Chad Risko

Synthesis, characterization, and transistor response of semiconducting silole polymers with substantial hole mobility and air stability. Experiment and theory.

Realizing p-channel semiconducting polymers with good hole mobility, solution processibility, and air stability is an important step forward in the chemical manipulation of charge transport in polymeric solids and in the development of low-cost printed electronics. We report here the synthesis and full characterization of the dithienosilole- and dibenzosilole-based homopolymers, poly(4,4-di-n-hexyldithienosilole) (TS6) and poly(9,9-di-n-octyldibenzosilole) (BS8), and their mono- and bithiophene copolymers, poly(4,4-di-n-hexyldithienosilole-alt-(bi)thiophene) (TS6T1, TS6T2) and poly(9,9-di-n-octyldibenzosilole-alt-(bi)thiophene) (BS8T1, BS8T2), and examine in detail the consequences of introducing dithienosilole and dibenzosilole cores into a thiophene polymer backbone. We demonstrate air-stable thin-film transistors (TFTs) fabricated under ambient conditions having hole mobilities as large as 0.08 cm(2)/V x s, low turn-on voltages, and current on/off ratios > 10(6). Additionally, unencapsulated TFTs fabricated under ambient conditions are air-stable, an important advance over regioregular poly(3-hexylthiophene) (P3HT)-based devices. Density functional theory calculations provide detailed insight into the polymer physicochemical and charge transport characteristics. A direct correlation between the hole injection barrier and both TFT turn-on voltage and TFT polymer hole mobility is identified and discussed, in combination with thin-film morphological characteristics, to explain the observed OTFT performance trends.

Unification of trap-limited electron transport in semiconducting polymers

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 10(23) traps per m(3) centred at an energy of ~3.6 eV below the vacuum level, with a typical distribution width of ~0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer.

A quantum-chemical perspective into low optical-gap polymers for highly-efficient organic solar cells

The recent and rapid enhancement in power conversion efficiencies of organic-based, bulk-heterojunction solar cells has been a consequence of both improved materials design and better understanding of the underlying physical processes involved in photocurrent generation. In this Perspective, we first present an overview of the application of quantum-chemical techniques to study the intrinsic material properties and molecular- and nano-scale processes involved in device operation. In the second part, these quantum-chemical tools are applied to an oligomer-based study on a collection of donor–acceptor copolymers that have been used in the highest-efficiency solar cell devices reported to date. The quantum-chemical results are found to be in good agreement with the empirical data related to the electronic and optical properties. In particular, they provide insight into the natures of the electronic excitations responsible for the near-infrared/visible absorption profiles, as well as into the energetics of the low-lying singlet and triplet states. These results lead to a better understanding of the inherent differences among the materials, and highlight the usefulness of quantum chemistry as an instrument for material design. Importantly, the results also point to the need to continue the development of integrated, multiscale modeling approaches to provide a thorough understanding of the materials properties.

Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals

We report the electrical transport characteristics of conjugated oligonaphthalenefluoreneimine (ONI) wires having systematically varied lengths up to 10 nm. Using aryl imine addition chemistry, ONI wires were built from gold substrates by extending the conjugation length through imine linkages between highly conjugated building blocks of alternating naphthalenes and fluorenes. The resistance and current-voltage characteristics of ONI wires were measured as a function of molecular length, temperature, and electric field using conducting probe atomic force microscopy (CP-AFM). We have observed a transition in direct current (DC) transport from tunneling to hopping near 4 nm as previously established for oligophenyleneimine (OPI) wires. Furthermore, we have found that long ONI wires are less resistive than OPI wires. The single-wire conductivity of ONI wires is approximately 1.8 +/- 0.1 x 10(-4) S/cm, a factor of approximately 2 greater than that of OPI wires, and consistent with the lower transport activation energy ( approximately 0.58 eV versus 0.65 eV or 13 versus 15 kcal/mol). Quantum chemical calculations reveal that charge is preferentially localized on the fluorene subunits and that the molecules are substantially twisted. Overall, this work confirms that imine addition chemistry can be used to build molecular wires long enough to probe the hopping transport regime. The versatility of this chemistry, in combination with CP-AFM, opens up substantial opportunities to probe the physical organic chemistry of hopping conduction in long conjugated molecules.

Use of X-Ray Diffraction, Molecular Simulations, and Spectroscopy to Determine the Molecular Packing in a Polymer-Fullerene Bimolecular Crystal

The molecular packing in a polymer: fullerene bimolecular crystal is determined using X-ray diffraction (XRD), molecular mechanics (MM) and molecular dynamics (MD) simulations, 2D solid-state NMR spectroscopy, and IR absorption spectroscopy. The conformation of the electron-donating polymer is significantly disrupted by the incorporation of the electron-accepting fullerene molecules, which introduce twists and bends along the polymer backbone and 1D electron-conducting fullerene channels.

