Investigation of doped cuprous halides for photovoltaic and display applications

The thesis mainly focuses on the growth and optoelectronic characterisation of the doped cuprous halides (CuX) with high UV/blue emission properties and the light harvesting in the CuBr/Si based heterojunction photovoltaic (PV) cells. Since cuprous halides are short wavelength emitters with high excitonic binding energies, growth of lower resistivity, highly luminescent p and n-type films are essential for the development of the future excitonic based light emitting devices with these materials. We describe the deposition and characterisation of the Zn doped n-type CuCl and oxygen doped p-type CuBr with higher carrier concentration using pulsed dc magnetron sputtering and thermal evaporation followed by oxygen plasma exposure, respectively. The structural and morphological properties of the n-type Zn doped CuCl films are investigated using XRD, SEM and AFM measurements. Our findings show that, the crystallinity of CuCl increases with doping of Zn and the maximum is obtained for the 3 % doped film, beyond which crystallinity decreases. The Zn doping has no deleterious impact on the structural and luminescent properties of CuCl up to a doping percentage of 5. An order of magnitude reduction in the resistivity of the CuCl films is obtained as a result of Zn doping. The resistivity, n-type carrier concentration and carrier mobility corresponding to the 3 % Zn doped films were, respectively 6 Ωcm, ~ 9.8×1018 cm-3 and 0.1 cm2V-1s-1. The influence of Zn doping on the electronic structure of CuCl is investigated using photoemission spectroscopic studies. Our studies on the Cu and Cl core level spectra show the presence of trace amounts of Cu2+ species in the undoped CuCl sample along with the major Cu+ species. This is verified by the observation of the satellite and shoulder peaks in the Cu 2p core level and a higher binding energy tail in the Cl 2p core level spectra. The disappearance of the cupric species with doping of Zn (3 %) is also observed. Furthermore, the shifting of the valence band towards the higher binding energy confirms the filling of the conduction band owing to the Zn doping, which explains the improvement of the conductivity of the doped films. vii We have successfully deposited p-type CuBr films by doping of oxygen. Our findings show that, there is no significant influence on the structural properties of the CuBr up to an oxygen plasma exposure time of 5 min. The investigation of the optical properties confirms good luminescence of the CuBr films up to a plasma exposure time of 3 min. The electrical characterisations of the oxygen plasma exposed films reveal that, oxygen act as a good acceptor for CuBr. The resistivity of the 3 min oxygen plasma exposed samples reduces to the order of ~ 1 Ωcm. SIMS analysis shows that, the diffusion of oxygen is quite good in the CuBr films. The realization of the heterojunction PV cell based on p-CuBr/n-Si is investigated. The I-V characteristics confirm the rectification behaviour of the p-n diode with a turn on voltage of ~ 1 V. The photoresponse properties of the heterojunction are studied by measuring the I-V characteristics under illumination. The photogenerated carrier formation is confirmed by the increased reverse current under illumination of the heterojunction. The wavelength dependence of the photo current was also studied using LED illumination. Development of ultrathin transparent Cr contacts, useful for the electroluminescent device fabrication was also discussed. This can be used as an alternative to well-known Indium Tin Oxide films for the future fabrication of the CuX-based display devices

Highly transparent and reproducible nanocrystalline ZnO and AZO thin films grown by room temperature pulsed-laser deposition on flexible zeonor plastic substrates

Zeonor plastics are highly versatile due to exceptional optical and mechanical properties which make them the choice material in many novel applications. For potential use in flexible transparent optoelectronic applications, we have investigated Zeonor plastics as flexible substrates for the deposition of highly transparent ZnO and AZO thin films. Films were prepared by pulsed laser deposition at room temperature in oxygen ambient pressures of 75, 150 and 300 mTorr. The growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of as grown films with thicknesses∼65–420 nm were recorded for the three oxygen pressures. The growth rates were found to be highly linear both as a function of film thickness and oxygen pressure, indicating high reproducibility. All the films were optically smooth, hydrophobic and nanostructured with lateral grain shapes of∼150 nm wide. This was found compatible with the deposition of condensed nanoclusters, formed in the ablation plume, on a cold and amorphous substrate. Films were nanocrystalline (wurtzite structure), c-axis oriented, with average crystallite size∼22 nm for ZnO and∼16 nm for AZO. In-plane compressive stress values of 2–3 GPa for ZnO films and 0.5 GPa forAZO films were found. Films also displayed high transmission greater than 95% in some cases, in the 400–800 nmwavelength range. The low temperature photoluminescence spectra of all the ZnO and AZO films showed intense near band edge emission. A considerable spread from semi-insulating to n-type conductive was observed for the films, with resistivity∼103 Ω cm and Hall mobility in 4–14 cm2 V−1 s−1 range, showing marked dependences on film thickness and oxygen pressure. Applications in the fields of microfluidic devices and flexible electronics for these ZnO and AZO films are suggested

