Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals

Enabling Fast Photoresponse in Hybrid Perovskite/MoS2 Photodetectors by Separating Local Photocharge Generation and Recombination

Interfacing CH3NH3PbI3 (MAPbI3) with 2D van der Waals materials in lateral photodetectors can suppress the dark current and driving voltage, while the interlayer charge separation also renders slower charge dynamics. In this work, we show that more than one order of magnitude faster photoresponse time can be achieved in MAPbI3/MoS2 lateral photodetectors by locally separating the photocharge generation and recombination through a parallel channel of single-layer MAPbI3. Photocurrent (Iph) mapping reveals electron diffusion lengths of about 20 μm in single-layer MAPbI3 and 4 μm in the MAPbI3/MoS2 heterostructure. The illumination-power scaling of Iph and time-resolved photoluminescence studies point to the dominant roles of the heterostructure region in photogeneration and single-layer MAPbI3 in charge recombination. Our results shed new light on the material design that can concurrently enhance photoresponsivity, reduce driving voltage, and sustain high operation speed, paving the path for developing high-performance lateral photodetectors based on hybrid perovskites.

Arising 2D Perovskites for Ionizing Radiation Detection

2D perovskites have greatly improved moisture stability owing to the large organic cations embedded in the inorganic octahedral structure, which also suppresses the ions migration and reduces the dark current. The suppression of ions migration by 2D perovskites effectively suppresses excessive device noise and baseline drift and shows excellent potential in the direct X-ray detection field. In addition, 2D perovskites have gradually emerged with many unique properties, such as anisotropy, tunable bandgap, high photoluminescence quantum yield, and wide range exciton binding energy, which continuously promote the development of 2D perovskites in ionizing radiation detection. This review aims to systematically summarize the advances and progress of 2D halide perovskite semiconductor and scintillator ionizing radiation detectors, including reported alpha (α) particle, beta (β) particle, neutron, X-ray, and gamma (γ) ray detection. The unique structural features of 2D perovskites and their advantages in X-ray detection are discussed. Development directions are also proposed to overcome the limitations of 2D halide perovskite radiation detectors.

Single-Crystal Perovskite Solar Cells Exhibit Close to Half A Millimeter Electron-Diffusion Length

Single-crystal halide perovskites exhibit photogenerated-carriers of high mobility and long lifetime, making them excellent candidates for applications demanding thick semiconductors, such as ionizing radiation detectors, nuclear batteries, and concentrated photovoltaics. However, charge collection depreciates with increasing thickness; therefore, tens to hundreds of volts of external bias is required to extract charges from a thick perovskite layer, leading to a considerable amount of dark current and fast degradation of perovskite absorbers. However, extending the carrier-diffusion length can mitigate many of the anticipated issues preventing the practical utilization of perovskites in the abovementioned applications. Here, single-crystal perovskite solar cells that are up to 400 times thicker than state-of-the-art perovskite polycrystalline films are fabricated, yet retain high charge-collection efficiency in the absence of an external bias. Cells with thicknesses of 110, 214, and 290 µm display power conversion efficiencies (PCEs) of 20.0, 18.4, and 14.7%, respectively. The remarkable persistence of high PCEs, despite the increase in thickness, is a result of a long electron-diffusion length in those cells, which was estimated, from the thickness-dependent short-circuit current, to be ≈0.45 mm under 1 sun illumination. These results pave the way for adapting perovskite devices to optoelectronic applications in which a thick active layer is essential.

Tuning the Band Gap in the Halide Perovskite CsPbBr3 through Sr Substitution

The ability to continuously tune the band gap of a semiconductor allows its optical properties to be precisely tailored for specific applications. We demonstrate that the band gap of the halide perovskite CsPbBr₃ can be continuously widened through homovalent substitution of Sr²⁺ for Pb²⁺ using solid-state synthesis, creating a material with the formula CsPb_1-xSr_xBr₃ (0 ≤ x ≤ 1). Sr²⁺ and Pb²⁺ form a solid solution in CsPb_1-xSr_xBr₃. Pure CsPbBr₃ has a band gap of 2.29(2) eV, which increases to 2.64(3) eV for CsPb_0.25Sr_0.75Br₃. The increase in band gap is clearly visible in the color change of the materials and is also confirmed by a shift in the photoluminescence. Density-functional theory calculations support the hypothesis that Sr incorporation widens the band gap without introducing mid-gap defect states. These results demonstrate that homovalent B-site alloying can be a viable method to tune the band gap of simple halide perovskites for absorptive and emissive applications such as color-tunable light-emitting diodes, tandem solar cells, and photodetectors.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ (MAPbI₃) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI₃. Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI₃ polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI₃ and are indispensable for optoelectronic device development with better performance.

