Al-doped ZnO Electrodes Research Articles

Low-voltage, self-powered and broadband photodetector with Ohmic, transparent and cost-effective AZO electrodes on vertical aligned MoS2 flakes

The transparent Ohmic electrodes are the extreme necessity to develop cutting-edge broadband photodetectors. Conventional metal electrodes reflect incident light and diminish overall performance of photodetector. Here, we report cost-effective, transparent Ohmic Al-doped ZnO (AZO) electrodes to develop self-powered, highly responsive photodetectors from visible to near-infrared regions using vertically-aligned MoS2 (VA- MoS2) flakes. The MoS2 photodetector with AZO electrode (AZO-MoS2) shows a high responsivity of 40.67 A/W at 800 nm at low biasing voltage of 0.5 V. The self-driven behavior is observed in a broad range from 400 to 1100 nm with high responsivity of 38.18 mA/W at 800 nm. Moreover, AZO-MoS2 device has fast rise and decay time. To understand impact of Ohmic AZO electrode in comparison to traditional metal electrodes, we have also investigated conventional Au-MoS2 PDs fabricated under identical dimensions. The AZO-MoS2 PD demonstrated 100 times higher responsivity than conventional Au-MoS2 PD at 800 nm. We have investigated band-alignment between AZO and MoS2 using ultraviolet spectroscopy (UPS) and confirmed ohmic nature between the AZO and MoS2. These results indicate superior performance of AZO-MoS2 PD device in comparison to traditional metal electrodes.

Investigation on Transparent, Conductive ZnO:Al Films Deposited by Atomic Layer Deposition Process.

Transparent electrodes are a core component for transparent electron devices, photoelectric devices, and advanced displays. In this work, we fabricate fully-transparent, highly-conductive Al-doped ZnO (AZO) films using an atomic layer deposition (ALD) system method of repeatedly stacking ZnO and Al2O3 layers. The influences of Al cycle ratio (0, 2, 3, and 4%) on optical property, conductivity, crystallinity, surface morphology, and material components of the AZO films are examined, and current conduction mechanisms of the AZO films are analyzed. We found that Al doping increases electron concentration and optical bandgap width, allowing the AZO films to excellently combine low resistivity with high transmittance. Besides, Al doping induces preferred-growth-orientation transition from (002) to (100), which improves surface property and enhances current conduction across the AZO films. Interestingly, the AZO films with an Al cycle ratio of 3% show preferable film properties. Transparent ZnO thin film transistors (TFTs) with AZO electrodes are fabricated, and the ZnO TFTs exhibit superior transparency and high performance. This work accelerates the practical application of the ALD process in fabricating transparent electrodes.

Light‐Assisted Resistive Switching Memory Device with Reduced SET/RESET Voltage Using Sol–Gel TiO2 on Al‐Doped ZnO Electrode

Herein, light‐assisted resistive switching (RS) characteristic of low‐cost sol–gel TiO2 film‐based metal–insulator–metal (MIM) configuration using Al‐doped ZnO (AZO) as the bottom electrode is demonstrated. Unprecedentedly, instead of the costly transparent conducting electrode such as tin‐doped indium oxide, AZO electrode is exploited in this study due to its dual role as a transparent and an additional oxygen vacancy (VO)‐rich electrode. The Al/TiO2/AZO device shows electroforming free bipolar RS characteristics, which can be explained by the formation (rupture) of conducting filaments composed of VO. The memory window is increased three times under the blue light illumination compared to dark conditions. Interestingly, the SET and RESET voltages are decreased from 5.2 to 4.5 V and −3.9 to −3.3 V, respectively, under the blue light illumination with respect to the dark condition. The enhancement in the RS effect under the blue light illumination is explained in terms of the photoresponsive properties of TiO2. The MIM device exhibits good switching repeatability with stable retention characteristics, making them attractive for next‐generation nonvolatile resistive random access memory devices.

