In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites ...(MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film‐growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.
Watching the defects: Defects play a pivotal role in the overall performance of perovskite solar cells. This Review focuses on central questions of “what defects exist in metal halide perovskites” and “how can one reduce detrimental defects towards high‐performance perovskite solar cells”.
The current challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. For instance, efficient generation ...of charges, extraction, and transport with minimum recombination through interlayer interfaces is crucial to attain high-efficiency solar cell devices. Furthermore, intralayer interfaces may be present in the form of grain boundaries within a film composed of the same material, for example, a polycrystalline perovskite layer. The adjacent grains may assume different crystal orientations and/or have different chemical compositions, which impacts charge excitation and dynamics and thereby the overall solar cell performance. In this Perspective, we present case studies to demonstrate (1) how surfaces and interfaces can impact material properties and device performance and (2) how these issues can be investigated by surface science techniques, such as scanning probe microscopy, photoelectron spectroscopy, and so forth. We end this Perspective by outlining the future research directions based on the reported results as well as the new trends in the field.
Abstract
Rational design of heterostructures opens up new opportunities as an ideal catalyst system for lithium polysulfides conversion in lithium-sulfur battery. However, its traditional fabrication ...process is complex, which makes it difficult to reasonably control the content and distribution of each component. In this work, to rationally design the heterostructure, the atomic layer deposition is utilized to hybridize the TiO
2
-TiN heterostructure with the three-dimensional carbon nanotube sponge. Through optimizing the deposited thickness of TiO
2
and TiN layers and adopting the annealing post-treatment, the derived coaxial sponge with uniform TiN-TiO
2
heterostructure exhibits the best catalytic ability. The corresponding lithium-sulfur battery shows enhanced electrochemical performance with high specific capacity of 1289 mAh g
−1
at 1 C and capacity retention of 85% after 500 cycles at 2 C. Furthermore, benefiting from the highly porous structure and interconnected conductive pathways from the sponge, its areal capacity reaches up to 21.5 mAh cm
−2
.
Organic–inorganic halide perovskite materials (e.g., MAPbI3, FAPbI3, etc.; where MA = CH3NH3 +, FA = CH(NH2)2 +) have been studied intensively for photovoltaic applications. Major concerns for the ...commercialization of perovskite photovoltaic technology to take off include lead toxicity, long-term stability, hysteresis, and optimal bandgap. Therefore, there is still need for further exploration of alternative candidates. Elemental composition engineering of MAPbI3 and FAPbI3 has been proposed to address the above concerns. Among the best six certified power conversion efficiencies reported by National Renewable Energy Laboratory on perovskite-based solar cells, five are based on mixed perovskites (e.g., MAPbI1–x Br x , FA0.85MA0.15PbI2.55Br0.45, Cs0.1FA0.75MA0.15PbI2.49Br0.51). In this paper, we review the recent progress on the synthesis and fundamental aspects of mixed cation and halide perovskites correlating with device performance, long-term stability, and hysteresis. In the outlook, we outline the future research directions based on the reported results as well as related topics that warrant further investigation.
Lithium-ion batteries (LIBs) continue to draw vast attention as a promising energy storage technology due to their high energy density, low self-discharge property, nearly zero-memory effect, high ...open circuit voltage, and long lifespan. In particular, high-energy density lithium-ion batteries are considered as the ideal power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the automotive industry, in recent years. This review discusses key aspects of the present and the future battery technologies on the basis of the working electrode. We then discuss how lithium-ion batteries evolve to meet the growing demand on high charge capacity and electrode stability. An account of a stand-alone energy device (off-grid system) that combines an energy harvesting technology with a lithium-ion battery is also provided. The main discussion is categorized into three perspectives such as the evolution from the conventional to the advanced LIBs (
e.g.
, Li-rich transition metal oxide and Ni-rich transition metal oxide batteries), to the state-of-the-art LIBs (
e.g.
, Li-air, Li-sulfur batteries, organic electrode batteries, solid-state batteries, and Li-CO
2
batteries), and to the hybridized LIBs (
e.g.
, metal halide perovskite batteries).
Key insights into the evolution of lithium-ion batteries: present, future, and hybridized technologies.
There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low ...efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn
Ge
I
) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N
atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.
