Factors controlling charge recombination under dark and light conditions in dye sensitised solar cells

A simple and powerful approach for assessing the recombination losses in dye sensitised solar cells (DSSCs) across the current voltage curve (j-V) as a function of TiO(2) electron concentration (n) is demonstrated. The total flux of electrons recombining with iodine species in the electrolyte and oxidised dye molecules can be thought of as a recombination current density, defined as j(rec) = j(inj)-j where j(inj) is the current of electrons injected from optically excited dye states and j is the current density collected at cell voltage (V). The electron concentration at any given operating conditions is determined by charge extraction. This allows comparison of factors influencing electron recombination rates at matched n. We show that j(rec) is typically 2-3 times higher under 1 sun equivalent illumination (j(inj) > 0) relative to dark (j(inj) = 0) conditions. This difference was increased by increasing light intensity, electrolyte iodine concentration and electrolyte solvent viscosity. The difference was reduced by increasing the electrolyte iodide concentration and increasing the temperature. These results allowed us to verify a numerical model of complete operational cells (Barnes et al., Phys. Chem. Chem. Phys., DOI: 10.1039/c0cp01554g) and to relate the differences in j(rec) to physical processes in the devices. The difference between j(rec) in the light and dark can be explained by two factors: (1) an increase in the concentration of electron acceptor species (I(3)(-) and/or I(2)) when current is flowing under illumination relative to dark conditions where the current is flowing in the opposite direction, and (2) a non-trivial contribution from electron recombination to oxidised dye molecules under light conditions. More generally, the technique helps to assign the observed relationship between the components, processing and performance of DSSCs to more fundamental physical processes.     

Instability of nanostructures patterned in polystyrene under high electric field gradients

A nanolithography technique based on the lateral displacement of electrically biased AFM tip was developed for nanostructures formation of 30-100 nm in width and 1-10 nm in height in the polystyrene (PS) films. It was demonstrated that the nanostructures patterned in annealed PS films (90K Mw) show slow exponential relaxation between 55 and 265 h depending on their size. Relaxation of the nanostructures in non-annealed films usually occurred in minutes. It was observed that in the annealed samples a negative electric charge accumulated in the areas where the nanostructures formed while in the non-annealed samples only the positive charge in exposed areas was detected using the electric force microscopy. After 320 h of monitoring under the humidity maintained between 25 and 27% it was suggested that slow dynamical changes of the nanostructures can be attributed to the negative electric charge dissipation in the annealed samples.     

Perfect matchings in random intersection graphs

Let W ₁,??,W _n be independent random subsets of [m]={1,??,m}. Assuming that each W _i is uniformly distributed in the class of d-subsets of?[m] we study the uniform random intersection graph G _s (n,m,d) on the vertex set {W ₁,??W _n }, defined by the adjacency relation: W _i ??W _j whenever |W _i ??W _j |?Rs. For even?n we show that as n,m???? the edge density threshold for the property that G _s (n,m,d) contains a perfect matching is asymptotically the same as that for G _s (n,m,d) being connected.     

Patterning of indium-tin oxide on glass with picosecond lasers

The results of patterning of the indium-tin oxide (ITO) film on the glass substrate with high repetition rate picosecond lasers at various wavelengths are presented. Laser radiation initiated the ablation of the material, forming grooves in ITO. Profile of the grooves was analyzed with a phase contrast optical microscope, a stylus type profiler, scanning electron microscope (SEM) and atomic force microscope (AFM). Clean removal of the ITO film was achieved with the 266 nm radiation when laser fluence was above the threshold at 0.20 J/cm², while for the 355 nm radiation, the threshold was higher, above 0.46 J/cm². The glass substrate was damaged in the area where the fluence was higher than 1.55 J/cm². The 532 nm radiation allowed getting well defined grooves, but a lot of residues in the form of dust were generated on the surface. UV radiation with the 266 nm wavelength provided the widest working window for ITO ablation without damage of the substrate. Use of UV laser radiation with fluences close to the ablation threshold made it possible to minimize surface contamination and the recast ridge formation during the process.     

