Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells

Perovskite Quantum Heterostructure Constructed by Halide Mixing between a Single CsPbI3 Nanocrystal and an Individual CsPbBr3 Microplate

Ion migration is significantly enhanced in lead-halide perovskites with a soft crystal lattice, which can promote the formation of a heterogeneous interface between two such materials with different halide-anion compositions. Here we have deposited a single CsPbI3 nanocrystal (NC) on top of an individual CsPbBr3 microplate to create a mixed-halide CsPbBrxI3-x (0 < x < 3) NC by means of the anion exchange process. The formation of a CsPbBrxI3-x/CsPbBr3 heterostructure is confirmed by the much-enlarged geometric volume of the CsPbBrxI3-x NC as compared to the original CsPbI3 one, as well as by its capability of receiving photogenerated excitons from the CsPbBr3 microplate with a larger bandgap energy. The quantum nature of this heterostructure is reflected from single-photon emission of the composing CsPbBrxI3-x NC, which can also be bulk-like during phase segregation to demonstrate a red shift in the photoluminescence peak that is opposite to the common trend observed in smaller-sized mixed-halide NCs.

Methanol as an anti-solvent to improve the low open-circuit voltage of CsPbBr3 perovskite solar cells prepared with water

CsPbBr3 has received more and more attention in the field of optoelectronic devices due to its excellent stability. To address the cost and environmental concerns associated with the use of toxic methanol, water has been explored as a substitute solvent for CsBr in the preparation of CsPbBr3 perovskite solar cells (PSCs). In this study, we utilized methanol as an anti-solvent of the CsBr/H2O solution to regulate the detrimental effects of water on the CsPbBr3 film and control the crystallization process. From results of the experiment, it was found that methanol anti-solvent treatment greatly improved the crystallization of the CsPbBr3 film, increased the grain size, and reduced the defect density. After the introduction of methanol anti-solvent treatment, the power conversion efficiency (PCE) increased from 6.09% to 7.91%, while the open-circuit voltage (Voc) increased from 1.18 V to 1.39 V. Furthermore, we incorporated 2-hydroxyethylurea into the CsPbBr3 PSCs to improve the wettability of PbBr2 towards the CsBr/H2O solution and ensure the formation of pure-phase CsPbBr3 films. The introduction of 2-hydroxyethylurea resulted in an additional increase in Voc from 1.19 V to 1.42 V. The PCE further improved from 6.56% to 8.62% after methanol anti-solvent treatment. These results demonstrate that methanol treatment effectively addresses the low Voc issue observed in CsPbBr3 PSCs prepared with water as a solvent. Importantly, this approach significantly reduces the reliance on methanol compared to conventional fabrication methods for CsPbBr3 PSCs. Overall, this work presents a promising pathway for achieving high Voc and efficiency in CsPbBr3 PSCs by utilizing water as a solvent.

Light-enhanced oxygen degradation of MAPbBr3 single crystal

Organometal halide perovskites are promising materials for optoelectronic applications, whose commercial realization depends critically on their stability under multiple environmental factors. In this study, a methylammonium lead bromide (MAPbBr3) single crystal was cleaved and exposed to simultaneous oxygen and light illumination under ultrahigh vacuum (UHV). The exposure process was monitored using X-ray photoelectron spectroscopy (XPS) with precise control of the exposure time and oxygen pressure. It was found that the combination of oxygen and light accelerated the degradation of MAPbBr3, which could not be viewed as a simple addition of that by oxygen-only and light-only exposures. The XPS spectra showed significant loss of carbon, bromine, and nitrogen at an oxygen exposure of 1010 Langmuir with light illumination, approximately 17 times of the additive effects of oxygen-only and light-only exposures. It was also found that the photoluminescence (PL) emission was much weakened by oxygen and light co-exposure, while previous reports had shown that PL was substantially enhanced by oxygen-only exposure. Measurements using a scanning electron microscope (SEM) and focused ion beam (FIB) demonstrated that the crystal surface was much roughened by the co-exposure. Density functional theory (DFT) calculations revealed the formation of superoxide and oxygen induced gap state, suggesting the creation of oxygen radicals by light illumination as a possible microscopic driving force for enhanced degradation.

Doping strategies for inorganic lead-free halide perovskite solar cells: progress and challenges

In recent years, remarkable advancements have been achieved in the field of halide perovskite solar cells (PSCs). However, the commercialization of PSCs has been impeded by challenges such as Pb leakage and the instability of hybrid organic-inorganic perovskites (HOIPs). Hence, the future lies in the development of environmentally friendly inorganic lead-free halide perovskites (LFHPs) based on elements like Sn, Ge, Bi, Sb, and Cu, which show great promise for photovoltaic applications. However, LFHP photovoltaic cells still face challenges such as low efficiency, poor film quality, and stability in comparison to HOIPs. These limitations significantly hinder their further development. To address these issues, element doping strategies, including cationic and anionic doping, as well as the use of additives, are frequently employed. These strategies aim to improve film quality, passivate defects, reduce the band gap, and enhance device performance and stability. In this paper, we aim to provide a comprehensive review of the recent research progress in doping strategies for LFHPs.