2,2′,7,7′‐tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐MeOTAD) hole transport material (HTM) is a milestone in the history of perovskite solar cells (PSCs). Proper choice of ...HTMs is key factor for efficient charge extraction and stability in solar cells. Spiro‐MeOTAD is proven to be the most suitable HTM for testing PSCs due to its facile implementation and high performance. Similarly, spiro‐MeOTAD is receiving attention in other applications other than in solar cells due to its desirable properties. However, spiro‐MeOTAD is under debate regarding the topics of cost‐performance, long‐term stability, degradation issues (induced by temperature, additives, film quality, and environmental conditions), coating technologies compatibility, reliance on additives, and hysteresis. In this review, the advent of spiro‐MeOTAD, and related aforementioned issues about spiro‐MeOTAD are discussed. In addition, spiro‐MeOTAD properties, alternative and new additives, other applications, and new HTMs that is comparable or outperforms spiro‐MeOTAD in PSCs are summarized. In the outlook, the future research directions based on reported results that warrant further investigations are outlined.
Spiro‐MeOTAD is the most commonly used hole transport material for perovskite solar cells. In this article, various aspects of spiro‐MeOTAD including its properties, additives, stability, alternatives, its applications in perovskite solar cells as well as other applications are reviewed.
Lightweight and flexible energy storage devices are urgently needed to persistently power wearable devices, and lithium-sulfur batteries are promising technologies due to their low mass densities and ...high theoretical capacities. Here we report a flexible and high-energy lithium-sulfur full battery device with only 100% oversized lithium, enabled by rationally designed copper-coated and nickel-coated carbon fabrics as excellent hosts for lithium and sulfur, respectively. These metallic carbon fabrics endow mechanical flexibility, reduce local current density of the electrodes, and, more importantly, significantly stabilize the electrode materials to reach remarkable Coulombic efficiency of >99.89% for a lithium anode and >99.82% for a sulfur cathode over 400 half-cell charge-discharge cycles. Consequently, the assembled lithium-sulfur full battery provides high areal capacity (3 mA h cm
), high cell energy density (288 W h kg
and 360 W h L
), excellent cycling stability (260 cycles), and remarkable bending stability at a small radius of curvature (<1 mm).
The thermal stability and decomposition products of formamidinium, a widely used organic cation in perovskite solar cell formulation, were investigated. The thermal degradation experiments of ...formamidinium-based perovskites and their halide precursors were carried out under helium atmosphere and vacuum at a constant heating rate of 20 °C min −1 . In addition, pulsed heating steps were employed under illumination/dark conditions to simulate a more realistic working temperature condition for photovoltaic devices. The identification of gas decomposition products was based on the quadrupole mass spectrometry technique. The released amounts of sym -triazine, formamidine, and hydrogen cyanide (HCN) were observed to highly depend on the temperature. For the experimental conditions used in this study, sym -triazine was obtained as the thermal product of degradation at temperatures above 95 °C. Below this temperature, only formamidine and HCN generation routes were observed. The energy pathways of formamidinium thermal degradation under photovoltaic working temperature conditions were further assessed by density functional theory calculations. The results indicated that formamidinium was more resilient to thermal degradation and the release of irreversible decomposition products compared to methylammonium because of a larger enthalpy and activation energy obtained for the decomposition reactions. The HCN instantaneous concentration observed during the low temperature heating tests and the estimations of the maximum release of HCN achievable per meter-square of an FA based perovskite based solar cell were compared to acute exposure guideline levels of airborne HCN concentration.
Hybrid lead halide perovskites have emerged as promising active materials for photovoltaic cells. Although superb efficiencies have been achieved, it is widely recognized that long-term stability is ...a key challenge intimately determining the future development of perovskite-based photovoltaic technology. Herein, we present reversible and irreversible photodecomposition reactions of methylammonium lead iodide (MAPbI 3 ). Simulated sunlight irradiation and temperature (40–80 °C) corresponding to solar cell working conditions lead to three degradation pathways: (1) CH 3 NH 2 + HI (identified as the reversible path), (2) NH 3 + CH 3 I (the irreversible or detrimental path), and (3) a reversible Pb(0) + I 2 (g) photodecomposition reaction. If only the reversible reactions (1) and (3) take place and reaction (2) can be avoided, encapsulated MAPbI 3 can be regenerated during the off-illumination timeframe. Therefore, to further improve operational stability in hybrid perovskite solar cells, detailed understanding of how to mitigate photodegradation and thermal degradation processes is necessary. First, encapsulation of the device is necessary not only to avoid contact of the perovskite with ambient air, but also to prevent leakage of volatile products released from the perovskite. Second, careful selection of the organic cations in the compositional formula of the perovskite is necessary to avoid irreversible reactions. Third, selective contacts must be as chemically inert as possible toward the volatile released products. Finally, hybrid halide perovskite materials are speculated to undergo a dynamic formation and decomposition process; this can gradually decrease the crystalline grain size of the perovskite with time; therefore, efforts to deposit highly crystalline perovskites with large crystal sizes may fail to increase the long-term stability of photovoltaic devices.
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