Suppressing generation of iodine impurity via an amidine additive in perovskite solar cells

An amidine additive (DBU) was introduced into the precursor to suppress the formation of iodine impurity for high-performance perovskite solar cells.

Synergistic enhancement of Cs and Br doping in formamidinium lead halide perovskites for high performance optoelectronics

Cs and Br doped FAPbI₃ shows better phase stability as well as optoelectronic properties, furnishing it with good optoelectronic performance.

Improved Efficiency and Stability of Perovskite Solar Cells Induced by CO Functionalized Hydrophobic Ammonium-Based Additives

Because of the rapid rise of the efficiency, perovskite solar cells are currently considered as the most promising next-generation photovoltaic technology. Much effort has been made to improve the efficiency and stability of perovskite solar cells. Here, it is demonstrated that the addition of a novel organic cation of 2-(6-bromo-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)ethan-1-ammonium iodide (2-NAM), which has strong Lewis acid and base interaction (between CO and Pb) with perovskite, can effectively increase crystalline grain size and reduce charge carrier recombination of the double cation FA_0.83 MA_0.17 PbI_2.51 Br_0.49 perovskite film, thus boosting the efficiency from 17.1 ± 0.8% to 18.6 ± 0.9% for the 0.1 cm² cell and from 15.5 ± 0.5% to 16.5 ± 0.6% for the 1.0 cm² cell. The champion cell shows efficiencies of 20.0% and 17.6% with active areas of 0.1 and 1.0 cm² , respectively. Moreover, the hysteresis behavior is suppressed and the stability is improved. The result provides a promising route to further elevate efficiency and stability of perovskite solar cells by the fine tuning of triple organic cations.

Solution grown double heterostructure on a large hybrid halide perovskite crystal

Growing modulation-doped layers on hybrid perovskite crystals is achieved using a solvothermal process by limiting the inherent halide ion diffusion.

CH3NH3Pb1−xEuxI3 mixed halide perovskite for hybrid solar cells: the impact of divalent europium doping on efficiency and stability

The crucial role of the impact of divalent europium doping in perovskite solar cells is investigated in this work.

Acetone vapour-assisted growth of 2D single-crystalline organic lead halide perovskite microplates and their temperature-enhanced photoluminescence

We adopt an acetone vapour-assisted method to grow high quality single-crystalline microplates of two-dimensional (2D) perovskite, 2-phenylethylammonium lead bromide [(C₆H₅C₂H₄NH₃)₂PbBr₄]. The microplates, converted from the spin-coated films, are well-defined rectangles. Temperature dependent photoluminescence (PL) spectroscopy shows that the band gap PL is enhanced markedly with increasing temperature up to 218 K, accompanied by the quenching of the PL related to the trap states, which perhaps results from the exciton–phonon couplings. The optical phonon energy around 50 meV and the exciton binding energy around 120 meV are derived by fitting the band gap PL linewidths and intensities at different temperatures, respectively.

We report an acetone vapour-assisted method to grow single-crystalline 2D perovskite microplates and find their temperature-enhanced photoluminescence.

Synthesis and optical properties of ordered-vacancy perovskite cesium bismuth halide nanocrystals

Vacancy-ordered cesium bismuth halide nanocrystals were synthesized using hot-injection methods.

Extremely Low Operating Current Resistive Memory Based on Exfoliated 2D Perovskite Single Crystals for Neuromorphic Computing

Extremely low energy consumption neuromorphic computing is required to achieve massively parallel information processing on par with the human brain. To achieve this goal, resistive memories based on materials with ionic transport and extremely low operating current are required. Extremely low operating current allows for low power operation by minimizing the program, erase, and read currents. However, materials currently used in resistive memories, such as defective HfO_x, AlO_x, TaO_x, etc., cannot suppress electronic transport (i.e., leakage current) while allowing good ionic transport. Here, we show that 2D Ruddlesden-Popper phase hybrid lead bromide perovskite single crystals are promising materials for low operating current nanodevice applications because of their mixed electronic and ionic transport and ease of fabrication. Ionic transport in the exfoliated 2D perovskite layer is evident via the migration of bromide ions. Filaments with a diameter of approximately 20 nm are visualized, and resistive memories with extremely low program current down to 10 pA are achieved, a value at least 1 order of magnitude lower than conventional materials. The ionic migration and diffusion as an artificial synapse is realized in the 2D layered perovskites at the pA level, which can enable extremely low energy neuromorphic computing.

