This PDF file contains the front matter associated with SPIE Proceedings Volume 8811, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

We present the design, construction and implementation of a modular microscopy device that transforms a basic inverted fluorescence microscope into a versatile single-molecule imaging system. The device uses Convex Lens- Induced Confinement (CLIC) to improve background rejection and extend diffusion-limited observation time. To facilitate its integration into a wide range of laboratories, this implementation of the CLIC device can use a standard flow-cell, into which the sample is loaded. By mechanically deforming the flow-cell, the device creates a tunable, wedge-shaped imaging chamber which we have modeled using finite element analysis simulations and characterized experimentally using interferometry. A powerful feature of CLIC imaging technology is the ability to examine single molecules under a continuum of applied confinement, from the nanometer to the micrometer scale. We demonstrate, using freely diffusing λ-phage DNA, that when the imposed confinement is on the scale of individual molecules their molecular conformations and diffusivity are altered significantly. To improve the flow-cell stiffness, seal, and re-usability, we have innovated the fabrication of thin PDMS-bonded flow-cells. The presented flow-cell CLIC technology can be combined with surface-lithography to provide an accessible and powerful approach to tune, trap, and image individual molecules under an extended range of imaging conditions. It is well-suited to tackling open problems in biophysics, biotechnology, nanotechnology, materials science, and chemistry.

Modulation excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) together with resistance measurements has been carried out to study water isotopic exchange on undoped SnO₂ materials as a function of CO concentration. We compare two materials synthesized via hydrothermal treatment and different only in their precursors: SnO₂ Ac synthesized from tin(IV) hydroxide acetate and SnO₂ Cl from tin(IV) chloride pentahydrate. DRIFTS and resistance measurements were performed simultaneously in an environmental chamber at 300 oC and in a flow of humid air. The annealed materials were found to have similar particle sizes (16±7 nm), crystallite sizes (12±2 nm) and pore size distribution (9±1 nm). However, sensor tests showed notably higher responses to CO in the presence of water vapor for SnO₂ Ac. Electronic effect of CO chemisorption quantitatively correlates with consumption of bridging hydroxyls on the latter surface upon increasing concentration of CO from 0 to 500 ppm in humid air. No such correlation was found for SnO₂ Cl. Water desorption kinetics was found to be slower for the latter by ca. 30 % with respect to SnO₂ Ac. Low activity of surface OH groups and consequently low sensor signals of SnO₂ Cl were proposed to originate from traces of Cl ions found in the material after the synthesis despite negative Cl test before the hydrothermal treatment.

Organic-inorganic hybrid systems based on lead halide compounds have recently encountered considerable success as light absorbers in solid-state solar cells. Herein we show how fundamental mechanistic processes in mesoporous oxide films impregnated with CH₃NH₃PbI₃ can be investigated by time resolved techniques. In particular, charge separation reactions such as electron injection into the titanium dioxide film and hole injection into the hole transporting material spiro-OMeTAD as well as the corresponding charge recombination reactions were scrutinized. Femtosecond transient absorption spectroscopy and time-resolved terahertz spectroscopy were applied to CH₃NH₃PbI₃ deposited either on TiO₂ or Al₂O₃ mesoporous films and infiltrated with the hole transporting material spiro-OMeTAD.

Simultaneous adsorption of dye molecules and coadsorbates is important for the fabrication of high-efficiency dyesensitized solar cells, but its mechanism is not well understood. Herein, we use a quartz crystal microbalance with dissipation technique (QCM-D) to study dynamically and quantitatively the sensitization of TiO₂ in situ. We investigate dye loading for a ruthenium(II) polypyridyl complex (Z907), of a triphenylamine-based D-π-A dye (Y123), and of a ullazine sensitizer (JD21), as well as the simultaneous adsorption of the latter two with the coadsorbate chenodeoxycholic acid. By combining the QCM-D technique with fluorescence measurements, we quantify molar ratios between the dye and coadsorbate. Furthermore, we will present first studies using liquid-phase AFM on the adsorbed dye monolayer, thus obtaining complementary microscopic information that may lead to understanding of the adsorption mechanism on the molecular scale.

