Investigation of the Effect of the Passivation on the Charge Transfer from MAPbI3 to Electron Transport Layer Using a Heterodyne Transient Grating Spectroscopic Technique

We fabricated MAPbI3 perovskite thin films with ZnO on a glass substrate, in which a passivation layer (Phenethylammonium iodide (PEAI); p-methoxyphenethylammonium iodide (CH3O-PEAI); 2-methoxyethylammonium iodide (MEAI)) was inserted between two layers. In order to understand the effect of the insertion of each passivation material on the transfer efficiency of the photo-generated electrons from MAPbI3 to ZnO, we observed the near-field heterodyne transient grating (NF-HD-TG) responses of each film and investigated the component arising from the recombination of the trapped electrons at the ZnO surface. Based on the accelerated recombination between photo-generated holes remaining in the MAPbI3 layer and surface-trapped electrons in ZnO and the increase in the number of the trapped electrons in ZnO when either CH3O-PEAI or PEAI was applied, we successfully revealed that the charge transfer efficiency was enhanced by the insertion of the passivation materials including a benzene ring stabilizing the defect states. Particularly, it was demonstrated that CH3O-PEAI showed the highest increase in the charge transfer efficiency, which could be attributed to the high electron density in the benzene ring, resulting from the existence of the electron donating group, CH3O, and its role in the effective transition from 3D to 2D perovskite phases.

The Promise and Challenges of Inverted Perovskite Solar Cells

Recently, there has been an extensive focus on inverted perovskite solar cells (PSCs) with a p-i-n architecture due to their attractive advantages, such as exceptional stability, high efficiency, low cost, low-temperature processing, and compatibility with tandem architectures, leading to a surge in their development. Single-junction and perovskite-silicon tandem solar cells (TSCs) with an inverted architecture have achieved certified PCEs of 26.15% and 33.9% respectively, showing great promise for commercial applications. To expedite real-world applications, it is crucial to investigate the key challenges for further performance enhancement. We first introduce representative methods, such as composition engineering, additive engineering, solvent engineering, processing engineering, innovation of charge transporting layers, and interface engineering, for fabricating high-efficiency and stable inverted PSCs. We then delve into the reasons behind the excellent stability of inverted PSCs. Subsequently, we review recent advances in TSCs with inverted PSCs, including perovskite-Si TSCs, all-perovskite TSCs, and perovskite-organic TSCs. To achieve final commercial deployment, we present efforts related to scaling up, harvesting indoor light, economic assessment, and reducing environmental impacts. Lastly, we discuss the potential and challenges of inverted PSCs in the future.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

Advances in Mixed Tin-Lead Narrow-Bandgap Perovskites for Single-Junction and All-Perovskite Tandem Solar Cells

Organic-inorganic metal-halide perovskites have received great attention for photovoltaic (PV) applications owing to their superior optoelectronic properties and the unprecedented performance development. For single-junction PV devices, although lead (Pb)-based perovskite solar cells have achieved 26.1% efficiency, the mixed tin-lead (Sn-Pb) perovskites offer more ideal bandgap tuning capability to enable an even higher performance. The Sn-Pb perovskite (with a bandgap tuned to ≈1.2 eV) is also attractive as the bottom subcell for a tandem configuration to further surpass the Shockley-Queisser radiative limit for the single-junction devices. The performance of the all-perovskite tandem solar cells has gained rapid development and achieved a certified efficiency up to 29.1%. In this article, the properties and recent development of state-of-the-art mixed Sn-Pb perovskites and their application in single-junction and all-perovskite tandem solar cells are reviewed. Recent advances in various approaches covering additives, solvents, interfaces, and perovskite growth are highlighted. The authors also provide the perspective and outlook on the challenges and strategies for further development of mixed Sn-Pb perovskites in both efficiency and stability for PV applications.

Chelating Dual Interface for Efficient and Stable Crystal Growth and Iodine Defect Management in Sn-Pb Perovskite Solar Cells

Suppressing Sn2+ oxidation and rationally controlling the crystallization process of tin-lead perovskite (Sn-Pb PVK) films by suitable bonding methods have emerged as key approaches to achieving efficient and stable Sn-Pb perovskite solar cells (PSCs). Herein, the chelating coordination is performed at the top and bottom interfaces of Sn-Pb PVK films. The chelation strength is stronger toward Sn2+ than Pb2+ by introducing oligomeric proanthocyanidins (OPC) at the bottom interface. This difference in chelation strength resulted in a spontaneous gradient distribution of Sn/Pb within the perovskite layer during crystallization, particularly enhancing the enrichment of Sn2+ at the bottom interface and facilitating the extraction and separation of photogenerated charge carriers in PSCs. Simultaneously, this top-down distribution of gradually increasing Sn content slowed down the crystallization rate of Sn-Pb PVK films, forming higher-quality films. On the top interface of the PVK, trifluoroacetamidine (TFA) was used to inhibit the generation of iodine vacancies (VI) through chelating with surface-uncoordinated Pb2+/Sn2+, further passivating defects while suppressing the oxidation of Sn2+. Ultimately, the PSCs with simultaneous chelation at both top and bottom interfaces achieved a power conversion efficiency (PCE) of 23.31% and an open-circuit voltage (VOC) exceeding 0.90 V. The stability of unencapsulated target devices in different environments also improved.

Promoting Ruddlesden-Popper Perovskite Formation by Tailoring Spacer Intramolecular Interaction for Efficient and Stable Solar Cells

Low-dimensional Ruddlesden-Popper phase (LDRP) perovskites are widely studied in the field of photovoltaics due to their tunable energy-band properties, enhanced photostability, and improved environmental stability compared to the 3D perovskites. However, the insulating spacers with weak intramolecular interaction used in LDRP materials limit the out-of-plane charge transport, leading to poor device performance of LDRP perovskite solar cells (PSCs). Here, a functional ligand, 3-guanidinopropanoic acid (GPA), which is capable of forming strong intramolecular hydrogen bonds through the carboxylic acid group, is employed as an organic spacer for LDRP PSCs. Owing to the strong interaction between GPA molecules, high-quality LDRP (GPA)2(MA)n-1PbnI3n+1 film with promoted formation of n = 5 phase, improved crystallinity, preferential vertical growth orientations, reduced trap-state density, and prolonged carrier lifetime is achieved using GPAI as the dimensionality regulator compared to butylamine hydroiodide (BAI). As a result, GPA-based LDRP PSC exhibits a champion power conversion efficiency of 18.16% that is much superior to the BA-based LDRP PSC (15.43%). Importantly, the optimized GPA-based LDRP PSCs without encapsulation show enhanced illumination, thermal, storage, and humidity stability compared to BA-based ones. This work provides new insights into producing high n value LDRP films and their efficient and stable PSCs.

