Understanding the layer stacking in two-dimensional covalent organic frameworks remains a significant challenge. A study now reports the on-surface synthesis of bilayer covalent organic frameworks where the interlayer orientation — the twist angle — can be modulated through tuning the monomer structure.

The synthesis of new donor-acceptor polymers is reported through Knoevenagel condensation of indacenetetraone with aromatic dialdehydes. The obtained polymers possess exceptionally rigid planar backbones with a rotational barrier (for single C & horbar;C bonds) of 16-20 kcal mol-1, the highest among known conjugated polymers. These structural features confer several unusual properties, including remarkably narrow absorption bands with very high absorption coefficient (up to approximate to 3x105 cm-1) and efficient fluorescence in the red to near-IR region (lambda max 694-850 nm, quantum yield 10-29%). Systematic spectroscopic, electrical, and microstructural characterization supported by Density Functional Theory (DFT) calculations reveals the effects of heteroatoms and the conjugation lengths on the optoelectronic properties of these materials. The polymers exhibit ambipolar charge transport in field-effect transistors, which can be tailored to show unipolar p-type (0.1 cm2 V-1 s-1) and n-type (0.05 cm2 V-1 s-1) behavior by adjusting the energy levels at the polymer/metal contacts. Additionally, their application is demonstrated as acceptors in all-polymer organic photovoltaic devices with commercial donor polymers, achieving a power conversion efficiency (PCE) of 11%.

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

Two near-infrared (NIR) emissive π-conjugated covalent organic frameworks (COFs) pTANG1 and pTANG2 are synthesized using Knoevenagel condensation of trioxaazatriangulenetricarbaldehyde (TATANG) with benzene- and biphenyldiacetonitriles, respectively. The morphology of the COFs is affected by the size of TATANG precursor crystals. Donor-acceptor interactions in these COFs result in small bandgaps (≈1.6 eV) and NIR emission (λmax = 789 nm for pTANG1). pTANG1 can absorb up to 9 molecules of water per unit cell, which is accompanied by a marked quenching of the NIR emission, suggesting applications as humidity sensors. p-Doping with magic blue significantly increases the electrical conductivities of the COFs by up to 8 orders of magnitude, with the room temperature conductivity of pTANG1 reaching 0.65 S cm-1, the highest among reported C═C linked COFs. 1H NMR relaxometry, temperature-dependent fluorescence spectroscopy, and DFT calculations reveal that the higher rigidity of the shorter phenylene linker is responsible for the more extended conjugation (red-shifted emission, higher electrical conductivity) of pTANG1 compared to pTANG2.

We report here that methyl-substituted hexamethoxytrityl (HMT) and the derived trioxatriangulene (TOTA) salts react with aldehydes, forming π-extended tristyryl-substituted HMT and TOTA dyes via a dynamic Knoevenagel condensation. These cations undergo a reversible electrochemical (or chemical) reduction, forming neutral radicals, including the first persistent TOTA radical. This reaction represents a promising platform to generate novel π-conjugated systems.

Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon nanomaterials. The irreversible nature of this reaction prevents the "self-healing" of defects, and a detailed knowledge of its mechanism is therefore essential to enable the growth of extended ordered structures. However, the dynamics of the Ullmann polymerization remain largely unexplored, as coupling events occur on a timescale faster than conventional scanning probe microscopy imaging frequencies. Here, we reveal the dynamics of these surface events using high-speed variable-temperature scanning tunneling microscopy (STM) (10 frames per second). Performing the measurements at the onset reaction temperatures provides an unprecedented description of the evolution of organometallic (OM) and covalent surface species during the Ullmann polymerization of para-dibromobenzene on Cu(110). Our results demonstrate the existence of an intermediate OM phase with Cu adatoms that inhibits the polymerization. These observations now complete the picture of the pathways of on-surface Ullmann polymerization, which includes the complex interplay of the phenylene moieties and metal atoms. Our work demonstrates the unique capability of high-speed STM to capture the dynamics of molecular self-assembly and coupling.

