Interfacial Engineering and Photon Downshifting of CsPbBr3 Nanocrystals for Efficient, Stable, and Colorful Vapor Phase Perovskite Solar Cells

Ruddlesden-Popper Perovskite Nanocrystals as Interface Modification Layer for Efficient Perovskite Solar Cells

Perovskite nanocrystals are advantageous for interfacial passivation of perovskite solar cells (PSCs), but the insulating long alkyl chain surface ligands impede the charge transfer, while the conventional ligand exchange would possibly introduce surface defects to the nanocrystals. In this work, we reported novel in situ modification of CsPbBr3 nanocrystals using a short chain conjugated molecule 2-methoxyphenylethylammonium iodide (2-MeO-PEAI) for interfacial passivation of PSCs. Transmission electron microscopy studies with atomic resolution unveil the transformation from cubic CsPbBr3 to Ruddlesden-Popper phase (RPP) nanocrystals due to halogen exchange. Synergic passivation by the RPP nanocrystals and 2-MeO-PEA+ has led to suppressed interface defects and enhanced charge carrier transport. Consequently, PSCs with in situ modified RPP nanocrystals achieved a champion power conversion efficiency of 24.39%, along with an improvement in stability. This work brings insights into the microstructural evolution of perovskite nanocrystals, providing a novel and feasible approach for interfacial passivation of PSCs.

Halide Perovskite Crystallization Processes and Methods in Nanocrystals, Single Crystals, and Thin Films

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High-performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (C_min ). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation-dominated process by rapidly increasing the concentration over C_min , single crystals are mainly prepared via the growth-dominated process by keeping the concentration between solubility and C_min , while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

Second-phase-induced fluorescence quenching in non-equivalent substituted red phosphors

Concentration quenching, which generally originates from serious energy migrations among the uniformly distributed luminescent centers in the host matrix, is a key factor to influence the luminescence properties of materials. Different from previous reports, we demonstrate a novel fluorescence-quenching mechanism attributable to the second-phase Eu₂W₂O₉ in non-equivalent substituted SrWO₄:xEu³⁺ phosphors. The crystal structure, elemental distribution, and luminescence properties of the as-prepared SrWO₄:xEu³⁺ phosphors are systematically investigated. A second-phase Eu₂W₂O₉ is confirmed when the Eu³⁺-doping concentration exceeds 20%, which produces the new structure defects and energy-transfer paths, resulting in fluorescence quenching in this material. This finding gives a new perspective to analyze the concentration-quenching mechanism of the non-equivalent substituted phosphors and can help in the design of new, efficient luminescence materials. In addition, the as-prepared SrWO₄:xEu³⁺ phosphors exhibit a strong intrinsic excitation in the range of 355-425 nm, which is accompanied by the Commission Internationale de I'Eclairage (CIE) coordinates at (0.653, 0.347) and stable color purity of up to 94.52%. A packaged white light-emitting diode with CIE chromaticity coordinates of (0.398, 0.335), correlated color temperature of 3132 K, and color rendering index of 84.3 is fabricated by SrWO₄:20%Eu³⁺ phosphors with blue BAM:Eu²⁺ and green YAGB:Tb³⁺ phosphors in a near-ultraviolet chip.

Selection, Preparation and Application of Quantum Dots in Perovskite Solar Cells

As the third generation of new thin-film solar cells, perovskite solar cells (PSCs) have attracted much attention for their excellent photovoltaic performance. Today, PSCs have reported the highest photovoltaic conversion efficiency (PCE) of 25.5%, which is an encouraging value, very close to the highest PCE of the most widely used silicon-based solar cells. However, scholars have found that PSCs have problems of being easily decomposed under ultraviolet (UV) light, poor stability, energy level mismatch and severe hysteresis, which greatly limit their industrialization. As unique materials, quantum dots (QDs) have many excellent properties and have been widely used in PSCs to address the issues mentioned above. In this article, we describe the application of various QDs as additives in different layers of PSCs, as luminescent down-shifting materials, and directly as electron transport layers (ETL), light-absorbing layers and hole transport layers (HTL). The addition of QDs optimizes the energy level arrangement within the device, expands the range of light utilization, passivates defects on the surface of the perovskite film and promotes electron and hole transport, resulting in significant improvements in both PCE and stability. We summarize in detail the role of QDs in PSCs, analyze the perspective and associated issues of QDs in PSCs, and finally offer our insights into the future direction of development.

