistic strategy.We utilize organic spacer cations n-butylammonium（BA⁺）to achieve self-assembly quasi-2D perovskite crystals through van der Waals interactions between BA⁺cations;simultaneously,we add a small amount of organic spacer cation Octylammonium（OA⁺）with longer carbon chains to tune the morphology and phase distribution of the films.The results of UV-vis absorption and photoluminescence show that the addition of OA⁺does not affect the composition of the phase in the film but reduces the small-n phases（n<4）at the bottom of the quasi-2D perovskite film.This could be the result of different crystallization dynamic equilibrium states due to the difference in solubility of BAI and OAI in organic solvents.Due to the wide optical bandgap of the small-n phases,the small-n phases will act as hole traps when located at the bottom of the film,hindering the transport of holes from the perovskite active layer to the PEDOT:PSS.Therefore,reducing the small n-phase at the film bottom after adding OA⁺improves the carrier lifetime in the quasi-2D perovskite from 16.53 ns to 36.23 ns.At the same time,OA⁺can promote the anisotropic growth of perovskite crystals,allowing a more compact and uniform perovskite morphology,which is beneficial for charge transport.By adopting the synergy strategy between BA⁺and OA⁺,the PCE of the n=4 quasi-2D perovskite(BA₂MA₃Pb₄I₁₃)is increased from 10.81%to 11.90%.The J_SC of the device slightly reduces from 19.04 m A/cm² to 18.71 m A/cm² due to the insulating properties of organic spacer cations.However,the V_OC and FF are improved from 0.94 V and 0.60 to 0.99 V and 0.64,respectively.The improvement of V_OC and FF is attributed to the improvement of phase purity and film morphology.Finally,the stability test indicates that the device shows consistent stability before and after incorporating OA⁺,the PCE of the device remains at 60%and 80%of the initial efficiency after keeping in the air for 400 h and in the nitrogen glove box for 1200 h,respectively.