Musa Mohamed Zahidi

Performance of an Ultraviolet Photoconductive Sensor Using Well-Aligned Aluminium-Doped Zinc-Oxide Nanorod Arrays Annealed in an Air and Oxygen Environment

Ultraviolet (UV) photoconductive sensors were fabricated using an aluminium (Al)-doped zinc-oxide (ZnO) nanorod array with a diameter between 40 and 150 nm and thickness of approximately 1.1 µm. The nanorod arrays were prepared using a sonicated sol–gel immersion and annealed at 500 °C under different ambient conditions of air and oxygen. The annealing process induced the formation of nanoholes on the nanorod surfaces, which increased the nanorod surface area. The nanoholes existed in larger quantities on the nanorod surfaces annealed in air compared with the nanorods annealed in an oxygen environment. This condition reduced the rise and decay time constants of the air-annealed UV sensor. However, the sample annealed in an oxygen ambient shows the highest responsivity of 1.55 A/W for UV light (365 nm, 5 mW/cm2) under a 10 V bias mainly due to defect reduction and improvement in stoichiometric properties. To the best of our knowledge, a UV photoconductive sensor using this ZnO nanostructure has not yet been reported.

Effects of annealing environments on the solution-grown, aligned aluminium-doped zinc oxide nanorod-array-based ultraviolet photoconductive sensor

We have fabricated metal-semiconductor-metal- (MSM-) type ultraviolet (UV) photoconductive sensors using aluminium- (Al-) doped zinc oxide (ZnO) nanorod arrays that were annealed in different environments: air, oxygen, or a vacuum. The Al-doped ZnO nanorods had an average diameter of 60 nm with a thickness of approximately 600nm that included the seed layer (with thickness ∼200 nm). Our results show that the vacuum-annealed nanorod-array-based UV photoconductive sensor has the highest photocurrent value of 2.43 × 10-4 A. The high photocurrent is due to the high concentration of zinc (Zn) interstitials in the vacuum-annealed nanorod arrays. In contrast, the oxygen-annealing process applied to the Al-doped ZnO nanorod arrays produced highly sensitive UV photoconductive sensors, in which the sensitivity reached 55.6, due to the surface properties of the oxygen-annealed nanorods, which have a higher affinity for oxygen adsorption than the other samples and were thereby capable of reducing the sensors dark current. In addition, the sensor fabricated using the oxygen-annealed nanorod arrays had the lowest rise and decay time constants. Our result shows that the annealing environment greatly affects the surface condition and properties of the Al-doped ZnO nanorod arrays, which influences the performance of the UV photoconductive sensors.

Controllable Growth of Vertically Aligned Aluminum-Doped Zinc Oxide Nanorod Arrays by Sonicated Sol--Gel Immersion Method depending on Precursor Solution Volumes

Aluminium (Al)-doped zinc-oxide (ZnO) nanorod arrays have been successfully prepared using a novel and low-temperature sonicated sol–gel immersion method. The photoluminescence (PL) spectrum reveals the appearance of two emission peaks from the nanorod that are centred at 381 and 590 nm. The nanorod has a hexagonal structure with a flat-end facet, as observed using field-emission electron microscopy (FESEM). Interestingly, all samples have similar surface morphologies and diameter sizes of 40 to 150 nm after immersion in different precursor-solution volumes. The thickness-measurement results show that the thicknesses of the samples increase after immersion in higher precursor-solution volumes. We show for the first time that the growth of nanorod arrays along the c-axis can be controlled using different precursor volumes, and its growth mechanism is discussed. X-ray diffraction (XRD) spectra indicate that the prepared nanorods are ZnO with a hexagonal wurtzite structure that grows preferentially along the c-axis.