Controllable Synthesis of Two-Dimensional Ruddlesden-Popper-Type Perovskite Heterostructures

Two-dimensional Ruddlesden-Popper type perovskites (2D perovskites) have recently attracted increasing attention. It is expected that 2D perovskite-based heterostructures can significantly improve the efficiency of the optoelectronic devices and extend the material functionalities; however, rational synthesis of such heterostructures has not been realized to date. We report on a general low-temperature synthetic strategy for the synthesis of 2D perovskite-based lateral and vertical (n-CH₃(CH₂)₃NH₃)₂PbI₄/(n-CH₃(CH₂)₃NH₃)₂(CH₃NH₃)Pb₂I₇ heterostructures for the first time. A combination of solution synthesis and gas-solid phase intercalation approach allows us to efficiently synthesize both lateral and vertical heterostructures with great flexibility. X-ray diffraction, photoluminescence, and photoluminescence excitation mapping and electrical transport measurement studies reveal the successful synthesis of lateral and vertical heterostructures with precisely spatial-modulation control and distinguishable interfaces. Our studies not only provide an efficient synthetic strategy with great flexibility, enabling us to create 2D perovskite-based heterostructures, but also offer a platform to investigate the physical processes in those heterostructures.

Impact of grain boundaries on efficiency and stability of organic-inorganic trihalide perovskites

Organic–inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies. To further improve device performance, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide thin films with different efficiencies by light-stimulated microwave impedance microscopy. The microwave signals are largely uniform across grains and grain boundaries, suggesting that microstructures do not lead to strong spatial variations of the intrinsic photo-response. In contrast, the measured photoconductivity and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation process begins with the disintegration of grains rather than nucleation and propagation from visible boundaries between grains. Our findings provide insights to improve the electro-optical properties of perovskite thin films towards large-scale commercialization.

Probing the nanoscale photoconductivity of methylammonium lead triiodide is important for understanding the microstructures of the solar cell devices, but scanning probe methods suffer from sample degradation. Here Chu et al. solve the problem with noncontact microwave impedance microscopy.

High-Performance Green Light-Emitting Diodes Based on MAPbBr3-Polymer Composite Films Prepared by Gas-Assisted Crystallization

The morphology of perovskite films has a significant impact on luminous characteristics of perovskite light-emitting diodes (PeLEDs). To obtain a highly uniform methylammonium lead tribromide (MAPbBr₃) film, a gas-assisted crystallization method is introduced with a mixed solution of MAPbBr₃ precursor and polymer matrix. The ultrafast evaporation of the solvent causes a high degree of supersaturation which expedites the generation of a large number of nuclei to form a MAPbBr₃-polymer composite film with full surface coverage and nano-sized grains. The addition of the polymer matrix significantly affects the optical properties and morphology of MAPbBr₃ films. The PeLED made of the MAPbBr₃-polymer composite film exhibits an outstanding device performance of a maximum luminance of 6800 cd·m^-2 and a maximum current efficiency of 1.12 cd·A^-1. Furthermore, 1 cm² area pixel of PeLED displays full coverage of a strong green electroluminescence, implying that the high-quality perovskite film can be useful for large-area applications in perovskite-based optoelectronic devices.

Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites

Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor-liquid-solid (VLS) growth of lead halide (PbX₂; X = Cl, Br, or I) NWs and conversion to LHP. We present a kinetic model of the PbX₂ NW growth process in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI₂, we show that the NWs are single-crystalline, oriented in the ⟨1̅21̅0⟩ direction, and composed of a stoichiometric PbI₂ shaft with a spherical Pb tip. Low-temperature vapor-phase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI₃) perovskite while maintaining the NW morphology. Single-NW experiments comparing measured extinction spectra with optical simulations show that the NWs exhibit a strong optical antenna effect, leading to substantially enhanced scattering efficiencies and to absorption efficiencies that can be more than twice that of thin films of the same thickness. Further development of the self-catalyzed VLS mechanism for lead halide and perovskite NWs should enable the rational design of nanostructures for various optoelectronic technologies, including potentially unique applications such as hot-carrier solar cells.

