Incorporation of a Boron-Nitrogen Covalent Bond Improves the Charge-Transport and Charge-Transfer Characteristics of Organoboron Small-Molecule Acceptors for Organic Solar Cells.

An organoboron small-molecular acceptor (OSMA) MB←N containing a boron-nitrogen coordination bond (B←N) exhibits good light absorption in organic solar cells (OSCs). In this work, based on MB←N, OSMA MB-N, with the incorporation of a boron-nitrogen covalent bond (B-N), was designed. We have systematically investigated the charge-transport properties and interfacial charge-transfer characteristics of MB-N, along with MB←N, using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT). Theoretical calculations show that MB-N can simultaneously boost the open-circuit voltage (from 0.78 V to 0.85 V) and the short-circuit current due to its high-lying lowest unoccupied molecular orbital and the reduced energy gap. Moreover, its large dipole shortens stacking and greatly enhances electron mobility by up to 5.91 × 10-3 cm2·V-1·s-1. Notably, the excellent interfacial properties of PTB7-Th/MB-N, owing to more charge transfer states generated through the direct excitation process and the intermolecular electric field mechanism, are expected to improve OSCs performance. Together with the excellent properties of MB-N, we demonstrate a new OSMA and develop a new organoboron building block with B-N units. The computations also shed light on the structure-property relationships and provide in-depth theoretical guidance for the application of organoboron photovoltaic materials.

Ultrathin Covalent Organic Framework Anchored on Graphene for Enhanced Organic Pollutant Removal.

Covalent organic frameworks (COFs) are of great potential as adsorbents owing to their tailorable functionalities, low density and high porosity. However, their intrinsically stacked two-dimensional (2D) structure limits the full use of their complete surface for sorption, especially the internal pores. The construction of ultrathin COFs could increase the exposure of active sites to the targeted molecules in a pollutant environment. Herein, an ultrathin COF with a uniform thickness of ca. 2 nm is prepared employing graphene as the surface template. The resulting hybrid aerogel with an ultralow density (7.1 mg cm-3 ) exhibits the ability to remove organic dye molecules of different sizes with high efficiency. The three-dimensional (3D) macroporous structure and well-exposed adsorption sites permit rapid diffusion of solution and efficient adsorption of organic pollutants, thereby, greatly contributing to its enhanced uptake capacity. This work highlights the effect of COF layer thickness on adsorption performance.

Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study.

The design of new molecules with theoretical chemistry methods and further obtaining a fundamental understanding of the structure-property relationship are important for the development of high-efficiency hole-transporting materials (HTMs). Herein, the effect of semi-locked and fully-locked cores was systematically investigated based on two conformation-tunable tetrathienylethene (TTE) and tetraphenylethylene (TPE) units. Our results show that the highest occupied molecular orbital (HOMO) levels of the locked TTE-2 and TTE-3 are clearly down-shifted compared with that of the unlocked TTE-1, which is due to the decreased electronic conjugation between the locked cores and the triphenylamine (TPA) arms, whereas the same situation is not found for TPE-3 due to the twisted core configuration. Compared with the TTE-series, the TPE-series exhibits less optical absorption in the visible light region and enhanced stability. Meanwhile, the hole mobility of the designed HTMs displays an increased trend from the unlocked core to the semi-locked and fully-locked cores due to the gradually increasing hole transfer integral with enhanced molecular planarity. In addition, we also found that the reorganization energy of the locked TTE cores is obviously lowered compared to that of the unlocked one, which plays an important role in increasing the hole mobility. In summary, this work can provide some useful clues for designing high-efficiency two-dimensional HTMs, and two potential promising candidates (TTE-3 and TPE-3) are proposed.

Understanding the effects of the co-sensitizing ratio on the surface potential, electron injection efficiency, and Förster resonance energy transfer.

