Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C₆₀ Thin-Film Photovoltaics

We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C_60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible <i>E</i>₂₂ and near-infrared <i>E</i>₁₁ and <i>E</i>₁₁ + <i>X</i> resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” <i>E</i>₁₁ + <i>X</i> and <i>E</i>₂₂ excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the <i>E</i>₁₁ transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm^–2, indicating negligible losses via trion, charge-exciton, and charge–charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the <i>E</i>₁₁ transition. The high IQE across multiple spectral windows, invariant photoresponsivity, and attractive open circuit voltage relative to the 1.18 eV optical bandgap demonstrate the future promise of using monochiral and multichiral semiconducting carbon nanotube films for broadband solar photovoltaic applications

Graphene Growth Dynamics on Epitaxial Copper Thin Films

Graphene chemical vapor deposition on copper is phenomenologically complex, yielding diverse crystal morphologies, including lobes, dendrites, stars, and hexagons, of various orientations depending on conditions. We present a comprehensive study of the evolution of these morphologies as a function of the copper surface orientation, absolute pressure, hydrogen-to-methane ratio (H₂:CH₄), and nucleation density. Growth was studied on ultrasmooth, epitaxial copper films inside copper enclosures to minimize copper polycrystallinity and roughness and decrease the graphene nucleation density. At low pressure and low H₂:CH₄, circular graphene islands initially form. After exceeding ∼1.0 μm, Mullins-Sekerka instabilities evolve into dendrites extending hundreds of micrometers in the ⟨100⟩, ⟨111⟩, and ⟨110⟩ directions on Cu(100), Cu(110), and Cu(111), respectively, indicating mass transport limited growth. Twin boundaries perturb the preferential growth direction on Cu(111) and alter graphene morphology. Increasing H₂:CH₄ results in compact islands that reflect the copper symmetry. At atmospheric pressure and low H₂:CH₄, Mullins-Sekerka instabilities develop but with multiple preferred orientations. Increasing H₂:CH₄ results in more hexagonal islands. Every growth regime can be tuned to yield continuous monolayers with a D:G Raman ratio <0.1. The understanding gained from this study provides a roadmap to rationally tailor the structure, morphology, and orientation of graphene crystals

Dissociating Excitons Photogenerated in Semiconducting Carbon Nanotubes at Polymeric Photovoltaic Heterojunction Interfaces

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have strong near-infrared and visible absorptivity and exceptional charge transport characteristics, rendering them highly attractive semiconductor absorbers for photovoltaic and photodetector technologies. However, these applications are limited by a poor understanding of how photogenerated charges, which are bound as excitons in s-SWCNTs, can be dissociated in large-area solid-state devices. Here, we measure the dissociation of excitons in s-SWCNT thin films that form planar heterojunction interfaces with polymeric photovoltaic materials using an exciton dissociation-sensitive photocapacitor measurement technique that is advantageously insensitive to optically induced thermal photoconductive effects. We find that fullerene and polythiophene derivatives induce exciton dissociation, resulting in electron and hole transfer, respectively, away from optically excited s-SWCNTs. Significantly weaker or no charge transfer is observed using wider gap polymers due to insufficient energy offsets. These results are expected to critically guide the development of thin film s-SWCNT-based photosensitive devices

Enrichment of Single-Walled Carbon Nanotubes by Diameter in Density Gradients

The bulk enrichment and separation of single-walled carbon nanotubes (SWNTs) by diameter has been achieved through ultracentrifugation of DNA-wrapped SWNTs in aqueous density gradients. The separation is identified by the visual formation of colored bands of SWNTs in the density range of 1.11−1.17 g cm-3. The optical absorbance spectra of the separated SWNTs indicate that SWNTs of decreasing diameter are increasingly more buoyant. This nondestructive and scalable separation strategy is expected to impact the fields of molecular electronics, optoelectronics, and sensing where SWNTs of a monodisperse band gap are essential

Templating Highly Crystalline Organic Semiconductors Using Atomic Membranes of Graphene at the Anode/Organic Interface

Charge and energy transport in organic semiconductors is highly anisotropic and dependent on crystalline ordering. Here, we demonstrate a novel approach for ordering crystalline organic semiconductors, with orientations optimized for optoelectronics applications, by using a single monolayer of graphene as a molecular template. We show, in particular, that large-area graphene can be integrated on metals and oxides to modify their surface energies and used to template copper phthalocyanine (CuPc), a prototypical organic semiconductor. On unmodified substrates, thermally evaporated films of CuPc are small-grained, and the molecules are in the “standing-up” (100) orientation. On graphene modified substrates, CuPc is templated in favorable “lying-down” (112̅) and (012̅) orientations with drastically larger crystal sizes. This results in an 86% increase in the absorption coefficient at 700 nm and should furthermore result in enhanced energy and charge transport. The use of highly conductive and transparent (>95%) graphene membranes as templates is expected to be a foundation for developing future planar and nanostructured organic light-emitting diodes and organic photovoltaics with improved performance

Efficiently Harvesting Excitons from Electronic Type-Controlled Semiconducting Carbon Nanotube Films

We have employed thin films of highly purified semiconducting carbon nanotubes as near-infrared optical absorbers in heterojunction photovoltaic and photodetector devices with the electron acceptor C60. In comparison with previous implementations of more electrically heterogeneous carbon nanotube/C60 devices, we have realized a 10× gain in zero-bias quantum efficiency (QE) and even more substantial gains in power conversion efficiency (ηp). The semiconducting nanotube/C60 heterojunctions are highly rectifying with a peak external QE, internal QE, and ηp of 12.9 ± 1.3, 91 ± 22, and 0.6%, respectively, in the near-infrared. We show that the device efficiency is determined by the effective length scale for exciton migration in the nanotube films, confirm the high internal QE via photoluminescence quenching, and demonstrate that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm. These results will guide the development of next-generation high-performance carbon nanotube-based solar cells and photosensitive devices

Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents

Arrays of aligned semiconducting single-walled carbon nanotubes (s-SWCNTs) with exceptional electronic-type purity were deposited at high deposition velocity of 5 mm min^–1 by a novel “dose-controlled, floating evaporative self-assembly” process with excellent control over the placement of stripes and quantity of s-SWCNTs deposited. This approach uses the diffusion of organic solvent on the water–air interface to deposit aligned s-SWCNT (99.9%) tubes on a partially submerged hydrophobic substrate, which is withdrawn vertically from the surface of water. By decoupling the s-SWCNT stripe formation from the evaporation of the bulk solution and by iteratively applying the s-SWCNTs in controlled “doses”, we show through polarized Raman studies that the s-SWCNTs are aligned within ±14°, are packed at a density of ∼50 s-SWCNTs μm^–1, and constitute primarily a well-ordered monodispersed layer. The resulting field-effect transistor devices show high performance with a mobility of 38 cm² V^–1 s^–1 and on/off ratio of 2.2 × 10⁶ at 9 μm channel length

Assembly and Alignment of High Packing Density Carbon Nanotube Arrays Using Lithographically Defined Microscopic Water Features

High packing density aligned arrays of semiconducting carbon nanotubes (CNTs) are required for many electronics applications. Past work has shown that the accumulation of CNTs at a water–solvent interface can drive array self-assembly. Previously, the confining interface was a large-area, macroscopic feature. Here, we report on the CNT assembly on microscopic water features. Water microdroplets are formed on 10–100 μm wide hydrophilic stripes patterned on a substrate. Exposure to CNTs dispersed in solvent accumulates CNTs at the microdroplet–solvent interface, driving their alignment and deposition at the microdroplet–solvent–substrate contact line. Compared with macroscopic methods in which the contact line uncontrollably moves across the substrate as it is pulled out of the liquids, the hydrophilic patterns and microdroplets allow pinning of the contact line. As CNTs deposit, the contact line self-translates, allowing for dense CNT packing. We realize monolayer CNT arrays aligned within ±3.9° at density of 250 μm–1 and field effect transistors with a high current density of 1.9 mA μm–1 and transconductance of 1.2 mS μm–1 at −0.6 V drain bias and 60 nm channel length

Emergent Dispersion and Sorting Behaviors of Carbon Nanotubes When Combining Short Conjugated Oligomers with Nonconjugated Coil Segments

Single-walled carbon nanotubes have unique electronic properties and the potential to advance the microelectronics industry. For these applications, semiconducting CNTs with high purity (>99.99%) and specific diameters or band gaps are required. High-molecular-weight conjugated polymers, especially polyfluorenes and their derivatives, have shown an exceptional ability to selectively wrap semiconducting over metallic CNTs while enriching a reduced number of diameters, whereas lower-molecular-weight analogues (<10,000 g/mol) form much less stable dispersions. Here, we report coil-conjugated-coil triblock copolymers (PS28-b-PFO16-b-PS28 and PS69-b-PFO16-b-PS69) that combine low-molecular-weight fluorene oligomers with polystyrene coils. Neither the short, conjugated oligomers nor the polystyrene coils alone have the ability to act as stable wrappers, but when combined, stable CNT dispersions are obtained with yields comparable to or exceeding those of high-molecular-weight PFO, with a semiconducting selectivity shifted to larger diameters. These results open the door to low-molecular-weight wrappers for CNT sorting and to solution-processing with wrappers with modified conformations that have the potential to alter the interactions of CNTs with their environment

Hydrodynamic Characterization of Surfactant Encapsulated Carbon Nanotubes Using an Analytical Ultracentrifuge

The hydrodynamic properties of surfactant encapsulated single-walled carbon nanotubes (SWNTs) have been characterized by optically measuring their spatial and temporal redistribution in situ in an analytical ultracentrifuge. The measured redistribution profiles are fit to the Lamm equation, thus determining the sedimentation, diffusion, and hydrodynamic frictional coefficients of the surfactant encapsulated SWNTs. For sodium cholate encapsulated SWNTs, we demonstrate that the technique of analytical ultracentrifugation can be utilized to determine the linear packing density of surfactant molecules along the length of the SWNTs, 3.6 ± 0.8 nm−1, and the anhydrous molar volume of the surfactant molecules on the SWNT surfaces, 270 ± 20 cm3 mol−1. Additionally, analytical ultracentrifugation is used to measure and compare the sedimentation rates of bundled and isolated carbon nanotubes. This study should serve as a guide for designing centrifuge-based processing procedures for preparing samples of SWNTs for a wide variety of applications and studies. Additionally, the results obtained here should aid in understanding the hydrodynamic properties of SWNTs and the interactions between SWNTs and surfactants in aqueous solution