David A. Egger

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Role of Dispersive Interactions in Determining Structural Properties of Organic−Inorganic Halide Perovskites: Insights from First- Principles Calculations

A microscopic picture of structure and bonding in organic-inorganic perovskites is imperative to understanding their remarkable semiconducting and photovoltaic properties. On the basis of a density functional theory treatment that includes both spin-orbit coupling and dispersive interactions, we provide detailed insight into the crystal binding of lead-halide perovskites and quantify the effect of different types of interactions on the structural properties. Our analysis reveals that cohesion in these materials is characterized by a variety of interactions that includes important contributions from both van der Waals interactions among the halide atoms and hydrogen bonding. We also assess the role of spin-orbit coupling and show that it causes slight changes in lead-halide bonding that do not significantly affect the lattice parameters. Our results establish that consideration of dispersive effects is essential for understanding the structure and bonding in organic-inorganic perovskites in general and for providing reliable theoretical predictions of structural parameters in particular.

Are Mobilities in Hybrid Organic–Inorganic Halide Perovskites Actually “High”?

We present an experimental and theoretical viewpoint on the electronic carrier mobilities of typical hybrid organic-inorganic perovskites (HOIPs). While these mobilities are often quoted as high, a review of them shows that although otherwise the semiconducting properties of HOIPs are impressively good, mobilities of HOIPs used in most solar cells are actually not that high. This is especially apparent if they are compared to those of inorganic semiconductors used in other high efficiency solar cells. We critically examine possible causes and focus on electron-lattice coupling mechanisms that are active at room temperature, and can lead to carrier scattering. From this, we propose scattering due to acoustic phonons or polarons as possible causes, but also point out the difficulties with each of these in view of additional experimental and theoretical findings in the literature. Further research in this direction will contribute to making HOIP solar cells even more efficient than they already are.

Hybrid Organic–Inorganic Perovskites on the Move

Conspectus Hybrid organic–inorganic perovskites (HOIPs) are crystals with the structural formula ABX3, where A, B, and X are organic and inorganic ions, respectively. While known for several decades, HOIPs have only in recent years emerged as extremely promising semiconducting materials for solar energy applications. In particular, power-conversion efficiencies of HOIP-based solar cells have improved at a record speed and, after only little more than 6 years of photovoltaics research, surpassed the 20% threshold, which is an outstanding result for a solution-processable material. It is thus of fundamental importance to reveal physical and chemical phenomena that contribute to, or limit, these impressive photovoltaic efficiencies. To understand charge-transport and light-absorption properties of semiconducting materials, one often invokes a lattice of ions displaced from their static positions only by harmonic vibrations. However, a preponderance of recent studies suggests that this picture is not sufficient for HOIPs, where a variety of structurally dynamic effects, beyond small harmonic vibrations, arises already at room temperature. In this Account, we focus on these effects. First, we review structure and bonding in HOIPs and relate them to the promising charge-transport and absorption properties of these materials, in terms of favorable electronic properties. We point out that HOIPs are much “softer” mechanically, compared to other efficient solar-cell materials, and that this can result in large ionic displacements at room temperature. We therefore focus next on dynamic structural effects in HOIPs, going beyond a static band-structure picture. Specifically, we discuss pertinent experimental and theoretical findings as to phase-transition behavior and molecular/octahedral rearrangements. We then discuss atomic diffusion phenomena in HOIPs, with an emphasis on the migration of intrinsic and extrinsic ionic species. From this combined perspective, HOIPs appear as highly dynamic materials, in which structural fluctuations and long-range ionic motion have an unusually strong impact on charge-transport and optical properties. We highlight the potential implications of these effects for several intriguing phenomenological observations, ranging from scattering mechanisms and lifetimes of charge carriers to light-induced structural effects and ionic conduction.

High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide

Cesium lead iodide possesses an excellent combination of band gap and absorption coefficient for photovoltaic applications in its perovskite phase. However, this is not its equilibrium structure under ambient conditions. In air, at ambient temperature it rapidly transforms to a nonfunctional, so-called yellow phase. Here we show that chloride doping, particularly at levels near the solubility limit for chloride in a cesium lead iodide host, provides a new approach to stabilizing the functional perovskite phase. In order to achieve high doping levels, we first co-deposit colloidal nanocrystals of pure cesium lead chloride and cesium lead iodide, thereby ensuring nanometer-scale mixing even at compositions that potentially exceed the bulk miscibility of the two phases. The resulting nanocrystal solid is subsequently fused into a polycrystalline thin film by chemically induced, room-temperature sintering. Spectroscopy and X-ray diffraction indicate that the chloride is further dispersed during sintering and a polycrystalline mixed phase is formed. Using density functional theory (DFT) methods in conjunction with nudged elastic band techniques, low-energy pathways for interstitial chlorine diffusion into a majority-iodide lattice were identified, consistent with the facile diffusion and fast halide exchange reactions observed. By comparison to DFT-calculated values (with the PBE exchange-correlation functional), the relative change in band gap and the lattice contraction are shown to be consistent with a Cl/I ratio of a few percent in the mixed phase. At these incorporation levels, the half-life of the functional perovskite phase in a humid atmosphere increases by more than an order of magnitude.

