Yingzhi Jin

We report a D-A-π-A-D molecular configuration for the low-cost and efficient dopant-free hole transport materials (HTMs). The suitable energy level and surface passivation effects of DTB-FL HTM effectively inhibit recombination loss and improve charge collection property at hole extraction interface. By replacing the p-doped Spiro with DTB-FL HTM, the perovskite solar cells exhibit higher efficiencies of 21.5% (0.04 cm2 active area) and 19.6% (1.0 cm2) with superior long-term stability under different harsh environments.

The power conversion efficiencies (PCEs) of single‐junction organic solar cells have now reached over 18%. Recent progress that has been made in understanding the morphology and the device photophysics of high performing polymer:non‐fullerene acceptor blends and some of the major challenges that must be overcome to attain PCEs of over 20% are highlighted.

Abstract

The power conversion efficiencies (PCEs) of single‐junction organic solar cells (OSC) have now reached over 18%. This rapid recent progress can be attributed to the development of new nonfullerene electron acceptors (NFAs) that are paired with suitable high performing polymer electron donors. Substantial improvements in the PCEs and long‐term stability enabled by NFA OSCs have allowed the development and integration of these systems into many niche and novel applications. Here, the recent progress that has been made in understanding the device photophysics of high performing polymer:NFA blends is highlighted. As the bulk heterojunction morphology is intrinsically linked to the device photophysics, this review focuses on studies that have provided noteworthy morphological insights using advanced techniques such as solid‐state NMR and resonant soft X‐ray scattering. Through this, some of the major challenges that must be overcome to attain PCEs of over 20% in NFA OSCs are addressed.

Conformation effects of Y6‐type acceptors are systematically studied based on asymmetric design strategies. Z‐shape and W‐shape conformations‐based acceptors can help reduce energy loss in devices through significantly suppressed nonradiative energy loss. Benefiting from the high open‐circuit voltage of BP5T‐4F in the devices, ternary organic solar cells based on PM6:BP5T‐4F:CH1007 achieve a 17.2% efficiency.

Abstract

Y6, as a state‐of‐the‐art nonfullerene acceptor (NFA), is extensively optimized by modifying its side chains and terminal groups. However, the conformation effects on molecular properties and photovoltaic performance of Y6 and its derivatives have not yet been systematically studied. Herein, three Y6 analogs, namely, BP4T‐4F, BP5T‐4F, and ABP4T‐4F, are designed and synthesized. Owing to the asymmetric molecular design strategies, three representative molecular conformations for Y6‐type NFAs are obtained through regulating the lateral thiophene orientation of the fused core. It is found that conformation adjustment imposes comprehensive effects on the molecular properties in neat and blend films of these NFAs. As a result, organic solar cells (OSCs) fabricated with PM6:BP4T‐4F, PM6:BP5T‐4F, and PM6:ABP4T‐4F show high power conversion efficiency of 17.1%, 16.7%, and 15.2%, respectively. Interestingly, these NFAs with different conformations also show reduced energy loss (E _loss) in devices via gradually suppressed nonradiative E _loss. Moreover, by employing a selenium‐containing analog, CH1007, as the complementary third component, ternary OSCs based on PM6:BP5T‐4F:CH1007 (1:1.02:0.18) achieve a 17.2% efficiency. This work helps shed light on engineering the molecular conformation of NFAs to achieve high efficiency OSCs with reduced voltage loss.

A new class of polymer acceptors (P _As, P(BDT2BOY5‐X)) consisting of benzodithiophene (BDT) and non‐fullerene small molecule‐accepting units is developed, which shows excellent material compatibility with an efficient BDT‐based polymer donor (P _D). The resulting all‐polymer solar cells show excellent photovoltaic efficiency, thermal stability, and mechanical robustness at the same time, benefitting from the high chemical and molecular compatibilities between P _D and P _A.

