Blended films of poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) were annealed at 70°C under rotational magnetic field (RMF) (∼350 G with 600 RPM) for 10 min. At low temperatures, the films prepared with RMF contained new spectral features. The photoluminescence (PL) spectra of films prepared under RMF had two peaks (1.59 and 1.73 eV), whereas films prepared without RMF had a single PL band at 1.75 eV. Furthermore, the photoinduced absorption (PIA) spectra of films prepared with RMF showed an additional PIA band at 1.15 eV, along with the well-known PIA band at 1.35 eV due to excited state absorption by polarons. We conclude that the spectral features in MEH-PPV/PCBM films were due to charge transfer complexes (CTC), the formation of which is promoted by RMF. Our results suggested that applying proper magnetic field while preparing polymer/fullerene film could improve the formation of CTC. We discuss the relevance to solar cell applications.
Organometallic hybrid perovskites have attracted intense attentions recently, as a new family of solution processable semiconductors for photoenergy conversion and light emission purposes. Specifically, CH3NH3PbBr3 is one type of emissive perovskites, but its quantum efficiency largely depends on the preparation procedure. Here, we use the fluorescence lifetime imaging (FLIM) technique to investigate the relation between microscopic structures and photoluminescence (PL) in CH3NH3PbBr3 polycrystalline films. By dripping poor solvents (chlorobenzene or chloroform) to accelerate the crystallization during the film preparation, we could decrease the crystal domain size from 10 μm down to 500 nm, and the corresponding PL intensity increases significantly. From the FLIM characterization of these films, we find that the PL emission is mostly from the edge of crystal domains. The PL dynamics indicates that the radiative decay of edge state is much more efficient than that of the bulk state, and the bulk state of photoexcitation undergoes an energy transfer to the edge state. This finding explains the origin of enhanced PL from CH3NH3PbBr3 films when treated with poor solvents and provides useful information for further improvement on the PL efficiency of hybrid perovskite materials.
In an imaging system based on a coherent source of moderate power density, images can be blurred when a biased photorefractive crystal is applied at the focal point of the imaging lens. In the frequency domain of the original images,
the intensity patterns are diffracted through the photorefractive crystal with varied bias voltage. The high intensity region,
which is usually the center or low frequency region of the intensity patterns, is more readily focused or defocused, resulting in blurred images in perception. Such blurred images could not be simply recovered by defocusing methods,
which can only indistinguishably focus or defocus the whole intensity patterns. However, the blurred images may be deblurred to certain extent for recovery if a second photorefractive crystal with bias voltage is employed at the focal point of a tandem imaging system. The mechanism of deblurring is similar to that of blurring: the blurred images are
transferred through the frequency domain again using an imaging lens, where the second biased photorefractive crystal
diffracts the intensity patterns to revert the sensitive region where previously gets focused or defocused. In this work,
theoretical analyses are presented in detail to explain the blurring-deblurring mechanism using two biased photorefractive crystals and compatible experimental results are obtained and illustrated. Considering the blurring and
deblurring function subgroups of the experiment setup can be potentially developed into encryption and decryption units
compatible with far field propagation, the technology presented herein may be promising to find applications in secure laser-based free-space communication systems.
π-conjugated polymers (PCPs) are attractive candidates as gain media in laser applications due to their high photoluminescence quantum efficiency in broad spectral range. However, the self-absorption of long-lived excited states was considered to be a limitation for achieving more effective organic lasers. Moreover, the morphology of films is found to be crucial to their optical and electrical properties recently. In this work, we studied amplified spontaneous emission (ASE) of a typical PCP, namely, Poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) films with a 10 ns 532 nm pulse laser focused by a cylindrical lens for obtaining an excitation area in the form of a 100 μm wide and 1 cm long stripe. In an as cast MEH-PPV film, the thresholds increase with the temperatures increase due to the thermal torsion and vibration mode shorten the conjugation chain. On the other hand, a MEH-PPV film which is annealed in Nitrogen at 350 K of half hour, the ASE is not observed at both 300 K and 77 K, for annealing will form π- stacks which increase the interchain interaction. Further analysis suggests that interchain excimers instead of intrachain excitons may be more primary to optical properties in annealed MEH-PPV film. Our measurements suggest that the morphology of the film instead of long lived photoexcitation with lifetime sensitive to the temperature is more crucial to threshold of ASE, as well as, to PCPs lasers.