Biopharmaceutical Evaluation of Capsules with Lyophilized Apple Powder

Apples are an important source of biologically active compounds. Consequently, we decided to model hard gelatin capsules with lyophilized apple powder by using different excipients and to evaluate the release kinetics of phenolic compounds. The apple slices of “Ligol” cultivar were immediately frozen in a freezer (at −35°C) with air circulation and were lyophilized with a sublimator at the pressure of 0.01 mbar (condenser temperature, −85°C). Lyophilized apple powder was used as an active substance filled into hard gelatin capsules. We conducted capsule disintegration and dissolution tests to evaluate the quality of apple lyophilizate-containing capsules of different encapsulating content. Individual phenolic compounds can be arranged in the following descending order according to the amount released from the capsules of different compositions: chlorogenic acid > rutin > avicularin > hyperoside > phloridzin > quercitrin > (−)-epicatechin > isoquercitrin. Chlorogenic acid was the compound that was released in the highest amounts from capsules of different encapsulating content: its released amounts ranged from 68.4 to 640.3 μg/mL. According to the obtained data, when hypromellose content ranged from 29% to 41% of the capsule mass, the capsules disintegrated within less than 30 min, and such amounts of hypromellose did not prolong the release of phenolic compounds. Based on the results of the dissolution test, the capsules can be classified as fast-dissolving preparations, as more than 85% of the active substances were released within 30 min.     

Local probabilities of randomly stopped sums of power-law lattice random variables

Lithuanian Mathematical Journal - Let X1 and N ≥ 0 be integer-valued power-law random variables. For a randomly stopped sum SN = X1+?+XN of independent and identically distributed...     

Pulsed-laser generation of gold nanoparticles with on-line surface plasmon resonance detection

Size of nanoparticles is an important parameter for their applications. The real-time monitoring is required for reliable and reproducible production of nanoparticles with controllable size. We present results of our research on development of the system for the online nanoparticle characterization during their production by a laser. The laser ablation chamber which allows measurements of surface plasmon resonance spectra during the nanoparticle generation process has been designed and fabricated. The online characterization system was tested by producing and modification of gold nanoparticles. Nanoparticles were generated by nanosecond-laser (wavelength 1064 nm) ablation of gold target in deionized water, and optimal conditions for the highest nanoparticle productivity were estimated. The mean diameter of nanoparticles was determined using their absorption spectra measured in the real-time during the ablation experiments and from the TEM images analysis, and it varied from 20 to 45 nm. The mismatch between nanoparticle diameters, estimated using these two methods, is due to the polydispersity of the generated nanoparticles. The further experiments of laser-induced modification of colloidal gold nanoparticles were carried out using second harmonic (wavelength 532 nm) of nanosecond Nd:YAG laser and alteration in nanoparticle size were acquired by the online measurement system.     

Relaxation pathways of excited N-(triphenylmethyl)salicylidenimine in solutions

Excited state relaxation of N-(triphenylmethyl)-salicylidenimine (MS1) in protic and aprotic solvents has been investigated by means of absorption pump-probe spectroscopy with femtosecond time resolution and fluorescence spectroscopy with picosecond time resolution. Short-lived excited states and long-lived photoproducts have been identified from the differential absorption spectra. Excited states and photoproducts were different under excitation of enol-closed and cis-keto tautomers. As a result, the commonly accepted excited state relaxation model of aromatic anils, which assumes an ultrafast transformation of excited enol-closed tautomers into cis-keto tautomers, has been modified. Performed quantum chemical calculations suggest that hydrogen-bonded ethanol molecules facilitate formation of cis-keto tautomers and are responsible for their different relaxation pathways in comparison with relaxation of excited enol-closed tauromers. Fluorescence decay on a nanosecond time scale was attributed to aggregated MS1 molecules.     

Phototuning Selectively Hole and Electron Transport in Optically Switchable Ambipolar Transistors

One of the grand challenges in organic electronics is to develop multicomponent materials wherein each component imparts a different and independently addressable property to the hybrid system. In this way, the combination of the pristine properties of each component is not only preserved but also combined with unprecedented properties emerging from the mutual interaction between the components. Here for the first time, that tri‐component materials comprised of an ambipolar diketopyrrolopyrrole‐based semiconducting polymer combined with two different photochromic diarylethene molecules possessing ad hoc energy levels can be used to develop organic field‐effect transistors, in which the transport of both, holes and electrons, can be photo‐modulated. A fully reversible light‐switching process is demonstrated, with a light‐controlled 100‐fold modulation of p‐type charge transport and a tenfold modulation of n‐type charge transport. These findings pave the way for photo‐tunable inverters and ultimately for completely re‐addressable high‐performance circuits comprising optical storage units and ambipolar field‐effect transistors.