Key Factors Affecting the Stability of CsPbI3 Perovskite Quantum Dot Solar Cells: A Comprehensive Review

Air-Processed Perovskites Enabled by an Interface-Reconstruction Strategy for High-Performance Light-Emitting Diodes

Fabricating high-performance perovskites in ambient air is desirable for low-cost and large-scale patterned manufacture of light-emitting diodes (LEDs). However, perovskites inherently exhibit high sensitivity to moisture and oxygen, which considerably hinders their fabrication under ambient air conditions. Here, we demonstrated an interface-reconstruction strategy which enabled air-processed CsPbI3 quantum dot (QD) films for high-performance LEDs. Ethyl acetate/tris(1-naphthyl)phosphine oxide (EA/TNPO) treatment was used to polish and passivate the interface between the QD film and the electron transport layer, thereby providing a protective coating against moisture and oxygen. We fabricated LEDs based on the spin-coated QD films, achieving a maximum external quantum efficiency (EQE) of 20.2% and a long half-life of over 100 days. We also fabricated LEDs based on the inkjet-printed QD films, highlighting the practical application potential for patterning techniques. Our work develops high-quality air-processed perovskite films and demonstrates their great prospect for low-cost optoelectronic devices in the future.

A Brief Review of Perovskite Quantum Dot Solar Cells: Synthesis, Property and Defect Passivation

Perovskite quantum dot solar cells (PQDSCs), as the promising candidate for the next generation of solar cell, have garnered the significant attention over the past decades. However, the performance and stability of PQDSCs are highly dependent on the properties of interfaces between the perovskite quantum dots (PQDs) and the other layers in the device. This work provides a brief overview of PQDSCs, including the synthesis of PQDs, the characteristics and preparation methods of PQDs, the photoelectric properties as the light absorption layer and optimization methods for PQDSCs with high efficiency. Future directions and potential applications are also highlighted.

Post-Degradation Recovery of CsPbI3 Quantum Dot Solar Cells

The stability of perovskite quantum dot solar cells is one of the key challenges of this technology. This study reveals the unique degradation behavior of cesium lead triiodide (CsPbI3) quantum dot solar cells. For the first time, it is shown that the oxygen-induced degradation and performance loss of CsPbI3 quantum dot photovoltaic devices can be reversed by exposing the degraded samples to humidity, allowing the performance to recover and even surpass the initial performance. By careful characterization and analysis throughout the degradation and recovery process, the underlying physical and chemical mechanisms that govern the evolution of the device performance could be identified. It is shown that the ligand shell of the quantum dots, rather than the instability of the semiconducting material itself, is the driving factor in these mechanisms. This highlights the important role of surface chemistry and ligand design in enhancing perovskite quantum dot photovoltaics.

Synthesis, characterization, and practical applications of perovskite quantum dots: recent update

This review paper provides an in-depth analysis of Perovskite quantum dots (PQDs), a class of nanomaterials with unique optical and electronic properties that hold immense potential for various technological applications. The paper delves into the structural characteristics, synthesis methods, and characterization techniques of PQDs, highlighting their distinct advantages over other Quantum Dots (QDs). Various applications of PQDs in fields such as solar cells, LEDs, bioimaging, photocatalysis, and sensors are discussed, showcasing their versatility and promising capabilities. The ongoing advancements in PQD research and development point towards a bright future for these nanostructures in revolutionizing diverse industries and technologies.

Discovery of a Thermodynamic-Control Two-Dimensional Cs6Pb5I16 Perovskite with a Unique Green Emission Color via Dynamic Structural Transformation

New perovskite materials of two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs6Pb5I16 nanosheets were successfully obtained from the structural transformation of 2D PR-phase Cs7Pb6I19 nanosheets. The 2D RP-phase Cs6Pb5I16 perovskite nanosheets exhibited unique green emission with an emission wavelength of ∼500 nm. The crystal structure of the 2D RP-phase Cs6Pb5I16 perovskite nanosheets was determined by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy, and atomic force microscopy. The time-dependent photoluminescence measurements and XRD spectra were used to observe the optical and structure transformations from 2D Cs7Pb6I19 (n = 6) to 2D Cs6Pb5I16 (n = 5) perovskites. The in situ XRD measurements confirmed that γ-phase CsPbI3 was released during the structural transformation. Moreover, temperature-dependent in situ XRD measurements were employed to examine the kinetic energy involved in the structural transformation from the n = 6 form to the n = 5 form. Specifically, an intermediate structure from n = 6 to n = 5 was also identified. Most importantly, 2D Cs6Pb5I16 (n = 5) was more structurally thermodynamically stable than 2D Cs7Pb6I19 (n = 6). This study provides an essential route for the discovery of new types of perovskite structures during structural transformation.