Multiple absorbers that function in different absorption regions (near infra-red (NIR) and UV-Visible (UV-Vis)) have been widely used in solar cell applications to enhance the light-harvesting. Herein, two special co-sensitizing Models 1 and 2, which feature either saturated dye IQ21 or saturated co-sensitizer S2, have been added to a TiO2 surface to explore the effect of the altered sensitizing sequence, namely the co-sensitizing ratio of IQ21/S2 and S2/IQ21 on the electrostatic potential variation (ΔV), electron injection efficiency (ηinj'), and Förster resonance energy transfer (FRET), using density functional theory and first-principle molecular dynamics simulations. The ΔV related to the open-circuit voltage (Voc) is insensitive in both Models 1 and 2. However, the absorption (λabs) and ηinj' associated with the short-circuit density (Jsc) display a significant deviation (the λabs for 1 is red-shifted compared to that of 2, and the ηinj' for 1 is improved by 56%). Meanwhile, Model 1 manifests a suppressed FRET and potentially favors co-sensitizer S2 functioning as the electron-injector and not the energy-donor. Another two possible Models 3 and 4 that feature a reduced adsorption of IQ21 and S2 relative to 1 and 2 were considered further, and the result mirrors the main trend in 1 and 2, except for the ηinj'. Overall, it implies that sensitizing a larger absorber with NIR features to saturate it first, then introducing a smaller absorber with UV-Vis features, can potentially improve the electron injection and diminish electron-hole recombination considerably. Our results provide a comprehensive analysis of the active role of an optimized sensitizing sequence to improve the conversion efficiency.

Boosting the hole transport of conductive polymers by regulating the ion ratio in ionic liquid additives.

Poly(3,4-ethylenedioxythiophene) (PEDOT) has aroused great interest in organic electrics because of its high electrical conductivity and mechanical flexibility. To improve the charge transport, it can act as an ionic liquid (IL) additive due to its ion characteristics and high electrical conductivity. Herein, we investigated the hole-transport performance of PEDOT treated by ILs featuring specific ion ratios (4 : 1, 3 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3, and 1 : 4) of the cation and anion through classical dynamics simulations and quantum mechanics computations. The hole mobility of the amorphous PEDOT, constituting nine EDOT monomers, could be improved to 16.81, 18.03, and 10.14 cm2 V-1 s-1 when synergistically regulating the ion ratio to 2 : 1, 3 : 1, and 4 : 1. Consequently, these ratios potentially achieved nearly a 100-fold improvement in the electrical conductivity with respect to the pristine system. The improvements mainly stemmed from the fact that decreasing the amount of anions in ILs and prolonging the chain length of PEDOT yielded an ordered face-to-face π-π stacking. The electronic coupling and charge excitation further confirmed that the anions play an active role in tunneling the hole transport in ILs/heterogeneous PEDOT, and the highest occupied molecular orbital (HOMO) energy level of PEDOT was up-shifted significantly after treatment by the ratios of 2 : 1, 3 : 1, and 4 : 1, which favored the electron-donating ability and was in line with the extraordinary enhancement of the hole mobility. Our results imply that regulating the ion ratio in ILs is a novel strategy for modulating the electronic properties and π-stacked morphology of PEDOT.

Electron Density Learning of Z-Bonds in Ionic Liquids and Its Application.

Ionic liquids (ILs) exhibit fascinating properties due to special Z-bonds and have been widely used in electrochemical systems. The local Z-bond networks potentially cause a discrepancy in electrochemical properties. Understanding the correlations between the Z-bond energy (EZ-bond) and the electrochemical properties is helpful to identify appropriate ILs. It is difficult to estimate the correlations from single density functional theory calculations or molecular dynamic simulations. In this work, a machine learning model targeting the electronic density (ρBCP) of Z-bonds has been trained successfully, as expected for use in systems above the nanoscale size. The connection between the EZ-bond and the electrochemical potential window in ILs@TiO2, as well as that between the EZ-bond and the charge carrier mobility in ILs-PEDOT:Tos@SiO2, was separately investigated. This study highlights an efficient model for predicting ρBCP in nanoscale systems and anticipates exploring the connection between Z-bonds and the electrochemical properties of IL-based systems.

