Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C₆₀ layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C₆₀ increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz SWCNT hole mobility of >1.3 cm² V^–1 s^–1. These studies improve understanding of s-SWCNT photoresponses in solar cells and photodetectors

Quenching of the Perylene Fluorophore by Stable Nitroxide Radical-Containing Macromolecules

Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern–Volmer approach but rather necessitates a more complex model

Polymer-Free Carbon Nanotube Thermoelectrics with Improved Charge Carrier Transport and Power Factor

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have recently attracted attention for their promise as active components in a variety of optical and electronic applications, including thermoelectricity generation. Here we demonstrate that removing the wrapping polymer from the highly enriched s-SWCNT network leads to substantial improvements in charge carrier transport and thermoelectric power factor. These improvements arise primarily from an increase in charge carrier mobility within the s-SWCNT networks because of removal of the insulating polymer and control of the level of nanotube bundling in the network, which enables higher thin-film conductivity for a given carrier density. Ultimately, these studies demonstrate that highly enriched s-SWCNT thin films, in the complete absence of any accompanying semiconducting polymer, can attain thermoelectric power factors in the range of ∼400 μW m^–1 K^–2, which is on par with that of some of the best single-component organic thermoelectrics demonstrated to date

Photoinduced Energy and Charge Transfer in P3HT:SWNT Composites

Using steady-state photoluminescence and transient microwave conductivity (TRMC) spectroscopies, photoinduced energy and charge transfer from poly(3-hexylthiophene) (P3HT) to single-walled carbon nanotubes (SWNTs) are reported. Long-lived charge carriers are observed for excitons generated in the polymer due to interfacial electron transfer, while excitation of the SWNTs results in short-lived carriers confined to the nanotubes. The TRMC-measured mobility of electrons injected into the SWNTs exhibits a surprisingly small lower limit of 0.057 cm²/(V s), which we attribute to carrier scattering within the nanotube that inhibits resonance of the microwave electric field with the confined carriers. The observation of charge transfer and the lifetime of the separated carriers suggest that the primary photoinduced carrier generation process does not limit the performance of organic photovoltaic (OPV) devices based on P3HT:SWNT composites. With optimization, blends of P3HT with semiconducting SWNTs (s-SWNTs) may exhibit promise as an OPV active layer and could provide good solar photoconversion power efficiencies

Complementary operation.

<p>Schematic diagram of the MSET for different magnetization orientations. (a) <i>ϕ</i> = 0° the magnetization is in-plane and (b) <i>ϕ</i> = 90° the magnetization is out-of-plane. (c) Coulomb blockade oscillations as a function of the direction of the back-gate voltage <i>V</i>_<i>gs</i> and the applied magnetic field orientation <i>ϕ</i> for B = 0.7 T. The-dashed blue and red lines indicate the operating points. (d) MSET Ids-Vgs transfer function at <i>ϕ</i> = 0°. The logic 0 (1) has been selected at a low (high) current level, n-type SET. (e) MSET Ids-Vgs transfer function at <i>ϕ</i> = 90°. The logic outputs have been inverted, p-type SET.</p

Single-device logic.

<p>(a) <i>V</i>_<i>ds</i> − <i>V</i>_<i>gs</i> map of the drain current for <i>ϕ</i> = 0° showing the characteristic Coulomb diamonds. Red and blue frames sketch the implemented logic gates for <i>ϕ</i> = 0° and 90° respectively. (b-c) AND-OR set of reprogrammable logic gates. AND gate implemented at <i>ϕ</i> = 0° (b) and OR gate at <i>ϕ</i> = 90° (c) with <i>V</i>_<i>ds</i> (input A) 0(1) defined as −132(−220) <i>μ</i>V and <i>V</i>_<i>gs</i> (input B) 0(1) defined as −96(0) <i>μ</i>V. (d-e) NAND-NOR set of reprogrammable logic gates. NAND gate implemented at <i>ϕ</i> = 0° (d) and NOR gate at <i>ϕ</i> = 90° (e) with <i>V</i>_<i>sd</i> (input A) 0(1) defined as 220(132) <i>μ</i>V and <i>V</i>_<i>gs</i> (input B) 0(1) defined as 128(224) <i>μ</i>V.</p

Logic at the multiple device level considering identical SETs and the logic inputs defined in Fig 2.

<p>The inputs A and B are defined as taken as the SET gate values. (a) A series pull-down network performs the OR operation at <i>ϕ</i> = 0° and NAND at <i>ϕ</i> = 90°. (b) Parallel pull-down network performs the AND operation at <i>ϕ</i> = 0° and NOR at <i>ϕ</i> = 90°.</p

Device structure.

<p>(a) Schematic cross-section of the device sketching the magnetization orientation of the (Ga,Mn)As back-gate layer. (b) SEM image of the device. The aluminium island is separated from the source and drain leads by AlO_<i>x</i> tunnel junctions. Side gates were not used in this experiment. (c) Drain current (<i>I</i>_<i>ds</i>) oscillations as a function of the back gate voltage (<i>V</i>_<i>gs</i>).</p

Mediating Photochemical Reaction Rates at Lewis Acidic Rare Earths by Selective Energy Loss to 4f-Electron States

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 the f-electron states of Dy3+ act as an energy sink. Complexes RE(hfac)3(NMMO)2 (RE = Dy (2-Dy) and Y (2-Y), hfac = hexafluoroacetylacetonate, and NMMO = N-methylmorpholine-N-oxide) showed variable rates of oxygen atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation, as monitored by 1H and 19F NMR spectroscopies. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction or lack thereof. Competing sensitization pathways leading to excited-state deactivation in 2-Dy through energy transfer to the 4f electron manifold ultimately slows the OAT reaction at this metal cation. The measured rate differences between the open-shell Dy3+ and closed-shell Y3+ complexes demonstrate that using established principles of 4f ion sensitization may deliver new, selective modalities for differentiating the RE elements that do not depend on cation size