Optoelectronic Neural Interfaces Based on Quantum Dots

Optoelectronic modulation of neural activity is an emerging field for the investigation of neural circuits and the development of neural therapeutics. Among a wide variety of nanomaterials, colloidal quantum dots provide unique optoelectronic features for neural interfaces such as sensitive tuning of electron and hole energy levels via the quantum confinement effect, controlling the carrier localization via band alignment, and engineering the surface by shell growth and ligand engineering. Even though colloidal quantum dots have been frontier nanomaterials for solar energy harvesting and lighting, their application to optoelectronic neural interfaces has remained below their significant potential. However, this potential has recently gained attention with the rise of bioelectronic medicine. In this review, we unravel the fundamentals of quantum-dot-based optoelectronic biointerfaces and discuss their neuromodulation mechanisms starting from the quantum dot level up to electrode-electrolyte interactions and stimulation of neurons with their physiological pathways. We conclude the review by proposing new strategies and possible perspectives toward nanodevices for the optoelectronic stimulation of neural tissue by utilizing the exceptional nanoscale properties of colloidal quantum dots.

Recent Advances in Colloidal Quantum Dots or Perovskite Quantum Dots as a Luminescent Downshifting Layer Embedded on Solar Cells

The solar cell has a poor spectral response in the UV region, which affects its power conversion efficiency (PCE). The utilization of a luminescent downshifting (LDS) layer has been suggested to improve the spectral response of the photovoltaics in the short wavelength region through photoluminescence (PL) conversion and antireflection effects, which then enhance the PCE of the solar cell. Recently, colloidal quantum dots (CQDs) or perovskite quantum dots (PQDs) have been gaining prime importance as an LDS material due to their eminent optical characteristics, such as their wide absorption band, adjustable visible emission, short PL lifetime, and near-unity quantum yields. However, the instability of QDs that occurs under certain air, heat, and moisture conditions limits its commercialization. Thus, in this review, we will focus on the physical and optical characteristics of QDs. Further, we will discuss different synthesis approaches and the stability issues of QDs. Different approaches to improve the stability of QDs will be discussed in detail alongside the recent breakthroughs in QD-based solar cells for various applications and their current challenges. We expect that this review will provide an effective gateway for researchers to fabricate LDS-layer-based solar cells.

Perovskite Quantum Dots in Solar Cells

Perovskite quantum dots (PQDs) have captured a host of researchers' attention due to their unique properties, which have been introduced to lots of optoelectronics areas, such as light-emitting diodes, lasers, photodetectors, and solar cells. Herein, the authors aim at reviewing the achievements of PQDs applied to solar cells in recent years. The engineering concerning surface ligands, additives, and hybrid composition for PQDSCs is outlined first, followed by analyzing the reasons of undesired performance of PQDSCs. Subsequently, a novel overview that PQDs are utilized to improve the photovoltaic performance of various kinds of solar cells, is provided. Finally, this review is summarized and some challenges and perspectives concerning PQDs are also discussed.

Quantum Dot Interface-Mediated CsPbIBr2 Film Growth and Passivation for Efficient Carbon-Based Solar Cells

CsPbIxBry-based all-inorganic perovskite materials are a potential candidate for stable semitransparent and tandem structured photovoltaic devices. However, poor film (morphological and crystalline) quality and interfacial recombination lead consequently to a decline in the photoelectric conversion performance of the applied solar cells. In this work, we incorporated PbS quantum dots (QDs) at the interface of electron transporting layer (ETL) SnO₂ and perovskite to modulate the crystallization of CsPbIBr₂ and the interfacial charge dynamics in carbon-based solar cells. The as-casted PbS QDs behave as seeds for lattice-matching the epitaxial growth of pinhole-free CsPbIBr₂ films. The modified films with reduced defect density exhibit facilitated carrier transfer and suppressed charge recombination at the ETL/perovskite interface, contributing to an enhanced device efficiency from 7.00 to 9.09% and increased reproducibility and ambient stability. This strategic method of QD-assisted lattice-matched epitaxial growth is promising to prepare high-quality perovskite films for efficient perovskite solar cells.

CsPbBr3 Nanocrystal Induced Bilateral Interface Modification for Efficient Planar Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have drawn tremendous attention owing to their remarkable photovoltaic performance and simple preparation process. However, conventional wet-chemical synthesis methods inevitably create defects both in the bulk and at the interfaces of perovskites, leading to recombination of charge carriers and reduced stability. Herein, a bilateral interface modification to perovskites by doping room-temperature synthesized CsPbBr3 nanocrystals (CN) is reported. The ultrafast transient absorption measurement reveals that CN effectively suppresses the defect at the SnO2 /perovskite interface and boosts the interfacial electron transport. Meanwhile, the in situ Kelvin probe force microscopy and contact potential difference characterizations verify that the CN within the upper part of the perovskites enhances the built-in electric field, facilitating oriented migration of the carriers within the perovskite. Combining the superiorities of CN modifiers on both sides, the bilaterally modified CH3 NH3 PbI3 -based planar PSCs exhibit optimal power conversion efficiency exceeding 20% and improved device stability.

