Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin

We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal–organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP–PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.

Direct Measurement of the Triplet Exciton Diffusion Length in Organic Semiconductors

We present a new method to measure the triplet exciton diffusion length in organic semiconductors. N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) has been used as a model system. Triplet excitons are injected into a thin film of NPD by a phosphorescent thin film, which is optically excited and forms a sharp interface with the NPD layer. The penetration profile of the triplet excitons density is recorded by measuring the emission intensity of another phosphorescent material (detector), which is doped into the NPD film at variable distances from the injecting interface. From the obtained triplet penetration profile we extracted a triplet exciton diffusion length of 87±2.7  nm. For excitation power densities >1  mW/mm2 triplet-triplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor. The proposed sample structure can be further used to study excitonic spin degree of freedom.

Exciton diffusion length in narrow bandgap polymers

We developed a new method to accurately extract the singlet exciton diffusion length in organic semiconductors by blending them with a low concentration of methanofullerene[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The dependence of photoluminescence (PL) decay time on the fullerene concentration provides information on both exciton diffusion and the nanocomposition of the blend. Experimentally measured PL decays of blends based on two narrow band gap dithiophene–benzothiadiazole polymers, C–PCPDTBT and Si–PCPDTBT, were modeled using a Monte Carlo simulation of 3D exciton diffusion in the blend. The simulation software is available for download. The extracted exciton diffusion length is 10.5 ± 1 nm in both narrow band gap polymers, being considerably longer than the 5.4 ± 0.7 nm that was measured with the same technique in the model compound poly(3-hexylthiophene) as a reference. Our approach is simple, fast and allows us to systematically measure and compare exciton diffusion length in a large number of compounds.

Exciton Quenching Close to Polymer-Vacuum Interface of Spin-Coated Films of Poly(p-phenylenevinylene) Derivative

Polymer-fullerene bilayer heterostructures are suited to study excitonic processes in conjugated polymers. Excitons are efficiently quenched at the polymer-fullerene interface, whereas the polymer-vacuum interface is often considered as an exciton-reflecting interface. Here, we report about efficient exciton quenching close to the polymer-vacuum interface of spin-coated MDMO-PPV (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene]) films. The quenching efficiency is estimated to be as high as that of the polymer-fullerene interface. This efficient quenching is consistent with enhanced intermolecular interactions close to the polymer-vacuum interface due to the formation of a “skin layer” during the spin-coating procedure. In the skin layer, the polymer density is higher; that is, the intermolecular distances are shorter than in the rest of the film. The effect of exciton quenching at the polymer-vacuum interface should be taken into account when the thickness of the polymer film is on the order of the exciton diffusion length; in particular, in the determination of the exciton diffusion length.

Cyclometalated red iridium(III) complexes containing carbazolyl-acetylacetonate ligands: efficiency enhancement in polymer LED devices

The design, synthesis, photophysical and significantly improved electrooptical properties of a series of red emitting cyclometalated iridium(III) complexes containing carbazolyl-acetylacetonate ligands are described.

Effect of thermal annealing on exciton diffusion in a diketopyrrolopyrrole derivative

We show that the method we have developed for measuring the singlet exciton diffusion length in blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) can be applied not only to polymeric materials, but also to small molecule organic semiconductors. Small organic molecules have a large potential for molecular re-organization upon thermal annealing. Here we show that the exciton diffusion length is decreased upon annealing from 9 to 3 nm in a thin film of a diketopyrrolopyrrole derivative. Such a variation is attributed to exciton delocalization effects in the crystalline domains that are formed during the annealing process as well as to the exciton quenching at grain boundaries.

Exploring the Origin of the Temperature-Dependent Behavior of PbS Nanocrystal Thin Films and Solar Cells

Temperature-dependent studies of the electrical and optical properties of cross-linked PbS nanocrystal (NC) solar cells can provide deeper insight into their working mechanisms. It is demonstrated that the overall effect of temperature on the device efficiency originates from the temperature dependence of the open-circuit voltage and the short-circuit current, while the fill factor remains approximately constant. Extensive modeling provides signs of band-like transport in the inhomogeneously coupled NC active layer and shows that the charge transport is dominated by diffusion. Moreover, via low temperature absorption and photoluminescence (PL) measurements, it is shown that the optical properties of PbS thin films before and after benzenedithiol (BDT) treatment exhibit very distinct behavior. After BDT treatment, both the optical density (OD) and PL are shifted to lower energies, indicating the occurrence of electronic wave function overlap between adjacent NCs. Decrease of the temperature leads to additional red-shift of the OD and PL spectra, which is explained by the well-known temperature dependence of the PbS NCs’ bandgap. Moreover, BDT treated PbS NCs show unusual properties, such as decrease of the PL signal and broadening of the spectra at low temperatures. These features can be attributed to the partial relaxation of the quantum confinement and the opening of new radiative and nonradiative pathways for recombination at lower temperatures due to the presence of trap states.

Systematic study of exciton diffusion length in organic semiconductors by six experimental methods

Six experimental methods have been used to investigate the exciton diffusion length in materials with systematic chemical modifications. We find that exciton diffusion length correlates with molecular ordering. We discuss situations in which certain experimental techniques are more appropriate