Efficient singlet fission has been reported in polycrystalline thin films of 1,3-diphenylisobenzofuran. Two conformational polymorphs have been characterized by various spectroscopic techniques. Thin films exhibit 200% triplet quantum yield at 77 K, as determined by ultrafast transient absorption measurements. Rapid excimer formation presents a significant energetic barrier to triplet formation.

Molecular orientation plays a significant role in determining the performance of small molecule solar cells. Key photovoltaic processes in these cells are strongly dependent on how the molecules are oriented in the active layer. We isolate contributions arising from the bulk molecular orientations vs. those from interfacial orientations in ZnPc/C60 bilayer systems and we probe these contributions by comparing device pairs in which only the bulk or the interface differ. By controlling the orientation in the bulk the current can be strongly modulated, whereas controlling the interfacial molecular orientation and degree of intermixing mediate the voltage.

As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.

Non-geminate charge recombination is a significant source of carrier loss in organic photovoltaic systems. Recent experiments by Rao, et al. (Nature, 2013 500, 435-439) suggest that the recombination of triplet charge-transfer (³CT) states can be suppressed by careful control of the molecular order in the vicinity of the phase boundary between donor and acceptor materials polymer/fullerene bulk-heterojunction devices. In short, recombination of ³CT states is effectively suppressed when the fullerene phase exhibits a high degree of local order near the interface. Here we report upon our theoretical model that connects energetic disorder, dimensionality, and wave function localization to show that inhomogeneous broadening introduces strong coupling between the interfacial ³CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.

The two-dimensional coherence function corresponding to positively charged polarons (“holes”) in poly(3- hexylthiophene) π-stacks is calculated based on a Holstein-style Hamiltonian which treats electronic coupling, vibronic coupling and disorder on equal footing. Assuming a model of isotropic site-energy disorder, the hole is found to be delocalized between 1-2 nm along the polymer chain and between 0.5-1 nm along the stacking axis.

Since charge separation in organic photovoltaics takes place between n- and p-type semiconductors, their interface should be maximized within the active layer, while charge percolation pathways to the electrodes should be ensured. One way to obtain an ideal and thermodynamically stable morphology is to covalently link n- and p-type semiconductors, provided a sufficiently high internal order is achieved. To this end, we have synthesized a perylene-quaterthiophene-perylene triad substituted with poly(isobutene) segments that induce order at the nanoscale via microphase segregation. Charge separation under selective illumination on the quaterthiophene or perylene moiety was investigated by steady state and transient absorption spectroscopy, and related to the molecular packing and phase morphology deduced from electron and grazing incidence X-ray diffractions. Photoluminescence was quenched due to charge separation within the triad. In solution, these charges quickly recombined, but were shown to have a longer lifetime in films, which is beneficial for charge collection in photovoltaic devices. We have also investigated whether alignment of the supramolecular aggregates affects the photophysics.

We present a depth-resolved X-ray photoemission spectroscopy study of the Poly(9,9’-dioctylfluorene-cobenzothiadiazole): Perylene tetracarboxylic diimide blend (briefly, F8BT:PDI), employed for the realization of the light harvesting layer in organic photovoltaic devices. We address the problem of the vertical distribution of PDI molecules in the blend, relevant for the optimization of the photo-generated charge collection in such devices. The depth resolution is obtained by sputtering the organic layer with Ar⁺ ions. A thorough investigation of the effects of different sputtering treatments on the F8BT:PDI film surface is presented. Changes in the stoichiometry of the organic layer, as well as the cleavage of molecular bonds are detected, even after mild sputtering. In particular, we report about the formation of a carbon-rich surface layer. Finally, a method is proposed for the calculation of the PDI concentration, which relies on the detection of specific chemical markers and is robust against sputter-induced artifacts. As a case study, we evaluated the PDI concentration in a 10 nm thick F8BT:PDI layer spin coated on indium tin oxide.

Semiconducting carbon nanotubes are an exciting new material for solar cells. Mesoscale films can now be assebled and made into devices in which the semiconducting tubes are the photoactive layer, analogous to organic dyes or quantum dots in dye-sensitized solar cells. In order to understand their exciton transport properties, we are studying the photophysics of these films. Obtaining a comprehensive picture of the pathways, rates, and bottlenecks is challenging because in many cases this relaxation spans a wide range of energies. The standard approach to study such a wide frequency range is to use a tunable pump pulse to excite each electronic transition in turn, one after another. We have developed two-dimensional white light spectroscopy (2D WL) which allows us to simultaneously examine a spectral range spanning roughly 500-1400 nm. The spectra resolve energy transfer between all possible combinations of excitonic states in the chirality-selected nanotubes, thereby providing an instantaneous and comprehensive snapshot of the dynamical pathways. The new physics we uncover has important implications in the development of carbon nanotube electronics and optoelectronics.