Sn-doped ZnO for efficient and stable quantum dot light-emitting diodes via a microchannel synthesis strategy

ZnO nanocrystals (NCs) are widely employed as an electron transport layer (ETL) in quantum-dot light-emitting diodes (QLEDs). However, the excessive electron mobility, abundant surface defects and poor reproducibility of ZnO NC synthesis are currently the primary restrictive factors influencing the development of QLEDs. In this study, we developed Sn(IV)-doped ZnO NCs as the ETL for constructing highly efficient and long lifetime QLEDs. The introduction of Sn can reduce the surface hydroxyl oxygen defects and alter the electron transport properties of NCs, and thus is beneficial for improving the efficiency of hole-electron recombination in the emitting layer. Meanwhile, a microchannel (MC) reactor is utilized to finely control the synthesis of Zn0.96Sn0.04O NCs, enabling us to achieve uniform size distribution and consistent production reproducibility. Using the Sn(IV)-doped ZnO NCs as the ETL has led to a remarkable enhancement of external quantum efficiency (EQE) for the fabricated red QLED, from 9.2% of the ZnO only device to 15.5% of the Zn0.96Sn0.04O device. Furthermore, the T70 (@1000 cd m-2) of the Zn0.96Sn0.04O device reached 78 h, which is 1.77-fold higher than that of the ZnO only device (44 h). The present work provides an alternative ETL for efficient and stable QLEDs.

A comprehensive review of the current progresses and material advances in perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted ample consideration from the photovoltaic community owing to their continually-increasing power conversion efficiency (PCE), viable solution-processed methods, and inexpensive materials ingredients. Over the past few years, the performance of perovskite-based devices has exceeded 25% due to superior perovskite films achieved using low-temperature synthesis procedures along with evolving appropriate interface and electrode-materials. The current review provides comprehensive knowledge to enhance the performance and materials advances for perovskite solar cells. The latest progress in terms of perovskite crystal structure, device construction, fabrication procedures, and challenges are thoroughly discussed. Also discussed are the different layers such as ETLs and buffer-layers employed in perovskite solar-cells, seeing their transmittance, carrier mobility, and band gap potentials in commercialization. Generally, this review delivers a critical assessment of the improvements, prospects, and trials of PSCs.

Compositional gradient engineering and applications in halide perovskites

Organic-inorganic halide perovskites (HPs) have attracted respectable interests as active layers in solar cells, light-emitting diodes, photodetectors, etc. Besides the promising optoelectronic properties and solution-processed preparation, the soft lattice in HPs leads to flexible and versatile compositions and structures, providing an effective platform to regulate the bandgaps and optoelectronic properties. However, conventional solution-processed HPs are homogeneous in composition. Therefore, it often requires the cooperation of multiple devices in order to achieve multi-band detection or emission, which increases the complexity of the detection/emission system. In light of this, the construction of a multi-component compositional gradient in a single active layer has promising prospects. In this review, we summarize the gradient engineering methods for different forms of HPs. The advantages and limitations of these methods are compared. Moreover, the entropy-driven ion diffusion favors compositional homogeneity, thus the stability issue of the gradient is also discussed for long-term applications. Furthermore, applications based on these compositional gradient HPs will also be presented, where the gradient bandgap introduced therein can facilitate carrier extraction, and the multi-components on one device facilitate functional integration. It is expected that this review can provide guidance for the further development of gradient HPs and their applications.

Highly Efficient Red Quantum Dot Light-Emitting Diodes by Balancing Charge Injection and Transport

Quantum dot light-emitting diodes (QLEDs) have promising commercial value and application prospects in the fields of displays and lighting. However, a charge-transfer imbalance always exists in the devices. In this work, the high-efficiency red QLEDs were obtained via employing the mixtures of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) and 4,4'-bis(carbazole-9-yl)-1,1'-biphenyl (CBP) as hole-transport layers (HTLs) by solution processing. The optimized mixing concentration of CBP is 20 wt %. The corresponding red QLED exhibited a maximum luminance of 963 433 cd m^-2, a maximum current efficiency of 38.7 cd A^-1, an external quantum efficiency of 30.0%, a central wavelength of 628 nm with a narrow full width at half-maximum (fwhm) of 24 nm, and a 5-fold T₅₀ lifetime enhancement at an extremely high luminance of 200 000 cd m^-2. The characteristics of carrier-only devices with QD emissive layers (QD EMLs) and impedance characteristics of QLEDs demonstrate that these advances are chiefly ascribed to the more balanced charge transport and efficient hole-electron recombination in EML. We anticipate that our results could offer a low-cost and simple solution-processed method for preparing high-performance QLEDs.

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

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)A_n-1 Pb_n I_3n+1 composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.

Developments on Perovskite Solar Cells (PSCs): A Critical Review

This review provides detailed information on perovskite solar cell device background and monitors stepwise scientific efforts applied to improve device performance with time. The work reviews previous studies and the latest developments in the perovskite crystal structure, electronic structure, device architecture, fabrication methods, and challenges. Advantages, such as easy bandgap tunability, low charge recombination rates, and low fabrication cost, are among the topics discussed. Some of the most important elements highlighted in this review are concerns regarding commercialization and prototyping. Perovskite solar cells are generally still lab-based devices suffering from drawbacks such as device intrinsic and extrinsic instabilities and rising environmental concerns due to the use of the toxic inorganic lead (Pb) element in the perovskite (ABX3) light-active material. Some interesting recommendations and possible future perspectives are well articulated.