Efficient and stabilized molecular doping of hole-transporting materials driven by a cyclic-anion strategy for perovskite solar cells

Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) is commonly used as an effective dopant to improve the performance of the hole-transporting material (HTM) in n-i-p perovskite solar cells (PSCs). However, the ultra-hygroscopic and migratory nature of Li-TFSI leads to inferior stability of PSCs. Here, we report on a strategy to regulate the anion unit in Li-TFSI from linear to cyclic, constructing a new dopant, lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (Li-CYCLIC), for the state-of-the-art poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). Mechanistic and experimental results reveal that the cyclic anion CYCLIC- exhibits stronger interaction with Li+ and PTAA˙+ compared with the linear anion TFSI-, thus significantly restraining the moisture absorption and migration of Li+ and improving the thermodynamic stability of PTAA˙+CYCLIC-. With this molecular engineering, the resulting PSCs based on Li-CYCLIC obtained an improved efficiency, along with remarkably enhanced stability, retaining 96% of the initial efficiency after over 1150 hours under continuous 1 sun illumination in an N2 atmosphere, yielding an extrapolated T 80 of over 12 000 hours. In a broader context, the proposed strategy of linear-to-cyclic doping provides substantial guidance for the subsequent advancement in the development of effective dopants for photoelectric devices.

All-Inorganic Perovskite Solar Cells: Defect Regulation and Emerging Applications in Extreme Environments

All-inorganic perovskite solar cells (PSCs), such as CsPbX3, have garnered considerable attention recently, as they exhibit superior thermodynamic and optoelectronic stabilities compared to the organic-inorganic hybrid PSCs. However, the power conversion efficiency (PCE) of CsPbX3 PSCs is generally lower than that of organic-inorganic hybrid PSCs, as they contain higher defect densities at the interface and within the perovskite light-absorbing layers, resulting in higher non-radiative recombination and voltage loss. Consequently, defect regulation has been adopted as an important strategy to improve device performance and stability. This review aims to comprehensively summarize recent progresses on the defect regulation in CsPbX3 PSCs, as well as their cutting-edge applications in extreme scenarios. The underlying fundamental mechanisms leading to the defect formation in the crystal structure of CsPbX3 PSCs are firstly discussed, and an overview of literature-adopted defect regulation strategies in the context of interface, internal, and surface engineering is provided. Cutting-edge applications of CsPbX3 PSCs in extreme environments such as outer space and underwater situations are highlighted. Finally, a summary and outlook are presented on future directions for achieving higher efficiencies and superior stability in CsPbX3 PSCs.

Defects and Defect Passivation in Perovskite Solar Cells

Perovskite solar cells have made significant strides in recent years. However, there are still challenges in terms of photoelectric conversion efficiency and long-term stability associated with perovskite solar cells. The presence of defects in perovskite materials is one of the important influencing factors leading to subpar film quality. Adopting additives to passivate defects within perovskite materials is an effective approach. Therefore, we first discuss the types of defects that occur in perovskite materials and the mechanisms of their effect on performance. Then, several types of additives used in perovskite solar cells are discussed, including ionic compounds, organic molecules, polymers, etc. This review provides guidance for the future development of more sustainable and effective additives to improve the performance of solar cells.

Stabilizing Top Interface by Molecular Locking Strategy with Polydentate Chelating Biomaterials toward Efficient and Stable Perovskite Solar Cells in Ambient Air

The instability of top interface induced by interfacial defects and residual tensile strain hinders the realization of long-term stable n-i-p regular perovskite solar cells (PSCs). Herein, one molecular locking strategy is reported to stabilize top interface by adopting polydentate ligand green biomaterial 2-deoxy-2,2-difluoro-d-erythro-pentafuranous-1-ulose-3,5-dibenzoate (DDPUD) to manipulate the surface and grain boundaries of perovskite films. Both experimental and theoretical evidence collectively uncover that the uncoordinated Pb2+ ions, halide vacancy, and/or I─Pb antisite defects can be effectively healed and locked by firm chemical anchoring on the surface of perovskite films. The ingenious polydentate ligand chelating is translated into reduced interfacial defects, increased carrier lifetimes, released interfacial stress, and enhanced moisture resistance, which should be liable for strengthened top interface stability and inhibited interfacial nonradiative recombination. The universality of the molecular locking strategy is certified by employing different perovskite compositions. The DDPUD modification achieves an enhanced power conversion efficiency (PCE) of 23.17-24.47%, which is one of the highest PCEs ever reported for the devices prepared in ambient air. The unsealed DDPUD-modified devices maintain 98.18% and 88.10% of their initial PCEs after more than 3000 h under a relative humidity of 10-20% and after 1728 h at 65 °C, respectively.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

Dimensionality Engineering of Organic-Inorganic Halide Perovskites for Next-Generation X-Ray Detector

The next-generation X-ray detectors require novel semiconductors with low material/fabrication cost, excellent X-ray response characteristics, and robust operational stability. The family of organic-inorganic hybrid perovskites (OIHPs) materials comprises a range of crystal configuration (i.e., films, wafers, and single crystals) with tunable chemical composition, structures, and electronic properties, which can perfectly meet the multiple-stringent requirements of high-energy radiation detection, making them emerging as the cutting-edge candidate for next-generation X-ray detectors. From the perspective of molecular dimensionality, the physicochemical and optoelectronic characteristics of OIHPs exhibit dimensionality-dependent behavior, and thus the structural dimensionality is recognized as the key factor that determines the device performance of OIHPs-based X-ray detectors. Nevertheless, the correlation between dimensionality of OIHPs and performance of their X-ray detectors is still short of theoretical guidance, which become a bottleneck that impedes the development of efficient X-ray detectors. In the review, the advanced studies on the dimensionality engineering of OIHPs are critically assessed in X-ray detection application, discussing the current understanding on the "dimensionality-property" relationship of OIHPs and the state-of-the-art progresses on the dimensionality-engineered OIHPs-based X-ray detector, and highlight the open challenges and future outlook of this field.

Halogen-Site Regulation in Cs3Bi2X9 Quantum Dots for Efficient and Selective Oxidation of Benzyl Alcohol Driven by Solar Light

Sluggish charge kinetics and low selectivity limit the solar-driven selective organic transformations under mild conditions. Herein, an efficient strategy of halogen-site regulation, based on the precise control of charge transfer and molecule activation by rational design of Cs3Bi2X9 quantum dots photocatalysts, is proposed to achieve both high selectivity and yield of benzyl-alcohol oxidation. In situ PL spectroscopy study reveals that the Bi─Br bonds formed in the form of Br-associated coordination can enhance the separation and transfer of photoexcited carriers during the practical reaction. As the active center, the exclusive Bi─Br covalence can benefit the benzyl-alcohol activation for producing carbon-centered radicals. As a result, the Cs3Bi2Br9 with this atomic coordination achieves a conversion ratio of 97.9% for benzyl alcohol and selectivity of 99.6% for aldehydes, which are 56.9- and 1.54-fold higher than that of Cs3Bi2Cl9. Combined with quasi-in situ EPR, in situ ATR-FTIR spectra, and DFT calculation, the conversion of C6H5-CH2OH to C6H5-CH2* at Br-related coordination is revealed to be a determining step, which can be accelerated via halogen-site regulation for enhancing selectivity and photocatalytic efficiency. The mechanistic insights of this research elucidate how halogen-site regulation in favor of charge transfer and molecule activation toward efficient and selective oxidation of benzyl alcohol.