Abstract

All‐polymer solar cells (all‐PSCs) are a highly attractive class of photovoltaics for wearable and portable electronics due to their excellent morphological and mechanical stabilities. Recently, new types of polymer acceptors (P _As) consisting of non‐fullerene small molecule acceptors (NFSMAs) with strong light absorption have been proposed to enhance the power conversion efficiency (PCE) of all‐PSCs. However, polymerization of NFSMAs often reduces entropy of mixing in PSC blends and prevents the formation of intermixed blend domains required for efficient charge generation and morphological stability. One approach to increase compatibility in these systems is to design P _As that contain the same building blocks as their polymer donor (P _D) counterparts. Here, a series of NFSMA‐based P _As [P(BDT2BOY5‐X), (X = H, F, Cl)] are reported, by copolymerizing NFSMA (Y5‐2BO) with benzodithiophene (BDT), a common donating unit in high‐performance P _Ds such as PBDB‐T. All‐PSC blends composed of PBDB‐T P _D and P(BDT2BOY5‐X) P _A show enhanced molecular compatibility, resulting in excellent morphological and electronic properties. Specifically, PBDB‐T:P(BDT2BOY5‐Cl) all‐PSC has a PCE of 11.12%, which is significantly higher than previous PBDB‐T:Y5‐2BO (7.02%) and PBDB‐T:P(NDI2OD‐T2) (6.00%) PSCs. Additionally, the increased compatibility of these all‐PSCs greatly improves their thermal stability and mechanical robustness. For example, the crack onset strain (COS) and toughness of the PBDB‐T:P(BDT2BOY5‐Cl) blend are 15.9% and 3.24 MJ m^–3, respectively, in comparison to the PBDB‐T:Y5‐2BO blends at 2.21% and 0.32 MJ m^–3.

Nonfullerene acceptors dominate organic solar cell research due to their promising high device efficiencies. However, key challenges for achieving high stability in commercially viable devices still remain. In this review, recent progress and challenges toward stable organic solar cells are discussed correlating molecular design and device engineering to device stability.

Abstract

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

Highly efficient CsPbI_1.5Br_1.5 perovskite solar cells (PSCs) are achieved via introducing fluorescein isothiocyanate (FITC) organic dye as passivator. FITC not only reduces the metal ion related trap states but also improves film crystallinity, resulting in an enhancement of device efficiency from 12.3% to 14.05%. In addition, it is demonstrated that CsPbI_1.5Br_1.5 perovskite shows the optimal halide composition for inorganic PSCs.

Abstract

All‐inorganic perovskite solar cells (PSCs) have recently received growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the thermodynamic phase instability and relatively low device efficiency pose challenges. Herein, highly efficient and stable CsPbI_1.5Br_1.5 compositional perovskite‐based inorganic PSCs are fabricated using an organic dye, fluorescein isothiocyanate (FITC), as a passivator. The carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interactions with the under‐coordinated Pb²⁺ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, perovskite films with superior optoelectronic properties, prolonged carrier lifetime, reduced trap density, and improved stability are obtained. The resulting device yields a champion efficiency of 14.05% with negligible hysteresis, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells reported thus far. In addition, FITC can be generally adopted as attractive passivator to improve the performance of CsPbI₂Br‐ and CsPbIBr₂‐based PSCs. Furthermore, with a comprehensive comparison of mixed‐halide inorganic perovskites, it is demonstrated that CsPbI_1.5Br_1.5 compositional perovskite is a promising candidate with the optimal halide composition for high‐performance inorganic PSCs.

In poly(3,4‐ethylenedioxythiophene) (PEDOT) thin films with a highly face‐on orientation, the charge transport between chains within a crystallite becomes a rate‐limiting factor, which is highly sensitive to the π–π stacking distance. Engineering π–π stacking distance in PEDOT films grown by water‐assisted oxidative chemical vapor deposition (oCVD) yields a record high electrical conductivity of 7520 ± 240 S cm⁻¹.

Abstract

Engineering the texture and nanostructure to improve the electrical conductivity of semicrystalline conjugated polymers must address the rate‐limiting step for charge carrier transport. In highly face‐on orientation, the charge transport between chains within a crystallite becomes rate‐limiting, which is highly sensitive to the π–π stacking distance and interchain charge transfer integral. Here, face‐on oriented semicrystalline poly(3,4‐ethylenedioxythiophene) (PEDOT) thin films are grown via water‐assisted (W‐A) oxidative chemical vapor deposition (oCVD). Combining W‐A with the volatile oxidant, antimony pentachloride, yields an optimized electrical conductivity of 7520 ± 240 S cm⁻¹, a record for PEDOT thin films. Systematic control of π–π stacking distance from 3.50 Å down to 3.43 Å yields an electrical conductivity enhancement of ≈1140%. The highest electrical conductivity also corresponds to minimum in Urbach energy of 205 meV, indicating superior morphological order. The figure of merit for transparent conductors, σ_dc/σ_op, reaches a maximum value of 94, which is 1.9× and 6.7× higher than oCVD PEDOT grown without W‐A and utilizing vanadium oxytrichloride and iron chloride oxidizing agents, respectively. The W‐A oCVD is single‐step all‐dry process and provides conformal coverage, allowing direct growth on mechanical flexible, rough, and structured surfaces without the need for complex and costly transfer steps.