High-efficiency planar heterojunction perovskite solar cell produced by using 4-morpholine ethane sulfonic acid sodium salt doped SnO2

Perovskite solar cells (PSCs) have become a promising photovoltaic (PV) technology. Meanwhile, developing an electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of PSCs. Here, a 4-morpholine ethane sulfonic acid sodium salt (MES Na⁺) doped SnO₂ ETL is utilized in planar heterojunction PSCs. The results show that the MES Na⁺ doped ETL can improve the crystallinity, and absorbance of perovskite films, and passivate interface defects between the perovskite film and SnO₂ ETL. The doping effect accounts for the enhancement of conductivity and the decreasing work function of SnO₂. When 10 mg mL^-1 MES Na⁺ was added to the SnO₂ precursor solution, the device showed the best performance Jsc, Voc, and FF of the PSCs values, which were 23.88 mA cm^-2, 1.12 V and 78.69%, respectively, and the PCE was increased from 17.43% to 21.05%. This doping ETL strategy provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.

Determining Out-of-Plane Hole Mobility in CuSCN via the Time-of-Flight Technique To Elucidate Its Function in Perovskite Solar Cells

Copper(I) thiocyanate (CuSCN) is a stable, low-cost, solution-processable p-type inorganic semiconductor used in numerous optoelectronic applications. Here, for the first time, we employ the time-of-flight (ToF) technique to measure the out-of-plane hole mobility of CuSCN films, enabled by the deposition of 4 μm-thick films using aerosol-assisted chemical vapor deposition (AACVD). A hole mobility of ∼10^-3 cm²/V s was measured with a weak electric field dependence of 0.005 cm/V^1/2. Additionally, by measuring several 1.5 μm CuSCN films, we show that the mobility is independent of thickness. To further validate the suitability of our AACVD-prepared 1.5 μm-thick CuSCN film in device applications, we demonstrate its incorporation as a hole transport layer (HTL) in methylammonium lead iodide (MAPbI₃) perovskite solar cells (PSCs). Our AACVD films result in devices with measured power conversion efficiencies of 10.4%, which compares favorably with devices prepared using spin-coated CuSCN HTLs (12.6%), despite the AACVD HTLs being an order of magnitude thicker than their spin-coated analogues. Improved reproducibility and decreased hysteresis were observed, owing to a combination of excellent film quality, high charge-carrier mobility, and favorable interface energetics. In addition to providing a fundamental insight into charge-carrier mobility in CuSCN, our work highlights the AACVD methodology as a scalable, versatile tool suitable for film deposition for use in optoelectronic devices.

Exploring Transport Behavior in Hybrid Perovskites Solar Cells via Machine Learning Analysis of Environmental-Dependent Impedance Spectroscopy

Hybrid organic-inorganic perovskites are one of the promising candidates for the next-generation semiconductors due to their superlative optoelectronic properties. However, one of the limiting factors for potential applications is their chemical and structural instability in different environments. Herein, the stability of (FAPbI3 )0.85 (MAPbBr3 )0.15 perovskite solar cell is explored in different atmospheres using impedance spectroscopy. An equivalent circuit model and distribution of relaxation times (DRTs) are used to effectively analyze impedance spectra. DRT is further analyzed via machine learning workflow based on the non-negative matrix factorization of reconstructed relaxation time spectra. This exploration provides the interplay of charge transport dynamics and recombination processes under environment stimuli and illumination. The results reveal that in the dark, oxygen atmosphere induces an increased hole concentration with less ionic character while ionic motion is dominant under ambient air. Under 1 Sun illumination, the environment-dependent impedance responses show a more striking effect compared with dark conditions. In this case, the increased transport resistance observed under oxygen atmosphere in equivalent circuit analysis arises due to interruption of photogenerated hole carriers. The results not only shed light on elucidating transport mechanisms of perovskite solar cells in different environments but also offer an effective interpretation of impedance responses.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

A Review of Integrated Systems Based on Perovskite Solar Cells and Energy Storage Units: Fundamental, Progresses, Challenges, and Perspectives

With the remarkable progress of photovoltaic technology, next-generation perovskite solar cells (PSCs) have drawn significant attention from both industry and academic community due to sustainable energy production. The single-junction-cell power conversion efficiency (PCE) of PSCs to date has reached up to 25.2%, which is competitive to that of commercial silicon-based solar cells. Currently, solar cells are considered as the individual devices for energy conversion, while a series connection with an energy storage device would largely undermine the energy utilization efficiency and peak power output of the entire system. For substantially addressing such critical issue, advanced technology based on photovoltaic energy conversion-storage integration appears as a promising strategy to achieve the goal. However, there are still great challenges in integrating and engineering between energy harvesting and storage devices. In this review, the state-of-the-art of representative integrated energy conversion-storage systems is initially summarized. The key parameters including configuration design and integration strategies are subsequently analyzed. According to recent progress, the efforts toward addressing the current challenges and critical issues are highlighted, with expectation of achieving practical integrated energy conversion-storage systems in the future.

The curious case of ion migration in solid-state and liquid electrolyte-based perovskite devices: unveiling the role of charge accumulation and extraction at the interfaces

Electrochemical impedance spectroscopy (EIS) has been extensively used for the detailed investigation and understanding of the plethora of physical properties of variegated electrochemical and solid-state systems. Over the past few years, EIS has revealed many significant findings in hybrid halide perovskite (HHP)-based optoelectronic devices too. Photoinduced ion-migration, negative capacitance, anomalous mid-frequency capacitance, hysteresis, and instability to heat, light and moisture in HHP-based devices are among the few issues addressed by the IS technique. However, performing EIS in perovskite devices presents new challenges related to multilayer solid-state device geometry and complicated material properties. The ions in the perovskite behave in a specified manner, which is dictated by the energy-levels of the transport layer. Electronic-ionic coupling is one of the major challenges to understand ion transport kinetics in solid-state devices. In this work, we have performed impedance measurements in both solid-state (S-S) and liquid-electrolyte (L-E) device geometry to unfold the effect of charge transport layers on the ac ionic conductivity in perovskite materials. We have modelled the impedance spectra using the electrical equivalent circuit (EEC) and compared the behaviour of ions in different controlling environments. It was concluded that the AC as well as dc ionic conductivity and the accumulation of ions in the perovskite material are highly influenced by the nature of the interface in different device geometry. Charge accumulation in the S-S device gives rise to large polarisation, thereby negative capacitance or any inductive loop can be observed in the Nyquist plot while in the L-E device the presence of an electric double layer at the perovskite/electrolyte interface reduces the surface polarisation effect. Ionic conductivity is hopping limited in the low field regime and diffusion limited in the high field regime in the S-S device. Moreover, the perovskite/electrolyte based devices are promising candidates for electrolyte gated field-effect transistors, perovskite-based supercapacitors and electrochemical cells for water splitting or CO₂ reduction.

