Challenges and Emerging Opportunities in High‐Mobility and Low‐Energy‐Consumption Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) are the basic elements of organic circuits towards flexible, printable, and wearable electronics. Low‐energy‐consumption OFETs with high mobility are the prerequisite for practical applications. After 30 years of development, OFETs have progressed rapidly, from field‐effect materials to devices, and from individual device to small‐ and medium‐scale integration. Here, a brief retrospective of OFETs' development over the past decades, and the emerging opportunities and challenges from device physics, multifunctional materials to integrated application are presented.     

Lateral Organic Solar Cells with Self‐Assembled Semiconductor Nanowires

Solution‐processable organic semiconductor nanowires (NWs) offer a potentially powerful strategy for producing large‐area printed flexible devices. Here, the fabrication of lateral organic solar cells (LOSC) using solution‐processed organic NW blends on a flexible substrate to produce a power source for use in flexible integrated microelectronics is reported. A high photocarrier generation and an efficient charge sweep out are achieved by incorporating 1D self‐assembled poly(3‐hexylthiophene) NWs into the active layer, and an MoO₃ interfacial layer with high work function is introduced to increase the built‐in potential. These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the channel. The utility of the LOSCs for high power source applications is demonstrated by using interdigitated electrode patterns that consist of multiple devices connected in parallel or in series. High photovoltage‐producing LOSC modules on plastic substrates for use in flexible optoelectronic devices are successfully fabricated. The LOSCs described here offer a new device architecture for use in highly flexible photoresponsive energy devices.     

A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.     

Plasmonic Backscattering Effect in High‐Efficient Organic Photovoltaic Devices

A universal strategy for efficient light trapping through the incorporation of gold nanorods on the electron transport layer (rear) of organic photovoltaic devices is demonstrated. Utilizing the photons that are transmitted through the active layer of a bulk heterojunction photovoltaic device and would otherwise be lost, a significant enhancement in power conversion efficiency (PCE) of poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:phenyl‐C₇₁‐butyric acid methyl ester (PCDTBT:PC₇₁BM) and poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b] thiophenediyl]] (PTB7):PC₇₁BM by ≈13% and ≈8%, respectively. PCEs over 8% are reported for devices based on the PTB7:PC₇₁BM blend. A comprehensive optical and electrical characterization of our devices to clarify the influence of gold nanorods on exciton generation, dissociation, charge recombination, and transport inside the thin film devices is performed. By correlating the experimental data with detailed numerical simulations, the near‐field and far‐field scattering effects are separated of gold nanorods (Au NRs), and confidently attribute part of the performance enhancement to the enhanced absorption caused by backscattering. While, a secondary contribution from the Au NRs that partially protrude inside the active layer and exhibit strong near‐fields due to localized surface plasmon resonance effects is also observed but is minor in magnitude. Furthermore, another important contribution to the enhanced performance is electrical in nature and comes from the increased charge collection probability.     

2D Materials for Large‐Area Flexible Thermoelectric Devices

The rapid development of the concept of the “Internet of Things (IoT)” requires wearable devices with maintenance‐free batteries, and thermoelectric energy conversion based on large‐area flexible materials has attracted much attention. Among large‐area flexible materials, 2D materials, such as graphene and related materials, are promising for thermoelectric applications due to their excellent transport properties and large power factors. In this Review, both single‐crystalline and polycrystalline 2D materials are surveyed using the experimental reports on thermoelectric devices of graphene, black phosphorus, transition metal dichalcogenides, and other 2D materials. In particular, their carrier‐density dependent thermoelectric properties and power factors maximized by Fermi level tuning techniques are focused. The comparison of the relevant performances between 2D materials and commonly used thermoelectric materials reveals the significantly enhanced power factors in 2D materials. Moreover, the current progress in thermoelectric module applications using large‐area 2D material thin films is summarized, which consequently offers great potential for the use of 2D materials in large‐area flexible thermoelectric device applications. Finally, important remaining issues and future perspectives, such as preparation methods, thermal transports, device designs, and promising effects in 2D materials, are discussed.     

Ultra‐High Voltage Multijunction Organic Solar Cells for Low‐Power Electronic Applications

Low power electronics are an ideal application for organic photovoltaics (OPV) where a low‐cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra‐high voltage small molecule multijunction devices with open circuit voltage (V_OC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi‐layer stacks and ensure current balancing is achieved between sub‐cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub‐cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) and SubPc/Cl₆‐SubPc were selected both for their high V_OC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro‐energy cell type battery under both low illumination intensity white light and monochromatic illumination.     

Plasmonic Back Reflectors: A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.     

