Non-Fused π-Extension of Endcaps of Small Molecular Acceptors Enabling High-Performance Organic Solar Cells.

The modular structure of small molecular acceptors (SMAs) allows for versatile modifications of the materials and boosts the photovoltaic efficiencies of organic solar cells (OSCs) in recent years. As a critical component, the endcaps of SMAs have been intensively investigated and modified to control the molecular aggregation and photo-electronic conversion. However, most of the studies focus on halogenation or π-fusion extension of the endcap moieties, but overlook the non-fused π-extension approach, which could be a promising strategy to balance the self-aggregation and compatibility behaviors. Herein, we reported two new acceptors namely BTP-Th and BTP-FTh based on non-fused π-extension of the endcap by chlorinated-thiophene, of which the latter molecule has better co-planarity and crystallinity because of the intramolecular noncovalent interactions. Paired with donor PBDB-T, the optimal device of BTP-FTh reveals a greater efficiency of 14.81% that that of BTP-Th (13.91%). Nevertheless, the BTP-Th based device realizes a lower energy loss, enabling BTP-Th as a good candidate to serve as guest acceptor. As a result, the ternary solar cells of PM6:BTP-eC9:BTP-Th output a champion efficiency up to 18.71% with enhanced open-circuit voltage. This study highlights the significance of rational decoration of endcaps for the design of high-performance SMAs and photovoltaic cells.

Efficient Dual Mechanisms Boost the Efficiency of Ternary Solar Cells with Two Compatible Polymer Donors to Exceed 19.

Ternary strategyopens a simple avenue to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The introduction of wide bandgap polymer donors (PDs) as third component canbetter utilize sunlight and improve the mechanical and thermal stability of active layer. However, efficient ternary OSCs (TOSCs) with two PDs are rarely reported due to inferior compatibility and shortage of efficient PDs match with acceptors. Herein, two PDs-(PBB-F and PBB-Cl) are adopted in the dual-PDs ternary systems to explore the underlying mechanisms and improve their photovoltaic performance. The findings demonstrate that the third components exhibit excellent miscibility with PM6 and are embedded in the host donor to form alloy-like phase. A more profound mechanism for enhancing efficiency through dual mechanisms, that are the guest energy transfer to PM6 and charge transport at the donor/acceptor interface, has been proposed. Consequently, the PM6:PBB-Cl:BTP-eC9 TOSCs achieve PCE of over 19%. Furthermore, the TOSCs exhibit better thermal stability than that of binary OSCs due to the reduction in spatial site resistance resulting from a more tightly entangled long-chain structure. This work not only provides an effective approach to fabricate high-performance TOSCs, but also demonstrates the importance of developing dual compatible PD materials.

Structure-Activity Relationship between Crystallinity and Carrier Transport of Two-Dimensional Donor Units in Organic Solar Cells.

Benzo[1,2-b:4,5-b']dithiophene (BDT) and its derivatives have made important contributions to constructing high-performance polymers. However, it is difficult to clarify the real role of donor units due to the interference of strong electronegativity and crystallinity of acceptor units in the D-A copolymer. Here, we design a cyclohexane-substituted dithieno[3,2-f:2',3'-h]quinoxaline (DTQ)-based acceptor unit with successfully destroyed crystallinity and charge transport. Three donor-dominated materials PQH-BTF, PQH-BTCl, and PQH-BFCl are obtained. It is found that the materials exhibit obvious differences after destroying the crystallization and charge transport of the acceptor unit, and the real role of different two-dimensional donor units in designed polymers is confirmed. The backbone BDF exhibits much stronger intermolecular interactions compared to BDT, while the side chain ThF demonstrates a higher crystallization capacity than that of ThCl. More interestingly, it can be inferred that the molecular backbone is likely to construct miscible-phase crystallization (D-A crystal) while the side chain tends to demonstrate a capacity for pure-phase crystallization (D-D crystal) in a 2D donor system. Different crystallization leads to different exciton transport: pure-phase crystallization is conducive to the reduction of trap-assisted recombination, while miscible crystallization is beneficial to the reduction of bimolecular recombination. This work can help to choose donor units more accurately when preparing D-A copolymers.

Embedded Host/Guest Alloy Aggregations Enable High-Performance Ternary Organic Photovoltaics.