CsPbI3 Perovskite Quantum Dot-Based WORM Memory Device with Intrinsic Ternary States

The migration of mobile ionic halide vacancies is usually considered detrimental to the performance and stability of perovskite optoelectronic devices. Taking advantage of this intrinsic feature, we fabricated a CsPbI3 perovskite quantum dot (PQD)-based write-once-read-many-times (WORM) memory device with a simple sandwich structure that demonstrates intrinsic ternary states with a high ON/OFF ratio of 103:102:1 and a long retention time of 104 s. Through electrochemical impedance spectroscopy, we proved that the resistive switching is achieved by the migration of mobile iodine vacancies (VIs) under an electric field to form conductive filaments (CFs). Using in situ conductive atomic force microscopy, we further revealed that the multilevel property arises from the different activation energies for VIs to migrate at grain boundaries and grain interiors, resulting in two distinct pathways for CFs to grow. Our work highlights the potential of CsPbI3 PQD-based WORM devices, showcasing intrinsic multilevel properties achieved in a simple device structure by rationally controlling the drift of ionic defects.

Efficient open-air synthesis of Mg2+-doped CsPbI3 nanocrystals for high-performance red LEDs

Inorganic CsPbI3 perovskite nanocrystals (NCs) exhibit remarkable optoelectronic properties for illumination. However, their poor stability has hindered the development of light-emitting diodes (LEDs) based on these materials. In this study, we propose a facile method to synthesize Mg2+-doped CsPbI3 NCs with enhanced stability and high photoluminescence (PL) intensity under ambient air conditions. Theoretical calculations confirm that doped NCs possess stronger formation energy compared to undoped NCs. The undoped CsPbI3 NCs emit red light at approximately 653 nm. We optimize the doping ratio to 1/30, which significantly enhances the photoluminescence of single-particle CsPbI3 NCs. Subsequently, we fabricate a red LED by combining the CsPbI3 NCs with a blue chip. The resulting LED, based on the doped CsPbI3 NCs, exhibits excellent performance with a high luminance of 4902.22 cd m-2 and stable color coordinates of (0.7, 0.27). This work not only presents a simple process for synthesizing perovskite NCs but also provides a design strategy for developing novel red LEDs for various applications.

Perovskite Colloidal Nanocrystal Solar Cells: Current Advances, Challenges, and Future Perspectives

The power conversion efficiencies (PCEs) of polycrystalline perovskite (PVK) solar cells (SCs) (PC-PeSCs) have rapidly increased. However, PC-PeSCs are intrinsically unstable without encapsulation, and their efficiency drops during large-scale production; these problems hinder the commercial viability of PeSCs. Stability can be increased by using colloidal PVK nanocrystals (c-PeNCs), which have high surface strains, low defect density, and exceptional crystal quality. The use of c-PeNCs separates the crystallization process from the film formation process, which is preponderant in large-scale fabrication. Consequently, the use of c-PeNCs has substantial potential to overcome challenges encountered when fabricating PC-PeSCs. Research on colloidal nanocrystal-based PVK SCs (NC-PeSCs) has increased their PCEs to a level greater than those of other quantum-dot SCs, but has not reached the PCEs of PC-PeSCs; this inferiority significantly impedes widespread application of NC-PeSCs. This review first introduces the distinctive properties of c-PeNCs, then the strategies that have been used to achieve high-efficiency NC-PeSCs. Then it discusses in detail the persisting challenges in this domain. Specifically, the major challenges and solutions for NC-PeSCs related to low short-circuit current density Jsc are covered. Last, the article presents a perspective on future research directions and potential applications in the realm of NC-PeSCs.