Imidazoanthraquinone Derivative as a Surface Passivator for Enhanced and Stable Perovskite Solar Cells

Hybrid organic-inorganic perovskites have been investigated for their potential to serve in next-generation perovskite solar cells (PSCs). While PSC technology is approaching commercialization, thermal and moisture stabilities remain a concern. Here, we describe the assembly of PSCs using an imidazoanthraquinone derivative (AQ) as a small organic additive to enhance the device performance and stability. Unlike polymer additives, AQ is easy to synthesize and is more economical. AQ was synthesized because it has both carbonyl and imidazole functional groups. The presence of C=O and N-H groups results in coordination interaction with Pb2+ and I- of the perovskite. Addition of the AQ molecule to methylammonium lead iodide leads to the formation of a superior crystalline perovskite film with fewer defects and enhanced stability under humid conditions. The use of optimized perovskite films enhanced device power conversion efficiency (PCE = 17.21%) compared to pristine perovskite (PCE = 13.88%). Unencapsulated optimized devices retained 90% of the initial power conversion efficiency for 30 days at a relative humidity of nearly 35%. The optimized films also exhibited superior thermal stability to that of pristine perovskite films.

Unraveling the Molecular Size Effect on Surface Engineering of Perovskite Solar Cells

Surface engineering in perovskite solar cells, especially for the upper surface of perovskite, is widely studied. However, most of these studies have primarily focused on the interaction between additive functional groups and perovskite point defects, neglecting the influence of other parts of additive molecules. Herein, additives with -NH3 + functional group are introduced at the perovskite surface to suppress surface defects. The chain lengths of these additives vary to conduct a detailed investigation into the impact of molecular size. The results indicate that the propane-1,3-diamine dihydroiodide (PDAI2 ), which possesses the most suitable size, exhibited obvious optimization effects. Whereas the molecules, methylenediamine dihydroiodide (MDAI2 ) and pentane-1,5-diamine dihydroiodide (PentDAI2 ) with unsuitable size, lead to a deterioration in device performance. The PDAI2 -treated devices achieved a certified power conversion efficiency (PCE) of 25.81% and the unencapsulated devices retained over 80% of their initial PCE after 600 h AM1.5 illumination.

Experimental and theoretical study of improved mesoporous titanium dioxide perovskite solar cell: The impact of modification with graphene oxide

The present study serves experimental and theoretical analyses in developing a hybrid advanced structure as a photolysis, which is based on electrospun Graphene Oxide-titanium dioxide (GO-TiO2) nanofibers as an electron transfer material (ETMs) functionalized for perovskite solar cell (PVSCs) with GO. The prepared ETMs were utilized for the synthesis of mixed-cation (FAPbI3)0.8(MAPbBr3)0.2. The effect of GO on TiO2 and their chemical structure, electronic and morphological characteristic were investigated and discussed. The elaborated device, namely ITO/Bl-TiO2/3 wt% GO-TiO2/(FAPbI3)0.8(MAPbBr3)0.2/spiro-MeTAD/Pt, displayed 20.14% disposition and conversion solar energy with fill factor (FF) of 1.176%, short circuit current density (Jsc) of 20.56 mA/cm2 and open circuit voltage (VOC) 0.912 V. The obtained efficiency is higher than titanium oxide (18.42%) and other prepared GO-TiO2 composite nanofibers based ETMs. The developed materials and device would facilitate elaboration of advanced functional materials and devices for energy storage applications.

Breaking the bottleneck of lead-free perovskite solar cells through dimensionality modulation

The emerging perovskite solar cell (PSC) technology has attracted significant attention due to its superior power conversion efficiency (PCE) among the thin-film photovoltaic technologies. However, the toxicity of lead and poor stability of lead halide materials hinder their commercialization. In this case, after a decade of effort, various categories of lead-free perovskites and perovskite-like materials have been developed, including tin halide perovskites, double perovskites, defect-structured perovskites, and rudorffites. However, the performance of the corresponding devices still falls short of expectations, especially their PCE. The limitations mainly originate from either the unstable lattice structure of these materials, which causes the distortion of their octahedra, or their low dimensionality (e.g., structural and electronic dimensionality)-correlated poor carrier transport and self-trapping effect, accelerating nonradiative recombination. Therefore, understanding the relationship between the structures and performance in these emerging candidates and leveraging these insights to design or modify new lead-free perovskites is of great significance. Herein, we review the variety of dimensionalities in different categories of lead-free perovskites and perovskite-like materials and conclude that dimensionality is an important aspect among the crucial indexes that determine the performance of lead-free PSCs. In addition, we summarize the modulation of both structural and electronic dimensionality, and the corresponding enhanced optoelectronic properties in different categories. Finally, perspectives on the future development of lead-free perovskites and perovskite-like materials for photovoltaic applications are provided. We hope that this review will provide researchers with a concise overview of these emerging materials and help them leverage dimensionality to break the bottleneck in photovoltaic applications.

Enhancing Stability and Efficiency of Inverted Inorganic Perovskite Solar Cells with In-Situ Interfacial Cross-Linked Modifier

Inverted inorganic perovskite solar cells (PSCs) is potential as the top cells in tandem configurations, owing to the ideal bandgap, good thermal and light stability of inorganic perovskites. However, challenges such as mismatch of energy levels between charge transport layer and perovskite, significant non-radiative recombination caused by surface defects, and poor water stability have led to the urgent need for further improvement in the performance of inverted inorganic PSCs. Herein, the fabrication of efficient and stable CsPbI3-x Brx PSCs through surface treatment of (3-mercaptopropyl) trimethoxysilane (MPTS), is reported. The silane groups in MPTS can in situ crosslink in the presence of moisture to build a 3-dimensional (3D) network by Si-O-Si bonds, which forms a hydrophobic layer on perovskite surface to inhibit water invasion. Additionally, -SH can strongly interact with the undercoordinated Pb2+ at the perovskite surface, effectively minimizing interfacial charge recombination. Consequently, the efficiency of the inverted inorganic PSCs improves dramatically from 19.0% to 21.0% under 100 mW cm-2 illumination with MPTS treatment. Remarkably, perovskite films with crosslinked MPTS exhibit superior stability when soaking in water. The optimized PSC maintains 91% of its initial efficiency after aging 1000 h in ambient atmosphere, and 86% in 800 h of operational stability testing.

Self-Healing Behavior of the Metal Halide Perovskites and Photovoltaics

Perovskite solar cells have achieved rapid progress in the new-generation photovoltaic field, but the commercialization lags behind owing to the device stability issue under operational conditions. Ultimately, the instability issue is attributed to the soft lattice of ionic perovskite crystal. In brief, metal halide perovskite materials are susceptible to structural instability processes, including phase segregation, component loss, lattice distortion, and fatigue failure under harsh external stimuli such as high humidity, strong irradiation, wide thermal cycles, and large stress. Developing self-healing perovskites to further improve the unsatisfactory operational stability of their photoelectric devices under harsh stimuli has become a cutting-edge hotspot in this field. This self-healing behavior needs to be studied more comprehensively. Therefore, the self-healing behavior of the metal halide perovskites and photovoltaics is classified and summarized in this review. By discussing recent advances, underlying mechanisms, strategies, and existing challenges, this review provides perspectives on self-healing of perovskite solar cells in the future.

