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 conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT), and especially its complex with poly(styrene sulfonate) (PEDOT:PSS), is perhaps the most well-known example of an organic conductor. It is highly conductive, largely transmissive to light, processible in water, and highly flexible. Much recent work on this ubiquitous material has been devoted to increasing its deformability beyond flexibility-a characteristic possessed by any material that is sufficiently thin-toward stretchability, a characteristic that requires engineering of the structure at the molecular- or nanoscale. Stretchability is the enabling characteristic of a range of applications envisioned for PEDOT in energy and healthcare, such as wearable, implantable, and large-area electronic devices. High degrees of mechanical deformability allow intimate contact with biological tissues and solution-processable printing techniques (e.g., roll-to-roll printing). PEDOT:PSS, however, is only stretchable up to around 10%. Here, the strategies that have been reported to enhance the stretchability of conductive polymers and composites based on PEDOT and PEDOT:PSS are highlighted. These strategies include blending with plasticizers or polymers, deposition on elastomers, formation of fibers and gels, and the use of intrinsically stretchable scaffolds for the polymerization of PEDOT.

Stretchable Conductive Polymers and Composites Based on PEDOT and PEDOT:PSS.

Substantial effort has been devoted to both scientific and technological developments of wearable, flexible, semitransparent, and sensing electronics (e.g., organic/perovskite photovoltaics, organic thin-film transistors, and medical sensors) in the past decade. The key to realizing those functionalities is essentially the fabrication of conductive electrodes with desirable mechanical properties. Conductive polymers (CPs) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have emerged to be the most promising flexible electrode materials over rigid metallic oxides and play a critical role in these unprecedented devices as transparent electrodes, hole transport layers, interconnectors, electroactive layers, or motion-sensing conductors. Here, the current status of research on PEDOT:PSS is summarized including various approaches to boosting the electrical conductivity and mechanical compliance and stability, directly linked to the underlying mechanism of the performance enhancements. Along with the basic principles, the most cutting edge-progresses in devices with PEDOT:PSS are highlighted. Meanwhile, the advantages and plausible problems of the CPs and as-fabricated devices are pointed out. Finally, new perspectives are given for CP modifications and device fabrications. This work stresses the importance of developing CP films and reveals their critical role in the evolution of these next-generation devices featuring wearable, deformable, printable, ultrathin, and see-through characteristics.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201900813

PEDOT:PSS for Flexible and Stretchable Electronics: Modifications, Strategies, and Applications

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations.

Semi-transparent perovskite solar cells are highly attractive for a wide range of applications, such as bifacial and tandem solar cells; however, the power conversion efficiency of semi-transparent devices still lags behind due to missing suitable transparent rear electrode or deposition process. Here we report a low-temperature process for efficient semi-transparent planar perovskite solar cells. A hybrid thermal evaporation-spin coating technique is developed to allow the introduction of PCBM in regular device configuration, which facilitates the growth of high-quality absorber, resulting in hysteresis-free devices. We employ high-mobility hydrogenated indium oxide as transparent rear electrode by room-temperature radio-frequency magnetron sputtering, yielding a semi-transparent solar cell with steady-state efficiency of 14.2% along with 72% average transmittance in the near-infrared region. With such semi-transparent devices, we show a substantial power enhancement when operating as bifacial solar cell, and in combination with low-bandgap copper indium gallium diselenide we further demonstrate 20.5% efficiency in four-terminal tandem configuration.

