High power conversion efficiencies in state-of-the-art nonfullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low ...nonradiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to the fact that the CT states can borrow considerable oscillator strengths from the energy-close local excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor–acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce nonradiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures to further improve the NF OSC performance.
Organic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18 % efficiency for the single‐junction ...devices based on the state‐of‐the‐art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy (Eb) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small Eb of −0.11–0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak Eb in Y6. Thus, charge generation in NFA‐based OSCs shows little dependence on interfacial driving forces.
Direct photogeneration of free charge carriers enabled by remarkably low exciton binding energies is demonstrated in the state‐of‐the‐art nonfullerene acceptor of Y6 by a joint experimental and theoretical study. This results in efficient charge generation under small interfacial energy offsets in the high‐efficiency nonfullerene organic solar cells.
Reducing the driving force of exciton dissociation into charge-transfer states is one effective solution to minimize energy loss and thus to improve power conversion efficiencies for organic solar ...cells. Traditionally, the driving force should be larger than 0.3 eV to achieve efficient exciton dissociation. Recent experiments have shown that excitons can be effectively dissociated, whereas the energy offsets between donor and acceptor are extremely small, but the mechanisms are not understood yet. Here, we use system-optimized long-range corrected functional with solid-state electronic polarization to investigate exciton binding energies of 14 typical nonfullerene small molecule acceptors in organic solar cells. The results point to that the driving forces for dissociation of the acceptor excitons into charge-transfer states are linearly correlated to the exciton binding energies. The smaller the exciton binding energy, the lower driving force required. Moreover, primarily owing to the largest dielectric constants, IDT- or IDTT-based fused-ring acceptors have the smallest exciton binding energies with respect to other acceptors, i.e., DPP-, PDI-, and BFI-based systems. The influence of conjugation lengths, strengths of electron-donating and withdrawing units, and molecular volumes on the dielectric constants are analyzed in detail. Our work rationalizes the experimental observations and would be helpful for designing active materials to reduce energy loss for organic solar cells.
Electrocatalysis by atomic catalysts is a major focus of chemical and energy conversion effort. Although transition-metal-based bulk electrocatalysts for electrochemical application on energy ...conversion processes have been reported frequently, anchoring the stable transition-metal atoms (e.g. nickel and iron) still remains a practical challenge. Here we report a strategy for fabrication of ACs comprising only isolated nickel/iron atoms anchored on graphdiyne. Our findings identify the very narrow size distributions of both nickel (1.23 Å) and iron (1.02 Å), typical sizes of single-atom nickel and iron. The precision of this method motivates us to develop a general approach in the field of single-atom transition-metal catalysis. Such atomic catalysts have high catalytic activity and stability for hydrogen evolution reactions.
Abstract
The high voltage losses (
$${V}_{{loss}}$$
V
l
o
s
s
), originating from inevitable electron-phonon coupling in organic materials, limit the power conversion efficiency of organic solar ...cells to lower values than that of inorganic or perovskite solar cells. In this work, we demonstrate that this
$${V}_{{loss}}$$
V
l
o
s
s
can in fact be suppressed by controlling the spacing between the donor (D) and the acceptor (A) materials (DA spacing). We show that in typical organic solar cells, the DA spacing is generally too small, being the origin of the too-fast non-radiative decay of charge carriers (
$${k}_{{nr}}$$
k
n
r
), and it can be increased by engineering the non-conjugated groups, i.e., alkyl chain spacers in single component DA systems and side chains in high-efficiency bulk-heterojunction systems. Increasing DA spacing allows us to realize significantly reduced
$${k}_{{nr}}$$
k
n
r
and improved device voltage. This points out a new research direction for breaking the performance bottleneck of organic solar cells.
In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π-π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing ...efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π-π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π-π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.
