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Chiroptical properties in the bulk state can be tuned by controlling the formation of chiral ordered nanostructures. Here, we present a series of discrete oligodimethylsiloxane-helicene-pyrene block molecules with varying o DMS lengths and study the nanostructures formed in both bulk and solution, including their chiroptical properties. In bulk, ordered 2D nanostructures self-assemble, driven by phase segregation induced by the siloxane oligomers, with clear differences in the properties of the racemic and enantiopure versions. Moreover, intermolecular pyrene interactions lead to excimer emission. As a result, up to a 5-fold increase in circularly polarized luminescence is observed in the solid state as compared to the solution, accompanied by a clear influence of the pyrene excimer emission on the overall emission process. Interestingly, in the ordered lamellar packing achieved from long o DMS units, the excimer emission shows very little net circular polarization, while in the disordered state achieved from shorter o DMS units, this excimer emission displays a significant degree of circular polarization. These results demonstrate that functionalizing chiroptical building blocks with discrete oligodimethylsiloxane chains is a versatile strategy to control photophysical properties and modulate chiroptical emission in bulk. This approach advances the integration of chiroptical materials into devices, enabling diverse applications ranging from optoelectronics to communication technologies.

Singlet fission (SF), which involves the conversion of a singlet excited state into two triplet excitons, holds great potential to boost the efficiency of photovoltaics. However, losses due to triplet-triplet annihilation hamper the efficient harvesting of SF-generated triplet excitons, which limits an effective implementation in solar energy conversion schemes. A fundamental understanding of the underlying structure-property relationships is thus crucial to define design principles for cutting-edge SF materials, yet it remains elusive. Herein, we harness helical supramolecular polymers decorated with pentacene side groups to elucidate intermolecular SF dynamics in solution and promote the formation of long-lived mobile triplets. By leveraging the hydrogen bonding-driven assembly of benzene-1,3,5-tricarboxamide (BTA) cores into one-dimensional scaffolds, we direct the organization of appended pentacene motifs into long-range ordered helical frameworks. Dynamic interactions between weakly coupled SF pendants mediate singlet conversion within hundreds of picoseconds, affording triplet quantum yields well above 100%. Moreover, analysis of triplet dynamics with a Monte Carlo simulation model reveals that triplet diffusion along the supramolecular fibers is favored over annihilation, resulting in independent triplets exhibiting considerably slow decay on the time scale of tens of microseconds. The molecular packing within the assembly is tuned by subtle changes in monomer design to increase the rate and efficiency of SF while ensuring exceptionally long-lived mobile triplets, allowing to maintain triplet quantum yields exceeding 100% for at least 100 ns. This work opens new opportunities to exploit self-assembled supramolecular polymers as functional templates to achieve long-lived SF-generated triplets.

Molecular scaffolds that enable the combinatorial synthesis of new supramolecular building blocks are promising targets for the construction of functional molecular systems. Here, we report a supramolecular scaffold based on boroxine that enables the formation of chiral and ordered 1D supramolecular polymers, which can be easily functionalized for circularly polarized luminescence. The boroxine monomers are quantitatively synthesized in situ, both in bulk and in solution, from boronic acid precursors and cooperatively polymerize into 1D helical aggregates stabilized by threefold hydrogen-bonding and π-π stacking. We then demonstrate amplification of asymmetry in the co-assembly of chiral/achiral monomers and the co-condensation of chiral/achiral precursors in classical and in situ sergeant-and-soldiers experiments, respectively, showing fast boronic acid exchange reactions occurring in the system. Remarkably, co-condensation of pyrene boronic acid with a hydrogen-bonding chiral boronic acid results in chiral pyrene aggregation with circularly polarized excimer emission and g-values in the order of 10 -3 . Yet, the electron deficiency of boron in boroxine makes them chemically addressable by nucleophiles, but also sensitive to hydrolysis. With this sensitivity in mind, we provide first insights into the prospects offered by boroxine-based supramolecular polymers to make chemically addressable, functional, and adaptive systems., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Spontaneous phase separation is a promising strategy for the development of novel electronic materials, as the resulting well-defined morphologies generally exhibit enhanced conductivity. Making these structures adaptive to external stimuli is challenging, yet crucial as multistate reconfigurable switching is essential for neuromorphic materials. Here, a modular and scalable approach is presented to obtain switchable phase-separated viologen-siloxane nanostructures with sub-5 nm features. The domain spacing, morphology, and conductivity of these materials can be tuned by ion exchange, repeated pulsed photoirradiation and electric stimulation. Counterion exchange triggers a postsynthetic modification in domain spacing of up to 10%. Additionally, in some cases, 2D to 1D order-order transitions are observed with the latter exhibiting a sevenfold decrease in conductivity with respect to their 2D lamellar counterparts. Moreover, the combination of the viologen core with tetraphenylborate counterions enables reversible and in situ reduction upon light irradiation. This light-driven reduction provides access to a continuum of conducting states, reminiscent of long-term potentiation. The repeated voltage sweeps improve the nanostructures alignment, leading to increased conductivity in a learning effect. Overall, these results highlight the adaptivity of phase-separated nanostructures for the next generation of organic electronics, with exciting applications in smart sensors and neuromorphic devices., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