Impact of Chemical Doping on Optical Responses in Bismuth-Doped CH3NH3PbBr3 Single Crystals: Carrier Lifetime and Photon Recycling

We studied the optical responses of organic-inorganic halide perovskite CH₃NH₃PbBr₃ single crystals doped with heterovalent Bi³⁺ ions (electron densities up to 2.3 × 10¹² cm^-3). The Bi doping causes no significant changes in the band gap energy but leads to an enhanced Urbach tail and photoluminescence blue shift. On the basis of the time-resolved photoluminescence measurements, we attribute the PL response to a shorter carrier lifetime induced by Bi doping, which results in a reduced photon recycling effect (i.e., the repeated emission and reabsorption of photons inside the crystal). We discuss the physical relation between Bi concentration and the optical and electric properties of Bi-doped CH₃NH₃PbBr₃ and reveal the unique nature of the Bi³⁺ ion as a dopant in halide perovskites.

Metal-Halide Perovskite Transistors for Printed Electronics: Challenges and Opportunities

Following the unprecedented rise in photovoltaic power conversion efficiencies during the past five years, metal-halide perovskites (MHPs) have emerged as a new and highly promising class of solar-energy materials. Their extraordinary electrical and optical properties combined with the abundance of the raw materials, the simplicity of synthetic routes, and processing versatility make MHPs ideal for cost-efficient, large-volume manufacturing of a plethora of optoelectronic devices that span far beyond photovoltaics. Herein looks beyond current applications in the field of energy, to the area of large-area electronics using MHPs as the semiconductor material. A comprehensive overview of the relevant fundamental material properties of MHPs, including crystal structure, electronic states, and charge transport, is provided first. Thereafter, recent demonstrations of MHP-based thin-film transistors and their application in logic circuits, as well as bi-functional devices such as light-sensing and light-emitting transistors, are discussed. Finally, the challenges and opportunities in the area of MHPs-based electronics, with particular emphasis on manufacturing, stability, and health and environmental concerns, are highlighted.

Thin single crystal perovskite solar cells to harvest below-bandgap light absorption

The efficiency of perovskite solar cells has surged in the past few years, while the bandgaps of current perovskite materials for record efficiencies are much larger than the optimal value, which makes the efficiency far lower than the Shockley–Queisser efficiency limit. Here we show that utilizing the below-bandgap absorption of perovskite single crystals can narrow down their effective optical bandgap without changing the composition. Thin methylammonium lead triiodide single crystals with tuned thickness of tens of micrometers are directly grown on hole-transport-layer covered substrates by a hydrophobic interface confined lateral crystal growth method. The spectral response of the methylammonium lead triiodide single crystal solar cells is extended to 820 nm, 20 nm broader than the corresponding polycrystalline thin-film solar cells. The open-circuit voltage and fill factor are not sacrificed, resulting in an efficiency of 17.8% for single crystal perovskite solar cells.

Thin films of halide perovskites are promising for solar cell technology but they do not perform well at the band edge due to the low optical absorption. Herein, Chen et al. fabricate a high efficiency single crystal perovskite solar cell with thicker single crystals to harvest the below-bandgap photons.

Tailoring the Mesoscopic TiO2 Layer: Concomitant Parameters for Enabling High-Performance Perovskite Solar Cells

Architectural control over the mesoporous TiO₂ film, a common electron-transport layer for organic-inorganic hybrid perovskite solar cells, is conducted by employing sub-micron sized polystyrene beads as sacrificial template. Such tailored TiO₂ layer is shown to induce asymmetric enhancement of light absorption notably in the long-wavelength region with red-shifted absorption onset of perovskite, leading to ~20% increase of photocurrent and ~10% increase of power conversion efficiency. This enhancement is likely to be originated from the enlarged CH₃NH₃PbI₃(Cl) grains residing in the sub-micron pores rather than from the effect of reduced perovskite-TiO₂ interfacial area, which is supported from optical bandgap change, haze transmission of incident light, and one-diode model parameters correlated with the internal surface area of microporous TiO₂ layers. With the templating strategy suggested, the necessity of proper hole-blocking method is discussed to prevent any direct contact of the large perovskite grains infiltrated into the intended pores of TiO₂ scaffold, further mitigating the interfacial recombination and leading to ~20% improvement in power conversion efficiency compared with the control device using conventional solution-processed hole blocking TiO₂. Thereby, the imperatives that originate from the structural engineering of the electron-transport layer are discussed to understand the governing elements for the improved device performance.

