We report on a flexible coherent random laser based on poly(9,9-dioctylfluorene) (PF8) deposited on a surface corrugated poly-phthalate-carbonate (PPC) substrate. We show that the feedback for lasing is due to a combination between the scattering at the rough PF8–PPC interface and the scattering at the PF8–air interface, due to PF8 thickness fluctuations.

The maximization of the optical gain of organic active materials is a crucial step in the development of organic lasers. In this paper we demonstrate that the amplified spontaneous emission (ASE) of regio regularpoly(3-hexylthiophene) (rrP3HT):poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) thin films is strongly affected by the film morphology, controlled by changing the solvent used for the spin-coating deposition. The solvent variation results in ASE threshold changes of up to 8-fold due to the variation in the uniformity of the two polymers mixing, determining the optical gain, and the morphology uniformity, affecting the losses. Our results demonstrate that the morphology optimization is a very important step in the development of organic materials with high optical gain

Confocal Laser Scanning Microscopy (CLSM) has been used as a fast, user-friendly, and noninvasive tool for characterizing the phase composition differences at the substrate and air interfaces in thick films of polymer blends. A clearly different phase composition at the blend/glass interface and at the blend/air interface has been detected. We show that PCBM preferentially accumulates at the glass/blend interface, while P3HT preferentially accumulates at the blend/air interface, by comparing the integrated signal intensity of the luminescence coming from both interfaces. Our results demonstrate that CLSM can be used conveniently for the fast identification of a preferential phase segregation at interfaces in polymer blends. This is useful in the research field on devices (like sensors or planar waveguides) that are based on very thick layers (thickness higher than 1 micron).

Charge transients induced by optical pulses in CdTe detectors are used to investigate carrier dynamics and collection properties. Various features have been observed in the signals induced by optical excitation in the wavelength range 500nm - 1650nm, depending on the absorption, and the transport mechanism involved. A systematic comparison between charge transients recorded for irradiations through cathode and anode contacts, allows to point out the role of defects near the surface, instability effects, deep level transitions into the bulk, and finally internal photoelectric effects at the contacts.

Abstract: 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.

Reversible oxygen induced emission quenching of both the Spontaneous Emission (SE) and the Amplified Spontaneous Emission (ASE) of poly(9,9-dioctylfluorene) waveguides is demonstrated. We show that ASE shows a stronger quenching than SE, up to about 6.2 times, but also a stronger decrease when the excitation density increases. We conclude that the fast increase of the ASE decay rate is the main process in determining the ASE detection sensitivity, limiting the potentiality of sensitivity improvement of ASE with respect to SE.

We demonstrate a static fabrication approach to make free-standing ordered arrays of fluorescent nanofibres through control of the transverse electrospinning field. The alignment and the density of the nanofibre arrays are optimised by careful design of both the source and collector electrode geometries which can control the transverse electric field over the full path of the jet. In doing so, we fabricate suspended fluorescent nanofibres with an aspect ratio of 10^4, and with a substantially increased density and order parameter (by a factor of ∼10 compared to random deposition). Electrostatic modelling suggests that the field distribution of the component is the main contribution to the ordering between the plates. This method offers increased efficiency for the creation of ordered fibres collected over a small area and the characterisation of their photoluminescent properties

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 Frster resonant energy transfer (FRET) and for about 37% to reabsorption by the NCs of the PF8 luminescence. The 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

Amongst the different optoelectronic applications of conjugated polymers, the development of new active materials for optically pumped organic lasers is still an open question particularly in the blue-near UV spectral range. We investigate the emission properties of poly[(9,9-dioctylfluorene-2,7-dyil)- alt-p-phenylene] (PFP) neat films under nanosecond pump. We demonstrate that thanks to the introduction of a phenylene moiety between two fluorene units it is possible to obtain Amplified Spontaneous Emission (ASE) with a lower threshold and a blue shifted wavelength with respect to poly(9,9-dioctylfluorene) (PFO). We demonstrate efficient ASE with a minimum threshold as low as 23 μJcm−2 and a minimum ASE wavelength of 436 nm. A maximum net optical gain of about 26 cm−1 is measured at an excitation density of 0.23 mJcm−2. These results make the PFP a good active material for optically pumped deep blue organic lasers.

The Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique is emerging as an alternative route to the conventional methods for depositing organic materials, although the MAPLE-deposited films very often present high surface roughness and characteristic morphological features. Films of the blue-emitting polymer, poly(9,9-dioctylfluorene)—PFO, have been deposited by MAPLE to investigate the influence of the laser fluence and repetition rate on both their topography and emission properties. The laser fluence has been changed from 150 up to 450 mJ/cm2 , while laser repetition rates of 2 and 10 Hz have been considered. The interplay/relationship between the topography and the emission properties of the MAPLE-deposited films has been studied by confocal microscopy, photoluminescence spectrometry and atomic force microscopy. It has been found that under high irradiation (fluence of 450 mJ/cm2) conditions, the sample surface is characterized by bubbles presenting the intrinsic PFO blue emission. Instead, while improvements in the film morphology can be observed for lowered fluence and laser repetition rate, green emission becomes predominant in such conditions. Such result is very interesting to better understand the MAPLE ablation mechanism, which is discussed in this study.