An Efficient Ultra‐Flexible Photo‐Charging System Integrating Organic Photovoltaics and Supercapacitors

Flexible and biocompatible integrated photo‐charging devices consisting of photovoltaic cells and energy storage units can provide an independent power supply for next‐generation wearable electronics or biomedical devices. However, current flexible integrated devices exhibit low total energy conversion and storage efficiency and large device thickness, hindering their applicability towards efficient and stable self‐powered systems. Here, a highly efficient and ultra‐thin photo‐charging device with a total efficiency approaching 6% and a thickness below 50 µm is reported, prepared by integrating 3‐µm‐thick organic photovoltaics on 40 µm‐thick carbon nanotube/polymer‐based supercapacitors. This flexible photo‐charging capacitor delivers much higher performance compared with previous reports by tuning the electrochemical properties of the composite electrodes, which reduce the device thickness to 1/8 while improving the total efficiency by 15%. The devices also exhibit a superior operational stability (over 96% efficiency retention after 100 charge/discharge cycles for one week) and mechanical robustness (94.66% efficiency retention after 5000 times bending at a radius of around 2 mm), providing a high‐power and long‐term operational energy source for flexible and wearable electronics.     

Design of Nonfullerene Acceptors with Near‐Infrared Light Absorption Capabilities

A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm^?2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.     

Understanding the Effects of Molecular Dopant on n‐Type Organic Thermoelectric Properties

Molecular doping is a powerful method to fine‐tune the thermoelectric properties of organic semiconductors, in particular to impart the requisite electrical conductivity. The incorporation of molecular dopants can, however, perturb the microstructure of semicrystalline organic semiconductors, which complicates the development of a detailed understanding of structure–property relationships. To better understand how the doping pathway and the resulting dopant counterion influence the thermoelectric performance and transport properties, a new dimer dopant, (N‐DMBI)₂, is developed. Subsequently, FBDPPV is then n‐doped with dimer dopants (N‐DMBI)₂, (RuCp*mes)₂, and the hydride‐donor dopant N‐DMBI‐H. By comparing the UV–vis–NIR absorption spectra and morphological characteristics of the doped polymers, it is found that not only the doping mechanism, but also the shape of the counterion strongly influence the thermoelectric properties and transport characteristics. (N‐DMBI)₂, which is a direct electron‐donating dopant with a comparatively small, relatively planar counterion, gives the best power factor among the three systems studied here. Additionally, temperature‐dependent conductivity and Seebeck coefficient measurements differ between the three dopants with (N‐DMBI)₂ yielding the best thermoelectric properties. The results of this study of dopant effects on thermoelectric properties provide insight into guidelines for future organic thermoelectrics.     

Contact Doping with Sub‐Monolayers of Strong Polyelectrolytes for Organic Photovoltaics

Barriers to charge transfer at electrode‐semiconductor contacts are ubiquitous and limit the applicability of organic semiconductors in electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers, but the soft nature of organic materials allows for small molecule dopants to diffuse and migrate, degrading the performance of the device and limiting effective interfacial doping. Here, it is demonstrated that contact doping in organic electronics is possible through ionic polymer dopants, which resist diffusion or migration due to their large size. Sub‐monolayer deposition of non‐conjugated strong polyelectrolytes, e.g., sulfonated poly(sulfone)s, at the anode‐semiconductor interface of organic photovoltaics enables efficient hole extraction at the anode. The performance of contact‐doped organic photo­voltaics nearly matches the performance of devices composed of traditional hole transport layers such as poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The degree of sulfonation of the dopant polymer and the thickness of the ionic dopant layer is shown to be critical for optimizing doping and the efficiency of the device.     

Modulation of End Groups for Low‐Bandgap Nonfullerene Acceptors Enabling High‐Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E_g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V_oc of the devices is reduced but the J_sc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs.     

Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge‐Transfer Excitons

The role of excess excitation energy on long‐range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high‐energy charge‐transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in BHJ devices. Here, the population dynamics of charge‐separated polarons upon excitation of high‐energy polymer states and low‐energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond time scales are studied. It is observed that the charge‐separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges.     

Indoor Thin‐Film Photovoltaics: Progress and Challenges

Energy generation and consumption have always been an important component of social development. Interests in this field are beginning to shift to indoor photovoltaics (IPV) which can serve as power sources under low light conditions to meet the energy needs of rapidly growing fields, such as intelligence gathering and information processing which usually operate via the Internet‐of‐things (IoT). Since the power requirements for this purpose continue to decrease, IPV systems under low light may facilitate the realization of self‐powered high‐tech electronic devices connected through the IoT. This review discusses and compares the characteristics of different types of IPV devices such as those based on silicon, dye, III‐V semiconductors, organic compounds, and halide perovskites. Among them, specific attention is paid to perovskite photovoltaics which may potentially become a high performing IPV system due to the fascinating photophysics of the halide perovskite active layer. The limitations of such indoor application as they relate to the toxicity, stability, and electronic structure of halide perovskites are also discussed. Finally, strategies which could produce highly functional, nontoxic, and stable perovskite photovoltaics devices for indoor applications are proposed.     

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady‐state mobilities. However, the suitability of steady‐state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady‐state mobilities. This demonstrates that under realistic operating conditions the steady‐state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance.     

