Preparation And Properties Of Perovskite Solar Cells Based On Interface Engineering Strategy

With the development and progress of society,the growing global population further stimulates the greater demand for energy in all countries.However,the high dependence on fossil fuels has caused serious environmental pollution.Solar energy is a kind of clean and renewable energy with huge reserves.Converting solar energy into electricity through solar cells is one of the feasible solutions to effectively solve energy problems.Although the sun provides an adequate energy supply,the performance and production costs of solar cells still need to be further improved and optimized.In recent years,perovskite solar cells（PSCs）stand out in many solar fields due to their excellent photoelectric performance,and become the dark horse in photovoltaic devices,so far,the efficiency of single-junction organic-inorganic hybrid PSCs has exceeded 26.1%.However,there are still some problems in PSCs,such as non-radiative recombination of interfacial carriers caused by buried defects and energy barriers in electron transport layer（ETL）materials,charge recombination caused by grain boundary and interfacial defects in perovskite films,and carrier transport imbalance between perovskite layers and each layers.These problems limit the further improvement of PSCs performance.In this paper,the interface of n-i-p type PSCs is optimized.Small organic molecules with different functional groups are introduced into PSCs,the energy level matching between layers is adjusted,the surface morphology of perovskite films is improved,and the efficiency of carrier extraction and transport is increased,so as to obtain high-performance PSCs devices.The main research contents include:（1）A multifunctional dopant was used to passivate the defects between ETL and perovskite layer,doping 4-morpholine ethane sulfonic acid sodium salt（MES Na⁺）into the SnO₂ precursor solution to prepare planar heterojunction PSCs.The experimental results show that the introduction of MES Na⁺can improve the conductivity of ETL,which is beneficial to the extraction and transmission of electron,and reduce the work function of ETL,improve the energy level matching,passivate the interface defects between SnO₂ ETL and perovskite,reduce the interface non-radiative recombination,and obtain high quality perovskite films.Finally,the PSCs based on SnO₂-MES Na⁺(10 mg m L^-1)obtain the best effect.The champion power conversion efficiency（PCE）is increased from 17.43%of the control device to 21.05%of the optimized device.（2）The non-radiative recombination of interface carriers caused by buried defects and energy barriers in ETL materials seriously hinders the further improvement of the efficiency and stability of PSCs.The improvement of buried interface（in this paper,the interface is between ETL and perovskite layer）is a simple and effective method to solve the above problems.Therefore,we used choline chloride（CC）,acetylcholine chloride（AC）and phosphocholine chloride sodium salt（PCSS）to study the effects of choline derivatives with different functional groups on SnO₂ ETL.The introduction of dopants improves the energy level matching between SnO₂ ETL and perovskite layer,and acts as a bridge through synergistic effect to form an ETL film with uniform morphology,which is conducive to enhancing interface contact and passivating defects.The results show that compared with CC（containing-OH group）and AC（containing C=O group）molecules,the PCSS with functional groups such as P=O and Na⁺are more beneficial to improve the performance of PSCs,which is mainly due to the stronger interaction between PCSS with SnO₂ ETL and perovskite layer.Finally,the champion PCE based on the SnO₂-PCSS ETL device reached 23.06%and the open circuit voltage(V_OC)was as high as 1.20 V,while the PCE based on the SnO₂ ETL device was 20.55%.In addition,after 500 h of storage at 25°C and 30-40%relative humidity（RH）,the unpackaged device based on the SnO₂-PCSS ETL retained 94%of its initial efficiency,while the control device based on the SnO₂ ETL only retained 80%.This study provides a meaningful reference for the design and selection of ideal pre-buried additive molecules.（3）SnO₂ ETL was modified by a multifunctional small organic molecule 2-amino-1,3-propanediol（APDO）,which eliminated the inherent defects of SnO₂ and passivated the interfacial charge recombination.In addition,the introduction of APDO can regulate the crystallization process of perovskite crystals,obtain high-quality perovskite films,which is conducive to improving the performance of PSCs.We pre-buried the SnO₂ETL by two methods,labeled SnO₂-APDO（doping）and SnO₂/APDO（ETL upper surface modification）.The experimental results show that the SnO₂-APDO ETL has more advantages in defect passivation and grain growth regulation than SnO₂/APDO ETL.Finally,the control device based on the rigid ITO substrate and using the MAPb I₃obtained the PCE of 20.17%,while the optimized device based on SnO₂/APDO ETL and SnO₂-APDO ETL obtained the champion PCE of 21.08%and 21.93%,respectively.At 25°C and 30-40%RH,unpackaged device based on SnO₂-APDO ETL retain 93%of its original efficiency after 500 h of storage.At the same time,the control device based on flexible PET substrate obtained the PCE of 18.87%,while the flexible device with SnO₂-APDO ETL obtained the champion PCE of 20.64%.The above experiments show that the use of APDO to passivate the defects of SnO₂ ETL and perovskite films is an effective strategy to achieve high photovoltaic performance devices.（4）The performance of PSCs was improved by using two organic small molecules,2-chlorothiazole-4-carboxylic acid（SN）and 2,5-pyridine-dicarboxylic acid（PDCA）,to passivate the interface defects between the perovskite layer and hole transport layer（HTL）.In the first study,the SN molecule based on the five-membered thiazole ring,due to its donor atoms such as sulfur（S）and nitrogen（N）,and-COOH functional group,can react with the uncoordinated Pb²⁺and I^-vacancy through lone pair electron to promote the formation of stronger covalent bonds at the interface,thus passivating the inherent defects of the perovskite layer and reducing the non-radiative recombination sites.Finally,the champion PCE of SN treated PSCs was 22.55%.In the second study,the interface modification strategy of PDCA molecule based on six-membered pyridine rings.Since N donor atom contained in pyridine rings and functional groups such as-COOH on both sides also have ability to effectively passivate the defects of perovskite films,the introduction of PDCA can increase the grain size of perovskite,prolong the life-time of carriers,and improve the stability of PSCs.Finally,the champion PCE of PSCs treated with PDCA was 22.45%.Therefore,interface passivation strategy is one of the effective methods to improve the performance of PSCs.