Characterization Of Charge Transfer Research Articles

(Invited) Characterization of Charge Transfer and Recombination Processes at Metal Oxide Semiconductors for Solar Water Splitting

Metal oxide (MO) semiconductors based on abundant, low-cost materials are attractive for application in photoelectrochemical water splitting systems related to their inherent stability, specifically against oxidation. In addition, these materials provide a wide-ranging versatility in terms of composition, physical and chemical properties, and configuration of multi-component architectures. The semiconducting MO must at least comply with three requirements: (i) absorb sunlight efficiently; (ii) the valence and conduction band edges must be situated at the appropriate energies; and (iii) the electrochemical reaction must have favorable kinetics compared with charge separation, transport and recombination kinetics. Many systems have been evaluated for photo-oxidation of water in the past 10 years, with some of the more popular materials being Fe2O3, WO3, BiVO4, TiO2, etc.In order to improve the efficiency of a solar water splitting system, the specific loss mechanisms need to be elucidated; based on these insights strategies may then be developed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied in combination with steady-state techniques. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be deduced. The kinetic model applicable to the specific system determines the interpretation of the results. On the other hand, theoretical modeling is a very powerful method to validate the interpretation of the results, and to complement the experimental studies in order to obtain a clear picture of the dominant processes and effects.In this presentation, the results from IMPS studies on a variety of systems will be discussed and strategies to improve performance are presented and tested, both experimentally and theoretically: (i) planar, compact WO3 / BiVO4 heterojunction systems are compared with nanorod array WO3 / BiVO4 heterojunctions, with and without a hole scavenger; (ii) CuBi2O4 with and without catalysts and an alternative electron acceptor; and (iii) specific examples of other materials to highlight the proposed interpretations. The results are compared with calculations of the IMPS response using a classic kinetic model and a continuity equation approach in order to incorporate the effects of charge transport and electric field within the electrode.The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. We also thank the Ministerio de Ciencia e Innovación, the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund of the European Union through Project PID2019-110430GB-C22. AR thanks the Spanish Ministry of Education, Culture and Sports via a PhD grant (FPU2017-03684). Finally, we would like thank Dr. Antonio Zapien and the NANOMXCN initiative to foment scientific collaboration between Mexico and China for support.

Characterization of Charge Transfer in Excited States of Extended Clusters of π-Stacked Donor and Acceptor Complexes in Lock-Arm Supramolecular Ordering.

Lock-arm supramolecular ordering cocrystals formed by π-stacked materials constitute an interesting class of materials, which exhibits ferroelectric behavior at room temperature. To characterize the charge transfer in excited states, two complexes with π-stacked donors and acceptors, the 1,5-naphthalene diol (NDI) donor and pyromellitic diimide with diethylene glycol arms (PDIA) acceptor, 5-amino-1-naphthol (AMN) donor and PDIA acceptor, were investigated. The electronic excitations were calculated using the scaled opposite-spin variant of ADC(2), time-dependent density functional theory (TD-DFT) using a long-range corrected (LC) functional (ωB97xD), and the TD-LC approach within density-functional-based tight binding (TD-LC-DFTB). Face-to-face mixed stacks and edge-to-face crossed stacks up to hexamers were investigated. The calculations show that the ground state of the complexes does not possess significant CT character. On the other hand, the lowest excited state (S1) shows in all clusters a strong charge transfer. In several cases, the second excited state and also higher excited singlet states possess significant CT character. The orbitals involved in the excitation are mostly well localized and located on adjacent donor/acceptor pairs. Comparing different stacking directions, the vertical excitation energies for the NDI-PDIA crossed stacks are larger than those for the mixed stacks by 0.2-0.4 eV. In the case of the AMN-PDIA system, the energy differences are smaller (∼0.1 eV) with mostly the same energetic ordering as for the NDI-PDIA case. Strong red shifts in vertical fluorescence emission transitions have been computed, which could even lead to intersection between ground and first excited states, resulting in ultrafast radiationless decay and fluorescence quenching.

Excited State Dynamics of Cold Protonated Cytosine Tautomers: Characterization of Charge Transfer, Intersystem Crossing, and Internal Conversion Processes

Charge transfer reactions are ubiquitous in chemical reactivity and often viewed as ultrafast processes. For DNA, femtochemistry has undeniably revealed the primary stage of the deactivation dynamics of the locally excited state following electronic excitation. We here demonstrate that the full time scale excited state dynamics can be followed up to milliseconds through an original pump-probe photodissociation scheme applied to cryogenic ion spectroscopy. Protonated cytosine is chosen as a benchmark system in which the locally excited 1ππ* state decays in the femtosecond range toward long-lived charge transfer and triplet states with lifetimes ranging from microseconds to milliseconds, respectively. A three-step mechanism (1ππ* → 1CT → 3ππ*) is proposed where internal conversion from each state can occur leading ultimately to fragmentation in the ground electronic state.

Towards a Parameter-Free Characterization of Charge Transfer via Hopping: The Case of tris(8-Hydroxyquinolato) Aluminum

By using calculations based on density functional theory, we investigate the physical factors determining the elemental charge transfer in Alq3, taken as a prototype of molecular systems in condensed phase. The effect of the environment on the hopping of the charge carrier is evaluated self-consistently in a model in which an Alq3 dimer is embedded in an ensemble of permanent and polarizable dipoles, including orientational disorder and the presence of impurities. The results indicate that the origin of the activation barrier is mainly extrinsic and that the local orientation of the dipole moments plays a major role. The influence of nonadiabaticity is also studied and found to be more important for a hole than for an electron.

Characterization of Charge Transfer and Charge Transport in Polypyrrole Film Electrodes

Abstract The a.c. response of polypyrrole film electrodes in 0.2 M KNO3 was investigated at different applied potentials and film thicknesses. The results of impedance analysis on the basis of the Randles equivalent circuit indicated that the electron exchange between a metal and the polymer controlled the rate of charge-transfer process at the film electrode. Accordingly, the oxidation of the polymer proceeds from the back of the film to its surface. The resistance due to electric migration in the film strongly depended on the concentration of the supporting electrolyte; this dependence suggests that the migration is ionic. This effect may be due to a change in the solvent swelling of the polymer. The oxidation of the polymer brought about each decrease in the charge-transfer resistance at a metal–polymer interface and the limiting resistance associated with the diffusion of charge carriers. The low-frequency pseudocapacitance of the polymer increased with increasing oxidation level and film thickness, which was expected from a finite diffusion model.

Characterization of charge-transfer devices with position-dependent small-signal transfer inefficiency

Models for the small-signal response of a CTD with position-dependent transfer inefficiency are developed. The small-signal inefficiency E. can become position-dependent, for example, when sizeable amount of thermally generated charge is added to the transferred charge packets at each stage. Expressions for the frequency response and for the step response are derived in terms of single-transfer inefficiencies. In order to describe the overall performance of a CTD by means of an unique parameter, an effective transfer inefficiency is defined which only depends upon the input bias charge and generation conditions. Based upon the small-signal response models an experimental characterization and performance prediction technique is presented. Bucket-brigade delay lines with different design parameters are used as experimental test vehicles. It is shown that for optimum performance the devices must be operated at some optimum input bias charge level.