Conjugated polymers are appealing materials for cost-effective production of flexible electronic devices, but the efficiency of these devices may be compromised by exciton quenching by hole polarons. Exciton migration was monitored directly in single poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) chains embedded in a hole-injection device by applying super-resolution point-spread function fitting techniques to the fluorescence image of each chain as holes were reversibly injected into the polymer. Correlated excitation polarization spectroscopy techniques reported the orientation of the longitudinal axis of the rod-like polymers. The wide-field microscopy images are diffraction-limited in one dimension but slightly elongated in the direction of the longitudinal axis of the rod-like polymer in most cases and at every depth of fluorescence quenching explored.

Applications of conjugated polymers in photovoltaics and displays drive the need to understand how morphology affects emission and charge migration. Due to the inherent complexity of polymers, parallel studies of oligomer aggregates are required to ‘build-up' an understanding of the polymer features. Fluorescence lifetime imaging microscopy (FLIM) is used to probe variations in vibronic patterns and emission lifetime between individual aggregates and trends in these properties as a function of aggregate size. This technique yields insight into the structure and packing properties of these materials in the aggregated state.

The plant pigment betanin is investigated as a dye-sensitizer on TiO₂ with regard to its potential to undergo twoelectron oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide intermediate which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on nanocrystalline TiO² and ZrO₂ films were performed on femtosecond and nanosecond time-scales and provide evidence for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dyesensitized solar cells.

Ultrafast transient absorption spectroscopy is used in conjunction with chemical doping experiments to study the photo-generation of charges in hybrid thin films composed of PCPDTBT and CdSe quantum dots. We show how we use chemical doping experiments to de-convolute the spectral signatures of the transient states in the near infrared.

Density functional theory calculations have been carried out to investigate the effect of the atomic under-coordination on the bond contraction, lattice strain, and electron configuration of Cuboctahedral and Marks decahedral structures of silver and copper nanoclusters. Our calculated results are consistent in trend with experimental measurements including extended X-ray-absorption fine structure (EXAFS), scanning tunneling microscope/spectroscopy (STM/S), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectra (UPS). This agreement approved the prognostications made on the bond-order-length-strength (BOLS) correlation and nonbonding electron polarization (NEP), suggesting that atomic under-coordination at the surface of nanoclusters cause bond contraction, which then leads to lattice strain, charge densification, core electron entrapment, as well as polarization of valence charge. The results of this work will contribute to the understanding of the intriguing properties of Ag and Cu nanoclusters.

Spectroelectrochemical photoluminescence (SEPL) is used to investigate surface electron and hole traps on anatase nanoparticles, anatase nanosheets and rutile nanowires in aqueous and nonaqueous environments. In aqueous environment there is an overvoltage for occupying surface electron traps in rutile and anatase samples. For anatase, this overvoltage is larger on (101) nanoparticles than on (001) nanosheets. The electrochemical energy levels of electron traps determined by SEPL in acetonitrile are consistent with emitted photon energies as determined by the photoluminescence spectrum. Our results show how the contacting solvent and particle morphology can influence the redox potential of surface electron traps and thus guide further research on improving the performance of nano- TiO₂ in applications such as dye–sensitized solar cells and solar water splitting.

Organic-inorganic hybrid materials have recently attracted a lot of attention for optoelectronic applications as they allow combining the flexibility and processibility of organic molecular systems with the good stability and charge-transport properties of inorganic crystalline materials. In this contribution, we report on the use of ultrafast vis-pump / infrared(IR)-probe spectroscopy to investigate the optical signature of charge carrier dynamics in CH₃NH₃PbI₃ perovskite films. We observed that this perovskite shows a complex response which involves strong ground-state bleach contribution. This behaviour differs from most inorganic semiconductors and quantum dot materials that show strong photoinduced absorption in IR upon carrier generation,. The bleaching probably results from a partial filling of intragap states next to the conduction band edge. It may also indicate the existence of a Burstein–Moss type effect in perovskite materials.

