In recent years, lead-free metal-halide perovskite photovoltaics has attracted ever-growing attention, in view of its potential to replicate the outstanding properties of lead-halide perovskite photovoltaics, but without the toxicity burden of the latter. Despite a research effort much smaller in scale than that pursued with lead-based perovskites, considerable progress has been achieved in lead-free perovskite photovoltaics, with the highest power conversion efficiencies now being in the region of 13%. In this Perspective, we first discuss the state of the art of lead-free perovskite photovoltaics and additionally highlight promising directions and strategies that could lead to further progress in material exploration and understanding as well as in photovoltaic efficiency. Furthermore, we point out the widespread lack of experimental data on the fundamental optoelectronic properties of lead-free halide perovskite absorbers (e.g., charge carrier mobility, defect parameters, Urbach energy, and the impact of dimensionality). All of this currently hampers a rational approach to further improving their performance and points to the need for a concerted effort that could bridge this knowledge gap. Additionally, this Perspective brings to the fore the manifold photovoltaic opportunities—thus far largely unexplored with lead-free perovskite absorbers—beyond single-junction outdoor photovoltaics, which may potentially enable the realization of their full potential. The exploration of these opportunities (tandem photovoltaics, indoor photovoltaics, and building-integrated and transparent photovoltaics) could energize the investigation of existing and new classes of lead-free perovskite absorbers beyond current paradigms and toward high photovoltaic performance.

In the wake of lead-halide perovskite research, bismuth-and antimony-based perovskite-inspired semiconducting materials are attracting increasing attention as safer and potentially more robust alternatives to lead-based archetypes. Of particular interest are the group IB-group VA halide compositions with a generic formula A x B y X x+3y (A + = Cu + /Ag + ; B 3+ = Bi 3+ /Sb 3+ ; X -= I -/Br -), i.e., silver/copper pnictohalides and derivatives thereof. This family of materials forms 3D structures with much higher solar cell efficiencies and greater potential for indoor photovoltaics than the lower-dimensional bismuth/ antimony-based perovskite-inspired semiconductors. Furthermore, silver/ copper pnictohalides are being investigated for applications beyond photovoltaics, e.g., for photodetection, ionization radiation detection, memristors, and chemical sensors. Such versatility parallels the wide range of possible compositions and synthetic routes, which enable various structural, morphological, and optoelectronic properties. This manuscript surveys the growing research on silver/copper pnictohalides, highlighting their composition-structure-property relationships and the status and prospects of the photovoltaic and optoelectronic devices based thereon. The authors hope that the insights provided herein might accelerate the development of eco-friendly and stable perovskiteinspired materials for next-generation photovoltaics and optoelectronics.

A zinc oxide (ZnO)-reduced graphene oxide (rGO) composite thin film memristive device is reported. Further, it has been shown that it is possible to implement Hebbian learning rules like, the spiketiming-dependent plasticity, using this device. Furthermore, a circuit on PCB is developed; this circuit can imitate the biological spike firing scheme and activate the memristor synapse. The fabricated device along with the custom made circuit can be extended for developing future neuromorphic circuit applications.

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