The long lived phtoexcitation (polarons) dynamics that range from microseconds to milliseconds in a typical organic photovoltaic (OPV) material, the blends of regio-regular poly (3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been investigated using continuous-wave photoinduced absorption (cw-PIA) spectroscopy. In this time regime, whether above-gap (AG) or below-gap (BG) excitation, the delocalized polarons (DP) and localized polarons (LP) all exhibit dispersive bimolecular recombination process which is limited by multi-trap states; however, compared to AG, BG excitation presented the narrower distribution profile in trap polaron states. Furthermore, the recombination in RR-P3HT/PCBM films was weak temperature dependence with small thermal activation energy, the value Δ for DPs and LPs of 25 meV (45 meV) and 13 meV (42 meV) using AG (BG) excitation, respectively. We considered that polarons from BG excitation are not intrinsically different with polarons from AG one in millisecond time regime, therefore may contribute to the power conversion efficiency using appropriate materials and device structures. For completeness, cw-PIA spectroscopy of poly[2-methoxy, 5-(2’-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) and PCBM blended films with amorphous morphology were also studied. It is found that polarons also exhibited multi-trap limited bimolecular recombination, but Δ for polarons in amorphous MEH-PPV/PCBM was up to ~160 meV. We concluded that polarons in RR-P3HT/PCBM blend are basically transport by tunneling, but the recombination in MEH-PPV/PCBM blend near room temperature is mainly thermal activated process. Overall, it is demonstrated that cw-PIA spectroscopy with thermal-activated-recombination analysis can be applied to evaluate polymer (dis)order in bulk heterojunction films.
Various spectroscopy techniques such as absorption, photoluminescence and photoinduced absorption (PIA) spectroscopy, were used to study the photophysics in poly [2-methoxy-5-(20-ethyl-hexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) films, which were dropped cast on glass substrates using its toluene solution and being annealed at various temperatures. With the analysis of temperature dependence PL intensities, we conclude that PL emission around 680 nm at low temperature is due to intertain excimers instead of intrachain excitons for 450 K annealed film;On the other hand, this relative intensity difference is much smaller in both unannealed and 500 K annealed films, in which the morphology is amorphous and microcrystalline, respectively. We conclude that the interchain photoexcitations play crucial roles in the photophysics of MEH-PPV films. The further measurements on PIA spectrum of MEH-PPV films suggest that the interchain photoexciation is also important for the generation of triplet excitons.
We have made lead (II) tetrakis (4-cumylphenoxy) phthalocyanine (PbTCPc) films with various concentrations in plasticized poly (methyl methacrylate) (PMMA) and poly (acrylic tetraphenyl diaminobiphenylamine) (PATPD) matrices, and investigated nonlinear transmission properties with a Nd:YAG laser (532 nm, 5ns). Our results demonstrate a solid state PbTCPc nonlinear absorber, whose performance is comparable to that of low concentration solutions of the dye, that can be fabricated by mixing the dye in a plasticized polymer matrix.
In this paper, a novel electron node for microring is analyzed based on plasma dispersion modulation effect. The gearshaped double-sector electron node is designed for microring embedded with resonator disks (MERD). Silvaco, twodimensional (2-D) semiconductor device simulator, has been used to analyze the relationship between concentration changes of free hole (electron) and output voltage. The optical investigation was carried out by CMT tools, giving rise to a complete evaluation of the electro-optically properties for our device. We simulate three positions of the MERD cross section with two dopant concentrations. The results show that spacing between electron nodes has important impact to resonator frequency shift. Fixing the concentrations of P and N at 1e20/cm3, the resonator frequency shift can be reached to 0.09nm when the spacing between electron nodes is 2 µm.
Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t)-α, with α in the range of 0.7 to 1. The PL emission of S-NTs in D2O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.
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