Janusz Schinke

Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes

We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A(-1), whereas maximum luminance values are as high as 8000 and 3000 cd m(-2) for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime.

Understanding S-shaped current-voltage characteristics of organic solar cells: Direct measurement of potential distributions by scanning Kelvin probe

We present a comparison of the potential distribution along the cross section of bilayer poly(3-hexylthiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (P3HT/PCBM) solar cells, which show normal and anomalous, S-shaped current-voltage (IV) characteristics. We expose the cross sections of the devices with a focussed ion beam and measure them with scanning Kelvin probe microscopy. We find that in the case of S-shaped IV-characteristics, there is a huge potential drop at the PCBM/Al top contact, which does not occur in solar cells with normal IV-characteristics. This behavior confirms the assumption that S-shaped curves are caused by hindered charge transport at interfaces.

Processing Follows Function: Pushing the Formation of Self- Assembled Monolayers to High-Throughput Compatible Time Scales

Self-assembled monolayers (SAMs) of organic molecules can be used to tune interface energetics and thereby improve charge carrier injection at metal-semiconductor contacts. We investigate the compatibility of SAM formation with high-throughput processing techniques. Therefore, we examine the quality of SAMs, in terms of work function shift and chemical composition as measured with photoelectron and infrared spectroscopy and in dependency on molecular exposure during SAM formation. The functionality of the SAMs is determined by the performance increase of organic field-effect transistors upon SAM treatment of the source/drain contacts. This combined analytical and device-based approach enables us to minimize the necessary formation times via an optimization of the deposition conditions. Our findings demonstrate that SAMs composed of partially fluorinated alkanethiols can be prepared in ambient atmosphere from ethanol solution using immersion times as short as 5 s and still exhibit almost full charge injection functionality if process parameters are chosen carefully. This renders solution-processed SAMs compatible with high-throughput solution-based deposition techniques.

Naphthalene Tetracarboxydiimide-Based n-Type Polymers with Removable Solubility via Thermally Cleavable Side Chains

Multilayer solution-processed devices in organic electronics show the tendency of intermixing of subsequently deposited layers. Here, we synthesize naphthalene tetracarboxydiimide (NDI)-based n-type semiconducting polymers with thermally cleavable side chains which upon removal render the polymer insoluble. Infrared and photoelectron spectroscopy were performed to investigate the pyrolysis process. Characterization of organic field-effect transistors provides insight into charge transport. After the pyrolysis homogeneous films could be produced which are insoluble in the primary solvent. By varying curing temperature and time we show that these process parameters govern the amount of side chains in the film and influence the device performance.

Improving performance, stability, and processability of OFETs with printed Ag electrodes by means of a novel, multipurpose self-assembled monolayer(Conference Presentation)

We present a novel SAM-forming molecule bisjulolidyldisulfide that reduces the WF of metal surfaces by ~1.2 eV and can lower the barrier for electron injection to organic semiconductors. Applied to Au and Ag surfaces, including inkjet-printed Ag on PET, we characterized bisjulolidyldisulfide monolayers by means of photoelectron spectroscopy (PES) and sessile drop technique, as well as their influence on the performance of n-type OFETs. Next a strong reduction of the contact resistance by two orders of magnitude, we found that this SAM treatment extends the shelf lifetime of ambient-stored OFET devices. Also, it improves the wettability and thereby facilitates solution processing of a subsequent layer with respect to the untreated surface. The full electrical functionality of bisjulolidyldisulfide SAMs was found to become manifest with only one minute of immersion in ethanol solution. PES measurements suggests that the surface coverage is thorough on Au, but only fractional on Ag, especially on printed Ag. However, the quality of SAM-treated bottom contacts in n-type OFETs is very similar for all three investigated metal surfaces (Au and Ag evaporated and printed Ag). This is especially important for printed Ag-electrodes, as their surface was found to be significantly worse for device performance in comparison to their evaporated Ag counterpart. Using this surface treatment we realized integrated unipolar n-type ring oscillators with inkjet printed Ag electrodes.