Abnormal Synergetic Effect of Organic and Halide Ions on the Stability and Optoelectronic Properties of a Mixed Perovskite via In Situ Characterizations

Insight into ion-induced stability degradation in all-perovskite tandem photovoltaics: quantitative characterization and effective manipulation strategies

Ion-induced stability degradation is a critical factor limiting the power conversion efficiency and commercialization potential of perovskite-based solar cells. To mitigate ion migration in perovskite-based devices, various strategies, including defect passivation and ion optimization, have been extensively investigated from the device fabrication and integration perspectives, which, however, are often costly and may negatively impact device efficiency. Notably, all-perovskite tandem solar cells (TSCs), which exhibit more complex ion dynamics compared to single-junction devices, remain poorly understood. In this study, we employ comprehensive photoelectric coupling simulations in conjunction with a self-assembled light source compensation setup to elucidate ion dynamics in all-perovskite TSCs. Specifically, the hysteresis behavior at a defined scan rate is used to quantitatively characterize the extent of ion migration. Our findings reveal that enhanced ion migration occurs in current-limited sub-cells, whereas ion migration in higher-current sub-cells is attenuated due to differential voltage assignments. This study provides valuable insights into the device physics of ion migration in TSCs, facilitating effective control of ion behavior and offering essential guidance for the design of highly efficient and stable TSCs.

Amphipathic Astaxanthin Additive for Low Voltage-loss Perovskite Solar Cells With Enhanced Quasi-Fermi Level Splitting and Solar Hydrogen Production Application

Even though the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is nearly approaching the Schottky-Queisser limit, low open-circuit voltage (Voc) and severe Voc loss problems continue to impede the improvement of PCEs. Astaxanthin (ASTA) additive is introduced in the formamidinium lead triiodide (FAPbI3) perovskite film as an additive, which can facilitate the transportation of charge carriers and interact with Pb2+ by its distinctive groupings. Furthermore, the addition of ASTA decreases the defect's active energy, regulates the deep-level defect by filling up the grain boundaries (GBs), and promotes the crystallization of perovskite film. Remarkably, an enhanced quasi-Fermi level splitting (QFLS) of 1.164 eV and a reduced Voc loss of only 96 mV are realized. The champion PCE of 24.56% is attained by ASTA-modified PSCs on the basis of 22.75% PCE. Moreover, the PSCs that underwent ASTA modification demonstrate improved operational stability, ensuring consistent output in real-world scenarios. Furthermore, PSCs with an active area of 1 cm2 are used for water electrolysis to produce hydrogen and exhibit a PCE of 22.41%. This work offers an environmentally benign solution to address the inherent issues of FAPbI3 PSCs and lays the groundwork for the development of a prospective solar hydrogen production application.

Investigation of perovskite materials for solar cells using scanning tunneling microscopy

The issue of energy scarcity has become more prominent due to the recent scientific and technological advancements. Consequently, there is an urgent need for research on sustainable and renewable resources. Solar energy, in particular, has emerged as a highly promising option because of its pollution-free and environment-friendly characteristics. Among the various solar energy technologies, perovskite solar cells have attracted much attention due to their lower cost and higher photoelectric conversion efficiency (PCE). However, the inherent instability of perovskite materials hinders the commercialization of such devices. The utilization of scanning tunneling microscopy/spectroscopy (STM/STS) can provide valuable insights into the fundamental properties of different perovskite materials at the atomic scale, which is crucial for addressing this challenge. In this review, we present the recent research progress of STM/STS analysis applied to various perovskites for solar cells, including halide perovskites, two-dimensional Ruddlesden-Popper perovskites, and oxide perovskites. This comprehensive overview aims to inspire new ideas and strategies for optimizing solar cells.

Bromine Incorporation Affects Phase Transformations and Thermal Stability of Lead Halide Perovskites

Mixed-cation and mixed-halide lead halide perovskites show great potential for their application in photovoltaics. Many of the high-performance compositions are made of cesium, formamidinium, lead, iodine, and bromine. However, incorporating bromine in iodine-rich compositions and its effects on the thermal stability of the perovskite structure has not been thoroughly studied. In this work, we study how replacing iodine with bromine in the state-of-the-art Cs0.17FA0.83PbI3 perovskite composition leads to different dynamics in the phase transformations as a function of temperature. Through a combination of structural characterization, cathodoluminescence mapping, X-ray photoelectron spectroscopy, and first-principles calculations, we reveal that the incorporation of bromine reduces the thermodynamic phase stability of the films and shifts the products of phase transformations. Our results suggest that bromine-driven vacancy formation during high temperature exposure leads to irreversible transformations into PbI2, whereas materials with only iodine go through transformations into hexagonal polytypes, such as the 4H-FAPbI3 phase. This work sheds light on the structural impacts of adding bromine on thermodynamic phase stability and provides new insights into the importance of understanding the complexity of phase transformations and secondary phases in mixed-cation and mixed-halide systems.

