Interface Modification And Crystallization Control In Printable Mesoscopic Perovskite Solar Cells

Perovskite solar cells（PSCs）have attracted the attention from both academics and industries with a certified power conversion efficiency（PCE）reaching 25.5%in less than a decade.According to the structure of devices,PSCs could be divided into mesoscopic PSCs,planar PSCs and printable mesoscopic PSCs.Among these three types of devices,printable mesoscopic PSCs can be fabricated by the screen-printing method and adopt the carbon as the material of counter electrode,which could further decrease the cost of PSCs and exhibit the good stability.However,the crystallization of perovskite is hard to control in the mesoscopic scaffold,which leads to lots of interface loss inside devices and restricts the further improvment of printable mesoscopic PSCs.In this work,the performance of printable mesoscopic PSCs is enhanced via interface modification and crystallization control of perovskite inside the mesopores.（1）To separate Ti O₂ and carbon electrode,a thick insulation layer of Zr O₂ mesoscopic layer is inserted between the Ti O₂ layer and carbon electrode in printable mesoscopic PSCs,which could effectively prevent the short circuit and decrease the carrier recombination.However,the thick Zr O₂ layer could damage the reproducibility of devices.To solve this problem,an ultrathin layer of Al₂O₃ was deposited on the surface of Ti O₂ through the spraying pyrolysis,which could assist separating Ti O₂ and carbon electrode.With the interface modification of Al₂O₃,the thickness of Zr O₂ layer decreased from 3μm to 1.2μm without sacrifice of PCE.In addition,the usage of perovskite precursor solution was also reduced,which could help control the environmental pollution in the fabrication process.The trap density of Ti O₂ was lower after being modified with Al₂O₃ and the interface contact was improved between Ti O₂ and perovskite,which accelerated the transition of carriers.（2）To get the black phase of FA-based perovskite and better crystallization inside the mesoscopic scaffold,Cs⁺and Rb⁺were used to partially substitute FA⁺of FAPb I₃.Through tuning the ratio of Cs⁺and Rb⁺,it was Cs⁺that mainly influenced the formation of black phase and the Rb⁺could enhance the FF of devices.For the solvent,through comparing mixed solvents made of DMF,DMSO,GBL and formamide,the property of solvent including solubility,volatility and the capacity to form the intermediate during the annealing process could influence the crystallization of perovskite.By adjusting the content of perovskite and the ratio of mixed solvent,the black phase of FA-based perovskite could be achieved at the temperature of 70℃combining Cs_0.1Rb_0.05FA_0.85Pb I₃·5%Pb I₂ with the mixed solvent（DMF:DMSO:formamide=7.5:2:0.5,v:v:v）,which could realize the dense pore-filling in the mesoscopic scaffold.（3）To study the crystallization process of perovskite inside the mesoscopic scaffold,the situation of perovskite crystallization was attained at different annealing stages through combining fast crystallization and slow crystallization.Ostwald ripening could help the perovskite to grow up in the mesoscopic scaffold,which could enhance the crystallinity of perovskite and realize the dense pore-filling during the slow crystallization.In the fast crystallization,many small crystals could form in the mesoscopic scaffold,which could exhibit the bad crystallinity and lead to voids inside devices.Finally,through slowing down the crystallization rate with solvent evaporation controlled crystallization（SECC）,the printable mesoscopic PSC achieved a stabilized PCE of 16.26%using the Cs0.1Rb0.05FA0.85Pb I3·5%Pb I2.（4）To explore the universality of SECC method in printable mesoscopic PSCs,Cs Pb Br₃ solar cells were fabricated with one-step method using SECC.By comparing properties of Cs Pb Br₃ films and devices,it was confirmed that SECC could also be used to control the crystallization and pore-filling of Cs Pb Br₃ in the mesoscopic scaffold.The average PCE was improved from 1.26%to 4.28%with SECC.Besides,there were no Cs Pb₂Br₅ and Cs₄Pb Br₆ existing with one-step method,which could appear in the two-step method.The result reveals that it is easier to get the pure Cs Pb Br₃ with one-step method using SECC.