Efficient red‐emitting aggregation‐induced emission luminogens with tunable organelle‐specific anchoring of PIZ‐CN and PID‐CN are designed and synthesized. By virtue of their large Stokes shift, excellent photostability, and low stimulated emission depletion (STED) saturation intensity, the dynamic motions of lysosomes and mitochondria are recorded with high resolution under low STED power.

Abstract

Lysosomes and mitochondria play an important role in maintaining cell homeostasis. Visualizing the long‐term activities of lysosomes and mitochondria on the nanometer scale in live cells is essential for further understanding their functions but remains challenging due to the limitations of existing fluorescent probes, such as aggregation‐caused quenching (ACQ) effect, limited signal‐to‐noise ratio from fluorescence “always on” in the process of targeting organelle and poor photobleaching resistance. Herein, two efficient red‐emitting aggregation‐induced emission (AIE) luminogens are reported, which showed “off‐on” fluorescence characteristic and specific lysosomes as well as mitochondria targeting capability. Owing to their AIE characteristics, a Stokes’ shift larger than 100 nm, good biocompatibility, and excellent photostability, the AIE luminogens have been successfully utilized for high fidelity imaging of lysosomes and mitochondria. By virtue of these two probes, stimulated emission depletion (STED) images of dynamic lysosomal fusion and mitochondrial fission with a high resolution of 65.6 nm are obtained. Furthermore, the interactions between lysosomes and mitochondria in the process of mitophagy are recorded. This study also provides practical guidance for designing specific organelle targeting probes to support live cell dynamic super‐resolution imaging.

The moisture instability and unscalable fabrication protocols are still unsolved and blocking FACs‐based perovskite solar cells’ further applications. Here, high‐quality FACsPbI₃ films are fabricated by crown ether tailoring (which chelated with Cs⁺/Pb²⁺ ions) to inhibit the moisture invasion and stabilize the α‐phase FACsPbI₃, producing large‐area perovskite films and improving solar module performance.

Abstract

FACs‐based (FA⁺, formamidinium and Cs⁺, cesium) perovskite solar cells have gained great attention due to their remarkable light and thermal stabilities toward practical application of perovskite modules. However, the moisture instability and difficulty in scalable fabrication are still the main obstacles blocking their photovoltaic applications in current status. Here, the employment of novel interaction between crown ether with metal cations is introduced to tailor the uniform growth and inhibit moisture invasion during the crystallization of α‐phase FACsPbI₃, yielding the successful synthesis of high‐quality perovskite films in a large scale. Consequently, perovskite solar cells (PSC) modules in the total area of 4 × 4 and 10 × 10 cm² are readily fabricated with respective champion efficiencies of 16.69% and 13.84% and excellent stability over 1000 h. This facile scaling‐up strategy assisted by crown ether has shown great promise for pursuing efficient and highly stable large‐area PSC modules.

Dense carbon nanotube (CNT)‐polymer composites with tailored polymer distribution are obtained by interfacial polymerization, performed in situ within CNT networks. This method is rapid and enables control of the polymer morphology, suggesting its viability for large‐scale fabrication of CNT‐polymer composites with improved mechanical properties.

Abstract

Composites of polymers and organized carbon nanotube (CNT) networks have been proposed as next‐generation lightweight structural materials, yet polymer infiltration of CNT networks often results in stress‐concentrating heterogeneities, due to local CNT aggregation or incomplete infiltration. Herein, it is demonstrated that dense CNT‐polymer composites with tailored polymer distribution can be obtained by interfacial polymerization (IP), performed in situ within CNT networks. Three regimes of the in situ interfacial polymerization (ISIP) process are identified: a reaction‐limited regime where the polymer forms beads on the CNTs; a uniformly‐filled regime with polymer throughout the CNT network; and a transport‐limited regime with polymer only near the outer surface of the network. Uniform polyamide‐CNT composite sheets obtained by this method have a Young's modulus of 31 GPa and a tensile strength of 776 MPa, which is a two‐fold increase compared to the pristine CNT sheets. Premature failure of the composites is attributed to large voids in the pristine CNT sheets, suggesting that further improved mechanical properties can be achieved with a more homogeneous CNT network. Nevertheless, the rapid rate and overall controllability of ISIP suggest its viability for formation of polymers within CNT networks via roll‐to‐roll methods.