In assemblies of identical molecules or chromophores, electronic excitations can be described as excitons, bound electron-hole pairs that can move from site to site as a pair in a coherent manner. The understanding of excitons is crucial when trying to engineer favorable photophysical properties through structuring organic molecular matter. In recent decades, limitations of the concept of an exciton have become clear. The exciton can hybridize with phonon and photons. To clarify these issues, the exciton is discussed within the broader context of the gauge properties of the electromagnetic force., (© 2023 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

Supramolecular chiral organization gives π-conjugated molecules access to fascinating specific interactions with circularly polarized light (CPL). Such a feature enables the fabrication of high-performance chiral organic electronic devices that detect or emit CPL directly. Herein, it is shown that chiral fused-ring electron-acceptor BTP-4F single-crystal-based phototransistors demonstrate distinguished CPL discrimination capability with current dissymmetry factor exceeding 1.4, one of the highest values among state-of-the-art direct CPL detectors. Theoretical calculations prove that the chirality at the supramolecular level in these enantiomeric single crystals originates from chiral exciton coupling of a unique quasi-2D supramolecular organization consisting of interlaced molecules with opposite helical conformation. Impressively, such supramolecular organization produces a higher dissymmetry factor along the preferred growth direction of the chiral single crystals in comparison to that of the short axis direction. Furthermore, the amplified, inverted, and also anisotropic current dissymmetry compared to optical dissymmetry is studied by finite element simulations. Therefore, a unique chiral supramolecular organization that is responsible for the excellent chiroptical response and anisotropic electronic properties is developed, which not only enables the construction of high-performance CPL detection devices but also allows a better understanding of the structure-property relationships in chiral organic optoelectronics., (© 2023 Wiley-VCH GmbH.)

The preparation of the first enantiopure cycloplatinated complexes bearing a bidentate, helicenic N-heterocyclic carbene and a diketonate ancillary ligand is presented, along with their structural and spectroscopic characterization based on both experimental and computational studies. The systems exhibit long-lived circularly polarized phosphorescence in solution and in doped films at room temperature, and also in a frozen glass at 77 K, with dissymmetry factor g lum values ≥10 -3 in the former and around 10 -2 in the latter.

When irradiating a molecular material containing photo-isomerizable groups with pure circularly polarized light, a particular handedness may get imprinted into the material. To study the mechanism and kinetics of this process in situ and operando, we have developed a new chiroptical tool where the circular polarization of the incident circularly polarized light is monitored after transmission through the photoactive layer. Practical limits to the resolution and sensitivity of the measurements as well as its calibration are discussed. To aid interpretation of experimental results, we present kinetic Monte Carlo simulations on a model for the active material involving photo-induced reorientation of molecules in a cholesteric organization. The simulations support the interpretation of a transient minimum in the degree of circular polarization of the transmitted light in terms of a nematic transient state during photo-inversion of a cholesteric organization in the molecular material., (© 2023 The Authors. Chirality published by Wiley Periodicals LLC.)

