First-Principles Calculations of the Rotational Motion and Hydrogen Bond Capability of Large Organic Cations in Hybrid Perovskites

The organic cation dynamics in organic–inorganic hybrid perovskites affect the unique physical properties of these materials. To date, the rotational dynamics of methylammonium (CH₃NH₃⁺) and formamidinium (CH­(NH₂)₂⁺) have been studied both experimentally and from first-principles calculations. Recently, a novel hybrid perovskite with large organic cation guanidinium (C­(NH₂)₃⁺, GA), which exhibited extraordinarily long carrier lifetimes, was reported. In order to analyze physical properties of GA, we examined the detailed rotational potential energy surfaces and rotational energy barriers of GA in cubic-phase GASnI₃ and alternative perovskites using first-principles calculations. The analysis revealed that the principal rotations of GA involve six hydrogen bonds between the organic cation and the inorganic framework in the crystals. Our results suggest that GA can effectively passivate under-coordinated iodine ions using its high hydrogen bond capability, which is consistent with the experimental speculation that GA can suppress iodine defects by the hydrogen bonds

Theoretical Study on Rotational Controllability of Organic Cations in Organic–Inorganic Hybrid Perovskites: Hydrogen Bonds and Halogen Substitution

The organic cation dynamics in organic–inorganic hybrid perovskites strongly affect the power energy conversion and unique physical properties of these materials. To date, the first-principles rotational potential energy surface (PES) of formamidinium (FA) has not been reported. Thus, we examined the rotational energy barriers for FA in cubic-phase perovskites (FABX₃ (B = Sn/Pb; X = Cl/Br/I)) by density functional theory and compared these with those of methylammonium. The calculated rotational PES of FAPbI₃ indicates that FA rotates around the N–N bond axis (φ) with a low energy barrier, whereas the energy barrier for FA rotation around the axis penetrating the C atom and the center of gravity of FA (θ) is high. Moreover, the φ and θ rotational barriers of FA increase with halogen substitution. Thus, we reveal important design rules for controlling the rotational barrier and orientation by forming hydrogen bonds and halogen substitution

Automatic High-Throughput Screening Scheme for Organic Photovoltaics: Estimating the Orbital Energies of Polymers from Oligomers and Evaluating the Photovoltaic Characteristics

A theoretical search for organic photovoltaic materials is immensely helpful for easily identifying candidate materials and ultimately achieving high power conversion efficiencies for solar cells. In this study, an automatic scheme for screening organic photovoltaic (OPV) materials has been developed. This scheme mainly includes three steps, namely, the automatic generation of thiophene-based semiconducting polymers composed of donor and acceptor units, estimation of orbital levels by Hückel-based models, and an evaluation of photovoltaic characteristics. A numerical assessment confirmed that the screening tool could be applied to any calculations with a basis set that includes diffuse functions. An examination of 380 donor–acceptor-type polymers demonstrated that the geometric effects such as effective conjugation length and distortion in the polymers affected the orbital levels and were important to consider in the scheme for screening an ideal OPV material. In addition, the photovoltaic characteristics were evaluated and promising acceptor units for photovoltaic materials were obtained. Thus, the proposed methodology was suitable for high-throughput screening of promising donor/acceptor units

Theoretical Insights into the Electronic Structures and Stability of Dimetallofullerenes M₂@<i>I</i>_<i>h</i>‑C₈₀

We present a theoretical study on the structural and electronic properties of a series of neutral and anionic species of M₂@<i>I</i>_<i>h</i>-C₈₀ (M = Sc, Y, La, Gd, Lu). Molecular orbital analysis suggests that the unpaired electron appears on the cage for neutral M₂@<i>I_h</i>-C₈₀ (M = Y, Gd, Lu), and is governed by the level of a metal-based (interstitial) orbital. We showed that anionization is an effective means to stabilize the neutral dimetallofullerenes because of the disappearance of the unpaired electron on the cage. Our theoretical studies on the paramagnetic di-EMF reveals that a strong ferromagnetic interaction is possible for [Gd₂@<i>I_h</i>-C₈₀]⁻. We have also investigated the absorption spectra and found that the anions [M₂@<i>I</i>_<i>h</i>-C₈₀]⁻ with the <i>D</i>_2<i>h</i> cage symmetry result in similar absorption spectra irrespective of the kinds of metals present (M = Y, La, Gd), while the absorption spectrum for [Sc₂@<i>I</i>_h-C₈₀]⁻ with the strong interaction between the Sc metal and cage, which leads to the <i>C</i>_2<i>h</i> symmetry, is different from those of <i>D</i>_2<i>h</i>

Analyses of Thiophene-Based Donor–Acceptor Semiconducting Polymers toward Designing Optical and Conductive Properties: A Theoretical Perspective