The properties of hydrazine-treated carbon nanotubes (CNTs) were investigated by synchrotron photoelectron spectroscopy. The surfactant-free CNTs used in this study were synthesized by alcohol catalytic chemical vapor deposition. When the CNTs subject to the vapor-phase hydrazine treatment and the 80° C-baking treatment were probed by ultraviolet photoelectron spectroscopy (UPS), the results showed (i) damaged π-bonding and (ii) the shift of the CNTs’ Fermi level toward the conduction band. A further 350° C-baking treatment on the hydrazine-treated CNTs could restore the damaged π-bonding and cause the CNTs’ Fermi level to shift back toward the valence band. The results obtained from UPS indicated that the above interaction between hydrazine and CNTs was a thermally metastable chemical adsorption. When the CNTs subject to the vapor-phase hydrazine treatment and the 80° C-baking treatment were probed by X-ray photoelectron spectroscopy (XPS), the results showed a significant increase in the spectral intensity of the signal corresponding to C-N bonding in the XPS profile. A further 350° C-baking treatment on the hydrazine-treated CNTs could essentially eliminate the spectral intensity of the signal corresponding to C-N bonding in the XPS profile. Our experimental results show that the transient fate of the thermally metastable C-N bonding is associated with the nitrogenous radicals, such as nitrene and amidogen, thermally decomposed from hydrazine. The chemical association of nitrogenous radicals with CNTs generates metastable amino/aziridino derivatization on the surface of CNTs, which will disrupt the continuum of CNTs' graphitic domains. Upon further baking, the disruptive functionalization can be eliminated to restore the graphitic sp²-carbon bonding structure.

Photo-induced charge transfer from an Indium Tin Oxide (ITO) contact into [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₀BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) is measured. Charge transfer peaks are observed for a series of excitation energies below the PCBM absorption edge. If charge transfer is blocked using a tunnel barrier or an applied electric field, the peaks disappear. The observed transitions are similar to those predicted by theoretical calculations of the absorption spectra for negatively charged C₆₀ and C₇₀ chains. This observation suggests that charge transfer occurs preferentially at the polaronic transition energies in the PCBM, providing a means for polaronic state spectroscopy.

We report the fabrication and characterization of solution-processed organic bilayer field effect transistors with a middle-contact configuration. P3HT and PCBM were chosen for a hole and electron transporting material, respectively. The P3HT:PCBM bilayer FET with a middle contact structure showed only p-type behavior with a hole mobility (μ_h) of ~ 10^-3 cm² V^-1 s^-1, which is a different result compared to the conventional top-contact ambipolar device. Electron injection was enabled by using a thin CPE layer beneath the PCBM layer. The thickness of the CPE layer is critical for achieving balanced hole and electron mobilities and provides an important variable for consideration in future optimization studies. More detailed mechanisms on the role of CPE layer and the middle-contact structure are currently under investigation.

In this report we discuss the great potential of Eutectic Gallium Indium (EGaIn) as conformal soft top electrode. EGaIn is a liquid eutectic supporting a skin (∼ 1 nm-thick) of self-limiting oxide Ga₂O₃ as a non-damaging, conformal top-contact. In the last half of decade EGaIn has been used by several group to form molecular junctions and study charge transport properties in self-assembled monolayer (SAMs). We compared the current density (J) versus applied bias (V) for three different self-assembled monolayers (SAMs) of ethynylthiophenol- functionalized anthracene derivatives with approximately the same thickness and diverse conjugation: linear- conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) by using liquid eutectic Ga-In (EGaIn). This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts, however it may also have limited the maximum values of J observed for AC. We measure values of J for AH and AQ which are not significantly different (J ≈ 10^-1 A/cm² at V = 0.4 V). For AC, however, J is one (using log-averages) or two (using Gaussian mean) orders of magnitude higher than both AH and AQ. Our results are also in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules transport calculation, based on chemisorbed between Au contacts which predict currents, I, that are two orders of magnitude higher for AC than AH at 0 < |V| < 0.4 V. We ascribe these observations to quantum-interference effects. The agreement between the theoretical predictions on single-molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.

Bulk heterojunction (BHJ) organic solar cells are a promising alternative energy technology, but a thorough understanding of charge transport behavior in BHJ materials is necessary in order to design devices with high power conversion efficiencies. Parameters such as carrier mobilities, carrier concentrations, and the recombination coefficient have traditionally been successfully measured using vertical structures similar to organic photovoltaic (OPV) cells. We have developed a lateral BHJ device which complements these vertical techniques by allowing spatially resolved measurement along the transport direction of charge carriers. This is essential for evaluating the effect of nanoscale structure and morphology on these important charge transport parameters. Nanomorphology in organic BHJ films has been controlled using a variety of methods, but the effect of these procedures has been infrequently correlated with the charge transport parameter of the BHJ material. Electron beam lithography has been used to create lateral device structures with many voltage probes at a sub-micron resolution throughout the device channel. By performing in-situ potentiometry, we can calculate both carrier mobilities and determine the effect of solvent choice and annealing procedure on the charge transport in BHJ system. Spin coated P3HT:PCBM films prepared from solutions in chloroform and o-xylene are characterized using this technique.

Nanoscale thin film morphology has been identified as an important factor in organic solar cell device func- tionality and efficiency. To better understand the limiting factors, it is important to work at the length scale of these processes. A study of thin films of organic molecules with Kelvin probe force microscopy (KPFM) to observe charge distribution and non-contact atomic force microscopy (NC-AFM) to simultaneously obtain structural information is presented. This allows investigation of the structure-function relationships in molecu- lar photovoltaics at the nanometer scale. PTCDI (3,4,9,10-perylenetetracarboxylic diimide) and CuPc (copper phthalocyanine) are used as organic molecules and are precisely grown on alkali halide substrates.

