Norman Mechau

Inkjet Printed, High Mobility Inorganic-Oxide Field Effect Transistors Processed at Room Temperature

Printed electronics (PE) represents any electronic devices, components or circuits that can be processed using modern-day printing techniques. Field-effect transistors (FETs) and logics are being printed with intended applications requiring simple circuitry on large, flexible (e.g., polymer) substrates for low-cost and disposable electronics. Although organic materials have commonly been chosen for their easy printability and low temperature processability, high quality inorganic oxide-semiconductors are also being considered recently. The intrinsic mobility of the inorganic semiconductors are always by far superior than the organic ones; however, the commonly expressed reservations against the inorganic-based printed electronics are due to major issues, such as high processing temperatures and their incompatibility with solution-processing. Here we show a possibility to circumvent these difficulties and demonstrate a room-temperature processed and inkjet printed inorganic-oxide FET where the transistor channel is composed of an interconnected nanoparticle network and a solid polymer electrolyte serves as the dielectric. Even an extremely conservative estimation of the field-effect mobility of such a device yields a value of 0.8 cm(2)/(V s), which is still exceptionally large for a room temperature processed and printed transistor from inorganic materials.

Appropriate choice of channel ratio in thin-film transistors for the exact determination of field-effect mobility

Field-effect mobilities are the most important figures of merit to evaluate the feasibility of semiconductors for thin-film transistors (TFTs). They are, however, sometimes extracted from TFTs with the active semiconductor area undefined and in small channel ratios without the effect of the fringing electric field at the ends of source/drain electrodes taken into account. In this letter, it is demonstrated that the field-effect mobilities extracted from undefined nanoparticulate zinc oxide (ZnO) TFTs at the channel ratio of 2.5 are overestimated by 418%, and the choice of large channel ratios gives the real value of field-effect mobilities.

Influence of interface roughness on the performance of nanoparticulate zinc oxide field-effect transistors

Nanoparticulate zinc oxide is regarded as one of the most promising inorganic materials for printable field-effect transistors (FETs), which work in the n-channel enhancement mode, due to the compatibility with solution, low-temperature, and high throughput processes. Since nanoparticulate films are composed of the nanoparticles and their agglomerates, the roughness of the interface to the insulating layer, where the channel of the FETs is formed, is a critical issue. Thus, the influence of the interface roughness on the field-effect mobility of the FETs is investigated in conjunction with film roughness and capacitance analyses. The field-effect mobility increases almost by a factor of 50, from 2.0×10−4 to 8.4×10−3 cm2 V−1 s−1, even if the reduction in the average roughness of the films is as small as 1.7 nm.

Electrical resistivity of nanocrystalline Al-doped zinc oxide films as a function of Al content and the degree of its segregation at the grain boundaries

Highly transparent and conducting Al-doped ZnO (AZO) films are prepared via sol-gel method with a broad range of nominal Al-doping. The film porosity and morphology is determined by the rate of temperature ramping during the drying of the gel phase. The minimum resistivity is observed to occur around 1.5–2 at. % Al-doped films, irrespective of the morphology and microstructure. It is found by local chemical analysis that Al tends to segregate at the grain boundaries and above a critical concentration, the segregated Al starts to dominate the electronic transport in nanocrystalline AZO. The optical measurements corroborate these findings showing a systematic increase in carrier density only up to 1.5–2 at. % Al-doping. It is concluded that the presence of the resistivity minimum is not merely determined by a solubility limit but is a result of the interplay between the changing carrier concentration and carrier scattering at the segregated Al.

The Compromises of Printing Organic Electronics: A Case Study of Gravure‐Printed Light‐Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are fabricated by gravure printing. The compromise between device performance and printing quality is correlated to the ink formulation and the printing process. It is shown that the rheological properties of the ink formulations of LECs can be tailored without changing the chemical composition of the material blend.

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.

Ultraviolet photodetector arrays assembled by dielectrophoresis of ZnO nanoparticles.

Sensitive and fast ultraviolet sensor arrays have been produced by dielectrophoretic assembling of ZnO nanoparticles. The sub-micron device dimensions induce low operating voltage and low power consumption. The devices are long-term stable and operate in air, oxygen and nitrogen. We have determined the absorption and desorption dynamics from the time-resolved photoresponse and conclude that oxygen or carbon dioxide are the photodesorbed species. We could derive the charge carrier concentration and mobility of the device from measurements of the low-bias and high-bias photocurrent. The presence of defects is discussed by comparing electroluminescence spectra from biased devices with photoluminescence spectral maps of ZnO dispersions.

Trap states and space charge limited current in dispersion processed zinc oxide thin films

The electric transport properties of nanoparticulate zinc oxide (ZnO) thin films are investigated in nitrogen and ambient atmosphere with respect to the effects of polymer adsorbates, in order to study the origin of hysteresis behavior of ZnO thin film transistors. A strong dependence on the polymer adsorbate of the conductivity in nitrogen atmosphere is observed. Utilizing the space charge limited current theory, the trap depth and concentration in the films have been estimated. According to this analysis, the low conductivity of polymer free thin films in ambient atmosphere is caused by an increase in deep traps, compensating free charge carriers and not by a reduction in donorlike defect states. Furthermore, polymeric additives seem to induce similar trap states, which make the transport properties less sensitive against atmospheric influences. However, the strongly compensated semiconductor created in this way, causes a slow trap and release behavior resulting in a strong hysteresis in the transistor ...

Influence of stabilizers in ZnO nanodispersions on field-effect transistor device performance

In order to build printable inorganic electronic devices, semiconducting suspensions are needed, which can be processed at low temperatures using low-cost manufacturing techniques. Stabilized suspensions made of zinc oxide (ZnO) nanoparticles were used to fabricate field-effect transistors by spin coating. The performance of the devices is strongly affected by the nature and concentration of the compounds added to stabilize the nanodispersions. An increase in the field-effect mobility by more than one order of magnitude is obtained upon increasing the stabilizer concentration from 3 to 13 wt %. A further increase in the concentration above 13 wt % results in a decrease in the field-effect mobility. This behavior can be explained by changes in the morphology, the particle-particle junction, and the passivation of surface defect sites.

Ink-Jet-Printed Organic Semiconductor Distributed Feedback Laser

Ink-jet-printed organic distributed feedback (DFB) lasers are realized by employing light-emitting copolymer and suitable organic solvents to meet the demands of printability and optical amplification in a nanopatterned conjugated polymer slab waveguide. We demonstrate the accurate lateral positioning of ink-jet-printed patches of the gain material on a polymer substrate with 500×500 µm2 grating areas. We also printed patches of large lateral dimension of 6 mm2 on a silica grating. The high uniformity of the film thickness leads to a laser wavelength variation of less than 3 nm over the whole area.