Thickness-Dependent Characteristics of Aluminium-Doped Zinc Oxide Nanorod-Array-Based, Ultraviolet Photoconductive Sensors

Aluminium (Al)-doped zinc oxide (ZnO) nanorod arrays were prepared on a seed-layer-coated glass substrate by a sonicated sol–gel immersion method. We have shown, for the first time, that the thickness of the nanorod arrays can be increased incrementally without greatly affecting the diameter of the nanorods, by increasing the number of immersions. The field-emission scanning electron micrographs and thickness measurements revealed that the nanorods had diameters within the range from 40 to 150 nm and thicknesses from 629 to 834 nm with immersion times ranging from 1 to 5 h. The photoluminescence (PL) spectra revealed that the ZnO nanorod quality was enhanced with long immersion times as shown by an improvement in the ratio of the UV peak intensity to the visible emission peak intensity, or IUV/Ivis. The thickness-dependent characteristic of Al-doped ZnO nanorod-array-based, UV photoconductive sensors was studied; minimising the thickness of the nanorod arrays was found to provide high responsivity and good performance. Our experiments showed that a decrease in the thickness of the nanorod arrays improved the responsivity and response time of the UV sensors, with a maximum responsivity of 2.13 A/W observed for a 629-nm-thick nanorod film.

ZnO Nanorod Arrays Synthesised Using Ultrasonic-Assisted Sol-Gel and Immersion Methods for Ultraviolet Photoconductive Sensor Applications

Zinc oxide (ZnO) nanomaterials have emerged as one of the most promising materials for electronic devices such as solar cells, light-emitting devices, transistors, and sensors. The diverse structures of ZnO nanomaterials produce unique, useful, and novel characteristics that are applicable for high-performance devices. The ZnO nanorod array is a beneficial structure that has become extremely important in many applications due to its porosity, large surface area, high electron mobility, and variety of feasible techniques. The chemistry and physical tuning of its surface state, including processes such as annealing and chemical treatments, enhance its functionality and sensitivity and consequently improve the device performance. These useful characteristics of ZnO nanorod arrays enable the fabrication of ultraviolet (UV) photoconductive sensors with high responsivity and reliability. Although there are many techniques available to synthesise the ZnO nanorod arrays, solution-based methods offer many advantages, including the capacity for low-temperature processing, large-scale deposition, low cost, and excellent ZnO crystalline properties. In this chapter, the synthesis of ZnO nanorod arrays via ultrasonic-assisted sol-gel and immersion methods will be discussed for application to UV photoconductive sensors. The optical, structural, and electrical properties of deposited ZnO nanorod arrays will be reviewed, and the performance of the synthesised ZnO nanorod array-based UV photoconductive sensors will be discussed.

Effects of Aluminium Doping and Electrode Distance on the Performance of Aligned Zinc Oxide Nanorod Array-Based Ultraviolet Photoconductive Sensors

Metal–semiconductor–metal (MSM) ultraviolet (UV) photoconductive sensors were fabricated using undoped and aluminium (Al)-doped zinc oxide (ZnO) nanorod arrays prepared via the sonicated sol–gel immersion method. Notably, the nanorod diameter decreased with Al-doping, varying in the size range of 30 to 70 nm, compared with undoped ZnO, which had a size range of 80–120 nm. The Al-doped nanorod arrays exhibited optical properties superior to those of the undoped ZnO, with an average transmittance of 85% in the visible region; the Al-doped arrays also showed good UV absorption properties. Photoluminescence measurements indicate that the suppression of defects was observed for the Al-doped ZnO nanorod, as shown by a bigger IUV/Ivis of 1.24 compared to that of undoped ZnO (0.97). According to the experimental results, the UV responsivity was significantly improved by the Al-doping, with the highest value of 373 A/W obtained at an inter-electrode distance of 0.07 mm under UV light (365 nm, 750 µW/cm2) with a 10 V bias. In addition, the responsivity of the UV sensor also significantly improved when the inter-electrode distances were reduced from 2.00 to 0.07 mm.

