Matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit layers of poly(9,9-dioctylfluorene) (PFO) to study the relation between the solvent properties (laser light absorption, boiling temperature and solubility parameters) and the morphology of the deposited films. To this end, the polymer was diluted (0.5 wt%) in tetrahydrofuran-THF, toluene and toluene/hexane mixtures. The thickness of the films was equal to 70 +/- 20 nm. The morphology and uniformity of the films was investigated by Atomic Force Microscopy and by the photoluminescence emission properties of the polymer films, respectively. It is shown that, although the solubility parameters of the solvents are important in controlling the film roughness and morphology, the optical absorption properties and boiling temperature play a very important role, too. In fact, for matrices characterized by the same total solubility parameter, lower roughness values are obtained for films prepared using solvents with lower penetration depth of the laser radiation and higher boiling temperatures.

In this work two quantum dot (QD) solar cell structures have been proposed and compared as potential solutions for the realization of the Intermediate Band Solar Cell concept: the well known dot/barrier material system InAs / GaAs and an engineered InAlGaAs/AlGaAs combination. The Al-based structures have been obtained by a suitably developed growth procedure with the aim of increasing island density and engineering the absorption spectrum and the energy band profile in the near infrared region. Along with tunability of the confined electron energy levels, the proposed Al-based structures exhibit transport features, such as reduced edge recombination losses and lower reverse saturation current density with respect to the InAs/GaAs QD system, which can be useful for enhancing device performances. © 2014 AEIT.

We investigated the excitation density dependence of the photoluminescence spectra of hybrid poly(9,9-dioctylfluorene)-CdSe/ZnS nanocrystals (PF8-NCs) thin films. We demonstrate that this experiment allows the determination of the efficiency of all the CdSe/ZnS NCs excitation processes and that the presence of amplified spontaneous emission (ASE) from the PF8 leads to a strong dependence of the NC excitation processes from the laser excitation density. Below the PF8 ASE threshold only about 6% of the excitons in the NCs are due to pump laser absorption, while about 94% of the NC excitation is due to the interaction with the PF8, and it is due for about 58% to PF8 -> NC Forster resonant energy transfer (FRET) and for about 37% to reabsorption by the NCs of the PF8 luminescence. ne presence of PF8 ASE significantly modifies this scenario by strongly decreasing the FRET importance and strongly increasing the reabsorption one. The interplay between reduced FRET and increased reabsorption overall decreases the NC excitation due to PF8 indicating that ASE from the donors should be avoided if efficient NCs excitation under strong pumping is wished.

We investigate the Amplified Spontaneous Emission (ASE) properties of a prototypical host-guest polymer polymer blend, namely poly(9,9-dioctylfluorene) (PF8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) blend, with different concentration ratio. We show that the initial F8BT content increase causes an increase of the F8BT ASE threshold, even leading to ASE suppression for F8BT contents between 25% and 75%. ASE is then recovered upon further increase of the F8BT relative content. We demonstrate that the ASE properties of the PF8:F8BT are dominated by morphology effects, like submicrometric phase segregation, determining the net gain of the active waveguides.

The use of nanofluids as working fluids in direct absorption solar collector is growing up and the study of optical properties of nanoparticles is an important step for the success of this new technology. In this paper we report optical absorption measurements performed on several metal oxide nano- particles (ZnO,CeO2, andFe2O3) as a function of temperature in the range 25-500 °C, in order to study their optical properties, and to investigate how several heating cycles could affect nanoparticle structural stability and absorption characteristics. These are quite important issues to be investigated in order to assess the possibility to use such metal-oxide nanoparticles as gas-based high temperature nanofluid in Concentrated Solar Power (CSP).

A promisingnewgenerationofsolarthermalcollectorabletoenhancethethermalefficiency istheDASC (Direct absorberSolarCollector).Inthispaperwereportopticalabsorptionmeasurementsperformedon severalwater-basednanofluids (Al2O3, CuO,TiO2, ZnO,CeO2, andFe2O3) asafunctionofnanoparticles concentration. Thesemeasurementsareoffundamentalimportancetoassessthepossibilitytousethe abovementionedmetal-oxidenanoparticlesinliquid-basednanofluids fordirectabsorptionlowtem- perature flat panelsolarcollector.Theobtainedresultsshowdifferentopticalbehaviorsofthenanofluids depending onnanoparticlesmaterialandconcentration.Inallmeasurementsthetransmittancerises passing fromvisibletoinfraredregionandinsomecases,whenthenanoparticlesconcentrationistoo low,theextinctiondistancegrowsuptovalueslargerthanthetypicaldiameterofasolarreceiver.

We report on nitrogen dioxide (NO2) sensing measurements by means of zinc oxide films presenting different morphologies. The variation in the photoluminescence emission of the films is employed as transduction mechanism to detect the presence of NO2 gas molecules at room temperature. The significant role of film morphology on the sensing properties is presented and possible limits in the use of ZnO nanostructures for NO2 detection at high gas concentration (>20 ppm) and low gas flow (50 ml/min), where a worsening of the sensor response is observed, are discussed. These features are ascribed to a likely incomplete reversibility of the NO2 adsorption process and examined in connection with the mechanisms of interaction between NO2 molecules and ZnO.

