Phase Modulation Of Light-absorbing Layer And Stability In All-Inorganic Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials exhibit unique photoelectric properties,and the corresponding perovskite solar cells(PSCs)have demonstrated excellent energy conversion efficiency.However,due to the existence of volatile organic A-site cations,PSCs show inferior photostability,thermal stability and ambient stability,and this hampers their commercial applications.All-inorganic perovskite solar cells using Cs+ ions substituting organic cations at the A-site have excellent thermal stability.Nevertheless,although the thermal stability of all-inorganic CsPbX3 perovskite is rather excellent,its crystal structure is unstable andδ phase transition is easy to occur due to smaller tolerance factor of CsPbI3 perovskite,which affects its ambient stability.In order to improve the ambient stability of CsPbI3 perovskite,using mixed halogen anions at the X-site to construct CsPbIxBr3-x perovskite can increase the tolerance factor and improve phase stability.But the introduction of Br-will aggravate the halogen ion migration and halogen phase segregation,which will deteriorate the photostability of the device.Thus it is also necessary to develop suitable strategies to suppress this phenomenon.On the other hand,introducing Sn2+ which has smaller ionic radius than Pb2+ at the B-site is another feasible way to increase the tolerance factor of all-inorganic CsPbX3 perovskite.Meanwhile,bandgap of CsPbX3 perovskite can be reduced by introducing Sn2+,facilitating its absorption of sunlight.However,the introduction of Sn2+ ions accelerates the crystallization process of perovskite,making it difficult to obtain high-quality perovskite films.Aiming to solve the above-mentioned problems,in this dissertation we focus on phase modulation and stability improvement of all-inorganic perovskite solar cells,and carried out the following three works:1.This work studied the origin of phase segregation in CsPbIBr perovskite in depth and found that halogen phase segregation is not only induced by light but also occurs in the crystallization process of CsPbI2Br perovskite films,which is called intrinsic phase segregation.Low annealing temperature and fast crystallization rate exacerbates intrinsic phase segregation.In order to solve this problem,polymethyl methacrylate(PMMA)was introduced as an additive in the CsPbI2Br perovskite layer via anti-solvent dipping.The introduction of PMMA can control the crystallization,thereby suppressing the intrinsic phase segregation during the crystallization process,thereby further eliminating its light-induced phase separation.PMMA additive not only improved the device efficiency from 14.42%to 15.88%,but also improved the photostability.With the addition of 0.05 mg/mL PMMA,PCE decreased by only about 17%after 400 hours of continuous light illumination,whereas the control device without any additive exhibited a～36%PCE decrease under the same condition.2.The reasons for the poor thermal stability of n-i-p CsPbI2.25Br0.75 perovskite solar cells based on ZnO electron transport layer were investigated in depth.This work found that the residues such as acetate group and hydroxyl groups on the surface of ZnO could induce δ phase transition of CsPbI2.25Br0.75 perovskite when heated at 85℃ in N2 atmosphere.Increasing annealing temperature to 300℃ can effectively remove these residual groups and improve crystallinity of ZnO.Furthermore,this work used a bis-dimethylaminofunctionalized fullerene derivative PCBDMAM to modify ZnO.PCBDMAM can not only improve the surface morphology of ZnO and passivate the oxygen vacancies,but also prohibit δ-phase transform of perovskite under thermal stress,consequently leading to obvious efficiency enhancement(from 15.44%to 17.04%)of CsPbI2.25Br0.75 PSC devices.Besides,the PSC device maintains initial efficiency of 73%after 700 hours under the heating condition of 85℃ in N2 atmosphere,which is the best thermal stability of similar devices reported in the literature.3.For all-inorganic Pb-Sn mixed PSCs,lead iodide(PbI2)has been commonly used as the lead source,however it is hard to obtain phase-pure CsPbxSn1-xX3(X=I,Br)perovskite films with specific composition because of accelerated and inhomogeneous crystallization process of perovskite.Besides,the stronger lewis acidity of Sn2+makes SnI2 reacts faster with CsI than PbI2,leading to uneven distribution of Pb and Sn elements in the final perovskite films and consequently inferior operational stability of PSC devices.To solve this problem,lead acetate(Pb(Ac)2)was applied as an alternative lead source substituting PbI2,affording homogeneous phase-pure CsPb0.4Sn0.6I2.4Br0.6 perovskite.Pb(Ac)2 has comparable Pb release rate to SnI2,preventing from inhomogeneous crystallization process of perovskite and consequently homogeneous distribution of Pb and Sn elements in the final perovskite films is achieved.These advantages lead to significant PCE enhancement of the inverted-structure CsPb0.4Sn0.6I2.4Br0.6 PSC device,delivering a champion PCE of 13.98%(certified at 13.88%),which is much higher than that of the control device based on PbI2 lead source(10.91%)and among the highest values of all-inorganic Pb-Sn based PSCs reported up to now.Furthermore,Pb(Ac)2 lead source benefits simultaneous improvements of the thermal and operational stabilities of CsPb0.4Sn0.6I2.4Br0.6PSC devices due to increased quality of perovskite film.