Ultrafast charge-transfer in organic photovoltaic interfaces: geometrical and functionalization effects

Ultrafast Interlayer Charge Separation, Enhanced Visible-Light Absorption, and Tunable Overpotential in Twisted Graphitic Carbon Nitride Bilayers for Water Splitting

Moiré pattern superlattice formed by 2D van der Waals layered structures have attracted great attention for diverse applications. In experiments, the enhancement of catalytic performance in twisted bilayer systems is reported while its mechanism remains unclear. From high-accuracy first-principles and time-dependent ab initio nonadiabatic molecular dynamics calculations, ultrafast interlayer charge transfer within 120 fs, excellent charge separation, improved visible-light absorption, and satisfactory overpotentials for the hydrogen evolution and oxygen evolution reactions in twisted graphitic carbon nitride (g-C₃ N₄ ) bilayers are found, which are beneficial to photocatalytic, photo-electrocatalytic, or electrocatalytic water splitting. This work provides insightful guidance to advanced nanocatalysis based on twisted layered materials.

Effect of dihydronaphthyl-based C60 bisadduct as third component materials on the photovoltaic performance and charge carrier recombination of binary PBDB-T : ITIC polymer solar cells

A dihydronaphthyl-based C60 bisadduct (NCBA) acceptor was introduced as a third component to typical poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b0]dithiophene))-alt-(5,5-(10,30-di-2-thienyl-50,70-bis(2-ethylhexyl)benzo[10,20-c:40,50-c0]dithiophene-4,8-dione))] (PBDB-T): 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20,30-d0]-s-indaceno[1,2-b:5,6-b0]-dithiophene (ITIC) binary polymer solar cells (PSCs). NCBA plays a bridging role between the lowest unoccupied molecular orbital (LUMO) of PBDB-T and ITIC and provides more routes for charge carrier transfer at the interface between PBDB-T and ITIC, whereupon a higher open-circuit voltage (VOC) could be realized upon the addition of NCBA relative to the neat ITIC as an electron acceptor. With the strong visible light absorption in the range from 300 to 520 nm of the NCBA molecule, it had the effect of apparently complementary visible light absorption compared with the binary PBDB-T : ITIC layer. The crystallinity and surface morphology of the PBDB-T : NCBA : ITIC (1 : 0.1 : 0.9) thin films was similar to that of the binary PBDB-T : ITIC layer, which guaranteed suitable efficient exciton dissociation and charge carrier transport. The photocurrent density versus effective voltage (Jph-Veff) curves, short-circuit current density (JSC), and VOC as a function of incident light intensity as well as the transient photovoltage (TPV) and transient photocurrent (TPC) were measured, and the results illustrated the effects of NCBA as third component materials in terms of efficient exciton dissociation and reduced charge carrier recombination and loss. The PBDB-T : NCBA : ITIC (1 : 0.1 : 0.9)-based PSCs showed an optimized PCE value of 9.56% and better thermal stability after 10 h thermal annealing treatment (the normalized PCE value was 92.5% of the initial PCE value).

Rotational superstructure in van der Waals heterostructure of self-assembled C60 monolayer on the WSe2 surface

Hybrid van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials and self-assembled organic molecules are promising systems for electronic and optoelectronic applications with enhanced properties and performance. Control of molecular assembly is therefore paramount to fundamentally understand the nucleation, ordering, alignment, and electronic interaction of organic molecules with 2D materials. Here, we report the formation and detailed study of highly ordered, crystalline monolayers of C₆₀ molecules self-assembled on the surface of WSe₂ in well-ordered arrays with large grain sizes (∼5 μm). Using high-resolution scanning tunneling microscopy (STM), we observe a periodic 2 × 2 superstructure in the C₆₀ monolayer and identify four distinct molecular appearances. Using vdW-corrected ab initio density functional theory (DFT) simulations, we determine that the interplay between vdW and Coulomb interactions as well as adsorbate-adsorbate and adsorbate-substrate interactions results in specific rotational arrangements of the molecules forming the superstructure. The orbital ordering through the relative positions of bonds in adjacent molecules creates a charge redistribution that links the molecule units in a long-range network. This rotational superstructure extends throughout the self-assembled monolayer and opens a pathway towards engineering aligned hybrid organic/inorganic vdW heterostructures with 2D layered materials in a precise and controlled way.

A theoretical exploration of the effect of fluorine and cyano substitutions in diketopyrrolopyrrole-based polymer donor for organic solar cells

A series of polymer donor materials 1-5 based on diketopyrrolopyrrole and thiophene unit which have been widely used in organic solar cells (OSCs) were investigated based on quantum chemical calculations. The effect of fluorine and cyano substitutions in polymer donor materials was focused on. Based on the investigation on electronic structures and optical properties of the reported molecules 1 and 2 and the analysis on some parameters relevant to charge dissociation ability at donor/acceptor interface constituted by 1 and 2 with PC₆₁BM such as intermolecular charge transfer and recombination, driving force and Coulombic bound energy, we explained why fluorine substitution can improve OPV efficiency through strengthening eletron-withdrawing ability from a theoretical perspective. Then we designed cyano-substituted polymers 3-5 with the aim of obtaining better photovoltaic donor materials. The results reveal that our attempt to design donor materials which can balance large open-circuit voltage (V_oc) and high short-circuit current (J_sc) in OSCs has worked out. It is worth noting that the substitutions of fluorine and cyano groups synergistically reduce energy gap and HOMO energy level of polymers 3 and 4. Moreover, 3/PC₆₁BM and 4/PC₆₁BM heterojunctions show over 10⁷ and 10⁴ times higher than 1/PC₆₁BM on the ratios of intermolecular charge transfer and recombination rates (k_inter-CT/k_inter-CR). Thus, our work here may provide an efficient strategy to design promising donor materials in OPVs and we hope it could be useful in the future experimental synthesis.