Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes

The hole-injection and recombination photophysics for NiO sensitized with RuP ([Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, that is, quenching of the dye excited state that results in a detectable charge-separated electron–hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid nongeminate recombination enabled by the presence of latent holes in the surface of the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway

Synthetically Encoding 10 nm Morphology in Silicon Nanowires

Si nanowires (NWs) have been widely explored as a platform for photonic and electronic technologies. Here, we report a bottom-up method to break the conventional “wire” symmetry and synthetically encode a high-resolution array of arbitrary shapes, including nanorods, sinusoids, bowties, tapers, nanogaps, and gratings, along the NW growth axis. Rapid modulation of phosphorus doping combined with selective wet-chemical etching enabled morphological features as small as 10 nm to be patterned over wires more than 50 μm in length. This capability fundamentally expands the set of technologies that can be realized with Si NWs, and as proof-of-concept, we demonstrate two distinct applications. First, nanogap-encoded NWs were used as templates for Noble metals, yielding plasmonic structures with tunable resonances for surface-enhanced Raman imaging. Second, core/shell Si/SiO₂ nanorods were integrated into electronic devices that exhibit resistive switching, enabling nonvolatile memory storage. Moving beyond these initial examples, we envision this method will become a generic route to encode new functionality in semiconductor NWs

Light-Harvesting Polymers: Ultrafast Energy Transfer in Polystyrene-Based Arrays of π‑Conjugated Chromophores

Energy transfer along a nonconjugated polymer chain is studied with a polystyrene-based copolymer of oligo­(phenylene-ethynylene) (OPE) donor and thiophene-benzothiadiazole (TBT) acceptor pendants. The graft copolymers are prepared from reversible addition–fragmentation transfer polymerization (RAFT) and copper­(I)-catalyzed azide–alkyne “click” reaction. The singlet energy transfer from donor to accept is studied via fluorescence emission and ultrafast transient absorption spectroscopy. Near unity quenching of the OPE excited state by the TBT moiety occurs on multiple time scales (2–50 ps) dependent on where the initial exciton is formed on the polymer

Application of Degenerately Doped Metal Oxides in the Study of Photoinduced Interfacial Electron Transfer

Degenerately doped In₂O₃:Sn semiconductor nanoparticles (<i>nano</i>ITO) have been used to study the photoinduced interfacial electron-transfer reactivity of surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ (RuP²⁺) molecules as a function of driving force over a range of 1.8 eV. The metallic properties of the ITO nanoparticles, present within an interconnected mesoporous film, allowed for the driving force to be tuned by controlling their Fermi level with an external bias while their optical transparency allowed for transient absorption spectroscopy to be used to monitor electron-transfer kinetics. Photoinduced electron transfer from excited-state -RuP^2+* molecules to <i>nano</i>ITO was found to be dependent on applied bias and competitive with nonradiative energy transfer to <i>nano</i>ITO. Back electron transfer from <i>nano</i>ITO to oxidized -RuP³⁺ was also dependent on the applied bias but without complication from inter- or intraparticle electron diffusion in the oxide nanoparticles. Analysis of the electron injection kinetics as a function of driving force using Marcus–Gerischer theory resulted in an experimental estimate of the reorganization energy for the excited-state -RuP^3+/2+* redox couple of λ* = 0.83 eV and an electronic coupling matrix element, arising from electronic wave function overlap between the donor orbital in the molecule and the acceptor orbital(s) in the <i>nano</i>ITO electrode, of <i>H</i>_ab = 20–45 cm^–1. Similar analysis of the back electron-transfer kinetics yielded λ = 0.56 eV for the ground-state -RuP^3+/2+ redox couple and <i>H</i>_ab = 2–4 cm^–1. The use of these wide band gap, degenerately doped materials provides a unique experimental approach for investigating single-site electron transfer at the surface of oxide nanoparticles

Watching Photoactivation in a Ru(II) Chromophore–Catalyst Assembly on TiO₂ by Ultrafast Spectroscopy