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.

Simultaneous photoluminescence (PL) and external quantum efficiency (EQE) confocal mapping is used to investigate the correlation between the local PL and the EQE in a regioregular poly(3-exylthiophene):poly(9,9-dioctylfluorene-co-benzothiadiazole) inverted bulk heterojunction solar cell. We show that the charge generation and charge collection are strongly non-uniform on a length scale up to 100 μm. Our results evidence that organic solar cells optimization requires not only the control of the submicrometric active materials arrangement but also the control of the large scale device uniformity.

We investigate the encapsulation performances of polyethylene terephthalate-poly (ethylene vinyl acetate) laminating pouches, thermally laminated on flexible poly(9,9-dioctylfluorene)(PF8) active waveguides showing amplified spontaneous emission (ASE). We show that the lamination process does not cause significant degradation of the PF8 emission properties and preserves the ASE properties of the waveguide, with an ASE threshold increase of only 15%. A clear increase of the ASE operational lifetime up to 2.3 times is observed after lamination, together with stable ASE threshold for more than 6 months after the device realization, thus proposing thermal lamination as a straightforward technique to encapsulate organic active waveguides, preserving the possibility to realize flexible devices.

In the past 30 years, organic conjugated molecules received a lot of attention in research because of their unique combination of active properties typical of semiconductors and the technological appeal typical of plastic materials. Among the different applications proposed for organic materials, organic lasers are quickly approaching the performance required for application, while the research on novel active materials is still ongoing. The book presents the current state of the art of the understanding of the physics of optical gain in organic systems and provides a complete description of the most recent advances in organic lasers, including both the structures currently closest to application as well as the fascinating and unconventional systems. The chapters are written by authors with wide experience in the field. The book starts with the basic aspects of the optical gain process and then progressively introduces the most advanced research topics, discussing the state of the art of active material development and the physics and development of fascinating systems for organic lasers, such as random lasers and microcavities in strong coupling, and concludes with a description of the physics and technology of external cavity and distributed feedback lasers. The book is unique that it covers basic aspects, technological aspects, and systems still a subject of basic science research.

We investigated the fluorescence (FL) dependence on the environment oxygen content of poly(9,9-dioctylfluorene) (PF8) thin films. We show that the PF8 interactions with oxygen are not limited to the known irreversible photo-oxidation, resulting in the formation of Keto defects, but also reversible FL quenching is observed. This effect, which is stronger for the Keto defects than for the PF8, has been exploited for the realization of a prototype oxygen sensor based on FL quenching. The sensing sensitivity of Keto defects is comparable with the state of the art organic oxygen sensors based on phosphorescence quenching.

We investigated the effects of photodegradation on the emission properties of a poly(9,9-dioctylfluorene) (PF8) active waveguide operating in air. We demonstrate that the PF8 degradation results in an unexpected wavelength dependent intensity variation, with an intensity enhancement of the PF8 0–0 line, an exponential decrease of the amplified spontaneous emission (ASE) intensity stronger than the one expected due to exciton quenching and a weak intensity quenching in the green emission range. We ascribe these results to the PF8 exciton quenching and keto defect formation due to photodegradation, affecting not only the exciton densities but also the waveguide losses, reducing the intrinsic PF8 self-absorption and increasing the keto defect absorption.

Il libro si rivolge agli studenti che stanno per iniziare gli studi universitari nell'area Tecnico-Scientifica. Gli argomenti trattati sono preliminari a quelli dei corsi universitari di Fisica e mirano ad evidenziare le conoscenze preliminari necessarie per poter affrontare con successo gli studi universitari. Il libro contiene numerosi esercizi svolti e alcuni test simili a quelli di recupero organizzati in numerose sedi universitarie.

We show that anthocyanines extracted from 3 different local cultivar of grapes characteristic of the South Italian region named ''Salento'' can effectively act as nature-friendly and eco-sustainable sensitizers in Dye-sensitized Solar Cells (DSSC) type devices. The natural pigments have been extracted in a simple way by immersing both the whole fruits and the fruit epicarps in methanol and in an ultrasonic bath, without any further purification step. The measured open circuit voltage and the fill factors are among the best values reported in literature. We compare the effectiveness of the extraction process from fermented and from fresh grapes, showing that the devices realized starting from the fresh fruit epicarps perform slightly better than those realized starting from the fermented fruit. We correlate this behavior with an interaction of different dye molecules in the device.

