Speciation of Lanthanide Metal Ion Dopants in Microcrystalline All-Inorganic Halide Perovskite CsPbCl3.

Lanthanides are versatile modulators of optoelectronic properties owing to their narrow optical emission spectra across the visible and near-infrared range. Their use in metal halide perovskites (MHPs) has recently gained prominence, although their fate in these materials has not yet been established at the atomic level. We use cesium-133 solid-state NMR to establish the speciation of all nonradioactive lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Sm2+, Eu3+, Eu2+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) in microcrystalline CsPbCl3. Our results show that all lanthanides incorporate into the perovskite structure of CsPbCl3 regardless of their oxidation state (+2, +3).

Direct Observation of Contact Reaction Induced Ion Migration and its Effect on Non-Ideal Charge Transport in Lead Triiodide Perovskite Field-Effect Transistors.

The migration of ionic defects and electrochemical reactions with metal electrodes remains one of the most important research challenges for organometal halide perovskite optoelectronic devices. There is still a lack of understanding of how the formation of mobile ionic defects impact charge carrier transport and operational device stability, particularly in perovskite field-effect transistors (FETs), which tend to exhibit anomalous device characteristics. Here, the evolution of the n-type FET characteristics of one of the most widely studied materials, Cs0.05 FA0.17 MA0.78 PbI3, is investigated during repeated measurement cycles as a function of different metal source-drain contacts and precursor stoichiometry. The channel current increases for high work function metals and decreases for low work function metals when multiple cycles of transfer characteristics are measured. The cycling behavior is also sensitive to the precursor stoichiometry. These metal/stoichiometry-dependent device non-idealities are correlated with the quenching of photoluminescence near the positively biased electrode. Based on elemental analysis using electron microscopy the observations can be understood by an n-type doping effect of metallic ions that are created by an electrochemical interaction at the metal-semiconductor interface and migrate into the channel. The findings improve the understanding of ion migration, contact reactions, and the origin of non-idealities in lead triiodide perovskite FETs.

Multi-Length Scale Structure of 2D/3D Dion-Jacobson Hybrid Perovskites Based on an Aromatic Diammonium Spacer.

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)An-1 Pbn I3n+1 composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.

Crystal-Size-Induced Band Gap Tuning in Perovskite Films.

A comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto-electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo-induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state-of-the-art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

A combined molecular dynamics and experimental study of two-step process enabling low-temperature formation of phase-pure α-FAPbI3.

It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable α phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure α-formamidinium lead iodide.

Supramolecular Modulation of Hybrid Perovskite Solar Cells via Bifunctional Halogen Bonding Revealed by Two-Dimensional 19F Solid-State NMR Spectroscopy.

There has been an ongoing effort to overcome the limitations associated with the stability of hybrid organic-inorganic perovskite solar cells by using different organic agents as additives to the perovskite formulations. The functionality of organic additives has been predominantly limited to exploiting hydrogen-bonding interactions, while the relevant atomic-level binding modes remain elusive. Herein, we introduce a bifunctional supramolecular modulator, 1,2,4,5-tetrafluoro-3,6-diiodobenzene, which interacts with the surface of the triple-cation double-halide perovskite material via halogen bonding. We elucidate its binding mode using two-dimensional solid-state 19F NMR spectroscopy in conjunction with density functional theory calculations. As a result, we demonstrate a stability enhancement of the perovskite solar cells upon supramolecular modulation, without compromising the photovoltaic performances.

Guanine-Stabilized Formamidinium Lead Iodide Perovskites.

Formamidinium (FA) lead iodide perovskite materials feature promising photovoltaic performances and superior thermal stabilities. However, conversion of the perovskite α-FAPbI3 phase to the thermodynamically stable yet photovoltaically inactive δ-FAPbI3 phase compromises the photovoltaic performance. A strategy is presented to address this challenge by using low-dimensional hybrid perovskite materials comprising guaninium (G) organic spacer layers that act as stabilizers of the three-dimensional α-FAPbI3 phase. The underlying mode of interaction at the atomic level is unraveled by means of solid-state nuclear magnetic resonance spectroscopy, X-ray crystallography, transmission electron microscopy, molecular dynamics simulations, and DFT calculations. Low-dimensional-phase-containing hybrid FAPbI3 perovskite solar cells are obtained with improved performance and enhanced long-term stability.

Bifunctional Organic Spacers for Formamidinium-Based Hybrid Dion-Jacobson Two-Dimensional Perovskite Solar Cells.

Three-dimensional (3D) perovskite materials display remarkable potential in photovoltaics owing to their superior solar-to-electric power conversion efficiency, with current efforts focused on improving stability. Two-dimensional (2D) perovskite analogues feature greater stability toward environmental factors, such as moisture, owing to a hydrophobic organic cation that acts as a spacer between the inorganic layers, which offers a significant advantage over their comparatively less stable 3D analogues. Here, we demonstrate the first example of a formamidinium (FA) containing Dion-Jacobson 2D perovskite material characterized by the BFA n-1Pb nI3 n+1 formulation through employing a novel bifunctional organic spacer (B), namely 1,4-phenylenedimethanammonium (PDMA). We achieve remarkable efficiencies exceeding 7% for (PDMA)FA2Pb3I10 based 2D perovskite solar cells resisting degradation when exposed to humid ambient air, which opens up new avenues in the design of stable perovskite materials.

Revealing the detailed path of sequential deposition for metal halide perovskite formation.