This paper examines the ultrafast dynamics of the initial photoactivation step in a molecular assembly consisting of a chromophore (denoted [Ru_a^II]²⁺) and a water-splitting catalyst (denoted [Ru_b^II]²⁺) anchored to TiO₂. Photoexcitation of the chromophore is followed by rapid electron injection from the Ru­(II) metal-to-ligand charge-transfer (MLCT) excited state. The injection process was followed via the decay of the bpy radical anion absorption at 375 nm. Injection is ∼95% efficient and exhibits multiple kinetic components with decay times ranging from <250 fs to 250 ps. Electron injection is followed by the transfer of the oxidative equivalent from the chromophore to the catalyst (Δ<i>G</i> = −0.28 eV) with a transfer time of 145 ps. In the absence of subsequent photoexcitation events, the charge-separated state undergoes electron-transfer recombination on the microsecond time scale

Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces

Photoinduced, interfacial electron injection and back electron transfer between surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ and degenerately doped In₂O₃:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (Δ<i>G</i>^o′). By controlling the external bias, Δ<i>G</i>^o′ was varied from 0 to −1.8 eV for electron injection and from −0.3 to −1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using Marcus–Gerischer theory gave an experimental estimate of λ = 0.56 eV for the reorganization energy of the surface-bound Ru^III/II couple in acetonitrile with 0.1 M LiClO₄ electrolyte

Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

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 nonacceptor-substituted thiophene oligomers, <b>T</b>_{<b><i>n</i></b>}, and corresponding NDI end-capped series, <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> (where <i>n</i> is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the <b>T</b>_{<b><i>n</i></b>} and <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond–microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the <b>T</b>_{<b><i>n</i></b>} oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, ³<b>T</b>_{<b><i>n</i></b>}*, to <i>N</i>,<i>N</i>-dimethylviologen (MV²⁺) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, <b>T</b>_{<b><i>n</i></b>}^+•. The fluorescence of the <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> oligomers is quenched nearly quantitatively, and no long-lived transients are observed by nanosecond transient absorption. These findings suggest that rapid photoinduced electron transfer and charge recombination occurs, NDI-¹(T_<i>n</i>)*-NDI → NDI-(T_<i>n</i>)^+•-NDI^–• → NDI-T_<i>n</i>-NDI. Preliminary femtosecond–picosecond transient absorption studies on <b>T</b>_<b>4</b><b>NDI</b>_<b>2</b> reveal that both forward electron transfer and charge recombination occur with <i>k</i> > 10¹¹ s^–1, consistent with both reactions being nearly activationless. Analysis with semiclassical electron transfer theory suggests that both reactions occur at near the optimum driving force where −Δ<i>G</i> ∼ λ

Reversible Strain-Induced Electron–Hole Recombination in Silicon Nanowires Observed with Femtosecond Pump–Probe Microscopy

Strain-induced changes to the electronic structure of nanoscale materials provide a promising avenue for expanding the optoelectronic functionality of semiconductor nanostructures in device applications. Here we use pump–probe microscopy with femtosecond temporal resolution and submicron spatial resolution to characterize charge–carrier recombination and transport dynamics in silicon nanowires (NWs) locally strained by bending deformation. The electron–hole recombination rate increases with strain for values above a threshold of ∼1% and, in highly strained (∼5%) regions of the NW, increases 6-fold. The changes in recombination rate are independent of NW diameter and reversible upon reduction of the applied strain, indicating the effect originates from alterations to the NW bulk electronic structure rather than introduction of defects. The results highlight the strong relationship between strain, electronic structure, and charge–carrier dynamics in low-dimensional semiconductor systems, and we anticipate the results will assist the development of strain-enabled optoelectronic devices with indirect-bandgap materials such as silicon

π‑Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated 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 on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond–nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond–nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC₆₁BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low

Ultrafast Recombination Dynamics in Dye-Sensitized SnO₂/TiO₂ Core/Shell Films

Interfacial dynamics are investigated in SnO₂/TiO₂ core/shell films derivatized with a Ru­(II)-polypyridyl chromophore ([Ru^II(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺, <b>RuP</b>) 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 recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO₂ core and must tunnel through the TiO₂ shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for the overall efficiencies of dye-sensitized photoelectrosynthesis cells (DSPECs)