Application of Atomic Layer Deposition for Fabrication of Solar Cells

Atomic layer deposition (ALD) is exceptional deposition technique because it allows for growth of high quality films on large are substrates at low deposition temperatures. In our contribution ALD was employed for preparation of transparent conducting electrodes and for encapsulation of fabricated solar cells. Al-doped ZnO films were used for transparent conducting electrodes. Deposition was carried out at temperatures between 150 and 250 °C on Si and quartz substrates. Diethyl zinc and trimethyl aluminum were used as precursors and water vapors as reactant. Al-doping was performed by inserting Al2O3 cycles in ZnO growth. Deposition of 1 Al2O3 layer per 7 deposited ZnO layers gave the best results. Resistivity of the Al-doped ZnO films depended on the deposition temperature and film thickness. Transition electron microscopy revealed typical columnar growth of the films with fine grained polycrystalline region close to the substrate. The films with the thickness above 100 nm prepared at 250 °C showed resistivity of 1 mΩcm. This corresponds to the sheet resistance of 40 Ω/square. Hall measurement revealed electron concentration of 4*1020 cm-3 and mobility of 15 cm2Vs-1 for the films with resistivity of 1 mΩcm. Work function of 4.4 eV was measured by Kelvin probe. Optical transmittance of the films was higher than 80% in the wavelength range between 400 and 900 nm. Al-doped ZnO transparent electrodes were used for fabrication of perovskite-type conventional planar solar cell. CH3NH3 PbI3 perovskite was used as absorbing layer, Spiro-OMeTAD as hole transport layer and Au/Ag as top contact layer. Properties of the solar cell with Al-doped ZnO electrode are compared with inverted planar solar cell using indium tin oxide as a transparent electrode. Inorganic-organic hybrid perovskite solar cells suffer from poor stability in ambient atmosphere. Oxygen and moisture are believed to be reason for the solar cell degradation. To protect the cells against ambient atmosphere influence ALD prepared Al2O3 films were employed for encapsulation. Trimethyl aluminum was used as precursor while either water vapors or ozone were applied as reactants. Deposition of the Al2O3 films took place at 50°C. Power conversion efficiency of the solar cell encapsulated by 24 nm Al2O3 thin film deposited using water vapors as reactant decreased to 60% while unprotected solar cell degraded down to 10% of its initial value after 20 days in ambient atmosphere. In our contribution we compare properties of perovskite solar cell encapsulated by ALD Al2O3 films prepared using water vapors or ozone as reactants.

Dye-sensitized solar cells based on Al-doped ZnO photoelectrodes sensitized with rhodamine

Dye sensitized solar cell (DSSC) study of rhodamine 6G dye has been carried out by photocurrent-voltage (I-V) and electrochemical impedance spectroscopy (EIS) characterizations based on Al doped ZnO (AZO) electrodes. The Al concentration was varied from 1 to 5 at.% for Al doped ZnO. The absorption spectrum of the dye shows remarkable absorption in the range of 538–650 nm indicating its possible use as a photosensitizer for ZnO. The I-V characteristics measured in the dark and under 80 W simulated illumination revealed enhanced DSSC performance for the AZO electrode with 2 at.% Al concentration. The electrochemical impedance spectra of Al doped ZnO samples was in good agreement with earlier results. This is a clear indication that rhodamine dye-sensitized AZO electrodes are superior to unsensitized ones in DSSC application.

Al-doped ZnO/Ag-nanowire Composite Electrodes for Flexible 3-Dimensional Nanowire Solar Cells

Silver nanowires in conjunction with sputter-coated Al-doped ZnO (AZO) thin films were used as a composite transparent top electrode for hybrid radial-junction ZnO nanowire/a-Si:H p-i-n thin-film solar cells. Solar cells with the composite nanowire top contacts attained a short-circuit current density (Jsc) of 13.9 mA/cm2 and a fill factor (FF) of 62% on glass substrates while a Jsc of 13.0 mA/cm2 and FF of 62% was achieved on plastic substrates. The power conversion efficiency (PCE) of the 3-dimensional solar cells improved by up to 60% compared to using AZO electrodes alone due to enhanced coverage of the top electrode over the 3-D structures, decreasing the series resistance of the device by 5×. The composite layer also showed a 10× reduction in sheet resistance compared to the AZO thin-film contact under applied mechanical strain.