The ternary strategy has been intensively studied to improve the power conversion efficiencies of organic photovoltaics. Thereinto, the location of the guest component plays a critical role, but few reports have been devoted to this concern. Hereon, the distribution of LA1 as a guest acceptor in a variety of ternary scenarios is reported and the dominating driving forces of managing the guest distribution and operating modes are outlined. Governed by the appropriate relationship of compatibility, crystallinity, and surface energies between host and guest acceptors, as well as interfacial interactions between donor and dual acceptors, most of the LA1 molecules permeate into the internal of host acceptor phases, forming embedded host/guest alloy-like aggregations. The characteristic distributions greatly optimize the morphologies, maximize energy transfer, and enhance exciton/charge behaviors. Particularly, PM6:IT-4F:LA1 ternary cells afford high efficiency of 15.27% with impressive fill factors (FF) over 81%. The popularization studies further verify the superiority of the LA1-involved alloy structures, and with the Y6-family acceptor as the host component, an outstanding efficiency of 19.17% is received. The results highlight the importance of guest distribution in ternary systems and shed light on the governing factors of distributing the guests in ternary cells.

Over 19% Efficiency Organic Solar Cells by Regulating Multidimensional Intermolecular Interactions.

Research on organic solar cells (OSCs) has progressed through material innovation and device engineering. However, well-known and ubiquitous intermolecular interactions, and particularly their synergistic effects, have received little attention. Herein, the complicated relationship between photovoltaic conversion and multidimensional intermolecular interactions in the active layers is investigated. These interactions are dually regulated by side-chain isomerization and end-cap engineering of the acceptors. The phenylalkyl featured acceptors (LA-series) exhibit stronger crystallinity with preferential face-on interactions relative to the alkylphenyl attached isomers (ITIC-series). In addition, the PM6 and LA-series acceptors exhibit moderate donor/acceptor interactions compared to those of the strongly interacting PM6/ITIC-series pairs, which helps to enhance phase separation and charge transport. Consequently, the output efficiencies of all LA series acceptors are over 14%. Moreover, LA-series acceptors show appropriate compatibility, host/guest interactions, and crystallinity relationships with BTP-eC9, thereby leading to uniform and well-organized "alloy-like" mixed phases. In particular, the highly crystalline LA23 further optimizes multiple interactions and ternary microstructures, which results in a high efficiency of 19.12%. Thus, these results highlight the importance of multidimensional intermolecular interactions in the photovoltaic performance of OSCs.

Tracking Electron Dynamics of Single Molecules in Scanning Tunneling Microscopy Junctions with Laser Pulses.

On the basis of real-time simulations, we devise a method to extend the capability of scanning tunneling microscopy (STM) to track the electronic dynamics of molecules on a material's surface with the ultrafast temporal resolution of laser pulses. The intrinsic mechanism of visualization of electronic dynamics by measuring tunneling charge is attributed to the interference between the electronic oscillations stimulated by pump and probe pulses. The charge-transfer rate from molecule to the surrounding environment can be estimated with the decay time of electronic dynamics, which can also be detected by measuring the tunneling charge across the STM junction. Moreover, it is found that the tunneling charge can be varied precisely by changing the carrier-envelope phase (CEP) of the pulses, and this phase-dependence of tunnelling diminishes as the duration of incident laser pulses increases. The proposed scheme provides an alternative means for visualization of electron dynamics of single molecules by measuring tunnelling charges.

Enhanced photocurrent in heterostructures formed between CH3NH3PbI3 perovskite films and graphdiyne.

Extending photoabsorption to the near-infrared region (NIR) of the spectrum remains a major challenge for the enhancement of the photoelectric performance of perovskites. In this work, we propose a model of van der Waals heterostructures formed by CH3NH3PbI3 perovskite films and graphdiyne (GDY) to improve the photocurrent in the NIR. To obtain better insights into the properties of GDY/perovskite heterostructures, we first determine its electronic properties using the first principles calculations. The charge transfer between GDY and perovskites leads to a built-in electrical field that facilitates the separation and the transport of the photogenerated carriers. Then, the non-equilibrium Green's function (NEGF) is used to calculate the photocurrents of perovskite slabs with and without GDY. The photocurrents of GDY/perovskite heterostructures are nearly an order of magnitude larger than that of pristine perovskites in NIR due to the synergistic effect between GDY and perovskites. Furthermore, a polarization-sensitive photocurrent is obtained for a GDY/PbI2 heterostructure.

Tunable Photoresponse by Gate Modulation in Bilayer Graphene Nanoribbon Devices.