Fabrication-Method-Dependent Excited State Dynamics in CH3NH3PbI3 Perovskite Films

Understanding the excited-state dynamics in perovskite photovoltaics is necessary for progress in these materials, but changes in dynamics depending on the fabrication processes used for perovskite photoactive layers remain poorly characterised. Here we report a comparative study on femtosecond transient absorption (TA) in CH₃NH₃PbI₃ perovskite films fabricated by various solution-processing methods. The grain sizes and the number of voids between grains on each film varied according to the film synthesis method. At the low excitation fluence of 0.37 μJ cm⁻², fast signal drops in TA dyanmics within 1.5 ps were observed in all perovskite films, but the signal drop magnitudes differed becuase of the variations in charge migration to trap states and band gap renormalisation. For high excitation fluences, the buil-up time of the TA signal was increased by the activated hot-phonon bottleneck, while the signal decay rate was accelerated by fluence-dependent high-order charge recombination. These fluence-dependent dynamics changed for different perovskite fabrication methords, indicating that the dynamics were affected by morphological features such as grain sizes and defects.

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research "Molecular Ultrafast Science and Technology," a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here.

Hybrid Organic-Inorganic Perovskite Photodetectors

Hybrid organic-inorganic perovskite materials garner enormous attention for a wide range of optoelectronic devices. Due to their attractive optical and electrical properties including high optical absorption coefficient, high carrier mobility, and long carrier diffusion length, perovskites have opened up a great opportunity for high performance photodetectors. This review aims to give a comprehensive summary of the significant results on perovskite-based photodetectors, focusing on the relationship among the perovskite structures, device configurations, and photodetecting performances. An introduction of recent progress in various perovskite structure-based photodetectors is provided. The emphasis is placed on the correlation between the perovskite structure and the device performance. Next, recent developments of bandgap-tunable perovskite and hybrid photodetectors built from perovskite heterostructures are highlighted. Then, effective approaches to enhance the stability of perovskite photodetector are presented, followed by the introduction of flexible and self-powered perovskite photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. A comprehensive summary of the research status on perovskite photodetectors is hoped to push forward the development of this field.

A Review on Organic-Inorganic Halide Perovskite Photodetectors: Device Engineering and Fundamental Physics

The last eight years (2009-2017) have seen an explosive growth of interest in organic-inorganic halide perovskites in the research communities of photovoltaics and light-emitting diodes. In addition, recent advancements have demonstrated that this type of perovskite has a great potential in the technology of light-signal detection with a comparable performance to commercially available crystalline Si and III-V photodetectors. The contemporary growth of state-of-the-art multifunctional perovskites in the field of light-signal detection has benefited from its outstanding intrinsic optoelectronic properties, including photoinduced polarization, high drift mobilities, and effective charge collection, which are excellent for this application. Photoactive perovskite semiconductors combine effective light absorption, allowing detection of a wide range of electromagnetic waves from ultraviolet and visible, to the near-infrared region, with low-cost solution processability and good photon yield. This class of semiconductor might empower breakthrough photodetector technology in the field of imaging, optical communications, and biomedical sensing. Therefore, here, the focus is specifically on the critical understanding of materials synthesis, design, and engineering for the next-stage development of perovskite photodetectors and highlighting the current challenges in the field, which need to be further studied in the future.

High-Performance Flexible Photodetectors based on High-Quality Perovskite Thin Films by a Vapor-Solution Method

Organometal halide perovskites are new light-harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low-temperature process capability. However, the preparation of high-quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor-solution method is developed to achieve uniform and pinhole-free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as-prepared high-quality perovskite thin films, high-performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W^-1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin-film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high-quality perovskite thin films opens up a new avenue to develop high-performance flexible optoelectronic devices.

Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells

Organic-inorganic hybrid perovskite (OIHP) solar cells have achieved comparable efficiencies to those of commercial solar cells, although their instability hinders their commercialization. Although encapsulation techniques have been developed to protect OIHP solar cells from external stimuli such as moisture, oxygen, and ultraviolet light, understanding of the origin of the intrinsic instability of perovskite films is needed to improve their stability. We show that the OIHP films fabricated by existing methods are strained and that strain is caused by mismatched thermal expansion of perovskite films and substrates during the thermal annealing process. The polycrystalline films have compressive strain in the out-of-plane direction and in-plane tensile strain. The strain accelerates degradation of perovskite films under illumination, which can be explained by increased ion migration in strained OIHP films. This study points out an avenue to enhance the intrinsic stability of perovskite films and solar cells by reducing residual strain in perovskite films.

