New strategies to develop High-Efficiency Lead-Free wide bandgap perovskite solar cells

• Pb-free WBG PSCs reliable for next generation tandem technology as front sub-cells. • Grain engineering strategies to control crystallization related defects is reviewed. • Defect passivation via additive engineering in Sn/Sn-Ge WBG is broadly reviewed. • Strategies to overcome interfacial recombination, V oc deficit discussed in detail. • Perspective on new approaches for high efficiency Sn/Sn-Ge WBG PSCs is discussed. Over the past decade, the world has witnessed tremendous achievement in power conversion efficiency (PCE) enhancement for Pb-perovskite solar cells (PSCs). However, spectral loss limits the performance, which can be overcome with tandem solar cells (TSCs) and hence with excellent optoelectronic properties and bandgap tunability Pb-perovskites have been widely used as front/rear sub-cells in TSCs, where variable bandgap sub-cells can utilize maximum spectral radiations. Though the operational instability issue has been well controlled by structural optimizations, yet Pb toxicity is a major challenge in commercializing Pb-PSCs. To resolve the issue of toxicity, Pb-free perovskites have gained immense consideration, especially Sn 2+ having similar physical properties to Pb 2+ and non-toxic feature of Sn 2+ , Ge 2+ , Bi 3+ , or Sb 3+ perovskites. In TSCs, the wide bandgap (WBG ≥ 1.6 eV) sub-cell plays a critical role in harnessing shorter wavelength photons and is responsible to achieve high Voc. To nurture Pb-free all-PTSC technology, it is urgent to develop high-performance Pb-free WBG PSCs. To understand the seriousness herein, this review focus on the existing challenges in Pb-free WBG PSCs and discuss new strategies that have been implemented with state-of the art fabrication techniques to enhance device performance. By discussing the issues of trap densities, defects formation, bandgap mismatching, V oc deficits, and poor stability, we focused on the strategies to overcome these issues through compositional engineering, and defect passivation via additive and interfacial engineering. In conclusive remarks, we discussed our viewpoint with innovative ideas that can be applied to collectively focus on this area to achieve high-performance Pb-free WBG PSCs.