Even though perovskite solar cells have reached 22% efficiency within a very short span, the presence of lead is a major bottleneck to its commercial application. Tin and Germanium based perovskites failed to be viable replacements due to the instability of their +2 oxidation states. Antimony could be a possible replacement, forming perovskites with structure A3M2X9. However, solution processing of Cs, organic ammonium based Sb perovskites result in the formation of the dimer phase with poor charge transport properties. Here we demonstrate that Rb can template the formation of the desired layered phase irrespective of processing methodologies, enabling the demonstration of efficient lead-free perovskite solar cells.

Future brain-machine interfaces, prosthetics, and intelligent soft robotics will require integrating artificial neuromorphic devices with biological systems. Due to their poor biocompatibility, circuit complexity, low energy efficiency, and operating principles fundamentally different from the ion signal modulation of biology, traditional Silicon-based neuromorphic implementations have limited bio-integration potential. Here, we report the first organic electrochemical neurons (OECNs) with ion-modulated spiking, based on all-printed complementary organic electrochemical transistors. We demonstrate facile bio-integration of OECNs with Venus Flytrap (Dionaea muscipula) to induce lobe closure upon input stimuli. The OECNs can also be integrated with all-printed organic electrochemical synapses (OECSs), exhibiting short-term plasticity with paired-pulse facilitation and long-term plasticity with retention >1000 s, facilitating Hebbian learning. These soft and flexible OECNs operate below 0.6 V and respond to multiple stimuli, defining a new vista for localized artificial neuronal systems possible to integrate with bio-signaling systems of plants, invertebrates, and vertebrates.

Bismuth-based double perovskite Cs2AgBiBr6 is regarded as a potential candidate for low-toxicity, high-stability perovskite solar cells. However, its performance is far from satisfactory. Albeit being an indirect bandgap semiconductor, we observe bright emission with large bimolecular recombination coefficient (reaching 4.5 ± 0.1 × 10−11 cm3 s−1) and low charge carrier mobility (around 0.05 cm2 s−1 V−1). Besides intermediate Fröhlich couplings present in both Pb-based perovskites and Cs2AgBiBr6, we uncover evidence of strong deformation potential by acoustic phonons in the latter through transient reflection, time-resolved terahertz measurements, and density functional theory calculations. The Fröhlich and deformation potentials synergistically lead to ultrafast self-trapping of free carriers forming polarons highly localized on a few units of the lattice within a few picoseconds, which also breaks down the electronic band picture, leading to efficient radiative recombination. The strong self-trapping in Cs2AgBiBr6 could impose intrinsic limitations for its application in photovoltaics.

Solution-processed lead halide perovskites have established themselves as one of the most important absorber materials in solar cells with power conversion efficiencies now exceeding 22%. [1] Unfortunately, the over reliance on highly toxic Pb 2+ remains a key issue for widespread commercial applications. The significant concentration of Pb 2+ in high performing halide perovskites and its water solubility make it highly hazardous compound to the environment. [2] Another apparent issue is the inherent instability of lead-based halide perovskites in ambient atmosphere. [3] Although the stability of Pb-based halide perovskites has improved impressively in recent times along with the simultaneous development of passivation techniques, the fabrication of lead-based halide perovskites still requires stringent environmental control. [4] To address these potential issues, there is an increased interest toward leadfree halide perovskites and their analogues in photovoltaics. Replacing Pb 2+ with Sn 2+ or Ge 2+ could minimize the toxicity associated with lead-based halide perovskite; however, the increased environmental instability of these compounds poses significant challenges in solar cell development. [5] Even after incorporating 2D/3D mixtures of perovskites, the efficiency of the highest performing Sn-based system reduced to nearly 50% of its original value within 3 d under 20% humidity. [6] Considering atmospheric stability, trivalent cations such as bismuth and antimony-based ternary halides were also investigated as potential absorber materials due to their inherent atmospheric stability and low toxicity. [7] The incorporation of protonated cations such as MA or Cs with Bi-I octahedra forms Bi-based ternary halides (structural formula A 3 Bi 2 I 9 : A = Cs, MA) exhibits high absorption coefficients and facile solution processability. Nevertheless, the photovoltaic performances of Bi-based ternary halides remained poor mostly due to high optical bandgap and low electronic dimensionality. [8] Replacement of the A-site protonated cations with transition metals such as Ag or Cu is a promising strategy to improve the dimensionality. These transition metals also take part in bonding with Bi-I octahedra, resulting in complex halide bismuthates. In comparison to Bismuth-based ternary halides have recently gained a lot of attention as lead-free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low-dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide-based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air-stable lead-free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting ...

Double perovskite halides are a class of materials with diverse chemistries that are amenable to solution-based synthesis routes, and display a range of properties for diverse potential applications. Starting from a consideration of the octahedral and tolerance factors of 2000 candidate double-perovskite compounds, we compute structural, electronic and transport properties of 1000 compounds using first-principles 1 calculations. The computational results have been assembled in a database that is accessible through the Materials Project online. As one potential application, double perovskites are promising candidates in the search for lead-free halide photovoltaic absorbers. We present the application of our database to aid the discovery of new double perovskite halide photovoltaic materials. Forty compounds from five categories were identified as promising solar absorber candidates and the complex chemical trends for band gap within each category are analyzed, to provide guidelines for the use of subtitutional alloying as a means of tuning the electronic structure. Other possible applications of the database are also discussed briefly.