Research On Defects Passivation Of Perovskite Solar Cells By Surface Modification

Organic-inorganic hybrid perovskite solar cells,as one kind of the third-generation thin-film solar cells,have been attracting amounts of attention in recent years with the advantage of high efficiency,low cost,solution processing,and flexible fabrication.After ten years of booming development,the efficiency of perovskite solar cells has reached 25.2%,which is very promising.This is mainly ascribed to the characteristics of high absorption coefficient,adjustable band gap,long carrier diffusion distance,low exciton binding energy,and excellent bipolar transportation of perovskite.However,the solution-processed perovskite inevitably introduces a large number of defects,forming shallow-level defects and deep-level defects between the band gaps of the perovskite,which affects the carrier transportation behavior and reduces the efficiency of the device.The defects act as a channel for ion migration,causing hysteresis behavior in the perovskite solar cell.In addition,the water and oxygen in the air invade the perovskite through the position of defects and promote the decomposition of perovskite,which is not conducive to the stability of the device.In order to improve the efficiency and stability of the perovskite solar cell,it is very effective to passivate the defects of the perovskite.Herein,we focuse on how to passivate the defects of perovskite films to improve the efficiency and stability of the devices,and three works were carried out through the following methods:(1)TiO2 has been widely utilized as electron transportation layer in regular perovskite solar cells.However,the TiO2 layer usually suffers from sufficient electron trap states,low electron mobility and inavoidable catalytic activity.Besides,TiO2 as the growtn substrates of perovskite film,its interface properties have an important influence on the crystallization,morphology and defects of peovskite,and ultimately affect the performance of the device.Herein,mesoporous TiO2 layer is modified by tetraethylammonium/p-toluenesulfonate(abbreviated as TEATS)for the first time,leading to a significant photoelectric conversion efficiencies(PCE)enhancement from 19.14%to 20.69%for perovskite solar cells.A series of measurements were carried out to investigate the effects of TEATS modification on the energy band structures of TiO2 as well as the CsMAFA perovskite layer atop,unveiling that TEATS modification benefits defect passivation of TiO2 film along with decrease of the work function of TiO2.Besides,TEATS modification helps to improve the wettability of perovskite precursors on m-TiO2 substrate and reduce the surface energy,affording improved quality of perovskite film with improved crystallinity and larger grain size.Consequently,the trap states existed in perovskite film can be passivated and the interfacial charge recombination is suppressed.This further benefits the improvement of the ambient stability of devices.(2)Large amounts of defects are mainly concentrated on the surface and grain boundaries of perovskite films due to the polycrystalline nature of solution-processed perovskite layers,detrimental to the performance of perovskite solar cells.Herein,we apply sodium p-toluenesulfonate(abbreviated as STS)in surface modification of perovskite layer for the first time,leading to efficient surface passivation of perovskite film and consequently significant enhancements of both efficiency and stability of mixed-cation PVKSC devices.Upon incorporating STS atop of perovskite layer,power conversion efficiency of Cs0.05MA0.12FA0.83PbI2.55Bro.45(abbreviated as CsMAFA)mesoporous-structure mixed-cation PVKSC devices improves from 18.70%to 20.05%with reduced hysteresis.The sulfonate(-S03-)anion of STS can coordinate with the Pb2+of CsMAFA perovskite,and Na+cation of STS can electrostatically interact with the anions(I-/Br-)of CsMAFA perovskite,resulting in surface passivation of the dangling bonds and vacancy defects on the perovskite surface with reduced electron and hole trap state densities.Besides,STS modification induces an upshift of the valence band of perovskite,facilitating hole extraction from the perovskite layer to the hole transport layer with suppressed interfacial charge recombination.Moreover,such a trap state passivation and enhanced hydrophobic of perovskite film leads to improvement of the ambient stability of PVKSC devices.(3)Methylammonium chloride(MACl)is utilized to modify the surface of perovskite film and the perovskite is post-annealed immediately after the surface modification of MACl.It is found that both the surface and bulk defects are passivated,leading to a significant photoelectric conversion efficiencies enhancement from 20.18%to 21.61%for CsMAFA based PVKSCs.A series of characterization indicates that MACl post-treatment induces Cl-entering the perovskite lattice and forming gradient distribution of Cl-in perovskite films,resulting in efficient defects passivation via filling the vacancies of the perovskite film.After post treated by MAC1,the peak of PbI2 remaining in the CsMAFA perovskite film is weaken to disappear,and the perovskite gradually changes from n-type to intrinsic type,which enhances the built-in potential of PCKSC devices.As a result,the mobility of electron and hole carrers is increased by 2-3 times,promoting decreased interfacial charge recombination.Moreover,the ions migration is suppressed with MACl post treatment and this further improves the ambient stability of devices.