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Zinc oxide (ZnO) is a n-type transparent semiconductor which can be processed by low cost techniques (such as spray-pyrolysis and spin-coating) and can be applied as the active layer of thin-films transistors (TFTs). The electrical properties of ZnO films are strongly affected when the device is exposed to room conditions and/or UV-light, suggesting possible applications as UV or/and gas sensors. Atmospheric oxygen molecules adsorbed on ZnO surface act as charge traps, decreasing the material conductivity. The incidence of UV-light causes an increase of the material conductivity due to the photogeneration of electron-hole pairs via direct band-to-band transitions (classic photoconductivity process) and due to the desorption of oxygen molecules, which presents a relatively slower response and is a less understood mechanism. In the current paper, we study the influence of environmental parameters, such as temperature, humidity and UV-light intensity, on the electrical properties of spin-coated ZnO thin films to understand the role of the desorption mechanism on the photoconductivity process. The analysis of the device current vs. time curves shows the existence of two light-induced desorption mechanisms: i) one which increases the electrical conductivity of the ZnO film (desorption-like process) and ii) a second one which decreases the conductivity (adsorption-like process). A Plackett-Burman design of experiment (DOE) was used to study the influence of characterization factors like UV intensity, temperature and humidity on electrical parameters obtained from the experimental curves. We observed that the desorption-like process is a first order mechanism, exhibiting desorption rate proportional to n(t), where n(t) represents the adsorbate concentration as a function of the time, whereas the adsorption-like mechanism exhibits a desorption rate proportional to the forth power of n(t).

In the present study, we analyze the influence of temperature and active layer thickness on the electrical properties of electroluminescent devices comprising a polymeric conductive blend (poly(3,4 ethylenedioxythiophene):polystyrene sulfonate, PEDOT:PSS), an inorganic electroluminescent material (manganese doped zinc orthosilicate, Zn2SiO4:Mn) and an organosilicon material (3-glicidoxypropyltrimethoxysilane, GPTMS), manufactured at different weight ratios of the component materials. The devices were obtained by depositing the active layer by drop-casting onto ITO-coated (RF-sputtering) glass substrates and thermally evaporating gold top electrodes in high vacuum. The results show that 90 wt% Zn2SiO4:Mn is required to observe high electroluminescence from the fabricated devices and that the optimum performance (turn-on voltage of 33 V, luminous efficacy of 24 cd/A and maximum luminance of almost 2000 cd/m2) was achieve for a (9.5/0.5/90) (GPTMS/PEDOT:PSS/Zn2SiO4:Mn) weight ratio. The device turn-on voltage found to be as proportional to the thickness of the active layer, indicating that the electroluminescence occurs by a field-effect mechanism. The temperature variation in the 100–300 K range allowed us to develop a theoretical model for the device operation, where the charge carrier transport in the active layer is well described by the variable range hopping model, with luminous efficacy nearby independent of the temperature.

In the current work, we evaluate the influence of the processing parameters on the electrical properties of aluminium zinc oxide (AZO) thin films produced by airbrush spray-pyrolysis deposition technique. Spray-deposited AZO thin-films were produced with Al:Zn molar ratios varying from 0 % (pure ZnO) up to 30 %, using aluminium acetate and zinc acetate as organic precursors and water as solvent. Thermogravimetric analysis (TGA) and infrared spectroscopy (FTIR-ATR) were used to monitor the metal-oxide formation from the organic precursors as a function of the temperature. The results show that a temperature of 400 °C is necessary to completely degrade the organic phase and to obtain the desired inorganic metal-oxides films. The electrical properties of the TMOs were evaluated by d.c. current-voltage (I-V) analysis using planar thermally evaporated Al electrodes on top of the TMO layer, with different aspect ratios (1/18, 2/9, 5/13, 5/9 and 8/9). The lowest sheet resistance was obtained for AZO films at a molar Al concentration of 5 %. We also observed that, after carrying out a post-annealing treatment (30 mbar, 150 °C) the samples presented a decrease on the sheet resistance superior to 60 %, in comparison to the samples before the treatment.

Metal oxides like zinc oxide (ZnO) are promising materials for the active layer of thin-film transistors (TFTs) used in the drive circuit of next-generation large-area active matrix displays due to their high electronic mobility, high transmittance in the optical visible range and processability. Traditional deposition techniques employ RF sputtering or pulsed-laser deposition (PLD), which are relatively sophisticated techniques. The deposition of very thin (less than 50 nm thick) layers of ZnO using soluble organic precursors have been extensively investigated recently as an alternative to traditional deposition methods. Solution-based deposition processes include simple and affordable techniques like dip-coating, spin-coating, spray-pyrolysis and ink-jet printing. Spray-pyrolysis is particularly interesting due to the high film uniformity, low cost and high device performance. We carried out several experiments analyzing the performance of ZnO based devices using zinc acetate as organic precursor to confirm that spray pyrolysis deposition is a suitable technique for production of high-performance and reproducible TFTs. Moreover, we observed that device performance can significantly vary with little modifications on the deposition parameters, even for the same active layer composition and pyrolysis temperature. Electrical parameters, as the electrical mobility and the on/off ratio, varied several orders of magnitude, whereas the threshold voltage varied up to 20 V for the tested devices. Deposition parameters as the nozzle height during the deposition, nozzle air pressure and deposition time were varied until we obtained devices with optimum electrical performance. Optimized devices presented mobilities in the order of 1 cm2.V-1.s-1, on/off ratio of about 106 and relatively low operation voltages. A statistical analysis of a great number of devices manufactured using the same deposition parameters was also carried out to assure the reproducibility of the deposition technique.

Solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) obtained via hydrolysis/pyrolysis of an organic precursor present an excellent technique to obtain high performance electronic devices with low manufacturing cost. In the current work, we propose the use of an alternative deposition method, based on a polymeric precursor route (known as Pechini), to obtain solution-processed ZnO compact films as the active layer of TFTs. The elimination of the organic phase and the formation of inorganic thin-films was carried out by thermal treatment at different temperatures (ranging from 200oC to 500oC) and at different times (from 5 min to 2 hours), being monitored by UV-vis and infrared (IR) optical absorption spectroscopy. It was observed that, for temperatures above 400oC and treatment times superior to 30 min, the organic phase was completely eliminated, remaining only the inorganic (metal oxide) phase. The optical bandgap of the resulting ZnO films, determined from UV-vis absorption, is about 3.4 eV. The electrical characteristics (output and transfer curves) of the obtained devices demonstrate the feasibility of Pecchini method to build solution-processed metal oxide TFTs. The results for the electrical mobility of the majority charge-carriers (electrons) and for the threshold voltage were 0.39 cm2.V-1.s-1 and 0.45 V, respectively.