The performance of photovoltaic devices comprising of a donor-acceptor copolymer (benzothiadiazole-fluorenediketopyrrolopyrrole-BTD-F-DKPP) and phenyl-C60-butyric acid methyl ester (PCBM) has been investigated. The ascast devices performed with very poor power conversion efficiency. Upon application of an additional top-cast treatment by methanol, considerable increase in the device performance was observed. Indeed, using transient photocurrent we found improved charge extraction of the methanol treated devices. In order to check, whether this effect could be obtained by removing smaller molecular weight fractions from the copolymer, we washed out these components by fractionation with methanol. However, this treatment only resulted in minor improvements and thus cannot be assigned to be the major effect/cause.

In this contribution we combine optical modeling and device fabrication/characterization techniques to demonstrate that semitransparent metal electrodes can improve light harvesting in organic photovoltaic (OPV) devices. We show that inverted P3HT:PCBM solar cells using a thin ~8 nm silver film as a front electrode outperform the ITO-based devices, despite the lower transmittance of silver films in comparison to ITO. The variation of silver thickness allows tailoring the field distribution inside the cell, which leads to a broad resonance window where the absorption is enhanced. Thereby the short-circuit current was increased by 84% and the solar-cell efficiency was doubled. These results show that semitransparent metal electrodes can be efficiently used for light trapping and also form a very promising alternative to ITO in OPV devices. The stacked silver electrodes used in this work are flexible and can be easily produced on a large scale, including printing techniques.

Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer: fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.

We report the photovoltaic performance of a low-bandgap polymer:perylene diimide (PDI) photovoltaic blend and study the exciton to charge carrier conversion in the photoactive layer by Vis-NIR broadband transient absorption spectroscopy over a dynamic range from pico- to microseconds. Power conversion efficiencies of 1.2 % are obtained from the polymer:PDI blends with a maximum EQE of about 30 %, which is significantly below the performance of the same polymer with fullerene as acceptor indicating that severe loss processes exist that limit the photocurrent. From the evolution of the transient absorption spectra we conclude that the photovoltaic performance of the polymer:PDI blends is mainly limited by inefficient exciton harvesting and dissociation at the interface. However, once free charge carriers are generated in the blend they can be extracted as photocurrent as their recombination occurs on a timescale similar to the time typically needed for charge extraction from the photoactive layer. Hence, strategies to improve the efficiency of polymer:PDI blends should aim at increasing exciton harvesting at the heterojunction and the dissociation efficiency into free charges at the interface.

The dynamics of mobile charge carrier generation in polymer bulk heterojunction films is of vital importance to the development of more efficient organic photovoltaics. As with conventional semiconductors, the optical signatures of mobile carriers lie in the far-infrared (1-30 THz) although the electrodynamics deviate strongly from the Drude model. The key time scales for the process are sub-100 fs to picoseconds, and is a challenge to perform low energy spectroscopy on these time scales as it is less than the period of oscillation for the probing light. In this work, we demonstrate sub-100 fs spectroscopy of a polymer bulk heterojunction film P3HT:PCBM using a single-cycle, phase-locked and coherently detected multi-THz transient as a probe pulse following femtosecond excitation at 400 nm. By observing changes to the reflected THz transients from the film surface following photoexcitation, we can extract the complex optical conductivity spectrum for the film in snapshots of 40 fs following photoexcitation. We find that for our excitation conditions mobile charges are created in less than 120 fs and are characterized by a spectrum which is characteristic of a two dimensional delocalized polaron. A large fraction of mobile carriers relax to a localized state on a 1 ps time scale. Pump energy dependent photon-to- mobile carrier conversion efficiency supports hot exciton dissociation as a mechanism for such fast mobile carrier generation.

In this work, we study the nature of long-lived photoexcitations in intercalated, partially and predominantly non-intercalated semicrystalline poly(2,5-bis(3-tetradecyl-thiophen-2-yl)thieno [3,2,-b]thiophene) (pBTTT):phenyl-C61 -butyric acid methyl ester (PC61BM) blend films by quasi-steady-state photoinduced absorption (PIA) spectroscopy. We find that polarons are generated in these microstructures. However, the polarons generated in partially and predominantly non-intercalated films (1.7 eV) are at higher energy than in intercalated film (1.4 eV). After comparing with the polaron generation in neat pBTTT polymer film, we propose that the polarons generated in partially and predominantly non-intercalated film are delocalized charges, and the polarons generated in intercalated film are localized charges. Furthermore, we also find that the polarons generated in the partially non-intercalated film have the longest lifetime.