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Controlling the multi-level assembly and morphological properties of conjugated polymers through structural manipulation has contributed significantly to the advancement of organic electronics. In this work, a redox active conjugated polymer, TPT-TT, composed of alternating 1,4-(2-thienyl)-2,5-dialkoxyphenylene (TPT) and thienothiophene (TT) units is reported with non-covalent intramolecular S⋯O and S⋯H-C interactions that induce controlled main-chain planarity and solid-state order. As confirmed by density functional theory (DFT) calculations, these intramolecular interactions influence the main chain conformation, promoting backbone planarization, while still allowing dihedral rotations at higher kinetic energies (higher temperature), and give rise to temperature-dependent aggregation properties. Thermotropic liquid crystalline (LC) behavior is confirmed by cross-polarized optical microscopy (CPOM) and closely correlated with multiple thermal transitions observed by differential scanning calorimetry (DSC). This LC behavior allows us to develop and utilize a thermal annealing treatment that results in thin films with notable long-range order, as shown by grazing-incidence X-ray diffraction (GIXD). Specifically, we identified a first LC phase, ranging from 218 °C to 107 °C, as a nematic phase featuring preferential face-on π-π stacking and edge-on lamellar stacking exhibiting a large extent of disorder and broad orientation distribution. A second LC phase is observed from 107 °C to 48 °C, as a smectic A phase featuring sharp, highly ordered out-of-plane lamellar stacking features and sharp tilted backbone stacking peaks, while the structure of a third LC phase with a transition at 48 °C remains unclear, but resembles that of the solid state at ambient temperature. Furthermore, the significance of thermal annealing is evident in the ∼3-fold enhancement of the electrical conductivity of ferric tosylate-doped annealed films reaching 55 S cm -1 . More importantly, thermally annealed TPT-TT films exhibit both a narrow distribution of charge-carrier mobilities (1.4 ± 0.1) × 10 -2 cm 2 V -1 s -1 along with a remarkable device yield of 100% in an organic field-effect transistor (OFET) configuration. This molecular design approach to obtain highly ordered conjugated polymers in the solid state affords a deeper understanding of how intramolecular interactions and repeat-unit symmetry impact liquid crystallinity, solution aggregation, solution to solid-state transformation, solid-state morphology, and ultimately device applications.

Free charge generation after photoexcitation of donor or acceptor molecules in organic solar cells generally proceeds via (1) formation of charge transfer states and (2) their dissociation into charge separated states. Research often either focuses on the first component or the combined effect of both processes. Here, we provide evidence that charge transfer state dissociation rather than formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between singlet and charge transfer states. We investigate devices based on dilute donor content blends of (fluorinated) ZnPc:C 60 and perform density functional theory calculations, device characterization, transient absorption spectroscopy and time-resolved electron paramagnetic resonance measurements. We draw a comprehensive picture of how energies and transitions between singlet, charge transfer, and charge separated states change upon ZnPc fluorination. We find that a significant reduction in photocurrent can be attributed to increasingly inefficient charge transfer state dissociation. With this, our work highlights potential reasons why low offset fullerene systems do not show the high performance of non-fullerene acceptors., (© 2024. The Author(s).)

An organic photovoltaic bulk heterojunction comprises of a mixture of donor and acceptor materials, forming a semi-crystalline thin film with both crystalline and amorphous domains. Domain sizes critically impact the device performance; however, conventional X-ray scattering techniques cannot detect the contrast between donor and acceptor materials within the amorphous intermixing regions. In this study, we employ neutron scattering and targeted deuteration of acceptor materials to enhance the scattering contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 system reveals a new length scale, indicating short-range aggregation of Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirm that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as shown by capacitance spectroscopy. Our findings uncover the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance., (© 2024. The Author(s).)

Organic photovoltaic cells using Y6 non-fullerene acceptors have recently achieved high efficiency, and it was suggested to be attributed to the charge-transfer (CT) nature of the excitations in Y6 aggregates. Here, by combining electroabsorption spectroscopy measurements and electronic-structure calculations, we find that the charge-transfer character already exists in isolated Y6 molecules but is strongly increased when there is molecular aggregation. Surprisingly, it is found that the large enhanced charge transfer in clustered Y6 molecules is not due to an increase in excited-state dipole moment, Δμ, as observed in other organic systems, but due to a reduced polarizability change, Δp. It is proposed that such a strong charge-transfer character is promoted by the stabilization of the charge-transfer energy upon aggregation, as deduced from density functional theory and four-state model calculations. This work provides insight into the correlation between molecular electronic properties and charge-transfer characteristics in organic electronic materials., (© 2024. The Author(s).)