/pdf/low-temperature-processed-efficient-semi-transparent-planar-3guzujd8r1.pdf

Low-temperature-processed efficient semi-transparent planar perovskite solar cells for bifacial and tandem applications

Thick, uniform, easily processed, highly conductive polymer films are desirable as electrodes for solar cells as well as polymer capacitors. Here, a novel scalable strategy is developed to prepare highly conductive thick poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (HCT-PEDOT:PSS) films with layered structure that display a conductivity of 1400 S cm−1 and a low sheet resistance of 0.59 ohm sq−1. Organic solar cells with laminated HCT-PEDOT:PSS exhibit a performance comparable to the reference devices with vacuum-deposited Ag top electrodes. More importantly, the HCT-PEDOT:PSS film delivers a specific capacitance of 120 F g−1 at a current density of 0.4 A g−1. All-solid-state flexible symmetric supercapacitors with the HCT-PEDOT:PSS films display a high volumetric energy density of 6.80 mWh cm−3 at a power density of 100 mW cm−3 and 3.15 mWh cm−3 at a very high power density of 16160 mW cm−3 that outperforms previous reported solid-state supercapacitors based on PEDOT materials.

Free-Standing Conducting Polymer Films for High-Performance Energy Devices.

A low-cost, light-weight and flexible power supply is highly desirable for portable electronic devices. All-plastic solar cells in which all the layers are fabricated sequentially from organic synthetic inks could meet these requirements. Here, we report that fully solution-processed all-plastic multijunction solar cells can be easily fabricated layer by layer without the need for sophisticated patterning. The key for the high-yield fully solution-processed multijunction cells is the control and tuning of the conductivity of the charge-recombination layer of PEDOT:PSS/PEI. The all-plastic multijunction solar cells achieve a PCE of 6.1 ± 0.4% and a high open-circuit voltage of 5.37 V. These all-plastic multijunction solar cells are successfully used to drive liquid-crystal displays, full-color light-emitting diodes and for water splitting under different light illumination conditions.

Flexible all-solution-processed all-plastic multijunction solar cells for powering electronic devices

Polyethylenimine (PEI) has been widely used to produce low-work-function electrodes. Generally, PEI modification is prepared by spin coating from 2-methoxyethanol solution. In this work, we explore the method for PEI modification on indium tin oxide (ITO) by dipping the ITO sample into PEI aqueous solution for organic solar cells. The PEI prepared in this method could reduce the work function of ITO as effectively as PEI prepared by spin coating from 2-methoxyethanol solution. H2O as the processing solvent is more environmentally friendly and much cheaper compared to the 2-methoxyethanol solvent. The dipping method is also compatible with large-area samples. With low-work-function ITO treated by the dipping method, solar cells with a simple structure of glass/ITO/PEI(dipping)/P3HT:ICBA/PEDOT:PSS(vacuum-free processing) display a high open-circuit voltage of 0.86 ± 0.01, a high fill factor of 66 ± 2%, and power conversion efficiency of 4.4 ± 0.3% under 100 mW/cm2 illumination.

Polyethylenimine Aqueous Solution: A Low-Cost and Environmentally Friendly Formulation to Produce Low-Work-Function Electrodes for Efficient Easy-to-Fabricate Organic Solar Cells

We report perovskite solar cells with a new device structure that employ highly conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the top electrode replacing commonly used metal electrodes. The PEDOT:PSS top electrode is prepared from its aqueous solution through a transfer-lamination technique rather than direct spin-coating, which converts the CH3NH3PbI3 into PbI2. Perovskite solar cells with the structure of glass/FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD/PEDOT:PSS yield a maximum open-circuit voltage (VOC) of 1.02 V, and a maximum power conversion efficiency (PCE) of 11.29% under AM1.5 100 mW/cm2 illumination. The whole device was fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of perovskite solar cells.

Metal electrode-free perovskite solar cells with transfer-laminated conducting polymer electrode

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Free-Standing Conducting Polymer Films for High-Performance Energy Devices.

Flexible all-solution-processed all-plastic multijunction solar cells for powering electronic devices

Polyethylenimine Aqueous Solution: A Low-Cost and Environmentally Friendly Formulation to Produce Low-Work-Function Electrodes for Efficient Easy-to-Fabricate Organic Solar Cells

Metal electrode-free perovskite solar cells with transfer-laminated conducting polymer electrode