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The properties of polycrystalline materials are often dominated by defects; two-dimensional (2D) crystals can even be divided and disrupted by a line defect1–3. However, 2D crystals are often required to be processed into films, which are inevitably polycrystalline and contain numerous grain boundaries, and therefore are brittle and fragile, hindering application in flexible electronics, optoelectronics and separation1–4. Moreover, similar to glass, wood and plastics, they suffer from trade-off effects between mechanical strength and toughness5,6. Here we report a method to produce highly strong, tough and elastic films of an emerging class of 2D crystals: 2D covalent organic frameworks (COFs) composed of single-crystal domains connected by an interwoven grain boundary on water surface using an aliphatic bi-amine as a sacrificial go-between. Films of two 2D COFs have been demonstrated, which show Young’s moduli and breaking strengths of 56.7 ± 7.4 GPa and 73.4 ± 11.6 GPa, and 82.2 ± 9.1 N m−1and 29.5 ± 7.2 N m−1, respectively. We predict that the sacrificial go-between guided synthesis method and the interwoven grain boundary will inspire grain boundary engineering of various polycrystalline materials, endowing them with new properties, enhancing their current applications and paving the way for new applications.

Porphyrin-based two-dimensional metal–organic frameworks (2D MOFs) are of high interest for extraordinary optoelectronic devices due to their well-ordered architecture and appealing photochemical properties. However, the high-throughput synthesis of 2D MOF thin films for device applications remains challenging because of the slow nucleation and crystallization process in classical interfacial synthesis. Here, we report a microwave-assisted interfacial methodology for the rapid construction (∼3 min) of highly crystalline 2D MOF thin films and demonstrate their promising potential in biomimetic optical sensors. The resultant 2D MOF films exhibit a lateral size of up to ∼23 cm2and controllable thickness in the range of 2–40 nm. The crystal domain size of the 2D MOF can be up to ∼4 μm, which boasts a long-range molecular ordering and appealing optical responsive properties. The 2D MOF films enable the fabrication of biomimetic optical sensors with light intensity-dependent autonomous photoadaptation without the assistance of voltage gating. Furthermore, an artificial machine vision system consisting of a 4 × 8 2D MOF-sensor array is developed to implement adaptive image recognition under bright- and dim-light stimuli with an accuracy of over 90%.

sp2carbon-conjugated covalent organic frameworks (sp2c-COFs) with superb in-plane π-conjugations, high chemical stability, and robust framework structure are expected to be ideal films/membranes for a wide range of applications including energy-related devices and optoelectronics. However, so far, sp2c-COFs have been mainly limited to microcrystalline powders, and this consequently hampered their performances in devices. Herein, we report a simple and robust methodology to fabricate large-area, free-standing, and crystalline sp2c-COF films (TFPT–TMT and TB–TMT) on various solid substrates (e.g., fluorine-doped tin oxide, aluminum sheet, polyacrylonitrile membrane) by self-assembly monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation (namely, SI-SBMAP). The resultant sp2c-COF films show lateral sizes up to 120 cm2and tunable thickness from tens of nanometers to a few micrometers. Owing to the robust framework and highly ordered quasi-1D channels, the sp2c-COF membrane-based osmotic power generator presents an output power density of 14.1 W m–2under harsh conditions, outperforming most reported COF membranes as well as commercialized benchmark devices (5 W m–2). This work demonstrates a simple and robust interfacial methodology for the fabrication of sp2c-COF films/membranes for green energy applications and potential optoelectronics.

Solution-processed metal oxide (MO) thin films have been extensively studied for use in thin-film transistors (TFTs) due to their high optical transparency, simplicity of fabrication methods, and high electron mobility. Here, we report, for the first time, the improvement of the electronic properties of solution-processed indium oxide (InOx) films by the subsequent addition of an organic p-type semiconductor material, here 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), yielding organic–inorganic hybrid TFTs. The addition of TIPS-pentacene not only improves the electron mobility by enhancing the charge carrier percolation pathways but also improves the electronic and temporal stability of the IDS(VG) characteristics as well as reduces the number of required spin-coating steps of the InOxprecursor solution. Very interestingly, the introduction of 10 nm TIPS-pentacene films on top of 15 nm InOxlayers allows the fabrication of either enhancement- or depletion-mode devices with only minimal changes to the fabrication process. Specifically, we find that when the TIPS-pentacene layer is added on top of the source/drain electrodes, resulting in devices with embedded source/drain electrodes [embedded electrode TFTs (EETFTs)], the devices exhibit an enhancement-mode behavior with an average mobility (μ) of 6.4 cm2V–1s–1, a source–drain current ratio (Ion/Ioff) of around 105, and a near-zero threshold voltage (VTH). When on the other hand the TIPS-pentacene layer is added before the source–drain electrodes, i.e., in top-contact electrode TFTs (TCETFTs), a very clear depletion mode behavior is observed with an average μ of 6.3 cm2V–1s–1, an Ion/Ioffratio of over 105, and a VTHof −80.3 V. Furthermore, a logic inverter is fabricated combining the enhancement (EETFTs)- and depletion (TCETFTs)-mode transistors, which shows a potential for the construction of organic–inorganic hybrid electronics and circuits.

