Ellen Moons

Conjugated polymer blends: linking film morphology to performance of light emitting diodes and photodiodes

Blending is a technique known in polymer technology that takes advantage of the processibility of polymers to produce new solid materials or composites with specific structural and physical properties, distinct from the ones of their components. In thin films of polymer blends interesting morphologies are formed because of phase separation. For conjugated polymers, i.e. solution-processible semiconductors, blending also opens a way to optimize the performance of opto-electronic devices, bringing about technological benefits. It is therefore crucial to achieve understanding of the effect film morphology has on the device performance, and, ultimately, to achieve control over the phase separation in a blend, so that structures can be designed that yield the desired device performance. Light-emitting diodes (LEDs) made of polymer blends have shown strongly enhanced electroluminescence (EL) efficiencies, as compared to pure homopolymers. Colour conversion, white light emission, polarized light emission, emission line narrowing, and voltage-tunable colours are other effects that have been observed in blends containing light-emitting polymers. Although the enhanced EL efficiency is attributed to F?rster-type energy transfer in numerous reports, the exciton dynamics behind this effect is not well understood. Here we review the formation and morphology of thin films of conjugated polymer blends, as well as modern microscopic and spectroscopic techniques to study them. Furthermore, we attempt to link the film morphology to the electronic performance of electroluminescent and photovoltaic devices and discuss energy and charge transfer phenomena at the interfaces. We also report some new results, specifically for polyfluorene blends in LEDs. This article was originally intended for publication in Issue 42 of this volume, which was a special issue on Conjugated Polymers: Issue 42

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophenebased dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO2 thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2 0 -bithiophene), P4UBT, or poly(3-undecyl-2,2 0 -bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO2 films. The dense TiO2 thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)2 Bis(2,2 0 bipyridyl-4,4 0 dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO2 surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO2 and a liquid electrolyte, or

Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a comparison

Abstract We report a comparison of the work functions of thin films of indium tin oxide (ITO), carried out by means of ultraviolet photoelectron spectroscopy (UPS) and by measurements of the contact potential difference with respect to a gold reference electrode (Kelvin probe (KP) method). We investigated commercially available ITOs both “as-received”, and after certain surface treatments, such as oxygen plasma. First, we find measurable discrepancies between KP values measured with three different instruments, and between the KP and the UPS values. Secondly, and unexpectedly, we find that the KP, although more sensitive than UPS, does not detect certain differences between ITOs with different surface treatments. We discuss the results in view of the different environments in which the measurements are carried out (UHV for the UPS and air/Ar for the Kelvin method), of the effects which may be induced by the high-energy photon irradiation in the UPS measurement, and of the stability of the gold probe work function in gas ambient. We conclude that UPS is better-suited for absolute work function determination, although KP remains a convenient and inexpensive tool for fast screening of contact potential differences.

Device performance of APFO-3/PCBM solar cells with controlled morphology

Polymer/fullerene solar cells with three different device structures: A) diffuse bilayer, B) spontaneously formed multilayer, and C) vertically homogenous thin films, are fabricated. The photocurrent/voltage performance is compared and it is found that the self-stratified structure (B) yields the highest energy conversion efficiency.

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

De‐mixing of Polyfluorene‐Based Blends by Contact with Acetone: Electro‐ and Photo‐luminescence Probes

De-mixing of Polyfluorene Based Blends by Contact with Acetone: Electro-and Photoluminescence Probes

Ultraviolet–visible near-field microscopy of phase-separated blends of polyfluorene-based conjugated semiconductors

We have used fluorescence scanning near-field microscopy to characterize polymer blends for electroluminescent applications, and thereby identify compositional nonhomogeneities. In particular, we have focused on the binary system constituted by poly(9,9′-dioctylfluorenealt-benzothiadiazole) and poly(9,9′-dioctylfluorene) (PFO), known to give efficiencies of up to 22 cd/A in light-emitting devices with suitable electrodes. Our primary aim was the assignment of the morphological features revealed in shear-force and atomic-force images of spin-coated films, and suggestive of phase separation on a 300-nm-length scale. From analysis of the fluorescence images (325 and 488 nm excitation), and quantitative correlation of optical and topographic data, we identify the raised features with PFO-rich regions. However, the limited variation in fluorescence intensity reveals a high extent of mixing within each phase on the length scale accessible in our experiment, approximately 100 nm for our focused-ion-beam-processe...

Ohmic contacts to p-CuInSe 2 crystals

We report here on the optimization of ohmic contacts to p-CuInSe2 (CISe) single crystals. A low resistance ohmic contact is required to minimize current losses due to series resistance; e.g. in Schottky diodes. Both In-Ga (eutectic)/CISe and gold (evaporatedVCISe contacts have been fabricated on crystals with different orientations and bulk properties. Gold contacts were found to have a lower resistance and to be more stable than In-Ga ones, from the slope of the linear current-voltage plot of the junctions. The resistance of the Au/CISe ohmic contact was decreased by etching the CISe crystal surface chemically in a 0.5% solution of Br2 in methanol for 30 sec at room temperature, prior to gold deposition, while that of the In-Ga contact increased by this etch. Wetting experiments and contact angle measurements showed evidence for changes in the polarity of the surface due to chemical etches.

Ordering domains of spin cast blends of conjugated and dielectric polymers on surfaces patterned by soft- and photo-lithography

Spin casting polymer blends of conjugated and dielectric macromolecules onto chemically patterned metal and oxidized silicon surfaces might provide a simple method to fabricate polymer-based circuitries that can be integrated with conventional electronics. Such solution-processing of the blend components offers simultaneous deposition and pattern-directed alignment of the phase separated polymer domains. The alignment is driven by self-organization guided by preferential surface segregation. Here we demonstrate that the laterally arranged domain structures in spin cast films of the conjugated poly(3-alkylthiophenes) (P3ATs): P3BT, P3DDT and regioregular R-P3HT, blended with dielectric polystyrene (PS), can be ordered by three different surface templates. The templates are formed by a patterned self-assembled monolayer (SAM), micro-contact printed on the surface of interest, i.e. hexadecanethiols on gold (for alignment of P3DDT/PS blend) and octadecyltrichlorosilanes on oxidized silicon (for R-P3HT/PS). Additionally gold lines are micro-patterned on SiO2 with photo-lithography (for P3BT/PS mixture). The forces driving pattern-directed self-organization of the polymers are discussed based on complementary studies of preferential surface segregation, observed for blend films spin cast on homogeneous surfaces that correspond to the different regions of the surface templates.

Optical and Morphological Investigations of Non-homogeneity in Polyfluorene Blends

Abstract We report investigations of organic light-emitting diodes with an active layer consisting of poly(9,9-dioctylfluorene), PFO, blended with a green emitting derivative (F8BT). We used indium tin oxide (ITO) anodes with and without a hole transporting layer, consisting of doped poly(3,4-ethylene dioxythiophene). We also studied the incorporation of an oxadiazole based hole-blocking/electron transporting layer, HBL, namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole, PBD, between the emissive layer and the Ca/Al cathodes. The presence of this layer reduces the range of radiative recombination in a region closer to the HBL interface, and therefore, allows investigation of depth-dependent structural non-uniformities of the emissive layer. We find marginal alterations of the electroluminescence spectra depending on the weight concentration of the F8BT in the range 5–25%. Efficient energy transfer from the host (PFO) to the guest (F8BT) ensures that the emission spectra are dominated by the green component. However, we find that blue emission from the PFO host is much more intense for the structures incorporating the HBL than for those without it. We propose that phase separation phenomena of the blend induced by the spin-coating of the PBD-based layer, are responsible for the higher weight of the blue component.