Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part II: The intrinsic electronic midgap states

We propose a structural model that treats in a unified fashion both the atomic motions and electronic excitations in quenched melts of pnictide and chalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we argued these quenched melts represent aperiodic $pp\sigma$-networks that are highly stable and, at the same time, structurally degenerate. These networks are characterized by a continuous range of coordination. Here we present a systematic way to classify these types of coordination in terms of discrete coordination defects in a parent structure defined on a simple cubic lattice. We identify the lowest energy coordination defects with the intrinsic midgap electronic states in semiconductor glasses, which were argued earlier to cause many of the unique optoelectronic anomalies in these materials. In addition, these coordination defects are mobile and correspond to the transition state configurations during the activated transport above the glass transition. The presence of the coordination defects may account for the puzzling discrepancy between the kinetic and thermodynamic fragility in chalcogenides. Finally, the proposed model recovers as limiting cases several popular types of bonding patterns proposed earlier, including: valence-alternation pairs, hypervalent configurations, and homopolar bonds in heteropolar compounds.Comment: 17 pages, 15 figures, revised version, final version to appear in J. Chem. Phy

Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the ppσpp\sigma-network

Semiconductor glasses exhibit many unique optical and electronic anomalies. We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508 (2010)) in which several of these anomalies arise from deep midgap electronic states residing on high-strain regions intrinsic to the activated transport above the glass transition. Here we demonstrate at the molecular level how this scenario is realized in an important class of semiconductor glasses, namely chalcogen and pnictogen containing alloys. Both the glass itself and the intrinsic electronic midgap states emerge as a result of the formation of a network composed of $\sigma$-bonded atomic $p$-orbitals that are only weakly hybridized. Despite a large number of weak bonds, these $pp\sigma$-networks are stable with respect to competing types of bonding, while exhibiting a high degree of structural degeneracy. The stability is rationalized with the help of a hereby proposed structural model, by which $pp\sigma$-networks are symmetry-broken and distorted versions of a high symmetry structure. The latter structure exhibits exact octahedral coordination and is fully covalently-bonded. The present approach provides a microscopic route to a fully consistent description of the electronic and structural excitations in vitreous semiconductors.Comment: 22 pages, 17 figures, revised version, final version to appear in J. Chem. Phy

Small Polarons in Two-Dimensional Pnictogens: A First-Principles Study

We report the first-principles study of small polarons in the most stable twodimensional pnictogen allotropes: blue and black phosphorene and arsenene. While both cations and anions of small hydrogen-passivated clusters show charge localization and local lattice distortions, only the hole polaron in the blue allotrope is stable in the infinite size cluster limit. The adiabatic polaron relaxation energy is found to be 0.1 eV for phosphorene and 0.15 eV for arsenene. The polaron is localized on lone-pair orbitals with half of the extra charge distributed among 13 atoms. In the blue phosphorene, these orbitals form the valence band’s top with a relatively flat band dispersion. However, in the black phosphorene, lone-pair orbitals hybridize with bonding orbitals, which explains the difference in hole localization strength between the two topologically equivalent allotropes. The polaron’s adiabatic barriers for motion are small compared to the most strongly coupled phonon frequency, implying the polaron barrierless motion

Lowest-Energy Crystalline Polymorphs of P3HT

We systematically study low-energy crystalline polymorphs of the archetypal conjugated polymer, regioregular poly-3-hexylthiophene (rr-P3HT) using the best available density functional theory methods benchmarked against the ab initio coupled cluster method. A comprehensive conformational search is performed for two-dimensional π-stacks being the most rigid structural unit of bulk P3HT. We have identified a number of nearly isoenergetic polymorphs below the energy level of room-temperature amorphous structures and well below the energy of optimized best-fit experimental models. Classical molecular dynamics simulations show that these crystals retain their structure at least at 200 K. At room temperature, although the conjugated backbone of the π-stack remains ordered, aliphatic side chains are melted, transforming from low-energy folded conformations to high-entropy fully unfolded structures. Our study shows that P3HT is a statistically frustrated system with multiple competing interactions, which complicates fabrication of highly ordered bulk forms but gives structural flexibility of glasses