Organic solar cells offer several advantages over conventional photovoltaics, such as flexibility, low cost, and abundant materials, and are thus seen as a promising choice for energy harvesting. A rather particular case refers to diluted systems, where a small number of electron-donating molecules are dispersed within an acceptor matrix, hampering hole transport in the absence of percolation paths. Nonetheless, diluted systems present a reasonable performance, thereby raising discussions on how this is achieved. Here, we investigate the hole transport by measuring the hole mobility in several systems with varying donor concentrations. We found an unexpected correlation between the hole mobility and the energy offset between the donor HOMO and the acceptor LUMO (instead of the acceptor HOMO), with the mobility increasing as the offset decreases. Such a correlation is supported by molecular modeling suggesting a contribution from the acceptor LUMO-mediated superexchange mechanism. These results corroborate our experimental observation, hinting at a previously neglected mechanism of transport, which depends on the coupling between the donor HOMO and the acceptor LUMO.

Blending organic molecules to tune their energy levels is currently being investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy and electron affinity can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here we show that the energy gap of organic semiconductors can also be tuned by blending. We use oligothiophenes with different numbers of thiophene rings as an example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller, effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy for optoelectronic devices.

The shape of the mid-IR absorption spectrum provides valuable information about the “hole” polaron coherence length in doped and undoped conjugated polymer films. In poly(3-hexylthiophene) (P3HT) films, the spectrum generally consists of a narrow, low-energy peak A (700–1000 cm–1) followed by a much broader, higher-energy peak B (2500–5000 cm–1). By using a theory based on the Holstein Hamiltonian for mobile holes in P3HT, the IR line-shape is successfully reproduced for several recently measured spectra recorded in doped and undoped films, confirming the association of an enhanced peak ratio (A/B) with extended polaron coherence. Emphasis is placed on the origin of components polarized along the intra- and interchain directions and their dependence on the spatial distribution of disorder as well as the position of the dopant relative to the π-stack. The model is further adapted to treat donor–acceptor copolymers where the local HOMO energy varies periodically from donor unit to acceptor unit. The calculated line shape for a diketopyrrolopyrrole-based copolymer agrees well with the recently measured spectrum.

Semiconducting donor–acceptor copolymers are considered to be a promising material class for solution-coated, large-scale organic electronic applications. A large number of works have shown that the best-performing organic field-effect transistors (OFETs) are obtained on low-surface-energy substrates. The meniscus instabilities that occur when coating on such surfaces considerably limit the effective deposition speeds. This represents a limiting factor for the upscaling of device fabrication for mass production, an issue that needs to be addressed if organic electronic devices are ever to become commercially relevant. In this work, we present a method to increase the accessible window of coating speeds for the solution shearing of donor–acceptor semiconductor polymers for the fabrication of OFETs. By incorporating a piezo crystal that is capable of producing high-frequency vibrations into the coating head, we are able to mitigate contact line instabilities due to the depinning of the contact line, thereby suppressing the commonly encountered “stick-and-slip” phenomenon.

Highly efficient 2D (interfacial) doping of organic semiconductors, poly(3-hexylthiophene) (P3HT) and TIPS-pentacene, was achieved by a polyelectrolyte-supported layer-by-layer assembly of the dual-mode functional dopant CN6-CP•–K+, having an anionic group for its fixation onto oppositely charged surfaces/molecules as well as electron-deficient groups providing its p-doping ability. Polyelectrolyte-supported dopant layers were used to generate conductive channels at the bottom or at the top of semiconducting films. Unlike to the case of sequentially processed P3HT films doped by F4TCNQ (Moulé, J.Chem. Mater.2015, 27, 5765; Koech, P. K.J. Mater. Chem. C2013, 1, 1876; Schwartz, B. J.J. Phys. Chem. Lett.2015, 6, 4786), the use of more polar CN6-CP•–K+dopant and ultrathin polycation separation interlayer enables predominantly interfacial kind of doping placement with no or minimal intercalation of the dopant into the semiconductor bulk. The layered structure of the doped film was proved by transmission electron microscopy of the cross-section and it agrees well with other data obtained in this work. The interfacial doping enabled an impressive conductivity of 13 S/cm even for ultrathin P3HT films. We propose to explain the superior efficiency of the interfacial doping compared to the bulk doping in terms of unperturbed morphology of the semiconductor and high mobility of charge carriers, which are spatially separated from the dopant phase.

