Theoretical Design of Dithienopicenocarbazole-Based Molecules by Molecular Engineering of Terminal Units Toward Promising Non-fullerene Acceptors.

Dithienopicenocarbazole (DTPC), as the kernel module in A-D-A non-fullerene acceptors (NFA), has been reported for its ultra-narrow bandgap, high power conversion efficiency, and extremely low energy loss. To further improve the photovoltaic performance of DTPC-based acceptors, molecular engineering of end-capped groups could be an effective method according to previous research. In this article, a class of acceptors were designed via bringing terminal units with an enhanced electron-withdrawing ability to the DTPC central core. Their geometrical structures, frontier molecular orbitals, absorption spectrum, and intramolecular charge transfer and energy loss have been systematically investigated on the basis of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Surprisingly, NFA 4 highlights the dominance for its increased open circuit voltages while NFA 2, 7, and 8 exhibit great potential for their enhanced charge transfer and lower energy loss, corresponding to a higher short-circuit current density. Our results also manifest that proper modifications of the terminal acceptor with extensions of π-conjugation might bring improved outcomes for overall properties. Such a measure could become a feasible strategy for the synthesis of new acceptors, thereby facilitating the advancement of organic solar cells.

Dual Interfacial Engineering Enables Efficient and Reproducible CsPbI2Br All-Inorganic Perovskite Solar Cells.

CsPbI2Br all-inorganic perovskite has shown superior photovoltaic properties particularly excellent phase and thermal stability, while the complicated film growth process requires additional research. Herein, the nucleation and crystallization process of the CsPbI2Br perovskite film is assisted by methyl acetate anti-solvent treatment. Additionally, a tailored SnO2 nanoparticle/TiO2 nanocrystal structured double electron transport layers (ETLs) is designed to remove the interfacial energy barrier, thus enhancing charge transfer and decrease charge recombination at the CsPbI2Br/ETL interfaces. Through synergistically dual interfacial engineering, we have demonstrated the preparation of a compact CsPbI2Br polycrystalline film with ordered and homogeneous grain size as well as ideal interfacial energy level alignment. In consequence, stable CsPbI2Br all-inorganic perovskite solar cells with the best power conversion efficiency of 15.86% has been successfully achieved together with a high open-circuit voltage of 1.23 V and a fill factor of 82.29%. We believe that the results here demonstrate efficient approaches to achieve high-quality inorganic perovskites for promoting their optoelectronic applications.

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations.

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti2CO2 monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti2CO2 monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti2CO2. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti2CO2 is calculated to be 1.76-1.67 mA cm-2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti2CO2, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

Electronic properties and oxygen reduction reaction catalytic activity of h-BeN2 and MgN2 by first-principles calculations.

Currently, identifying suitable oxygen reduction reaction (ORR) catalysts in novel two-dimensional (2D) materials has attracted more and more research attention. Here, we have studied the catalytic activities of 2D h-BeN2 and MgN2 monolayers for ORR by using first-principles calculations. The calculated results reveal that the direct quasiparticle bandgap of BeN2 monolayer is 3.32 eV, and the indirect bandgap of MgN2 is 3.42 eV. 2D h-BeN2 and MgN2 exhibit high exciton binding energies of 1.07 and 0.83 eV respectively, and their optical properties are determined by bound exciton transitions due to the strong quantum confinement effects. Importantly, h-BeN2 and MgN2 monolayers with positive-charged (+1.6 e) metal atom (Be/Mg) on the surface exhibit excellent adsorption ability for O2 and ORR intermediates, and show better CO tolerance than Pt(111). The calculated free energy plots are always downhill for ORR catalyzed by BeN2 in both acid and alkaline environments, and by MgN2 in alkaline environments. The detailed reaction mechanism analyses show that high-efficient four-electron pathway is the optimal pathway for ORR catalyzed by BeN2 in acid environments. Surprisingly, there is a low overpotential of 0.45 eV for ORR catalyzed by BeN2 in the acid solution and no overpotential in the alkaline solution. Our studies found for the first time that 2D h-BeN2 shows huge potential as a non-precious metal ORR catalyst in acid and alkaline environments.