The efficiency optimization of bulk heterojunction solar cells requires the control of the local active materials arrangement in order to obtain the best compromise between efficient charge generation and charge collection. Here, we investigate the large scale (10–100 μm) inhomogeneity of the photoluminescence (PL) and the external quantum efficiency (EQE) in inverted all-polymer solar cells (APSC) with regioregular poly(3-hexylthiophene) (P3HT):poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) active blends. The morphology and the local active polymer mixing are changed by depositing the active layer from four different solvents and by thermal annealing. The simultaneous PL and EQE mapping allowed us to inspect the effects of local irregularities of active layer thickness, polymer mixing, polymer aggregation on the charge generation and collection efficiencies. In particular, we show that the increase of the solvent boiling point affects the EQE non-uniformity due to thickness fluctuations, the density non-uniformity of rrP3HT aggregate phase, and the blend components clustering. The thermal annealing leads to a general improvement of EQE and to an F8BT clustering in all the samples with locally decrease of the EQE. We estimate that the film uniformity optimization can lead to a total EQE improvement between 2.7 and 6.3 times.

The time evolution of the series resistance of bulk heterojunction solar cells realized and stored in inert atmosphere as well as in air has been monitored by fitting the experimental current vs. voltage characteristic to the analytical solution of the real diode equation obtained by means of the Lambert W function. The method allows to determine if and at what extent the standard one diode model can be used to acceptably describe the organic device in the field of a preliminary simple and fast evaluation of the series resistance evolution with time, without the need of complex and time consuming numerical fitting algorithms.

The importance of interface effects for organic devices has long been recognized, but getting detailed knowledge of the extent of such effects remains a major challenge because of the difficulty in distinguishing from bulk effects. This paper addresses the interface effects on the emission efficiency of poly(p-phenylene vinylene) (PPV), by producing layer-by-layer (LBL) films of PPV alternated with dodecylbenzenesulfonate. Films with thickness varying from ∼15 to 225 nm had the structural defects controlled empirically by converting the films at two temperatures, 110 and 230 °C, while the optical properties were characterized by using optical absorption, photoluminescence (PL), and photoluminescence excitation spectra. Blueshifts in the absorption and PL spectra for LBL films with less than 25 bilayers (<40–50 nm) pointed to a larger number of PPV segments with low conjugation degree, regardless of the conversion temperature. For these thin films, the mean free-path for diffusion of photoexcited carriers decreased, and energy transfer may have been hampered owing to the low mobility of the excited carriers. The emission efficiency was then found to depend on the concentration of structural defects, i.e., on the conversion temperature. For thick films with more than 25 bilayers, on the other hand, the PL signal did not depend on the PPV conversion temperature. We also checked that the interface effects were not caused by waveguiding properties of the excited light. Overall, the electronic states at the interface were more localized, and this applied to film thickness of up to 40–50 nm. Because this is a typical film thickness in devices, the implication from the findings here is that interface phenomena should be a primary concern for the design of any organic device.

Cesius lead halide perovskite colloidal nanocrystals are among the most promising perovskite systems for light-emitting devices applications due to their high fluorescence quantum yield and high optical gain at room temperature. In this letter, we report on the first investigation of temperature dependence of amplified spontaneous emission (ASE) properties of thin films of CsPbBr3 nanocrystals. We demonstrate that ASE is strongly temperature-dependent, with a complex variation in temperature of the ASE intensity, threshold, and peak wavelength. The joint investigation of the photoluminescence (PL) spectra below and above the ASE threshold allows us to conclude that the temperature increase results in the formation of disordered subdomains emitting in the low-energy tail of the PL spectra, leading to the existence of three emission regimes with transitions at about 90 and 170 K, with individually different temperature dependences.

We investigate the thickness dependence of the amplified spontaneous emission (ASE) threshold and operational lifetime in air-poly(9,9-dioctylfluorene)(PF8)-glass asymmetric active waveguides. We show that the ASE threshold decreases with the film thickness up to about 200 nm, and increases for higher thicknesses. The ASE operational lifetime increases with the thickness up to about 300 nm, and it is almost thickness independent for higher thickness. We show that the observed results are related to the guided mode confinement in the waveguide and to the spatial overlap between the guided modes and the excited region in the film.

L’invenzione riguarda un metodo per la realizzazione di strati attivi basati su molecole organiche in grado di mostrare emissione spontanea amplificata (ASE) in bande spettrali multiple.Si tratta dello sviluppo di realizzazione di sorgenti di ASE e laser operanti simultaneamente ad almeno due colori distinti,combinando la realizzazione di uno strato attivo uniforme con materiale con emissione del primo colore e un secondo materiale attivo con emissione di colore diverso, di cui si controlli l’aggregazione su scala micrometica.