Lowest-Energy Crystalline Polymorphs of P3HT

We systematically study low-energy crystalline polymorphs of the archetypal conjugated polymer, regioregular poly-3-hexylthiophene (rr-P3HT) using the best available density functional theory methods benchmarked against the ab initio coupled cluster method. A comprehensive conformational search is performed for two-dimensional π-stacks being the most rigid structural unit of bulk P3HT. We have identified a number of nearly isoenergetic polymorphs below the energy level of room-temperature amorphous structures and well below the energy of optimized best-fit experimental models. Classical molecular dynamics simulations show that these crystals retain their structure at least at 200 K. At room temperature, although the conjugated backbone of the π-stack remains ordered, aliphatic side chains are melted, transforming from low-energy folded conformations to high-entropy fully unfolded structures. Our study shows that P3HT is a statistically frustrated system with multiple competing interactions, which complicates fabrication of highly ordered bulk forms but gives structural flexibility of glasses

Microcrystal Electron Diffraction for Molecular Design of Functional Non-Fullerene Acceptor Structures

Understanding the relationship between molecular structure and solid-state arrangement informs about the design of new organic semiconductor (OSC) materials with improved optoelectronic properties. However, determining their atomic structure remains challenging. Here, we report the lattice organization of two non-fullerene acceptors (NFAs) determined using microcrystal electron diffraction (MicroED) from crystals not traceable by X-ray crystallography. The MicroED structure of o-IDTBR was determined from a powder without crystallization, and a new polymorph of ITIC-Th is identified with the most distorted backbone of any NFA. Electronic structure calculations elucidate the relationships between molecular structures, lattice arrangements, and charge-transport properties for a number of NFA lattices. The high dimensionality of the connectivity of the 3D wire mesh topology is the best for robust charge transport within NFA crystals. However, some examples suffer from uneven electronic coupling. MicroED combined with advanced electronic structure modeling is a powerful new approach for structure determination, exploring polymorphism and guiding the design of new OSCs and NFAs

Ab Initio Study of a Molecular Crystal for Photovoltaics: Light Absorption, Exciton and Charge Carrier Transport

Using ab initio methods we examine the molecular and solid-state electronic properties of a recently synthesized small-molecule donor, <i>p</i>-DTS­(PTTh₂)₂, which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC₇₀BM acceptor, <i>p</i>-DTS­(PTTh₂)₂ can be used to fabricate high-efficiency bulk heterojunction organic solar cells. A precise picture of molecular structure and interchromophore packing is provided via a single-crystal X-ray diffraction study; such details cannot be easily obtained with donor materials based on conjugated polymers. In first-principles approaches we are limited to a single-crystallite scale. At this scale, according to our investigation, the principal properties responsible for the high efficiency are strong low-energy light absorption by individual molecules, large exciton diffusion length, and fast disorder-resistant hole transport along π-stacks in the crystallite. The calculated exciton diffusion length is substantially larger than the average crystallite size in previously characterized device active layers, and the calculated hole mobility is 2 orders of magnitude higher than the measured device-scale mobility, meaning that the power conversion “losses” on a single-crystallite scale are minimal

Ab Initio Study of a Molecular Crystal for Photovoltaics: Light Absorption, Exciton and Charge Carrier Transport

Using ab initio methods we examine the molecular and solid-state electronic properties of a recently synthesized small-molecule donor, <i>p</i>-DTS­(PTTh₂)₂, which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC₇₀BM acceptor, <i>p</i>-DTS­(PTTh₂)₂ can be used to fabricate high-efficiency bulk heterojunction organic solar cells. A precise picture of molecular structure and interchromophore packing is provided via a single-crystal X-ray diffraction study; such details cannot be easily obtained with donor materials based on conjugated polymers. In first-principles approaches we are limited to a single-crystallite scale. At this scale, according to our investigation, the principal properties responsible for the high efficiency are strong low-energy light absorption by individual molecules, large exciton diffusion length, and fast disorder-resistant hole transport along π-stacks in the crystallite. The calculated exciton diffusion length is substantially larger than the average crystallite size in previously characterized device active layers, and the calculated hole mobility is 2 orders of magnitude higher than the measured device-scale mobility, meaning that the power conversion “losses” on a single-crystallite scale are minimal