Impact of Molecular Packing on Electronic Polarization in Organic Crystals: The Case of Pentacene vs TIPS-Pentacene

Polarization energy corresponds to the stabilization of the cation or anion state of an atom or molecule when going from the gas phase to the solid state. The decrease in ionization energy and increase in electron affinity in the solid state are related to the (electronic and nuclear) polarization of the surrounding atoms and molecules in the presence of a charged entity. Here, through a combination of molecular mechanics and quantum mechanics calculations, we evaluate the polarization energies in two prototypical organic semiconductors, pentacene and 6,13-bis(2-(tri-isopropylsilyl)ethynyl)pentacene (TIPS-pentacene). Comparison of the results for the two systems reveals the critical role played by the molecular packing configurations in the determination of the polarization energies and provides physical insight into the experimental data reported by Lichtenberger and co-workers (J. Amer. Chem. Soc. 2010, 132, 580; J. Phys. Chem. C 2010, 114, 13838). Our results underline that the impact of packing configurations, well established in the case of the charge-transport properties, also extends to the polarization properties of π-conjugated materials.

Three-Dimensional Packing Structure and Electronic Properties of Biaxially Oriented Poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) Films

We use a systematic approach that combines experimental X-ray diffraction (XRD) and computational modeling based on molecular mechanics and two-dimensional XRD simulations to develop a detailed model of the molecular-scale packing structure of poly(2,5-bis (3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT-C(14)) films. Both uniaxially and biaxially aligned films are used in this comparison and lead to an improved understanding of the molecular-scale orientation and crystal structure. We then examine how individual polymer components (i.e., conjugated backbone and alkyl side chains) contribute to the complete diffraction pattern, and how modest changes to a particular component orientation (e.g., backbone or side-chain tilt) influence the diffraction pattern. The effects on the polymer crystal structure of varying the alkyl side-chain length from C(12) to C(14) and C(16) are also studied. The accurate determination of the three-dimensional polymer structure allows us to examine the PBTTT electronic band structure and intermolecular electronic couplings (transfer integrals) as a function of alkyl side-chain length. This combination of theoretical and experimental techniques proves to be an important tool to help establish the relationship between the structural and electronic properties of polymer thin films.

Intermixing at the Pentacene‐Fullerene Bilayer Interface: A Molecular Dynamics Study

Organic photovoltaics (OPVs) hold promise as a technology for low-cost, large-area power conversion. [ 1–4 ] Though conceptually straightforward, the processes required for effi cient operation of an organic solar cell – photon absorption and exciton (electron-hole pair) formation, dissociation of the exciton into separated charges, and collection of the charges – are inherently complex due to the characteristics of the π -conjugated organic materials, i.e., weak van der Waals intermolecular interactions, low dielectric constants, strong electron-electron interactions, and large electron-vibration couplings. The effective separation of the electron-hole pair requires that the active layer consist of two components, an electron-rich (hole-transport) donor material and an electron-defi cient (electron-transport) acceptor material. [ 5 ] This condition leads to a key bottleneck, both in terms of operation and basic understanding, as there exists a delicate interplay between charge-separation and (geminate and non-geminate) charge-recombination processes at the interface between the donor and acceptor materials. While the importance of the donor–acceptor interface has long been recognized, recent experimental evidence has shone new light on the morphological complexity; this is particularly the case in polymer-fullerene bulk-heterojunction (BHJ) solar cells, [ 6 , 7 ] and more recently in molecule-molecule and polymermolecule bilayers. [ 8 , 9 ] In addition to the possible miscibility of the two components, the deposition protocol (e.g., chemical vapor deposition, solution casting, or spin coating to name a few [ 2 , 10 ] ) and post-processing procedures (e.g., removal of solvent additives or solvent and thermal annealing [ 11 , 12 ] ) can impact the interface morphology. This morphological variability leads to diffi culties, then, in recognizing the underlying physical processes at the interface as the relevant electronic states and electrostatic interactions are highly dependent on the molecular packing confi gurations between the donor and acceptor molecules or chain segments. [ 3 , 13–16 , 17 ] Hence, to achieve optimal OPV performance, one must be able to understand and ultimately control the morphology of these heterojunctions. [ 12 ]

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

All-optical switching--controlling light with light--has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π-conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π-conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

Heteroannulated acceptors based on benzothiadiazole

Increasing the acceptor strength of the widely used acceptor benzothiadiazole (BT) by extending the heterocyclic core is a promising strategy for developing new and stronger acceptors for materials in organic electronics and photonics. In recent years, such heteroannulated BT acceptors have been incorporated into a wide variety of materials that have been used in organic electronic and photonic devices. This review critically assesses the properties of these materials. Although heteroannulation to form acceptors, such as benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (BBT), does result in materials with significantly higher electron affinity (EA) relative to BT, in many cases the extended BT systems also exhibit lower ionization energy (IE) than BT. Both the significantly higher EA and lower IE limit the efficacy of these materials in applications such as bulk heterojunction organic photovoltaics (BHJ-OPV) based on C60. Although the relatively high EA may enable some applications such as air stable organic field effect transistors (OFET), more widespread use of heteroannulated BT acceptors will likely require the ability to moderate or retain the high EA while increasing IE.