Exciton dynamics in hybrid polymer/QD blends

Abstract The prospect of exploiting quantum dots (QDs) properties (tunable absorption spectrum, multiple exciton generation) while maintaining the flexible structure of polymer systems opens new possibilities in the photovoltaic field. Although charge transport dynamics in pristine polymer and QDs systems have been quite well established lately, a complete understanding of the charge transfer process between QDs and polymers when they are in blends is still lacking. In this work we used static and ultrafast fluorescence spectroscopy together with Atomic force Microscopy (AFM) to study the exciton dynamics in polymer/QDs films. Specifically we used poly(3-hexylthiophene) (P3HT) as the hole conducting donor material and the core shell CdSe(ZnS) QDs as the electron acceptor material. The QDs surface has been treated with two different capping ligands treatments: one based on the use of pyridine and the other one on hexanoic acid. The influence of the two different methods on the exciton dynamics and on the morphology will also be discussed. Blends containing differently treated P3HT/CdSe(ZnS) wt% ratios have been prepared producing films having uniform morphology and good intermixing, as proved by AFM measurements. Ultrafast fluorescence decays allowed us to compare the exciton dynamics in the polymer pristine respect to the treated P3HT/CdSe(ZnS) films. Efficient fluorescence quenching has been shown by both kind of blends respect to the pure polymer

Effect of tungsten disulfide (WS_2) nanotubes on structural, morphological and mechanical properties of poly(L-lactide) (PLLA) films

Poly(L-lactide) (PLLA) is a semicrystalline, biocompatible and biodegradable polymer widely employed in many applications (food packaging, biomedical devices, drug delivery systems). This work deals with nanocomposites of PLLA and tungsten disulfide (WS_2) nanotubes (NTs) as a novel material to obtain thinner and stronger bioresorbable vascular scaffolds. We studied the influence of WS_2 NTs on the mechanical properties of PLLA-WS_2 films. Polarized optical microscopy reveals a high degree of orientation of the polymer molecules in stretched films that further increases with a post-stretching annealing treatment. At the same time, X-ray diffraction (XRD) and Raman spectroscopy confirm enhancement of the crystallinity induced by the WS_2 NTs

Effect of tungsten disulfide (WS_2) nanotubes on structural, morphological and mechanical properties of poly(L-lactide) (PLLA) films

Poly(L-lactide) (PLLA) is a semicrystalline, biocompatible and biodegradable polymer widely employed in many applications (food packaging, biomedical devices, drug delivery systems). This work deals with nanocomposites of PLLA and tungsten disulfide (WS_2) nanotubes (NTs) as a novel material to obtain thinner and stronger bioresorbable vascular scaffolds. We studied the influence of WS_2 NTs on the mechanical properties of PLLA-WS_2 films. Polarized optical microscopy reveals a high degree of orientation of the polymer molecules in stretched films that further increases with a post-stretching annealing treatment. At the same time, X-ray diffraction (XRD) and Raman spectroscopy confirm enhancement of the crystallinity induced by the WS_2 NTs

Fabrication and biocompatibility analysis of flexible organic light emitting diodes on poly(lactic acid) substrates: toward the development of greener bio-electronic devices

The use of biodegradable and biocompatible materials for electronic applications is a research topic of great interest, offering the possibility to develop bio-electronic devices and reduce e-wastes. In this work, poly(lactic acid) (PLA) films were prepared to be employed as substrates in flexible optoelectronic devices and their functionality was tested in indium tin oxide-free organic light emitting diodes (OLEDs). The PLA substrates were fabricated by extrusion and characterized in terms of morphological, optical and wettability properties. The films showed optical transmittance of about 90% in the visible region and surface roughness of about 12 nm, optical and morphological parameters suitable for OLED applications. Different structures were fabricated on top of the PLA substrates by hybrid technology through solution-based and thermal evaporation deposition methods. Good electro-optical properties were detected in iridium complexes-based devices, with current efficiencies of about 14 and 1 cd/A in structures with tris[2-phenylpyridinato-C2,N]iridium(III) (Ir[ppy]3) and bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium (FIrPic) as emitting layer, respectively. The biocompatibility assay showed that the encapsulated devices do not release toxic substances and their biocompatibility can be improved by selecting different encapsulating glues. The results of this study show the potentiality of PLA as substrate for the fabrication of biocompatible and biodegradable optoelectronic devices opening new routes for the development of advanced bio-electronic system

Analysis of the persistent photoresponse of C8BTBT transistors in the near-bandgap spectral region

The variations in physical parameters of an organic field-effect transistor having dioctylbenzothieno[2,3-b]benzothiophene (C8BTBT) as the channel semiconductor were investigated under different light irradiation conditions at wavelengths of 350 nm, 370 nm, 400 nm and by increasing exposure doses. The progress of the electro-optical history of the transistor was evaluated by repeating I–V scan cycles both in the dark and under light exposure. The information recorded upon different exposure times was used to detect the photoactivated charge-trapping effects. The device showed a stable I–V response in the dark bias (VDS = −10 V, −10 V ≤ VGS ≤ +10 V) conditions and a persistent threshold voltage (VT) shift under illumination at all irradiation wavelengths. We suggested that the observed dose-dependent VT drifts were due to charge retention in trap sites within the organic semiconductor. The threshold voltage was recognized as the main parameter affected by charge retention. VT variations were modelled versus time through a single exponential revealing a maximum in charge relaxation times for irradiations at wavelengths of 370 nm, in proximity of the C8BTBT bandgap energy. Furthermore, bias-stress effects and persistent photoinduced VT drifts were found to depend on comparable characteristic times. Therefore, a common nature for both the bias-stress decay and relaxation from photoexcitation mechanisms is likely