Grain Boundary Regulation And Stability Study For Perovskite Solar Cells

In recent years,the perovskite solar cells（PVSCs）have developed rapidly and the power conversion efficiency（PCE）has been continuously broken through.At present,the certified PCE has reached 24.2%.However,the perovskite materials and devices can degenerate under water,oxygen,heat,light conditions,and their poor stability limits their commercial applications.Currently,polycrystalline perovskite films are basically used in small perovskite devices and large area modules,and its defect density is about five orders of magnitude higher than that of monocrystalline perovskite films.The defects of perovskite film are derived from pinholes,vacancies,surface roughness,grain boundaries（GBs）and anomalous crystal lattice.There are many charge trap states in GBs,which adversely affect the transportation of charge carriers.In addition,external water and oxygen will penetrate perovskite film through GBs,accelerating the decomposition of perovskite and affecting the stability of PVSCs.Therefore,grain boundary regulation can further improve the efficiency and stability of PVSCs.There are many charge trap states in the grain boundary of polycrystalline perovskite films,which can act as a non-radiative recombination center and reduce the lifetime of charge carriers.These energy losses severely limit device performance.The semiconductor molecule F4TCNQ was applied to modify the GBs of perovskite film.F4TCNQ molecules fill the GBs and vacancies of perovskite film.The cyanogroup and metal lead can form Lewis adduct,which can passivate grain boundary defects.The F4TCNQ can be used as an electron transport material,and it is also commonly used as dopant of hole transport material to improve the conductivity.Therefore,the introduction of F4TCNQ can promote charge transfer and improve charge mobility.As a result,the PCE of F4TCNQ incorporated devices reaches 16.6%and the filling factor is up to 80%.Moreover,the fluorine atoms in F4TCNQ can prevent water from penetrating into the grain boundary and improve the moisture stability of the device.The AIE（aggregation-induced emission）polymer PTN-Br was applied to modify the GBs of FASnI₃ film to form bulk heterojunction.The PTN-Br molecule with suitable highest occupied molecular orbital energy level can fill into the grain boundaries of perovskite film,serving as a hole-transport medium between grains.The gradient band alignment is formed between the FASnI₃ absorption layer and the PEDOT:PSS hole transport layer,which contributes to the excellent hole transportation and higher open-circuit voltage and enhance device performance.Moreover,PTN-Br molecules can provide lone pair electrons to form Lewis adduct with uncoordinated tin atoms,thus passivating the defect state.These advantages significantly improve the open circuit voltage and filling factor of the perovskite device,and the device efficiency reaches 7.94%.Furthermore,the PTN-Br incorporated device shows better UV stability because of UV barrier and passivating effect of PTN-Br.Finally,a novel device structure is designed,and a self-encapsulating device with water stability is prepared by laminating perovskite sub-cell.The novel approach has many advantages.First of all,the perovskite films are sandwiched between two glass substrates and encapsulated with UV curing adhesive,which can effectively prevent perovskite degradation caused by water and oxygen.Secondly,a vacuum-evaporation metal-free electrode is adopted,which is beneficial to avoid electrode-related degradation and reduce fabrication cost.Thirdly,the laminated perovskite device is semi-transparent,which is favor of efficient solar light absorption.The self-encapsulated laminated devices with CB treatment show power conversion efficiency of 6.9%and excellent stability even if soaking in water for 24 h.