Improved Comprehensive Performance Of CsPbI₂Br Solar Cells By Additive Engineering

In the last decade,organic-inorganic hybrid perovskite solar cells（PSCs）have shown rapid development owing to the good comprehensive optoelectronic properties,such as high absorption coefficients,long electron-hole diffusion lengths and tunable bandgaps for the related photoactive materials.The current power conversion efficiency（PCE）of the organic-inorganic hybrid PSCs is comparable to that of commercial crystalline silicon solar cells.However,hybrid PSCs suffer from unstable performance due to the decomposition of the fragile cations（CH₃NH₃⁺or NH=CHNH₃⁺）under light,thermal and moisture attack,which severely hinders the commercialization of the related solar cells.As a unique alternative,all-inorganic perovskites（Cs Pb X₃,X=I,Br）,substituting the organic cations with the Cs cation,show better light and thermal stability.Among all the Cs-based inorganic perovskites,Cs Pb I₂Br has a suitable bandgap of 1.91 e V,thus showing huge potential in photovoltaic（PV）application,especially for tandem and semitransparent PV devices.Although the performance of Cs Pb I₂Br devices has made significant progress,which,however,is still far away from hybrid PSCs.As the core part of the PSCs,the quality of the Cs Pb I₂Br film directly determines the PCE and stability of the device.Therefore,in this thesis,we aim to improve the quality of perovskite films by additive engineering,and thus to prepare high-efficiency and stable Cs Pb I₂Br PSCs.The effects of different types of additives on the morphology,defects,optical and electrical properties of the Cs Pb I₂Br films are systematically investigated,and meanwhile the related mechanisms are discussed.The main research contents and results are as follows:（1）Carbon nanodots（CNDs）are introduced into the Cs Pb I₂Br precursor solution as an additive to improve the PCE and operation stability of the related PSCs.It is found that the crystallinity can be enhanced,and the trap density can be notably suppressed after introducing CNDs with an optimal quantity.In addition,the improved interface contact between the Cs Pb I₂Br film and the neighboring layers due to the CND addition,thus leads to the enhanced charge separation and transport at the corresponding interface.The demonstration device with a configuration of ITO/Sn O₂/CND-added Cs Pb I₂Br/P3HT/Ag can thus deliver the average PCE of 13.77%（the highest PCE:14.69%）,compared with that（12.14%）of the control device without CND addition.Moreover,thanks to the enhanced humidity resistance of the Cs Pb I₂Br film,the CND-added PSCs exhibit superior stability,i.e.,～86%retention of the initial PCE after 160h aging in air with the humidity of 20–30%,to the control device.（2）The Ti₃C₂T_x MXene nanosheets are introduced in the Cs Pb I₂Br layer as an additive,to prepare large-grained,defect-free and high crystalline perovskite film,thus leading to the reduced the nonradiative recombination of carriers.Together with the formation of the Schottky junction between MXene nanosheets and Cs Pb I₂Br,enhanced separation and transport of the photogenerated electron-hole pairs can be realized.Thanks to these positive effects owing to the addition of Ti₃C₂T_x MXene into Cs Pb I₂Br,the solar cell with the MXene-added Cs Pb I₂Br layer delivers a significantly improved PCE of 15.10%.In addition,the MXene-added Cs Pb I₂Br layers exhibit the enhanced humidity resistance.（3）Low-cost Si O₂ nanoparticles are embedded into the Cs Pb I₂Br photoactive layer,to improve the comprehensive performance of the inorganic PSCs.A systematic study indicates that the improved crystallization quality,optimized the surface morphology due to the Si O₂ nanoparticle addition,thus leading to the reduced trap density,as well as enhanced resistance to moisture and Ag diffusion for the Cs Pb I₂Br layers.In addition,the embedded Si O₂ nanoparticles as light scattering centers,lead to the enhanced light absorption,thus increasing the short-circuit current density(J_sc)of the device.As a result,～18.39%enhancement in PCE,i.e.,from 12.94%to 15.32%is achieved for the device with the Si O₂ nanoparticle addition compared to the control device with J_scincreasing from 14.82 m A/cm² to 16.21 m A/cm².In addition,the Si O₂ nanoparticle-embedded Cs Pb I₂Br device exhibits improved operation stability,i.e.,～83%retention of the initial PCE after aging in air with the relative humidity of 20-30%for 160 h,as compared to the control device.（4）A dual-function strategy,in which the Cs Pb I₂Br films are treated by Cs Pb Br₃nanocrystal（NC）addition and interfacial modification,to construct“electron bridges”and gradient heterojunction to improve the performance of the PSCs is proposed.The“electron bridges”formed by the introduced Cs Pb Br₃ NCs are suggested to provide an extra transport channel for the photogenerated electrons in the Cs Pb I₂Br layer,thus facilitating the electron transport.Meanwhile,the Cs Pb Br₃ NC surface-modified Cs Pb I₂Br layer forms a gradient heterojunction between the Cs Pb I₂Br layer and the hole transport layer,enhancing the hole extraction.In addition,the treatment by Cs Pb Br₃NCs reduces the defect at the bulk and surface of the Cs Pb I₂Br film.As a result,the PCE of 17.03%is achieved for the demonstration device with a simple configuration of ITO/Sn O₂/Cs Pb Br₃ NC-added Cs Pb I₂Br/Cs Pb Br₃ NCs/P3HT/Ag.Furthermore,the hole-conductor-free carbon-based Cs Pb I₂Br solar cell with the Cs Pb Br₃ NC treatment delivers a PCE of 13.84%.