Charge transfer in superbase n-type doping of PCBM induced by deprotonation.

N-type electronic doping of organic semiconductors (OSCs) by using superbase compounds shows high doping efficiency (H. Wei, Z. Cheng, T. Wu, Y. Liu, J. Guo, P.-A. Chen, J. Xia, H. Xie, X. Qiu, T. Liu, B. Zhang, J. Hui, Z. Zeng, Y. Bai and Y. Hu, Adv. Mater. 2023, 35, 2300084). While a deprotonation reaction is believed to trigger the doping process, the detailed mechanism therein is not yet fully understood. In the present work we theoretically study the electronic structure of the deprotonated Phenyl-C61-butyric acid methyl ester (PCBM) molecule, as well as the charge transfer (CT) between PCBM and its deprotonated species. We find that deprotonated PCBM without formation of a new bond between the deprotonated side chain and fullerene induces electronic structure with broken spin symmetry, where an in-gap state is singly occupied by an unpaired electron. A second scenario that we find to be possible is the formation of a new bond between the deprotonated side chain and a fullerene. This leads to a spin symmetric electronic structure with partially localized in-gap state, which is expected to contribute less to the effective doping. These results show that the deprotonated PCBM species without new bond formation predominantly accounts for the effective n-type doping of PCBM, an insight that will be useful for optimization of this recently discovered doping method.

Strain-Induced Indirect-to-Direct Bandgap Transition, Photoluminescence Enhancement, and Linewidth Reduction in Bilayer MoTe2.

Two-dimensional (2D) layered materials provide an ideal platform for engineering electronic and optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain on various spectral features of bilayer MoTe2 photoluminescence (PL). We found that bilayer MoTe2 can be converted from an indirect to a direct bandgap material through strain engineering, resulting in a photoluminescence enhancement by a factor of 2.24. Over 90% of the PL comes from photons emitted by the direct excitons at the maximum strain applied. Importantly, we show that strain effects lead to a reduction of the overall linewidth of PL by as much as 36.6%. We attribute the dramatic decrease of linewidth to a strain-induced complex interplay among various excitonic varieties such as direct bright excitons, trions, and indirect excitons. Our experimental results on direct and indirect exciton emission features are explained by theoretical exciton energies that are based on first-principles electronic band structure calculations. The consistent theory-experimental trend shows that the enhancement of PL and the reduction of linewidth are the consequences of the increasing direct exciton contribution with the increase of strain. Our results demonstrate that strain engineering can lead to a PL quality of the bilayer MoTe2 comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe2 more suitable for silicon-photonics integration due to the reduced silicon absorption.

Protonation-induced charge transfer and polaron formation in organic semiconductors doped by Lewis acids.

Lewis-acid doping of organic semiconductors (OSCs) opens up new ways of p-type doping and has recently become of significant interest. As for the mechanistic understanding, it was recently proposed that upon protonation of the OSC backbone, electron transfer occurs between the protonated polymer chain and a neutral chain nearby, inducing a positive charge carrier in the latter [B. Yurash, D. X. Cao, V. Brus et al., Nat. Mater., 2019, 18, 1327-1334]. To further clarify the underlying microscopic processes on a molecular level, in the present work, we theoretically analyze the influence of protons on the electronic properties of the widely used PCPDT-BT copolymer as a typical example. While we find that single protonation leads to formation of a localized polaron, double protonation leads to the release of a more delocalized polaron via an intrachain electron transfer. We also demonstrate the possibility of an interchain electron transfer. The vertical excitation spectra simulated for an ensemble of protonated polymers with different amounts of protons enable a detailed interpretation of the experimental observations and contribute to a molecular-level interpretation of the Lewis-acid doping process.

Microscopic Study of Molecular Double Doping.

Double doping, in which a single dopant molecule induces two charge carriers in an organic semiconductor (OSC), was recently experimentally observed and promises to enhance the efficiency of molecular doping. Here we present a theoretical investigation of p-type molecular double doping in a CN6-CP:bithiophene-thienothiophene OSC system. Our analysis is based on density functional theory (DFT) calculations for the electronic ground state. In a molecular complex with two OSC oligomers and one CN6-CP dopant molecule, we explicitly demonstrate double integer charge transfer and find the formation of two individual polarons on the OSC molecules and a dianion dopant molecule. We show that the vibrational modes and related infrared absorption spectrum of this complex can be traced back to those of the charged dopant and OSC molecules in their isolated forms. The near-infrared optical absorption spectrum calculated by time-dependent DFT shows features of both typical intramolecular polaron excitations and weak intermolecular charge transfer excitations associated with the doping-induced polaron states.