Surface Energy Engineering of Buried Interface for Highly Stable Perovskite Solar Cells with Efficiency Over 25

The abundant oxygen-related defects (e.g., O vacancies, O-H) in the TiO2 electron transport layer results in high surface energy, which is detrimental to effective carrier extraction and seriously impairs the photovoltaic performance and stability of perovskite solar cells. Here, novel surface energy engineering (SEE) is developed by applying a surfactant of heptadecafluorooctanesulfonate tetraethylammonium (HFSTA) on the surface of the TiO2 . Theoretical calculations show that the HFSTA-TiO2 is less prone to form O vacancies, leading to lower surface energy, thus improving the carrier-extraction efficiency. The experimental results show that superior perovskite film is obtained due to the reduced heterogeneous nucleation sites and improved crystallization process on the modified TiO2 . Furthermore, the flexible long alkyl chains in HFSTA considerably relieve the compressive stresses at the buried interface. By combining the passivation of TiO2 , crystallization process modulation, and stress relief, a champion PCE up to 25.03% is achieved. The device without encapsulation sustains 92.2% of its initial PCE after more than 2500 h storage under air ambient with relative humidity of 25-30%. The SEE of a buried interface paves a new way toward high-efficiency, stable perovskite solar cells.

Defect Passivation and Lithium Ion Coordination Via Hole Transporting Layer Modification for High Performance Inorganic Perovskite Solar Cells

Metal halide inorganic perovskite solar cells (PSCs) have great potential to achieve high efficiency with excellent thermal stability. However, the surface defect traps restrain the achievement of high open circuit voltage (VOC ) and power conversion efficiency (PCE) of the devices due to the severe nonradiative charge recombination. Moreover, the state-of-the-art hole transporting layer (HTL) significantly hampers device moisture stability, even though it renders the highest solar cell efficiency. Herein, a one-stone-two-birds strategy is proposed using a biocompatible material tryptamine (TA) as an additive in HTL. First, TA bearing electron rich moieties can favorably passivate the surface defects of inorganic perovskite films, significantly reducing trap density and prolonging charge lifetime. It results in a drastic improvement of VOC from 1.192 to 1.251 V, with a VOC loss of 0.48 V. The corresponding PSCs achieve a 21.8% PCE under 100 mW cm-2 illumination. Second, TA in HTL can coordinate with lithium cations, retarding their reaction with moisture and increasing the moisture stability of HTL. Consequently, the black phase of inorganic perovskite films is well preserved, and the corresponding PSCs maintain 90% of the initial PCE after 800 h storage at relative humidity of 25-35%, much higher than the control devices.

Characterizing Spatial and Energetic Distributions of Trap States Toward Highly Efficient Perovskite Photovoltaics

Due to their greater opt electric performance, perovskite photovoltaics (PVs) present huge potential to be commercialized. Perovskite PV's high theoretical efficiency expands the available development area. The passivation of defects in perovskite films is crucial for approaching the theoretical limit. In addition to creating efficient passivation techniques, it is essential to direct the passivation approach by getting precise and real-time information on the trap states through measurements. Therefore, it is necessary to establish quantitative characterization methods for the trap states in energy and 3D spaces. The authors cover the characterization of the spatial and energy distributions of trap states in this article with an eye toward high-efficiency perovskite photovoltaics. After going over the strategies that have been created for characterizing and evaluating trap states, the authors will concentrate on how to direct the creative development of characterization techniques for trap states assessment and highlight the opportunities and challenges of future development. The 3D space and energy distribution mappings of trap states are anticipated to be realized. The review will give key guiding importance for further approaching the theoretical efficiency of perovskite photovoltaics, offering some future research direction and technological assistance for the development of appropriate targeted passivation technologies.

Managing Interfacial Charged Defects with Multiple Active Sited Macrocyclic Valinomycin for Efficient and Stable Inverted Perovskite Solar Cells

The unavoidably positively and negatively charged defects at the interface between perovskite and electron transport layer (ETL) often lead to severe surface recombination and unfavorable energy level alignment in inverted perovskite solar cells (PerSCs). Inserting interlayers at this interface is an effective approach to eliminate charged defects. Herein, the macrocyclic molecule valinomycin (VM) with multiple active sites of ─C═O, ─NH, and ─O─ is employed as an interlayer at the perovskite/ETL contact to simultaneously eliminate positively and negatively charged defects. Combined with a series of theoretical calculations and experimental analyzes, it is demonstrated that the ─C═O and ─O─ groups in VM can immobilize the uncoordinated Pb2+ to manage the positively charged defect and the formation of N─H···I hydrogen bonding can recompense the formamidine vacancies to eliminate the negatively charged defect. In addition, the VM interlayer induces a favorable downshift band bending at the perovskite/ETL interface, facilitating charge separation and boosting charge transfer. Thanks to the reduced charged defects and favorable energy level alignment, the fabricated inverted PerSC delivers an outstanding power conversion efficiency of 24.06% with excellent long-term ambient and thermal stability. This work demonstrates that managing charged defects via multiple functional groups and simultaneously regulating energy level alignment is a reliable strategy to boost the performance of PerSCs.

Strengthened Surface Modification for High-Performance Inorganic Perovskite Solar Cells with 21.3% Efficiency

Metal halide inorganic perovskites show excellent thermal stability compared to organic-inorganic perovskites. However, the performance of inorganic perovskite solar cells (PSCs) is far from theoretical values, together with unsatisfactory stability, mainly due to the poor interfacial properties. In this work, a facial but effective method is reported to realize high-performance inorganic PSCs by post-modifying the perovskite surface with 2-thiophene ethylamine (TEA). It is found that amine group from TEA can favorably interact with the undercoordinated Pb2+ via Lewis acid-based coordination, while thiophene ring with electron-rich sulfur assists such interaction by functioning as an electron donor. The synergetic interaction allows TEA to passivate perovskite film defects more efficiently, as compared to phenethylamine (PEA) with less electron-donating ability. Moreover, perovskite valence band is slightly upward shift to match with hole transport material and facilitate hole transfer. These combinations result in a reduced non-radiative charge recombination and improved charge carrier lifetime. Consequently, PSCs with TEA modification shows a drastic improvement of VOC by 54 mV, yielding a champion PCE of 21.3%, much higher than the control PSCs (19.3%), along with improved ambient stability. This work demonstrates that surface modifier with an electron-rich moiety is critical for achieving efficient and stable inorganic PSCs.