Detection of the circular polarization of light is possible using chiral semiconductors, yet the mechanisms remain poorly understood. Semi-transparent chiral photodiodes allow for a simple experiment to investigate the basis of their selectivity: changing the side from which the diode is illuminated. A reversal of circular selectivity is observed in photocurrent generation when changing the direction of illumination on organic, bulk-heterojunction cells. The change in selectivity can be explained by a space-charge limitation on the collection of photocarriers in combination with preferential absorption of one of the circular polarizations of near-infrared light by the chiral non-fullerene acceptor. The space-charge limitation is supported by detailed measurements of frequency and intensity dependence of dc and ac photocurrents., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Remote measurement of vital sign parameters like heartbeat and respiration rate represents a compelling challenge in monitoring an individual's health in a noninvasive way. This could be achieved by large field-of-view, easy-to-integrate unobtrusive sensors, such as large-area thin-film photodiodes. At long distances, however, discriminating weak light signals from background disturbance demands superior near-infrared (NIR) sensitivity and optical noise tolerance. Here, we report an inherently narrowband solution-processed, thin-film photodiode with ultrahigh and controllable NIR responsivity based on a tandem-like perovskite-organic architecture. The device has low dark currents (<10 -6 mA cm -2 ), linear dynamic range >150 dB, and operational stability over time (>8 hours). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically filtered silicon-based sensors. We demonstrate its potential in remote monitoring by measuring the heart rate and respiration rate from distances up to 130 cm in reflection.

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (J d ). Recently, using thermal admittance or spectral photocurrent measurements, J d has been attributed to thermal charge generation mediated by mid-gap states. Here, the temperature dependence of J d in state-of-the-art OPDs is reported with J d down to 10 -9  mA cm -2 at -0.5 V bias. For a variety of donor-acceptor bulk-heterojunction blends it is found that the thermal activation energy of J d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid-gap states. Ultra-sensitive sub-bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor-acceptor interface mediated by intra-gap states near the band edges., (© 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Non-fullerene acceptor molecules developed for organic solar cells feature a very intense absorption band in the near-infrared. In the solid phase, the strong interaction between light and the transition dipole moment for molecular excitation should induce formation of polaritons. The reflection spectra for polycrystalline films of a non-fullerene acceptor with a thienothienopyrrolo-thienothienoindole core of the so-called Y6 type indeed show a signature of polaritons. A local minimum in the middle of the reflection band is associated with the allowed molecular transition. The minimum in reflection allows efficient entry of light into the solid, resulting in a local maximum in external quantum efficiency of a photovoltaic cell made of the pure acceptor.

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

In their thermotropic liquid-crystalline state, molecular semiconductors can show charge transport with high carrier mobility. Polymerization of the corresponding mesogens into a cross-linked network often deteriorates the charge transport. Here, we report that mesogens consisting of a terthiophene core and discrete oligodimethylsiloxane side-chains terminated by acrylate units can be photopolymerized in the columnar phase with retention of nanoscale order and charge transport capabilities. We argue that the strong tendency for microphase segregation protects the semiconducting block from reacting with free radicals during polymerization. This work provides new insights into the design of electroactive materials with charge transport properties.

Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light-driven molecular motors as photo-controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non-invasively by light, opening possibilities for pixilated displays with externally addressable polarization., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Control over molecular motion is facilitated in materials with highly ordered nanoscale structures. Here we report on the fabrication of cholesteric liquid-crystal networks by circularly polarized light irradiation, without the need for chiral dopant or plasticizer. The polymer network is obtained by photopolymerization of a smectic achiral diacrylate mesogen consisting of an azobenzene core and discrete oligodimethylsiloxane tails. The synchronous helical photoalignment and photopolymerization originate from the cooperative movement of the mesogens ordered in well-defined responsive structures, together with the flexibility of the oligodimethylsiloxane blocks. The resulting thin films show excellent thermal stability and light-induced memory features with reversible responses. Additionally, we demonstrate the fabrication of photo-patterned films of liquid-crystal networks with opposite helical senses. These findings provide a new method to make light-controllable chiroptical materials with exciting applications in optics and photonics., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The assembly of donor-acceptor molecules via charge transfer (CT) interactions gives rise to highly ordered nanomaterials with appealing electronic properties. Here, we present the synthesis and bulk co-assembly of pyrene (Pyr) and naphthalenediimide (NDI) functionalized oligodimethylsiloxanes (oDMS) of discrete length. We tune the donor-acceptor interactions by connecting the pyrene and NDI to the same oligomer, forming a heterotelechelic block molecule (NDI-oDMSPyr), and to two separate oligomers, giving Pyr and NDI homotelechelic block molecules (Pyr-oDMS and NDI-oDMS). Liquid crystalline materials are obtained for binary mixtures of Pyr-oDMS and NDI-oDMS, while crystallization of the CT dimers occurred for the heterotelechelic NDI-oDMS-Pyr block molecule. The synergy between crystallization and phase-segregation coupled with the discrete length of the oDMS units allows for perfect order and sharp interfaces between the insulating siloxane and CT layers composed of crystalline CT dimers. We were able to tune the lamellar domain spacing and donor-acceptor CT interactions by applying pressures up to 6 GPa on the material, making the system promising for soft-material nanotechnologies. These results demonstrate the importance of the molecular design to tune the CT interactions and stability of a CT material.