High-performance perovskite photodetectors based on CsPbBr3 microwire arrays

All inorganic perovskite materials have drawn extensive attention, owing to their outstanding performance, facile solution-processed method, and potential applications in optoelectronic devices. However, uncontrollable morphology, high defect density, and instability of perovskites prepared via solution-processed method are the main challenges for their large-scale production and commercialization. Herein, we prepared large-scale CsPbBr₃ microwire arrays with highly ordered morphology and high crystalline quality by a template-assisted method. The photodetectors based on CsPbBr₃ microwire arrays exhibited remarkable on/off photocurrent ratio of 9.02×10³, high detectivity of 1.59×10¹³ Jones, high responsivity of 4.55 A/W, and fast response speed of 4.9/3 ms. More importantly, the photocurrent of the photodetectors hardly changed in air after being stored for two months, indicating remarkable stability. This study demonstrates that CsPbBr₃ microwire arrays provide the possibility for preparing large-scale and high-performance optoelectronic devices.

Lignin Nanoparticles: Promising Sustainable Building Blocks of Photoluminescent and Haze Films for Improving Efficiency of Solar Cells

Films with the capacity for photoluminescence and haze, which can convert UV to visible light and enhance light management, are of great importance for optoelectronic devices. Here, taking advantage of the inherent fluorescence and self-assembly properties of lignin, we have developed a sustainable lignin-derived multifunctional dopant (L-MS-NPs) for fabricating optical films with haze, fluorescence, and room-temperature phosphorescence (RTP) together with poly(vinyl alcohol) (PVA). The optical films are used to improve the light-harvesting efficiency of solar cells. Specifically, attributed to the robust morphology in the film matrix, L-MS-NPs cause a rough morphology in the surface of an L-MS-NPs/PVA composite film, which eventually triggers the great optical haze. Additionally, L-MS-NPs inherit fluorescence properties from lignin and show fluorescence emission when embed in the film matrix. Moreover, the PVA film matrix can stabilize the excited triplet state, which finally induces RTP of L-MS-NPs. The combined haze, fluorescence, and RTP properties of the L-MS-NPs/PVA composite film enhances the power conversion efficiency (PCE) of dye-sensitized solar cells from ∼3.9 to ∼4.1%.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Easy-to-process and high-performance colorful perovskite solar cells using a multilayer planar filter

Color-rendering manipulation of solar cells is drawing increasing interest, since the integration of color displaying can promote various advanced applications. However, the dual functionality of high-performance operation and easy processing remain a challenge. Here we propose a colorful perovskite solar cell (PSC) based on purely planar layers. The photonic crystal (PC), which does not interfere with the PSC processing, enables the display of high-purity colors and maintaining the number of PC layers at 4-6. The fabricated PSC with a four-layer PC successfully displays red-green-blue (RGB) colors, with the power-conversion efficiency of 10.94%, 11.01%, and 13.70%, respectively. Further study indicates that by employing a six-layer PC the PSC can obtain excellent color-displaying effect with the color gamut up to 81.8% of the standard RGB. It also shows that the design has a good tolerance to the deviation of layer thickness.

The cation-anion co-exchange in CsPb1-x Fe x (Br1-y Cl y )3 nanocrystals prepared using a hot injection method

All inorganic perovskite nanocrystals (NCs) have wide practical applications for their remarkable optoelectronic properties. To obtain blue-emitting perovskites with high photoluminescence quantum yield and room-temperature ferromagnetism, CsPb_1-x Fe _x (Br_1-y Cl _y )₃ NCs were synthesized using a hot injection method. The effects of the cation-anion co-exchange on the structural, luminescent and magnetic properties of CsPbBr₃ NCs were studied by X-ray diffraction spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, field emission scanning electron microscopy, and vibrating sample magnetometer. The results indicated that there was cation-anion co-exchange in CsPb_1-x Fe _x (Br_1-y Cl _y )₃ NCs, while the band-edge energies and PLQY were mainly affected by the anion exchange. The ferromagnetism of CsPb_1-x Fe _x (Br_1-y Cl _y )₃ NCs had been observed at room temperature, and there was an increase in saturation magnetization with increasing Fe concentration.