Inverted structural quantum dot light-emitting diodes based on Al-doped ZnO electrode

As an indium-free transparent conducting film, Al-doped zinc oxide (AZO) was prepared by magnetron sputtering technique, exhibiting good electrical, optical and surface characteristics. UPS/XPS measurements show that AZO and zinc oxide nanoparticles (ZnO NPs) have matched energy level that can facilitate the electron injection from AZO to ZnO NPs. Inverted structural green quantum dot light-emitting diodes based on AZO cathode were fabricated, which exhibits a maximum luminance up to 178 000 cd m−2, and a maximum current efficiency of 10.1 cd A−1. Therewith, combined with the simulated space-charge-limited current (SCLC) theory, the measured current density–voltage characteristics of charge-only devices were analyzed. It demonstrated that AZO and ZnO NPs had much better electron injection efficiency than ITO, showing a electron injection efficiency close to 100%. By studying the relationship between the electric field and the current density, the measured curve of AZO-based devices nearly fits the theoretical curve of SCLC and the AZO electrode has a better ohmic contact with ZnO NPs than ITO.

Enhanced Performance in Al-Doped ZnO Based Transparent Flexible Transparent Thin-Film Transistors Due to Oxygen Vacancy in ZnO Film with Zn-Al-O Interfaces Fabricated by Atomic Layer Deposition.

Highly conductive and optical transparent Al-doped ZnO (AZO) thin film composed of ZnO with a Zn-Al-O interface was fabricated by thermal atomic layer deposition (ALD) method. The as-prepared AZO thin film exhibits excellent electrical and optical properties with high stability and compatibility with temperature-sensitive flexible photoelectronic devices; film resistivity is as low as 5.7 × 10-4 Ω·cm, the carrier concentration is high up to 2.2 × 1021 cm-3. optical transparency is greater than 80% in a visible range, and the growth temperature is below 150 °C on the PEN substrate. Compared with the conventional AZO film containing by a ZnO-Al2O3 interface, we propose that the underlying mechanism of the enhanced electrical conductivity for the current AZO thin film is attributed to the oxygen vacancies deficiency derived from the free competitive growth mode of Zn-O and Al-O bonds in the Zn-Al-O interface. The flexible transparent transistor based on this AZO electrode exhibits a favorable threshold voltage and Ion/Ioff ratio, showing promising for use in high-resolution, fully transparent, and flexible display applications.

Flexible High Performance Hybrid Azo/Ag-Nanowire/Azo Sandwich Structured Transparent Conductors for Flexible Cu(In,Ga)Se2 Solar Cell Applications

Transparent conducting oxides (TCOs) are an important class of materials used in a wide range of optoelectronic technologies, including solar cells, flat-panel displays and organic light emitting diodes, and presently there is a strong drive toward the development of solar cell technologies on flexible platforms that are robust, portable and flexible enough to conform to unusual shapes, particularly building structures. Tin-doped indium oxide (ITO) is currently the commonly chosen TCO for use in non-flexible applications; however, its brittleness limits its application on flexible substrates, whilst the scarcity of indium maintains an undesirably high cost for use in solar cell applications. As such, it is important that ITO alternatives are developed with suitably flexible mechanical and electronic properties, which can also be fabricated at reasonable costs. The chalcopyrite absorber, copper indium gallium selenide, Cu(In,Ga)Se2, has a very high absorption coefficient (~104 cm-1), requiring only thin-films (~1 μm) for efficient solar cell fabrication, making it thin enough for flexible applications. However the lack of a high quality flexible TCO has, to date, limited its use in such applications. In this work, we present a new approach for the fabrication of high performance transparent electrodes suitable for flexible CIGS solar cell applications, using a stacked Al-doped ZnO/Ag-nanowire/Al-doped ZnO (AAA) sandwich structure.1 In our AAA electrode the Al-doped ZnO (AZO) layers act as a broad area charge carrier collector and protective encapsulant for the Ag-nanowire network. Concurrently, the Ag-nanowires provide low-resistance, long-range pathways that aid the extraction of charge carriers, but more importantly, they aid in maintaining a low resistivity for our total electrode structure, by maintaining charge transport pathways across cracks that develop in the AZO during repeated strenuous bending. By optimising of our Ag-nanowire network density, AZO deposition and AAA stack structure we are able produce a transparent electrode with comparable resistivity and transparency performance to that of ITO. These combined benefits enable us to fabricate CIGS solar cells that are able to maintain 95% of their initial power conversion efficiency, following 1000 bending cycles. In comparison, devices fabricated using equivalent thickness AZO and ITO electrodes are only able to maintain 57 and 5%, respectively, due to crack formation and delamination of the films. This AAA sandwich structure electrode could therefore serve as a high-performance electrode for numerous flexible optoelectronic applications. 1. W.-C. Tsai, S. R. Thomas, C.-H. Hsu, Y.-C. Huang, J.-Y. Tseng, T.-T. Wu, C.-h. Chang, Z. M. Wang, J.-M. Shieh, C.-H. Shen and Y.-L. Chueh, Journal of Materials Chemistry A, 2016.

