Interfacial Optimization Of Electron Transport Layer To Improve Photovoltaic Performance Of Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells（PSCs）have made significant breakthroughs in power conversion efficiency（PCE）due to the outstanding semiconductor properties of perovskite absorbing materials,such as low exciton energy,high absorption coefficient,long intrinsic photogenerated carrier diffusion length,and tunable band gap.The latest record certified PCE for PSCs with n-i-p structure has reached 25.7%,which is close to the highest value of monocrystalline silicon solar cells,but still below the theoretical Shockley-Queiser limit efficiency（33%）.The buried interface between the electron transport layer（ETL）and the perovskite layer serves as both a growth interface and a charge transfer interface,which plays a decisive role in the crystal growth and carrier transport process.Introducing an appropriate interface modification layer at the buried interface of PSCs can synergistically optimize the photoelectric properties of ETL,improve the interfacial charge transfer performance,and regulate the crystal growth kinetics of the upper polycrystalline perovskite thin film at the same time.This not only simplifies the device manufacturing process,but also effectively improves the photovoltaic performance of PSCs.Therefore,in this thesis,three functional molecules were selected to modify the interface of ETL/perovskite layer,and the effects of different functional molecules on the interface of ETL,perovskite layer,and ETL/perovskite layer were studied,revealing the relevant physical mechanisms of different modified molecules affecting the photovoltaic performance of devices.The innovative achievements are as follows:1.The self-assembled monolayer（SAM）of L-histidine（L-His）was introduced on Sn O₂ETL using spin-coating method to synergistically optimize the ETL and ETL/perovskite layer interface,which successfully achieved the following four functions:The carboxyl group in L-His coordinated with Sn⁴⁺in Sn O₂,filled the oxygen vacancy（V_O）and improved the crystal quality and conductivity of Sn O₂ETL;L-His deposition improved the infiltration of perovskite and formed a perovskite film with larger grain size and better crystalline quality;The proton nuclear magnetic resonance（¹H NMR）confirmed that the hydrogen bond formed between perovskite and L-His enhanced the interfacial interaction;L-His also effectively modulated the energy level structure of the ETL/perovskite interface,which not only accelerated the charge transport,but also increased the open circuit voltage(V_OC).As a result,the PCE of MAPb I₃-based PSCs based on Sn O₂/L-His ETL reached 21.04%when the optimal concentration of L-His was 0.06 M.This bio-friendly modification technique is highly compatible with the large-scale production of PSCs.2.The sodium trifluoromethanesulfonate（STFS）SAM layer containing free Na⁺was introduced on Sn O₂ETL using spin-coating method.The free Na⁺was used to optimize the ETL,perovskite layer and ETL/perovskite layer interface simultaneously to improve the photovoltaic performance of PSCs.The results show that:The acidic sulfonic acid group in STFS coordinated with Sn⁴⁺in Sn O₂,filled V_Oand improved the crystal quality and conductivity of Sn O₂ETL.The STFS modified layer reduced the energy level barrier of Sn O₂ETL so that the energy level of ETL and perovskite become more matched,further accelerating the charge transfer.Moreover,during the deposition and annealing process of perovskite film,free Na⁺was proved to diffuse into the perovskite layer,which effectively improved the crystal quality of the perovskite film,reduced the density of defects and inhibited carrier recombination.When the optimal concentration of STFS was 0.08 M,the V_OCof MAPb I₃-based PSCs was 1.16 V,the PCE was increased to 21.13%,and the stability was also improved.3.A sodium 2-iodobenzenesulfonate（2-iodo-）SAM layer was introduced onto Sn O₂ETL by spin-coating method.Similar to the previous work,on the basis of optimizing the ETL,perovskite layer and ETL/perovskite layer synchronously,I end-groups were used to further passivate the intermediate level defects（iodine vacancies,V_I）in the top perovskite layer,thereby achieving the goal of deep integration to improve the photovoltaic performance of PSCs.The results show that:The acidic sulfonic acid group coordinated with Sn⁴⁺in Sn O₂and filled the V_O,so that the crystal quality and conductivity of Sn O₂ETL were improved;The introduction of 2-iodo-effectively adjusted the interfacial energy level structure and lowered the energy level barrier of Sn O₂films,accelerating the charge transport;The I end-group also formed a coordination bond with the V_Idefect in the top perovskite film,which reduced the intermediate energy level amd further suppressed the non-radiative recombination at the interface;In addition,the free Na⁺diffused into the perovskite layer from the interface during the deposition and annealing process of perovskite film,which effectively passivated the defects at the grain boundaries of the perovskite film,suppressed the carrier recombination and improved the PCE of PSCs.When the optimal concentration of 2-iodo-was 0.15 M,the V_OCof MAPb I₃-based PSCs was 1.17 V and the PCE was increased to 21.25%.