Photoemission Studies on the Environmental Stability of Thermal Evaporated MAPbI3 Thin Films and MAPbBr3 Single Crystals

Hybrid organic inorganic perovskites have been considered as a potential candidate for the next generational solar cell due to their outstanding optoelectronic properties and rapid development in recent years. However, the biggest challenge to prevent them from massive commercial use is their long-term stability. Photoemission spectroscopy has been widely used to investigate properties of the perovskites, which provide critical insights to better understand the degradation mechanisms. In this article, we review mainly our photoemission studies on the degradation processes of perovskite thin films and single crystals with different environmental factors, such as gases, water, and light by monitoring changes of chemical composition and electronic structure. These studies on the effects by different environmental parameters are discussed for the understanding of the stability issues and the possible solutions.

Review of Interface Passivation of Perovskite Layer

Perovskite solar cells (PSCs) are the most promising substitute for silicon-based solar cells. However, their power conversion efficiency and stability must be improved. The recombination probability of the photogenerated carriers at each interface in a PSC is much greater than that of the bulk phase. The interface of a perovskite polycrystalline film is considered to be a defect-rich area, which is the main factor limiting the efficiency of a PSC. This review introduces and summarizes practical interface engineering techniques for improving the efficiency and stability of organic-inorganic lead halide PSCs. First, the effect of defects at the interface of the PSCs, the energy level alignment, and the chemical reactions on the efficiency of a PSC are summarized. Subsequently, the latest developments pertaining to a modification of the perovskite layers with different materials are discussed. Finally, the prospect of achieving an efficient PSC with long-term stability through the use of interface engineering is presented.

A review of experimental and computational attempts to remedy stability issues of perovskite solar cells

Photovoltaic technology using perovskite solar cells has emerged as a potential solution in the photovoltaic makings for cost-effective manufacturing solutions deposition/coating solar cells. The hybrid perovskite-based materials possess a unique blend from low bulk snare concentrations, ambipolar, broad optical absorption properties, extended charge carrier diffusion, and charge transport/collection properties, making them favourable for solar cell applications. However, perovskite solar cells devices suffer from the effects of natural instability, leading to their rapid degradation while bared to water, oxygen, as well as ultraviolet rays, are irradiated and in case of high temperatures. It is essential to shield the perovskite film from damage, extend lifetime, and make it suitable for device fabrications. This paper focuses on various device strategies and computational attempts to address perovskite-based solar cells' environmental stability issues.

CsPbBrI2 perovskites with low energy loss for high-performance indoor and outdoor photovoltaics

Over the years, the efficiency of inorganic perovskite solar cells (PSCs) has increased at an unprecedented pace. However, energy loss in the device has limited a further increase in efficiency and commercialization. In this work, we used (NH₄)₂C₂O₄·H₂O to treat CsPbBrI₂ perovskite film during spin-coating. The CsPbBrI₂ underwent secondary crystallization to form high quality films with micrometer-scale and low trap density. (NH₄)₂C₂O₄·H₂O treatment promoted charge transfer capacity and reduced the ideal factor. It also dropped the energy loss from 0.80 to 0.64 eV. The resulting device delivered a power conversion efficiency (PCE) of 16.55% with an open-circuit voltage (V_oc) of 1.24 V, which are largely improved compared with the reference device which exhibited a PCE of 13.27% and a V_oc of 1.10 V. In addition, the optimized treated device presented a record indoor PCE of 28.48% under a fluorescent lamp of 1000 lux, better than that of the reference device (19.05%).

Tunable transition metal complexes as hole transport materials for stable perovskite solar cells

We developed a high-performance hole transport material based on transition metal complexes for perovskite solar cells, which exhibits excellent photostability.

Effects of Bromine Doping on the Structural Properties and Band Gap of CH3NH3Pb(I1-x Br x )3 Perovskite

An experimental and theoretical study is reported to investigate the influence of bromine doping on CH₃NH₃Pb(I_1-x Br _x )₃ perovskite for Br compositions ranging from x = 0 to x = 0.1, in which the material remains in the tetragonal phase. The experimental band gap is deduced from UV-vis absorption spectroscopy and displays a linear behavior as a function of bromine concentration. Density functional theory calculations are performed for five different series of randomly doped structures in order to simulate the disorder in bromine doping sites. The computations predict a linear variation of the lattice parameters, supercell volume, density, band gap, and formation energy in the considered doping range. The calculated evolution of the band gap as the function of Br doping is in excellent agreement with the experimental data, provided that different Br doping configurations are included in the simulations. The analysis of the structural and electronic properties shows a correlation between the increase of the band gap and the increased distortion of the Pb(I_1-x Br _x )₆ octahedrons. Additionally, the simulations suggest that in CH₃NH₃Pb(I_1-x Br _x )₃ bromine doping is likely to occur at both the equatorial and apical positions of the octahedrons.

