Defect Passivation Strategies

This chapter introduces various passivation strategies for perovskite solar cells (PSCs) to enhance their stability and performance. Passivation, which involves coating a material to make it less reactive or corroded by the environment, is crucial for reducing defects in PSCs. The chapter covers chemical, physical, energetic, and field-effect passivation methods. Chemical passivation, the primary method, uses additives to form bonds with defects, reducing non-radiative recombination and ion migration. Physical passivation, including strain relaxation, thermal annealing, and surface polishing, improves crystal quality and reduces defects without harmful chemicals. Energetic passivation prevents the formation of superoxide anions by introducing mid-gap states to capture photoexcited electrons. Field-effect passivation involves inserting dielectric layers to create interfacial band-bending, which protects charge carriers from recombination. The chapter highlights the advantages and limitations of each method, providing a comprehensive overview of the strategies used to optimize PSCs’ electronic and optoelectronic properties. Passivation is coating a material so that it becomes “passive”, that is, less readily affected or corroded by the environment. There are various defects introduced by the change of material structure and composition from precursor to perovskites in solution methods. Organic or inorganic compounds are used as passivating additives or interface modifiers to reduce various defects on the surface and in the bulk of perovskite solar cells. This chapter introduces the passivation strategies of perovskite solar cells in recent years, including chemical, physical, energetic, and field-effect passivation. Chemical passivation is the primary technique employed to enhance the performance of perovskite solar cells. The ionic nature of hybrid perovskites enables passivation approach to reduce various charged defects. Molecules possessing a chemical passivation function act during or subsequent to the film formation step to establish covalent and ionic bonds with under-coordinated metal cations or halide anions, thereby inactivating their electronic and/or chemical activity. This further inhibits undesirable non-radiative recombination and ion migration. Physical passivation employs a physical process or a physical treatment. Furthermore, physical passivation avoids chemical processes that may occur during chemical passivation and cause potential damage to other components of the device. Most physical passivation strategies are highly precise and time-efficient to be compatible with scaling up perovskite solar cells and modules. Both chemical and physical passivation techniques are effective in reducing defects in perovskite materials, as they reduce charge trapping from deep-level defects and suppress the migration of shallow-level defects. Energetic passivation is the introduction of mid-gap states by the passivator to capture photoexcited electrons before oxygen, in order to avoid the formation of superoxide anion radicals in photo-excited perovskites. When exposed to both light and oxygen, the active superoxide radical reacts with organic cations to degrade the hybrid perovskite layer, severely limiting the lifetime of perovskite solar cells. Field-effect passivation is the process of inserting a dielectric layer with a strong dipole or high work function at the interface of the perovskite absorber and transport layer. The resultant interfacial band-bending shields prevent photo-generated charge carriers from recombination at active regions by repelling or separating free charges from the interface. This decreases the probability of charge trapping at defect sites and minority carrier recombination at the interface.