The optical response by NO2 gas adsorption at different concentrations has been investigated, at room temperature, in ZnO nanostructured films grown by controlled vapor phase deposition. The variation (quenching) in the photoluminescence signal from excitonic and defects bands, due to the interactions between the oxidizing gas molecules and the sample surface, has been detected and dynamic responses and calibration curves as a function of gas concentration have been obtained and analyzed for each band. We showed that the sensing response results larger in excitonic band than in defect one and that the emission signal rises from two different quenchable and unquenchable states. A simple model was proposed in order to explain the quenching processes on the emission intensity and to correlate them to the morphological features of the samples. Finally, the reversibility of the quenching effects has also been tested at high gas concentration. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3700251]

The quest for single-stage deposition of CuInGaSe2 (CIGS) is an open race to replace very effective but capital intensive thin film solar cell manufacturing processes like multiple-stage coevaporation or sputtering combined with high pressure selenisation treatments. In this paper the most recent achievements of Low Temperature Pulsed Electron Deposition (LTPED), a novel single stage deposition process by which CIGS can be deposited at 250 °C, are presented and discussed. We show that selenium loss during the film deposition is not a problem with LTPED as good crystalline films are formed very close to the melting temperature of selenium. The mechanism of formation of good ohmic contacts between CIGS and Mo in the absence of any MoSe2 transition layers is also illustrated, followed by a brief summary of the measured characteristics of test solar cells grown by LTPED. The 17% efficiency target achieved by lab-scale CIGS devices without bandgap modulation, antireflection coating or K-doping is considered to be a crucial milestone along the path to the industrial scale-up of LTPED. The paper ends with a brief review of the open scientific and technological issues related to the scale-up and the possible future applications of the new technology.

In this work, we report on the competition between two-step two photon absorption, carrier recombination,and escape in the photocurrent generation mechanisms of high quality InAs/GaAs quantumdot intermediate band solar cells. In particular, the different role of holes and electrons ishighlighted. Experiments of external quantum efficiency dependent on temperature and electricalor optical bias (two-step two photon absorption) highlight a relative increase as high as 38% at10K under infrared excitation. We interpret these results on the base of charge separation by phononassisted tunneling of holes from quantum dots. We propose the charge separation as an effectivemechanism which, reducing the recombination rate and competing with the other escapeprocesses, enhances the infrared absorption contribution. Meanwhile, this model explains why thermalescape is found to predominate over two-step two photon absorption starting from 200 K,whereas it was expected to prevail at lower temperatures (70 K), solely on the basis of the relativelylow electron barrier height in such a system. VC 2016 AIP Publishing LLC.

In this work, the evolution of the Au assisted-growth of ZnO nanorods deposited by vapour phase deposition both on sapphire and on indium-tin-oxide on glass (ITO-glass) substrates has been studied. Our investigation demonstrates that the growth proceeds first as a 3D growth, giving rise to a buffer layer, few microns thick, formed by ZnO grains with different orientation. Then a 1D transition occurs with the nucleation of a dense array of vertically aligned nanorods. A different degree of crystalline order and nanorods alignment was found between the samples grown on ITO-glass and sapphire substrates, which was ascribed to the different morphology that the Au seed layer acquires on the two different substrates. A semi-quantitative analysis of the ZnO crystalline orientation was carried out by X-ray diffraction (XRD) measurements performed at fixed incidence configuration and supported by high resolution scanning electron microscopy (HR-SEM) investigations on focused ion beam (FIB) prepared cross-sections.

We have investigated the optical properties ofcolloidal seed-grown CdSe (seed)/CdTe (arms) nanotetrapodsboth experimentally and computationally. The tetrapods exhibita type-II transition arising from electrons localized in the CdSeseed region and holes delocalized in the CdTe arms, along witha residual type-I recombination in long-arm tetrapods.Experimentsand theory helped to identify the origin of both types oftransitions and their size dependence. In particular, timeresolvedexperiments performed at 10 K evidenced a sizedependent,long living type-II radiative emission arising fromthe peculiar electron-hole wave function localization. Temperature-dependent photoluminescence (PL) studies indicate that, at high temperature (>150 K), the main process limiting the PLquantum efficiency of the type-I PL is thermal escape of the charge carriers through efficient exciton-optical phonon coupling. Thetype-II PL instead is limited both by thermal escape and by the promotion of electrons from the conduction band of the seed regionto that of the arms, occurring at T > 200 K.

Various kinds of zinc oxide (ZnO) nanostructures, such as columns, pencils, hexagonal pyramids, hexagonal hierarchical structures, as well as smooth and rough films, were grown by pulsed laser deposition using KrF and ArF excimer lasers, without use of any catalyst. ZnO films were deposited at substrate temperatures from 500 to 700A degrees C and oxygen background pressures of 1, 5, 50, and 100 Pa. Quite different morphologies of the deposited films were observed using scanning electron microscopy when different laser wavelengths (248 or 193 nm) were used to ablate the bulk ZnO target. Photoluminescence studies were performed at different temperatures (down to 7 K). The gas sensing properties of the different nanostructures were tested against low concentrations of NO(2). The variation in the photoluminescence emission of the films when exposed to NO(2) was used as transduction mechanism to reveal the presence of the gas. The nanostructured films with higher surface-to-volume ratio and higher total surface available for gas adsorption presented higher responses, detecting NO(2) concentrations down to 3 ppm at room temperature.