Catalytic electron donor-acceptor (EDA) complexes have recently emerged as a powerful and sustainable alternative to iridium- and ruthenium-based photoredox synthetic methods. Yet, these complexes remain underexplored and reliant on the use of meticulously designed acceptors that require previous installation. Herein, we report a novel EDA complex employing tris(4-methoxyphenyl) amine as a catalytic donor for the sulfonylation of alkenes using inexpensive and readily available sulfonyl chlorides. Applying this operationally simple, visible-light-mediated general platform, we report both the redox-neutral and net-reductive functionalization of more than 60 substrates, encompassing vinylic or allylic sulfonylation, hydrosulfonylation, and sulfamoylation of activated and unactivated alkenes and alkynes.

We report a rapid, room-temperature mechanochemical synthesis of 2- and 3-dimensional boroxine covalent organic frameworks (COFs), enabled by using trimethylboroxine as a dehydrating additive to overcome the hydrolytic sensitivity of boroxine-based COFs. The resulting COFs display high porosity and crystallinity, with COF-102 being the first example of a mechanochemically-prepared 3D COF, exhibiting a surface area exceeding 2,400 m2 g–1. Mechanochemistry enabled a >20-fold reduction in solvent use and ~100-fold reduction in reaction time compared with solvothermal methods, providing target COFs quantitatively with no additional work-up besides vacuum drying. Real-time Raman spectroscopy permitted the first quantitative kinetic analysis of COF mechanosynthesis, while transferring the reaction design to Resonant Acoustic Mixing (RAM) enabled synthesis of multi-gram amounts of the target COFs.

Quantum dots (QDs) are promising building blocks for luminescent solar concentrators (LSCs), yet most QD-based LSCs suffer from toxic metal composition and color tinting. UV-selective harvesting QDs can enable visible transparency, but their development is restricted by large reabsorption losses and low photoluminescence quantum yield (PLQY). The developed here Ag, Mn: ZnInS2/ZnS QDs show a high PLQY of 53% due to the passivating effect of ZnS shell. These QDs selectively absorb UV light and emit orange-red light with a large Stokes shift of 180 nm. A LSC of 5 × 5 × 0.2 cm3, fabricated using a poly(lauryl methacrylate) (PLMA) as a matrix, maintains 87% of integrated PL after 7 h of UV exposure. The QD-PLMA achieved 90.7% average visible transparency (AVT) and a color rendering index (CRI) of 95.8, which is close to plain PLMA (AVT = 90.8%; CRI = 99.5), yielding excellent visible light transparency. Incorporating Si-PVs at LSC edges, the Ag, Mn: ZIS/ZnS QD-LSC achieved an optical efficiency of 1.42%, ranking competitively among high-performing UV-harvesting LSCs.

Small organic molecules can possess extremely high hyperpolarizabilities. Their potential use in nonlinear photonics has, however, been limited by the fact that their bulk second-order nonlinearities often vanish in thin film form due to the centrosymmetric arrangement that results from most fabrication processes. The typical approach to overcome this problem has been to use electric field poling, which comes at the cost of considerably increased complexity. In polar films, however, molecules can spontaneously adopt an asymmetric out-of-plane orientation distribution that breaks the centrosymmetry. This phenomenon is at the origin of the spontaneous orientation polarization observed in organic thin films, a phenomenon that has recently attracted considerable attention from the organic optoelectronics community. In this work we show that spontaneous orientation can be leveraged to obtain evaporated thin films with bulk second-order nonlinear coefficients of chi 33 (2) similar or equal to 20 pm/V, on par with the inorganic nonlinear materials commonly used in integrated photonics. Additionally, we show that the evaporation rate and substrate treatments can be used to tune the nonlinear properties of these films. Finally, we show that the codeposition of a molecule possessing a large hyperpolarizability with a host molecule known for its strong spontaneous orientation can favor the spontaneous orientation of the nonlinear molecule and lead to large nonlinearities. This technique can lead to films with stronger nonlinearities than in neat films, even at low concentrations of nonlinear compounds (as low as 23%). This work paves the way for the direct integration of evaporated organic semiconductor thin films for second-order nonlinear optical processes on optical chips and metasurfaces, without the need for electrical poling.