Quantifying Efficiency Limitations in All-Inorganic Halide Perovskite Solar Cells

While halide perovskites have excellent optoelectronic properties, their poor stability is a major obstacle toward commercialization. There is a strong interest to move away from organic A-site cations such as methylammonium and formamidinium toward Cs with the aim of improving thermal stability of the perovskite layers. While the optoelectronic properties and the device performance of Cs-based all-inorganic lead-halide perovskites are very good, they are still trailing behind those of perovskites that use organic cations. Here, the state-of-the-art of all-inorganic perovskites for photovoltaic applications is reviewed by performing detailed meta-analyses of key performance parameters on the cell and material level. Key material properties such as carrier mobilities, external photoluminescence quantum efficiency, and photoluminescence lifetime are discussed and what is known about defect tolerance in all-inorganic is compared relative to hybrid (organic-inorganic) perovskites. Subsequently, a unified approach is adopted for analyzing performance losses in perovskite solar cells based on breaking down the losses into several figures of merit representing recombination losses, resistive losses, and optical losses. Based on this detailed loss analysis, guidelines are eventually developed for future performance improvement of all-inorganic perovskite solar cells.

Probing Longitudinal Carrier Transport in Perovskite Thin Films via Modified Transient Reflection Spectroscopy

Accurate characterization of the longitudinal (along the thickness direction) carrier transport property is of significant importance for evaluating the quality and performance of perovskite thin films. Herein, we report the development of a modified transient reflection (TR) spectroscopy method to realize the direct observation and determination of the longitudinal carrier transport process in MAPbI₃ polycrystalline thin films. Unlike the traditional TR spectroscopy, the carrier transport dynamics along the film thickness is resolved by making the pump (excitation) and probe beams spatially separated on each side of the film, so that the carrier transport from the excitation side to the probe side is directly captured. Utilizing this method, the longitudinal carrier diffusion coefficients (D) in various perovskite films with different thicknesses and grain sizes (extracted from SEM images) are determined, showing D values of ∼1.5 to 1.8 cm² s⁻¹ (∼0.5 to 0.8 cm² s⁻¹) for films with grain size larger (smaller) than the thickness. This empirical correlation between the longitudinal D and film thickness/grain size provides a reference for quick quality screening and evaluation of perovskite polycrystalline thin films.

A back-excitation transient reflection spectroscopy was developed to visualize longitudinal carrier transport in perovskite films, showing that the longitudinal diffusion coefficient decreases sharply with increasing thickness-to-grain-size ratio.

Halide Perovskites: A New Era of Solution-Processed Electronics

Organic-inorganic mixed halide perovskites have emerged as an excellent class of materials with a unique combination of optoelectronic properties, suitable for a plethora of applications ranging from solar cells to light-emitting diodes and photoelectrochemical devices. Recent works have showcased hybrid perovskites for electronic applications through improvements in materials design, processing, and device stability. Herein, a comprehensive up-to-date review is presented on hybrid perovskite electronics with a focus on transistors and memories. These applications are supported by the fundamental material properties of hybrid perovskite semiconductors such as tunable bandgap, ambipolar charge transport, reasonable mobility, defect characteristics, and solution processability, which are highlighted first. Then, recent progresses on perovskite-based transistors are reviewed, covering aspects of fabrication process, patterning techniques, contact engineering, 2D versus 3D material selection, and device performance. Furthermore, applications of perovskites in nonvolatile memories and artificial synaptic devices are presented. The ambient instability of hybrid perovskites and the strategies to tackle this bottleneck are also discussed. Finally, an outlook and opportunities to develop perovskite-based electronics as a competitive and feasible technology are highlighted.

Transport Modeling of Locally Photogenerated Excitons in Halide Perovskites

Excitons have fundamental impacts on optoelectronic properties of semiconductors. Halide perovskites, with long carrier lifetimes and ionic crystal structures, may support highly mobile excitons because the dipolar nature of excitons suppresses phonon scattering. Inspired by recent experimental progress, we perform device modeling to rigorously analyze exciton formation and transport in methylammonium lead triiodide under local photoexcitation by using a finite element method. Mobile excitons, coexisting with free carriers, can dominate photocurrent generation at low temperatures. The simulation results are in excellent agreement with the experimentally observed strong temperature and gate dependence of carrier diffusion. This work signifies that efficient exciton transport can substantially influence charge transport in the family of perovskite materials.