Small molecule-driven directional movement enabling pin-hole free perovskite film via fast solution engineering

Organolead trihalide perovskite materials have been widely used as light absorbers in efficient photovoltaic cells. Solution engineering is a fast and effective method to fabricate perovskite films. Here, we report a fast precipitation of a pin-hole free perovskite film by small molecule-driven directed diffusion engineering. Solvent molecules diffuse easily and quickly by colliding with small molecules, e.g. helium. Fully compact perovskite films and highly efficient perovskite solar cells are achieved, and the devices show remarkable stability of ca. 90% original efficiency after more than 1000 hours of testing. The small molecule driving directed diffusion offers a promising fast precipitation of a perovskite film and highly efficient, stable perovskite solar cells.

Structural and Photophysical Properties of Methylammonium Lead Tribromide (MAPbBr3) Single Crystals

The structural and photophysical characteristics of MAPbBr₃ single crystals prepared using the inverse temperature crystallization method are evaluated using temperature-dependent X-ray diffraction (XRD) and optical spectroscopy. Contrary to previous research reports on perovskite materials, we study phase transitions in crystal lattice structures accompanied with changes in optical properties expand throughout a wide temperature range of 300–1.5 K. The XRD studies reveal several phase transitions occurred at ~210 K, ~145 K, and ~80 K, respectively. The coexistence of two different crystallographic phases was observed at a temperature below 145 K. The emission peaks in the PL spectra are all asymmetric in line shape with weak and broad shoulders near the absorption edges, which are attributed to the Br atom vacancy on the surface of the crystals. The time-resolved PL measurements reveal the effect of the desorption/adsorption of gas molecules on the crystal surface on the PL lifetimes. Raman spectroscopy results indicate the strong interplays between cations and different halide atoms. Lastly, no diamagnetic shift or split in emission peaks can be observed in the magneto-PL spectra even at an applied magnetic field up to 5 T and at a temperature as low as 1.5 K.

Wafer-scale reliable switching memory based on 2-dimensional layered organic-inorganic halide perovskite

Recently, organic-inorganic halide perovskite (OHP) has been suggested as an alternative to oxides or chalcogenides in resistive switching memory devices due to low operating voltage, high ON/OFF ratio, and flexibility. The most studied OHP is 3-dimensional (3D) MAPbI₃. However, MAPbI₃ often exhibits less reliable switching behavior probably due to the uncontrollable random formation of conducting filaments. Here, we report the resistive switching property of 2-dimensional (2D) OHP and compare switching characteristics depending on structural dimensionality. The dimensionality is controlled by changing the composition of BA₂MA_n-1Pb_nI_3n+1 (BA = butylammonium, MA = methylammonium), where 2D is formed from n = 1, and 3D is formed from n = ∞. Quasi 2D compositions with n = 2 and 3 are also compared. Transition from a high resistance state (HRS) to a low resistance state (LRS) occurs at 0.25 × 10⁶ V m^-1 for 2D BA₂PbI₄ film, which is lower than those for quasi 2D and 3D. Upon reducing the dimensionality from 3D to 2D, the ON/OFF ratio significantly increases from 10² to 10⁷, which is mainly due to the decreased HRS current. A higher Schottky barrier and thermal activation energy are responsible for the low HRS current. We demonstrate for the first time reliable resistive switching from 4 inch wafer-scale BA₂PbI₄ thin film working at both room temperature and a high temperature of 87 °C, which strongly suggests that 2D OHP is a promising candidate for resistive switching memory.

Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling

Photon recycling is a fundamental physical process that becomes especially important for photovoltaic devices that operate close to the radiative limit. This implies that the externally measured radiative decay rate deviates from the internal radiative recombination rate of the material. In the present Letter, the probability of photon recycling in organic lead halide perovskite films is manipulated by modifying the underlying layer stacks. We observe recombination kinetics by time-resolved photoluminescence that is controlled by the optical design of the chosen layer structure. Quantitative simulations of decay rates and emission spectra show excellent agreement with experimental results if we assume that the internal bimolecular recombination coefficient is ∼66% radiative.