The modern picture of negative charge carriers on conjugated polymers invokes the formation of a singly occupied (spin-up/spin-down) level within the polymer gap and a corresponding unoccupied level above the polymer conduction band edge. The energy splitting between these sublevels is related to on-site Coulomb interactions between electrons, commonly termed Hubbard U . However, spectral evidence for both sublevels and experimental access to the U value is still missing. Here, we provide evidence by n-doping the polymer P(NDI2OD-T 2 ) with [RhCp*Cp] 2 , [N-DMBI] 2 , and cesium. Changes in the electronic structure after doping are studied with ultraviolet photoelectron and low-energy inverse photoemission spectroscopies (UPS, LEIPES). UPS data show an additional density of states (DOS) in the former empty polymer gap while LEIPES data show an additional DOS above the conduction band edge. These DOS are assigned to the singly occupied and unoccupied sublevels, allowing determination of a U value of ∼1 eV.

Realizing high luminescence dissymmetry factor ( g ) in circularly polarized luminescence (CPL) materials remains a big challenge, which necessitates understanding systematically how their molecular structure controls the CPL. Here we investigate representative organic chiral emitters with different transition density distributions and reveal the pivotal role of transition density in CPL. We rationalize that to obtain large g -factors, two conditions should be simultaneously satisfied: (i) the transition density for the S 1 (or T 1 )-to- S 0 emission must be delocalized over the entire chromophore; and (ii) the chromophore inter-segment twisting must be restricted and tuned to an optimal value (∼50°). Our findings offer molecular-level insights into the CPL of organic emitters, with potential applications in the design of chiroptical materials and systems with strong CPL effects., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources─relying only on backfilled hydrogen gas and ion gauge filaments─and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br 3 )DTPA) as monomer precursors, we find that atomic hydrogen (H • ) blocks aryl-aryl bond formation to such an extent that we suspect this reaction may be a factor in limiting the ultimate size of 2D COFs created through on-surface synthesis. Conversely, we show that control of the relative monomer and hydrogen fluxes can be used to produce large self-assembled islands of monomers, dimers, or macrocycle hexamers, which are of interest in their own right. On-surface synthesis of oligomers, from a single precursor, circumvents potential challenges with their protracted wet-chemical synthesis and with multiple deposition sources. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D COF (synthesized in the absence of atomic hydrogen) as the end point in an evolution of electronic structures from the monomer.

Designing N-coordinated porous single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is a promising approach to achieve enhanced energy conversion due to maximized atom utilization and higher activity. Here, we report two Co(II)-porphyrin/ [2,1,3]-benzothiadiazole (BTD)-based covalent organic frameworks (COFs; Co@ rhm -PorBTD and Co@ sql -PorBTD), which are efficient SAC systems for O 2 electrocatalysis (ORR). Experimental results demonstrate that these two COFs outperform the mass activity (at 0.85 V) of commercial Pt/C (20%) by 5.8 times (Co@ rhm -PorBTD) and 1.3 times (Co@ sql -PorBTD), respectively. The specific activities of Co@ rhm -PorBTD and Co@ sql -PorBTD were found to be 10 times and 2.5 times larger than that of Pt/C, respectively. These COFs also exhibit larger power density and recycling stability in Zn-air batteries compared with a Pt/C-based air cathode. A theoretical analysis demonstrates that the combination of Co-porphyrin with two different BTD ligands affords two crystalline porous electrocatalysts having different d-band center positions, which leads to reactivity differences toward alkaline ORR. The strategy, design, and electrochemical performance of these two COFs offer a pyrolysis-free bottom-up approach that avoids the creation of random atomic sites, significant metal aggregation, or unpredictable structural features.

In contrast to closed-shell luminescent molecules, the electronic ground state and lowest excited state in organic luminescent radicals are both spin doublet, which results in spin-allowed radiative transitions. Most reported luminescent radicals with high photoluminescent quantum efficiency (PLQE) have a donor-acceptor (D-A•) chemical structure where an electron-donating group is covalently attached to an electron-withdrawing radical core (A•). Understanding the main factors that define the efficiency and stability of D-A• type luminescent radicals remains challenging. Here, we designed and synthesized a series of tri(2,4,6-trichlorophenyl)methyl (TTM) radical derivatives with donor substituents varying by their extent of conjugation and their number of imine-type nitrogen atoms. The experimental results suggest that the luminescence efficiency and stability of the radicals are proportional to the degree of conjugation but inversely proportional to the number of imine nitrogen atoms in the substituents. These experimental trends are very well reproduced by density functional theory calculations. The theoretical results indicate that both the luminescence efficiency and radical stability are related to the energy difference between the charge transfer (CT) and local-excitation (LE) states, which decreases as either the number of imine nitrogen atoms in the substituent increases or its conjugation length decreases., (© 2022 Wiley-VCH GmbH.)

