The rapid development of solar cells (SCs) based on organic-inorganic hybrid metal triiodide perovskite (MTP) materials holds great promise for next-generation photovoltaic devices. The demonstrated power conversion efficiency of the SCs based on MTP (PSCs for short) has reached over 20%. An MTP material is a kind of soft ionic solid semiconductor. The intrinsic optoelectronic properties of MTP are greatly determined by several factors, such as the crystalline phase, doping type, impurities, elemental composition, and defects in its crystal structure. In the development of PSCs, a good understanding and smart engineering of the defects in MTP have been demonstrated to be a key factor for the fabrication of high-efficiency PSCs. In this review, we start with a brief introduction to the types of defects and the mechanisms for their formation in MTP. Then, the positive and negative impacts of defects on the important optoelectronic features of MTP are presented. The optoelectronic properties mainly include charge recombination, charge transport, ion migration, and structural stability. Moreover, commonly used techniques for the characterization of the defects in MTP are systematically summarized. Recent progress on the state-of-the-art defect engineering approaches for the optimization of PSC devices is also summarized, and we also provide some perspectives on the development of high-efficiency PSCs with long-term stability through the optimization of the defects in MTP.

All-inorganic double-metal perovskite materials have recently gained much attention due to their three dimensionality (3D) and non-toxic nature to replace lead-based perovskite materials. Among all those double perovskite materials, theoretical works have demonstrated that Cs AgBiBr shows high stability and possesses a suitable band gap for solar-cell applications. However, the film-forming ability of Cs AgBiBr is found to be the utmost challenge hindering its development in thin-film solar-cell devices. In this work, a high-quality Cs AgBiBr film with ultra-smooth morphology, micro-sized grains, and high crystallinity is realized via anti-solvent dropping technology and post-annealing at high temperature. After optimization, the first example of an inverted planar heterojunction solar-cell device based on Cs AgBiBr exhibits a power conversion efficiency of 2.23 % with V =1.01 V, J =3.19 mA/cm , and FF=69.2 %. Besides, the device shows no hysteresis and a high stability.

A conjugated large-volume cation is adopted as an additive to modify FASnI 3 film with much improved film quality. Lead-free PSC devices with PCE of 9.61% on 0.09 cm 2 and 7.08% on 1 cm 2 can be achieved. The PSC devices also show robust stability with self-healing ability. This work addresses the promise of Sn-based PSCs and takes a big step forward in the field of ecofriendly lead-free photovoltaic devices.