Synergistic Surface Defect Passivation of Ionic Liquids for Efficient and Stable MAPbI3-Based Inverted Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells are fabricated using polycrystalline perovskite thin films, which possess high densities of point and surface defects. The surface defects of perovskite thin films are the key factors that affect the device performance. Therefore, the reduction of harmful defects is the primary task for improving device performance. Therefore, in this study, high-quality perovskite thin films are prepared using an ionic liquid, dithiocarbamate diethylamine (DADA), to passivate the interface. The electron-rich sulfur atom in the DADA molecule chelates with the uncoordinated lead ion in the perovskite films, and the diethylammonium cation forms a hydrogen bond with the free iodine ion, which further improves the passivation. The synergistic passivation and improved morphology of the perovskite thin films substantially reduce the number of charged defects on the film surface and prolong the carrier lifetime. In addition, the DADA surface treatment increases the work function of the perovskite film, which is beneficial for carrier transport. Under standard solar-lighting conditions, the power conversion efficiency (PCE) of the device increases from 19.13 to 21.36%, and the fill factor is as high as 83.17%. Owing to both the hydrophobicity of DADA molecules and the passivation of ion defects, the PCE of the device remains above 80%, even for the device stored for 500 h in air at a relative humidity of 65%, and the device stability is substantially improved.

Luminescence and Degeneration Mechanism of Perovskite Light-Emitting Diodes and Strategies for Improving Device Performance

Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.

Highly Reliable Textile-Type Memristor by Designing Aligned Nanochannels

Information-processing devices are the core components of modern electronics. Integrating them into textiles is the indispensable demand for electronic textiles to form close-loop functional systems. Memristors with crossbar configuration are regarded as promising building blocks to design woven information-processing devices that seamlessly unify with textiles. However, the memristors always suffer from severe temporal and spatial variations due to the random growth of conductive filaments during filamentary switching processes. Here, inspired by the ion nanochannels across synaptic membranes, a highly reliable textile-type memristor made of Pt/CuZnS memristive fiber with aligned nanochannels, showing small set voltage variation (<5.6%) under ultralow set voltage (≈0.089 V), high on/off ratio (≈106 ), and low power consumption (0.1 nW), is reported. Experimental evidence indicate that nanochannels with abundant active S defects can anchor silver ions and confine their migrations to form orderly and efficient conductive filaments. Such memristive performances enable the resultant textile-type memristor array to have high device-to-device uniformity and process complex physiological data like brainwave signals with high recognition accuracy (95%). The textile-type memristor arrays are mechanically durable to withstand hundreds of bending and sliding deformations, and seamlessly unified with sensing, power-supplying, and displaying textiles/fibers to form all-textile integrated electronic systems for new generation human-machine interactions.

Molecular Synergistic Passivation for Efficient Perovskite Solar Cells and Self-Powered Photodetectors

The interface between the perovskite and electron-transporting material is often treated for defect passivation to improve the photovoltaic performance of devices. A facile 4-Acetamidobenzoic acid (containing an acetamido, a carboxyl, and a benzene ring)-based molecular synergistic passivation (MSP) strategy is developed here to engineer the SnOx /perovskite interface, in which dense SnOx are prepared using an E-beam evaporation technology while the perovskite is deposited with vacuum flash evaporation deposition method. MSP engineering can synergistically passivate defects at the SnOx /perovskite interface by coordinating with Sn4+ and Pb2+ with functional group CO in the acetamido and carboxyl. The optimized solar cell devices can achieve the highest efficiency of 22.51% based on E-Beam deposited SnOx and 23.29% based on solution-processed SnO2 , respectively, accompanied by excellent stability exceeding 3000 h. Further, the self-powered photodetectors exhibit a remarkably low dark current of 5.22 × 10-9  A cm-2 , a response of 0.53 A W-1 at zero bias, a detection limit of 1.3 × 1013  Jones, and a linear dynamic range up to 80.4 dB. This work proposes a molecular synergistic passivation strategy to enhance the efficiency and responsivity of solar cells and self-powered photodetectors.

Perovskite interface defect passivation with poly(ethylene oxide) for improving power conversion efficiency of the inverted solar cells

Inverted perovskite solar cells (PSCs) attract researchers' attention for their potential application due to the low-temperature fabrication, negligible hysteresis and compatibility with multi-junction cells. However, the low-temperature fabricated perovskite films containing excessive undesired defects are not benefit for improving the performance of the inverted PSCs. In this work, we used a simple and effective passivation strategy that Poly(ethylene oxide) (PEO) polymer as an antisolvent additive to modify the perovskite films. The experiments and simulations have shown that the PEO polymer can effectively passivate the interface defects of the perovskite films. The defect passivation by PEO polymers suppressed non-radiative recombination, resulting in an increase in power conversion efficiency (PCE) of the inverted devices from 16.07% to 19.35%. In addition, the PCE of unencapsulated PSCs after PEO treatment maintains 97% of its original stored in a nitrogen atmosphere for 1000 h.

Laterally Grown Strain-Engineered Semitransparent Perovskite Solar Cells with 16.01% Efficiency

Hybrid organometallic halide perovskite-based semitransparent solar cell research has garnered significant attention recently due to their promising applications for smart windows, tandem devices, wearable electronics, displays, and sustainable internet-of-things. Though considerable progress has been made, stability, controlling the crystalline qualities, and growth orientation in perovskite thin films play crucial roles in improving the photovoltaic (PV) performance. Recently, strain modulation within the perovskite gathers an immense interest that is achieved by the ex situ process. However, little work is reported on in situ strain modulation, which is presented here. Apart from the challenges in the fabrication of high-efficiency perovskite solar cell (PSC) devices under ambient conditions, the stability of organic hole-transporting materials needs urgent attention. Herein, a single-step deposition of formamidiniumchloride (FACl)-mediated CH₃NH₃PbI₃ (MAPbI₃) thin films without an inert atmosphere and CuI as the inorganic hole-transporting material is demonstrated for their potential application toward semitransparent PSCs. The FACl amount in MAPbI₃ (mg/mL) plays a critical role in controlling the crystallinity, growth orientations, and in situ strains, which modulate the charge carrier transport dynamics, thereby improving the efficiency of the PSC device. A photoconversion efficiency of 16.01% has been achieved from MAPbI₃ with 20 mg/mL of FACl additive incorporation. The modification of the structural, electronic, and optical properties and the origin of strain in the as-synthesized MAPbI₃ domains due to the addition of FACl are further validated with experimental findings in detail using density functional theory simulations.

Foldable Hole-Transporting Materials for Merging Electronic States between Defective and Perfect Perovskite Sites

Defective and perfect sites naturally exist within electronic semiconductors, and considerable efforts to reduce defects to improve the performance of electronic devices, especially in hybrid organic-inorganic perovskites (ABX₃ ), are undertaken. Herein, foldable hole-transporting materials (HTMs) are developed, and they extend the wavefunctions of A-site cations of perovskite, which, as hybridized electronic states, link the trap states (defective site) and valence band edge (perfect site) between the naturally defective and perfect sites of the perovskite surface, finally converting the discrete trap states of the perovskite as the continuous valence band to reduce trap recombination. Tailoring the foldability of the HTMs tunes the wavefunctions between defective and perfect surface sites, allowing the power conversion efficiency of a small cell to reach 23.22% and that of a mini-module (6.5 × 7 cm, active area = 30.24 cm² ) to reach as high as 21.71% with a fill factor of 81%, the highest value reported for non-spiro-OMeTAD-based perovskite solar modules.