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH₃NH₃PbI₃. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Toward Stable Solution-Processed High-Mobility p-Type Thin Film Transistors Based on Halide Perovskites

Organolead halide perovskites have drawn significant attention from the scientific community as one of the most attractive materials in optoelectronics, especially in the field of photovoltaics. In this study, we focus on using halide perovskites in processing thin film transistors (TFTs). Halide perovskites have high solution processability and excellent carrier transport characteristics, in particular for holes. The present work aims to fill a gap in oxide-based technology. It concerns the process of using high-stable and reliable p-type oxide-based devices to target CMOS technology (complementary metal-oxide-semiconductor). We report on a solution-processed high-performance TFT based on methylammonium lead iodide (CH₃NH₃PbI₃) perovskite semiconductor films, which shows promise for devices that can be simple to manufacture with high reliability, reproducibility, and excellent stability in atmospheric conditions. To achieve a highly stable perovskite semiconductor film, we introduce diethylsulfide in the perovskite precursor. The TFT shows a stable p-type behavior when operated at low voltages (≤-2 V) and has a current modulation of >10⁴, an almost negligible hysteresis, and average saturation mobility of about 18.8 cm² V^-1 s^-1, taken over 50 devices tested (the highest one measured was ∼23.2 cm² V^-1 s^-1). This is the highest value until now reported in the literature. In addition, we demonstrate that perovskite TFTs can be fabricated at temperatures as low as 150 °C on flexible substrates with a saturation mobility of ∼11.5 cm² V^-1 s^-1. The high-performance perovskite TFT with excellent stability is a promising candidate for the next generation of p-type transistors for a plethora of low-cost electronics applications.

Building Blocks of Hybrid Perovskites: A Photoluminescence Study of Lead-Iodide Solution Species

In this work, we present a detailed investigation of the optical properties of hybrid perovskite building blocks, [PbI2+n ]n- , that form in solutions of CH3 NH3 PbI3 and PbI2 . The absorbance, photoluminescence (PL) and photoluminescence excitation (PLE) spectra of CH3 NH3 PbI3 and PbI2 solutions were measured in various solvents and a broad concentration range. Both CH3 NH3 PbI3 and PbI2 solutions exhibit absorption features attributed to [PbI3 ]1- and [PbI4 ]2- complexes. Therefore, we propose a new mechanism for the formation of polymeric polyiodide plumbates in solutions of pristine PbI2 . For the first time, we show that the [PbI2+n ]n- species in both solutions of CH3 NH3 PbI3 and PbI2 exhibit a photoluminescence peak at about 760 nm. Our findings prove that the spectroscopic properties of both CH3 NH3 PbI3 and PbI2 solutions are dominated by coordination complexes between Pb2+ and I- . Finally, the impact of these complexes on the properties of solid-state perovskite semiconductors is discussed in terms of defect formation and defect tolerance.

Role of the Metal-Semiconductor Interface in Halide Perovskite Devices for Radiation Photon Counting

Halide perovskites are promising optoelectronic semiconductors. For applications in solid-state detectors that operate in low photon flux counting mode, blocking interfaces are essential to minimize the dark current noise. Here, we investigate the interface between methylammonium lead tri-iodide (MAPbI₃) single crystals and commonly used high and low work function metals to achieve photon counting capabilities in a solid-state detector. Using scanning photocurrent microscopy, we observe a large Schottky barrier at the MAPbI₃/Pb interface, which efficiently blocks dark current. Moreover, the shape of the photocurrent profile indicates that the MAPbI₃ single-crystal surface has a deep fermi level close to that of Au. Rationalized by first-principle calculations, we attribute this observation to the defects due to excess iodine on the surface underpinning emergence of deep band-edge states. The photocurrent decay profile yields a charge carrier diffusion length of 10-25 μm. Using this knowledge, we demonstrate a single-crystal MAPbI₃ detector that can count single γ-ray photons by producing sharp electrical pulses with a fast rise time of <2 μs. Our study indicates that the interface plays a crucial role in solid-state detectors operating in photon counting mode.

