Progress on defects and interfacial passivation layers in hybrid perovskite solar cells

Perovskite, as a class of semiconductor materials with unique optoelectronic properties, such as low exciton binding energy, long carrier diffusion length and tunable bandgap, is widely studied in many research fields. Particularly, organic–inorganic hybrid perovskite solar cells (PSCs) have attracted wide attention due to their low cost and high power conversion efficiencies. Currently, inherent defects in perovskites are restricting their further development. As is known to all, both deep level defects and shallow level defects play key roles in the dynamic process of carriers and the stability of perovskites. Recombination loss, unfavorable band bending, unwanted surface reaction and ionic migration caused by these defects, can lead to serious damages to the device lifetime and performance. In this review article, the formation processes and physical properties of the defects, including point defects, surface defects and defects around perovskite grain boundaries, as well as their effects on the PSC performances, are carefully summarized and analyzed. According to the previous studies, conduction band and valence band of perovskites and defect energy states in perovskites are all related to the anti-bonding coupling of Pb-s and I-p atom orbitals. Certain anti-site and interstitial defects caused by the wrong ionic bonds as well as the surface states can create deep level defects. Moreover, shallow level defects produced by the point defects and grain boundaries will aggravate the unfavorable ion migration and surface reaction, which will dramatically deteriorate the device lifetime and performance of the PSCs. Therefore, defect passivation is a crucial and powerful route to optimizing the power conversion efficiency and long-term stability of the PSCs. Moreover, recent advances in interfacial layer used for the defect passivation, categorized as Lewis acid, Lewis base, alkylammonium, polymer, metal oxide, 3D perovskite, graphene and inorganic salt, are described in detail. We comprehensively present the passivation mechanisms and significant effects of these materials in this paper. Note that the main passivation mechanisms of these materials have been summarized as following: (1) Reducing the density of surface defects via the interaction between functional groups of the passivation molecules and the defects; (2) forming passivation layer on the surface through in situ reaction between the passivation molecules and the perovskite during post-treatment; (3) optimizing the energy level arrangement of the PSCs by molecular dipole to promote the transport and extraction of photo carriers; (4) adding an insulating layer between the active layer and the charge transport layer to improve the stability of the perovskite and the PSCs; (5) controlling the crystallization and wettability of the perovskite film and each function layer through surface interaction to obtain flat and high-quality films. Lastly, some potential design strategies of passivation materials are proposed, such as intentionally integrating multifunction groups, polymerization of small organic molecules and exploring novel inorganic materials. The theoretical understanding of the defects properties and the mechanisms for defect passivation summarized by this review should be very helpful in further developing the PSCs.