Control of absorption and photocurrent conversion is of practical importance for the design of photoelectric devices. In this paper, using simulations, we demonstrate that the photoresponse of a bilayer graphene nanoribbon (GNR) device can be controlled by gate voltage modulation. A vertical gate field shifts the potential on the top and bottom layers in opposite directions, resulting in a continuous change of band gap with applied gate voltage. This field simultaneously facilitates separation of photoexcited electron-hole pairs and gives rise to a photocurrent in a selected photon energy range. The photoresponse of a bilayer GNR device can thus be tuned by adjusting the applied gate voltage. In addition, the light frequency range can be changed by using nanoribbons of different widths. These findings provide a basis for the design of adjustable optoelectronic devices using two-dimensional materials.

Graphdiyne as a Host Active Material for Perovskite Solar Cell Application.

This work demonstrates a novel photovoltaic application in which graphdiyne (GD) can be employed as a host material in a perovskite active layer for the first time. In the device fabrication, the best molar ratio for active materials is verified as PbI2/MAI/GD being 1:1:0.25, yielding a peak power-conversion efficiency of 21.01%. We find that graphdiyne, as the host material, exerts significant influence on the crystallization, film morphology, and a series of optoelectronic properties of the perovskite active layer. A uniform MAPbI3 film with highly crystalline qualities, large domain sizes, and few grain boundaries was realized with the introduction of graphdiyne. Moreover, the current-voltage hysteresis was negligible, and device stability was significantly improved as well. The results indicate that graphdiyne as the host active material presents great potential for the enhancement of the performance of perovskite solar cells.

Quantum mechanical modeling the emission pattern and polarization of nanoscale light emitting diodes.

Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green's function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems.

Toward the development of the potential with angular distortion for halogen bond: a comparison of potential energy surfaces between halogen bond and hydrogen bond.

As noncovalent intermolecular interactions, hydrogen bond (HB) and halogen bond (XB) are attracting increasing attention. In this work, the potential energy surfaces (PESs) of hydrogen and halogen bonds are compared. Twelve halogen-bonded and three hydrogen-bonded models are scanned for analysis using the MP2 level of theory. This work indicates that potential energy surfaces of both HB and XB have angular distortion. The potential well of XB is narrower than that of HB. With the elongation of the bond length, the potential energy surfaces get flatter. The best fitting functions for angular distortion and the flattening character of angular terms are also combined into a modified Buckingham potential. The testing results show that the essential features of the PES, including angular distortion and flattening character, have been reproduced. These results provide a better understanding of halogen and hydrogen bonds and the optimization of halogen bond force fields.

A new fluorescent pH probe for extremely acidic conditions.

A novel turn-off fluorescent probe based on coumarin and imidazole moiety for extremely acidic conditions was designed and developed. The probe with pKa=2.1 is able to respond to very low pH value (below 3.5) with high sensitivity relying on fluorescence quenching at 460 nm in fluorescence spectra or the ratios of absorbance maximum at 380 nm to that at 450 nm in UV-vis spectra. It can quantitatively detect pH value based on equilibrium equation, pH=pKa-log[(Ix-Ib)/(Ia-Ix)]. It had very short response time that was less than 1 min, good reversibility and nearly no interference from common metal ions. Moreover, using (1)H NMR analysis and theoretical calculation of molecular orbital, we verified that a two-step protonation process of two N atoms of the probe leaded to photoinduced electron transfer (PET), which was actually the mechanism of the fluorescence quenching phenomenon under strongly acidic conditions. Furthermore, the probe was also applied to imaging strong acidity in bacteria, E.coli and had good effect. This work illustrates that the new probe could be a practical and ideal pH indicator for strongly acidic conditions with good biological significance.

A polarizable ellipsoidal force field for halogen bonds.

The anisotropic effects and short-range quantum effects are essential characters in the formation of halogen bonds. Since there are an array of applications of halogen bonds and much difficulty in modeling them in classical force fields, the current research reports solely the polarizable ellipsoidal force field (PEff) for halogen bonds. The anisotropic charge distribution was represented with the combination of a negative charged sphere and a positively charged ellipsoid. The polarization energy was incorporated by the induced dipole model. The resulting force field is "physically motivated," which includes separate, explicit terms to account for the electrostatic, repulsion/dispersion, and polarization interaction. Furthermore, it is largely compatible with existing, standard simulation packages. The fitted parameters are transferable and compatible with the general AMBER force field. This PEff model could correctly reproduces the potential energy surface of halogen bonds at MP2 level. Finally, the prediction of the halogen bond properties of human Cathepsin L (hcatL) has been found to be in excellent qualitative agreement with the cocrystal structures.