We investigated tetracyanoanthracenediacenaphthalimides (TCDADIs) as n-type organic semiconductors (OSCs) and assessed their molecular self-assembly in forming monolayers and thin films using optical absorption spectroscopy, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and grazing incidence wide-angle X-ray scattering (GIWAXS). The absorption spectra, along with quantitative GIWAXS analysis, reveal the influence of molecular structure (alkyl chain length) and film processing conditions (annealing temperature and spin-coating speed) on the orientation of TCDADI molecules in films. Our findings indicate that increasing the spin-coating speed and annealing temperatures causes a transition from a mixed phase to a predominantly edge-on molecular orientation. This transition significantly enhances the electron mobility, from 0.01 to 0.05 cm2 V-1 s-1 for TCDADI-C16 and from 0.13 to 0.20 cm2 V-1 s-1 for TCDADI-C24. In addition, we highlight the potential of TCDADIs for photodetector applications, showing a photoresponse gain of over 2000 under white light.

Despite enormous interest in two-dimensional (2D) carbon allotropes, discovering stable 2D carbon structures with practically useful electronic properties presents a significant challenge. Computational modeling in this work shows that fusing azulene-derived macrocycles – azulenoid kekulenes (AK) – into graphene leads to the most stable 2D carbon allotropes reported to date, excluding graphene. Density functional theory predicts that placing the AK units in appropriate relative positions in the graphene lattice opens the 0.54 eV electronic bandgap and leads to the appearance of the remarkable 0.80 eV secondary gap between conduction bands – a feature that is rare in 2D carbon allotropes but is known to enhance light absorption and emission in 3D semiconductors. Among porous AK structures, one material stands out as a stable narrow-multigap (0.36 and 0.56 eV) semiconductor with light charge carriers ( m e = 0.17 m 0 , m h = 0.19 m 0 ), whereas its boron nitride analog is a wide-multigap (1.51 and 0.82 eV) semiconductor with light carriers ( m e = 0.39 m 0 , m h = 0.32 m 0 ). The multigap engineering strategy proposed here can be applied to other carbon nanostructures creating novel 2D materials for electronic and optoelectronic applications.

Planar and twisted self-assemblies of diketopyrrolopyrrole molecules are determined by aromatic substitutions as revealed by STM at solid–liquid interfaces.

We explore a series of furan-based non-fullerene acceptors and report their optoelectronic properties, solid-state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red-shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels-Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non-fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well-balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14%), outperforming all furan-based acceptors reported to date and all indacenodithiophene-based acceptors. Our findings show the possibility of photostable furan-based alternatives to the currently omnipresent thiophene-based photovoltaic materials.

Operational instability of organic field-effect transistors (OFETs) is one of the key limitations for applications of printed electronics. Environmental species, especially oxygen and water, unintentionally introduced in the OFET channel, can act as either dopants or traps for charge carriers, affecting the electrical character-istics and stability of devices. Here, we report that intentional doping of the benchmark p-type semiconducting polymer (DPP-DTT) with 2,4,5,7-tetranitrofluorenone (TeNF) markedly improves the opera-tional and environmental stability of OFETs. Electrical interrogation of DPP-DTT OFETs in various environments and at variable temper-atures shows suppression of electron-induced traps and increase of hole mobility in oxygen-rich environment, while the water molecules act as traps for positive charge carrier, reducing the hole mobility and significantly shifting the threshold voltage. Doping of DPP-DTT with TeNF suppresses both effects, resulting in environmentally independent performance and superior long-term stability of unencapsulated devices for up to 4 months in ambient air. Furthermore, the doped OFETs exhibit dramatically reduced hysteresis and bias-stressed current drop. Such improvement of the environmental and operational stabilities is ascribed to the mitigation of traps induced by the injected minority carrier (electrons) and the reduction of the majority carrier (hole) traps in doped polymer films due to enhanced microstructural order.