In Situ and Operando Characterization Techniques in Stability Study of Perovskite-Based Devices

Metal halide perovskite materials have demonstrated significant potential in various optoelectronic applications, such as photovoltaics, light emitting diodes, photodetectors, and lasers. However, the stability issues of perovskite materials continue to impede their widespread use. Many studies have attempted to understand the complex degradation mechanism and dynamics of these materials. Among them, in situ and/or operando approaches have provided remarkable insights into the degradation process by enabling precise control of degradation parameters and real-time monitoring. In this review, we focus on these studies utilizing in situ and operando approaches and demonstrate how these techniques have contributed to reveal degradation details, including structural, compositional, morphological, and other changes. We explore why these two approaches are necessary in the study of perovskite degradation and how they can be achieved by upgrading the corresponding ex situ techniques. With recent stability improvements of halide perovskite using various methods (compositional engineering, surface engineering, and structural engineering), the degradation of halide perovskite materials is greatly retarded. However, these improvements may turn into new challenges during the investigation into the retarded degradation process. Therefore, we also highlight the importance of enhancing the sensitivity and probing range of current in situ and operando approaches to address this issue. Finally, we identify the challenges and future directions of in situ and operando approaches in the stability research of halide perovskites. We believe that the advancement of in situ and operando techniques will be crucial in supporting the journey toward enhanced perovskite stability.

25.24%-Efficiency FACsPbI3 Perovskite Solar Cells Enabled by Intermolecular Esterification Reaction of DL-Carnitine Hydrochloride

Judicious tailoring of the interface between the SnO₂ electron-transport layer and the perovskite buried surface plays a pivotal role in obtaining highly efficient and stable perovskite solar cells (PSCs). Herein, a DL-carnitine hydrochloride (DL) is incorporated into the perovskite/SnO₂ interface to suppress the defect-states density. A DL-dimer is obtained at the interface by an intermolecular esterification reaction. For the SnO₂ film, the Cl^- in the DL-dimer can passivate oxygen vacancies (V_O ) through electrostatic coupling, while the N in the DL-dimer can coordinate with the Sn⁴⁺ to passivate Sn-related defects. For the perovskite film, the DL-dimer can passivate FA⁺ defects via hydrogen bonding and Pb-related defects more efficiently than the DL monomer. Upon DL-dimer modification, the interfacial defects are effectively passivated and the quality of the resultant perovskite film is improved. As a result, the DL-treated device achieves a gratifying open-circuit voltage (V_OC ) of 1.20 V and a champion power conversion efficiency (PCE) of 25.24%, which is a record value among all the reported FACsPbI₃ PSCs to date. In addition, the unencapsulated devices exhibit a charming stability, sustaining 99.20% and 90.00% of their initial PCEs after aging in air for 1200 h and continuously operating at the maximum power point tracking for 500 h, respectively.

Hydrogen Bonds in Precursor Solution: The Origin of the Anomalous J–V Curves in Perovskite Solar Cells

Perovskite Solar Cells are a promising solar energy harvesting technology due to their low cost and high-power conversion efficiency. A high-quality perovskite layer is fundamental for a highly efficient perovskite Solar Cell. Utilizing a gas quenching process (GQP) can eliminate the need for toxic, flammable, and expensive anti-solvents in the preparation of perovskite layers. It is a promising candidate technology for large scale preparation of perovskite layers, as it can be easily integrated in a production line by coupling up-scalable techniques. The GQP removes the need for polar solvents in the precursor solution layer by using nitrogen flow, rather than extracting them with non-polar solvents. The crystallization dynamics in this process can be significantly different. In this study, we found that the quality of perovskite crystal from GQP is much more sensitive to Lewis base molecules (LBMs) in the precursor solution than it is in anti-solvents technology. Thus, the processing parameters of the LBMs in anti-solvents technology cannot be directly transferred to the GQP. An XRD and 1H NMR study explains the origin of the S-shaped J–V curves and how these LBMs hinder the reaction between PbI2 and monovelent cations.