Lead-free cesium tin halide nanocrystals for light-emitting diodes and color down conversion

Organometal halide perovskites are attracting a great deal of attention because of their long carrier diffusion lengths, wide wavelength tunability, and narrow-band emission. However, the toxicity of lead has caused considerable environmental and health concerns. In this work, lead-free cesium tin halide nanocrystals are synthesized and investigated. CsSnBr3 and CsSnI3 nanocrystals, 25 and 7 nm in size, are synthesized by a facile hot injection method. Absorption spectroscopy, photoluminescence spectroscopy, and X-ray diffraction were used to understand their structural and optical properties. CsSnBr3 and CsSnI3 nanocrystals show emission peaks at 683 and 938 nm, respectively. These nanocrystals show shelf stability for a few months. Temperature-dependent photoluminescence is utilized to know more about fundamental physical parameters, such as exciton binding energy, charge carrier-phonon interactions and band gap. Light-emitting diodes and color down-conversion films are also demonstrated using these lead free perovskite nanocrystals.

Solution-Phase Halide Exchange and Targeted Annealing Kinetics in Lead Chloride Derived Hybrid Perovskites

Hybrid perovskites are a promising class of materials for a range of optoelectronic applications. Many material properties are dictated by the details of the synthetic process, yet a mechanistic understanding is lacking for the majority of these materials. We have studied the formation of methylammonium lead iodide films derived from a lead chloride precursor to understand both the casting solution chemistry and its influence on the final, largely chlorine-free, film. Using solution-phase extended X-ray absorption spectroscopy, we observe a halide exchange with the primary solution plumbate species identified as PbI_2.5Cl_0.33. The mixed halide plumbate solution species leads to formation of the crystalline intermediate phase of (CH₃NH₃)₂PbI₃Cl. Using in situ synchrotron X-ray diffraction, we show that compositional control of the casting solution can control the annealing kinetics of film formation. Our study demonstrates the importance of solution-phase chemistry and its impact on lead halide perovskite synthesis.

Symmetry Breaking Induced Anisotropic Carrier Transport and Remarkable Thermoelectric Performance in Mixed Halide Perovskites CsPb(I1-xBrx)3

We present a combination of first-principles calculations and the Boltzmann transport theory to understand the carrier transport and thermoelectric performance of mixed halide perovskite alloys CsPb(I_1-xBr_x)₃ with different Br compositions. Our computational results correlate the conduction band splitting in CsPb(I_1-xBr_x)₃ to the significant anisotropy in their carrier transport properties, such as effective masses and deformation potential constants. Such band splitting originates from the symmetry-broken crystal structures of CsPb(I_1-xBr_x)₃ polymorphs: with residue stresses/strains in asymmetric CsPb(I_1-xBr_x)₃, nondegenerate orbitals reconstruct the conduction band and reduce the Pb-halide antibonding character along certain directions. While the Seebeck coefficient (S) and the relaxation time-normalized electrical conductivity (σ/τ) show weak directional anisotropy, the carrier relaxation time (τ) is highly direction-dependent. The reconstruction of the conduction band finally leads to significantly anisotropic and enhanced thermoelectric power factors (PF = S²σ) in CsPb(I_1-xBr_x)₃ compared to those in pure CsPbI₃ and CsPbBr₃, showing anomalous nonlinear alloy behavior. A delicate balance between S²σ and combined measurement of the carrier effective mass and deformation potential constant, m*E_DP, is confirmed. The lattice thermal conductivities of CsPb(I_1-xBr_x)₃ are significantly suppressed compared to those of their pure counterparts due to strong mass disordering and strain fields upon halogen substitution. As a result, symmetry breaking in CsPb(I_1-xBr_x)₃ leads to anisotropy in carrier transport, high PF, and scattered phonon transport (ultralow thermal conductivity), concurrently contributing to their promising thermoelectric figures of merit (ZT) up to 1.7 at room temperature. The principles behind the asymmetry-induced factors would serve as new design concepts to tailor the thermoelectric properties of alloys, mixtures, superlattices, and low-dimensional materials.

Unique Behavior of Halide Double Perovskites with Mixed Halogens

Engineering halide double perovskite (A₂M⁺M³⁺X^VII₆) by mixing elements is a viable way to tune its electronic and optical properties. In spite of many emerging experiments on halide double perovskite alloys, the basic electronic properties of the alloys have not been fully understood. In this work, we chose Cs₂AgBiCl₆ as an example and systematically studied electronic properties of its different site alloys Cs₂Na_xAg_1-xBiCl₆, Cs₂AgSb_xBi_1-xCl₆, and Cs₂AgBi(Br_xCl_1-x)₆ (x = 0.25, 0.5, 0.75) by first-principles calculations. Interestingly, the halogen site alloy shows opposite behavior to M⁺ and M³⁺ cation site alloys; that is, Cs₂AgBi(Br_xCl_1-x)₆ displays virtual crystal behavior without substantial broadening, while Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆ show split-band behaviors with substantial broadening, which indicates that lifetimes of electrons and holes in Cs₂AgBi(Br_xCl_1-x)₆ would be longer than those in Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆. We further found that long lifetimes of electrons and holes are common for mixed halide perovskites. Moreover, the band alignment is provided to determine the band gap change of alloys and to understand the transport of electrons and holes when these pure compounds form heterostructures. Our systematical studies should be helpful for future optoelectronic applications of halide perovskites.

