Formation of electron traps in semiconducting polymers via a slow triple-encounter between trap precursor particles

Already in 2012, Blom et al. reported (Nature Materials 2012, 11, 882) in semiconducting polymers on a general electron-trap density of ≈3 × 1017 cm-3, centered at an energy of ≈3.6 eV below vacuum. It was suggested that traps have an extrinsic origin, with the water-oxygen complex [2(H2O)-O2] as a possible candidate, based on its electron affinity. However, further evidence is lacking and the origin of universal electron traps remained elusive. Here, in polymer diodes, the temperature-dependence of reversible electron traps is investigated that develop under bias stress slowly over minutes to a density of 2 × 1017 cm-3, centered at an energy of 3.6 eV below vacuum. The trap build-up dynamics follows a 3rd-order kinetics, in line with that traps form via an encounter between three diffusing precursor particles. The accordance between universal and slowly evolving traps suggests that general electron traps in semiconducting polymers form via a triple-encounter process between oxygen and water molecules that form the suggested [2(H2O)-O2] complex as the trap origin.

Theoretical study of the dissociative photodetachment dynamics of the hydrated superoxide anion cluster

The dissociative photodetachment of the hydrated superoxide anion cluster, O2-·H2O + hν → O2 + H2O + e-, is theoretically investigated using path-integral and ring-polymer molecular dynamics simulation methods, which can account for nuclear quantum effects. Full-dimensional potential energy surfaces for the anionic and lowest two neutral states (triplet and singlet spin states) are constructed based on extensive density-functional theory calculations. The calculated photoelectron spectrum agrees well with the experimental spectra measured for different photodetachment laser wavelengths. The calculated photoelectron-photofragment kinetic energy correlation spectrum also agrees well with previous experimental measurements. The dissociation mechanisms, including available energy partitioning and the importance of nuclear quantum effects in photodetachment, are discussed in detail.

Intermolecular interactions and proton transfer in the hydrogen halide-superoxide anion complexes

The superoxide radical anion O2(-) is involved in many important chemical processes spanning different scientific disciplines (e.g., environmental and biological sciences). Characterizing its interaction with various substrates to help elucidate its rich chemistry may have far reaching implications. Herein, we investigate the interaction between O2(-) (X[combining tilde] (2)Πg) and the hydrogen halides (X[combining tilde] (1)Σ) with coupled-cluster theory. In contrast to the short (1.324 Å) hydrogen bond formed between the HF and O2(-) monomers, a barrierless proton transfer occurs for the heavier hydrogen halides with the resulting complexes characterized as long (>1.89 Å) hydrogen bonds between halide anions and the HO2 radical. The dissociation energy with harmonic zero-point vibrational energy (ZPVE) for FHO2(-) (X[combining tilde] (2)A'') → HF (X[combining tilde] (1)Σ) + O2(-) (X[combining tilde] (2)Πg) is 31.2 kcal mol(-1). The other dissociation energies with ZPVE for X(-)HO2 (X[combining tilde] (2)A'') → X(-) (X[combining tilde] (1)Σ) + HO2 (X[combining tilde] (2)A'') are 25.7 kcal mol(-1) for X = Cl, 21.9 kcal mol(-1) for X = Br, and 17.9 kcal mol(-1) for X = I. Additionally, the heavier hydrogen halides can form weak halogen bonds H-XO2(-) (X[combining tilde] (2)A'') with interaction energies including ZPVE of -2.3 kcal mol(-1) for HCl, -8.3 kcal mol(-1) for HBr, and -16.7 kcal mol(-1) for HI.

25th anniversary article: charge transport and recombination in polymer light-emitting diodes

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.

On the gas-phase reaction between SO2 and O2−(H2O)0–3 clusters – an ab initio study

An ab initio study on the outcome of a collision between SO₂ and the O₂⁻(H₂O)_0–2 anion.

Correlated ab initio investigations on the intermolecular and intramolecular potential energy surfaces in the ground electronic state of the O2(-)(X2Πg)-HF(X1Σ+) complex

This work reports the first highly correlated ab initio study of the intermolecular and intramolecular potential energy surfaces in the ground electronic state of the O(2)(-)(X(2)Π(g))-HF(X(1)Σ(+)) complex. Accurate electronic structure calculations were performed using the coupled cluster method including single and double excitations with addition of the perturbative triples correction [CCSD(T)] with the Dunning's correlation consistent basis sets aug-cc-pVnZ, n = 2-5. Also, the explicitly correlated CCSD(T)-F12a level of theory was employed with the AVnZ basis as well as the Peterson and co-workers VnZ-F12 basis sets with n = 2 and 3. Results of all levels of calculations predicted two equivalent minimum energy structures of planar geometry and C(s) symmetry along the A" surface of the complex, whereas the A' surface is repulsive. Values of the geometrical parameters and the counterpoise corrected dissociation energies (Cp-D(e)) that were calculated using the CCSD(T)-F12a/VnZ-F12 level of theory are in excellent agreement with those obtained from the CCSD(T)/aug-cc-pV5Z calculations. The minimum energy structure is characterized by a very short hydrogen bond of length of 1.328 Å, with elongation of the HF bond distance in the complex by 0.133 Å, and D(e) value of 32.313 Kcal/mol. Mulliken atomic charges showed that 65% of the negative charge is localized on the hydrogen bonded end of the superoxide radical and the HF unit becomes considerably polarized in the complex. These results suggest that the hydrogen bond is an incipient ionic bond. Exploration of the potential energy surface confirmed the identified minimum and provided support for vibrationally induced intramolecular proton transfer within the complex. The T-shaped geometry that possesses C(2v) symmetry presents a saddle point on the top of the barrier to the in-plane bending of the hydrogen above and below the axis that connects centers of masses of the monomers. The height of this barrier is 7.257 Kcal/mol, which is higher in energy than the hydrogen bending frequency by 909.2 cm(-1). The calculated harmonic oscillator vibrational frequencies showed that the H-F stretch vibrational transition in the complex is redshifted by 2564 cm(-1) and gained significant intensity (by at least a factor of 30) with respect to the transition in the HF monomer. These results make the O(2)(-)-HF complex an excellent prototype for infrared spectroscopic investigations on open-shell complexes with vibrationally induced proton transfer.