In ferroelectrics, domain walls separate regions with different polarization. At domain walls, modified symmetry and electronic structure, or chemical segregation can lead to unusual, extremely localized functional properties, quite different from those of the parent phase. Particularly exciting has been the discovery of domain-wall-specific electrical conductivity in multiferroic BiFeO3 [1]. Here, I will present our generalization of these observations to Pb(Zr,Ti)O3 [2], highlighting the key role of oxygen vacancies, whose distribution can be modulated to reversibly control domain wall transport. For these studies, we have also developed carbon-nanotube-based probes with exceptional performance [3]. [1] Seidel, Nat. Mat. 8, 229 (2009) [2] Guyonnet, Adv. Mat. 23, 5377 (2011) [3] Lisunova, Nano Lett. 13, 4527 (2013)

Solution-processed hybrid organic-inorganic perovskite solar cells, a newcomer to the photovoltaic arena, have taken the field by storm with their extraordinary power conversion efficiencies exceeding 17%. In this paper, the photophysics and the latest findings on the carrier dynamics and charge transfer mechanisms in this new class of photovoltaic material will be examined and distilled. Some open photophysics questions will also be discussed.

We used both cw and transient spectroscopies for studying the optical properties and photoexcitations in the low bandgap copolymer PTB7 that has been used in organic photovoltaic applications (OPV). Surprisingly we observed two primary photoexcitations that are generated within ~150 fs (our time resolution); we identify them as singlet exciton (S₁) and triplet-pair (¹TT). The singlet exciton has been considered to be the only primary photoexcitation in regular π-conjugated polymers and is related with a transient absorption band that peaks at an energy value close to the exciton binding energy (~0.4 eV in PTB7). The TT pair is a novel photoexcitation species in low band-gap π-conjugated copolymers. It has an absorption band close to that of isolated triplet exciton, and may readily dissociate at the donoracceptor interfaces in the PTB7/fullerene blend. This finding may explain the underlying mechanism for the high obtained power conversion efficiency in OPV devices based on the PTB7 copolymer.

Bismuth Ferrite nanoparticles have been recently used to selectively interact with malignant cell DNA via in situ generated second harmonic in a novel theranostics protocol [Nanoscale 6(5), pp. 2929, 2014]. In this report, we extend the screening of biocompatibility of BFO uncoated uncoated nanoparticles and assess the nanoparticle- mediated production of reactive oxygen species as a function of excitation wavelength.

Super-resolution microscopy, the imaging of features below the Abbe diffraction limit, has been achieved by a number of methods in recent years. Each of these methods relies on breaking one of the assumptions made in the derivation of the diffraction limit. While uniform spatial illumination, linearity and time independence have been the most common cornerstones of the Abbe limit broken in super-resolution modalities, breaking the ‘classicality of light’ assumption as a pathway to achieve super-resolution has not been shown. Here we demonstrate a method that utilizes the antibunching characteristic of light emitted by Quantum Dots (QDs), a purely quantum feature of light, to obtain imaging beyond the diffraction limit. Measuring such high order correlations in the emission of a single QD necessitates stability at saturation conditions while avoiding damage and enhanced blinking. This ability was facilitated through new understandings that arisen from exploring the QD ‘blinking’ phenomena. We summarize here two studies that contributed to our current understanding of QD stability.

We report here on a recent time-resolved fluorescence study [1] of the interaction between flurbiprofen (FBP), a chiral non-steroidal anti-inflammatory drug, and human serum albumin (HSA), the main transport protein in the human body. We compare the results obtained for the drug-protein complex with those of various covalently linked flurbiprofentryptophan dyads having well-defined geometries. In all cases stereoselective dynamic fluorescence quenching is observed, varying greatly from one system to another. In addition, the fluorescence anisotropy decays also display a clear stereoselectivity. For the drug-protein complexes, this can be interpreted in terms of the protein microenvironment playing a significant role in the conformational relaxation of FBP, which is more restricted in the case of the (R)- enantiomer.

Nanoparticles have important biological and biomedical applications ranging from drug and gene delivery to biosensing. In the presence of extracellular proteins, a “corona” of proteins adsorbs on the surface of the nanoparticles, altering their interaction with cells, including immune cells. Nanoparticles are often functionalized with polyethylene glycol (PEG) to reduce this non-specific adsorption of proteins. To understand the change in protein corona that occurs following PEGylation, we first quantified the adsorption of blood serum proteins on bare and PEGylated gold nanoparticles using gel electrophoresis. We find a threefold decrease in the amount of protein adsorbed on PEGylated gold nanoparticles compared to the bare gold nanoparticles, showing that PEG reduces, but does not prevent, corona formation. To determine if the secondary structure of corona proteins was altered upon adsorption onto the bare and PEGylated gold nanoparticles, we use CD spectroscopy to characterize the secondary structure of bovine serum albumin following incubation with the nanoparticles. Our results show no significant change in protein secondary structure following incubation with bare or PEGylated nanoparticles. Further examination of the secondary structure of bovine serum albumin, α2-macroglobulin, and transferrin in the presence of free PEG showed similar results. These findings provide important insights for the use of PEGylated gold nanoparticles under physiological conditions.