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

Light is a form of energy that can be converted to electric and chemical energies. Thus, organic photovoltaics (OPVs), perovskite solar cells (PSCs), photocatalysts, and photodetectors have evolved as scientific and commercial enterprises. However, the complex photochemical reactions and multicomponent materials involved in these systems have hampered rapid progress in their fundamental understanding and material design. This review showcases the evaluation-oriented exploration of photo energy conversion materials by using electrodeless time-resolved microwave conductivity (TRMC) and materials informatics (MI). TRMC with its unique options (excitation sources, environmental control, frequency modulation, etc.) provides not only accelerated experimental screening of OPV and PSC materials but also a versatile route toward shedding light on their charge carrier dynamics. Furthermore, MI powered by machine learning is shown to allow extremely high-throughput exploration in the large molecular space, which is compatible with experimental screening and combinatorial synthesis.

Local Rearrangement of the Iodide Defect Structure Determines the Phase Segregation Effect in Mixed-Halide Perovskites

Mixed-halide perovskites represent a particularly relevant case within the family of lead-halide perovskites. Beyond their technological relevance for a variety of optoelectronic devices, photoinduced structural changes characteristic of this type of material lead to extreme photophysical changes that are currently the subject of intense debate. Herein we show that the conspicuous photoinduced phase segregation characteristic of these materials is primarily the result of the local and metastable rearrangement of the iodide sublattice. A local photophysical study comprising spectrally resolved laser scanning confocal microscopy is employed to find a correlation between the defect density and the dynamics of photoinduced changes, which extend far from the illuminated region. We observe that iodide-rich regions evolve much faster from highly defective regions. Also, by altering the material composition, we find evidence for the interplay between the iodide-related defect distribution and the intra- and interdomain migration dynamics giving rise to the complexity of this process.

Benign ferroelastic twin boundaries in halide perovskites for charge carrier transport and recombination

Grain boundaries have been established to impact charge transport, recombination and thus the power conversion efficiency of metal halide perovskite thin film solar cells. As a special category of grain boundaries, ferroelastic twin boundaries have been recently discovered to exist in both CH3NH3PbI3 thin films and single crystals. However, their impact on the carrier transport and recombination in perovskites remains unexplored. Here, using the scanning photocurrent microscopy, we find that twin boundaries have negligible influence on the carrier transport across them. Photoluminescence (PL) imaging and the spatial-resolved PL intensity and lifetime scanning confirm the electronically benign nature of the twin boundaries, in striking contrast to regular grain boundaries which block the carrier transport and behave as the non-radiative recombination centers. Finally, the twin-boundary areas are found still easier to degrade than grain interior.

Temperature and Gate Dependence of Carrier Diffusion in Single Crystal Methylammonium Lead Iodide Perovskite Microstructures

We investigate temperature-dependent photogenerated carrier diffusion in single-crystal methylammonium lead iodide microstuctures via scanning photocurrent microscopy. Carrier diffusion lengths increased abruptly across the tetragonal to orthorhombic phase transition and reached 200 ± 50 μm at 80 K. In combination with the microsecond carrier lifetime measured by a transient photocurrent method, an enormous carrier mobility value of 3 × 10⁴ cm²/V s was extracted at 80 K. The observed highly nonlocal photocurrent and the rapid increase of the carrier diffusion length at low temperatures can be understood by the formation and efficient transport of free excitons in the orthorhombic phase as a result of reduced optical phonon scattering due to the dipolar nature of the excitons. Carrier diffusion lengths were tuned by a factor of 8 by gate voltage and increased with increasing majority carrier (electron) concentration, consistent with the exciton model.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

Metal halide perovskites (MHPs) have recently attracted great attention from the scientific community due to their excellent photovoltaic performance as well as their tremendous potential for other optoelectronic applications such as light-emitting diodes, lasers, and photodetectors. Despite the rapid progress in device applications, a solid understanding of the photophysical properties behind the device performance is highly desirable for MHPs. Here, the properties of excitons and photogenerated charge carriers in MHPs are explored. The unique dielectric constant properties, crystal-liquid duality, and fundamental optical processes of MHPs are first discussed. The properties of excitons and related phenomena in MHPs are then detailed, including the exciton binding energy determined by various methods and their influence factors, exciton dynamics, exciton-photon coupling and related applications, and exciton-phonon coupling in MHPs. The properties of photogenerated free charge carriers in MHPs such as the carrier diffusion length, mobility, and recombination are described. Recent progress in various applications is also demonstrated. Finally, a conclusion and perspectives of future studies for MHPs are presented.