Thick-shell colloidal quantum dots (QDs) are promising building blocks for solar technologies due to their size/composition/shape-tunable properties. However, most well-performed thick-shell QDs suffer from frequent use of toxic metal elements including Pb and Cd, and inadequate light absorption in the visible and near-infrared (NIR) region due to the wide bandgap of the shell. In this work, eco-friendly AgInSe2 /AgInS2 core/shell QDs, which are optically active in the NIR region and are suitable candidates to fabricate devices for solar energy conversion, are developed. Direct synthesis suffers from simultaneously controlling the reactivity of multiple precursors, instead, a template-assisted cation exchange method is used. By modulating the monolayer growth of template QDs, gradient AgInSeS shell layers are incorporated into AgInSe2 /AgInS2 QDs. The resulting AgInSe2 /AgInSeS/AgInS2 exhibits better charge transfer than AgInSe2 /AgInS2 due to their favorable electronic band alignment, as predicted by first-principle calculations and confirmed by transient fluorescence spectroscopy. The photoelectrochemical cells fabricated with AgInSe2 /AgInSeS/AgInS2 QDs present ≈1.5-fold higher current density and better stability compared to AgInSe2 /AgInS2 . The findings define a promising approach toward multinary QDs and pave the way for engineering the QDs' electronic band structures for solar-energy conversion.

Abstract Despite enormous interest in 2D carbon allotropes, discovering stable 2D carbon structures with practically useful electronic properties presents a significant challenge. This work shows that fusing azulene-derived macrocycles - azulenoid kekulenes (AK) - into graphene leads to the most stable 2D carbon allotropes reported to date, excluding graphene. DFT modeling predicts that placing the AK units in appropriate relative positions in the graphene lattice opens the 0.54eV electronic bandgap and leads to the appearance of the remarkable 0.80eV secondary gap between conduction bands - a feature that is exceedingly rare in 2D carbon allotropes but is known to enhance light absorption and emission in 3D semiconductors. Among porous AK structures, one material stands out as a stable narrow-multigap (0.36 and 0.56eV) semiconductor with light charge carriers (me = 0.17 m0, mh = 0.19 m0), whereas its boron nitride analog is a wide-multigap (1.51 and 0.82eV) semiconductor with mobile carriers (me = 0.39 m0, mh = 0.32 m0). The multigap engineering strategy proposed here can be applied to other carbon nanostructures creating novel 2D materials for electronic and optoelectronic applications.

2D materials with high charge carrier mobility and tunable electronic band gaps have attracted intense research effort for their potential use as active components in nanoelectronics. 2D-conjugated polymers (2DCP) constitute a promising sub-class due to the fact that the electronic band structure can be manipulated by varying the molecular building blocks, while at the same time preserving the key features of 2D materials such as Dirac cones and high charge mobility. The major challenge for their use in technological applications is to fabricate mesoscale ordered 2DCP networks since current synthetic routes yield only small domains with a high density of defects. Here we demonstrate the synthesis of a mesoscale ordered 2DCP with semiconducting properties and Dirac cone structures via Ullmann coupling on Au(111). This material has been obtained by combining rigid azatriangulene precursors and a hot dosing approach which favours molecular diffusion and reduces the formation of voids in the network. These results open opportunities for the synthesis of 2DCP Dirac cone materials and their integration into devices.

CuGaS2/ZnS quantum dots with a Stokes shift of ∼190 nm and PLQY of 80% were developed for LSCs, which achieved an optical efficiency of 1.7%. A tandem configuration integrating near-infrared-active AgInS2/ZnS achieved an optical efficiency of 4.4%.

Two-dimensional conjugated polymers (2DCPs) are an emerging class of materials that exhibit properties similar to graphene yet do not have the limitation of zero bandgap. On-surface synthesis provides exceptional control on the polymerization reaction, allowing tailoring properties by choosing suitable monomers. Heteroatom-substituted triangulene 2DCPs constitute a playing ground for such a design and are predicted to exhibit graphene-like band structures with high charge mobility and characteristic Dirac cones in conduction or valence states. However, measuring these properties experimentally is challenging and requires long-range-ordered polymers, preferably with an epitaxial relationship with the substrate. Here, we investigate the electronic properties of a mesoscale-ordered carbonyl-bridged triphenylamine 2DCP (P2TANGO) and demonstrate the presence of a Dirac cone by combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations. Moreover, we measure the absolute energy position of the Dirac cone with respect to the vacuum level. We show that the bridging functionality of the triangulene (ether vs carbonyl) does not significantly perturb the band structure but strongly affects the positioning of the bands with respect to the Au(111) states and allows control of the ionization energy of the polymer. Our results provide proof of the controllable electronic properties of 2DCPs and bring us closer to their use in practical applications.