Probing Charge Recombination Dynamics in Organic Photovoltaic Devices under Open‐Circuit Conditions

Large perturbation transient photovoltage and impedance spectroscopy measurements are used to gain insights into recombination in organic photovoltaic devices. The combination of these two simple optoelectronic techniques enables characterization of recombination order as well as mobile and trapped charge evolution over a large range of carrier densities. The data show that trapped charge is approximately equal to total charge at low carrier densities in the high efficiency devices measured. Between low and high charge carrier density, the order of recombination is observed to vary from monomolecular to bimolecular to higher order. The new techniques and methods presented can be applied to any type of photovoltage device to gain insight into device operation and limitations.     

An All‐Organic Non‐aqueous Lithium‐Ion Redox Flow Battery

A non‐aqueous lithium‐ion redox flow battery employing organic molecules is proposed and investigated. 2,5‐Di‐tert‐butyl‐1,4‐bis(2‐methoxyethoxy)benzene and a variety of molecules derived from quinoxaline are employed as initial high‐potential and low‐potential active materials, respectively. Electrochemical measurements highlight that the choice of electrolyte and of substituent groups can have a significant impact on redox species performance. The charge‐discharge characteristics are investigated in a modified coin‐cell configuration. After an initial break‐in period, coulombic and energy efficiencies for this unoptimized system are ～70% and ～37%, respectively, with major charge and discharge plateaus between 1.8‐2.4 V and 1.7‐1.3 V, respectively, for 30 cycles. Performance enhancements are expected with improvements in cell design and materials processing.     

A Battery‐Like Self‐Charge Universal Module for Motional Energy Harvest

Wearable and portable electronics have brought great convenience. These battery‐powered commercial devices have a limited lifetime and require recharging, which makes more extensive applications challenging. Here, a battery‐like self‐charge universal module (SUM) is developed, which is able to efficiently convert mechanical energy into electrical energy and store it in one device. An integrated SUM consists of a power management unit and an energy harvesting unit. Compared to other mechanical energy harvesting devices, SUM is more ingenious, efficient and can be universally used as a battery. Under low frequency (5 Hz), a SUM can deliver an excellent normalized output power of 2 mW g^?1. After carrying several SUMs and jogging for 10 min, a commercial global positioning system module is powered and works continuously for 0.5 h. SUMs can be easily assembled into different packages for powering various commercial electronics, demonstrating the great application prospects of SUM as a sustainable battery‐like device for wearable and portable electronics.     

Evidencing Excellent Thermal‐ and Photostability for Single‐Component Organic Solar Cells with Inherently Built‐In Microstructure

Solution‐processed organic solar cells (OSCs) are promising low‐cost, flexible, portable renewable sources for future energy supply. The state‐of‐the‐art OSCs are typically fabricated from a bulk‐heterojunction (BHJ) active layer containing well‐mixed donor and acceptor molecules in the nanometer regime. However, BHJ solar cells suffer from stability problems caused by the severe morphological changes upon thermal or illumination stress. In comparison, single‐component organic solar cells (SCOSCs) based on a double‐cable conjugated polymer with a covalently stabilized microstructure is suggested to be a key strategy for superior long‐term stability. Here, the thermal‐ and photostability of SCOSCs based on a model double‐cable polymer is systematically investigated. It is encouraging to find that under 90 °C & 1 sun illumination, the performance of SCOSCs remains substantially stable. Transport measurements show that charge generation and recombination (lifetime and recombination order) hardly change during the aging process. Particularly, the SCOSCs exhibit ultrahigh long‐term thermal stability with 100% PCE remaining after heating at temperature up to 160 °C for over 400 h, indicating an excellent candidate for extremely rugged applications.     

Paper‐Based Energy‐Storage Devices Comprising Carbon Fiber‐Reinforced Polypyrrole‐Cladophora Nanocellulose Composite Electrodes

Composites of polypyrrole (PPy) and Cladophora nanocellulose, reinforced with 8 μm‐thick chopped carbon filaments, can be used as electrode materials to obtain paper‐based energy‐storage devices with unprecedented performance at high charge and discharge rates. Charge capacities of more than 200 C g^?1 (PPy) are obtained for paper‐based electrodes at potential scan rates as high as 500 mV s^?1, whereas cell capacitances of ～60–70 F g^?1 (PPy) are reached for symmetric supercapacitor cells with capacitances up to 3.0 F (i.e.,0.48 F cm^?2) when charged to 0.6 V using current densities as high as 31 A g^?1 based on the PPy weight (i.e., 99 mA cm^?2). Energy and power densities of 1.75 Wh kg^?1 and 2.7 kW kg^?1, respectively, are obtained when normalized with respect to twice the PPy weight of the smaller electrode. No loss in cell capacitance is seen during charging/discharging at 7.7 A g^?1 (PPy) over 1500 cycles. It is proposed that the nonelectroactive carbon filaments decrease the contact resistances and the resistance of the reduced PPy composite. The present straightforward approach represents significant progress in the development of low‐cost and environmentally friendly paper‐based energy‐storage devices for high‐power applications.