Theory of Hydrogen Migration in Organic–Inorganic Halide Perovskites†

Solar cells based on organic–inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current–voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites—interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin–Corbett mechanism. Our analysis highlights the structural flexibility of organic–inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells.

Local Polar Fluctuations in Lead Halide Perovskite Crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental–Theoretical Study

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional.

Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett.2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1–0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found—a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

Perovskite Solar Cells: Do We Know What We Do Not Know?

D 1−4, 2014 saw one of the largest gatherings of scientists and engineers on the topic of hybrid organic− inorganic perovskite solar cells, which dominated symposium W of the fall meeting of the Materials Research Society (MRS) in Boston on Perovskite-Based and Related Novel Material Solar Cells. The symposium included many contributions as well as invited talks, open discussion sessions (to highlight major themes and stimulate discussion of unpublished data), and a “rump session”. The latter was intended (and indeed mostly used) for last minute news in the field. Over the 4 days, well over 120 papers were presented, including some 50 posters, 3 of which were awarded the symposium’s poster prizes. Furthermore, on the first day of the symposium, Henry Snaith (University of Oxford) received the 2014 Outstanding Young Investigator award of the MRS and gave a meeting-wide lecture at Symposium X. In this Guest Commentary, we summarize the topics that, in our opinion, were the more salient ones of symposium W, with a focus on recent progress and future challenges in understanding the unique properties of hybrid perovskite solar cells. The salient issues that were discussed included hysteresis, which appears to be, at least in part, an interfacial phenomenon, primarily relevant for interfaces with oxides; ion migration, which remains a hypothesis because although it is supported by a variety of experimental results, it still lacks fully conclusive experimental evidence; materials preparation, where solvent annealing of the product of a 2-step process seems to be beneficial for cell efficiency, as is the presence of water; and electronic traps, where evidence for the presence of traps just below mid gap was presented, which is somewhat inconsistent with the high Voc of cells and may present a case of photoinduced effects. Furthermore, a variety of stability tests give promising short-term results, but the reported very low formation energy of hybrid perovskite materials (from the “binaries”) could be detrimental for long-term stability. For mixed I−Br perovskites that are relevant as “ideal bandgap” materials for spectral splitting and tandem cells, phase separation was reported, which, at least for the moment, cannot be explained well by thermodynamics or kinetics. An overall impression is that likely not everyone is, as yet, working with the same material as some effects of exposure to ambient could be explained by doping of the perovskite or holeconducting material. Other results remain puzzling, as even N2 was seen to change the material’s properties. Finally, not all ef f iciency measurements (and their results) appear to be comparable with each other. Subsequent to the first public report of hysteresis in the current−voltage curves of hybrid-perovskite-based solar cells by Hoke et al. at the MRS 2013 Fall meeting and its discussion in the 2013 “rump session” (see Figure 1), the phenomenon was reported in the literature and was the focus of a number of talks at this year’s symposium. Ideas that were brought up to explain it were analyzed in more detail in the discussion sessions of the symposium. Part of the interest stems from the need to arrive at a well-documented, easily reproducible measurement of cell efficiencies, part from the hope that revealing the origin of such “hysteretic behavior” may help to understand the mechanism behind the photovoltaic properties of hybrid perovskites. Furthermore, the hysteresis might be connected to the long-term stability of the devices. In several talks, layers of organic molecules (discussed in talk W4.06) and device geometry (see, e.g., talk W2.05) were reported to play an important, if not essential, role in the magnitude of the hysteresis effects. These findings suggest that the hysteresis is at least partly an interface phenomenon and highlight the importance of (inter-) sur-face passivation. On the other hand, several talks throughout the symposium discussed the complementary possibility that reorientation of the polar organic molecules, contained inside the inorganic cage, due to an external electric field could occur. One hypothesis is that such concerted dipole alignment results in ferroelectric domains, which could then be responsible for the observed hysteresis in the current−voltage curves (see talk W1.01). Hysteresis was shown to be independent of light intensity, implying that it is not arising entirely from stored chargesone of the other possible explanations (talk W1.06). While not stated explicitly, we got the impression that without TiO2 or other electron-conducting hole-blocking oxide layers, there is no or very little hysteresis, suggesting that interface phenomena play a dominant role. Although there is as of now no fully comprehensive explanation for the hysteretic behavior of hybrid perovskites, it was generally agreed in a number of discussion sessions that at the moment, for generating reliable physical insight in terms of I−V curves and solar cell efficiencies, it is highly important to report a number of other experimental parameters. These include a steady-state maximum power point, the scan rate, the scan direction, and the preconditioning of the device prior to measuring (e.g., was it electrically biased, illuminated, or measured immediately upon completion of cell fabrication).