High-performance hydrogen evolution reaction catalysis achieved by small core-shell copper nanoparticles.

In this work, we reported a comprehensive first-principles study on the hydrogen evolution reaction (HER) catalytic activity of Cu based core-shell nanoparticles (CSNPs). Cu55 nanoparticle (NP) and Mx@Cu55-x CSNPs (x = 1, 13) with Fe, Ru, and Os as core components were constructed and their structural parameters, stability, and electronic properties were calculated. We found that there exist competing effects during the binding of H atom on the NP surface. The competing effects between the decreasing of negative charge on surface Cu atoms and the surface expansion resulted from the change of core port size and atoms affect hydrogen adsorption oppositely. Among these CSNPs, Os13@Cu42 exhibited the best HER catalytic performance with moderate adsorption for H atoms (Eads = -0.20 eV), low Gibbs free energies of hydrogen adsorption (ΔGH = 0.03 eV) and acceptable reaction barrier for H2 generation (Eb = 0.32 eV), which makes it highly promising as HER electro-catalyst.

Metallophthalocyanine-Based Molecular Dipole Layer as a Universal and Versatile Approach to Realize Efficient and Stable Perovskite Solar Cells.

It is well known that tailoring the interfacial structure is very important for perovskite solar cells, especially for its performance and stability. Here, we report a universal and versatile method of modulating the energetic alignment between the perovskite and hole-transporting layer by introducing a multifunctional dipole layer based on metallophthalocyanine derivatives copperphthalocyanine (CuPc) or highly fluorinated copper hexadecafluorophthalocyanine (F16CuPc). Both molecules were introduced through an "antisolution" process to treat the surface of organic-inorganic CH3NH3PbI3 perovskite. The dipole layer can well align the interfacial energy levels, passivate the CH3NH3PbI3 surface, and fill the grain boundaries, resulting in greatly suppressed charge recombination. As a result, our planar CH3NH3PbI3 perovskite devices exhibit the best power conversion efficiency of 20.2%, with significantly enhanced open-circuit voltages ( Voc) of 1.112 V (CuPc) and 1.145 V (F16CuPc), which is a record high Voc value for CH3NH3PbI3 thin-film solar cells. More importantly, the use of highly fluorinated F16CuPc produces a significantly more hydrophobic surface, leading to drastically improved long-term stability under ambient conditions. We believe that our study offers a general approach to making multifunctional dipole layers, which are necessary for achieving both stable and efficient perovskite solar cells.

Study of the nanoscale morphology of polythiophene fibrils and a fullerene derivative.

Nanoscale blending of electron-donor and electron-acceptor materials in solution-processed bulk heterojunction organic photovoltaic devices is crucial for achieving high power conversion efficiency. We used a classic blend of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) as a model to observe the nanoscale morphology of the P3HT fibrils and PCBM nanoclusters in the mixture. Energy-filtered transmission electron microscopy (EFTEM) clearly revealed a nanoscopic phase separation. Randomly connected and/or nonconnected P3HT fibrous networks and PCBM domains, revealed by 2-dimensional micrographs, were observed by collecting electron energy loss spectra in the range of 19-30 eV. From EFTEM images, the average length and the diameter of P3HT fibrils were found to be approximately 70 ± 5 and 15 ± 2 nm, respectively. Combining the EFTEM, selected area electron diffraction, and X-ray diffraction results, the number and spacing of the ordered chains in P3HT fibrils were determined. There were 18 ± 3 repeating units of P3HT perpendicular to the fibril, ∼184 layers of π-π stacking along the fibril, and ∼9 layers of interchain stacking within the fibril. These conclusive observations provide insight into the number of molecules found in one instance of ordered-plane stacking. This information is useful for the calculation of charge transport in semicrystalline polymers. Using cross-section samples prepared with a focused ion beam technique, the vertical morphology of each phase was analyzed. By collecting 30 eV energy loss images, the phase separation in the P3HT/PCBM system was distinguishable. A higher P3HT concentration was observed at the top of the cell, near Al contact, which could possibly cause loss of carriers and recombination due to a mismatch in the P3HT and Al energy bands.