Metal halide perovskite photodiodes (PPDs) offer high responsivity and broad spectral sensitivity, making them attractive for low-cost visible and near-infrared sensing. A significant challenge in achieving high detectivity in PPDs is lowering the dark current density (J D ) and noise current (i n ). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of J D for lead-tin based PPDs with different bandgaps and electron-blocking layers (EBL), we demonstrate that while EBLs eliminate electron injection, they facilitate undesired thermal charge generation at the EBL-perovskite interface. The interfacial energy offset between the EBL and the perovskite determines the magnitude and activation energy of J D . By increasing this offset we realized a PPD with ultralow J D and i n of 5 × 10 -8  mA cm -2 and 2 × 10 -14  A Hz -1/2 , respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes., (© 2021. The Author(s).)

Light-induced halide segregation hampers obtaining stable wide-band-gap solar cells based on mixed iodide-bromide perovskites. So far, the effect of prolonged illumination on the performance of mixed-halide perovskite solar cells has not been studied in detail. It is often assumed that halide segregation leads to a loss of open-circuit voltage. By simultaneously recording changes in photoluminescence and solar cell performance under prolonged illumination, we demonstrate that cells instead deteriorate by a loss of short-circuit current density and that the open-circuit voltage is less affected. The concurrent red shift, increased lifetime, and higher quantum yield of photoluminescence point to the formation of relatively emissive iodide-rich domains under illumination. Kinetic Monte Carlo simulations provide an atomistic insight into their formation via exchange of bromide and iodide, mediated by halide vacancies. Localization of photogenerated charge carriers in low-energy iodide-rich domains and subsequent recombination cause reduced photocurrent and red-shifted photoluminescence. The loss in photovoltaic performance is diminished by partially replacing organic cations by cesium ions. Ultrasensitive photocurrent spectroscopy shows that cesium ions result in a lower density of sub-band-gap defects and suppress defect growth under illumination. These defects are expected to play a role in the development and recovery of light-induced compositional changes., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

It has long been surmised that the circular polarization of luminescence (CPL) emitted by a chiral molecule or a molecular assembly should vary with the direction in which the photon is emitted. Despite its potential utility, this anisotropic CPL has not yet been demonstrated at the level of single molecules or supramolecular assemblies. Here we show that conjugated polymers bearing chiral side chains self-assemble into solid microspheres with a twisted bipolar interior, which are formed via liquid-liquid phase separation and subsequent condensation into a cholesteric lyotropic liquid crystalline mesophase. The resultant microspheres, when dispersed in methanol, exhibit CPL with a g lum value as high as 0.23. The microspheres are mechanically robust enough to be handled with a microneedle under ambient conditions, allowing comprehensive examination of the angular anisotropy of CPL. The single microsphere is found to exhibit distinct angularly anisotropic birefringence and CPL with g lum up to ∼0.5 in the equatorial plane, which is 2.5-fold greater than that along the polar axis. Such optically anisotropic solid materials are important for the application to next-generation microlight-emitting and visualizing devices as well as for fundamental optics studies of chiral light-matter interaction.

Squaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co-assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co-assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin-filtering measurements using mc-AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities. Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively. Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale., (© 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The optical properties of two sets of donor-acceptor-donor molecules with terminal bithiophene donor units and a central diketopyrrolopyrrole (DPP) acceptor unit are studied. The two sets differ in the alkyl chains on the DPP, which are either branched at the α-carbon (3-pentyl) ( 1 - 4 ) or linear ( n -hexyl) ( 5 - 8 ). Within each set, the molecules differ by the absence or presence of n -hexyl chains on the terminal thiophene rings in the 3', 4', or 5' positions. While in solution, the optical spectra differ only subtly; they differ dramatically in the solid state. In contrast to 5 - 8 , 1 - 4 are nonplanar as a consequence of the sterically demanding 3-pentyl groups, which inhibit π-stacking of the DPP units. Using the crystal structures of 2 (brick layer stacking) and 6 (slipped stacking), we quantitatively explain the solid state absorption spectra. By computing the molecular transition charge density and solving the dispersion relation, the optical absorption of the molecules in the crystal is predicted and in agreement with experiments. For 2 , a single resonance frequency is obtained, while for 6 two transitions are seen, with the lower-energy transition being less intense. The results demonstrate how subtle changes in substitution exert large effects in optical properties., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)

