Photoinduced spontaneous free-carrier generation in semiconducting single-walled carbon nanotubes

Abstract

Strong quantum confinement and low dielectric screening impart single-walled carbon nanotubes with exciton-binding energies substantially exceeding kBT at room temperature. Despite these large binding energies, reported photoluminescence quantum yields are typically low and some studies suggest that photoexcitation of carbon nanotube excitonic transitions can produce free charge carriers. Here we report the direct measurement of long-lived free-carrier generation in chirality-pure, single-walled carbon nanotubes in a low dielectric solvent. Time-resolved microwave conductivity enables contactless and quantitative measurement of the real and imaginary photoconductance of individually suspended nanotubes. The conditions of the microwave conductivity measurement allow us to avoid the complications of most previous measurements of nanotube free-carrier generation, including tube–tube/tube–electrode contact, dielectric screening by nearby excitons and many-body interactions. Even at low photon fluence (approximately 0.05 excitons per μm length of tubes), we directly observe free carriers on excitation of the first and second carbon nanotube exciton transitions.

300% Enhancement of Carrier Mobility in Uniaxial‐Oriented Perovskite Films Formed by Topotactic‐Oriented Attachment

“…The impact of TOA‐perovskite growth on charge transport in perovskites was studied by comparing the photoconductance (Δ G ) of TOA‐ and SE‐perovskite films using fp‐TRMC. This technique has shown its versatility in characterizing charge‐carrier mobilities, charge‐carrier generation efficiency, and subsequent carrier‐recombination dynamics in various organic/inorganic material systems . In fp‐TRMC, Δ G ( t ) is related to the product (Φ( t )Σ µ ) of carrier‐generation yield (Φ( t )) and both electron ( µ e ) and hole ( µ h ) mobilities (Σ µ = µ e + µ h ).…”

Direct–indirect character of the bandgap in methylammonium lead iodide perovskite

“…This can be visualized with a so-called action spectrum, in which the wavelength-dependent Σμ is directly compared to the FA spectrum. 22,23 Figures 2b and 2d (dashed lines) show the action spectra for the CH3NH3PbI3 thin film at 300 K (b) and at 120 K (d), together with the temperature-dependent FA spectra (solid lines).…”

Independence of optical absorption on Auger ionization in single-walled carbon nanotubes revealed by ultrafast e–h photodoping

“…This prediction is considered as one of the possible sources of free carriers in recent photocurrent measurements, along with phonon mediated E 22 free carrier disassociation pathways [10]. Recent studies at very low excitation densities have indeed confirmed that single exciton disassociation pathways exist to generate free carriers [11]. However, it has yet to be shown whether the very efficient exciton-exciton annihilation (EEA) process in SWCNTs acts as an additional source of free carriers, as has been theorized.…”

“…The fluence dependence of the optical absorption and PL are measured under ultrafast (∼150 fs) E 22 resonant optical excitation. While this technique is not sensitive to linear sources of free-carriers, such as E 22 and E 11 disassociation [11], it is highly sensitive to nonlinear sources of free-carriers, namely density dependent Auger ionization. We show that, despite strong PL saturation and excitation fluences far exceeding the BGR theshhold calculated by Konabe and Okada [9], the optical absorption remains constant, indicating the Auger process does not lead to exciton disassociation.…”