The Opto-Electronic Functional Devices Based on Three-Dimensional Lead Halide Perovskites

These days, opto-electronic functional devices based on three-dimensional lead halide perovskites (LHPs) are emerging. LHPs could be spin-coated to other materials, making it very convenient to combine LHPs with different categories of materials including metals, semiconductors, and polymers and achieve high-level performances. In this review, we will discuss the development in the LHP-based functional devices in recent years. After a brief presentation of the LHP’s properties, we will focus on the functional devices including lasers, photodetectors, and modulators. Then the fabrication of the LHP-based devices will be presented, which is followed by the summary and outlook.

Identifying the functional groups effect on passivating perovskite solar cells

Many organic molecules with various functional groups have been used to passivate the perovskite surface for improving the efficiency and stability of perovskite solar cell (PSCs). However, the intrinsic attributes of the passivation effect based on different chemical bonds are rarely studied. Here, we comparatively investigate the passivation effect among 12 types of functional groups on para-tert-butylbenzene for PSCs and find that the open circuit voltage (V_OC) tends to increase with the chemical bonding strength between perovskite and these passivation additive molecules. Particularly, the para-tert-butylbenzoic acid (tB-COOH), with the extra intermolecular hydrogen bonding, can stabilize the surface passivation of perovskite films exceptionally well through formation of a crystalline interlayer with water-insoluble property and high melting point. As a result, the tB-COOH device achieves a champion power conversion efficiency (PCE) of 21.46%. More importantly, such devices, which were stored in ambient air with a relative humidity of ≃45%, can retain 88% of their initial performance after a testing period of more than 1 year (10,080 h). This work provides a case study to understand chemical bonding effects on passivation of perovskite.

Recent progress in hybrid perovskite solar cells through scanning tunneling microscopy and spectroscopy

Currently, sustainable renewable energy sources are urgently required to fulfill the cumulative energy needs of the world's 7.8 billion population, since the conventional coal and fossil fuels will be exhausted soon. Photovoltaic devices are a direct and efficient means to produce a huge amount of energy to meet these energy targets. In particular, hybrid-perovskite-based photovoltaic devices merit special attention not only due to their exceptional efficiency for generating appreciable energy but also their tunable band gaps and the ease of device fabrication. However, the commercialization of such devices suffers from the instability of the compositional materials. The cause of instability is the perovskite's structure and its morphology at the sub-molecular level; thereby revealing and eliminating these instabilities are a striking challenge. To address this issue, scanning tunneling microscopy/spectroscopy (STM/STS) presents a comprehensive method to allow the visualization of the morphology and electronic structure of materials at atomic-level resolution. Here, we review the recent developments of perovskite-based solar cells (PSCs), the STM/STS analysis of photoactive halide/hybrid and oxide materials, and the real-time STM/STS investigation of electronic structures with defects and traps that are believed to mainly affect device performances. The detailed STM/STS analysis can facilitate a better understanding of the properties of materials at the nanoscale. This informative study may hold great promise to advance the development of stable PSCs under atmospheric conditions.

Template-Assisted Formation of High-Quality α-Phase HC(NH2)2PbI3 Perovskite Solar Cells

Formamidinium (FA) lead halide (α-FAPbI3) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure α-FAPbI3 phase because of its easy transformation into a nondesirable δ-FAPbI3 phase. In the present study, a "perovskite" template (MAPbI3-FAI-PbI2-DMSO) structure is used to avoid and suppress the formation of δ-FAPbI3 phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI-PbI2-DMSO film with FAI before annealing. In situ X-ray diffraction in vacuum shows no detectable δ-FAPbI3 phase during the whole synthesis process when the sample is annealed from 100 to 180 °C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much-enhanced short-circuit current density (  J sc) of 24.99 mA cm-2 and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI3, with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.

Manipulating the Mixed-Perovskite Crystallization Pathway Unveiled by In Situ GIWAXS

Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA_0.83 MA_0.17 Pb(I_0.83 Br_0.17 )₃ ) during spin-coating is revealed by in situ grazing-incidence wide-angle X-ray scattering measurements, and three phase-formation stages are identified: I) precursor solution; II) hexagonal δ-phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI-PbI₂ -DMSO intermediate phases, and perovskite α-phase. The correlated device performance and ex situ characterizations suggest the existence of an "annealing window" covering the duration of stage II. The spin-coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs⁺ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs⁺ will bypass the formation of secondary phases in stage III by promoting the formation of α-phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed-perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.