Cesium Acetate-Induced Interfacial Compositional Change and Graded Band Level in MAPbI3 Perovskite Solar Cells

Compositional engineering and interfacial modifications have played pivotal roles in the accomplishment of high-efficiency perovskite solar cells (PSCs). Different interfaces in the PSCs influence the performance remarkably either by altering the crystallization of the active material or shifting the energy levels or improving the electrical contact. This work reports how a thin layer of cesium acetate on the TiO₂ electron transport layer (ETL) induces generation of a PbI₂-rich methylammonium lead iodide (MAPbI₃) composition at the ETL/MAPbI₃ interface, which downshifts the conduction band level of MAPbI₃ to create an energy level gradient favorable for carrier collection, resulting in higher photocurrent, fill factor, and overall power conversion efficiency.

Unveiling the Morphology Effect on the Negative Capacitance and Large Ideality Factor in Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes have almost reached the threshold for potential commercialization within a few years of research. However, there are still some unsolved puzzles such as large ideality factor and the presence of large negative capacitance especially at the low-frequency regime yet to be addressed. Here, we have fabricated a methylammonium lead tri-bromide perovskite n-i-p structure for light-emitting diodes from a smooth and textured emissive layer and demonstrated for the first time that these two factors are strongly dependent on the perovskite film morphology. Bias-dependent capacitance measurement also reveals the transition between negative to positive capacitance in textured films at the low-frequency regime. We have observed an anomalous capacitive behavior at the mid-frequency regime in smooth perovskite films but not in textured films. The relatively large ideality factor and anomalous capacitive behavior observed in perovskite light-emitting diodes are due to the presence of strong coupling between ions and electrons near the electrode interface. Therefore, the ideality factor and anomalous capacitance at the mid-frequency regime can be decreased by minimizing electronic-ionic coupling in textured perovskite films, while light outcoupling can be improved significantly.

Low-Temperature Synthesized Nb-Doped TiO2 Electron Transport Layer Enabling High-Efficiency Perovskite Solar Cells by Band Alignment Tuning

An Nb-doped TiO₂ (Nb-TiO₂) film comprising a double structure stacked with a bottom compact layer and top mesoporous layers was synthesized by treating a Ti precursor-coated substrate using a one-step low-temperature steam-annealing (SA) method. The SA-based Nb-TiO₂ films possess high crystallinity and conductivity, and that allows better control over the conduction band (CB) of TiO₂ for the electron transport layer (ETL) of the perovskite solar cells by the Nb doping level. Optimization of power conversion efficiency (PCE) for the Nb-TiO₂-based ETL was combined with the CB level tuning of the mixed-halide perovskite by changing the Br/I ratio. This band offset management enabled to establish the most suitable energy levels between the ETL and the perovskites. This method was applied to reduce the band gap of perovskites to enhance the photocurrent density while maintaining a high open-circuit voltage. As a result, the optimal combination of 5 mol % Nb-TiO₂ ETL and 10 mol % Br in the mixed-halide perovskite exhibited high photovoltaic performance for low-temperature device fabrication, achieving a high-yield PCE of 21.3%.

Realizing 22.3% EQE and 7-Fold Lifetime Enhancement in QLEDs via Blending Polymer TFB and Cross-Linkable Small Molecules for a Solvent-Resistant Hole Transport Layer

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl)))] (TFB) has been widely used as a hole transport layer (HTL) material in cadmium-based quantum dot light-emitting diodes (QLEDs) because of its high hole mobility. However, as the highest occupied molecular orbital (HOMO) energy level of TFB is -5.4 eV, the hole injection from TFB to the quantum dot (QD) layer is higher than 1.5 eV. Such a high oxidation potential at the QD/HTL interface may seriously degrade the device lifetime. In addition, TFB is not resistant to most solvents, which limits its application in inkjet-printed QLED display. In this study, the blended HTL consisting of TFB and cross-linkable small molecular 4,4'-bis(3-vinyl-9H-carbazol-9-yl)1,1'-biphenyl (CBP-V) was introduced into red QLEDs because of the deep HOMO energy level of CBP-V (-6.2 eV). Compared with the TFB-only devices, the external quantum efficiency (EQE) of devices with the blended HTL improved from 15.9 to 22.3% without the increase of turn-on voltage for spin-coating-fabricated devices. Furthermore, the blended HTL prolonged the T90 and T70 lifetime from 5.4 and 31.1 to 39.4 and 148.9 h, respectively. These enhancements in lifetime are attributed to the low hole-injection barrier at the HTL/QD interface and high thermal stability of the blended HTL after cross-linking. Moreover, the cross-linked blended HTL showed excellent solvent resistance after cross-linking, and the EQE of the inkjet-printed red QLEDs reached 16.9%.

Photoelectrochemical Water Splitting Reaction System Based on Metal-Organic Halide Perovskites

In the development of hydrogen-based technology, a key challenge is the sustainable production of hydrogen in terms of energy consumption and environmental aspects. However, existing methods mainly rely on fossil fuels due to their cost efficiency, and as such, it is difficult to be completely independent of carbon-based technology. Electrochemical hydrogen production is essential, since it has shown the successful generation of hydrogen gas of high purity. Similarly, the photoelectrochemical (PEC) method is also appealing, as this method exhibits highly active and stable water splitting with the help of solar energy. In this article, we review recent developments in PEC water splitting, particularly those using metal-organic halide perovskite materials. We discuss the exceptional optical and electrical characteristics which often dictate PEC performance. We further extend our discussion to the material limit of perovskite under a hydrogen production environment, i.e., that PEC reactions often degrade the contact between the electrode and the electrolyte. Finally, we introduce recent improvements in the stability of a perovskite-based PEC device.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Electronic structure of MAPbI3 and MAPbCl3: importance of band alignment

Since their first appearance, organic-inorganic perovskite absorbers have been capturing the attention of the scientific community. While high efficiency devices highlight the importance of band level alignment, very little is known on the origin of the strong n-doping character observed in the perovskite. Here, by means of a highly accurate photoemission study, we shed light on the energy alignment in perovskite-based devices. Our results suggest that the interaction with the substrate may be the driver for the observed doping in the perovskite samples.

