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.
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
) and noise current (i
). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of J
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
. By increasing this offset we realized a PPD with ultralow J
and i
of 5 × 10
mA cm
and 2 × 10
A Hz
, respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes.
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.
Functional supramolecular systems like carbonyl-bridged triarylamine (CBT) trisamides are known for their long-range energy transport at room temperature. Understanding the complex self-assembly ...processes of this system allows for control over generated structures using controlled supramolecular polymerization. Here, we present two novel CBT trisamides with (S)- or (R)-chiral side chains which show a two-pathway self-assembly behavior in solution. Depending on the thermal profile during the self-assembly process, two different stable states are obtained under otherwise identical conditions. A kinetically trapped state A is reached upon cooling to 7 °C, via a proposed isodesmic process. In addition, there is a thermodynamically stable state B at 7 °C that is induced by first undercooling to −5 °C, via a nucleation-elongation mechanism. In both cases, helical supramolecular aggregates comprising H-aggregated CBTs are formed. Additionally, controlled supramolecular polymerization was achieved by mixing the two different states (A and B) from the same enantiomer, leading to a conversion of the kinetically trapped state to the thermodynamically stable state. This process is highly enantioselective, as no conversion is observed if the two states consist of opposite enantiomers. We thus show the importance and opportunities emerging from understanding the pathway complexity of functional supramolecular systems.
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.
A facile route to soft matter self‐powered bulk heterojunction photodiode detectors sensitive to the circular polarization state of light is shown based on the intrinsic excitonic circular dichroism ...of the photoactive layer blend. As light detecting materials, enantiopure semiconducting small molecular squaraine derivates of opposite handedness are employed. Via Mueller matrix ellipsometry, the circular dichroism is proven to be of H‐type excitonic nature and not originating from mesoscopic structural ordering. Within the green spectral range, the photodiodes convert circular polarized light into a handedness‐dependent photocurrent with a maximum dissymmetry factor of ±0.1 corresponding to 5% overall efficiency for the polarization discrimination under short circuit conditions. On the basis of transfer matrix optical simulations, it is rationalized that the optical dissymmetry fully translates into a photocurrent dissymmetry for ease of device design. Thereby, the photodiode's ability to efficiently distinguish between left and right circularly polarized light without the use of external optical elements and voltage bias is demonstrated. This allows a straightforward and sustainable future design of flexible, lightweight, and compact integrated platforms for chiroptical imaging and sensing.
Circular polarized light (CPL) detection is facilitated by strong excitonic circular dichroism of chiral small molecular semiconductors in a bulk heterojunction (BHJ) photodiode. The supramolecular exciton chirality is conserved in the BHJ blend allowing direct, CPL sensitive photovoltaic conversion without the need for additional polarization manipulating elements.
Organic photodetectors (OPDs) have gained increasing interest as they offer cost‐effective fabrication methods using low temperature processes, making them particularly attractive for large area ...image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible‐light OPDs are proposed for use in indirect‐conversion X‐ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near‐infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (Jd) is crucial to improve important figures of merits such as the signal‐to‐noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to Jd in OPDs. Jd reduction is typically addressed by fine‐tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental Jd values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.
Organic photodetectors (OPDs) are promising for large area image detectors. Minimizing the dark current density (Jd) is crucial in most applications; nevertheless, a wide range of Jd values has been reported. Here, possible reasons that lead to this large variation are discussed. A quantitative analysis of intrinsic Jd processes shows that charge injection from the electrodes is the dominant mechanism.
The nature of the solubilizing alkyl side chains has a strong effect on the performance of semiconducting diketopyrrolopyrrole polymers in organic solar cells with fullerene acceptors. The effect ...relates to the width of semicrystalline polymer fibrils that form in these blends. If the width of the fibril is wider than the exciton diffusion length, fewer charges form and the efficiency drops.
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.
Photophysical properties of diketopyrrolopyrrole derivatives substituted with oligothiophenes are investigated. All compounds are found to be fluorescent both in solution and in the solid phase. At ...low temperature in the solid, fluorescence originates from excimer-like excited states. Comparison of absorption and fluorescence excitation spectra taken under matrix isolated conditions and on solid films show the presence of both J- and H-type absorption bands in the solid phase. Quantum-chemical calculations, including exciton–phonon coupling to account for deviations from the Born–Oppenheimer approximation, are performed to simulate the band shape of the lowest absorption band in the molecular solid. The joint presence of J- and H-bands is explained by the presence of two molecules in the unit cell. The Davydov splitting is substantial for molecules with linear alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but can be reduced to almost zero by introducing branching at the β-carbon of the alkyl side chain.
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