A hierarchical MXene‐based solar‐absorbing architecture is demonstrated. The rational integration of three categories of photothermal materials enables broadband light absorption, efficient light to heat conversion, low heat loss, rapid water transportation behavior, and much improved corrosion and oxidation resistance.

Abstract

A solar‐thermal water evaporation structure that can continuously generate clean water with high efficiency and good salt rejection ability under sunlight is highly desirable for water desalination, but its realization remains challenging. Here, a hierarchical solar‐absorbing architecture is designed and fabricated, which comprises a 3D MXene microporous skeleton with vertically aligned MXene nanosheets, decorated with vertical arrays of metal–organic framework‐derived 2D carbon nanoplates embedded with cobalt nanoparticles. The rational integration of three categories of photothermal materials enables broadband light absorption, efficient light to heat conversion, low heat loss, rapid water transportation behavior, and much‐improved corrosion and oxidation resistance. Moreover, when assembling with a hydrophobic insulating layer with hydrophilic channel, the MXene‐based solar absorber can exhibit effective inhibition of salt crystallization due to the ability to advect and diffuse concentrated salt back into the water. As a result, when irradiating under one sun, the solar‐vapor conversion efficiency of the MXene‐based hierarchical design can achieve up to ≈93.4%, and can remain over 91% over 100 h to generate clean vapor for stable and continuous water desalination. This strategy opens an avenue for the development of MXene‐based solar absorbers for sustainable solar‐driven desalination.

A donor‐acceptor‐donor structure‐based iridium(III) complex is synthesized for synergistic photodynamic and photothermal therapy of cancer. The complex can be triggered with 808 nm light, generate O₂ ^−• to relieve the oxygen‐dependence, and exbibit efficient reactive oxygen species (ROS) and heat generation with ROS quantum yield of 14.6% and photothermal conversion efficiency of 27.5%.

Abstract

Iridium(III) complexes are an important group of photosensitizers for photodynamic therapy (PDT). This work constructs a donor–acceptor–donor structure‐based iridium(III) complex (IrDAD) with high reactive oxygen species (ROS) generation efficiency, negligible dark toxicity, and synergistic PDT and photothermal therapy (PTT) effect under near‐infrared (NIR) stimulation. This complex self‐assembles into metallosupramolecular aggregates with a unique aggregation‐induced PDT behavior. Compared with conventional iridium(III) photosensitizers, IrDAD not only achieves NIR light deep tissue penetration but also shows highly efficient ROS and heat generation with ROS quantum yield of 14.6% and photothermal conversion efficiency of 27.5%. After conjugation with polyethylene glycol (PEG), IrDAD is formulated to a nanoparticulate system (IrDAD‐NPs) with good solubility. In cancer phototherapy, IrDAD‐NPs preferentially accumulate in tumor area and display a significant tumor inhibition in vivo, with 96% reduction in tumor volume, and even tumor elimination.

Flexible organic solar cells (OSCs) come to the forefront of organic electronics. It's critical to develop high‐merit flexible transparent electrodes (FTEs). The work covers the frontier progress of PEDOT:PSS, graphene, metallic nanostructures, metal oxide/metal/metal oxide, Mxene and hybrid electrodes. It raises the awareness for the importance of developing the FTEs and reveals the critical role in flexible OSCs.

Abstract

Substantial effort has been devoted to both chemical doping and design of flexible transparent electrodes (FTEs) for flexible organic solar cells (OSCs) in the past decade. Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate), graphene, metal nanostructures, metal oxide/ultrathin metal/metal oxide, Mxene, and their hybrid electrodes emerge to be the most promising flexible conducting materials over indium tin oxide. The FTE fabrications play a critical role in flexible OSCs. This feature review article summarizes the current status on the researches of the FTEs including various approaches and strategies to boost the conductivity, work function, mechanical flexibility, wettability, etc, which directly affect the performances of the flexible OSCs. The most cutting edge progresses on both FTEs and flexible OSCs are highlighted along the line. Advantages and plausible issues are pointed out. Perspectives are provided that can advance the developments of the flexible OSCs. This review raises the awareness for the importance of developing plenty of FTEs and reveals their critical role in flexible OSCs.