The separation of charge-transfer states into free charges at the donor/acceptor (D/A) interfaces plays a central role in organic solar cells (OSCs). Because of strong Coulomb attraction, the ...separation mechanisms are elusive, particularly for the high-efficiency non-fullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions because of strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. In particular, upon incorporation of fluorine substituents and electron-poor cores into ITIC, the increased polarization energies can completely compensate for the Coulomb attraction in the IT-4F- and Y6-based heterojunctions, leading to barrierless charge separation.
The design and synthesis of highly efficient deep red (DR) and near‐infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a ...great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline‐6,7‐dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA‐QCN. The TPA‐QCN molecule with orange‐red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light‐emitting devices (OLEDs) were fabricated by regulating TPA‐QCN dopant concentration in the emitting layers.
TPA‐QCN with thermally activated delayed fluorescence was designed and synthesized. The pure crystal and thin film show intense deep red (DR) and NIR emissions, respectively. The supramolecular structure and intermolecular interactions are beneficial to the reduction of non‐radiative transitions and enhancement of emission in the solid state. Doped and neat films were used for high‐performance DR and NIR organic light‐emitting devices (OLEDs).
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•Three n-aminomethylpyridine functionalized adsorbents were synthesized via radiation method.•Effect of aminomethyl position on the adsorption ability of n-AMPR was studied by DFT ...calculation.•Adsorption of Au(III) onto 2-AMPR proceed by adsorption–reduction mechanism.•Au(III) ions can be separated completely by 2-AMPR from simulation e-wastewater.•2-AMPR was a promising adsorbent for trace amount of Au(III) from e-wastewater.
Three novel n-aminomethylpyridine (n-AMP, n=2, 3, 4) functionalized adsorbents (n-AMPRs) for recovery of Au(III) were synthesized by radiation grafting of glycidyl methacrylate (GMA) onto microcrystalline cellulose microsphere, followed by ring-opening processes with n-AMP. The FTIR and XPS spectra manifested that n-AMP were successfully introduced into the novel adsorbents. Batch and column mode adsorption experiments against Au(III) were conducted to evaluate the adsorption abilities of the adsorbents. The adsorption isotherms of Au(III) were well fitted with Langmuir mode, and adsorption kinetics were well described by pseudo-second-order model. The results presented that n-AMPRs exhibited good adsorption abilities for Au(III) with adsorption capacities in the order of 2-AMPR>3-AMPR>4-AMPR. Moreover, the different adsorption abilities of three n-AMPRs for Au(III) ion were studied by DFT calculations. The adsorbents adsorbed Au(III) ion through anion exchange combined with chelation process, after which a particular reduction process for loaded Au(III) ion was also found. The dynamic experiment revealed that Au(III) in HCl media could be efficiently adsorbed by 2-AMPR in perturbation of equal amounts of Cu(II), Ni(II) and Cr(III), which demonstrated that 2-AMPR would be a promising adsorbent for trace amount of Au(III) ion from industrial effluents and e-wastes.
Quantitative understanding of the photophysical processes is essential for developing novel thermally activated delayed fluorescence (TADF) materials. Taking as an example ...1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene, a typical TADF-active molecule, we calculated the interconversion and decay rates of the lowest excited singlet and triplet states at different temperatures as well as the prompt and delayed fluorescence efficiencies at 300 K at the first-principles level. Our results can reproduce well the experimentally available data. It is found that the reverse intersystem crossing rate (k RISC) is sharply increased by 3 orders of magnitude, while the other rates increase slightly or remain unchanged when the temperature rises from 77 to 300 K. Importantly, k RISC reaches up to 1.23 × 106 s–1 and can compete with the radiative and nonradiative decay rates of S1 (1.11 × 107 and 2.37 × 105 s–1) at 300 K, leading to an occurrence of delayed fluorescence. In addition, our calculations indicate that it is the freely rotational motions of the carbazolyl between two cyano groups that are responsible for the interconversion between S1 and T1. The large torsional barriers of other three adjacent carbazolyl groups block the nonradiative decay channels of S1 → S0, leading to strong fluorescence. This work would provide useful insight into the molecular design of high-efficiency TADF emitters.
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