Many quantum magnetic materials suffer from structural imperfections. The effects of structural disorder on bulk properties are difficult to assess systematically from a chemical perspective due to the complexities of chemical synthesis. The recently reported S = 1/2 kagome lattice antiferromagnet, (CH 3 NH 3 ) 2 NaTi 3 F 12 , 1-Ti , with highly symmetric kagome layers and disordered interlayer methylammonium cations, shows no magnetic ordering down to 0.1 K. To study the impact of structural disorder in the titanium fluoride kagome compounds, (CH 3 NH 3 ) 2 KTi 3 F 12 , 2-Ti , was prepared. It presents no detectable structural disorder and only a small degree of distortion of the kagome lattice. The methylammonium disorder model of 1-Ti and order in 2-Ti were confirmed by atomic-resolution transmission electron microscopy. The antiferromagnetic interactions and band structures of both compounds were calculated based on spin-polarized density functional theory and support the magnetic structure analysis. Three spin-glass-like (SGL) transitions were observed in 2-Ti at 0.5, 1.4, and 2.3 K, while a single SGL transition can be observed in 1-Ti at 0.8 K. The absolute values of the Curie-Weiss temperatures of both 1-Ti (-139.5(7) K) and 2-Ti (-83.5(7) K) are larger than the SGL transition temperatures, which is indicative of geometrically frustrated spin glass (GFSG) states. All the SGL transitions are quenched with an applied field >0.1 T, which indicates novel magnetic phases emerge under small applied magnetic fields. The well-defined structure and the lack of structural disorder in 2-Ti suggest that 2-Ti is an ideal model compound for studying GFSG states and the potential transitions between spin liquid and GFSG states.

Molecular electronic spin qubits are promising candidates for quantum information science applications because they can be reliably produced and engineered via chemical design. Embedding electronic spin qubits within two-dimensional polymers (2DPs) offers the possibility to systematically engineer inter-qubit interactions while maintaining long coherence times, both of which are prerequisites to their technological utility. Here, we introduce electronic spin qubits into a diamagnetic 2DP by n -doping naphthalene diimide subunits with varying amounts of CoCp 2 and analyze their spin densities by quantitative electronic paramagnetic resonance spectroscopy. Low spin densities ( e.g. , 6.0 × 10 12 spins mm -3 ) enable lengthy spin-lattice ( T 1 ) and spin-spin relaxation ( T 2 ) times across a range of temperatures, ranging from T 1 values of 164 ms at 10 K to 30.2 μs at 296 K and T 2 values of 2.36 μs at 10 K to 0.49 μs at 296 K for the lowest spin density sample examined. Higher spin densities and temperatures were both found to diminish T 1 times, which we attribute to detrimental cross-relaxation from spin-spin dipolar interactions and spin-phonon coupling, respectively. Higher spin densities decreased T 2 times and modulated the T 2 temperature dependence. We attribute these differences to the competition between hyperfine and dipolar interactions for electron spin decoherence, with the dominant interaction transitioning from the former to the latter as spin density and temperature increase. Overall, this investigation demonstrates that dispersing electronic spin qubits within layered 2DPs enables chemical control of their inter-qubit interactions and spin decoherence times.

The ability to design and control the chemical characteristics of covalent organic frameworks (COFs) offers a new avenue for the development of functional materials, especially with respect to topological properties. Based on density functional theory calculations, by varying the core units through the choice of bridging groups [O, C═O, CH 2 , or C(CH 3 ) 2 ] and the linker units [acetylene, diacetylene, or benzene], we have designed heterotriangulene-based COFs that are predicted to be two-dimensional higher-order topological insulators (TIs). The higher-order TI characteristics of these COFs are identified via their topological invariants and the presence of in-gap topological corner modes and gapped edge states. The frontier molecular orbital energies of the building moieties play an important role in determining the size of the higher-order TI gap, which we find to be highly dependent on linker units. We also examined the deposition of the COFs on a boron nitride substrate to assess the feasibility of experimental observation of a higher-order TI phase in the organic layer. This work thus provides new insights into heterotriangulene-based COFs and guidance for the exploration of purely organic topological materials.

Record power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have been obtained with the organic hole transporter 2,2',7,7'-tetrakis( N , N -di- p -methoxyphenyl-amine)9,9'-spirobifluorene (spiro-OMeTAD). Conventional doping of spiro-OMeTAD with hygroscopic lithium salts and volatile 4- tert -butylpyridine is a time-consuming process and also leads to poor device stability. We developed a new doping strategy for spiro-OMeTAD that avoids post-oxidation by using stable organic radicals as the dopant and ionic salts as the doping modulator (referred to as ion-modulated radical doping). We achieved PCEs of >25% and much-improved device stability under harsh conditions. The radicals provide hole polarons that instantly increase the conductivity and work function (WF), and ionic salts further modulate the WF by affecting the energetics of the hole polarons. This organic semiconductor doping strategy, which decouples conductivity and WF tunability, could inspire further optimization in other optoelectronic devices.

2D polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors is enhanced by controllably n-doping the NDI units using cobaltocene (CoCp 2 ). Optical and transient microwave spectroscopy reveal that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp 2 , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. While the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10 -4  S cm -1 are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices., (© 2022 Wiley-VCH GmbH.)