Zn doped nanocrystalline CuCl thin films for optoelctronic applications

We report on the use of Zn as an n-type dopant in CuCl thin films for optoelectronic applications, wherein maximum n-type doping of the order of 1018 cm -3 has been achieved. Zn doped nanocrystalline CuCl thin films are successfully deposited on glass and Si substrates by pulsed dc magnetron sputtering. Structural and morphological properties are investigated using X-ray diffraction (XRD) studies and Scanning Electron Microscopy (SEM), respectively. The conductivity of the CuCl:Zn films is examined using the four point probe technique. An order of magnitude increase in the conductivity of CuCl, by the doping with Zn is reported herein. The doped CuCl films display strong room temperature cathodoluminescence (CL) at ~ 385nm, which is similar to that of the undoped films. Hall Effect measurements show an n-type conductivity of the doped films

Crystalline ZnO/amorphous ZnO core/shell nanorods: self-organized growth, structure, and novel luminescence

We have used pulsed-laser deposition, following a specific sequence of heating and cooling phases, to grow ZnO nanorods on ZnO buffer/Si (100) substrates, in a 600 mT oxygen ambient, without catalyst. In these conditions, the nanorods preferentially self-organize in the form of vertically aligned, core/shell structures. X-ray diffraction analyses, obtained from 2θ-ω and pole figure scans, shows a crystalline (wurtzite) ZnO deposit with uniform c-axis orientation normal to the substrate. Field emission SEM, TEM, HR-TEM and selective area electron diffraction (SAED) studies revealed that the nanorods have a crystalline core and an amorphous shell. The low-temperature (13 K) photoluminescence featured a strong I6 (3.36 eV) line emission, structured green band emission and a hitherto unreported broad emission at 3.331 eV. Further studies on the 3.331 eV band showed the involvement of deeply-bound excitonic constituents in a single electron-hole recombination. The body of structural data suggests that the 3.331 eV emission can be linked to the range of defects associated with the unique crystalline ZnO/amorphous ZnO core/shell structure of the nanorods. The relevance of the work is discussed in the context of the current production methods of core/shell nanorods and their domains of application

Low temperature growth technique for nanocrystalline cuprous oxide thin films using microwave plasma oxidation of copper

We report on the direct formation of phase pure nanocrystalline cuprous oxide (Cu2O) film with band gap ~ 2 eV by microwave plasma oxidation of pulsed dc magnetron sputtered Cu films and the highly controlled oxidation of Cu in to Cu2O and CuO phases by controlling the plasma exposure time. The structural, morphological and optoelectronic properties of the films were investigated. p-type Cu2O film with a grain size ~20-30 nm, resistivity of ~66 Ω cm and a hole concentration of ~2×1017 cm-3 is obtained for a plasma exposure time of 10 min without using any foreign dopants. The optical absorption coefficient (~105 cm-1) of the Cu2O film is also reported

Crystallographic characterisation of ultra-thin, or amorphous transparent conducting oxides:the case for raman spectroscopy.

The electronic and optical properties of transparent conducting oxides (TCOs) are closely linked to their crystallographic structure on a macroscopic (grain sizes) and microscopic (bond structure) level. With the increasing drive towards using reduced film thicknesses in devices and growing interest in amorphous TCOs such as n-type InGaZnO 4 (IGZO), ZnSnO 3 (ZTO), p-type Cu x CrO 2 , or ZnRh 2 O 4 , the task of gaining in-depth knowledge on their crystal structure by conventional X-ray diffraction-based measurements are becoming increasingly difficult. We demonstrate the use of a focal shift based background subtraction technique for Raman spectroscopy specifically developed for the case of transparent thin films on amorphous substrates. Using this technique we demonstrate, for a variety of TCOs CuO, a-ZTO, ZnO:Al), how changes in local vibrational modes reflect changes in the composition of the TCO and consequently their electronic properties

Femtosecond laser assisted crystallization of gold thin films.