Theoretical Study of Non-Fullerene Acceptors Using End-Capped Groups with Different Electron-Withdrawing Abilities toward Efficient Organic Solar Cells.

Acceptors in organic solar cells (OSCs) are of paramount importance. On the basis of the well-known non-fullerene acceptor Y6, six acceptors (Y6-COH, Y6-COOH, Y6-CN, Y6-SO2H, Y6-CF3, and Y6-NO2) were designed by end-capped manipulation. The effects of end-capped engineering on electronic properties, optical properties, and interfacial charge-transfer states were systematically studied by density functional theory, time-dependent density functional theory, and molecular dynamics. The designed acceptors possess suitable energy levels and improved optical properties. More importantly, the electron mobility of the new acceptors was greatly enhanced, even more than 20 times that of the parent molecule. Among them, Y6-NO2 with the lowest-lying frontier molecular orbitals and the largest red-shifted absorption was selected to construct interfaces with the donor PM6. PM6/Y6-NO2 exhibits stronger interfacial interactions and enhanced charge-transfer characteristics compared with PM6/Y6. This work not only enhances the understanding of the structure-property relationship for acceptors but also offers a set of promising acceptors for high-performance OSCs.

Machine Learning Screening of Efficient Ionic Liquids for Targeted Cleavage of the ß-O-4 Bond of Lignin.

Lignin conversion into high value-added chemicals is of great significance for maximizing the use of renewable energy. Ionic liquids (ILs) have been widely used for targeted cleavage of the C-O bonds of lignin due to their high catalytic activity. Studying the cleavage activity of each IL is impossible and time-consuming, given the huge number of cations and anions. Currently, the mainstream approach to determining the cleavage activity of one IL is to calculate the activation barrier energy (Ea) theoretically via transition state search, a process that involves the iterative determination of an appropriate "imaginary frequency". Machine learning (ML) has been widely used for catalyst design and screening, enabling accurate mapping from specified descriptors to target properties. To avoid complicated Ea calculations and to screen potential candidates, in this study, we selected nearly 103 ILs and guaiacylglycerol-ß-guaiacyl ether (GG) as the lignin model and used the ML technology to train models that can rapidly predict the cleavage activity of ILs. Taking the easily accessible bond dissociation energy (BDE) of the ß-O-4 bond in GG as the target, an ML model with r > 0.93 for predicting the catalytic activity of ILs was obtained. The change tendency of the BDE is consistent with the experimental yield of guaiacol, reflecting the reliability of the ML model. Finally, [C2MIM][Tyrosine] and [C3MIM][Tyrosine] as the optimal candidates for future applications were screened out. This is a novel strategy for predicting the catalytic activity of ILs on lignin without the need to calculate complicated reaction pathways while reducing time consumption. It is anticipated that the ML model can be utilized in future practical applications for targeted cleavage of lignin.

Ultralow Friction and High Robustness of Monolayer Ionic Liquids.

Ultralow friction between interacting surfaces in relative motion is of vital importance in many pure and applied sciences. We found that surfaces bearing ordered monolayer ionic liquids (ILs) can have friction coefficient µ values as low as 0.001 at pressures up to 78 MPa and exhibit good structure recoverability. This extreme lubrication is attributed primarily to the ordered striped structure driven by the "atomic-locking" effect between carbon atoms on the alkyl chain of ILs and graphite. The longer alkyl chain has lower µ values, and the stripe periodicity is decisive in reducing energy dissipation during the sliding process. In combination with simulation, the alternate atomic-scale ordered and disordered ionic regions were recognized, whose ratio fundamentally determines the µ values and lubrication mechanism. This finding is an important step toward the practical utilization of ILs with negligible vapor pressure as superlubricating materials in future technological applications operating under extreme conditions.

How to design more efficient hole-transporting materials for perovskite solar cells? Rational tailoring of the triphenylamine-based electron donor.