Diammonium Porphyrin-Induced CsPbBr3 Nanocrystals to Stabilize Perovskite Films for Efficient and Stable Solar Cells

Employing all-inorganic perovskite quantum dots (QDs) to treat organic-inorganic perovskite films has been well documented as a serviceable tactic to improve the performance of perovskite solar cells (PSCs). However, the inert molecule-coated QDs with zero-dimensional (0D) structure would limit further enhancement of the efficiency and stability of PSCs. Here, we employ a conductive diammonium porphyrin (ZnPy-NH₃Br) to treat CsPbBr₃ QDs coated on a three-dimensional perovskite film, thus constructing a stable 0D-two-dimensional perovskite capping layer. The generation of large-scale nanocube crystals by treating CsPbBr₃ nanocrystallites with ZnPy-NH₃Br in solution demonstrates such an assembly strategy. The formed capping layer can achieve efficient charge transport and separation. As a consequence, the best efficiency of an optimized device is up to 20.0%, which is superior to the control PSCs fabricated without modification (19.1%) and with pure CsPbBr₃ QD modification (19.5%). More importantly, the porphyrin-treated CsPbBr₃ QD-based devices retain over 65 or 85% of their initial efficiency when placed at 85 °C or 45% humidity tracking for 1000 h, respectively. Also, with the incorporation of QD-Por, the device retained 85% of the original efficiency when illuminated at AM 1.5 G for 450 h. Therefore, this work offered a facile avenue to modify perovskite films for fabricating highly efficient and stable PSCs.

Enhanced Optical Absorption and Interfacial Carrier Separation of CsPbBr3/Graphene Heterostructure: Experimental and Theoretical Insights

Controlling effective separation of carriers at the interface is a key element to realize highly efficient halogenated perovskite-based optoelectronic devices. Here, a comprehensive study of interfacial properties for CsPbBr₃ nanocrystals (NCs)/graphene heterostructure is performed by the combination of theoretical and experimental methods. Enhanced visible light absorption is observed experimentally in the CsPbBr₃ NCs/graphene heterostructure. The strong photoluminescence quenching phenomenon and improved photoresponse prove the efficient interfacial charge transfer from the perovskite CsPbBr₃ NC layer to the graphene side. Significantly, theoretical calculations suggest that an intrinsic built-in electric field, pointing from graphene toward CsPbBr₃, promotes the separation of photoinduced carriers at the CsPbBr₃ NCs/graphene interface and simultaneously inhibits the recombination of electron-hole pairs. Thus, the high optoelectronic performance can be obtained in the CsPbBr₃ NCs/graphene heterostructure, as shown in our experiment. Moreover, the CsPbBr₃ NCs/graphene heterostructure exhibits smaller effective mass than that of CsPbBr₃ NCs, indicating that the heterostructure does possess a high carrier mobility, which can further accelerate the separation of photogenerated carriers. Furthermore, the calculated results reveal that, accounting for the presence of the stronger built-in electric field, larger band bending value, and smaller effective mass, the PbBr₂/graphene interface can realize the separation of the photoinduced carriers more effectively than the CsBr/graphene interface and thus more efficiently facilitate electron transfer from the perovskite optical absorber side to the graphene electronic transport side. Our findings provide valuable insight into perovskite/graphene-based photodetector devices via the interface engineering project.

Fiber-Spinning-Chemistry Method toward In Situ Generation of Highly Stable Halide Perovskite Nanocrystals

All-inorganic halide perovskite nanocrystals (PNCs) have drawn increasing attention owing to their splendid optical properties. However, such nanomaterials suffer from intrinsic instability, greatly limiting their practical application. Meanwhile, environmental regulation has restricted the emissions of volatile organic compounds (VOCs), initiating a search for alternative approaches to PNC synthesis and film forming. Herein, fiber-spinning chemistry (FSC) is proposed for easy-to-perform synthesis of highly stable PNC fibrous films. The FSC process utilizes spinning fibers as reactors, reducing the generation of VOCs. This method enables the fabrication of CsPbX3 (X = Cl, Br, I) PNCs/poly(methyl methacrylate)/thermoplastic polyurethanes fibrous films at room temperature in one step, exhibiting tunable emission between 450 and 660 nm. Significantly, the in situ generation of PNCs in hydrophobic core-shell nanofibers results in highly improved fluorescence stability. PNCs/polymer fibrous films keep constant in photoluminescence (PL) after storage at atmosphere for 90 d and retain 82% PL after water immersion for 120 h (vs fluorescence quenching in 10 d in air or 5 h in water for pristine PNCs). The PNCs/polymer fibrous films endowed with superior optical stability and great flexibility show promising potentials in flexible optoelectronic applications. This work paves a facile way toward high-performance nanoparticles/polymer fibrous films.