Atomically Thin Sheets of Lead-Free 1D Hybrid Perovskites Feature Tunable White-Light Emission from Self-Trapped Excitons.

Low-dimensional organic-inorganic perovskites synergize the virtues of two unique classes of materials featuring intriguing possibilities for next-generation optoelectronics: they offer tailorable building blocks for atomically thin, layered materials while providing the enhanced light-harvesting and emitting capabilities of hybrid perovskites. This work goes beyond the paradigm that atomically thin materials require in-plane covalent bonding and reports single layers of the 1D organic-inorganic perovskite [C7 H10 N]3 [BiCl5 ]Cl. Its unique 1D-2D structure enables single layers and the formation of self-trapped excitons, which show white-light emission. The thickness dependence of the exciton self-trapping causes an extremely strong shift of the emission energy. Thus, such 2D perovskites demonstrate that already 1D covalent interactions suffice to realize atomically thin materials and provide access to unique exciton physics. These findings enable a much more general construction principle for tailoring and identifying 2D materials that are no longer limited to covalently bonded 2D sheets.

Density Functional Study on the Intercalation of Fullerenes into AnE-PV Copolymer Layers.

We investigated the intercalation of C60 into poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymers layers by density functional theory calculations in respect of crystal structures and electronic band structures. Based on the experimental observations, we found that the copolymer with branched side chains substituted next to the anthracene units and the linear side chains substituted to the vinylene units has a better tendency to intercalate with C60 than the reversely substituted copolymer. The calculated electronic band structures of the intercalated phase, featured by flat in-gap states resulting from C60 molecules, explain the experimentally observed variations of the photocurrent, photoluminescence, and electroluminescence yields with different ratio between PCBM and the two types of copolymers in the ternary blend.

Density Functional Study of Stacking Structures and Electronic Behaviors of AnE-PV Copolymer.

In this work, we report an in-depth investigation on the π-stacking and interdigitating structures of poly(p-anthracene-ethynylene)-alt-poly(p-phenylene-vinylene) copolymer with octyl and ethyl-hexyl side chains and the resulting electronic band structures using density functional theory calculations. We found that in the π-stacking direction, the preferred stacking structure, determined by the steric effect of the branched ethyl-hexyl side chains, is featured by the anthracene-ethynylene units stacking on the phenylene-vinylene units of the neighboring chains and vice versa. This stacking structure, combined with the interdigitating structure where the branched side chains of the laterally neighboring chains are isolated, defines the energetically favorable structure of the ordered copolymer phase, which provides a good compromise between light absorption and charge-carrier transport.

Relativistic effects and the unique low-symmetry structures of gold nanoclusters.

The atomic structures of bare gold clusters provide the foundation to understand the enhanced catalytic properties of supported gold nanoparticles. However, the richness of diverse structures and the strong relativistic effects have posed considerable challenges for a systematic understanding of gold clusters with more than 20 atoms. We use photoelectron spectroscopy of size-selected anions, in combination with first principles calculations, to elucidate the structures of gold nanoclusters in a critical size regime from 55 to 64 atoms (1.1-1.3 nm in diameter). Au(55)(-) is found to be a nonicosahedral disordered cluster as a result of relativistic effects that induce strong surface contractions analogous to bulk surface reconstructions, whereas low-symmetry core-shell-type structures are found for Au(56)(-) to Au(64)(-). Au(58) exhibits a major electron-shell closing and is shown to possess a low-symmetry, but nearly spherical structure with a large energy gap. Clear spectroscopic and computational evidence has been observed, showing that Au(58)(-) is a highly robust cluster and additional atoms are simply added to its surface from Au(59)(-) to Au(64)(-) without inducing significant structural changes. The unique low-symmetry structures characteristic of gold nanoclusters due to the strong relativistic effects allow abundant surface defects sites, providing a key structure-function relationship to understand the catalytic capabilities of gold nanoparticles.