Naphthalene Diimide Based n-Type Conjugated Polymers as Efficient Cathode Interfacial Materials for Polymer and Perovskite Solar Cells

A series of naphthalene diimide (NDI) based n-type conjugated polymers with amino-functionalized side groups and backbones were synthesized and used as cathode interlayers (CILs) in polymer and perovskite solar cells. Because of controllable amine side groups, all the resulting polymers exhibited distinct electronic properties such as oxidation potential of side chains, charge carrier mobilities, self-doping behaviors, and interfacial dipoles. The influences of the chemical variation of amine groups on the cathode interfacial effects were further investigated in both polymer and perovskite solar cells. We found that the decreased electron-donating property and enhanced steric hindrance of amine side groups substantially weaken the capacities of altering the work function of the cathode and trap passivation of the perovskite film, which induced ineffective interfacial modifications and declining device performance. Moreover, with further improvement of the backbone design through the incorporation of a rigid acetylene spacer, the resulting polymers substantially exhibited an enhanced electron-transporting property. Upon use as CILs, high power conversion efficiencies (PCEs) of 10.1% and 15.2% were, respectively, achieved in polymer and perovskite solar cells. Importantly, these newly developed n-type polymers were allowed to be processed over a broad thickness range of CILs in photovoltaic devices, and a prominent PCE of over 8% for polymer solar cells and 13.5% for perovskite solar cells can be achieved with the thick interlayers over 100 nm, which is beneficial for roll-to-roll coating processes. Our findings contribute toward a better understanding of the structure-performance relationship between CIL material design and solar cell performance, and provide important insights and guidelines for the design of high-performance n-type CIL materials for organic and perovskite optoelectronic devices.

Synthetic Manipulation of Hybrid Perovskite Systems in Search of New and Enhanced Functionalities

Over the past few years the organic-inorganic hybrid perovskite systems have emerged as a promising class of materials for photovoltaic and electroluminescent thin-film device applications, in view of their unique set of tunable optoelectronic properties. Importantly, these materials can be easily solution-processed at low temperatures and as such are amenable to facile molecular engineering. Thus, a variety of low-dimensional forms and quantum structures of these materials can be obtained through strategic synthetic manipulations through small molecule incorporation or molecular ion doping. In this Minireview, we specifically focus on these approaches and outline the possibilities of utilizing these for enhanced functionalities and newer application domains.

High Efficiency MAPbI3 Perovskite Solar Cell Using a Pure Thin Film of Polyoxometalate as Scaffold Layer

Here, we successfully used a pure layer of [SiW₁₁ O₃₉ ]^8- polyoxomethalate (POM) structure as a thin-film scaffold layer for CH₃ NH₃ PbI₃ -based perovskite solar cells (PSCs). A smooth nanoporous surface of POM causes outstanding improvement of the photocurrent density, external quantum efficiency (EQE), and overall efficiency of the PSCs compared to mesoporous TiO₂ (mp-TiO₂ ) as scaffold layer. Average power conversion efficiency (PCE) values of 15.5 % with the champion device showing 16.3 % could be achieved by using POM and a sequential deposition method with the perovskite layer. Furthermore, modified and defect-free POM/perovskite interface led to elimination of the anomalous hysteresis in the current-voltage curves. The open-circuit voltage decay study shows promising decrease of the electron recombination in the POM-based PSCs, which is also related to the modification of the POM/ perovskite interface and higher electron transport inside the POM layer.

CsPb2 Br5 Single Crystals: Synthesis and Characterization

CsPb₂ Br₅ is a ternary halogen-plumbate material with close characteristics to the well-reported halide perovskites. Owing to its unconventional two-dimensional structure, CsPb₂ Br₅ is being looked at broadly for potential applications in optoelectronics. CsPb₂ Br₅ investigations are currently limited to nanostructures and powder forms of the material, which present unclear and conflicting optical properties. In this study, we present the synthesis and characterization of CsPb₂ Br₅ bulk single crystals, which enabled us to finally clarify the material's optical features. Our CsPb₂ Br₅ crystal has a two-dimensional structure with Pb₂ Br₅^- layers spaced by Cs⁺ cations, and exhibits approximately 3.1 eV indirect band gap with no emission in the visible spectrum.

Factors Influencing the Mechanical Properties of Formamidinium Lead Halides and Related Hybrid Perovskites

The mechanical properties of formamidinium lead halide perovskites (FAPbX₃ , X=Br or I) grown by inverse-temperature crystallization have been studied by nanoindentation. The measured Young's moduli (9.7-12.3 GPa) and hardnesses (0.36-0.45 GPa) indicate good mechanical flexibility and ductility. The effects of hydrogen bonding were evaluated by performing ab initio molecular dynamics on both formamidinium and methylammonium perovskites and calculating radial distribution functions. The structural and chemical factors influencing these properties are discussed by comparison with corresponding values in the literature for other hybrid perovskites, including double perovskites. Our results reveal that bonding in the inorganic framework and hydrogen bonding play important roles in determining elastic stiffness. The influence of the organic cation becomes more important for structures at the limit of their perovskite stability, indicated by high tolerance factors.