A novel water‐surface drag coating method is developed for large‐area fabrication of high‐quality conjugated small‐molecule thin films with enhanced charge transport properties. 2,8‐Difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene thin films with millimeter‐sized single‐crystal domains and pure crystallographic orientations are realized, showing a significant enhancement (4.7 times) of carrier mobility compared to that of thin films fabricated by the conventional solution‐coating method.

Abstract

Electronic properties of organic semiconductor (OSC) thin films are largely determined by their morphologies and crystallinities. However, solution‐processed conjugated small‐molecule OSC thin films usually exhibit abundant grain boundaries and impure grain orientations because of complex fluid dynamics during solution coating. Here, a novel methodology, water‐surface drag coating, is demonstrated to fabricate high‐quality OSC thin films with greatly enhanced charge transport properties. This method utilizes the water surface to alter the evaporation dynamics of solution to enlarge the grain size, and a unique drag‐coating process to achieve the unidirectional growth of organic crystals. Using 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (Dif‐TES‐ADT) as an example, thin films with millimeter‐sized single‐crystal domains and pure crystallographic orientations are achieved, revealing a significant enhancement (4.7 times) of carrier mobility. More importantly, the resulting film can be directly transferred onto any desired flexible substrates, and flexible transistors based on the Dif‐TES‐ADT thin films show a mobility as high as 16.1 cm² V⁻¹ s⁻¹, which represents the highest mobility value for the flexible transistors reported thus far. The method is general for the growth of various high‐quality OSC thin films, thus opening up opportunities for high‐performance organic flexible electronics.

Recent advances in organic photovoltaic (OPV) materials and processing technologies are promising for transitioning of OPV devices from laboratory‐scale to large‐area industrial scale modules. Recent breakthroughs attained by development of nonfullerene acceptors have led to significant enhancement in power conversion efficiency. It is essential to elucidate degradation mechanisms of OPV devices for improving device long‐term stability.

Abstract

Organic solar cells based on bulk heterojunctions (BHJs) are attractive energy‐conversion devices that can generate electricity from absorbed sunlight by dissociating excitons and collecting charge carriers. Recent breakthroughs attained by development of nonfullerene acceptors result in significant enhancement in power conversion efficiency (PCEs) exceeding 17%. However, most of researches have focused on pursuing high efficiency of small‐area (<1 cm²) unit cells fabricated usually with spin coating. For practical application of organic photovoltaics (OPVs) from lab‐scale unit cells to industrial products, it is essential to develop efficient technologies that can extend active area of devices with minimized loss of performance and ensured operational stability. In this progress report, an overview of recent advancements in materials and processing technologies is provided for transitioning from small‐area laboratory‐scale devices to large‐area industrial scale modules. First, development of materials that satisfy requirements of high tolerability in active layer thickness and large‐area adaptability is introduced. Second, morphology control using various coating techniques in a large active area is discussed. Third, the recent research progress is also underlined for understanding mechanisms of OPV degradation and studies for improving device long‐term stability along with reliable evaluation procedures.

Multiple photoresponsive motions, excited‐state intramolecular proton transfer, and trans–cis isomerization, are combined into anil‐poly(ethylene terephthalate) systems to achieve photocontrolled function with the aid of a stretching process. The relationship between molecular structure and photodeformation property is investigated, to provide efficient strategies of molecular design for the further development of photoresponsive materials.

Abstract

With the combination of excited‐state intramolecular proton transfer and trans–cis isomerization as microscopic molecular motions under light stimulus, multiple photodeformable processes are achieved in anil‐poly(ethylene terephthalate) systems, including simple bending, dancing butterflies, and switches. The doping films can realize light‐driven contraction as large as 70% and bending angle of about 141°, upon a simple stretching process. The internal mechanism is confirmed by transient absorption spectra, and the relationship between molecular structure and photocontrolled motion is investigated by theoretical calculations and crystal analysis. This work provides a convenient approach by utilizing anils to fabricate reversible actuations with desirable geometries, greatly contributing to the applications and manufacturing of soft robots and related research.