The field of supramolecular polymers is rapidly expanding; however, the exploitation of these systems as functional materials is still elusive. To become competitive, supramolecular polymers must display microstructural order and the emergence of new properties upon copolymerization. To tackle this, a greater understanding of the relationship between monomers' design and polymer microstructure is required as well as a set of functional monomers that efficiently interact with one another to synergistically generate new properties upon copolymerization. Here, we present the first implementation of frustrated Lewis pairs into supramolecular copolymers. Two supramolecular copolymers based on π-conjugated O -bridged triphenylborane and two different triphenylamines display the formation of B-N pairs within the supramolecular chain. The remarkably long lifetime and the circularly polarized nature of the resulting photoluminescence emission highlight the possibility to obtain an intermolecular B-N charge transfer. These results are proposed to be the consequences of the enchainment of B-N frustrated Lewis pairs within 1D supramolecular aggregates. Although it is challenging to obtain a precise molecular picture of the copolymer microstructure, the formation of random blocklike copolymers could be deduced from a combination of optical spectroscopic techniques and theoretical simulation.

Hybrid organic-inorganic perovskites allow the synthesis of high-quality, nanostructured semiconducting films via easily accessible solution-based techniques. This has allowed tremendous development in optoelectronic applications, primarily solar cells and light-emitting diodes. Allowed by the ease of access to nanostructure, chirality has recently been introduced in semiconducting perovskites as a promising way to obtain advanced control of charge and spin and for developing circularly polarized light sources. Circular polarization of photoluminescence (CPL) is a powerful tool to probe the electronic structure of materials. However, CPL in chiral perovskites has been scarcely investigated, and a study in bulk thin films and at room temperature is still missing. In this work, we fabricate bromine-based chiral perovskites by using a bulky chiral organic cation mixed with CsBr, resulting in Ruddlesden-Popper perovskite thin films. We measure CPL on these films at room temperature and, by using unpolarized photoexcitation, we record a degree of circular polarization of photoluminescence in the order of 10 -3 and provide a full spectral characterization of CPL. Our results show that chirality is imparted on the electronic structure of the semiconductor; we hypothesize that the excess in polarization of emitted light originates from the charge in the photogenerated Wannier exciton describing an orbit in a symmetry-broken environment. Furthermore, our experiments allow the direct measurement of the magnetic dipole moment of the optical transition, which we estimate to be ≥0.1 μ B . Finally, we discuss the implications of our findings on the development of chiral semiconducting perovskites as sources of circularly polarized light.

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C 71 -butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices., Competing Interests: The authors declare no competing interest.

Poly(3,4-ethylenedioxythiophene) blended with polystyrenesulfonate and poly(styrenesulfonic acid), PEDOT:PSS, has found widespread use in organic electronics. Although PEDOT:PSS is commonly used in its doped electrically conducting state, the ability to efficiently convert PEDOT:PSS to its undoped nonconducting state is of interest for a wide variety of applications ranging from biosensors to organic neuromorphic devices. Exposure to aliphatic monoamines, acting as an electron donor and Brønsted-Lowry base, has been reported to be partly successful, but monoamines are unable to fully dedope PEDOT:PSS. Remarkably, some-but not all-polyamines can dedope PEDOT:PSS very efficiently to very low conductivity levels, but the exact chemical mechanism involved is not understood. Here, we study the dedoping efficacy of 21 different aliphatic amines. We identify the presence of two or more primary amines, which can participate in an intramolecular reaction, as the key structural motif that endows polyamines with high PEDOT:PSS dedoping strength. A multistep reaction mechanism, involving sequential electron transfer and deprotonation steps, is proposed that consistently explains the experimental results. Finally, we provide a simple method to convert the commonly used aqueous PEDOT:PSS dispersion into a precursor formulation that forms fully dedoped PEDOT:PSS films after spin coating and subsequent thermal annealing., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

Polymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure-property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (β 1 and β 2 ) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to β 1 , the β 2 polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced π-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The β 1 and β 2 polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.