Doping is a powerful tool to control the majority charge carrier density in organic field-effect transistors and the threshold voltage of these devices. Here, a surface doping approach is shown, where the dopant is deposited on the prefabricated polycrystalline semiconducting layer. In this study, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS), a fluorinated alkylsilane is used as a dopant, which is solution processable and much cheaper than conventional p-type dopants, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). In this work, the depositions from the gas phase and from solution are compared. Both deposition approaches led to an increased conductivity and to a shift in the threshold voltage to more positive values, both of which indicate a p-type doping effect. The magnitude of the threshold voltage shift could be controlled by the FTCS deposition time (from vapor) or FTCS concentration (from solution); for short deposition times and low concentrations, the off current stayed constant and the mobility decreased only slightly. In the low doping concentration regime, both approaches resulted in similar transistor characteristics, i.e., similar values of shift in the threshold and turn-on voltage as well as mobility, ION/IOFFratio and amount of introduced free charge carriers. In comparison with vapor deposition, the solution-based approach can be conducted with less material and in a shorter time, which is critical for industrial applications.

Diketopyrrolopyrrole (DPP)-based donor–acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP-based donor–acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (ECT) of the blend films lies exceptionally close to the singlet energy of the donor (ED*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (VOC) is therefore determined to be due to rather high nonradiative (≈ 418 ± 13 mV) and unavoidable radiative voltage losses (≈ 255 ± 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (JSC) covers a vast span from 7 to 18 mA cm–2for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor–acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.

It is commonly thought that inhomogeneities in the bulk heterojunction thickness of donor:acceptor organic solar cells are a major efficiency‐limiting factor in scaling to modules. This is primarily related to degradation of the charge collection efficiency associated with the thicker segments of the active layer and/or loss of fill factor from thin segments where shorting occurs. We study the former effect in two model polymer–fullerene organic solar cell blend systems with very different charge transport properties: poly(3‐n‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PC60BM) and poly[N‐9”‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PCDTBT:PC70BM). We find that junction thickness inhomogeneities do not cause substantial power losses in large area cells (5 × 5 cm2monolithic submodules) when the bimolecular recombination dynamics are suppressed, i.e., the system is non‐Langevin. This insight into the scaling physics of organic solar cells suggests that non‐Langevin materials will deliver more robust and viable large area manufacturing opportunities.

Skin-conformable photoplethysmogram (PPG) sensors enable continuous and accurate monitoring of physiological states to efficiently prevent cardiovascular-related diseases. Herein, novel PPG sensors consisting of polymer/oxide hybrid phototransistors, mini-light-emitting diodes, and a framework conformable to epidermis are developed. The key element, a heterojunction phototransistor for efficient energy usage, is composed of an indium gallium zinc oxide (IGZO)-based active layer for low-power consumption and a specific diketopyrrolopyrrole (DPP) polymer layer affording high near-infrared (NIR) light absorbability and hydrophobicity. Therefore, the phototransistors with NIR detectivity of 1.00 × 1013Jones, rapid photoresponse within the human heart rate range, high reliability against perspiration and mechanical stress, and low operating voltages (< 5 V) are achieved. Using the developed PPG sensors, the heart rate and oxygen saturation of human subjects are successfully detected, which is comparable to the commercial PPG sensors. Furthermore, controlling potential barrier energy at the interface between heterojunction layers, PPG sensors that operate separately at low and high heart rates are implemented for continuous monitoring. Consequently, a distinguished configuration of skin-conformable PPG sensors and a novel concept of an always-on cardiovascular monitoring system while consuming less power are suggested. The study contributes to the development of PPG sensors and may become a potential solution for Healthcare 4.0 applications.

Design of printed circuits becomes an increasingly important research field as applications of this emerging technology come into sight. However, significant demand of research and development exists because new technology-related requirements apply as well as new opportunities evolve. We illustrate recent progress and future developments in terms of device modelling, circuit design and simulation as well as circuitry concepts using practical examples.

The development of layer-oriented two-dimensional conjugated metal–organic frameworks (2D c-MOFs) enables access to direct charge transport, dial-in lateral/vertical electronic devices, and the unveiling of transport mechanisms but remains a significant synthetic challenge. Here we report the novel synthesis of metal-phthalocyanine-based p-type semiconducting 2D c-MOF films (Cu2[PcM–O8], M = Cu or Fe) with an unprecedented edge-on layer orientation at the air/water interface. The edge-on structure formation is guided by the preorganization of metal-phthalocyanine ligands, whose basal plane is perpendicular to the water surface due to their π–π interaction and hydrophobicity. Benefiting from the unique layer orientation, we are able to investigate the lateral and vertical conductivities by DC methods and thus demonstrate an anisotropic charge transport in the resulting Cu2[PcCu–O8] film. The directional conductivity studies combined with theoretical calculation identify that the intrinsic conductivity is dominated by charge transfer along the interlayer pathway. Moreover, a macroscopic (cm2size) Hall-effect measurement reveals a Hall mobility of ∼4.4 cm2V–1s–1for the obtained Cu2[PcCu–O8] film. The orientation control in semiconducting 2D c-MOFs will enable the development of various optoelectronic applications and the exploration of unique transport properties.