The negatively‐doped conducting polymer poly(benzimidazobenzophenanthroline) (BBL) obtained from highly abundant elements is found to be an efficient and stable electrocatalyst of the oxygen reduction reaction. In contrast to p‐doped conducting polymers, the n‐doped BBL provides electrocatalysis that fully reduces dioxygen into water via a (2+2)‐pathway. The BBL is a promising unified electrocatalyst for polymer‐based air cathodes in low‐cost and ecofriendly fuel cells.

Abstract

The oxygen reduction reaction (ORR) limits the efficiency of oxygen‐associated energy conversion in fuel cells and air‐metal batteries. Today, expensive noble metal catalysts are often utilized to enhance the ORR and the resulting conversion efficiency in those devices. Hence, there is an intensive research to find efficient electrodes, exhibiting a favorable electronic structure, for ORR based on abundant materials that can be manufactured using low cost processes. In that context, metal‐free carbon‐based nanostructures and conducting polymers have been actively investigated. The negatively doped poly(benzimidazobenzophenanthroline) (BBL) as an efficient and stable oxygen cathode material is reported here. Compared to the benchmark p‐doped conducting polymer poly(3,4‐ethylendioxythiophene) (PEDOT), the BBL provides electrocatalysis that fully reduces dioxygen into water, via a (2 + 2)‐electron transfer pathway with hydrogen peroxide (H₂O₂) as an intermediate; while PEDOT limits the ORR to H₂O₂. It is demonstrated that n‐doped BBL is a promising air electrode material for low‐cost and ecofriendly model fuel cells, without the need of any co‐catalysts, and its performance is found to be superior to p‐doped PEDOT air electrodes.

This essay discusses the feasibility and core challenges of achieving flexible self‐powered health monitoring systems by holistically analyzing the energy consumption of the essential building blocks of the relevant systems and the energy harvesting and storage capacities of state‐of‐the‐art flexible electronic devices.

Abstract

The rapid development of flexible electronics and sensing technologies have made it possible for the detection of various human vital signs and biomarkers through an extremely light‐weight and soft device attached to the skin. To support such sensing devices for multi‐parameter tracking and continuous operation, a powerful energy supply unit, which is compact, mobile, and with high energy density and self‐charging capability, is desired. The recently emerged energy harvesting devices have demonstrated the potential of utilizing ambient and human‐based energy sources to power sensor systems. In this context, this review aims to investigate the question of whether the capacity of current flexible energy devices can meet the energy demand of wireless flexible sensor systems in long‐term health monitoring applications. First, the total energy demand of typical sensor systems is estimated by analyzing the energy consumption of each building block of the relevant systems. The design of the energy supply architecture is discussed by considering batteries/supercapacitors as the energy storage unit and photovoltaic, thermoelectric, piezoelectric, and triboelectric devices as the energy‐harvesting unit. Based on the analysis, health monitoring protocols that could be readily realized with self‐powered system designs are suggested and the core challenges for further development of the technologies for practical applications are identified.

A comprehensive review of the advances in poly(3,4‐ethylenedioxythiophene) (PEDOT) fundamentals (synthesis and doping/dedoping), properties’ tuning (transmittance, conductivity, work function, chemical reactivity), film fabrication methods to versatile photovoltaic applications (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) is presented.

Abstract

Poly(3,4‐ethylenedioxythiophene) (PEDOT) is a very unique polymer. It can be very conductive, highly transparent, and environmentally stable. It is highly switchable between its oxidation state and neutral state. It forms a micellar complex with polystyrene sulfonate in aqueous conditions, and then can be solution‐processible and printable. Based on these advantages, PEDOT has been widely used as conductors and transparent electrodes in electronics and optoelectronics. The device performance is highly correlated with the structure and properties of PEDOT. In this review, advances in the synthesis, optoelectronic and chemical properties are comprehensively described and analyzed, as well as the strategies for tuning these properties to fulfill the requirement for device applications. Film processing techniques (printing and transfer printing) for the conducting polymer are also presented. Then, the applications of PEDOT as conductors for versatile organic and perovskite solar cells (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) are summarized. Finally future study directions for PEDOT in terms of conductivity enhancement and application‐oriented formulations are discussed.

A new concept in the fabrication of high performance transparent, flexible, zero‐junction resistance silver nanowire network electrodes that avoids the need for a masking, metal etching, or metal transfer, is presented. This unconventional approach is based on extreme local modulation of the condensation coefficient of silver vapor using two composites, one of which takes the form of an electrospun nanofiber network.