Designed with a symmetrical naphthatetrathiophene (NTT) core and triphenylamine (TPA)-based side arms, a series of novel organic small molecule hole-transporting materials are simulated for perovskite solar cells (PSCs) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. As a fundamental understanding, the energy level alignments and the charge transport behavior are explored for their potential applications. Our results show that, adding an oxygen-bridge between the neighboring phenyl groups of TPA side arms makes the highest occupied molecular orbital (HOMO) levels up-shift, whereas the carbon-carbon single bond stabilizes the HOMOs by about 0.3-0.4 eV. By structural tailoring of the TPA side arms, the HOMO levels of newly designed molecules range from -5.08 eV to -5.61 eV, which provides more possibilities for the interfacial energy regulation. Meanwhile, our results also indicate that the quasi-planar molecular architecture and the delocalized frontier molecular orbitals can effectively enhance the electronic coupling between adjacent molecules. In addition, the reorganization energies are distinctly lowered in the cases of the mixed carbon-carbon bond and oxygen-bridge, and the double oxygen-bridge models. As a result, these molecules with the additional carbon-carbon bond and oxygen-bridge exhibit high hole mobilities. Several promising candidates are proposed toward more efficient PSCs, and more importantly, this work offers some new insights for the design of organic small molecule materials.

Extending donor size in D-A-π-A organic dye for dye sensitized solar cells: Anti-aggregation and improving electron injection.

We have theoretically designed two D-A-π-A dyes 3 and 4 based on the efficient references 1 and 2 by introducing an extra electron donor unit (D2). Via calculating the electronic structures of isolated dyes, we obtain that dyes 3 and 4 possess stronger light-harvesting efficiency imparted by the fluorescence energy transfer of D2 part, maintain comparable lifetime of excited states, and shorten the electron injection time significantly with regard to 1 and 2. Meanwhile, dye 3 positively shifts the edge of virtual states of TiO2 in a larger extent compared to its counterparts. Then after considering the alignment morphology of multiple dyes adsorbed on TiO2 surface, we find that dyes 3 and 4 manifest the capability of anti-aggregation obviously, which is evidenced by the smaller quantity of intermolecular electronic coupling compared to that of dyes 1 and 2, definitively illustrating the prominent performance of novel dyes with the bulky D2 moiety. Finally, dye 3 is screened out as the potential candidate for future application.

The electron injection rate in CdSe quantum dot sensitized solar cells: from a bifunctional linker and zinc oxide morphology.

Herein, we have investigated the effect of both the bifunctional linker (L1, L2, L3, and L4) and ZnO morphology (porous nanoparticles (NPs), nanowires (NWs), and nanotubes (NTs-A and NTs-Z)) on the electron injection in CdSe QD sensitized solar cells by first-principles simulation. Via calculating the partitioned interfaces formed by different components (linker/QDs and ZnO/linker), we found that the electronic states of QDs and every ZnO substrate are insensitive to any linker, while the frontier orbitals of L1-L4 (with increased delocalization) manifest a systematical negative-shift. Because of the lowest unoccupied molecular orbital (LUMO) of L1 compared to its counterparts aligned in the region of the virtual states of QDs or the substrate with a high density of states, it always yields a stronger electronic coupling with QDs and varied substrates. After characterization of the complete ZnO/linker/QD system, we found that the electron injection time (τ) vastly depends on both the linker and substrate. On the one hand, L1 bridged QDs and every substrate always achieve the shortest τ compared to their counterpart associated cases. On the other hand, NW supported systems always yield the shortest τ no matter what the linker is. Overall, the NW/L1/QD system achieves the fastest injection by ∼160 fs. This essentially stems from the shortest molecular length of L1 decreasing the distance between QDs and the substrate, subsequently improving the interfacial coupling. Meanwhile, the NW supported cases generate the less sensitive virtual states for both the QDs and NWs, ensuring a less variable interfacial coupling. These facts combined can provide understanding of the effects contributed from the linker and the oxide semiconductor morphology on charge transfer with the aim of choosing an appropriate component with fast directional electron injection.