Biomedical applications of quantum dots (QDs) have become a subject of a considerable concern in the past few decades. The present study examines the stability and cytotoxicity of two QDs systems in cell culture medium in the presence and absence of a thin layer of ZnS shell. The two systems were built from core, CdSe QDs, surface modified with glutathione (GSH), named CdSe∼GSH and CdSe/ZnS∼GSH. Our results demonstrated that 0.7 nm layer of ZnS shell played a significant role in the stability of CdSe/ZnS~GSH QDs in supplemented cell culture medium (RPMI). Also, a significant improvement in the physicochemical properties of the core CdSe QDs was shown by maintaining their spectroscopic characteristics in RPMI medium due to the wide band gap of ZnS shell. Both systems showed insignificant reduction in cell viability of HFB-4 or MCF-7 cell lines in the dark which was attributed to the effective GSH coating. Following photoirradiation with low laser power (irradiance 10 mW cm^-2), CdSe~GSH QDs showed a significant decrease in cell viability after 60 min irradiation which may result from detachment of GSH molecules. Under the same irradiation condition, CdSe/ZnS~GSH QDs showed insignificant decrease in cell viability or after 2 h incubation from laser irradiation which was attributed to the strong binding between ZnS and GSH coatings. It can be concluded that the stability of CdSe core QDs was significantly improved in cell culture medium by encapsulation with a thin layer of ZnS shell whereas their cytotoxicity and photo-cytotoxicity are highly dependent on surface modification.

Gap states in organic semiconductors play a crucial role in determining Energy-Level Alignment and in many cases they act as charge trapping centers to result in serious lowering of charge mobility. Thus origin of gap states has gained increasing attention in order to realize higher mobility organic devises [1-4]. Bussolotti et al. have demonstrated recently that gap states in a pentacene thin film increase even by exposing the film to inert gas and confirmed that the gas exposure mediates structural defects in the film thus gap states [4]. The results have also indicated that preparation of highly-ordered organic thin film is necessary to improve the device performance, namely to decrease trapping states. To improve the ordering of molecule in the film, deposition of a template molecular underlayer is one of the simplest methods to increase the domain size of overlayer film and its crystallinity, and thus we expect improvement of the charge mobility [5]. Hinderhofer et al. reported recently that diindenoperylene (DIP; Figure 1a) could be used as a template layer to grow highly ordered and oriented C₆₀ film with its (111) plane parallel to the SiO₂ substrate [6]. Considering the hole mobility of DIP single crystal, which is quite low (~0.005 cm² V^-1S^-1 at room temperature [7]), it is expected for the DIP template C₆₀ thin film system that lower drain current would be achieved to improve the on/off ratios based on n type C₆₀ transistor and its electron mobility (especially on the negative Vgs region, compared to PEN modified C₆₀ transistors [8]).

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO₂ photoelectrodes on indium tin oxide (ITO) coated polymer substrates have drawn great attention due to its lightweight, flexibility and advantages in commercial applications. However, the thermal instability of polymer substrates limits the process temperature to below 150 °C. In order to assure high and firm interparticle connection between TiO₂ nanocrystals (TiO₂-NC) and polymer substrates, the post-treatment of flexible TiO₂ photoelectrodes (F-TiO₂-PE) by mechanical compression was employed. In this work, Degussa P25 TiO₂-NC was mixed with tert-butyl alcohol and DI-water to form TiO₂ paste. F-TiO₂-PE was then prepared by coating the TiO2 paste onto ITO coated polyethylene terephthalate (PET) substrate using doctor blade followed by low temperature sintering at 120 °C for 2 hours. To study the effect of mechanical compression, we applied 50 and 100 kg/cm2 pressure on TiO₂/PET to complete the fabrication of F-TiO₂-PE. The surface morphology of F-TiO₂-PE was characterized using scanning electron microscopy. The resultant F-TiO₂-PE sample exhibited a smooth, crack-free structure indicating the great improvement in the interparticle connection of TiO₂-NC. Increase of compression pressure could lead to the increase of DSSC photoconversion efficiency. The best photoconversion efficiency of 4.19 % (open circuit voltage (Voc) = 0.79 V, short-circuit photocurrent density (Jsc) = 7.75 mA/cm2, fill factor (FF) = 0.68) was obtained for the F-TiO₂-PE device, which showed great enhancement compared with the F-TiO₂-PE cell without compression treatment. The effect of compression in DSSC performance was vindicated by the electrochemical impedance spectroscopy measurement.