Passivation of 2D Cs2PbI2Cl2 Nanosheets for Efficient and Stable CsPbI3 Quantum Dot Solar Cells

Inorganic CsPbI3 perovskite quantum dots (PQDs) possess remarkable optical properties, making them highly promising for photovoltaic applications. However, the inadequate stability resulting from internal structural instability and the complex external surface chemical environment of CsPbI3 PQDs has hindered the development of CsPbI3 PQD solar cells (PQDSCs). In this work, the capping layer composed of inorganic two-dimensional (2D) Ruddlesden-Popper (RP) phase Cs2PbI2Cl2 nanosheets (NSs) is introduced, which may be effectively treated to improve the surface properties of the CsPbI3 PQD film. This modification serves to passivate defects by filling cesium and iodine vacancies while optimizing the energy band arrangement and preventing humidity intrusion, leading to the meliorative stability and photovoltaic performance. The optimized CsPbI3 PQDSCs achieve an enhanced power conversion efficiency (PCE) of 14.73%, with the superb stability of only a 16% efficiency loss after being exposed to ambient conditions (30 ± 5% RH) for 432 h.

Enhancement and Broadening of the Internal Electric Field of Hole-Transport-Layer-Free Perovskite Solar Cells by Quantum Dot Interface Modification

Hole-transport-layer-free perovskite solar cells have attracted strong interest due to their simple structure and low cost, but charge recombination is serious. Built-in electric field engineering is an intrinsic driver to facilitate charge separation transport and improve the efficiency of photovoltaic devices. However, the enhancement of the built-in electric field strength is often accompanied by the narrowing of the space charge region, which becomes a key constraint to the performance improvement of the device. Here, we propose an effective regulation method, the component engineering of quantum dots, to enhance the strength of the built-in electric field and broaden the range of space charge. By using all inorganic CsPbBrxI3-x (x = 0, 1, 2, 3) quantum dot interface modification to passivate the defects of MAPbI3 perovskite films, the regulation law of quantum dot components on the work function of perovskite films was revealed, and the mechanism of their influence on the internal electric field intensity and space charge region distribution was further clarified, thereby fundamentally solving the serious problem of charge recombination. As directly observed by electron-beam-induced current (EBIC), the introduction of CsPbBr2I quantum dots can effectively enhance the interfacial electric field intensity, widening the space charge region from 160 to 430 nm. Moreover, the efficiency of the hole-free transport layer perovskite solar cells modified by CsPbBr2I quantum dots was also significantly enhanced by 1.5 times. This is an important guideline for electric field modulation and efficiency improvement within photovoltaic devices with other simplified structures.

Cesium Cyclopropane Acid-Aided Crystal Growth Enables Efficient Inorganic Perovskite Solar Cells with a High Moisture Tolerance

While all-inorganic halide perovskites (iHPs) are promising photovoltaic materials, the associated water sensitivity of iHPs calls for stringent humidity control to reach satisfactory photovoltaic efficiencies. Herein, we report a moisture-insensitive perovskite formation route under ambient air for CsPbI2 Br-based iHPs via cesium cyclopropane acids (C3 ) as a compound introducer. With this approach, appreciably enhanced crystallization quality and moisture tolerance of CsPbI2 Br are attained. The improvements are attributed to the modified evaporation enthalpy of the volatile side product of DMA-acid initiated by Cs-acids. As such, the water-involving reaction is directed toward the DMA-acids, leaving the target CsPbI2 Br perovskites insensitive to ambient humidity. We highlight that by controlling the C3 concentration, the dependence of power conversion efficiency (PCE) in CsPbI2 Br devices on the humidity level during perovskite film formation becomes favorably weakened, with the PCEs remaining relatively high (>15 %) associated with improved device stability for RH levels changed from 25 % to 65 %. The champion solar cells yield an impressive PCE exceeding 17 %, showing small degradations (<10 %) for 2000 hours of shell storage and 300 hours of 85/85 (temperature/humidity) tests. The demonstrated C3 -based strategy provides an enabler for improving the long-sought moisture-stability of iHPs toward high photovoltaic device performance.