Enhance the light-harvesting capability of the ITO-free inverted small molecule solar cell by ZnO nanorods.

The ITO-free inverted SMPV1:PC71BM solar cells with an Al doped ZnO (AZO) transparent electrodes are fabricated. The AZO thin film prepared by pulsed laser deposition (PLD) technique exhibits high transmission (>85%) and low sheet resistance (~30 Ω/sq) and the power conversion efficiency (PCE) of devices based on AZO electrode can reach around 4%. To further enhance the light harvesting of the absorption layer of solar cells, ZnO nanorods interlayer is grown on the AZO layer before the deposition the active layer. The absorption spectrums of devices under various conditions are also simulated by RCWA method to identify the optical saturation length of the ZnO nanorods. The PCE of ITO-free inverted small molecule solar cell improved with ZnO nanorods can reach 6.6%.

Effect of Al-doped ZnO or Sn-doped In2O3 electrode on ferroelectric properties of (Pb,La)(Zr,Ti)O3 capacitors

Using chemical solution deposition, we fabricated ferroelectric (Pb,La)(Zr,Ti)O3 (PLZT) capacitors with either Al-doped ZnO (AZO) or Sn-doped In2O3 (ITO) top electrodes. Then, the effects of a thin conductive AZO or ITO buffer layer between the Pt bottom electrode and PLZT thin film were investigated in combination with an AZO or ITO top electrode (AZO/PLZT/AZO/Pt and ITO/PLZT/ITO/Pt). The H2 degradation resistance of the AZO/PLZT/AZO/Pt and ITO/PLZT/ITO/Pt capacitors was improved. For AZO/PLZT/AZO/Pt capacitors with 10- and 20-nm-thick buffer layers, the H2 degradation resistance was 91 and 81%, respectively, compared with 42–70% without a buffer layer. The H2 degradation resistance of ITO/PLZT/ITO/Pt capacitors with a 28-nm-thick buffer layer was improved to 85% from 60–76% without a buffer layer. The time-of-flight secondary ion mass spectrometry depth profile indicated that AZO is the better H2 barrier after forming gas (3% H2/balance N2) annealing.

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.

Indium-free transparent organic light emitting diodes with Al doped ZnO electrodes grown by atomic layer and pulsed laser deposition

We present highly efficient transparent organic light emitting diodes (OLEDs) with Al doped ZnO (AZO) electrodes prepared by atomic layer deposition and pulsed laser deposition (PLD). The power and current efficiencies exceed 27 lm/W and 44 cd/A at a brightness level of 100 cd/m2, respectively. At the same time, the transmissivity of the devices is above 73% in the visible part of the spectrum. Owing to an efficient WO3 buffer layer and an optimized PLD process for the deposition of the top AZO electrode, the OLEDs show leakage current densities as low as 3×10−5 mA/cm2 at a reverse bias of 6 V. Therefore, our study paves the way for indium-free, see-through OLED displays.

Aligning Capabilities for Vatical-Alignment (VA) Liquid Crystal Display using AZO Film

We have investigated liquid crystal (LC) aligning capabilities and electro-optical (EO) characteristics of transparent electrodes as Al-doped ZnO (AZO) substituting indium tin oxide (ITO). The experiment results show that a uniform vertical LC alignment on AZO electrodes based on a rubbed polyimide (PI) surface were achieved. A high pretilt angle of about <TEX>$88^{\circ}$</TEX> was obtained. EO performances of the VA cell on rubbed PI surfaces with AZO electrodes are almost the same as that of the VA cell with ITO electrodes. These results appeared that AZO electrodes as transparent electrodes of LCDs could substitute ITO electrodes.