Melissa Gish

• Doctor of Philosophy
• Scientist at National Renewable Energy Laboratory

Singlet fission, the process of transforming a singlet excited state into two lower energy triplet excited states, is a promising strategy for improving the efficiency of dye-sensitized solar cells. The difficulty in utilizing singlet fission molecules in this architecture is understanding and controlling the orientation of dyes on mesoporous metal...

Pi-conjugated semiconducting polymers have garnered attention for a variety of electronic and optoelectronic applications, such as field-effect transistors, thermoelectric generators, and photovoltaic devices. In these systems, the charge carrier dynamics are primarily governed by the properties of the polaron and the polymer microstructure. More r...

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push–pull polymer poly(NDI2OD-T2) (N2200) is one such material. W...

Innovation in optoelectronic semiconductor devices is driven by a fundamental understanding of how to move charges and/or excitons (electron–hole pairs) in specified directions for doing useful work, e.g., for making fuels or electricity. The diverse and tunable electronic and optical properties of two-dimensional (2D) transition metal dichalcogeni...

Energy transfer across the donor–acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to th...

A rhenium metal complex tethered to a silicon quantum dot allows exploration of the effects of catalyst anchoring and Re–Si energetic alignment on photocatalytic CO 2 reduction activity.

Ambient temperature growth on Si produces a polycrystalline ZnTiN 2 film while Sn-assisted growth on sapphire at elevated temperature results in a single-crystal-like ZnTiN 2 film with significantly reduced sub-bandgap absorption.

Manifesting chemical differences in individual rare earth (RE) element complexes is challenging due to the similar sizes of the tripositive cations and the corelike 4f shell. We disclose a new strategy for differentiating between similarly sized Dy3+ and Y3+ ions through a tailored photochemical reaction of their isostructural complexes in which th...

Photoelectrochemical fuel generation is a promising route to sustainable liquid fuels produced from water and captured carbon dioxide with sunlight as the energy input. Development of these technologies requires photoelectrode materials that are both photocatalytically active and operationally stable in harsh oxidative and/or reductive electrochemi...

β-Mn2V2O7 (β-MVO) was recently reported to be a promising candidate for photoelectrochemical (PEC) water splitting, with a suitable band gap and band edge positions and reasonable stability and photoactivity in preliminary tests in alkaline solution. Here, we present an in-depth evaluation of the PEC performance and stability of phase-pure nanostru...

Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the d...

The rate of singlet fission, the process of generating two triplet excitons with photoexcitation of one singlet exciton, depends on a combination of singlet/triplet energy balance and intermolecular coupling. Here, we perform carboxylic acid functionalization of anthradithiophene (ADT) derivatives that results in hydrogen bonds that drive molecular...

Singlet fission (SF), the generation of two triplet excitons per the absorption of one photon, is a promising strategy for increasing the efficiencies of solar cells beyond the theoretical Shockley-Queisser limit of 34%. Upon photon absorption by an SF molecule, the initially created singlet excited state (S1) interacts with a neighboring chromopho...

The photophysics of a series of thiophene oligomers (Tn) with and without naphthalene diimide (NDI) acceptor end groups were investigated using femtosecond transient absorption spectroscopy. Photoexcited thiophene oligomers (n = 4, 6, 8, 10 and12) exhibit complex length-dependent excited state dynamics on the picosecond timescale due to rapid struc...

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]⁴⁺, -[RuaII-RubII-OH2]⁴⁺, on nanocrystalline TiO2 electrodes, starting from either -[RuaII-RubII-OH2]⁴⁺ or -[RuaII-RubIII-OH2]⁵⁺, have been investigated. Transient absorption measurements for TiO2-[RuaII-RubII-OH2]⁴⁺ in 0.1 M...

A series of linear thiophene oligomers containing 4, 6, 8, 10 and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of non-acceptor subs...

A series of organometallic complexes and polymers have been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end-functionalized with cyclometallated platinu...

Interfacial dynamics are investigated in SnO2/TiO2 core/shell films derivatized with a Ru(II)-polypyridyl chromophore ([RuII(bpy)2(4,4’-(PO3H2)2bpy)]²⁺, RuP) using transient absorption methods. Electron injection from the chromophore into the TiO2 shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombin...

The dye sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuel...

Interfacial electron transfer at titanium dioxide (TiO2) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal to ligand charge transfer state (MLCT) energetics are tuned through chemical modification. The series of twelve complexes are of the form Ru(II)(bpy-A)(L)2(2+), where bpy-A is a bipyridine ligand functionaliz...

An isoindigo based π-conjugated oligomer and polymer that contain cyclometallated platinum(II) "auxochrome" units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers...

Surface-bound, PDI-based molecular assemblies have been synthesized on nanocrystalline TiO2 by reaction of a dianhydride with a surface-bound aniline and succinimide bonding. In a second step, the Fe(II) polypyridyl complex [Fe(II)(tpy-PhNH2)2](2+) was added to the outside of the film, also by succinimide bonding. Ultrafast transient absorption mea...

Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a "layer-by-layer" approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating...

We report a detailed kinetic analysis of ultrafast interfacial and intra-assembly electron transfer following excitation of an oligoproline scaffold functionalized by chemically linked light-harvesting chromophore [Ru(pbpy)2(bpy)]2+ (pbpy = 4,4′-(PO3H2)2-2,2′-bipyridine, bpy = 2,2′-bipyridine) and water oxidation catalyst [Ru(Mebimpy)(bpy)OH2]2+ (M...

Single water molecules have been isolated in solid Ar matrices at 4 K and studied by rovibrational spectroscopy using FTIR in the regions of the ν(1), ν(2), and ν(3) modes. Upon nuclear spin conversion at 4 K, essentially pure para-H(2)O was prepared, followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above th...

Single water molecules have been isolated in solid Ar matrices at 4 K and studied by ro-vibrational spectroscopy using FTIR in the regions of the nu1, nu2, and nu3 modes. Upon nuclear spin conversion at 4 K, essentially pure para-H2O was prepared followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the con...