In situ surface regulation of 3D perovskite using diethylammonium iodide for highly efficient perovskite solar cells

Surface passivation by constructing a 2D/3D structure is considered to be an effective strategy for suppressing non-radiative recombination and improving the device efficiency and stability. Herein, the 2D perovskite is formed in situ on the surface of a 3D perovskite via chemical interactions between diethylammonium iodide (DAI) and Pb-I octahedra, which greatly reduces the deep level defects and non-radiative recombination. Moreover, the 2D/3D structure can regulate the energy level alignment, enhance the charge extraction, and improve the open-circuit voltage. Finally, compared with the control device, the power conversion efficiency (PCE) of the DAI-treated device increases from 21.58 to 23.50%. The unencapsulated devices stored in air for more than 500 hours can still retain 97% of their initial PCE, revealing good long-term placement stability. This work provides a promising strategy to fabricate efficient PSCs through the in situ construction of 2D/3D perovskite heterojunctions.

Lead-Free Perovskite Homojunction-Based HTM-Free Perovskite Solar Cells: Theoretical and Experimental Viewpoints

Simplifying the design of lead-free perovskite solar cells (PSCs) has drawn a lot of interest due to their low manufacturing cost and relative non-toxic nature. Focus has been placed mostly on reducing the toxic lead element and eliminating the requirement for expensive hole transport materials (HTMs). However, in terms of power conversion efficiency (PCE), the PSCs using all charge transport materials surpass the environmentally beneficial HTM-free PSCs. The low PCEs of the lead-free HTM-free PSCs could be linked to poorer hole transport and extraction as well as lower light harvesting. In this context, a lead-free perovskite homojunction-based HTM-free PSC was investigated, and the performance was then assessed using a Solar Cell Capacitance Simulator (SCAPS). A two-step method was employed to fabricate lead-free perovskite homojunction-based HTM-free PSCs in order to validate the simulation results. The simulation results show that high hole mobility and a narrow band gap of cesium tin iodide (CsSnI₃) boosted the hole collection and absorption spectrum, respectively. Additionally, the homojunction's built-in electric field, which was identified using SCAPS simulations, promoted the directed transport of the photo-induced charges, lowering carrier recombination losses. Homojunction-based HTM-free PSCs having a CsSnI₃ layer with a thickness of 100 nm, defect density of 10¹⁵ cm^-3, and interface defect density of 10¹⁸ cm^-3 were found to be capable of delivering high PCEs under a working temperature of 300 K. When compared to formamidinium tin iodide (FASnI₃)-based devices, the open-circuit voltage (V_oc), short-circuit density (J_sc), fill factor (FF), and PCE of FASnI₃/CsSnI₃ homojunction-based HTM-free PSCs were all improved from 0.66 to 0.78 V, 26.07 to 27.65 mA cm^-2, 76.37 to 79.74%, and 14.62 to 19.03%, respectively. In comparison to a FASnI₃-based device (PCE = 8.94%), an experimentally fabricated device using homojunction of FASnI₃/CsSnI₃ performs better with V_oc of 0.84 V, J_sc of 22.06 mA cm^-2, FF of 63.50%, and PCE of 11.77%. Moreover, FASnI₃/CsSnI₃-based PSC is more stable over time than its FASnI₃-based counterpart, preserving 89% of its initial PCE. These findings provide promising guidelines for developing highly efficient and environmentally friendly HTM-free PSCs based on perovskite homojunction.

Probing the Genuine Carrier Dynamics of Semiconducting Perovskites under Sunlight

Understanding the nature of photogenerated carriers and their subsequent dynamics in semiconducting perovskites is important for the development of solar cell materials and devices. However, most ultrafast dynamic measurements on perovskite materials were conducted under high carrier densities, which likely obscures the genuine dynamics under low carrier densities in solar illumination conditions. In this study, we presented a detailed experimental study of the carrier density-dependent dynamics in hybrid lead iodide perovskites from femtosecond to microsecond using a highly sensitive transient absorption (TA) spectrometer. From the dynamic curves with low carrier density in the linear response range, we observed two fast trapping processes that occurred in less than 1 ps and tens of picoseconds, attributed to the shallow traps, and two slow decays with lifetimes of hundreds of nanoseconds and longer than 1 μs, related to the trap-assisted recombination and trapping at deep traps. Further TA measurements clearly show that PbCl₂ passivation can effectively reduce both shallow and deep trap densities. These results provide insights into the intrinsic photophysics of semiconducting perovskites with direct implications for photovoltaic and optoelectronic applications under sunlight.

Enhancing the stability and efficiency of MAPbI3 perovskite solar cells by theophylline-BF4 - alkaloid derivatives, a theoretical-experimental approach

Perovskite solar cells (PSCs) are an evolving photovoltaic field with the potential to disrupt the established silicon solar cell market. However, the presence of many transport barriers and defect trap states at the interfaces and grain boundaries has negative effects on PSCs; it decreases their efficiency and stability. The purpose of this work was to investigate the effects on efficiency and stability achieved by quaternary theophylline additives in MAPbI₃ PSCs with the structure FTO/TiO₂/perovskite/spiro-OMeTAD/Ag. The X-ray photoelectron spectroscopy (XPS) and theoretical calculation strategies were applied to study the additive's interaction in the layer. The tetrafluoroborinated additive results in an increase in device current density (J _SC) (23.99 mA cm^-1), fill factor (FF) (65.7%), and open-circuit voltage (V _OC) (0.95 V), leading to significant improvement of the power conversion efficiency (PCE) to 15.04% compared to control devices (13.6%). Notably, films exposed to controlled humidity of 30% using the tetrafluoroborinated additive maintained their stability for more than 600 hours (h), while the control films were stable for less than 240 hours (h).

Polaron states of the full-configuration defects in metal halide perovskites

The systematical analysis for varieties of defects with different depths and lattice relaxation strengths in metal halide perovskites (MHPs) is a challenging task. Here, we study the energy shifts of the full-configuration defects due to the polaron effect based on the all-coupling variational method in MHPs, where these polaron states are formed stemming from different defect species coupling with the longitudinal optical phonon modes via Fro¨hlich mechanism. We find that the polaron effect results in defect levels varying from tens to several hundreds of meV, which are very close to the correction of defect levels due to the defect-polaron effect, especially for these defects migration proved in the recent experiments in MHPs. These results provide the significant enlightenment not only for analyzing the radiation and non-radiation processes of carriers mediated by defects, but also for optimizing defect effect in the photovoltaic and photoelectric devices based on MHPs materials.

