Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals.

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P4Se3 nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P4Se3 molecules, arising from an orderly and close packing of P4Se3 molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P4Se3 nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm2 V-1 s-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

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.

Theoretical study of non-Hammett vs. Hammett behaviour in the thermolysis and photolysis of arylchlorodiazirines.

Arylchlorodiazirines (ACDA) are thermal and photochemical precursors of carbenes that form these molecules via nitrogen elimination. We have studied this reaction with multireference quantum chemical methods (CASSCF and CASPT2) for a series of ACDA derivatives with different substitution at the aromatic ring. The calculations explain the different reactivity trends found in the ground and excited state, with good correlation between the calculated barriers and the experimental reaction rates. The ground state mechanism can be described as a reverse cycloaddition with small charge transfer from the aromatic ring to the diazirine moiety. This is consistent with the lack of correlation between the Hammett σ descriptors and the experimental rates. In contrast, the excited state reaction is the cleavage of a single C-N bond mediated by small barriers of 4-6 kcal mol-1. The reaction path goes through a conical intersection with the ground state, which facilitates radiationless decay and explains the disappearance of the transient absorption signal measured experimentally. This leads to a diazomethane intermediate that ultimately yields the carbene. Electronically, excitation to S1 is characterized initially by significant charge transfer from the phenyl ring to the diazirine. The charge transfer is reversed during the C-N cleavage reaction, and this explains the preferential stabilization of the excited-state minimum by polar solvents and electron-donating substituents. Therefore, our calculations reproduce and explain the relationship found experimentally between the Hammett σ+ parameters and the life time of S1 (Y. L. Zhang, et al. J. Am. Chem. Soc., 2009, 131, 16652-16653).

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.

Influences of dynamic and static disorder on the carrier mobility of BTBT-C12 derivatives: a multiscale computational study.

The role of dynamic and static disorder has been widely discussed for carrier transport in organic semiconductors. In this work, we apply a multiscale approach by combining molecular dynamics simulations, quantum mechanics calculations and kinetic Monte-Carlo simulations to study the influence of dynamic and static disorder on the hole mobility of four didodecyl[1]benzothieno[3,2-b]benzothiophene (BTBT-C12) isomers. It is found that the dynamic disorder of transfer integral tends to decrease the mobility for quasi-1D (quasi one-dimensional) BTBT1 and BTBT4 isomers and increase the mobility for 2D (two-dimensional) BTBT2 and BTBT3 isomers, while the dynamic disorder of site energy tends to decrease mobility for all the four isomers; however, the reduction in 2D molecules is much less than that in quasi-1D molecules. Results show that trap defects could reduce the mobility for both the quasi-1D and 2D molecular structures significantly, even to several orders of magnitude. In addition, our work also reveals that there might exist two kinds of oxidation defects of the scatter type for the concerned isomers, which thus leads to greater reduction in mobility for the quasi-1D molecular structures than the 2D molecular structures. The study shows that the 2D molecular structures are favored over the quasi-1D or 1D molecular structure, and it is expected that these results could be used to shed light on device design in organic electronics.

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.