Abstract

Silver nanowire networks can offer exceptionally high performance as transparent electrodes for stretchable sensors, flexible optoelectronics, and energy harvesting devices. However, this type of electrode suffers from the triple drawbacks of complexity of fabrication, instability of the nanowire junctions, and high surface roughness, which limit electrode performance and utility. Here, a new concept in the fabrication of silver nanowire electrodes is reported that simultaneously addresses all three of these drawbacks, based on an electrospun nanofiber network and supporting substrate having silver vapor condensation coefficients of one and near‐zero, respectively. Consequently, when the whole substrate is exposed to silver vapor by simple thermal evaporation, metal selectively deposits onto the nanofiber network. The advantage of this approach is the simplicity, since there is no mask, chemical or dry metal etching step, or mesh transfer step. Additionally, the contact resistance between nanowires is zero and the surface roughness is sufficiently low for integration into organic photovoltaic devices. This new concept opens the door to continuous roll‐to‐roll fabrication of high‐performance fused silver nanowire electrodes for myriad potential applications.

The T2‐CNORH organic passivation layer (OPL) is used to obtain low energy loss organic photovoltaics. The T2‐CNORH‐deposited PM6:Y6 device exhibits a power conversion efficiency (PCE) of 15.5% with low non‐radiative energy loss (0.203 eV). Furthermore, the OPL improves various photoactive layer systems with a best PCE of 16.4% for the PM6:Y7 system.

Abstract

Despite the tremendous development of various high‐performing photoactive layers in organic photovoltaic (OPVs) cells, improving their performance remains the most important challenge in the field. Here, an effective and compatible strategy (i.e., the concept of vacuum deposition of an organic passivation layer (OPL) on the photoactive layer) is presented to enhance the efficiency of the state‐of‐the‐art photoactive systems, where easy‐deposition processable T2‐ORH and T2‐CNORH OPLs are used. After the deposition process, T2‐ORH forms 2D‐like edge‐on crystalline structure, and the 3D‐like face‐on crystalline growth is induced in T2‐CNORH. Resulting from its relatively higher crystalline features and increased wettability with the cathode interfacial material, the performance of T2‐CNORH‐deposited OPVs with both small and the scaled‐up areas surpass devices without OPL and with T2‐ORH. Experimental studies are conducted linking conductivity, electroluminescence quantum efficiency, carrier transport, and recombination dynamics to find the reasons for the performance difference. Furthermore, by applying the T2‐CNORH to other photoactive platforms, the efficiencies are enhanced by 4.4–9.0% relative to those of the corresponding control devices; an optimal 16.4% efficiency is achieved, which validates its great applicability for photoactive layers that will be developed in the near future.

Ionogels of WPU and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) have high thermoelectric performance, high mechanical stretchable, and high optical transparency. They have a high mechanical stretchability of up to 156% and high ionic thermovoltage of 34.5 mV K⁻¹. Their ionic figure of merit (ZT_i) can be up to 1.3 ± 0.2.

Abstract

Stretchable electronic materials and devices have important applications in flexible electronic systems including wearable electronics and bioelectronics. Convenient electricity generation such as thermoelectric conversion is required for the flexible electronic systems. Hence, it is development of high‐performance thermoelectric materials with high mechanical stretchability would be highly desirable. Here, stretchable and transparent ionogels with high thermoelectric properties are demonstrated. The ionogels made of elastomeric waterborne polyurethane and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) are prepared by solution processing. Their mechanical and electrical properties depend on the loading of EMIM:DCA. The ionogels with 40 wt% EMIM:DCA can have a high mechanical stretchability of up to 156%, low tensile strength of 0.6 MPa, and low Young's modulus of 0.6 MPa. They also exhibit a high ionic thermovoltage of 34.5 mV K⁻¹, high ionic conductivity of 8.4 mS cm⁻¹ and low thermal conductivity of 0.23 W m⁻¹ K⁻¹ at a relative humidity of 90%. As a result, it can have a high ionic figure of merit (ZT_i) of 1.3 ± 0.2. Both the thermovoltage and the ZT_i value are the highest for stretchable thermoelectric materials. They can be used in ionic thermoelectric capacitors to convert heat into electricity.