Singlet fission in pentacene creates two triplet excitons per absorbed photon. In a solar cell, each triplet can generate an electron-hole pair, and hence, external quantum efficiencies exceeding 100% have been reported for pentacene-fullerene solar cells. The energetics of this process are intriguing because the minimum photon energy loss, defined as the energy difference between the (triplet) exciton state and the open-circuit voltage, is less than 0.5 eV and distinctively smaller than that in most organic donor-acceptor solar cells. To investigate the energetics of this process, we analyze the effect of the energy of the lowest unoccupied molecular orbital (LUMO) for different fullerene derivatives. With the LUMO energy becoming less negative, the open-circuit voltage increases and charge generation decreases. For all but one of the fullerenes tested, the charge-transfer state energy is distinctively higher than the pentacene triplet energy, revealing that charge generation via singlet fission is actually endergonic. An elementary Marcus model for the rate of electron transfer provides a qualitative description of the experimental trends, in accordance with an endergonic charge transfer. Considering that charge generation from triplet states is endergonic, involvement of pentacene singlet states, either from direct photoexcitation or via triplet-triplet annihilation, cannot be excluded., Competing Interests: The authors declare no competing financial interest.

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near-infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single-junction and tandem OPDs are compared. In the latter, two nominally identical single-junction cells are processed on top of each other, effectively doubling the open-circuit voltage (V OC ). Both single-junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high-resolution retinal prostheses based on flexible OPD arrays., (© 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We investigate the effect of fluorination on the photovoltaic properties of an alternating conjugated polymer composed of 4,8-di-2-thienylbenzo[1,2- b :4,5- b ']dithiophene (BDT) and 4,7-bis([2,2'-bithiophen]-5-yl)-benzo-2-1-3-thiadiazole (4TBT) units in bulk-heterojunction solar cells. The unsubstituted and fluorinated polymers afford very similar open-circuit voltages and fill factor values, but the fluorinated polymer performed better due to enhanced aggregation which provides a higher photocurrent. The photovoltaic performance of both materials improved upon thermal annealing at 150-200 °C as a result of a significantly increased fill factor and open-circuit voltage, counteracted by a slight loss in photocurrent. Detailed studies of the morphology, light intensity dependence, external quantum efficiency and electroluminescence allowed the exploration of the effects of fluorination and thermal annealing on the charge recombination and the nature of the donor-acceptor interfacial charge transfer states in these films.

Chiral conjugated polymers bearing enantiopure side chains offer the possibility to harness the effect of chirality in organic electronic devices. However, its use is hampered by the low degree of circular polarization in absorption ( g abs ) in most of the conjugated polymer thin-films studied. Here we demonstrate a versatile method to significantly increase the g abs by using a few weight percentages of a commercially available achiral long-chain alcohol as an additive. This additive enhances the chiroptical properties in both absorption and emission by ca. 5-10 times in the thin-films. We envisage that the alcohol additive acts as a plasticizer which enhances the long-range chiral liquid crystalline ordering of the polymer chains, thereby amplifying the chiroptical properties in the thin-film. The application of this methodology to various conjugated polymers has been demonstrated., Competing Interests: The authors declare no competing financial interest.

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is frequently used as hole transport layer in planar p-i-n perovskite solar cells. Here we show that processing of a metal halide perovskite layer on top of PEDOT:PSS via spin coating of a precursor solution chemically reduces the oxidation state of PEDOT:PSS. This reduction leads to a lowering of the work function of the PEDOT:PSS and the perovskite layer on top of it. As a consequence, the solar cells display inferior performance with a reduced open-circuit voltage and a reduced short-circuit current density, which increases sublinearly with light intensity. The reduced PEDOT:PSS can be re-oxidized by thermal annealing of the PEDOT:PSS/perovskite layer stack in the presence of oxygen. As a consequence, thermal annealing of the perovskite layer in air provides solar cells with increased open-circuit voltage, short-circuit current density and high efficiency.

