Solution-processable metal–halide perovskites recently opened a new route toward low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors ...among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites, but despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials the appropriate absorption and transport properties are afforded by the multibandgap and multivalley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain.
We use symmetry analysis, density functional theory calculations, and k·p modeling to scrutinize Rashba and Dresselhaus effects in hybrid organic–inorganic halide perovskites. These perovskites are ...at the center of a recent revolution in the field of photovoltaics but have also demonstrated potential for optoelectronic applications such as transistors and light emitters. Due to a large spin–orbit coupling of the most frequently used metals, they are also predicted to offer a promising avenue for spin-based applications. With an in-depth inspection of the electronic structures and bulk lattice symmetries of a variety of systems, we analyze the origin of the spin splitting in two- and three-dimensional hybrid perovskites. It is shown that low-dimensional nanostructures made of CH3NH3PbX3 (X = I, Br) lead to spin splittings that can be controlled by an applied electric field. These findings further open the door for a perovskite-based spintronics.
Three-dimensional (3D) hybrid perovskites CH3NH3PbX3 (X = Br, I) have recently been suggested as new key materials for dye-sensitized solar cells (DSSC) leading to a new class of hybrid semiconductor ...photovoltaic cells (HSPC). Thanks to density functional theory calculations, we show that the band gap of these compounds is dominated by a giant spin–orbit coupling (SOC) in the conduction-band (CB). At room temperature, direct and isotropic optical transitions are associated to a spin–orbit split-off band related to the triply degenerated CB of the cubic lattice without SOC. Due to the strong SOC, the electronic states involved in the optical absorption are only slightly perturbed by local distortions of the lattice. In addition, band offset calculations confirm that CH3NH3PbX3/TiO2 is a reference material for driving electrons toward the electrode in HSPC. Two-dimensional (2D) hybrids are also suggested to reach further flexibility for light conversion efficiency. Our study affords the basic concepts to reach the level of knowledge already attained for optoelectronic properties of conventional semiconductors.
Hybrid Dion–Jacobson 2D Lead Iodide Perovskites Mao, Lingling; Ke, Weijun; Pedesseau, Laurent ...
Journal of the American Chemical Society,
03/2018, Letnik:
140, Številka:
10
Journal Article
Recenzirano
Odprti dostop
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 n–1Pb n I3n+1 (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)3Pb4I13 has the best power conversion efficiency (PCE) of 7.32%, which is much higher than that of the corresponding (4AMP)(MA)3Pb4I13.
Hybrid organic–inorganic perovskites emerged as a new generation of absorber materials for high-efficiency low-cost solar cells in 2009. Very recently, fully inorganic perovskite quantum dots also ...led to promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins. Currently, the record efficiency is obtained with CsPbI3, whose crystallographical characterization is still limited. Here, we show through high-resolution in situ synchrotron XRD measurements that CsPbI3 can be undercooled below its transition temperature and temporarily maintained in its perovskite structure down to room temperature, stabilizing a metastable perovskite polytype (black γ-phase) crucial for photovoltaic applications. Our analysis of the structural phase transitions reveals a highly anisotropic evolution of the individual lattice parameters versus temperature. Structural, vibrational, and electronic properties of all the experimentally observed black phases are further inspected based on several theoretical approaches. Whereas the black γ-phase is shown to behave harmonically around equilibrium, for the tetragonal phase, density functional theory reveals the same anharmonic behavior, with a Brillouin zone-centered double-well instability, as for the cubic phase. Using total energy and vibrational entropy calculations, we highlight the competition between all the low-temperature phases of CsPbI3 (γ, δ, β) and show that avoiding the order–disorder entropy term arising from double-well instabilities is key to preventing the formation of the yellow perovskitoid phase. A symmetry-based tight-binding model, validated by self-consistent GW calculations including spin–orbit coupling, affords further insight into their electronic properties, with evidence of Rashba effect for both cubic and tetragonal phases when using the symmetry-breaking structures obtained through frozen phonon calculations.
Quantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic ...perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled sizes of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach the atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed.
Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power ...conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.
Layered halide hybrid organic–inorganic perovskites (HOP) have been the subject of intense investigation before the rise of three-dimensional (3D) HOP and their impressive performance in solar cells. ...Recently, layered HOP have also been proposed as attractive alternatives for photostable solar cells and revisited for light-emitting devices. In this review, we combine classical solid-state physics concepts with simulation tools based on density functional theory to overview the main features of the optoelectronic properties of layered HOP. A detailed comparison between layered and 3D HOP is performed to highlight differences and similarities. In the same way as the cubic phase was established for 3D HOP, here we introduce the tetragonal phase with D 4h symmetry as the reference phase for 2D monolayered HOP. It allows for detailed analysis of the spin–orbit coupling effects and structural transitions with corresponding electronic band folding. We further investigate the effects of octahedral tilting on the band gap, loss of inversion symmetry and possible Rashba effect, quantum confinement, and dielectric confinement related to the organic barrier, up to excitonic properties. Altogether, this paper aims to provide an interpretive and predictive framework for 3D and 2D layered HOP optoelectronic properties.
In this review we examine recent theoretical investigations on 2D and 3D hybrid perovskites (HOPs) that combine classical solid-state physics concepts and density functional theory (DFT) simulations ...as a tool for studying their optoelectronic properties. Such an approach allows one to define a new class of semiconductors, where the pseudocubic high-temperature perovskite structure plays a central role. Bloch states and k.p. Hamiltonians yield new insight into the influence of lattice distortions, including loss of inversion symmetry, as well as spin–orbit coupling. Electronic band folding and degeneracy, effective masses, and optical absorption are analyzed. Concepts of Bloch and envelope functions, as well as confinement potential, are discussed in the context of layered HOP and 3D HOP heterostructures. Screening and dielectric confinements are important for room-temperature optical properties of 3D and layered HOP, respectively. Nonradiative Auger effects are analyzed for the first time close to the electronic band gap of 3D hybrid perovskites.
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