While organic self-assembled monolayers (SAMs) have been widely used to modify the work function of metal and metal-oxide surfaces, their application to tune the critical temperature of a superconductor has only been considered recently when SAMs were deposited on NbSe 2 monolayers (Calavalle et al 2021 Nano Lett. 21 136-143). Here, we describe the results of density functional theory calculations performed on the experimentally reported organic/NbSe 2 systems. Our objectives are: (i) to determine how the organic layers impact the NbSe 2 work function and electronic density of states; (ii) to understand the possible correlation with the experimental variations in superconducting behavior upon SAM deposition. We find that, upon adsorption of the organic monolayers, the work-function modulation induced by the SAM and interface dipoles is consistent with the experimental results. However, there occurs no significant difference in the electronic density of states near the Fermi level, a consequence of the absence of any charge transfer across the organic/NbSe 2 interfaces. Therefore, our results indicate that it is not a SAM-induced tuning of the NbSe 2 density of states near the Fermi level that leads to the tuning of the superconducting critical temperature. This calls for further explorations, both experimentally and theoretically, of the mechanism underlying the superconducting critical temperature variation upon formation of SAM/NbSe 2 interfaces., (© 2022 IOP Publishing Ltd.)

Fused-ring core nonfullerene acceptors (NFAs), designated "Y-series," have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid-state NMR, crystallography, and molecular modeling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs., (© 2021 Wiley-VCH GmbH.)

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = ∼22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, k nr , is on the order of 10 5 s -1 , approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing k nr . Furthermore, despite the high radiative efficiency and already comparatively slow k nr , we find that k nr is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.

Two-dimensional covalent organic frameworks (2D-COFs), also referred to as 2D polymer networks, display unusual electronic-structure characteristics, which can significantly enrich and broaden the fields of electronics and spintronics. In this Focus article, our objective is to lay the groundwork for the conceptual description of the fundamental relationships among the COF electronic structures, the symmetries of their 2D lattices, and the frontier molecular orbitals (MOs) of their core and linker components. We focus on monolayers of hexagonal COFs and use tight-binding model analyses to highlight the critical role of the frontier-MO symmetry, in addition to lattice symmetry, in determining the nature of the electronic bands near the Fermi level. We rationalize the intriguing feature that, when the core unit has degenerate highest occupied MOs [or lowest unoccupied MOs], the COF highest valence band [or lowest conduction band] is flat but degenerate with a dispersive band at a high-symmetry point of the Brillouin zone; the consequences of having such band characteristics are briefly described. Multi-layer and bulk 2D COFs are found to maintain the salient features of the monolayer electronic structures albeit with a reduced bandgap due to the interlayer coupling. This Focus article is thus meant to provide an effective framework for the engineering of flat and Dirac bands in 2D polymer networks.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as one of the most promising light-emitting materials for optoelectronic devices with outstanding performance. However, the facile detachment of surface capping organic ligands from these NCs leads to very poor colloidal stability and durability. This is mainly due to the weak interfacial interactions between the inorganic perovskite core and ligands, high density of surface defect states, and aggregation of NCs. Here, using a combination of time-resolved laser spectroscopy and density functional theory (DFT) calculations, we explored the major impact of surface orientations and terminations for both CsPbBr 3 and Cs 4 PbBr 6 NCs not only on the interfacial binding affinities with organic ligands but also on surface defect formation and NC aggregation. By rationalizing that surface trap states are responsible for the decrease in photoluminescence (PL) upon fabrication and purification, we propose a powerful ligand-engineering strategy for eliminating these trap states and preventing the aggregation of CsPbBr 3 and Cs 4 PbBr 6 NCs. Interestingly, we find that the surface orientation and dimensionality determine the degree of interfacial interactions between the inorganic perovskite core and ligands and subsequently control the overall PL intensity and NC stability. Our results demonstrate that a treatment of as-synthesized CsPbBr 3 NCs consisting of the addition of extra oleylammonium bromide (OAmBr) as a capping ligand, allows the CsPbBr 3 NCs to retain their green emission with increased PL intensity and quantum yields and improves colloidal durability. On the other hand, the ultraviolet emissions of Cs 4 PbBr 6 NCs are effectively increased upon addition of extra cesium oleate (CsOL) as the trap states induced by surface cesium ions are largely reduced by the formation of Cs-O bonds. Our work provides a robust and adequate ligand engineering approach to significantly enhance the optical behavior of perovskite NCs with different dimensionalities and various compositions and to achieve more efficient and stable light-harvesting devices.

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure-property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω-Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs., (© 2021 Wiley-VCH GmbH.)

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m -1 K -1 ) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Hot-carrier cooling processes of perovskite materials are typically described by a single parabolic band model that includes the effects of carrier-phonon scattering, hot phonon bottleneck, and Auger heating. However, little is known (if anything) about the cooling processes in which the spin-degenerate parabolic band splits into two spin-polarized bands, i.e., the Rashba band splitting effect. Here, we investigated the hot-carrier cooling processes for two slightly different compositions of two-dimensional Dion-Jacobson hybrid perovskites, namely, (3AMP)PbI 4 and (4AMP)PbI 4 (3AMP = 3-(aminomethyl)piperidinium; 4AMP = 4-(aminomethyl)piperidinium), using a combination of ultrafast transient absorption spectroscopy and first-principles calculations. In (4AMP)PbI 4 , upon Rashba band splitting, the spin-dependent scattering of hot electrons is responsible for accelerating hot-carrier cooling at longer delays. Importantly, the hot-carrier cooling of (4AMP)PbI 4 can be extended by manipulating the spin state of the hot carriers. Our findings suggest a new approach for prolonging hot-carrier cooling in hybrid perovskites, which is conducive to further improving the performance of hot-carrier-based optoelectronic and spintronic devices.