Contactless measurements of photocarrier transport properties in perovskite single crystals

The remarkable properties of metal halide perovskites arising from their impressive charge carrier diffusion lengths have led to rapid advances in solution-processed optoelectronics. Unfortunately, diffusion lengths reported in perovskite single crystals have ranged widely - from 3 μm to 3 mm - for ostensibly similar materials. Here we report a contactless method to measure the carrier mobility and further extract the diffusion length: our approach avoids both the effects of contact resistance and those of high electric field. We vary the density of quenchers - epitaxially included within perovskite single crystals - and report the dependence of excited state lifetime in the perovskite on inter-quencher spacing. Our results are repeatable and self-consistent (i.e. they agree on diffusion length for many different quencher concentrations) to within ± 6%. Using this method, we obtain a diffusion length in metal-halide perovskites of 2.6 μm ± 0.1 μm.

Several Orders of Magnitude Difference in Charge-Transfer Kinetics Induced by Localized Trapped Charges on Mixed-Halide Perovskites

Partial halide substitution in organolead halide perovskites MAPbX₃ (MA = CH₃NH₃⁺, X = Cl^-, Br^-, or I^-) leads to semiconductor heterostructures with precisely tuned band-gap energies, which facilitates efficient charge extraction or separation for high-performance solar cells and optoelectronic devices. In this study, partially iodide-substituted MAPbBr₃ perovskites were prepared through a halide-exchange reaction in the liquid phase, and in situ space- and time-resolved photoluminescence profiles were acquired by means of confocal microscopy. The rates of charge transfer from the bulk MAPbBr₃ to the surface MAPbBr_{3- x}I _x domains, which are widely distributed over a single crystal, were found to greatly depend on the excitation-power density. In particular, an abnormally slow charge-transfer process, lasting a few nanoseconds, was observed at higher excitation density. To explain the dependence of this rate on the excitation density, and its correlation with the charge-trapping rate in the bulk MAPbBr₃, we propose a plausible mechanism in which trap filling associated with surface-trapped holes induces band bending within the space charge region. This band bending modulates carrier dynamics near the surface, thereby leading to efficient charge extraction from the bulk. To validate the mechanism, the carrier dynamics was numerically simulated using a diffusion model that includes the effect of the localized electric field. Our findings provide significantly deeper insight into the carrier dynamics within heterostructured perovskites with nanoscale heterogeneities, and a robust route for manipulating the photogenerated charges in various types of perovskite devices.

Absorption enhancement in methylammonium lead iodide perovskite solar cells with embedded arrays of dielectric particles

In the field of hybrid organic-inorganic perovskite based photovoltaics, there is a growing interest in the exploration of novel and smarter ways to improve the cells light harvesting efficiency at targeted wavelength ranges within the minimum volume possible, as well as in the development of colored and/or semitransparent devices that could pave the way both to their architectonic integration and to their use in the flowering field of tandem solar cells. The work herein presented targets these different goals by means of the theoretical optimization of the optical design of standard opaque and semitransparent perovskite solar cells. In order to do so, we focus on the effect of harmless, compatible and commercially available dielectric inclusions within the absorbing material, methylammonium lead iodide (MAPI). Following a gradual and systematic process of analysis, we are capable of identifying the appearance of collective and hybrid (both localized and extended) photonic resonances which allow to significantly improve light harvesting and thus the overall efficiency of the standard device by above 10% with respect to the reference value while keeping the semiconductor film thickness to a minimum. We believe our results will be particularly relevant in the promising field of perovskite solar cell based tandem photovoltaic devices, which has posed new challenges to the solar energy community in order to maximize the performance of semitransparent cells, but also for applications focusing on architectonic integration.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

Diffusion Enhancement in Highly Excited MAPbI3 Perovskite Layers with Additives

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH₃NH₃PbI₃ (MAPbI₃) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement-carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties

Two-dimensional (2D) hybrid perovskites are stoichiometric compounds consisting of alternating inorganic metal-halide sheets and organoammonium cationic layers. This materials class is widely tailorable in composition, structure, and dimensionality and is providing an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In this Perspective, we describe semiconducting 2D perovskites containing lead and tin halide inorganic frameworks. In these 2D perovskites, charges are typically confined to the inorganic framework because of strong quantum and dielectric confinement effects, and exciton binding energies are many times greater than kT at room temperature. We describe the role of the heavy atoms in the inorganic framework; the geometry and chemistry of organic cations; and the "softness" of the organic-inorganic lattice on the electronic structure and dynamics of electrons, excitons, and phonons that govern the physical properties of these materials.