The straightforward synthesis and photophysical properties of a new series of heteroleptic iridium(III) bis(2-arylimidazole) picolinate complexes are reported. Each complex has been characterized by nuclear magnetic resonance, UV-vis, cyclic voltammetry, and photoluminescent angle dependency, and the emissive properties of each are described. The preferred orientation of transition dipoles in emitter/host thin films indicated more preferred orientation than homoleptic complex Ir(ppy) 3 ., Competing Interests: The authors declare no competing financial interest.

The reflection of infrared light by ionic crystals with cubic symmetry such as lithium fluoride, LiF, is analyzed in terms of phonon-polaritons. In contrast to the conventional view on phonon-polaritons that uses the Coulomb gauge and assumes a purely local dielectric response of the material, we here develop an alternative description making use of the Lorentz gauge. This involves retarded interactions between charges, implying a non-local response of the material to electromagnetic radiation. The resulting new phonon-polariton dispersion relation features polaritons with negative group velocity in the frequency range in between the transverse (ω T ) and longitudinal frequency (ω L ). By contrast, the conventional description predicts, in zero order, the absence of any propagating polaritons in the frequency interval between ω T and ω L . The new dispersion relation provides an efficient, zero-order description of the fine structure within the reststrahlen band of LiF. The local minimum near the middle of the reflectance band is due to excitation of a phonon-polariton whose energy and momentum matches that of the incoming photon. The Lorentz gauge description can also describe off-normal reflection and accounts for the experimentally observed widening of the reflection band with increasing angle of incidence.

The molecular organization in thermally annealed films of poly(9,9-bis((S)-3,7-dimethyloctyl)-2,7-fluorene-alt-benzothiadiazole) is investigated using polarized light. Measurement of linear polarization in transmission and reflection as a function of layer thickness and orientation directly show a left handed cholesteric organization with a pitch length of 600 nm. Results are corroborated by measurements of circularly polarized reflection and generalized ellipsometry and are compared to calculations of the optical properties based on the Maugin-Oseen-DeVries model. For wavelengths near the lowest allowed optical transition, light with the same handedness as the cholesteric arrangement (left) is found to be reflected and transmitted with a probability higher than right circularly polarized light. The high transmission for left polarized light is interpreted as an optical manifestation of the Borrmann effect.

We demonstrate a facile route to obtain high and broad-band circular polarization of electroluminescence in single-layer polymer OLEDs. As a light-emitting material we use a donor-acceptor polyfluorene with enantiomerically pure chiral side-chains. We show that upon thermal annealing the polymer self-assembles into a multidomain cholesteric film. By varying the thickness of the polymer emitting layer, we achieve high levels of circular polarization of electroluminescence (up to 40% excess of right-handed polarization), which are the highest reported for polymer OLEDs not using chiral dopants or alignment layers. Mueller matrix ellipsometry shows strong optical anisotropies in the film, indicating that the circular polarization of luminescence arises mainly after the photon has been generated, through selective scattering and birefringence correlated in the direction of the initial linear polarization of the photon. Our work demonstrates that chirally substituted conjugated polymers can combine photonic and semiconducting properties in advanced optoelectronic devices.

The efficiency of organic light emitting diodes (OLEDs) can be improved by controlling the orientation of the transition dipole moment of the emitters. Currently, no effective methods exist for orienting the transition dipole moments in solution processed active layers for OLEDs. We investigate the orientation of the transition dipole moment of small molecular emitters in a host matrix of poly(9,9-dioctylfluorene) (PFO) by means of angle dependent luminescence intensity measurements. The polymer chains of the host orient predominantly in the plane of the film. Fluorescent p -phenylenevinylene oligomers with 6 and 7 repeat units (OPV6, OPV7) are found to also orient preferentially horizontally. The orientation of the emitters can be improved by thermal annealing with up to 90% of transition dipole moments oriented in the plane of the film. The phosphorescent emitter Ir(MDQ) 2 (acac) shows a degree of horizontal orientation in the polymeric host matrix lower than that which is observed for oligomers, but as high as is observed for the same emitter in evaporated layers. A carbazole derivative capable of thermally activated delayed fluorescence shows a small preference for vertical orientation within the polymer host. The strong orientation of OPV6 and OPV7 in the oriented polymer host is rationalized in terms of their high aspect ratios. The use of PFO as host material in host/guest systems allows achieving horizontal orientation of transition dipole moments in solution processed oligomers and small molecular emitters.