Hybrid halide perovskites have become the “next big thing” in emerging semi­conductor materials, as the past decade witnessed their successful application in high-performance photo­voltaics. This resurgence has encompassed enormous and wide­spread development of the three-dimensional (3D) perovskites, spear­headed by CH3­NH3­PbI3. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse sub­group of semi­conducting hybrids that possesses even higher tunability and excellent photo­physical properties. In this Perspective, we begin with a historical flash­back to early reports before the “perovskite fever”, and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance opto­electronics. We approach the evolution of 2D halide perovskites from a structural perspective, providing a way to classify the diverse structure types of the materials, which largely dictate the unusual physical properties observed. We sort the 2D hybrid halide perovskites on the basis of two key components: the inorganic layers and their modification, and the organic cation diversity. As these two hetero­geneous components blend, either by synthetic manipulation (shuffling the organic cations or inorganic elements) or by application of external stimuli (temperature and pressure), the modular perovskite structure evolves to construct crystallographically defined quantum wells (QWs). The complex electronic structure that arises is sensitive to the structural features that could be in turn used as a knob to control the dielectric and optical properties the QWs. We conclude this Perspective with the most notable achievements in opto­electronic devices that have been demonstrated to date, with an eye toward future material discovery and potential technological developments.

The three-dimensional hybrid organic-inorganic perovskites have shown huge potential for use in solar cells and other optoelectronic devices. Although these materials are under intense investigation, derivative materials with lower dimensionality are emerging, offering higher tunability of physical properties and new capabilities. Here, we present two new series of hybrid two-dimensional (2D) perovskites that adopt the Dion-Jacobson (DJ) structure type, which are the first complete homologous series reported in halide perovskite chemistry. Lead iodide DJ perovskites adopt a general formula A'A PbI (A' = 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP), A = methylammonium (MA)). These materials have layered structures where the stacking of inorganic layers is unique as they lay exactly on top of another. With a slightly different position of the functional group in the templating cation 3AMP and 4AMP, the as-formed DJ perovskites show different optical properties, with the 3AMP series having smaller band gaps than the 4AMP series. Analysis on the crystal structures and density functional theory (DFT) calculations suggest that the origin of the systematic band gap shift is the strong but indirect influence of the organic cation on the inorganic framework. Fabrication of photovoltaic devices utilizing these materials as light absorbers reveals that (3AMP)(MA)PbI has the best power conversion efficiency (PCE) of 7.32%, which is much higher than that of the corresponding (4AMP)(MA)PbI.

Hybrid inorganic-organic perovskites are developing rapidly as high performance semiconductors. Recently, two-dimensional (2D) perovskites were found to have white-light, broadband emission in the visible range that was attributed mainly to the role of self-trapped excitons (STEs). Here, we describe three new 2D lead bromide perovskites incorporating a series of bifunctional ammonium dications as templates which also emit white light: (1) α-(DMEN)PbBr (DMEN = 2-(dimethylamino)ethylamine), which adopts a unique corrugated layered structure in space group Pbca with unit cell a = 18.901(4) Å, b = 11.782(2) Å, and c = 23.680(5) Å; (2) (DMAPA)PbBr (DMAPA = 3-(dimethylamino)-1-propylamine), which crystallizes in P2/c with a = 10.717(2) Å, b = 11.735(2) Å, c = 12.127(2) Å, and β = 111.53(3)°; and (3) (DMABA)PbBr (DMABA = 4-dimethylaminobutylamine), which adopts Aba2 with a = 41.685(8) Å, b = 23.962(5) Å, and c = 12.000(2) Å. Photoluminescence (PL) studies show a correlation between the distortion of the "PbBr" octahedron in the 2D layer and the broadening of PL emission, with the most distorted structure having the broadest emission (183 nm full width at half-maximum) and longest lifetime (τ = 1.39 ns). The most distorted member α-(DMEN)PbBr exhibits white-light emission with a color rendering index (CRI) of 73 which is similar to a fluorescent light source and correlated color temperature (CCT) of 7863 K, producing "cold" white light.

Hybrid organic-inorganic halide perovskites are under intense investigations because of their astounding physical properties and promises for optoelectronics. Lead bromide and chloride perovskites exhibit intrinsic white-light emission believed to arise from self-trapped excitons (STEs). Here, we report a series of new structurally diverse hybrid lead bromide perovskites that have broad-band emission at room temperature. They feature Pb/Br structures which vary from 1D face-sharing structures to 3D corner- and edge-sharing structures. Through single-crystal X-ray diffraction and low-frequency Raman spectroscopy, we have identified the local distortion level of the octahedral environments of Pb within the structures. The band gaps of these compounds range from 2.92 to 3.50 eV, following the trend of "corner-sharing < edge-sharing < face-sharing". Density functional theory calculations suggest that the electronic structure is highly dependent on the connectivity mode of the PbBr octahedra, where the edge- and corner-sharing 1D structure of (2,6-dmpz)PbBr exhibits more disperse bands and smaller band gap (2.49 eV) than the face-sharing 1D structure of (hep)PbBr (3.10 eV). Using photoemission spectroscopy, we measured the energies of the valence band of these compounds and found them to remain almost constant, while the energy of conduction bands varies. Temperature-dependent PL measurements reveal that the 2D and 3D compounds have narrower PL emission at low temperature (∼5 K), whereas the 1D compounds have both free exciton emission and STE emission. The 1D compound (2,6-dmpz)PbBr has the highest photoluminescence quantum yield of 12%, owing to its unique structure that allows efficient charge carrier relaxation and light emission.

High-performance and good-stability hollow Sn-based perovskite solar cells using ethylenediammonium and formamidinium cations.