Suppression And Stopping Of Ion Migration In Single- And Mixed-halide Perovskites

Ion migration is one of the main issues faced in almost all ionic perovskite materials.It is known to be the reason for current-voltage hysteresis and instability/degradation in single-halide perovskite(SHP)-based solar cells and phase segregation in mixed-halide perovskites(MHPs)-based light-emitting diodes(LEDs).Forperovskites solar cells(PSCs),it causes the maximum open-circuit voltage(VOC)to be pinned at the smaller bandgap Irich domains,and for perovskites light-emitting diodes(PeLEDs),it causes the emission wavelength to be pinned at the near-infrared range emitted from I-rich domains.Because of this limitation,the perovskite device’s long-term operation and phase stability remain an issue.Based on the above research background,we prepared and optimized perovskites films to improve the morphology,studied the effect of ion migration in the perovskite films under light illumination and a strong electric field,optimized the performance of the PeLEDs to further study the effect of ion migration under working conditions,and finally extended the effect of ion migration using a solar cell device structure and a state-of-theart PSCs.The specific research work is divided into the following parts:1.In chapter 3,we studied tin perovskites and found no ion migration in tin-based and tin-doped perovskites compared to lead-based perovskite films.This discovery was first confirmed experimentally using major characterization techniques,including photoluminescence(PL),fluorescence microscopy mapping,and Energy dispersive spectrometry(EDS)mapping and X-ray Diffraction(XRD)of the light illuminated and electric-field poled perovskite films under a long time.2.In chapter 4,we verified our study based on the activation energy(Ea)measurements,trap density experiment,theoretical calculations,and X-ray photoelectron spectroscopy(XPS)analyses.Our results were analyzed based on the different proposed models,and we concluded that all factors that cause halide migration or phase segregation in lead-based perovskites also exist in tin-based perovskites and that stronger tin-halide bond and higher barrier energy in contrast to lead-halide bond are the origins of the absence of ion migration.By using mixed lead-tin perovskites MHPs with different Ea values,the threshold Ea value was found to be around 0.65 eV,which serves as a guideline in designing intrinsically stable perovskites.3.Finally,in chapter 5,we studied ion migration(phase segregation)in tin-based PeLEDs and PSCs.Results showed that the LED devices made from tin-halide perovskites exhibited no ion migration(phase segregation)with high operational stability under working conditions.We also show that incorporating different additives improved the device’s performance of the stable tin-halide perovskite.Finally,we prove that ion migration is stopped in tin-halide perovskites using a solar cell device structure under the electric field generated by photovoltage and suppressed in tin-doped perovskites using state-of-the-art perovskite solar cells(PSCs),thus improving their operational stability.