Rationalization of passivation strategies toward high-performance perovskite solar cells

Lead halide perovskite solar cells (PSCs) have shown unprecedented development in efficiency and progressed relentlessly in improving stability. All the achievements have been accompanied by diverse passivation strategies to circumvent the pervasive defects in perovskite materials, which play crucial roles in the process of charge recombination, ion migration, and component degradation. Among the tremendous efforts made to solve these issues and achieve high-performance PSCs, we classify and review both well-established and burgeoning passivation strategies to provide further guidance for the passivation protocols in PSCs, including chemical passivation to eliminate defects by the formation of chemical bonds, physical passivation to eliminate defects by strain relaxation or physical treatments, energetic passivation to improve the stability toward light and oxygen, and field-effect passivation to regulate the interfacial carrier behavior. The subtle but non-trivial consequences from various passivation strategies need advanced characterization techniques combining synchrotron-based X-ray analysis, capacitance-based measurements, spatially resolved imaging, fluorescent molecular probe, Kelvin probe force microscope, etc., to scrutinize the mechanisms. In the end, challenges and prospective research directions on advancing these passivation strategies are proposed. Judicious combinations among chemical, physical, energetic, and field-effect passivation deserve more attention for future high-efficiency and stable perovskite photovoltaics.

Secondary Anti-Solvent Treatment for Efficient 2D Dion-Jacobson Perovskite Solar Cells

Surface defects-mediated nonradiative recombination plays a critical role in the performance and stability of perovskite solar cells (PSCs) and surface post-treatment is widely used for efficient PSCs. However, the commonly used surface passivation strategies are one-off and the passivation defect ability is limited, which can only solve part of the defects in the topmost surface area. Here, a secondary anti-solvent strategy is proposed to further reduce surface defects based on conventional surface passivation for the first time. Based on this, the crystallization quality of 2D Dion-Jacobson perovskite is enhanced and the surface defects density is further reduced by nearly two orders. In addition, a gradient structure of perovskite with n = 2 phases located at the top of the film and 3D-like phases located at the bottom of the film can also be obtained. The modulated perovskite film boosts the efficiency of 2D perovskites (n = 5) up to 19.55%. This strategy is also very useful in other anti-solvent processed perovskite dipping systems, which paves a promising avenue for minimizing surface defects toward highly efficient perovskite devices.

Continuous Modification of Perovskite Film by a Eu Complex to Fabricate the Thermal and UV-Light-Stable Solar Cells

Perovskite solar cells (PSCs) with simple and low-cost processability have shown promising photovoltaic performances. However, internal defects, external UV light, and heat sensitivity are principal obstacles on their way toward commercialization. Herein, we prepare an Eu complex and directly dope it into the perovskite precursor as a UV filter to decrease the photodegradation of PSCs. The formation of hydrogen bonds between the organic cation of perovskite and the -CF₃ in the Eu complex could restrain the escape of organic cations under heating. The Eu complex acts as a redox shuttle to reduce metallic lead (Pb⁰) and iodine (I⁰) defects when the PSCs have a long-time operation. Additionally, the ligand-containing aromatic rings could reduce the trace amount of I⁰ existing as electronic defects in perovskites and together with the long alkyl chain retard the moisture immersion into the PSCs. The best efficiency of PSCs modified by the Eu complex improves up to 20.9%. The excellent thermal stability and UV-light resistance are also realized. This strategy provides a method to design a passivator which continuously modifies the imperfections and inhibits the chemical chain reactions in perovskite film, thereby enhancing the performance and stability of PSCs.

Antisolvent Additive Engineering for Boosting Performance and Stability of Graded Heterojunction Perovskite Solar Cells Using Amide-Functionalized Graphene Quantum Dots

Additive and antisolvent engineering strategies are outstandingly efficient in enhancing perovskite quality, photovoltaic performance, and stability of perovskite solar cells (PSCs). In this work, an effective approach is applied by coupling the antisolvent mixture and multi-functional additive procedures, which is recognized as antisolvent additive engineering (AAE). The graphene quantum dots functionalized with amide (AGQDs), which consists of carbonyl, amine, and long hydrophobic alkyl chain functional groups, are added to the antisolvent mixture of toluene (T) and hexane (H) as an efficient additive to form the CH3NH3PbI3 (MAPI):AGQDs graded heterojunction structure. A broad range of analytical techniques, including scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, space charge limited current, UV-visible spectroscopy, external quantum efficiency, and time-of-flight secondary ion mass spectrometry, are used to investigate the effect of AAE treatment with AGQDs on the quality of perovskite film and performance of the PSCs. Importantly, not only a uniform and dense perovskite film with hydrophobic property is obtained but also defects on the perovskite surface are significantly passivated by the interaction between AGQDs and uncoordinated Pb2+. As a result, an enhanced power conversion efficiency (PCE) of 19.10% is achieved for the champion PSCs treated with AGQD additive, compared to the PCE of 16.00% for untreated reference PSCs. In addition, the high-efficiency PSCs based on AGQDs show high stability and maintain 89% of their initial PCE after 960 h in ambient conditions.

Blue Perovskite Nanocrystal Light-Emitting Diodes: Overcoming RuddlesdenPopper Fault-Induced Nonradiative Recombination via Post-Halide Exchange

Metal halide perovskites (MHPs) have gained traction as emitters owing to their excellent optical properties, such as facile bandgap tuning, defect tolerance, and high color purity. Nevertheless, blue-emitting MHP light-emitting diodes (LEDs) show only marginal progress in device efficiency compared with green and red LEDs. Herein, the origin of the drop in efficiency of blue-emitting perovskite nanocrystals (PNCs) by mixing halides and the genesis of Ruddlesden-Popper faults (RPFs) in CsPbBr_X Cl_3-X nanocrystals is investigated. Using scanning transmission electron microscopy and density functional theory calculations, the authors have found that RPFs induce possible nonradiative recombination pathways owing to the high chloride vacancy concentration nearby. The authors further confirm that the blue-emitting PNCs do not show RPFs post-halide exchange in the CsPbBr₃ nanocrystals. By introducing the post-halide exchange treatment, high-efficiency pure blue-emitting (464 nm) PNC-based LEDs with an external quantum efficiency of 2.1% and excellent spectral stability with a full-width at half-maximum of 14 nm are obtained.

Simultaneous Lattice Engineering and Defect Control via Cadmium Incorporation for High-Performance Inorganic Perovskite Solar Cells

Doping of all-inorganic lead halide perovskites to enhance their photovoltaic performance and stability has been reported to be effective. Up to now most studies have focused on the doping of elements in to the perovskite lattice. However, most of them cannot be doped into the perovskite lattice and the roles of these dopants are still controversial. Herein,the authors introduce CdI2 as an additive into CsPbI3-x Brx and use it as active layer to fabricate high-performance inorganic perovskite solar cells (PSCs). Cd with a smaller radius than Pb can partially substitute Pb in the perovskite lattice by up to 2 mol%. Meanwhile, the remaining Cd stays on the surface and grain boundaries (GB) of the perovskite film in the form of Cs2 CdI4-x Br-x , which is found to reduce non-radiative recombination. These effects result in prolonged charge carrier lifetime, suppressed defect formation, decreased GBs, and an upward shift of energybands in the Cd-containing film. A champion efficiency of 20.8% is achieved for Cd-incorporated PSCs, together with improved device ambient stability. This work highlights the importance of simultaneous lattice engineering, defectcontrol and atomic-level characterization in achieving high-performance inorganic PSCs with well-defined structure-property relationships.

