Providing Time to Transfer: Longer Lifetimes Lead to Improved Energy Transfer in Films of Semiconducting Carbon Nanotubes.

The performance of photovoltaic devices made using semiconducting carbon nanotubes is limited by the transverse exciton diffusion length, which is ultimately set by intertube energy transfer. In this paper, we study whether extending the exciton lifetime improves energy transfer, by allowing more time for exciton transfer between carbon nanotubes, and thereby device performance. To do so, we prepare nanotubes by either shear-force mixing or ultrasonication, leading to different lengths and defect densities. We create thin films that mix (6,5) and (7,5) nanotubes and quantify the relative amounts of energy transfer in them using two-dimensional white-light (2DWL) spectroscopy and photoluminescence excitation (PLE) spectroscopy. Cross-peaks appearing in 2DWL spectra and quenching of the (6,5) PLE signal upon mixing both quantify energy transfer from (6,5) to (7,5). In both spectroscopies, energy transfer between shear-force mixed tubes is ∼20% more efficient. The cross-peaks in 2DWL spectra grow in at the same rate regardless of the processing method with the all shear-force mixed sample ultimately reaching a larger cross-peak amplitude. Shear-force mixing methods instead of sonication have improved external quantum efficiency in carbon nanotube devices by 30%. The spectroscopic results observed here link energy transfer to exciton diffusion and correlate to device performance.

Impact of non-equilibrium molecular packings on singlet fission in microcrystals observed using 2D white-light microscopy.

Singlet fission, the process of splitting a singlet exciton into two triplet excitons, has been proposed as a mechanism for improving the efficiency of future photovoltaic devices. In organic semiconductors exhibiting singlet fission, the geometric relationship between molecules plays an important role by setting the intermolecular couplings that determine the system energetics. Here, we spatially image TIPS-pentacene microcrystals using ultrafast two-dimensional white-light microscopy and discover a low-energy singlet state sparsely distributed throughout the microcrystals, with higher concentrations at edges and morphological defects. The spectra of these singlet states are consistent with slip-stacked molecular geometries and increased charge-transfer couplings. The picosecond-timescale kinetics of these low-energy singlet states matches that of the correlated triplet-pair state, which we attribute to singlet/triplet-pair interconversion at these sites. Our observations support the conclusion that small populations of geometries with favourable energetics can play outsized roles in singlet fission processes.

Multidimensional Spectroscopy on the Microscale: Development of a Multimodal Imaging System Incorporating 2D White-Light Spectroscopy, Broadband Transient Absorption, and Atomic Force Microscopy.

The dynamics of electronic transitions in solid-state materials are closely linked to microscopic morphology, but it is challenging to simultaneously characterize their spectral and temporal response with high spatial resolution. We present a time-resolved nonlinear microscopy system using white-light supercontinuum pulses as a broadband light source. This system is capable of correlating nanometer scale sample morphology determined from atomic force topography measurements with broadband transient absorption hyperspectral images and ultrafast 2D white-light spectra, all with a spatial resolution of ≤1 µm. The experimental apparatus is described with a focus on the dispersion management strategies necessary to minimize the duration of optical pulses when implementing an AOM based pulse-shaping system covering a broad-spectral range in the VIS/NIR. Experiments on TIPS-pentacene organic semiconductor microcrystals are used to demonstrate the unique capabilities of this technique.

Two-Dimensional White-Light Spectroscopy Using Supercontinuum from an All-Normal Dispersion Photonic Crystal Fiber Pumped by a 70 MHz Yb Fiber Oscillator.

We report on a new broadband, ultrafast two-dimensional white-light (2DWL) spectrometer that utilizes a supercontinuum pump and a supercontinuum probe generated with a ytterbium fiber oscillator and an all-normal dispersion photonic crystal fiber (ANDi PCF). We demonstrate compression of the supercontinuum to sub-20 fs and the ability to collect high quality 2D spectra on films of single-walled carbon nanotubes. Two spectrometer designs are investigated. Supercontinuum from ANDi PCF provides a means to generate broadband pulse sequences for multidimensional spectroscopy without the need for an optical parametric amplifier.

Ultrafast Exciton Hopping Observed in Bare Semiconducting Carbon Nanotube Thin Films with Two-Dimensional White-Light Spectroscopy.

We observe ultrafast energy transfer between bare carbon nanotubes in a thin film using two-dimensional (2D) white-light spectroscopy. Using aqueous two-phase separation, semiconducting carbon nanotubes are purified from their metallic counterparts and condensed into a 10 nm thin film with no residual surfactant. Cross peak intensities put the time scale for energy transfer at <60 fs, and 2D anisotropy measurements determine that energy transfer is most efficient between parallel nanotubes, thus favoring directional energy flow. Lifetimes are about 300 fs. Thus, these results are in sharp contrast to thin films prepared from nanotubes that are wrapped by polymers, which exhibit picosecond energy transfer and randomize the direction of energy flow. Ultrafast energy flow and directionality are exciting properties for next-generation photovoltaics, photodetectors, and other devices.