Effect of Formamidinium/Cesium Substitution and PbI2 on the Long-Term Stability of Triple-Cation Perovskites

Altering cation and anion ratios in perovskites has proven an excellent means of tuning the perovskite properties and enhancing the performance. Recently, methylammonium/formamidinium/cesium triple-cation mixed-halide perovskites have demonstrated efficiencies up to 22 %. Similar to the widely explored methylammonium lead halide, excess PbI₂ is added to these perovskite films to enhance their performances. The excess PbI₂ is known to be beneficial for the performance. However, its impact on stability is less well known. Triple-cation perovskites deploy excess PbI₂ up to 8 %. Thus, it is imperative to analyze the role of excess PbI₂ in the degradation kinetics. In this study, the amount of PbI₂ in the triple-cation perovskite films is varied and the degradation kinetics monitored by X-ray diffraction and optical absorption spectroscopy. The inclusion of excess PbI₂ is shown to adversely affect the stability of the material. Faster degradation kinetics are observed for samples with higher PbI₂ contents. However, samples with excess PbI₂ also showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI₂ quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI₂ .

Understanding and Tailoring Grain Growth of Lead-Halide Perovskite for Solar Cell Application

The fundamental mechanism of grain growth evolution in the fabrication process from the precursor phase to the perovskite phase is not fully understood despite its importance in achieving high-quality grains in organic-inorganic hybrid perovskites, which are strongly affected by processing parameters. In this work, we investigate the fundamental conversion mechanism from the precursor phase of perovskite to the complete perovskite phase and how the intermediate phase promotes growth of the perovskite grains during the fabrication process. By monitoring the morphological evolution of the perovskite during the film fabrication process, we observed a clear rod-shaped intermediate phase in the highly crystalline perovskite and investigated the role of the nanorod intermediate phase on the growth of the grains of the perovskite film. Furthermore, on the basis of these findings, we developed a simple and effective method to tailor grain properties including the crystallinity, size, and number of grain boundaries, and then utilized the film with the tailored grains to develop perovskite solar cells.

Lead halide perovskites: Crystal-liquid duality, phonon glass electron crystals, and large polaron formation

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. This "crystal-liquid" duality implies that lead halide perovskites belong to phonon glass electron crystals, a class of materials believed to make the most efficient thermoelectrics. We show that the crystal-liquid duality and the resulting dielectric response are responsible for large polaron formation and screening of charge carriers, leading to defect tolerance, moderate charge carrier mobility, and radiative recombination properties. Large polaron formation, along with the phonon glass character, may also explain the marked reduction in hot carrier cooling rates in these materials.

Low-Noise and Large-Linear-Dynamic-Range Photodetectors Based on Hybrid-Perovskite Thin-Single-Crystals

Organic-inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin-single-crystal (TSC) photodetectors are fabricated with a vertical p-i-n structure. Due to the absence of grain-boundaries, the trap densities of TSCs are 10-100 folds lower than that of polycrystalline thin films. The photodetectors based on CH₃ NH₃ PbBr₃ and CH₃ NH₃ PbI₃ TSCs show low noise of 1-2 fA Hz^-1/2 , yielding a high specific detectivity of 1.5 × 10¹³ cm Hz^1/2 W^-1 . The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH₃ NH₃ PbBr₃ photodetectors show a linear response to green light from 0.35 pW cm^-2 to 2.1 W cm^-2 , corresponding to a linear dynamic range of 256 dB.

High-Performance Simple-Structured Planar Heterojunction Perovskite Solar Cells Achieved by Precursor Optimization

Planar perovskite solar cells (PSCs) with perovskite and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) heterojunction have attracted much interest because they can be fabricated by a low-temperature process and exhibit high power conversion efficiency (PCE). Various compositional engineering and interface engineering approaches have been applied to produce high-performance PSC devices. In this study, we found that high-quality lead halide crystal precursor has a significant effect on the perovskite film quality and it could enhance perovskite film light absorption, reduce crystal defects, and suppress charge recombination, resulting in enhanced device performance (from 8.9 to 15.0% for CH₃NH₃PbI₃ and from 14.2 to 18.0% for MA_0.7FA_0.3PbI_3-x Cl _x ). Moreover, electron and hole interlayer free devices were achieved by using high-quality PbI₂ crystals and the devices exhibited a high PCE of 13.6 and 10.4% for glass and flexible substrates, respectively. This is important for simplifying the perovskite solar cell fabrication process without complex interface engineering involved, especially for printed PSCs.