Manipulation of hot-carrier cooling dynamics in CsPbBr3 quantum dots via site-selective ligand engineering

Prolonging the lifetime of photoinduced hot carriers in lead-halide perovskite quantum dots (QDs) is highly desirable because it can help improve the photovoltaic conversion efficiency. Ligand engineering has recently become a promising strategy to achieve this; nevertheless, mechanistic studies in this field remain limited. Herein, we propose a new scenario of ligand engineering featuring Pb2+/Br- site-selective capping on the surface of CsPbBr3 QDs. Through joint observations of temperature-dependent photoluminescence, ultrafast transient absorption, and Raman spectroscopy of the two contrasting model systems of CsPbBr3 QDs (i.e., capping with organic ligand only vs hybrid organic/inorganic ligands), we reveal that the phononic regulation of Pb-Br stretching at the Br-site (relative to Pb-site) leads to a larger suppression of charge-phonon coupling due to a stronger polaronic screening effect, thereby more effectively retarding the hot-carrier cooling process. This work opens a new route for the manipulation of hot-carrier cooling dynamics in perovskite systems via site-selective ligand engineering.

Anomalous efficiency elevation of quantum-dot light-emitting diodes induced by operational degradation

Quantum-dot light-emitting diodes promise a new generation of high-performance and solution-processed electroluminescent light sources. Understanding the operational degradation mechanisms of quantum-dot light-emitting diodes is crucial for their practical applications. Here, we show that quantum-dot light-emitting diodes may exhibit an anomalous degradation pattern characterized by a continuous increase in electroluminescent efficiency upon electrical stressing, which deviates from the typical decrease in electroluminescent efficiency observed in other light-emitting diodes. Various in-situ/operando characterizations were performed to investigate the evolutions of charge dynamics during the efficiency elevation, and the alterations in electric potential landscapes in the active devices. Furthermore, we carried out selective peel-off-and-rebuild experiments and depth-profiling analyses to pinpoint the critical degradation site and reveal the underlying microscopic mechanism. The results indicate that the operation-induced efficiency increase results from the degradation of electron-injection capability at the electron-transport layer/cathode interface, which in turn leads to gradually improved charge balance. Our work provides new insights into the degradation of red quantum-dot light-emitting diodes and has far-reaching implications for the design of charge-injection interfaces in solution-processed light-emitting diodes.

Phase Stability Improvement of a γ-CsPbI3 Perovskite Solar Cell Utilizing a Barium Bis(trifluoromethanesulfonimide) Solution

The cesium lead iodide (CsPbI3) perovskite solar cell possesses a wide band gap ranging from 1.65 to 1.75 eV, which is suitable for integration into a tandem structure along with a low-band-gap silicon solar cell. Moreover, CsPbI3 has received considerable attention as a potential solution for the prevalent issues of low thermal stability of organic-inorganic perovskite solar cells and phase segregation encountered in conventional mixed halide wide-band-gap perovskite solar cells. Through the implementation of volatile additives, CsPbI3 has demonstrated substantial advancements in efficiency, process temperature, and stability. This study introduces a novel approach for barium (Ba)-doping by spraying an antisolvent containing barium bis(trifluoromethanesulfonimide) during the spin-coating process. By incorporating Ba2+ through this spraying technique, the formation of the delta phase in CsPbI3 is significantly suppressed; thereby, a power conversion efficiency of 18.56% is achieved, and a remarkable 93% of the initial efficiency is maintained after 600 h.

Stabilized Perovskite Quantum Dot Solids via Nonpolar Solvent Dispersible Covalent Ligands

The ligand exchange procedure of CsPbI3 perovskite quantum dots (PQDs) enables the fabrication of thick and conductive PQD solids that act as a photovoltaic absorber for solution-processed thin-film solar cells. However, the ligand-exchanged CsPbI3 PQD solids suffer from deterioration in photovoltaic performance and ambient stability due to the surface traps, such as uncoordinated Pb2+ sites on the PQD surface, which are generated after the conventional ligand exchange process using ionic short-chain ligands dissolved in polar solvents. Herein, a facile surface stabilization is demonstrated that can simultaneously improve the photovoltaic performance and ambient stability of CsPbI3 PQD photovoltaic absorber using covalent short-chain triphenylphosphine oxide (TPPO) ligands dissolved in a nonpolar solvent. It is found that the TPPO ligand can be covalently bound to uncoordinated Pb2+ sites and the nonpolar solvent octane can completely preserve the PQD surface components. Owing to their synergetic effects, the CsPbI3 PQD photovoltaic absorber stabilized using the TPPO ligand solution dissolved in octane exhibit higher optoelectrical properties and ambient stability than the control absorber. Consequently, CsPbI3 PQD solar cells composed of PQD photovoltaic absorbers fabricated via surface stabilization strategy provide an improved power conversion efficiency of 15.4% and an enhanced device stability.

