Singlet fission dynamics and optical spectra of pentacene and its derivatives

A multimode Brownian oscillator model is employed to investigate the absorption spectra of pentacene and its derivatives in solution and thin films. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using parameters obtained from fitting the absorption spectra of these pentacene derivatives, the singlet fission dynamics and two-dimensional electronic spectra of an ab initio Hamiltonian are investigated by Dirac-Frenkel time-dependent variation with multiple Davydov trial states. It is found that the periodic wave packet motion induced in the displaced excited state, and the accompanying vibrational relaxation, can be visualized by two-dimensional electronic spectra at short times.

Ultrashort laser pulse doubling by metal-halide perovskite multiple quantum wells

Multiple ultrashort laser pulses are widely used in optical spectroscopy, optoelectronic manipulation, optical imaging and optical signal processing etc. The laser pulse multiplication, so far, is solely realized by using the optical setups or devices to modify the output laser pulse from the optical gain medium. The employment of these external techniques is because the gain medium itself is incapable of modifying or multiplying the generated laser pulse. Herein, with single femtosecond laser pulse excitation, we achieve the double-pulsed stimulated emission with pulse duration of around 40 ps and pulse interval of around 70 ps from metal-halide perovskite multiple quantum wells. These unique stimulated emissions originate from one fast vertical and the other slow lateral high-efficiency carrier funneling from low-dimensional to high-dimensional quantum wells. Furthermore, such gain medium surprisingly possesses nearly Auger-free stimulated emission. These insights enable us a fresh approach to multiple the ultrashort laser pulse by gain medium.

Laser pulse multiplication is desired in many applications but has been challenging to realize by gain medium. Here Guo et al. achieve double-pulsed stimulated emission in quasi-2D metal-halide perovskites due to the two-channel carrier funneling effect in their multiple-quantum-wells structure.

Nucleation Control-Triggering Cocrystal Polymorphism of Charge-Transfer Complexes Differing in Physical and Electronic Properties

Binary charge-transfer complex polymorphs composed of perylene and 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo-[1,2-b:4,5-b']-dithiophene (DTTCNQ) were synthesized separately via a simple artificial nucleation-tailoring method, in both macroscopic and microscopic cocrystal engineering manners. The two polymorphs were testified to be independently thermosalient in the solid state, and the specific self-assembly derived from homogeneous or heterogeneous nucleation by assistance of governable thermodynamic/kinetic drive, leading to a change in the ordered p-n stacking structure. The as-prepared polymorphic microcrystals afforded a significantly varied (opto)electronic property: high n-type transporting and good photoresponsivity for β-complex, and ambipolar transporting with ignorable photoresponsivity for α-complex, attributing to the different charge-transfer and supramolecular alignment. This work provides us a new route to the exploitation of donor-acceptor complex family, making it possible to develop functional materials and devices based on variable supramolecular binary structures.

U-Shaped Helical Azaarenes: Synthesis, Structures, and Properties

The synthesis and properties of a series of U-shaped helical azaarenes are reported. Crystal structures of these helical azaarenes were obtained, and the solid-state structures unequivocally exhibited their helicity.

Theoretical Prediction of Chiral 3D Hybrid Organic-Inorganic Perovskites

Hybrid organic-inorganic perovskites (HOIPs), in particular 3D HOIPs, have demonstrated remarkable properties, including ultralong charge-carrier diffusion lengths, high dielectric constants, low trap densities, tunable absorption and emission wavelengths, strong spin-orbit coupling, and large Rashba splitting. These superior properties have generated intensive research interest in HOIPs for high-performance optoelectronics and spintronics. Here, 3D hybrid organic-inorganic perovskites that implant chirality through introducing the chiral methylammonium cation are demonstrated. Based on structural optimization, phonon spectra, formation energy, and ab initio molecular dynamics simulations, it is found that the chirality of the chiral cations can be successfully transferred to the framework of 3D HOIPs, and the resulting 3D chiral HOIPs are both kinetically and thermodynamically stable. Combining chirality with the impressive optical, electrical, and spintronic properties of 3D perovskites, 3D chiral perovskites is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.