Influence of Cathode Work Functions on the Photovoltaic Properties of MEH-PPV: TiO2 Bulk Heterojunction Solar Cell

Bulk heterojunction solar cell has received significant attention over the past decade due to low cost power generation and the potential to develop a clean renewable energy source [. We investigated the effect of different type of metal cathodes on the power conversion efficiency of bulk heterojunction solar cell based on a blend of conjugated polymer poly [2-methoxy 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with titanium dioxide (TiO2). In this case of study, Aluminum (Al) and gold (Au) has been chosen as the metal cathode due to the difference of work function and their wide application in hybrid solar cell. We show that the choice of metal cathode plays a role in determining overall device efficiency through their impact on short-circuit current, open circuit voltage and fill factor due to the influence of work function. It is found that the device employing Al metal cathode which has low work function is showing a comparable performance to the Au metal electrode with fill factor of over 20 % and a power conversion efficiency of 3.3x10-3 %. Overall it is demonstrated that the matching between the work function of the cathode and photoactive layer MEH-PPV: TiO2 is the most important factor towards best bulk heterojunction solar cell performance.

Electrical and Optical Properties of Zinc Oxide Thin Films by Sol‐Gel Dip‐Coating Technique

ZnO thin films were fabricated on glass substrate by sol‐gel method. A stable and homogeneous solution was prepared by dissolving zinc acetate dehydrate as a starting material in a solution of 2‐methoxyethanol and monoethanolamine (MEA). The precursor concentration was selected as the parameter to optimize the thin films quality. The molar ratio of zinc acetate dehydrate was kept at 1. The thin films were preheated at 300° C for 10 minutes after each coating. After depositing all layers, the thin films are annealed for 1 hour in order to obtain transparent ZnO thin films. It was found that the precursor concentration affect the IV characteristic, transmittance, and the thickness of the resultant ZnO thin films.

Quantum Confinement of Integrated Pulse Electrochemical Etching of Porous Silicon for Metal Semiconductor Metal Photodetector

Porous silicon (PS) was successfully synthesized via novel integrated pulsed electrochemical etching of an n-type (100) silicon (Si) substrate under various condition. The PS was etched using hydrofluoric acid (HF) based solution and the porosity was optimized by introducing electroless chemical etching process prior to photo electrochemical (PEC) anodization. In the electroless etching, a delay time (TD) of 2 min was applied. After that a cycle time (T) and pause time () of pulsed current were supplied throughout the 30 min PEC etching process. As grown Si and PS through conventional direct current (DC) anodization were also included for comparison. The result obtained showed that applying delay time helps to improve the uniformity and density of the porous structures. AFM indicated that the roughness of the Si increases as the dissolution of the Si occurred. Raman spectroscopy showed that an improvement in the crystalline quality of PS under pulse etching method compared to DC method indicated by the reduction of full width at half maximum (FWHM). A broad visible photoluminescence (PL) was observed from green to red with blue shift as nanocrystallite size decreases which constituted quantum confinement effect from the PS structures. Nickel (Ni) finger contact was deposited onto the PS to form metal semiconductor metal (MSM) photodetector. Ni/PS MSM photodetector by pulse method exhibited higher gain (2 times) compared to conventional Si device at 5 V bias.

The fabrication of solid state dye-sensitised solar cells with I2 doped CuI as the hole conductors

The fabrication (ss-DSSC) were done by employing the iodine doped CuI (I2:CuI) as the hole conductors. The hole conductor deposition was done by varying the I2:CuI weight in order to investigate its effect to the device performance. The brick-like structure with smooth faces and sharp edges were seen for the doped thin films. The high electrical resistivity of I2:CuI thin films were observed compared to the undoped CuI thin films which is caused by the surface traps create by iodine doping. The ss-DSSC fabricated with undoped CuI hole transport layer shows the highest efficiency of 1.05% which is in a good agreement with the resistivity value of CuI thin films. The cell fabricated with 40 mg I2:CuI shows the lowest conversion efficiency of 0.45%. The crystals size of CuI and its degree of crystallisation are greatly affect the solar cells performance.