In bulk-heterojunction organic solar cells, it is well established that the active-layer morphology significantly impacts device performance. However, morphology control remains very challenging. An interesting step in that direction was recently provided by the development of polymer donors that display a temperature-dependent aggregation behavior in solution; the aggregation characteristics were found indeed to play a determining role in the eventual active-layer morphology. Here, a combination of thermodynamic analyses, molecular dynamics simulations, and long-range corrected density functional theory calculations enables us (i) to establish the Flory-Huggins interaction parameter, χ, as a useful figure of merit that allows us to integrate the contributions from all inter-related molecular interactions and to describe the extent of polymer preaggregation in solution at room temperature; (ii) to correlate the χ values for various polymer solutions to the extent of polymer aggregation and the morphological characteristics of the active layers; and (iii) to assess how polymer-polymer and polymer-solvent intermolecular interactions contribute to the variations in χ values among different polymer solutions. We have chosen to examine four representative polymer donors (PBT4T-2OD, PBTff4T-2OD, PffBT4T-2OD, and PffBTff4T-2DT) in solution in chlorobenzene or dichlorobenzene. With χ as a robust bridge, our results provide an unprecedented, detailed description of the relationships among intermolecular interactions, extent of polymer solution aggregation, and morphological features of the active layers.

Emergent quantum phenomena in electronically coupled two-dimensional heterostructures are central to next-generation optical, electronic, and quantum information applications. Tailoring electronic band gaps in coupled heterostructures would permit control of such phenomena and is the subject of significant research interest. Two-dimensional polymers (2DPs) offer a compelling route to tailored band structures through the selection of molecular constituents. However, despite the promise of synthetic flexibility and electronic design, fabrication of 2DPs that form electronically coupled 2D heterostructures remains an outstanding challenge. Here, we report the rational design and optimized synthesis of electronically coupled semiconducting 2DP/2D transition metal dichalcogenide van der Waals heterostructures, demonstrate direct exfoliation of the highly crystalline and oriented 2DP films down to a few nanometers, and present the first thickness-dependent study of 2DP/MoS 2 heterostructures. Control over the 2DP layers reveals enhancement of the 2DP photoluminescence by two orders of magnitude in ultrathin sheets and an unexpected thickness-dependent modulation of the ultrafast excited state dynamics in the 2DP/MoS 2 heterostructure. These results provide fundamental insight into the electronic structure of 2DPs and present a route to tune emergent quantum phenomena in 2DP hybrid van der Waals heterostructures.

Neutral donor-acceptor (D-A • ) organic radicals have recently attracted a great deal of attention as promising luminescent materials due to their strong doublet emission. Here, we consider a series of emitters based on substituted triarylamine (TAA) donors and a radical-carrying perchlorotriphenylmethyl (PTM) acceptor. We evaluate, by means of quantum-chemical calculations and theoretical modeling, how chemical substitution affects the electronic structures and radiative and nonradiative decay rates. Our calculations show that the radiative decay rates are dominated in all instances by the electronic coupling between the lowest excited state, which has charge-transfer (CT) character, and the ground state. On the other hand, the nonradiative decay rates in the case of TAA-PTM radicals that have high CT energies are defined by the electronic hybridization of the CT state with local excitations (LE) on the PTM moiety; also, these nonradiative rates deviate significantly from the gap law dependence that is observed in the TAA-PTM radicals that have low CT energies. These findings underscore that hybridization of the emissive state with high-energy states can, in analogy with the intensity borrowing effect commonly invoked for radiative transitions, enhance as well the nonradiative decay rates. Our results highlight that in order to understand the emissive properties of D-A • radicals, it is required that the electronic hybridization of the CT states with both the ground and the LE states be properly considered.

The performance of organic light-emitting diodes based on thermally activated delayed fluorescence emitters depends on the efficiency of reverse intersystem crossing (RISC) processes, which are promoted by a small energy gap between the lowest singlet (S 1 ) and triplet (T 1 ) excited states and large spin-orbit couplings. Recently, it was proposed that the introduction of secondary donor units into 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) can significantly increase the mixing between triplet states with charge-transfer (CT) and local-excitation characteristics and consequently increase the spin-orbit couplings. Here, the results of long-range corrected density functional theory calculations show that the main impact on the RISC rates of substituting 5CzBN with secondary donors is due to a decrease in adiabatic singlet-triplet energy gaps and intramolecular reorganization energies rather than to a change in spin-orbit couplings. Our calculations underline that at least two singlet and three triplet excited states contribute to the ISC/RISC processes in 5CzBN and its derivatives. In addition, we find that in all emitters, the lowest singlet excited-state potential energy surface has a double-minimum shape.