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices.

Multiple-Trapping Model for the Charge Recombination Dynamics in Mesoporous-Structured Perovskite Solar Cells

The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO₂ -dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices.

Influence of the Grain Size on the Properties of CH3NH3PbI3 Thin Films

Hybrid perovskites have already shown a huge success as an absorber in solar cells, resulting in the skyrocketing rise in the power conversion efficiency to more than η = 22%. Recently, it has been established that the crystal quality is one of the most important parameters to obtain devices with high efficiencies. However, the influence of the crystal quality on the material properties is not fully understood. Here, the influence of the morphology on electronic properties of CH₃NH₃PbI₃ thin films is investigated. Postannealing was used to vary the average grain size continuously from ≈150 to ≈1000 nm. Secondary grain growth is thermally activated with an activation energy of E_a = 0.16 eV. The increase in the grain size leads to an enhancement of the photoluminescence, indicating an improvement in the material quality. According to surface photovoltage measurements, the charge-carrier transport length exhibits a linear increase with increasing grain size. The charge-carrier diffusion length is limited by grain boundaries. Moreover, an improved morphology leads to a drastic increase in power conversion efficiency of the devices.

Phonon Speed, Not Scattering, Differentiates Thermal Transport in Lead Halide Perovskites

Thermal management plays a critical role in the design of solid state materials for energy conversion. Lead halide perovskites have emerged as promising candidates for photovoltaic, thermoelectric, and optoelectronic applications, but their thermal properties are still poorly understood. Here, we report on the thermal conductivity, elastic modulus, and sound speed of a series of lead halide perovskites MAPbX₃ (X = Cl, Br, I), CsPbBr₃, and FAPbBr₃ (MA = methylammonium, FA = formamidinium). Using frequency domain thermoreflectance, we find that the room temperature thermal conductivities of single crystal lead halide perovskites range from 0.34 to 0.73 W/m·K and scale with sound speed. These results indicate that regardless of composition, thermal transport arises from acoustic phonons having similar mean free path distributions. A modified Callaway model with Born von Karmen-based acoustic phonon dispersion predicts that at least ∼70% of thermal conductivity results from phonons having mean free paths shorter than 100 nm, regardless of whether resonant scattering is invoked. Hence, nanostructures or crystal grains with dimensions smaller than 100 nm will appreciably reduce thermal transport. These results are important design considerations to optimize future lead halide perovskite-based photovoltaic, optoelectronic, and thermoelectric devices.

Symmetrization of the Crystal Lattice of MAPbI3 Boosts the Performance and Stability of Metal-Perovskite Photodiodes

Semiconducting lead triiodide perovskites (APbI₃ ) have shown remarkable performance in applications including photovoltaics and electroluminescence. Despite many theoretical possibilities for A⁺ in APbI₃ , the current experimental knowledge is largely limited to two of these materials: methylammonium (MA⁺ ) and formamidinium (FA⁺ ) lead triiodides, neither of which adopts the ideal, cubic perovskite structure at room temperature. Here, a volume-based criterion is proposed for cubic APbI₃ to be stable, and two perovskite materials MA_1-x EA_x PbI₃ (MEPI, EA⁺ = ethylammonium) and MA_1-y DMA_y PbI₃ (MDPI, DMA⁺ = dimethylammonium) are introduced. Powder and single-crystal X-ray diffraction (XRD) results reveal that MEPI and MDPI are solid solutions possessing the cubic perovskite structure, and the EA⁺ and DMA⁺ cations play similar roles in the symmetrization of the crystal lattice of MAPbI₃ . Single crystals of MEPI and MDPI are grown and made into plates of a range of thicknesses, and then into metal-perovskite photodiodes. These devices exhibit tripled diffusion lengths and about tenfold enhancement in stability against moisture, both relative to the current benchmark MAPbI₃ . In this study, the systematic approach to materials design and device fabrication greatly expands the candidate pool of perovskite semiconductors, and paves the way for high-performance, single-crystal perovskite devices including solar cells and light emitters.

