Charge photogeneration and transport in AgBis2 nanocrystal films for photovoltaics

Solution-processed AgBiS2 nanocrystal films are a promising material for nontoxic, earth-abundant solar cells. While solar cells with good device efficiency are demonstrated, so far, hardly anything is known about charge generation, transport, and recombination processes in these films. Here, a photoinduced time-resolved microwave conductivity study on AgBiS2 nanocrystal films is presented. By modeling the experimental data with density-dependent recombination processes, the product of the temperature-dependent electron and hole quantum yield and mobility, and the electron and hole recombination kinetics are determined

Tight-binding parameters for charge transfer along DNA

We systematically examine all the tight-binding parameters pertinent to charge transfer along DNA. The $\pi$ molecular structure of the four DNA bases (adenine, thymine, cytosine, and guanine) is investigated by using the linear combination of atomic orbitals method with a recently introduced parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are discussed and then used for calculating the corresponding wavefunctions of the two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO and LUMO energies of the bases are in good agreement with available experimental values. Our results are then used for estimating the complete set of charge transfer parameters between neighboring bases and also between successive base-pairs, considering all possible combinations between them, for both electrons and holes. The calculated microscopic quantities can be used in mesoscopic theoretical models of electron or hole transfer along the DNA double helix, as they provide the necessary parameters for a tight-binding phenomenological description based on the $\pi$ molecular overlap. We find that usually the hopping parameters for holes are higher in magnitude compared to the ones for electrons, which probably indicates that hole transport along DNA is more favorable than electron transport. Our findings are also compared with existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table

A handle on charge reorganization

Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ChemE/Opto-electronic Material

Opto-Electronic Properties of Conjugated Molecular Wires

Conjugated polymers are of considerable current interest because of their semi-conducting and light-emitting properties. These properties, combined with their relatively low cost and good processability as compared to inorganic semiconductors, make them attractive candidates for application in plastic electronic devices. Conjugated polymers are being considered for application as the semi-conducting layer in low-cost field-effect transistors (FETs) for the fabrication of flexible or disposable electronic devices. A second application using the light-emitting properties of conjugated polymers is as an emissive layer in light emitting diodes (LEDs) and large area displays. The third application that is being considered is the use of conjugated polymers as the light absorbing layer in photovoltaic devices. Finally, the fourth possible application is the use of conjugated polymers is as molecular wires in single molecule electronics. All of these applications critically rely on the ability of the conjugated polymers to transport charges. A thorough understanding of the properties of charge carriers and their mobility is therefore of obvious importance. Some of the applications involve electronically excited states of conjugated polymers. In LEDs the lowest singlet excited state is the emissive state, while in photovoltaic devices the excited state is the primarily formed species. Therefore it is also of interest to study the properties of conjugated polymers or oligomers in their electronically excited state. This thesis describes a combined experimental and theoretical study of the opto-electronic properties of isolated conjugated polymers in dilute solution. Measurements of the mobility of charges on isolated conjugated polymer chains in solution are discussed and compared to theoretical calculations. The electronic absorption spectra of charged conjugated oligomers were measured and are compared to calculated spectra obtained from quantum chemical calculations. The excited state polarizabilities of conjugated oligomers were calculated using quantum chemical methods and are compared to earlier measurements.Interfaculty Reactor Institut

Singlet Fission in Crystalline Organic Materials: Recent Insights and Future Directions

Singlet fission (SF) involves the conversion of one excited singlet state into two lower excited triplet states and has received considerable renewed attention over the past decade. This Perspective highlights recent developments and emerging concepts of SF in solid-state crystalline materials. Recent experiments showed the crucial role of vibrational modes in speeding up SF, and theoretical modeling has started to define an optimal energetic landscape and intermolecular orientation of chromophores for highly efficient singlet fission. A critical analysis of these developments leads to directions for future research to eventually find singlet fission chromophores with excellent optoelectronic properties.ChemE/Opto-electronic Material

Cooperative biexciton generation and destructive interference in coupled quantum dots using adiabatic rapid passage

We report numerical simulations of biexciton generation in coupled quantum dots (CQDs) placed in a static electric field and excited by a chirped laser pulse. Our simulations explicitly account for exciton-phonon interactions at finite temperature using a non-Markovian quantum jump approach to solve the excitonic dynamics. In the case of noninteracting quantum dots, the biexciton generation is severely limited by the biexciton binding energy. We demonstrate that the application of an axial electric field along the CQDs can yield a favorable excitonic level alignment that compensates for the biexciton binding energy and yields an optimum biexciton generation. On the contrary, well-defined values of the electric field lead to destructive quantum interference that completely inhibits the biexciton generation. We therefore demonstrate here the potential of chirped pulse excitations of CQDs for high-efficiency biexciton generation but also for the control of unique optoelectronic properties of complex quantum systems.ChemE/Chemical EngineeringApplied Science