Ultrastable, Deformable, and Stretchable Luminescent Organic-Inorganic Perovskite Nanocrystal-Polymer Composites for 3D Printing and White Light-Emitting Diodes

Organic-inorganic perovskite nanocrystals with excellent optoelectronic properties have been utilized in various applications, despite their stability issues. The perovskite materials are sensitive to environments such as polar solvents, moisture, and heat. Thus, they are not used for extrusion three-dimensional (3D) printing, as it is usually conducted in the ambient environment and requires heating to liquefy the printed materials. In this work, 11 thermoplastic polymers conventionally used for extrusion 3D printing were investigated to test their capability as protective encapsulation materials for perovskite nanocrystals. Three of them exhibited good protective properties, and one (polycaprolactone, PCL) of these three could be blended with perovskite nanocrystals to form perovskite nanocrystal-PCL composites, which were deformable and stretchable once heated. Because of the low melting point of PCL, the perovskite nanocrystals maintained their optical properties after 3D printing, and the printed objects were still having fluorescent behavior. Moreover, fluorescent micrometer-sized fibers based on the perovskite nanocrystal-PCL composites could also be simply prepared using cotton candy makers. Perovskite nanocrystal-PCL composite films with different emission wavelengths were incorporated with blue light-emitting diodes (LEDs) to realize white LEDs with Commission Internationale de l'Éclairage chromaticity coordinates of (0.33, 0.33).

Long-Term Stabilization of Two-Dimensional Perovskites by Encapsulation with Hexagonal Boron Nitride

Metal halide perovskites are known to suffer from rapid degradation, limiting their direct applicability. Here, the degradation of phenethylammonium lead iodide (PEA₂PbI₄) two-dimensional perovskites under ambient conditions was studied using fluorescence, absorbance, and fluorescence lifetime measurements. It was demonstrated that the long-term stability of two-dimensional perovskites could be achieved through the encapsulation with hexagonal boron nitride. While un-encapsulated perovskite flakes degraded within hours, the encapsulated perovskites were stable for at least three months. In addition, encapsulation considerably improved the stability under laser irradiation. The environmental stability, combined with the improved durability under illumination, is a critical ingredient for thorough spectroscopic studies of the intrinsic optoelectronic properties of this material platform.

Material and Interface Engineering for High-Performance Perovskite Solar Cells: A Personal Journey and Perspective

Hybrid organic-inorganic perovskite solar cells (PSCs) have become a shining star in the photovoltaic field due to their spectacular increase in power conversion efficiency (PCE) from 3.8 % to over 23 % in just few years, opening up the potential in addressing the important future energy and environment issues. The excellent photovoltaic performance can be attributed to the unique properties of the organometal halide perovskite materials, including high absorption coefficient, tunable bandgap, high defect tolerance, and excellent charge transport characteristics. The authors entered this field when pursuing research on dye-sensitized solar cells (DSCs) by leveraging nanorods arrays for vectorial transport of the extracted electrons. Soon after, we and others realized that while the organometal halide perovskite materials have excellent intrinsic properties for solar cells, interface engineering is at least equally important in the development of high-performance PSCs, which includes surface defect passivation, band alignment, and heterojunction formation. Herein, we will address this topic by presenting the historical development and recent progress on the interface engineering of PSCs primarily of our own group. This review is mainly focused on the material and interface design of the conventional n-i-p, inverted p-i-n and carbon electrode-based structure devices from our own experience and perspective. Finally, the challenges and prospects of this area for future development will also be discussed.

Phase Stability and Diffusion in Lateral Heterostructures of Methyl Ammonium Lead Halide Perovskites

Mixed halide hybrid organic-inorganic perovskites have band gaps that span the visible spectrum making them candidates for optoelectronic devices. Transport of the halide atoms in methyl ammonium lead iodide (MAPbI₃) and its alloys with bromine has been observed in both dark and under illumination. While halide transport upon application of electric fields has received much attention, less is known regarding bromide and iodide interdiffusion down concentration gradients. This work provides an upper bound on the bromide-iodide interdiffusion coefficient D_i in thin films of MAPb(Br_xI_1-x)₃ using a diffusion couples of lateral heterostructures. The upper bound of D_i was extracted from changes in the interface profiles of the heterostructures upon exposure to heat. The stability of thoroughly heated interfacial profiles suggests that the miscibility gap extends to higher temperatures and to a higher fractional composition of bromine than predicted by theory. The results of this work provide guidance for compositions of thermally stable heterostructures of hybrid halide perovskites.

Nuclei position-control and crystal growth-guidance on frozen substrates for high-performance perovskite solar cells

Nucleation and crystal growth are key stages for high-quality perovskite films that dominate the performance of perovskite solar cells. However, the position of nuclei in the films and the orientation of the crystal growth have not yet been intendedly controlled during their fabrication. In this study, we developed a method of spin-coating perovskite films on frozen substrates to control the position of the nuclei and the direction of the crystal growth at the same time. In this way, the position of the crystal nuclei and the growth orientation of the perovskite crystals in the perovskite films can be simultaneously controlled. A high-quality perovskite film with grains spanning vertically the entire film thickness has been obtained using this new method. And an efficient inverted planar solar cell (ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC61BM/BCP/Ag) with the highest power conversion efficiency of 17.14% and open-circuit voltage of 1.14 V has been achieved by using this technique.