We theoretically investigated the physical properties, including the frontier orbital and excitation energies, for thiophene-based semiconducting polymers composed of donor and acceptor units. Orbital analysis revealed that remarkably different behaviors of frontier orbital energies with respect to the degree of polymerization stems from the distribution of the frontier orbitals, which is insightful information for controlling the ionization potentials and electron affinities of semiconducting polymers. We also successfully estimated the frontier orbital energies of the polymers through a simple Hückel theory-based analytical model parametrized from calculations of relatively small oligomers. This simple model allows us to predict the highest occupied molecular orbital–lowest unoccupied molecular orbital gaps of a polymer at a low computational cost. The simulated absorption spectra of the thiophene-based semiconducting polymers were compared with the experimental spectra. The theoretically designed polymers were also investigated in terms of their frontier orbital energies and absorption spectra toward synthesizing promising polymers

Charge Dynamics at Heterojunction between Face-on/Edge-on PCPDTBT and PCBM Bilayer: Interplay of Donor/Acceptor Distance and Local Charge Carrier Mobility

The bulk heterojunction organic solar cell has shown much promise as a cost-effective energy harvesting device, while despite recent progress in boosted power conversion efficiency, critical photophysical process at the interface of electron donor and electron acceptor is subject of ongoing debate. Here we investigate the impact of polymer orientation of cychlopentadithiophene–benzothiadiazole copolymer (PCPDTBT) on the charge separation (CS) and recombination (CR) at the bilayer heterojunction of polymer and methanofullerene (PCBM). The charge carrier dynamics at contrasting face-on-rich or edge-on interface controlled via side-alkyl chain modification are monitored by flash-photolysis time-resolved microwave conductivity (TRMC). The data are analyzed using singlet exciton diffusion to donor–acceptor interface with quenching term at high excitation density. We show that CS is more efficient for the face-on-rich interface than edge-on, while CR is in turn retarded for the latter. Along with computational validation based on density functional theory, molecular dynamics, and the Marcus–Levich–Jortner model, our work provides a useful guide regarding interplay of polymer/fullerene interface, exciton/charge dynamics, and local charge carrier mobility

Anomalous Dielectric Behavior of a Pb/Sn Perovskite: Effect of Trapped Charges on Complex Photoconductivity

Organic–inorganic metal halide perovskites (MHPs) exhibit prominent electronic and optical properties benefiting the performance of solar cells and light-emitting diodes. However, the dielectric properties of these materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. Herein, we characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using time-resolved microwave conductivity (TRMC) measurements. At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO₃ and BaTiO₃) and characterizing its dependence on temperature, frequency, and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic holes/electrons by oriented dipoles of organic cations, which opens a hitherto unexplored route to the dynamical control of dielectric permittivity by photoirradiation

From Linear to Foldamer and Assembly: Hierarchical Transformation of a Coplanar Conjugated Polymer into a Microsphere

Despite the coplanar structure, a conjugated alternating copolymer forms amorphous, well-defined microspheres without π-stacked crystalline domains. Here, we gain insights into the mechanism of how the coplanar conjugated polymer forms amorphous microspheres by means of spectroscopic studies on the assembly/disassembly processes. The difference of the spectral profiles of photoabsorption and photoluminescence with varying solvent/nonsolvent composition clarifies that stepwise assembly takes place through the microsphere formation; [1] intrapolymer linear-to-folding transformation upon diffusion of polar nonsolvent and [2] interpolymer assembly of the foldamers upon further addition of the nonsolvent to form microspheres. As shown in various biopolymers such as proteins and DNA, such stepwise folding and assembly behaviors of conjugated polymers from primary to secondary and tertiary structure open a new way to create transformable functional materials

A Series of Layered Assemblies of Hydrogen-Bonded, Hexagonal Networks of <i>C</i>₃‑Symmetric π‑Conjugated Molecules: A Potential Motif of Porous Organic Materials

Hydrogen-bonded porous organic crystals are promising candidates for functional organic materials due to their easy construction and flexibility arising from reversible bond formation–dissociation. However, it still remains challenging to form porous materials with void spaces that are well-controlled in size, shape, and multiplicity because even well-designed porous frameworks often fail to generate pores within the crystal due to unexpected disruption of hydrogen bonding networks or interpenetration of the frameworks. Herein, we demonstrate that a series of <i>C</i>₃-symmetric π-conjugated planar molecules (<b>Tp</b>, <b>T12</b>, <b>T18</b>, and <b>Ex</b>) with three 4,4′-dicarboxy-<i>o</i>-terphenyl moieties in their periphery can form robust hydrogen-bonded hexagonal networks (H-HexNets) with dual or triple pores and that the H-HexNets stack without interpenetration to yield a layered assembly of H-HexNet (LA-H-HexNet) with accessible volumes up to 59%. Specifically, LA-H-HexNets of <b>Tp</b> and <b>T12</b> exhibit high crystallinity and permanent porosity after desolvation (activation): SA_BET = 788 and 557 m² g^–1, respectively, based on CO₂ sorption at 195 K. We believe that the present design principle can be applied to construct a wide range of two-dimensional noncovalent organic frameworks (2D-nCOFs) and create a pathway to the development of a new class of highly porous functional materials