Organic field-effect transistors (OFETs) are not only functional devices but also represent an important tool for measuring the charge-transport properties of organic semiconductors (OSs). Thus, efforts to understand the performance and characteristics of OFET devices are not only useful in helping achieve higher device efficiencies but also critical to ensuring accuracy in the evaluations of OS charge mobilities. These studies rely on OFET device models, which connect the measured current characteristics to the properties of the OSs. Developing such OFET models requires good knowledge of the charge-transport processes in OSs. In device active layers, the OS thin films are either amorphous (e.g. in organic light-emitting diodes and organic solar cells) or crystalline (e.g. those optimized for charge transport in OFETs). When the electronic couplings between adjacent OS molecules or polymer chain segments are weak, the charge-transport mechanism is dominated by hopping processes, which is the context in which we frame the discussion in this Review. Factors such as disorder, mobility anisotropy, traps, grain boundaries or film morphology all impact charge transport. To take these features fully into account in an OFET device model requires considering a nano-scale, molecular-level resolution. Here, we discuss the recent development of such molecular-resolution OFET models based on a kinetic Monte Carlo approach relevant to the hopping regime. We also briefly describe the applicability of these models to high-mobility OFETs, where we underline the need to extend them to incorporate aspects related to charge delocalization., (© The Author(s) 2020. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.)

Range-separated hybrid (RSH) functionals have become a tool of choice to study the intra- and inter -molecular electronic states in organic materials. These functionals provide the most accurate descriptions of the electronic structure when the range-separation parameter is optimally tuned (OT). However, since the range-separation parameter is molecule dependent, this approach faces consistency issues when applied to the multicomponent systems typically found in the active layers of organic solar cells or organic light-emitting diodes (OLEDs). Here, we investigate the performance of four common RSH functionals in the description of the excited states of three molecular compounds used as components of the active layer in a hyperfluorescence OLED device. Our results indicate that the excited-state energies of the investigated molecules show a very weak dependence on the range-separation parameter value when they are evaluated by means of a screened version of RSH functionals. In this instance, the excited states of all three molecular compounds can be derived accurately and consistently with the exact same functional.

Here we report that a covalent organic framework (COF), which contains 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties, undergoes color changes in the presence of solvents or solvent vapor that are rapid, passive, reversible, and easily detectable by the naked eye. A new visible absorption band appears in the presence of polar solvents, especially water, suggesting reversible conversion to another species. This reversibility is attributed to the ability of the diiminol to rapidly tautomerize to an iminol/ cis -ketoenamine and its inability to doubly tautomerize to a diketoenamine. Density functional theory (DFT) calculations suggest similar energies for the two tautomers in the presence of water, but the diiminol is much more stable in its absence. Time-dependent DFT calculations confirm that the iminol/ cis -ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.

Two-dimensional covalent organic frameworks (2D COFs) are composed of structurally precise, permanently porous, layered polymer sheets. 2D COFs have traditionally been synthesized as polycrystalline aggregates with small crystalline domains. Only recently have a small number of 2D COFs been obtained as single crystals, which were prepared by a seeded growth approach via the slow introduction of monomers, which favored particle growth over nucleation. However, these procedures are slow and operationally difficult, making it desirable to develop polymerization methods that do not require the continuous addition of reactants over days or weeks. Here, we achieve the rapid growth of boronate ester-linked COFs by chemically suppressing nucleation via addition of an excess of a monofunctional competitor, 4- tert -butylcatechol (TCAT), into the polymerization. In situ X-ray scattering measurements show that TCAT suppresses colloid nucleation, which enables seeded growth polymerizations in the presence of high monomer concentrations. Kinetic Monte Carlo simulations reveal that TCAT limits oligomers to sizes below the critical nucleus size and that in-plane expansion is restricted compared to out-of-plane oriented attachment of oligomers. The simulations are consistent with transmission electron micrographs, which show that the particles grow predominantly in the stacking direction. This mechanistic insight into the role of the modulators in 2D polymerizations enables the size and aspect ratio of COF colloids to be controlled under operationally simple conditions. This chemically controlled growth strategy will accelerate the discovery and exploration of COF materials and their emergent properties., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

Hot carrier (HC) cooling is a critical photophysical process that significantly influences the optoelectronic performance of hybrid perovskite-based devices. The hot carrier extraction at the device interface is very challenging because of its ultrashort lifetime. Here, ultrafast transient reflectance spectroscopy measurements and time-domain ab initio calculations show how the dielectric constant of the organic spacers can control and slow the HC cooling dynamics in single-crystal 2D Ruddlesden-Popper hybrid perovskites. We find that (EA) 2 PbI 4 (EA = HOC 2 H 4 NH 3 + ) that correspond to a high dielectric constant organic spacer has a longer HC cooling time compared to that of (AP) 2 PbI 4 (AP = HOC 3 H 6 NH 3 + ) and (PEA) 2 PbI 4 (PEA = C 6 H 5 C 2 H 4 NH 3 + ). The slow HC relaxation process in the former case can be ascribed to a stronger screening of the Coulomb interactions, a small nonradiative internal conversion within the conduction bands, as well as a weak electron-phonon coupling. Our findings provide a strategy to prolong the hot carrier cooling time in low-dimensional hybrid perovskite materials by using organic spacers with reduced dielectric confinement.

Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor-acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor-acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.

The charge-transfer electronic states appearing at the donor-acceptor interfaces in organic solar cells mediate exciton dissociation, charge generation, and charge recombination. To date, the characterization of their nature has been carried out on the basis of models that only involve the charge-transfer state and the ground state. Here, we demonstrate that it is essential to go beyond such a two-state model and to consider explicitly as well the electronic and vibrational couplings with the local absorbing state on the donor and/or acceptor. We have thus developed a three-state vibronic model that allows us: to provide a reliable description of the optical absorption features related to the charge-transfer states; to underline the erroneous interpretations stemming from the application of the semi-classical two-state model; and to rationalize how the hybridization between the local-excitation state and charge-transfer state can lead to lower non-radiative voltage losses and higher power conversion efficiencies.

An all-acceptor napthalenediimide-bithiazole-based co-polymer, P(NDI2OD-BiTz), was synthesized and characterized for application in thin-film transistors. Density functional theory calculations point to an optimal perpendicular dihedral angle of 90° between acceptor units along isolated polymer chains; yet optimized transistors yield electron mobility of 0.11 cm 2 /(V s) with the use of a zwitterionic naphthalene diimide interlayer. Grazing incidence X-ray diffraction measurements of annealed films reveal that P(NDI2OD-BiTz) adopts a highly ordered edge-on orientation, exactly opposite to similar bithiophene analogs. This report highlights an NDI and thiazole all-acceptor polymer and demonstrates high electron mobility despite its nonplanar backbone conformation.

Nonlinearity in the current characteristics of organic field-effect transistor (OFET) devices has become a serious issue for accurate evaluation of the charge-carrier mobilities in organic semiconductors. In particular, in the case of several high-mobility materials, a kink appears in the transfer curves, and this nonlinearity has been generally interpreted as the result of poor contacts. Here, we describe another possible origin for the appearance of such a kink. Extensive molecular-scale device simulations indeed demonstrate that the quasi-1D nature of charge transport often encountered in organic crystals or highly oriented polymers can lead to significant transport through the bulk and result in nonlinearity of the transfer current characteristics if the actual charge injection is away from the channel. When this is the case, the low-gate voltage regime in fact does not overestimate the charge mobility along the channel direction.

A well-defined series of long and soluble isoindigo thienoacene oligomers have been synthesized from a novel electron deficient building block: benzo[1,2-b:4,5-b']bis[b]benzothiophene bislactams. Extension of the π-conjugated systems facilitates control of the optical, electronic and device characteristics.

Zero-dimensional (0D) perovskites have recently opened a new frontier in device engineering for light conversion technologies due to their unprecedented high photoluminescence quantum yield as solids. Although many experimental and theoretical efforts have been made to understand their optical behavior, the origin of their green emission is still opaque. Here, we develop a complete experimental and theoretical picture of point defects in Cs-Pb-Br perovskites and demonstrate that bromide vacancies (V Br ) in prototype 0D perovskite Cs 4 PbBr 6 have a low formation energy and a relevant defect level to contribute to the midgap radiative state. Moreover, the state-of-the-art characterizations including atomic-resolution electron imaging not only confirm the purity of the 0D phase of Br-deficient green-emissive Cs 4 PbBr 6 nanocrystals (NCs) but also exclude the presence of CsPbBr 3 NCs impurities. Our findings provide robust evidence for defect-induced green luminescence in 0D perovskite NCs, which helps extend the scope of the utility of these bulk 0D quantum materials in optoelectronic applications.

In hybrid perovskite materials, the organic cations are one of the key structural components used to tune the electronic and optical properties of this promising class of materials. Here, we studied the strong impact of organic cations, methylammonium (MA) and formamidinium (FA), on halogen vacancy and interstitial migration, as well as surface degradation in cubic-phase MAPbBr 3 and FAPbBr 3 crystals using density functional theory calculations. We found Br vacancies and interstitials have much lower formation energies and higher density in MAPbBr 3 in comparison to FAPbBr 3 crystals. Moreover, the transition energy barrier for Br migration through vacancies within the bulk phase is lower in MAPbBr 3 than in FAPbBr 3 . We also found that FAPbBr 3 has a much higher rotation barrier of the organic cation than MAPbBr 3 , which points to a much stronger H-bonding with Br in the former case. Our results show that incorporating organic cations with the appropriate structure, shape, and strong H-bonding capabilities in hybrid perovskite crystals is very beneficial for suppressing ion migration and thus further improving the performance of hybrid perovskite-based devices.