We propose a novel low temperature annealing method for selective crystallization of gold thin films. Our method is based on a non-melt process using highly overlapped ultrashort laser pulses at a fluence below the damage threshold. Three different wavelengths of a femtosecond laser with the fundamental (1030 nm), second (515 nm) and third (343 nm) harmonic are used to crystallize 18-nm and 39-nm thick room temperature deposited gold thin films on a quartz substrate. Comparison of laser wavelengths confirms that improvements in electrical conductivity up to 40% are achievable for 18-nm gold film when treated with the 515-nm laser, and the 343-nm laser was found to be more effective in crystallizing 39-nm gold films with 29% improvement in the crystallinity. A two-temperature model provides an insight into ultrashort laser interactions with gold thin films and predicts that applied fluence was insufficient to cause melting of gold films. The simulation results suggest that non-equilibrium energy transfer between electrons and lattice leads to a solid-state and melt-free crystallization process. The proposed low fluence femtosecond laser processing method offers a possible solution for a melt-free thin film crystallization for wide industrial applications

Ultrashort laser sintering of printed silver nanoparticles on thin, flexible, and porous substrates

The fabrication of low-cost and mechanically robust flexible electronic patterns has increasingly gained attention due to their growing applications in flexible displays, touch screen panels, medical devices, and solar cells. Such applications require cost-effective deposition of metals in a well-controlled manner potentially using nanoparticles (NPs). The presence of solvent and precursors in NP based inks impacts the electrical conductivity of the printed pattern and a post-processing heating step is typically performed to restore the electrical properties and structure of the material. We report printing with picolitre droplet volumes of silver (Ag) NPs on flexible substrates using an acoustic microdroplet dispenser. The low-cost, controlled deposition of Ag ink is performed at room temperature on photopaper, polyimide and clear polyimide substrates. A localized, ultrashort pulsed laser with minimal heat affected zone is employed to sinter printed Ag patterns. For comparison, oven sintering is performed, and the results are analysed with scanning electron microscopy, four-point probe and Hall measurements. The femtosecond laser sintering revealed highly organized, connected nanostructure that is not achievable with oven heating. A significant decrease in sheet resistance, up to 93% in Ag NPs on clear polyimide confirms the laser sintering improves the connectivity of the printed film and as a result, the electrical properties are enhanced. The surface morphology attained by the laser sintering process is interpreted to be due to a joining of NPs as a result of a solid-state diffusion process in the near surface region of NPs

Pulsed plasma physical vapour deposition approach towards the facile synthesis of multilayer and monolayer graphene for anticoagulation applications

We demonstrate the growth of multilayer and single layer graphene on copper foil using bipolar pulsed direct current (DC) magnetron sputtering of a graphite target in pure Ar atmosphere. Single layer and few layer graphene films (SG and FLG) are deposited at temperatures ranging from 700-920 °C in less than 30 minutes. We find that the deposition and post-deposition annealing temperatures influence the layer thickness and quality of the graphene films formed. The films were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), High resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and optical transmission spectroscopy techniques. Based on the above studies, a diffusion controlled mechanism was proposed for the graphene growth. A single step whole blood assay was used to investigate the anticoagulant activity of graphene surfaces. Platelet adhesion, activation and morphological changes on the graphene/glass surfaces compared to bare glass were analysed using fluorescence microscopy and SEM techniques. We have found significant suppression of the platelet adhesion, activation and aggregation on the graphene covered surfaces compared to the bare glass, indicating the anticoagulant activity of the deposited graphene films. Our production technique represents an industrially relevant method for the growth of single and few layer graphene for various applications including the biomedical field

Effect of non-thermal plasma technology on microbial inactivation and total phenolic content of model liquid food and black pepper grains

pre-printThe objectives of this study were to investigate the effects of cold plasma technology on the growth and survival rates of vegetative cells and spores, and total phenolic content of black pepper grains. Plasma treatment was carried out using a non-thermal plasma jet system operating at 20 kHz using atmospheric air at a flow of 11 L/min. Two matrices were used, a model liquid food system and black pepper grains, both inoculated with Bacillus subtilis vegetative cells and spores. The samples were treated at 15 and 30 kV for 3–20 min. The plate count method was used to observe the colony-forming units at selected storage times i.e. at 1, 24 and 48 h post treatment at 4 °C. The highest log reduction was observed at 24 h post treatment, i.e. 2.92 log reduction. A 1 log reduction was achieved in the case of black pepper inoculated with spores for all selected storage times. No significant differences in total phenolic content were observed between treated and non-treated samples (p > 0.05). Optical emission spectroscopy was used to detect reactive species which could be responsible for cell death. Atomic oxygen, atomic nitrogen, hydroxyl radicals, nitrite oxide and nitrate were detected in light emitted from the plasma. Cell membrane damage caused by non-thermal plasma technology was observed using scanning electron microscopy. This study concludes that cold plasma technology has potential for industry application in food processing to reduce microbial loads in dried foods with limited impacts on food quality