In Situ Iodide Passivation Toward Efficient CsPbI3 Perovskite Quantum Dot Solar Cells

Highlights

The introduction of hydroiodic acid (HI) manipulates the dynamic conversion of PbI2 into highly coordinated species to optimize the nucleation and growth kinetics. The addition of HI enables the fabrication of CsPbI3 perovskite quantum dots with reduced defect density, enhanced crystallinity, higher phase purity, and near-unity photoluminescence quantum yield. The efficiency of CsPbI3 perovskite quantum dot solar cells was enhanced from 14.07% to 15.72% together with enhanced storage stability.

Abstract

All-inorganic CsPbI3 quantum dots (QDs) have demonstrated promising potential in photovoltaic (PV) applications. However, these colloidal perovskites are vulnerable to the deterioration of surface trap states, leading to a degradation in efficiency and stability. To address these issues, a facile yet effective strategy of introducing hydroiodic acid (HI) into the synthesis procedure is established to achieve high-quality QDs and devices. Through an in-depth experimental analysis, the introduction of HI was found to convert PbI2 into highly coordinated [PbIm]2−m, enabling control of the nucleation numbers and growth kinetics. Combined optical and structural investigations illustrate that such a synthesis technique is beneficial for achieving enhanced crystallinity and a reduced density of crystallographic defects. Finally, the effect of HI is further reflected on the PV performance. The optimal device demonstrated a significantly improved power conversion efficiency of 15.72% along with enhanced storage stability. This technique illuminates a novel and simple methodology to regulate the formed species during synthesis, shedding light on further understanding solar cell performance, and aiding the design of future novel synthesis protocols for high-performance optoelectronic devices.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-023-01134-1.

Suppressing Phase Segregation in CsPbIBr2 Films via Anchoring Halide Ions toward Underwater Solar Cells

Inorganic CsPbIBr₂ perovskite solar cells (PSCs) have accomplished many milestones, yet their progress has been constrained by ion migration and phase separation. This study explores the modulation of perovskite crystallization kinetics and halide ion migration through chlorobenzene (CB) antisolvent with bis(pentafluorophenyl)zinc (Zn(C₆F₅)₂) additive. The photoluminescence and absorption spectra reveal the significantly reduced phase segregaton in CsPbIBr₂ film treated by CB with Zn(C₆F₅)₂. Moreover, this research analyzes the CsPbIBr₂ film's free carrier lifetime, diffusion length, and mobility using time-resolved microwave conductivity and transient absorption spectroscopy after Zn(C₆F₅)₂ modification. Consequently, the modified CsPbIBr₂ PSCs offer a 12.57% power conversion efficiency (PCE), the highest value among CsPbIBr₂ PSCs with negligible hysteresis and prolonged stability. Furthermore, under 1-m-deep water, CsPbIBr₂ PSCs display a PCE of 14.18%. These findings provide an understanding of the development of phase-segregation-free CsPbIBr₂ films and showcase the prospective applications of CsPbIBr₂ PSCs in underwater power systems.

High-Performance Perovskite Quantum Dot Solar Cells Enabled by Incorporation with Dimensionally Engineered Organic Semiconductor

Perovskite quantum dots (PQDs) have been considered promising and effective photovoltaic absorber due to their superior optoelectronic properties and inherent material merits combining perovskites and QDs. However, they exhibit low moisture stability at room humidity (20-30%) owing to many surface defect sites generated by inefficient ligand exchange process. These surface traps must be re-passivated to improve both charge transport ability and moisture stability. To address this issue, PQD-organic semiconductor hybrid solar cells with suitable electrical properties and functional groups might dramatically improve the charge extraction and defect passivation. Conventional organic semiconductors are typically low-dimensional (1D and 2D) and prone to excessive self-aggregation, which limits chemical interaction with PQDs. In this work, we designed a new 3D star-shaped semiconducting material (Star-TrCN) to enhance the compatibility with PQDs. The robust bonding with Star-TrCN and PQDs is demonstrated by theoretical modeling and experimental validation. The Star-TrCN-PQD hybrid films show improved cubic-phase stability of CsPbI₃-PQDs via reduced surface trap states and suppressed moisture penetration. As a result, the resultant devices not only achieve remarkable device stability over 1000 h at 20-30% relative humidity, but also boost power conversion efficiency up to 16.0% via forming a cascade energy band structure.