Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

The low toxicity and a near-ideal choice of bandgap make tin perovskite an attractive alternative to lead perovskite in low cost solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low-dimensional Sn perovskites that exhibit markedly enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly oriented low-dimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 h and found no apprec...

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Highly Oriented Low-Dimensional Tin Halide Perovskites with Enhanced Stability and Photovoltaic Performance

Advances in hole transport materials engineering for stable and efficient perovskite solar cells

Hybrid organic-inorganic perovskite materials garner enormous attention for a wide range of optoelectronic devices. Due to their attractive optical and electrical properties including high optical absorption coefficient, high carrier mobility, and long carrier diffusion length, perovskites have opened up a great opportunity for high performance photodetectors. This review aims to give a comprehensive summary of the significant results on perovskite-based photodetectors, focusing on the relationship among the perovskite structures, device configurations, and photodetecting performances. An introduction of recent progress in various perovskite structure-based photodetectors is provided. The emphasis is placed on the correlation between the perovskite structure and the device performance. Next, recent developments of bandgap-tunable perovskite and hybrid photodetectors built from perovskite heterostructures are highlighted. Then, effective approaches to enhance the stability of perovskite photodetector are presented, followed by the introduction of flexible and self-powered perovskite photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. A comprehensive summary of the research status on perovskite photodetectors is hoped to push forward the development of this field.

Hybrid Organic-Inorganic Perovskite Photodetectors.

Hybrid organic-inorganic perovskite photovoltaics (PSCs) have attracted significant attention during the past decade. Despite the stellar rise of laboratory-scale PSC devices, which have reached a certified efficiency over 25% to date, there is still a large efficiency gap when transiting from small-area devices to large-area solar modules. Efficiency losses would inevitably arise from the great challenges of homogeneous coating of large-area high quality perovskite films. To address this problem, we provide an in-depth understanding of the perovskite nucleation and crystal growth kinetics, including the LaMer and Ostwald ripening models, which advises us that fast nucleation and slow crystallization are essential factors in forming high-quality perovskite films. Based on these cognitions, a variety of thin film engineering approaches will be introduced, including the anti-solvent, gas-assisted and solvent annealing treatments, Lewis acid-base adduct incorporation, etc., which are able to regulate the nucleation and crystallization steps. Upscaling the photovoltaic devices is the following step. We summarize the currently developed scalable deposition technologies, including spray coating, slot-die coating, doctor blading, inkjet printing and vapour-assisted deposition. These are more appealing approaches for scalable fabrication of perovskite films than the spin coating method, in terms of lower material/solution waste, more homogeneous thin film coating over a large area, and better morphological control of the film. The working principles of these techniques will be provided, which direct us that the physical properties of the precursor solutions and surface characteristics/temperature of the substrate are both dominating factors influencing the film morphology. Optimization of the perovskite crystallization and film formation process will be subsequently summarized from these aspects. Additionally, we also highlight the significance of perovskite stability, as it is the last puzzle to realize the practical applications of PSCs. Recent efforts towards improving the stability of PSC devices to environmental factors are discussed in this part. In general, this review, comprising the mechanistic analysis of perovskite film formation, thin film engineering, scalable deposition technologies and device stability, provides a comprehensive overview of the current challenges and opportunities in the field of PSCs, aiming to promote the future development of cost-effective up-scale fabrication of highly efficient and ultra-stable PSCs for practical applications.

Understanding of perovskite crystal growth and film formation in scalable deposition processes

We demonstrate the application of a low-temperature carbon counter electrode with good flexibility and high conductivity in fabricating perovskite solar cells. A modified two-step method was used for the deposition of nanocrystalline CH3NH3PbI3 under high relative humidity. The carbon counter electrode was printed on a perovskite layer directly, with different sizes of graphite powder being employed. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. We find that the existence of nano-graphite powder in the electrode has a noticeable influence on the back contact and cell performance. The prepared devices of hole-conductor-free perovskite heterojunction solar cells without encapsulation exhibit advantageous stability in air in the dark, with the optimal power conversion efficiency reaching 6.88%. This carbon counter electrode has the features of low-cost and low-temperature preparation, giving it potential for application in the large-scale flexible fabrication of perovskite solar cells in the future.

Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity

We report the encapsulation of low temperature carbon counter electrode based hole-conductor-free mesoscopic methylammonium lead iodide perovskite/TiO2 heterojunction solar cells with polydimethylsiloxane. The solar cells demonstrate improved photovoltaic performance, where we obtain an optimal short-circuit photocurrent JSC = 23.5 mA cm−2, open-circuit photovoltage VOC = 0.97 V, and fill factor FF = 0.474, corresponding to a light to electric power conversion efficiency of 10.8% under a standard AM 1.5 solar light of 100 mW cm−2 intensity. The results exhibit a remarkable 54% enhancement over those without encapsulation. The cross-sectional SEM images indicate that the PDMS layer can condense the carbon electrode by filling the gaps in the mesoscopic carbon film during the solidification of PDMS, resulting in an improved MAPbI3/carbon interface condition. The photoluminescence and electrical impedance spectroscopy measurements prove that the improved efficiency is ascribed to a more efficient charge transfer process and slower charge recombination between interfaces. In addition, the robust polydimethylsiloxane isolates water in air, avoiding the degradation of the CH3NH3PbI3 perovskite and leading to an impressive stability during a testing period of 3000 h. Our work paves the way for realizing low cost, highly efficient and stable hybrid photovoltaic cells.

Enhanced photovoltaic performance and stability of carbon counter electrode based perovskite solar cells encapsulated by PDMS

Different nanostructures of copper oxide (CuO) by in-situ growth on carbon clothes (CC) are prepared to develop ultrasensitive non-enzymatic glucose sensors. The electrochemical performance of the CuO-based electrodes for detecting glucose has been investigated by cyclic voltammetry (CV) and chronoamperometry. The CuO nanosheets (CuO NSs)/CC electrode demonstrates a high sensitivity of 4902 μA mM−1 cm−2 at an applied potential of 0.55 V (vs. Ag/AgCl/3 M KCl) in alkaline solution, and it shows 2973 μA mM−1 cm−2 and 1246 μA mM−1 cm−2 for the CuO nanowires (CuO NWs)/CC and CuO nanoparticles (CuO NPs)/CC, respectively. Ascribing to high conductivity of CC, high specific surface-area from CuO nanostructures, and facilitated charge transfer through in-situ grown structure, the electrodes demonstrate ultrasensitive, selective, stable and fast amperometric sensing (<3 s) of glucose, which presents a new strategy to develop non-enzymatic glucose sensors.

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Ultrasensitive non-enzymatic glucose sensors based on different copper oxide nanostructures by in-situ growth

Power packs integrating both photovoltaic parts and energy storage parts have gained great scientific and technological attention due to the increasing demand for green energy and the tendency for miniaturization and multifunctionalization in electronics industry. In this study, we demonstrate novel integration of perovskite solar cell and solid-state supercapacitor for power packs. The perovskite solar cell is integrated with the supercapacitor based on common carbon electrodes to hybridize photoelectric conversion and energy storage. The power pack achieves a voltage of 0.84 V when the supercapacitor is charged by the perovskite solar cell under the AM 1.5G white light illumination with a 0.071 cm2 active area, reaching an energy storage proportion of 76% and an overall conversion efficiency of 5.26%. When the supercapacitor is precharged at 1.0 V, an instant overall output efficiency of 22.9% can be achieved if the perovskite solar cell and supercapacitor are connected in series, exhibiting great poten...

Liu Zhiyong

Papers

Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity

Enhanced photovoltaic performance and stability of carbon counter electrode based perovskite solar cells encapsulated by PDMS

Ultrasensitive non-enzymatic glucose sensors based on different copper oxide nanostructures by in-situ growth

Novel Integration of Perovskite Solar Cell and Supercapacitor Based on Carbon Electrode for Hybridizing Energy Conversion and Storage

17.78% efficient low-temperature carbon-based planar perovskite solar cells using Zn-doped SnO2 electron transport layer