Full space device optimization for solar cells

Advances in computational materials have paved a way to design efficient solar cells by identifying the optimal properties of the device layers. Conventionally, the device optimization has been governed by single or double descriptors for an individual layer; mostly the absorbing layer. However, the performance of the device depends collectively on all the properties of the material and the geometry of each layer in the cell. To address this issue of multi-property optimization and to avoid the paradigm of reoccurring materials in the solar cell field, a full space material-independent optimization approach is developed and presented in this paper. The method is employed to obtain an optimized material data set for maximum efficiency and for targeted functionality for each layer. To ensure the robustness of the method, two cases are studied; namely perovskite solar cells device optimization and cadmium-free CIGS solar cell. The implementation determines the desirable optoelectronic properties of transport mediums and contacts that can maximize the efficiency for both cases. The resulted data sets of material properties can be matched with those in materials databases or by further microscopic material design. Moreover, the presented multi-property optimization framework can be extended to design any solid-state device.

Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH₃NH₃PbBr₃), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH₃NH₃PbBr₃ perovskite single crystals is 10¹⁵ cm⁻³_, only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.

Metal halide perovskites for optoelectronic devices have been extensively studied in two forms: single-crystals or polycrystalline thin films. Using spectroscopic approaches, Wenger et al. show that polycrystalline thin films possess similar optoelectronic properties to single crystals.

Influence of π-bridge conjugation on the electrochemical properties within hole transporting materials for perovskite solar cells

Hole transporting materials (HTMs) play an important role in most efficient perovskite solar cells (PSCs). In particular, donor-π-bridge-donor type oligomers (D-π-D) have been explored extensively as alternative and economical HTMs. In the present work, a series of triphenylamine-based derivatives as alternatives to the expensive Spiro-OMeTAD were explored by using first-principles calculations combined with the Marcus theory. The electronic structures, optical properties and hole mobilities of all the molecules were investigated to reveal the relationship between their charge-transport properties and the π-bridge conjugation. The HOMO levels decrease with the extension of the π-bridge conjugation length, which may lead to higher open-circuit voltages. Moreover, we employed a quantum mechanical (QM) methodology to estimate the carrier mobility for organic crystals. Specifically, an orientation function μ_Φ (V, λ, r, θ, γ; Φ) is first applied to quantitatively evaluate the overall carrier mobility of HTMs in PSCs. The theoretically calculated results validate that this model predicts the hole mobility of HTMs correctly. More importantly, it is revealed that enhancing the π-bridge conjugation in HTMs can improve the hole mobility, which will definitely improve the performance of PSCs. We hope that our theoretical investigation will offer a reliable calculation method to estimate the charge-transport properties of novel HTMs applied in perovskite solar cells.

Surface/Interface Carrier-Transport Modulation for Constructing Photon-Alternative Ultraviolet Detectors Based on Self-Bending-Assembled ZnO Nanowires

Surface/interface charge-carrier generation, diffusion, and recombination/transport modulation are especially important in the construction of photodetectors with high efficiency in the field of nanoscience. In the paper, a kind of ultraviolet (UV) detector is designed based on ZnO nanostructures considering photon-trapping, surface plasmonic resonance (SPR), piezophototronic effects, interface carrier-trapping/transport control, and collection. Through carefully optimized surface/interface carrier-transport modulation, a designed device with detectivity as high as 1.69 × 10¹⁶/1.71 × 10¹⁶ cm·Hz^1/2/W irradiating with 380 nm photons under ultralow bias of 0.2 V is realized by alternating nanoparticle/nanowire active layers, respectively, and the designed UV photodetectors show fast and slow recovery processes of 0.27 and 4.52 ms, respectively, which well-satisfy practical needs. Further, it is observed that UV photodetection could be performed within an alternative response by varying correlated key parameters, through efficient surface/interface carrier-transport modulation, spectrally resolved photoresponse of the detector revealing controlled detection in the UV region based on the ZnO nanomaterial, photodetection allowed or limited by varying the active layers, irradiation distance from one of the electrodes, standing states, or electric field. The detailed carrier generation, diffusion, and recombination/transport processes are well illustrated to explain charge-carrier dynamics contributing to the photoresponse behavior.

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.