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.
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.
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