Since the seminal work of Tang and Vanslyke in 1987 on small-molecule emitters and that of Friend and co-workers in 1990 on conjugated-polymer emitters, organic light-emitting diodes (OLEDs) have attracted much attention from academia as well as industry, as the OLED market is estimated to reach the $30 billion mark by the end of 2018. In these first-generation organic emitters, on the basis of simple spin statistics, electrical excitation resulted in the formation of ∼25% singlet excitons and ∼75% triplet excitons. Radiative decay of the singlet excitons to the singlet ground state leads to a prompt fluorescence emission, while the triplet excitons only lead to weak phosphorescence due to the very small spin-orbit couplings present in purely organic molecules. The consequence is a ca. 75% energy loss, which triggered wide-ranging efforts to try and harvest as many of the triplet excitons as possible. In 1998, Thompson, Forrest, and their co-workers reported second-generation OLED emitters based on coordination complexes with heavy transition metals (e.g., iridium or platinum). Here, the triplet excitons stimulate efficient and fast phosphorescence due to the strong spin-orbit couplings enabled by the heavy-metal atoms. Internal quantum efficiencies (IQE) up to 100% have been reported, which means that for every electron injected into the device, a photon is emitted. While these second-generation emitters are those mainly exploited in current OLED applications, there is strong impetus from both cost and environmental standpoints to find new ways of exploiting purely organic emitters, which in addition can offer greater flexibility to fine-tune the electronic and optical properties by exploiting the synthetic organic chemistry toolbox. In 2012, Adachi and co-workers introduced a promising strategy, based on thermally activated delayed fluorescence (TADF), to harvest the triplet excitons in purely organic molecular materials. These materials now represent the third generation of OLED emitters. Impressive photophysical properties and device performances have been reported, with internal quantum efficiencies also reaching nearly 100%. Our objectives in this Account are threefold: (i) to lay out a comprehensive description, at the molecular level, of the fundamental photophysical processes behind TADF emitters; (ii) to discuss some of the challenges facing the design of TADF emitters, such as the need to balance the efficiency of thermal activation of triplet excitons into the singlet manifold with the efficiency of radiative transition to the ground state; and (iii) to highlight briefly some of the recent molecular-design strategies that pave the way to new classes of TADF materials.

Flexibility and deformability are important factors influencing the assembly and application of two-dimensional covalent organic frameworks (2D COFs). On one hand, the formation of stable, extended 2D sheets is a prerequisite for the fabrication of high-quality 2D crystals. On the other hand, characterizing these properties will eventually provide a path toward the inclusion of electron-vibration couplings when evaluating their electronic properties such as their band structures. Here, atomistic molecular dynamics simulations are used to investigate the mechanical properties of 2D COF sheets, taking the prototypical COF-5 as a representative example. Large out-of-plane deformations are found, about 400% higher than those encountered in graphene. In addition, structural defects lead to significantly larger twists and deformations, which underlines the challenges in fabricating stand-alone, large-size 2D COF sheets. Stacking, on the other hand, effectively reduces the out-of-plane deformations and suppresses the role of defects, two aspects beneficial to the lateral extension of 2D COF sheets.

Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy. Langmuir-Blodgett (LB) films of ordered mono- and multilayers were transferred onto glass and silicon substrates, with layer quality, coverage, and intermolecular order controlled by layer compression pressure on the LB trough. Organic field-effect transistors and organic photovoltaics devices with active layers consisting of the amphiphilic conjugated D-A-D-type molecules were constructed to demonstrate that the LB technique is an effective layer-by-layer deposition approach to fabricate self-assembled, ordered thin films.

Well-defined structure-property relationships offer a conceptual basis to afford a priori design principles to develop novel π-conjugated molecular and polymer materials for nonlinear optical (NLO) applications. Here, we introduce the bond ellipticity alternation (BEA) as a robust parameter to assess the NLO characteristics of organic chromophores and illustrate its effectiveness in the case of streptocyanines. BEA is based on the symmetry of the electron density, a physical observable that can be determined from experimental X-ray electron densities or from quantum-chemical calculations. Through comparisons to the well-established bond-length alternation and π-bond order alternation parameters, we demonstrate the generality of BEA to foreshadow NLO characteristics and underline that, in the case of large electric fields, BEA is a more reliable descriptor. Hence, this study introduces BEA as a prominent descriptor of organic chromophores of interest for NLO applications.

Low-dimensional perovskites offer a rare opportunity to investigate lattice dynamics and charge carrier behavior in bulk quantum-confined solids, in addition to them being the leading materials in optoelectronic applications. In particular, zero-dimensional (0D) inorganic perovskites of the Cs 4 PbX 6 (X = Cl, Br, or I) kind have crystal structures with isolated lead halide octahedra [PbX 6 ] 4- surrounded by Cs + cations, allowing the 0D crystals to exhibit the intrinsic properties of an individual octahedron. Using both experimental and theoretical approaches, we studied the electronic and optical properties of the prototypical 0D perovskite Cs 4 PbBr 6 . Our results underline that this 0D perovskite behaves akin to a molecule, demonstrating low electrical conductivity and mobility as well as large polaron binding energy. Density functional theory calculations and transient absorption measurements of Cs 4 PbBr 6 perovskite films reveal the polaron band absorption and strong polaron localization features of the material. A short polaron lifetime of ~2 ps is observed in femtosecond transient absorption experiments, which can be attributed to the fast lattice relaxation of the octahedra and the weak interactions among them.