Lead-free hybrid perovskites for photovoltaics

Exploring Highly Efficient Broadband Self-Trapped-Exciton Luminophors: from 0D to 3D Materials

The review summarizes our recent reports on brightly-emitting materials with varied dimensionality (3D, 2D, 0D) synthesized using "green" chemistry and exhibiting highly efficient photoluminescence (PL) originating from self-trapped exciton (STE) states. The discussion starts with 0D emitters, in particular, ternary indium-based colloidal quantum dots, continues with 2D materials, focusing on single-layer polyheptazine carbon nitride, and further evolves to 3D luminophores, the latter exemplified by lead-free double halide perovskites. The review shows the broadband STE PL to be an inherent feature of many materials produced in mild conditions by "green" chemistry, outlining PL features general for these STE emitters and differences in their photophysical properties. The review is concluded with an outlook on the challenges in the field of STE PL emission and the most promising venues for future research.

Bismuth-Based Halide Perovskites for Photocatalytic H2 Evolution Application

Metal halide perovskites (MHPs), in particular lead-based perovskites, have earned recognized fame in several fields for their outstanding optoelectronic properties, including direct generation of free charge carriers, optimal ambipolar charge carrier transport properties, high absorption coefficient, point-defect tolerance, and compositional versatility. Nowadays, this class of materials represents a real and promising alternative to silicon for photovoltaic technologies. This worthy success led to a growing interest in the exploration of MHPs in other hot research fields, such as solar-driven photocatalytic water splitting towards hydrogen production. Nevertheless, many of these perovskites show air and moisture instability problems that considerably hinder their practical application for photocatalytic water splitting. Moreover, if chemical instability is a problem that can be in part mitigated by the optimization of the chemical composition and crystal structure, the presence of lead represents a real problem for the practical application of MHPs in commercial devices due to environmental and healthcare issues. To successfully overcome these problems, lead-free metal halide perovskites (LFMHPs) have gained increasing interest thanks to their optoelectronic properties, comparable to lead-based materials, and their more eco-friendly nature. Among all the lead-free perovskite alternatives, this mini-review considers bismuth-based perovskites and perovskite derivatives with a specific focus on solar-driven photocatalysis application for H₂ evolution. Special attention is dedicated to the structure and composition of the different materials and to the advantage of heterojunction engineering and the relative impact on the photocatalytic process.

Chemical Vapor Deposited Mixed Metal Halide Perovskite Thin Films

In this article, we used a two-step chemical vapor deposition (CVD) method to synthesize methylammonium lead-tin triiodide perovskite films, MAPb_1−xSn_xI₃, with x varying from 0 to 1. We successfully controlled the concentration of Sn in the perovskite films and used Rutherford backscattering spectroscopy (RBS) to quantify the composition of the precursor films for conversion into perovskite films. According to the RBS results, increasing the SnCl₂ source amount in the reaction chamber translate into an increase in Sn concentration in the films. The crystal structure and the optical properties of perovskite films were examined by X-ray diffraction (XRD) and UV-Vis spectrometry. All the perovskite films depicted similar XRD patterns corresponding to a tetragonal structure with I4cm space group despite the precursor films having different crystal structures. The increasing concentration of Sn in the perovskite films linearly decreased the unit volume from about 988.4 ų for MAPbI₃ to about 983.3 ų for MAPb₀.₃₉Sn₀.₆₁I₃, which consequently influenced the optical properties of the films manifested by the decrease in energy bandgap (E_g) and an increase in the disorder in the band gap. The SEM micrographs depicted improvements in the grain size (0.3–1 µm) and surface coverage of the perovskite films compared with the precursor films.

Ab initio modeling of 2D and quasi-2D lead organohalide perovskites with divalent organic cations and a tunable band gap

We describe theoretically the structure and properties of layered lead organohalide perovskites, considering purely bi-dimensional (2D) PbI₄ layers, and quasi-2D systems where the inorganic layers are formed by more than one lead iodide sheet. The intercalating organic dications were designed to have low lying virtual orbitals (LUMO), so as to induce in the perovskite the appearance of virtual bands, localized in the organic layer, either close to the inorganic conduction band bottom or valence band top, or in some cases in the middle of the inorganic band gap. Such a feature is quite uncommon for this class of materials, and deserves attention since it allows one to tune the effective band gap of the material, possibly leading to the absorption of visible light and influencing the optical properties deeply. We discuss the effect of functional groups on the organic cations, and of the different symmetries used in geometry optimizations: a careful analysis of the contributions to the dispersion curves and band gaps was performed. The charge carrier mobility is also discussed, computing the conductivity over relaxation time and the effective masses for all the systems, with particular attention to the features related to the unusual organic intra-gap bands. All the structures were optimized at the DFT level, with inclusion of dispersion effects; dispersion curves were computed with full relativistic potentials, and the band gaps corrected for long range coulombic effects at the GW level. A semiempirical approach, based on the integration of charge carrier group velocities over a dense grid of k-points, was used to compute the conductivities and effective masses.

A Potential Checkmate to Lead: Bismuth in Organometal Halide Perovskites, Structure, Properties, and Applications

The remarkable optoelectronic properties and considerable performance of the organo lead-halide perovskites (PVKs) in various optoelectronic applications grasp tremendous scientific attention. However, the existence of the toxic lead in these compounds is threatening human health and remains a major concern in the way of their commercialization. To address this issue, numerous nontoxic alternatives have been reported. Among these alternatives, bismuth-based PVKs have emerged as a promising substitute because of similar optoelectronic properties and extended environmental stability. This work communicates briefly about the possible lead-alternatives and explores bismuth-based perovskites comprehensively, in terms of their structures, optoelectronic properties, and applications. A brief description of lead-toxification is provided and the possible Pb-alternatives from the periodic table are scrutinized. Then, the classification and crystal structures of various Bi-based perovskites are elaborated on. Detailed optoelectronic properties of Bi-based perovskites are also described and their optoelectronic applications are abridged. The overall photovoltaic applications along with device characteristics (i.e., V OC, J SC, fill factor, FF, and power conversion efficiency, PCE), fabrication method, device architecture, and operational stability are also summarized. Finally, a conclusion is drawn where a brief outlook highlights the challenges that hamper the future progress of Bi-based optoelectronic devices and suggestions for future directions are provided.

Assembling Polyiodides and Iodobismuthates Using a Template Effect of a Cyclic Diammonium Cation and Formation of a Low-Gap Hybrid Iodobismuthate with High Thermal Stability

Exploiting a template effect of 1,4-diazacycloheptane (also known as homopiperazine, Hpipe), four new hybrid iodides, (HpipeH₂)₂Bi₂I₁₀·2H₂O, (HpipeH₂)I(I₃), (HpipeH₂)₃I₆·H₂O, and (HpipeH₂)₃(H₃O)I₇, were prepared and their crystal structures were solved using single crystal X-ray diffraction data. All four solid-state crystal structures feature the HpipeH₂²⁺ cation alternating with Bi₂I₁₀^4–, I₃^–, or I^– anions and solvent water or H₃O⁺ cation. HpipeH₂²⁺ assembles anionic and neutral building blocks into polymer structures by forming four strong (N)H···I and (N)H···O hydrogen bonds per cation, with the H···I distances ranging from 2.44 to 2.93 Å and H···O distances of 1.88–1.89 Å. These hydrogen bonds strongly affect the properties of compounds; in particular, in the case of (HpipeH₂)₂Bi₂I₁₀·2H₂O, they ensure narrowing of the band gap down to 1.8 eV and provide high thermal stability up to 240 °C, remarkable for a hydrated molecular solid.