Frenkel and Charge-Transfer Excitonic Couplings Strengthened by Thiophene-Type Solvent Enables Binary Organic Solar Cells with 19.8 % Efficiency.

Overcoming the trade-off between short-circuited current (Jsc) and open-circuited voltage (Voc) is important to achieving high-efficiency organic solar cells (OSCs). Previous works modulated the energy gap between Frenkel local exciton (LE) and charge-transfer (CT) exciton, which served as the driving force of exciton splitting. Differently, our current work focuses on the modulation of LE-CT excitonic coupling (tLE-CT) via a simple but effective strategy that the 2-chlorothiophene (2Cl-Th) solvent utilizes in the treatment of OSC active-layer films. The results of our experimental measurements and theoretical simulations demonstrated that 2Cl-Th solvent initiates tighter intermolecular interactions with non-fullerene acceptor in comparison with that of traditional chlorobenzene solvent, thus suppressing the acceptor's over-aggregation and retarding the acceptor crystallization with reduced trap. Critically, the resulting shorter distances between donor and acceptor molecules in the 2Cl-Th treated blend efficiently strengthen tLE-CT, which not only promotes exciton splitting but also reduces non-radiative recombination. The champion efficiencies of 19.8 % (small-area) with superior operational reliability (T80: 586 hours) and 17.0 % (large-area) were yielded in 2Cl-Th treated cells. This work provided a new insight into modulating the exciton dynamics to overcome the trade-off between Jsc and Voc, which can productively promote the development of the OSC field.

Asymmetric Acceptor-Donor-Acceptor Polymers with Fast Charge Carrier Transfer for Solar Hydrogen Production.

Construction of local donor-acceptor architecture is one of the valid means for facilitating the intramolecular charge transfer in organic semiconductors. To further accelerate the interface charge transfer, a ternary acceptor-donor-acceptor (A1 -D-A2 ) molecular junction is established via gradient nitrogen substituting into the polymer skeleton. Accordingly, the exciton splitting and interface charge transfer could be promptly liberated because of the strong attracting ability of the two different electron acceptors. Both DFT calculations and photoluminescence spectra elucidate the swift charge transfer at the donor-acceptor interface. Consequently, the optimum polymer, N3 -CP, undergoes a remarkable photocatalytic property in terms of hydrogen production with AQY405 nm =26.6 % by the rational design of asymmetric molecular junctions on organic semiconductors.

Ionothermal Synthesis of Triazine-Heptazine-Based Copolymers with Apparent Quantum Yields of 60 % at 420†nm for Solar Hydrogen Production from "Sea Water".

Polymeric carbon nitride (PCN), in either triazine or heptazine form, has been regarded as a promising metal-free, environmentally benign, and sustainable photocatalyst for solar hydrogen production. However, PCN in most cases only exhibits moderate activity owing to its inherent properties, such as rapid charge carrier recombination. Herein we present a triazine-heptazine copolymer synthesized by simple post-calcination of PCN in eutectic salts, that is, NaCl/KCl, to modulate the polymerization process and optimize the structure. The construction of an internal triazine-heptazine donor-acceptor (D-A) heterostructure was affirmed to significantly accelerate interface charge transfer (CT) and thus boost the photocatalytic activity (AQY=60 % at 420 nm). This study highlights the construction of intermolecular D-A copolymers in NaCl/KCl molten salts with higher melting points but in the absence of lithium to modulate the chemical structure and properties of PCN.

Null Exciton Splitting in Chromophoric Greek Cross†(+) Aggregate.

Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behaviour of π-conjugated materials. Though vital for optoelectronic applications, ideal Greek cross-dipole (α=90°) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2 ) which exhibits null exciton coupling mediated monomer-like optical characteristics in the crystalline state. In contrast, nonzero exciton coupling in X-type (α=70.2°, PTE-Br0 ) and J-type (α=0°, θ=48.4°, PTE-Br4 ) assemblies have perturbed optical properties. Additionally, the semi-classical Marcus theory of charge-transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE-Br2 . Precise rotation angle dependent optoelectronic properties in crystalline PTE-Br2 can have consequences in the rational design of novel π-conjugated materials for photonic and molecular electronic applications.

Polymerization-enhanced intersystem crossing: new strategy to achieve long-lived excitons.

For a singlet-triplet coupled molecular system, the efficiency of forward and reverse intersystem crossing processes can be enhanced by reducing the energy gap between the singlet and triplet excited states (ΔEST ), thus prolonging the exciton lifetimes. This has been proven beneficial for many emerging applications such as molecular luminescence, optoelectronics, and photonics. Here, a strategy is proposed to create small ΔEST by polymerizing fluorescent dye molecules, the efficacy of which is justified by density functional theory calculations and ultrafast spectroscopy. Thus, singlet-triplet exciton communication through polymerization-enhanced intersystem crossing is also proposed.

Discovery of ultra-weakly coupled ß-carotene J-aggregates by machine learning.

Carotenoid aggregates are omnipresent in natural world and can be synthesized in hydrophilic environments. Despite different types of carotenoid aggregates have been reported hitherto, the way to predict the formation of carotenoid aggregates, i.e. H- or J-aggregates, is still challenging. Here, for the first time, we established machine learning models that can predict the formation behavior of carotenoid aggregates. The models are trained based on a database containing different types of carotenoid aggregates reported in the literatures. With the help of these machine learning models, we found a series of unknown types of ß-carotene J-aggregates. These novel aggregates are ultra-weakly coupled and have absorption bands up to 700 nm, different from all the carotenoid aggregates reported previously. Our work demonstrates that the machine learning is a powerful tool to predict the formation behavior of carotenoid aggregates and can further lead into the discovery of new carotenoid aggregates for different applications.

Size Control of the Mechanism of Exciton Polarization in Metal Oxide Nanocrystals through Fermi Level Pinning.

Light-matter interaction in certain aliovalently doped metal oxide nanocrystals (NCs) results in the generation of localized surface plasmon resonance (LSPR) in the near- to mid-infrared, allowing for their implementation in various technologies, including photovoltaics, sensing, and electrochromics. These materials could also facilitate coupling between plasmonic and semiconducting properties, making them highly interesting for electronic and quantum information technologies. In the absence of dopants, free charge carriers can arise from native defects such as oxygen vacancies. Here we show using magnetic circular dichroism spectroscopy that the exciton splitting in In2O3 NCs is induced by both localized and delocalized electrons and that contributions from the two mechanisms are strongly dependent on the NC size, owing to Fermi level pinning and the formation of a surface depletion layer. In large NCs, the angular momentum transfer from delocalized cyclotron electrons to the excitonic states is the dominant mechanism of exciton polarization. This process diminishes with decreasing NC size, owing to the rapidly reduced volume of the plasmonic core. On the other hand, exciton polarization in small NCs is dominated by localized electron-spin-induced splitting of the excitonic states. This mechanism is independent of NC size, suggesting that wave functions of localized spin states on NC surfaces do not overlap with the excitonic states. The results of this work demonstrate that the effects of individual and collective electronic properties on excitonic states can be simultaneously controlled by NC size, making metal oxide NCs a promising class of materials for quantum, spintronic, and photonic technologies.