Surface Characterization of the Solution-Processed Organic-Inorganic Hybrid Perovskite Thin Films

The surface properties of organic-inorganic hybrid perovskites can strongly affect the efficiency and stability of corresponding devices. Even though different surface passivation methods are developed, the microscopic structures of solution-processed perovskite film surfaces are not systematically studied. This study uses low-temperature scanning tunneling microscopy to study the organic-inorganic hybrid perovskite thin films, MA_0.4 FA_0.6 PbI₃ and MAPbI₃ , synthesized by the spin-coating method. Flat surface structures, atomic steps, and crystal grain boundaries are resolved at an atomic resolution. The surface imperfections are also characterized, as well as the dominant defects. Simulations on different types of iodine vacancy configurations are performed by density functional theory calculations. In addition, it is observed that the surface iodine lattice structure is unstable during scanning. Tip scanning can also cause the vertical migration of surface iodine ions. The measurements provide the direct visualizations of the surface imperfections of the solution-processed perovskite films. They are essential for understanding the surface-related optoelectronic effects and rationally designing more efficient surface passivation methods.

Defect Pair Formation in FAPbI3 Perovskite Solar Cell Absorbers

Formamidinium lead iodide (FAPbI₃) based hybrid perovskite light absorbers have shown remarkable performance in recent years. Since they have unique set of optoelectronic characteristics, they are considered as a good candidate absorber material for future solar cell applications. Until recently, much research had focused on the quantitative analysis of point defects on halide-based perovskite solar cells. Studies show that understanding defect mechanisms in perovskites has a huge impact on efficiency and stability improvements; however, such mechanisms have not been fully understood yet. Here, using first-principles calculations, we investigate the possible defect pair formations in FAPbI₃, characterized by their formation energies and charge transitions. We found that while some donor and acceptor point defects are unstable and shallow when they are isolated, they form stable and deep-trap defect pairs and potentially limit the optoelectronic performance. We anticipate that our results will influence future discussions on the impact of defect formation on the performance and stability of perovskite solar cells.

Polar Species for Effective Dielectric Regulation to Achieve High-Performance CsPbI3 Solar Cells

Nonradiative losses caused by defects are the main obstacles to further advancing the efficiency and stability of perovskite solar cells (PSCs). There is focused research to boost the device performance by reducing the number of defects and deactivating defects; however, little attention is paid to the defect-capture capacity. Here, upon systematically examining the defect-capture capacity, highly polarized fluorinated species are designed to modulate the dielectric properties of the perovskite material to minimize its defect-capture radius. On the one hand, fluorinated polar species strengthen the defect dielectric-screening effect via enhancing the dielectric constant of the perovskite film, thus reducing the defect-capture radius. On the other, the fluorinated iodized salt replenishes the I-vacancy defects at the surface, hence lowering the defect density. Consequently, the power-conversion efficiency of an all-inorganic CsPbI₃ PSC is increased to as high as 20.5% with an open-circuit voltage of 1.2 V and a fill factor of 82.87%, all of which are among the highest in their respective categories. Furthermore, the fluorinated species modification also produces a hydrophobic umbrella yielding significantly improved humidity tolerance, and hence long-term stability. The present strategy provides a general approach to effectually regulate the defect-capture radius, thus enhancing the optoelectronic performance.

Ionic Liquid-Assisted Crystallization and Defect Passivation for Efficient Perovskite Solar Cells with Enhanced Open-Circuit Voltage

Perovskite materials have demonstrated many excellent properties in next-generation photovoltaic devices, but the intrinsic defects and the quality of perovskite film still limit the performance and stability of PSCs. Here, 1,3-dimethylimidazolium iodide (DMII) ionic liquid was employed as an additive to passivate the various defects and produce the high-quality perovskite film with enlarged grain sizes. DMII could act as an "ionic stabilizer" for passivating the point defects including the vacancies defects of organic cations and halogen anions of perovskite. At the same time, the extra problematic PbI₂ on surfaces and at grain boundaries of the perovskite film could also be reacted by DMII, leading to the reduction of recombination centers and trap states. On the other hand, the DMII ionic liquid with a "Ostwald ripening effect" could retard the crystallization process of perovskite crystals and yield better film quality with higher crystallinity, smoother morphology and larger grains. As a result, the optimal device achieved a champion power conversion efficiency (PCE) of 20.4 %. Particularly, the modified devices demonstrated a significant elevation in open-circuit voltage from 1.03 to 1.10 V. The hydrophobicity of perovskite films modified by DMII was enhanced and the un-encapsulated DMII devices retained 91 % of their initial PCE after aging 60 days under 15±5 % relative humidity.

Atomistic Mechanism of Surface-Defect Passivation: Toward Stable and Efficient Perovskite Solar Cells

Molecular engineering has been demonstrated to be a predominant strategy for augmenting the long-term stability and passivating adverse defects for perovskite solar cells (PSCs). Here, using density functional theory calculations combined with ab initio molecular dynamics (AIMD) simulations, the passivation effects of bidentate passivation molecules, 2-MP and 2-MDEP, on the iodine vacancy MAPbI₃ were comprehensively investigated. We demonstrate that 2-MDEP engenders stronger adsorption and localized charges on Pb atoms because the separated binding sites match with the MAPbI₃ lattice. Moreover, the activation barriers for ion migrations are improved by the passivation of 2-MP and 2-MDEP. Furthermore, AIMD simulations verify the improved structural stability and restrained nonradiative recombination after passivation. More importantly, the durable Pb-heteroatom interactions at the interface and stronger hydrophobicity endow 2-MDEP with more remarkable shielding effects against moisture compared to those of 2-MP. This work deepens our understanding of the passivation effects and paves the way for the design of passivation molecules toward the attainment of efficient and stable PSCs.

Defect-Polaron and Enormous Light-Induced Fermi-Level Shift at Halide Perovskite Surface

Halide perovskites intrinsically contain a large amount of point defects. The interaction of these defects with photocarriers, photons, and lattice distortion remains a complex and unresolved issue. We found that for halide perovskite films with excess halide vacancies, the Fermi level can be shifted by as much as 0.7 eV upon light illumination. These defects can trap photocarriers for hours after the light illumination is turned off. The enormous light-induced Fermi level shift and the prolonged electron trapping are explained by the capturing of photocarriers by halide vacancies at the surface of the perovskite film. The formation of this defect-photocarrier complex can result in lattice deformation and an energy shift in the defect state. The whole process is akin to polaron formation at a defect site. Our data also suggest that these trapped carriers increase the electrical polarizability of the lattice, presumably by enhancing the defect migration rate.