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.

Exploring water binding motifs to an excess electron via X2(-)(H2O) [X = O, F]

X(2)(-)(H(2)O) [X = O, F] is utilized to explore water binding motifs to an excess electron via ab initio calculations at the MP4(SDQ)/aug-cc-pVDZ + diffs(2s2p,2s2p) level of theory. X(2)(-)(H(2)O) can be regarded as a water molecule that binds to an excess electron, the distribution of which is gauged by X(2). By varying the interatomic distance of X(2), r(X1-X2), the distribution of the excess electron is altered, and the water binding motifs to the excess electron is then examined. Depending on r(X1-X2), both binding motifs of C(s) and C(2v) forms are found with a critical distance of ∼1.37 Å and ∼1.71 Å for O(2)(-)(H(2)O) and F(2)(-)(H(2)O), respectively. The energetic and geometrical features of O(2)(-)(H(2)O) and F(2)(-)(H(2)O) are compared. In addition, various electronic properties of X(2)(-)(H(2)O) are examined. For both O(2)(-)(H(2)O) and F(2)(-)(H(2)O), the C(s) binding motif appears to prevail at a compact distribution of the excess electron. However, when the electron is diffuse, characterized by the radius of gyration in the direction of the X(2) bond axis with a threshold of ∼0.84 Å, the C(2v) binding motif is formed.

Role of a distal pocket in the catalytic O2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

Five iron porphyrins with different superstructures were immobilized on self-assembled-monolayer (SAM)-coated interdigitated-array (IDAs) gold-platinum electrodes. The selectivity of the catalysts i.e., limited formation of partially reduced oxygen species (PROS) in the electrocatalytic reduction of dioxygen, is a function of 2 rates: (i) the rate of electron transfer from the electrode to the catalyst, which is controlled by the length, and conjugation of the linker from the catalyst to the electrode and (ii) the rate of bound oxygen (superoxide) hydrolysis, which correlates with the presence of a water cluster in the gas-binding pocket influencing the rate of oxygen binding; these factors are controlled by the nature of the porphyrin superstructure. The structurally biomimetic Tris-imidazole model is the most selective.

The water-oxygen dimer: First-principles calculation of an extrapolated potential energy surface and second virial coefficients

The systematic intermolecular potential extrapolation routine (SIMPER) is applied to the water-oxygen complex to obtain a five-dimensional potential energy surface. This is the first application of SIMPER to open-shell molecules, and it is the first use, in this context, of asymptotic dispersion energy coefficients calculated using the unrestricted time-dependent coupled-cluster method. The potential energy surface is extrapolated to the complete basis set limit, fitted as a function of intermolecular geometry, and used to calculate (mixed) second virial coefficients, which significantly extend the range of the available experimental data.

Structural, energetic, and spectroscopic features of lower energy complexes of superoxide hydrates O2(-)(H2O)(1-4)

The lower-energy portions of the potential energy surfaces of superoxide hydrates O2(-)(H2O)(1< or = n < or = 4) are thoroughly investigated at high computational levels. The structural, energetic and spectroscopic features of the stable superoxide hydrates on these potential energy surfaces are discussed, focusing in particular on some implications to their infrared spectra and the hydrogen bond trends. The present work reports the transition-state linkers between the most stable superoxide hydrates which are useful to understand the energetics of their mutual interconversions.

Photoelectron Imaging Study of the Effect of Monohydration on O2- Photodetachment

The photodetachment of the O(2)(-).H(2)O cluster anion at 780 and 390 nm is investigated in comparison with O(2)(-) using photoelectron imaging spectroscopy. Despite the pronounced shift in the photoelectron spectra, the monohydration has little effect on the photoelectron angular distributions: for a given wavelength and electron kinetic energy (eKE) range, the O(2)(-).H(2)O angular distributions are quantitatively similar to those for bare O(2)(-). This observation confirms that the excess electron in O(2)(-).H(2)O retains the overall character of the 2ppi(g) HOMO of O(2)(-). The presence of H(2)O does not affect significantly the partial wave composition of the photodetached electrons at a given eKE. An exception is observed for slow electrons, where O(2)(-).H(2)O exhibits a faster rise in the photodetachment signal with increasing eKE, as compared to O(2)(-). The possible causes of this anomaly are (i) the long-range charge-dipole interaction between the departing electron and the neutral O(2).H(2)O skeleton affecting the slow-electron dynamics; and (ii) the s wave contributions to the photodetachment, which are dipole-forbidden for pi(g)(-1) transitions in O(2)(-), but formally allowed in O(2)(-).H(2)O due to lower symmetry of the cluster anion and the corresponding HOMO.