300% Enhancement of Carrier Mobility in Uniaxial-Oriented Perovskite Films Formed by Topotactic-Oriented Attachment

Organic-inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large-grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single-crystal counterparts. Here, a facile topotactic-oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial-crystallographic texture, micrometer-grain morphology, high crystallinity, low trap density (≈4 × 10¹⁴ cm^-3 ), and unprecedented 9 GHz charge-carrier mobility (71 cm² V^-1 s^-1 ), is demonstrated. TOA-perovskite-based n-i-p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse-scan efficiency (19.7%). The TOA process is also applicable for growing other state-of-the-art perovskite alloys, including triple-cation and mixed-halide perovskites.

Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

We use scanning photocurrent microscopy and time-resolved microwave conductivity to measure the diffusion of holes and electrons in a series of lead bromide perovskite single crystals, APbBr₃, with A = methylammonium (MA), formamidinium (FA), and Cs. We find that the diffusion length of holes (L_D^h+ ∼ 10-50 μm) is on average an order of magnitude longer than that of electrons (L_D^e- ∼ 1-5 μm), regardless of the A-type cation or applied bias. Furthermore, we observe a weak dependence of L_D across the A-cation series MA > FA > Cs. When considering the role of the halide, we find that the diffusion of holes in MAPbBr₃ is comparable to that in MAPbI₃, but the electron diffusion length is up to five times shorter. This study shows that the disparity between hole and electron diffusion is a ubiquitous feature of lead halide perovskites. As with organic photovoltaics, this imbalance will likely become an important consideration in the optimization of lead halide perovskite solar cells.

Photocurrent Spectroscopy of Perovskite Layers and Solar Cells: A Sensitive Probe of Material Degradation

Optical absorptance spectroscopy of polycrystalline CH₃NH₃PbI₃ films usually indicates the presence of a PbI₂ phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI₂ phase restricts charge-carrier transport, suggesting that PbI₂ encapsulates CH₃NH₃PbI₃ grains. We also find that PbI₂ injects holes into the CH₃NH₃PbI₃ grains, increasing the apparent photosensitivity of PbI₂. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.

Substrate-dependent electronic structure and film formation of MAPbI3 perovskites

We present investigations on the interface formation between the hybrid perovskite MAPbI₃ and various substrates, covering a wide range of work functions. The perovskite films are incrementally evaporated in situ while the electronic structure is evaluated using photoelectron spectroscopy. Our results show that there is an induction period in the growth of the perovskite during which volatile compounds are formed, catalyzed by the substrate. The duration of the induction period depends strongly on the nature of the substrate material, and it can take up to 20–30 nm of formal precursor deposition before the surface is passivated and the perovskite film starts forming. The stoichiometry of the 2–3 nm thin passivation layer deviates from the expected perovskite stoichiometry, being rich in decomposition products of the organic cation. During the regular growth of the perovskite, our measurements show a deviation from the commonly assumed flat band condition, i.e., dipole formation and band bending dominate the interface. Overall, the nature of the substrate not only changes the energetic alignment of the perovskite, it can introduce gap states and influence the film formation and morphology. The possible impact on device performance is discussed.

Carrier Decay Properties of Mixed Cation Formamidinium-Methylammonium Lead Iodide Perovskite [HC(NH2)2]1-x[CH3NH3]xPbI3 Nanorods

Organic-inorganic lead iodide perovskites are efficient materials for photovoltaics and light-emitting diodes. We report carrier decay dynamics of nanorods of mixed cation formamidinium and methylammonium lead iodide perovskites [HC(NH₂)₂]_1-x[CH₃NH₃]_xPbI₃ (FA_1-xMA_xPbI₃) synthesized through a simple solution method. The structure and FA/MA composition ratio of the single-crystal FA_1-xMA_xPbI₃ nanorods are fully characterized, which shows that the mixed cation FA_1-xMA_xPbI₃ nanorods are stabilized in the perovskite structure. The photoluminescence (PL) emission from FA_1-xMA_xPbI₃ nanorods continuously shifts from 821 to 782 nm as the MA ratio (x) increases from 0 to 1 and is shown to be inhomogeneously broadened. Time-resolved PL from individual FA_1-xMA_xPbI₃ nanorods demonstrates that lifetimes of mixed cation FA_1-xMA_xPbI₃ nanorods are longer than those of the pure FAPbI₃ or MAPbI₃ nanorods, and the FA_0.4MA_0.6PbI₃ displays the longest average PL lifetime of about 2 μs. These results suggest that mixed cation FA_1-xMA_xPbI₃ perovskites are promising for high-efficiency photovoltaics and other optoelectronic applications.