Supramolecular Design of Donor-Acceptor Complexes via Heteroatom Replacement toward Structure and Electrical Transporting Property Tailoring

A feasible strategy relies on using heteroatom replacement, namely, chemical modification of an organic compound. Here we present this design concept for donor-acceptor complexes, which involves introducing nitrogen atoms to the middle ring of donor molecules to promote short contacts and reduce steric effect of the mixed framework. These nitrogen-modified complexes can possess a shorter molecular distance besides the mixed-stacking pathway, enlarged π-π interactions, or even a scarce 1:2.5 molar ratio through extra acceptor insertion. As a result, the unique 1:2 complex with nitrogen atoms on the different sides demonstrated stable electron field-effect mobility performance, whereas the binary system with no nitrogen replacement or N atoms on the identical sides displayed poor ambipolar properties. These results confirmed that heteroatom replacement was a powerful molecular design tool to fine-tune the molecular packing of organic donor-acceptor complexes and their corresponding electronic properties.

Synthesis, characterization and photophysical studies of a novel polycyclic diborane

A long polycyclic diborane (4) with a dihedral angle of 21.75° between the two naphthalene rings has been successfully prepared and fully characterized.

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Observation of A Raman mode splitting in few layer black phosphorus encapsulated with hexagonal boron nitride

We investigate the impact of encapsulation with hexagonal boron nitride (h-BN) on the Raman spectrum of few layer black phosphorus. The encapsulation results in a significant reduction of the line width of the Raman modes of black phosphorus, due to a reduced phonon scattering rate. We observe a so far elusive peak in the Raman spectra ∼4 cm^-1 above the A mode in trilayer and thicker flakes, which had not been observed experimentally. The newly observed mode originates from the strong black phosphorus inter-layer interaction, which induces a hardening of the surface atom vibration with respect to the corresponding modes of the inner layers. The observation of this mode suggests a significant impact of h-BN encapsulation on the properties of black phosphorus and can serve as an indicator of the quality of its surface.

Solvent Accommodation: Functionalities Can Be Tailored Through Co-Crystallization Based on 1:1 Coronene-F4TCNQ Charge-Transfer Complex

Because organic donor/acceptor blending systems play critical roles in ambipolar transistors, photovoltaics, and light-emitting transistors, it is highly desirable to precisely tailor the stacking of cocrystals toward different intrinsic structures and physical properties. Here, we demonstrated that the structure-stacking modes and electron-transport behaviors of coronene-F4TCNQ cocrystals (1:1) can be tuned through the solvent accommodation. Our results clearly show that the solvent accommodation not only enlarges the inner mixed packing (...DAD···) distances, leading to the depressed short-contact interactions including the side-by-side and face-by-face intermolecular interactions, but also switches off electron-transport behavior of coronene-F₄TCNQ cocrystals (1:1) in ambient atmosphere.

Effectiveness of External Electric Field Treatment of Conjugated Polymers in Bulk-Heterojunction Solar Cells

External electric field treatment (EFT) on P3HT:PCBM bulk heterojunction (BHJ) devices was recently found to be a viable approach for improving the power conversion efficiencies (PCEs) through modulating the blend nanomorphology. However, its effectiveness over the broad family of polymer-fullerene blends remains unclear. Herein, we investigate the effects of external EFT on various polymer-fullerene blends with distinct morphologies stemming from the difference in molecular structure of the polymers (i.e., semicrystalline vs amorphous) in a bid to establish a clear morphology-function-charge dynamics relationship to the photovoltaic performance. Our findings reveal that EFT promotes self-organization of the semicrystalline thiophene-based conjugated polymers (i.e., P3HT and P3BT) while it was ineffective for the amorphous polymers (i.e., PTB7 and PCPDTBT) even at the maximum applied E-field of 8 kV cm^-1. Transient absorption spectroscopy shows an improvement in the initial charge-carrier and polaron formation from delocalized excitons in the E-field treated semicrystalline blends compared to their untreated reference samples. Interfacial trap-assisted monomolecular and trap-free bimolecular recombination at nanosecond-microsecond time scale in the E-field treated P3BT:PC60BM devices are significantly suppressed. Importantly, our findings shed new light and provide guidelines on the effectiveness of utilizing external EFT to enhance the PCEs of a larger family of conjugated polymer-based BHJ OSCs.