Theory of hydrogen migration in organic-inorganic halide perovskites

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.

Hybrid Organic-Inorganic Perovskites on the Move

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.

What Is Moving in Hybrid Halide Perovskite Solar Cells?

Organic–inorganic semiconductors, which adopt the perovskite crystal structure, have perturbed the landscape of contemporary photovoltaics research. High-efficiency solar cells can be produced with solution-processed active layers. The materials are earth abundant, and the simple processing required suggests that high-throughput and low-cost manufacture at scale should be possible.

While these materials bear considerable similarity to traditional inorganic semiconductors, there are notable differences in their optoelectronic behavior. A key distinction of these materials is that they are physically soft, leading to considerable thermally activated motion.

In this Account, we discuss the internal motion of methylammonium lead iodide (CH₃NH₃PbI₃) and formamidinium lead iodide ([CH(NH₂)₂]PbI₃), covering: (i) molecular rotation-libration in the cuboctahedral cavity; (ii) drift and diffusion of large electron and hole polarons; (iii) transport of charged ionic defects. These processes give rise to a range of properties that are unconventional for photovoltaic materials, including frequency-dependent permittivity, low electron–hole recombination rates, and current–voltage hysteresis. Multiscale simulations, drawing from electronic structure, ab initio molecular dynamic and Monte Carlo computational techniques, have been combined with neutron diffraction measurements, quasi-elastic neutron scattering, and ultrafast vibrational spectroscopy to qualify the nature and time scales of the motions. Electron and hole motion occurs on a femtosecond time scale. Molecular libration is a sub-picosecond process. Molecular rotations occur with a time constant of several picoseconds depending on the cation. Recent experimental evidence and theoretical models for simultaneous electron and ion transport in these materials has been presented, suggesting they are mixed-mode conductors with similarities to fast-ion conducting metal oxide perovskites developed for battery and fuel cell applications. We expound on the implications of these effects for the photovoltaic action.

The temporal behavior displayed by hybrid perovskites introduces a sensitivity in materials characterization to the time and length scale of the measurement, as well as the history of each sample. It also poses significant challenges for accurate materials modeling and device simulations. There are large differences between the average and local crystal structures, and the nature of charge transport is too complex to be described by common one-dimensional drift-diffusion models. Herein, we critically discuss the atomistic origin of the dynamic processes and the associated chemical disorder intrinsic to crystalline hybrid perovskite semiconductors.

Mobile Ion Induced Slow Carrier Dynamics in Organic-Inorganic Perovskite CH₃NH₃PbBr₃

Here, we investigate photoluminescence (PL) and time-resolved photoluminescence (TRPL) in CH3NH3PbBr3 perovskite under continuous illumination, using optical and electro-optical techniques. Under continuous excitation at constant intensity, PL intensity and PL decay (carrier recombination) exhibit excitation intensity dependent reductions in the time scale of seconds to minutes. The enhanced nonradiative recombination is ascribed to light activated negative ions and their accumulation which exhibit a slow dynamics in a time scale of seconds to minutes. The observed result suggests that the organic-inorganic hybrid perovskite is a mixed electronic-ionic semiconductor. The key findings in this work suggest that ions are photoactivated or electro-activated and their accumulation at localized sites can result in a change of carrier dynamics. The findings are therefore useful for the understanding of instability of perovskite solar cells and shed light on the necessary strategies for performance improvement.

Ion Migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability

Organometal trihalide perovskites (OTPs) are emerging as very promising photovoltaic materials because the power conversion efficiency (PCE) of OTP solar cells quickly rises and now rivals with that of single crystal silicon solar cells after only five-years research. Their prospects to replace silicon photovoltaics to reduce the cost of renewable clean energy are boosted by the low-temperature solution processing as well as the very low-cost raw materials and relative insensitivity to defects. The flexibility, semitransparency, and vivid colors of perovskite solar cells are attractive for niche applications such as built-in photovoltaics and portable lightweight chargers. However, the low stability of current hybrid perovskite solar cells remains a serious issue to be solved before their broad application. Among all those factors that affect the stability of perovskite solar cells, ion migration in OTPs may be intrinsic and cannot be taken away by device encapsulation. The presence of ion migration has received broad attention after the report of photocurrent hysteresis in OTP based solar cells. As suggested by much direct and indirect experimental evidence, the ion migration is speculated to be the origin or an important contributing factor for many observed unusual phenomenon in OTP materials and devices, such as current-voltage hysteresis, switchable photovoltaic effect, giant dielectric constant, diminished transistor behavior at room temperature, photoinduced phase separation, photoinduced self-poling effect, and electrical-field driven reversible conversion between lead iodide (PbI2) and methylammonium lead triiodide (MAPbI3). Undoubtedly thorough insight into the ion-migration mechanism is highly desired for the development of OTP based devices to improve intrinsic stability in the dark and under illumination. In this Account, we critically review the recent progress in understanding the fundamental science on ion migration in OTP based solar cells. We look into both theoretical and experiment advances in answering these basic questions: Does ion migration occur and cause the photocurrent hysteresis in perovskite solar cells? What are the migrating ion species? How do ions migrate? How does ion migration impact the device efficiency and stability? How can ion migration be mitigated or eliminated? We also raise some questions that need to be understood and addressed in the future.

Impacts of Heterogeneous TiO2 and Al2O3 Composite Mesoporous Scaffold on Formamidinium Lead Trihalide Perovskite Solar Cells

Heterogeneous TiO2 and Al2O3 composites were employed as a mesoporous scaffold in formamidinium lead trihalide (FAPbI3-xClx)-based perovskite solar cells to modify surface properties of a mesoporous layer. It was found that the quality and morphology of the perovskite film were strongly affected by the TiO2/Al2O3 ratio in the mesoporous film. The conversion efficiency of the perovskite solar cell was improved by using a composite of TiO2 and Al2O3 in comparison with TiO2- and Al2O3-based cells, yielding 11.0% for a cell with a 7:3 TiO2/Al2O3 composite. Our investigation shows a change of electron transport path depending on a composition ratio of insulating Al2O3 to n-type semiconducting TiO2 in a mesoporous layer.

Systematic study on the impact of water on the performance and stability of perovskite solar cells

Sequentially spin-coated inverted perovskite solar cells were constructed with systematically varied concentrations of water in the PbI₂precursor solution. Surprisingly, small concentrations of water improved long-term stability, although with lower initial efficiency.

Material Innovation in Advancing Organometal Halide Perovskite Functionality

Organometal halide perovskites (OMHPs) have garnered much attention recently for their unprecedented rate of increasing power conversion efficiency (PCE), positioning them as a promising basis for the next-generation photovoltaic devices. However, the gap between the rapid increasing PCE and the incomplete understanding of the structure-property-performance relationship prevents the realization of the true potential of OMHPs. This Perspective aims to provide a concise overview of the current status of OMHP research, highlighting the unique properties of OMHPs that are critical for solar applications but still not adequately explained. Stability and performance challenges of OMHP solar cells are discussed, calling upon combined experimental and theoretical efforts to address these challenges for pioneering commercialization of OMHP solar cells. Various material innovation strategies for improving the performance and stability of OMHPs are surveyed, showing that the OMHP architecture can serve as a promising and robust platform for the design and optimization of materials with desired functionalities.