Unexpected hydrolytic transformation of new type hybrid bromobismuthates with methylpyrazinium dications

A series of new type hybrid bromobismuthates formed by various pyrazinium cations were isolated and studied. In the systems initially containing iodide anions and monocations of substituted pyrazines, the complexes based on doubly charged cations of substituted pyrazines instead of ones based on the corresponding monocations were surprisingly formed. The variation of substituted pyrazinium cations affects not only the crystal structures of hybrid bromobismuthates via tuning the nuclearity of the anions but also the hydrolytic stability of the compounds. A thorough structural study of hydrolytic transformations was performed for halobismuthates for the first time. The results revealed a stepwise course of the process affording several products. Spectral studies of the complexes evidence that the values of optical band gaps (E_g) are low in comparison with those for similar systems which is most likely due to the cooperative effect involving the nature of the corresponding cations together with the features of the supramolecular structures of the complexes.

Hydrogen Passivated Silicon Grain Boundaries Greatly Reduce Charge Recombination for Improved Silicon/Perovskite Tandem Solar Cell Performance: Time Domain Ab Initio Analysis

By performing nonadiabatic molecular dynamics simulations, we demonstrate that grain boundaries (GBs) can induce the indirect-to-direct transition of the silicon band gap. However, missing a silicon atom creates an electron trap state in the GBs. Electron trapping by the silicon vacancy occurs on tens of picoseconds followed by recombination of the trapped electron and valence band hole on sub-100 ps, which operates parallel to recombination of the free electron and hole on a similar time scale. The recombination is greatly accelerated by 2 orders of magnitude compared to the GBs without a silicon vacancy. Hydrogen passivation eliminates the trap state and notably delays the charge recombination due to an increased band gap and a shortened coherence time, extending the excited-state lifetime to sub-10 ns. Our study provides an atomistic description of how charge recombination in the silicon can be efficiently reduced, suggesting a rational route to enhance silicon/perovskite tandem solar cells performance.

Copper Iodide Interlayer for Improved Charge Extraction and Stability of Inverted Perovskite Solar Cells

Nickel oxide (NiO) is one of the most promising and high-performing Hole Transporting Layer (HTL) in inverted perovskite solar cells due to ideal band alignment with perovskite absorber, wide band gap, and high mobility of charges. At the same time, however, NiO does not provide good contact and trap-free junction for hole collection. In this paper, we examine this problem by developing a double hole transport configuration with a copper iodide (CuI) interlayer for efficient surface passivation. Transient photo-current (TPC) measurements showed that Perovskite/HTL interface with CuI interlayer has an improved hole injection; CuI passivation reduces the concentration of traps and the parasitic charge accumulation that limits the flow of charges. Moreover, we found that CuI protect the HTL/perovskite interface from degradation and consequently improve the stability of the cell. The presence of CuI interlayer induces an improvement of open-circuit voltage VOC (from 1.02 V to 1.07 V), an increase of the shunt resistance RSH (100%), a reduction of the series resistance RS (-30%), and finally a +10% improvement of the solar cell efficiency.

Fully Inorganic Ruddlesden-Popper Double Cl-I and Triple Cl-Br-I Lead Halide Perovskite Nanocrystals

The vast majority of lead halide perovskite (LHP) nanocrystals (NCs) are currently based on either a single halide composition (CsPbCl₃, CsPbBr₃, and CsPbI₃) or an alloyed mixture of bromide with either Cl^- or I^- [i.e., CsPb(Br:Cl)₃ or CsPb(Br:I)₃]. In this work, we present the synthesis as well as a detailed optical and structural study of two halide alloying cases that have not previously been reported for LHP NCs: Cs₂PbI₂Cl₂ NCs and triple halide CsPb(Cl:Br:I)₃ NCs. In the case of Cs₂PbI₂Cl₂, we observe for the first time NCs with a fully inorganic Ruddlesden-Popper phase (RPP) crystal structure. Unlike the well-explored organic-inorganic RPP, here, the RPP formation is triggered by the size difference between the halide ions. These NCs exhibit a strong excitonic absorption, albeit with a weak photoluminescence quantum yield (PLQY). In the case of the triple halide CsPb(Cl:Br:I)₃ composition, the NCs comprise a CsPbBr₂Cl perovskite crystal lattice with only a small amount of incorporated iodide, which segregates at RPP planes' interfaces within the CsPb(Cl:Br:I)₃ NCs. Supported by density functional theory calculations and postsynthetic surface treatments to enhance the PLQY, we show that the combination of iodide segregation and defective RPP interfaces are most likely linked to the strong PL quenching observed in these nanostructures. In summary, this work demonstrates the limits of halide alloying in LHP NCs because a mixture that contains halide ions of very different sizes leads to the formation of defective RPP interfaces and a severe quenching of LHP NC's optical properties.

Controlling the Phase Segregation in Mixed Halide Perovskites through Nanocrystal Size

Mixed halide perovskites are one of the promising candidates in developing solar cells and light-emitting diodes (LEDs), among other applications, because of their tunable optical properties. Nonetheless, photoinduced phase segregation, by formation of segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline size of CsPbBr3-x I x and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm-2) and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. These results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime and segregation rates. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices.