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Thin film semiconducting single walled carbon nanotube (s-SWCNT) photovoltaics suffer losses due to trapping and quenching of excitons by defects induced when dispersing s-SWCNTs into solution. We study these aspects by preparing photovoltaic devices from (6,5) carbon nanotubes isolated by different processes: extended ultrasonication, brief ultrasonication, and shear force mixing. Peak quantum efficiency increases from 28% to 38% to 49% as the processing harshness decreases and is attributed to both increasing s-SWCNT length and reducing sidewall defects. Fill-factor and open-circuit voltage also improve with shear force mixing, highlighting the importance of obtaining long, defect-free s-SWCNTs for efficient photoconversion devices.

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.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaic solar cells and photodetectors due to their strong optical absorptivity and high charge transport mobility. However, the external quantum efficiency (QE) of s-SWCNT/acceptor heterojunction solar cells has been limited by poor exciton harvesting efficiency. Exciton trapping and quenching at defects are a suspected source of loss. Here, we study the influence of defects on bilayer s-SWCNT/C 60 planar heterojunction photovoltaic devices via both experiment and modeling. First, diazonium chemistry is used to introduce covalent sp 3 sidewall defects to s-SWCNTs at various densities that are estimated using Raman and transient absorption spectroscopy. s-SWCNT/C 60 heterojunction photovoltaic cells are then fabricated that show a significant decrease in peak external QE (e.g., from 40% to 8%) with increasing defect density. Second, a diffusion-limited contact quenching Monte Carlo model is developed to assess the contributions of exciton quenching defects on exciton migration in bilayer s-SWCNT/C 60 heterojunction devices. The model indicates that current state-of-the-art s-SWCNT-based devices are defect limited and suggests that significant gains in exciton harvesting efficiency can be realized if more pristine, longer s-SWCNTs are utilized.

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.

Preparation of s-SWCNTs s-SWCNTs are selectively isolated from as-produced heterogeneous SWCNTs using methods adopted from Nish et al. 1 and Graf et al. 2 For (6,5)-enriched s-SWCNTs, poly[(9,9dioctylfluorenyl-2,7-diyl)-alt-cobipyridine)] (PFO-BPy) (American Dye Source, ADS153UV) is dissolved in toluene at a concentration of 1.5 mg mL-1 by magnetic stirring and heating at 80 ℃. CoMoCAT SG65i (Sigma) (0.5 mg/ml) is added to the above PFO-BPy/toluene solution. The mixtures are then processed in two ways: (1) sonicated at 35% amplitude for 60 min (Fisher model 500 horn sonicator, 400 W) or (2) shear force mixed (Silverson L5M-A) at 10,000 rpm for 24 hours. The resulting dispersion is centrifuged at 300,000g for 10 min to remove undispersed material. The top 90% supernatant is collected and further filtered through a 5 µm PTFE membrane to remove any additional residual impurities. The bottom pellet is mixed with fresh PFO-BPy/toluene solution and the above dispersion/centrifugation processes are repeated. The combined supernatant is transferred to a round-bottom flask and rotary evaporated to remove toluene under reduced pressure. The resultant s-SWCNT solids are redispersed in hot

We report on a new broadband, ultrafast twodimensional 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.

The properties of efficient solar cells fabricated with triple-cation perovskite placed between a mesoporous titania layer and a spiro-OMeTAD layer are studied by using devices either prepared under water-free drybox conditions or fabricated under ambient room humidity. The morphological studies indicate that the content of unreacted PbI 2 phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques ranging from femtoseconds to seconds reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the titania side than at the spiro-OMeTAD side for the cells prepared under ambient conditions. An increased hole injection rate correlates with higher photocurrent seen in the devices prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than at the interface with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution, indicating a change in recombination mechanism from bulk to surface recombination. We also found that the charge dynamics are not uniform within the whole perovskite layer. This feature has significant implications for understanding the operation and optimizing the performance of solar devices based on mixed cation perovskites.