Sequential deposition has been extensively used for the fabrication of perovskite solar cells. Nevertheless, fundamental aspects of the kinetics of methylammonium lead iodide perovskite formation remain obscure. We scrutinize the individual stages of the reaction and investigate the crystallization of the lead iodide film, which occurs before the intercalation of methylammonium iodide commences. Our study identifies the presence of mixed crystalline aggregates composed of perovskite and lead iodide during intercalation and structural reorganization. Furthermore, Ostwald ripening occurs in the film for reaction times beyond the point of conversion to perovskite. Using cross-sectional confocal laser scanning microscopy for the first time, we reveal that lead iodide in the over-layer and at the bottom of the mesoporous layer converts first. We identify unreacted lead iodide trapped in the mesoporous layer for samples of complete conversion. We acquire kinetic data by varying different parameters and find that the Avrami models best represent them. The model facilitates the rapid estimation of the reaction time for complete conversion for a variety of reaction conditions, thereby ascertaining a major factor previously determined by extensive experimentation. This comprehensive picture of the sequential deposition is essential for control over the perovskite film quality, which determines solar cell efficiency. Our results provide key insights to realize high-quality perovskite films for optoelectronic applications.

Poly(ethylene glycol)-[60]Fullerene-Based Materials for Perovskite Solar Cells with Improved Moisture Resistance and Reduced Hysteresis.

A series of [60]fullerenes covalently functionalized with the polymer poly(ethylene glycol) is presented. These new [60]fullerene-based materials have been incorporated as additives in CH3 NH3 PbI3 (MAPbI3 ), the most common organic-inorganic perovskite used in perovskite solar cells. The extensive photovoltaic study performed by using these materials shows several beneficial effects on the performance of these cells, including a reduction in hysteresis and an increased stability against moisture, whereby the solar cells retain up to 97 % of their initial power conversion efficiency in an ambient atmosphere.

Additives, Hole Transporting Materials and Spectroscopic Methods to Characterize the Properties of Perovskite Films.

The achievement of high efficiency and high stability in perovskite solar cells (PSCs) requires optimal selection and evaluation of the various components. After a brief introduction to the perovskite materials and their historical evolution, the first part is devoted to the hole transporting material (HTM), between photoelectrode and dark counter electrode. The basic requirements for an efficient HTM are stated. Subsequently, the most used HTM, spiro-OMeTAD, is compared to alternative HTMs, both small-molecule size species and electronically conducting polymers. The second part is devoted to additives related to the performance of the perovskite light-absorbing material itself. These are related either to the modification of the composition of the material itself or to the optimization of the morphology during the perovskite preparation stage, and their effect is in the enhancement of the power conversion efficiency, the long-term stability, or the reproducibility of the properties of the PSCs. Finally, a number of spectroscopic methods based on the UV-Vis part of the electromagnetic spectrum useful for characterizing the different perovskite material types are described in the last part of this review.

11% efficiency solid-state dye-sensitized solar cells with copper(II/I) hole transport materials.

Solid-state dye-sensitized solar cells currently suffer from issues such as inadequate nanopore filling, low conductivity and crystallization of hole-transport materials infiltrated in the mesoscopic TiO2 scaffolds, leading to low performances. Here we report a record 11% stable solid-state dye-sensitized solar cell under standard air mass 1.5 global using a hole-transport material composed of a blend of [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide)2 and [Cu (4,4',6,6'-tetramethyl-2,2'-bipyridine)2](bis(trifluoromethylsulfonyl)imide). The amorphous Cu(II/I) conductors that conduct holes by rapid hopping infiltrated in a 6.5 µm-thick mesoscopic TiO2 scaffold are crucial for achieving such high efficiency. Using time-resolved laser photolysis, we determine the time constants for electron injection from the photoexcited sensitizers Y123 into the TiO2 and regeneration of the Y123 by Cu(I) to be 25 ps and 3.2 µs, respectively. Our work will foster the development of low-cost solid-state photovoltaic based on transition metal complexes as hole conductors.

The effect of illumination on the formation of metal halide perovskite films.

Optimizing the morphology of metal halide perovskite films is an important way to improve the performance of solar cells when these materials are used as light harvesters, because film homogeneity is correlated with photovoltaic performance. Many device architectures and processing techniques have been explored with the aim of achieving high-performance devices, including single-step deposition, sequential deposition and anti-solvent methods. Earlier studies have looked at the influence of reaction conditions on film quality, such as the concentration of the reactants and the reaction temperature. However, the precise mechanism of the reaction and the main factors that govern it are poorly understood. The consequent lack of control is the main reason for the large variability observed in perovskite morphology and the related solar-cell performance. Here we show that light has a strong influence on the rate of perovskite formation and on film morphology in both of the main deposition methods currently used: sequential deposition and the anti-solvent method. We study the reaction of a metal halide (lead iodide) with an organic compound (methylammonium iodide) using confocal laser scanning fluorescence microscopy and scanning electron microscopy. The lead iodide crystallizes before the intercalation of methylammonium iodide commences, producing the methylammonium lead iodide perovskite. We find that the formation of perovskite via such a sequential deposition is much accelerated by light. The influence of light on morphology is reflected in a doubling of solar-cell efficiency. Conversely, using the anti-solvent method to form methyl ammonium lead iodide perovskite in a single step from the same starting materials, we find that the best photovoltaic performance is obtained when films are produced in the dark. The discovery of light-activated crystallization not only identifies a previously unknown source of variability in opto-electronic properties, but also opens up new ways of tuning morphology and structuring perovskites for various applications.

Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance.

All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb+) can be embedded into a "cation cascade" to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.

Enhancing Efficiency of Perovskite Solar Cells via N-doped Graphene: Crystal Modification and Surface Passivation.

Controlling the morphology and surface passivation in perovskite solar cells is paramount in obtaining optimal optoelectronic properties. This study incorporates N-doped graphene nanosheets in the perovskite layer, which simultaneously induces an improved morphology and surface passivation at the perovskite/spiro interface, resulting in enhancement in all photovoltaic parameters.