Perovskite materials have demonstrated remarkable optoelectronic properties, which have placed them at the crux of the photovoltaic technology. In this era of striving for clean and economical methods of energy production, lead-based perovskites have become the key materials in photovoltaics owing to their facile solution processability and phenomenal performance. Unfortunately, lead toxicity poses a major hurdle to their scalability and widespread commercialization. Hence, to fulfill the indisputable need for energy, lead-free perovskites are considered to be a boon to the future of photovoltaic technology. In this perspective, we provide a comprehensive assessment of lead-free perovskite solar cells enabled by hetero-valent substitutes. These comprise A₃B₂X₉-structured perovskites, halide double perovskites, and mixed metal halide–chalcogenide perovskites. Importantly, we emphasize the effects of cationic–anionic sites, metal substitutions, and solvents on the chemical and structural properties of A₃B₂X₉-structured perovskites. Moreover, we also focus on antimony(V) perovskite-like materials and antimony-based perovskite nanocrystals (NCs) in the fabrication of devices. Two types of halide double perovskites are described, including A₂B(I)B(III)X₆ and A₂B(IV)X₆ double perovskites. Subsequently, chalcogenide perovskite solar cells are also discussed. Overall, the primary purpose of this perspective is to explicitly describe lead-free perovskite solar cells enabled by hetero-valent substitutions more broadly as a category of next-generation optoelectronic materials. Finally, we propose that mixed halide–chalcogenide perovskites offer a promising pathway towards achieving highly efficient and stable perovskite solar cells.

The light soaking phenomenon, in chlorine-based mixed halide perovskite solar cells (PSC) with p-i-n configuration, is studied. Due to light soaking, the solar cells have shown improved open circuit voltage, fill factor (FF), and power conversion efficiency without much change in short circuit current. The devices have returned to their original state upon extended exposure to dark conditions. Capacitance spectroscopy is used to understand the effect of light soaking on various photovoltaic parameters. The observed increase of capacitance value with light soaking in the low frequency region of the capacitance-frequency ( C – F ) plot is elucidated, as the coupled effect of electronic and ionic processes involving defects. The decrement in the mid-frequency capacitance is attributed to an increase in the width of the depletion region which eventually facilitated the improvement in open circuit voltage and FF. Defect densities, and its distribution in the energy space, are deduced before and after light soaking. The defects density of states of PSC studied are found to reduce from 10 ¹⁶ to 10 ¹⁵ eV ⁻¹ cm ⁻³ , with a Gaussian distribution, and having a peak energy shift from 0.27 to 0.26 eV.

EuCl₃ doping in triple-cation perovskites boosts efficiency of photovoltaic cells from 27% to 30% at 1000 lx under indoor lighting. Furthermore, t50 lifetimes (times at which efficiencies half) more than doubled under thermal stress tests at 85 °C.

In the quest for finding non-toxic and stable lead-free perovskite nanocrystals (NCs), bismuth halide perovskites (BHP) have emerged as a promising alternative. This perspective presents an overview, challenges, and future opportunities in BHP NCs.

Semitransparent perovskite solar cells (ST-PSCs) for tandem applications typically use a buffer layer deposited via atomic layer deposition (ALD) to protect the cell stack from the damage induced by the sputtering of the transparent electrode. Here, we present a simple yet effective solution-processed buffer layer based on metal-oxide nanoparticles to mitigate sputter-induced damage. We exploit this strategy in a monolithic tandem integrating the optimized ST-PSC on a polished front-side/unpolished rear-side p-type silicon heterojunction (SHJ) solar cell. The intrinsic roughness on the backside significantly boosts the absorption, thus suppressing the need for a dedicated texturization step and leading to a final maximum efficiency of 25.3%. Our findings highlight the potential of solution-processed buffer layers as a practical and scalable solution to mitigate the sputtering damage, as well as the potential of silicon wafers with an unpolished rear surface for enhanced photocurrent.