F. P. Wenzl

Light-emitting electrochemical cells and light-emitting diodes based on ionic conductive poly(phenylene vinylene): a new chemical sensor system

Abstract Red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene] also named poly[2,5-bis (triethoxy-methoxy)-1,4-phenylene vinylene], (BTEM-PPV) are fabricated and characterized. BTEM-PPV combines good electronic conductivity with ionic conductivity in the oxidized and reduced state due to its conjugated backbone and oligo(ethylene oxide) attached as side chains. When applying this polymer in LECs one obtains devices of moderate brightness and with fast response times. This high performance is achieved without blending an additional ionic conductive polymer into the film. The response time of such devices driven with square waveform voltage pulses was determined to be 480 μs. The turn-on voltage for electroluminescence occurs at 2 V and at 3 V a brightness of about 35 cd/m 2 was obtained. Due to the covalent linkage of glyme like side chains to the PPV backbone, BTEM-PPV, complexed with metal ions, shows an ionochromic effect in the absorption spectra and also in the electroluminescence spectra, which can be a new approach to chemical sensors.

Photophysics of excitation energy transfer in highly fluorescent polymers

Abstract We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. The blue emitter laddertype poly(para-phenylene) (LPPP) is blended as a host with the red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We use transient and steady-state photoluminescence as well as near-steady-state photoinduced absorption to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. In addition, we describe analytical relations to determine the lifetime of these species from photomodulation spectroscopy.

Ionochromism in a light-emitting electrochemical cell with low response time based on an ionic conductive poly-phenylene vinylene

Characterizations of red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene], (BTEM-PPV), are presented, BTEM-PPV combines good electronic conductivity with ionic conductivity due to its conjugated backbone and side chains consisting of oligo(ethylene oxide). The use of this polymer in LECs leads to relatively bright light emitting devices with low response times which are obtained without blending an additional ionic conductive polymer into the film. The response times of the BTEM-LECs driven with a square wave form pulse were determined to be about 480 μs. The value for the turn-on voltage of the electroluminescence is 2 V and at 3 V a brightness of around 35 cd/m2 was obtained. BTEM-PPV complexed with metal ions shows an ionochromic effect in the absorption spectrum and also in the electroluminescence spectrum due to the covalent linkage of the glymelike side chains to the PPV backbone, which represents an approach toward chemical ...

Light-Emitting Electrochemical Cells with Microsecond Response Times Based on PPPs and Novel PPVs

The response times of light-emitting electrochemical cells (LEC) can be decreased to several microseconds by minimising phase separations in the active layer. This e.g. can be realised by optimising the phase morphology of the blend of an ion conductive polyethylene oxide and an electronic conductive laddertype PPP or by using a single polymer combining electron- and ion conduction properties, so that phase separations are excluded. Latter can be realised by a PPV with ionic conductive sidechains. We present efficient LECs yielding extremely low onset voltages.

Tracking of conduction phenomena and degradation in organic light emitting diodes by current noise measurements

Noise current analysis, both in time and frequency, is proposed as a means to sense variations of the microscopic conduction in organic light emitting diodes and to track their time evolution. The sensitivity of the technique would allow to correlate the carriers conduction properties with the corresponding changes in the microscopic morphology of the organic layers as obtained with structural or spectroscopic investigations. The method is shown to be very effective also in sensing the initial state and the growth of organic diodes catastrophic degradation in large advance to current monitoring or other techniques.

Long lived photoexcitation dynamics in a dendronically substituted poly(fluorene)

Polyfluorenes are possibly the most attractive class of conjugated polymers for blue polymer light emitting devices. Derivatives with dendron side chains have been found to significantly improve the color stability of the devices. Using a quasi-cw photoinduced absorption technique we present a qualitative and quantitative analysis of lifetimes and deactivation mechanisms for the triplet excited states in a dendronically substituted derivative. The triplet lifetimes increase by one order of magnitude in the dendronic derivative compared to those in a standard poly(dialkylfluorene), which has been characterized for comparison. The temperature dependence of triplet lifetimes is interpreted in terms of migration assisted deactivation of triplet excitations. We conclude that the dendrons inhibit the Dexter-transfers of triplet excitations between the polymer chains and so strongly hinder triplet migration.

Solution Processed Conjugated Polymer Multilayer Structures for Light Emitting Devices

We study the feasibility of semiconducting polymer nanospheres deposited from miniemulsions as an approach to form organic multilayer structures and devices from an all solution based process. A detailed study of the wetting and film forming properties of the dispersed semiconducting polymer nanospheres on different polar and non-polar organic surfaces is given. The transmission and fluorescence properties of the polymer multilayer structures are studied. Organic light emitting devices based on such multilayer structures are presented and their properties are discussed.

Self-absorption effects in a LEC with low Stokes shift

Abstract The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

Blue and Green Light-Emitting Electrochemical Cells with Microsecond Response Times

Blue and green light-emitting electrochemical cells (LECs) based on a ladder-type poly(paraphenylene) ( m-LPPP) are described. These LECs are characterized by an extremely low onset for light emission (between 2.2 and 2.7 V in reverse direction for blue emission) yielding an external quantum efficiency of approximately 0.3% and a brightness of about 250 cd/m 2 at 10 V. The LECs based on m-LPPP show a fast pulse response below 30 μs, which is ascribed to restricted motion of the ions in the LEC under pulse operation. Light-emitting diodes based on conjugated polymers (PLEDs) can be realized with promising efficiencies over the entire visible spectral range.1-6 The most attractive potential application of PLEDs is their use in emissive flat panel displays. In order to realize PLEDs with operating lifetimes which fulfill the values required for industrial applications, efficient devices with low operating voltages are necessary. Low operating voltages are more easily realized with small bandgap conjugated polymers, because the mismatch between the energy bands of the polymers and the work functions of the electrodes1 is smaller than for wide bandgap polymers. Thus, it has proved to be easier to realize red-orange and green PLEDs with low operating voltages and high efficiencies. 1,2 However, the fabrication of blue PLEDs with low turn-on voltages and sufficient lifetimes is absolutely necessary to realize multicolor flat panel displays. Moreover, one of the most promising ways to produce multicolor displays is based upon blue PLEDs, and the realization of red and green light emission through down-conversion.7-9

Current noise spectroscopy on mLPPP based organic light emitting diodes

Abstract Noise spectroscopy is presented to be a powerful tool to investigate the current flowing in organic light emitting diodes (oLEDs) with high sensitivity. Measurements can be performed over the whole bias range of interest, from reverse bias up to high values of forward bias voltage. From these measurements one can gain insight into the microscopic conduction processes dominating the device current and obtain valuable information for improved device modeling. In particular it is shown that the low frequency power spectrum of the tested oLEDs has a power law dependence around 1/f 1.3 almost irrespective of device characteristics and of measurement conditions. Additionally, noise spectra are also proposed as a means to sense the initial state and the growth of degradation phenomena in these devices. The onset of degradation is shown to be signaled by current spikes that reflect on a net increase of the white noise component of about three orders of magnitude in the power spectral density, when degradation is just hardly beginning to be visible as dark spots on the emitting surface.