Sol-gel derived zinc oxide films alloyed with cobalt and aluminium

ZnO films codoped with 5 at.% Co and 1 at.% Al were prepared by sol-gel technique on corning glass and silicon substrates with precursor sols of different pH values. The pH was varied from 5.4 to 11 by adding varying amounts of monoethanolamine to the sol. Since pH plays an important role in controlling the properties of films, we discuss in detail the effect of pH value on the structural, morphological and optical properties of the grown films. X-ray diffraction and atomic force microscopy images reveal that the size of crystallites increases with pH of the sol. The variation of pH in the reaction system influences the density of homogeneous nucleation and the crystal growth along the c-axis. High quality Co and Al codoped ZnO films annealed at 600 °C have been obtained using a sol with pH = 9. These sol-gel derived films find their suitability to be used as dilute magnetic semiconductors.     

One step sol-gel method for the formation of etch free texturing, anti-reflection coatings, and p-n junction for silicon solar cells

This study evaluated one step sol-gel process for the simultaneous formation of surface texturing without a series of chemical etching processes, as well as anti-reflection coatings without complicated vacuum processes and p-n junctions. The solutions for this process were prepared from a proper combination of precursors containing phosphorus and nano-size silicon dioxide particles. The specimen heat treated at 1000 °C showed the lowest reflectivity of 9% at a wavelength of 650 nm. In addition, the I-V test confirmed formation of a p-n junction. Therefore, the production of solar cells may be reduced to two steps, sol-gel and electrode printing.     

Effect of the interface on the electrical properties of an indium zinc oxide/SiOx multilayer

In this work, indium zinc oxide (IZO) films have been deposited on a polyethylene terephthalate substrate coated with an SiO_x film. Based on a comparative investigation of an IZO monolayer and an IZO/SiO_x multilayer, it is shown that oxygen has a great effect on the electrical properties of the thin films. A mechanism is described to explain the influence of the introduced SiO_x buffer layer. It is considered that an interfacial layer has come into being at the interface between the SiO_x layer and IZO layer, and the properties of this layer have been evaluated. Moreover, the electrical properties of the IZO/SiO_x multilayer have been successfully improved by controlling the oxygen content of the interfacial layer.     

Optical on-line monitoring for the long-term stabilization of a reactive mid-frequency sputtering process of Al-doped zinc oxide films

Closed-loop feedback control systems have shown to be able to stabilize optimum process conditions for reactively sputtered aluminum doped zinc oxide (ZnO:Al) films on glass, but the problem of long term drift has not been addressed so far. In the present work we describe an online control method which is able to detect process drifts and offers the possibility to adjust operation point settings in long-term operation. The control system is based on the evaluation of spectroscopic photometry measurements in the visible and near-infrared wavelength regime. The measured spectra are evaluated with respect to their band-gap and free carrier absorption.We found that the band gap and the plasma frequency are directly correlated with the oxygen partial pressure during deposition. Comparing the plasma frequency with Hall measurements, it was shown that the carrier concentration in the films can be monitored. This enables a control of free carrier absorption during a production process.     

Influence of assistant ion beam on the opto-electrical properties of molybdenum doped zinc oxide films deposited on polyethersulfone via dual ion beam sputtering

An alternative transparent conductive oxide, molybdenum doped zinc oxide (MZO) was deposited onto a flexible polyethersulfone (PES) substrate by using a dual ion beam sputtering system. One argon ion beam was used to sputter a MZO target and another assistant argon ion beam was for bombarding deposits simultaneously. The assistant ion source discharge voltage and current were changed respectively for investigating their influences on the conductivity of deposited MZO films. Changing the discharge voltage shows that, the film crystallinity, carrier concentration and mobility in films all increase with the discharge voltage and subsequently decrease when the applied voltage is over 100 V. Changing the discharge current also shows a similar trend. The film crystallinity and carrier concentration initially increase with the discharge current, and thereafter a minimum for 1.4 A, and a subsequent increase in resistivity is observed. According to the results, properly raising the discharge voltage and current of assistant ion source can improve both electrical conductivity and optical transparency of deposited MZO films, but the excess discharge voltage and current will cause the grain refinement which may retard the carrier mobility and result in the lower conductivity of MZO films.     

Low-emitting surfaces prepared by applying transparent aluminum-doped zinc oxide coatings via a sol-gel process

Polycrystalline transparent conductive oxide thin films based on aluminum-doped zinc oxide (AZO) have been prepared on soda-lime glass by using an inorganic sol-gel process and the dip-coating technique. The multilayered films were crystallized on the substrate and subsequently annealed in a reducing atmosphere to enhance the number of free charge carriers. Significant characteristics of the functional coatings, such as crystallinity, surface smoothness, layer thickness, electrical conductivity, as well as the optical properties in a large spectral range between 0.25 and 35 μm were analyzed. The obtained samples are highly transparent with a visual transmittance above 0.85 and show specific resistivities of up to 1.6·10^− 3 Ω·cm. Applying an optimized AZO-coating with at most 500 nm is sufficient to reduce the surface emittance from 0.89 to less than 0.45 in the infrared.     

Copper doped nickel oxide transparent p-type conductive thin films deposited by pulsed plasma deposition

Transparent p-type conductive Ni_0.9Cu_0.1O thin films were prepared by pulsed plasma deposition (PPD) method. The effects of substrate temperature and oxygen pressure on the structural, electrical and optical properties of the films were investigated respectively. The film deposited at room temperature exhibits the highest conductivity of 5.17 S cm⁻¹, with an average transmittance of 60% in the visible region. A transparent p-Ni_0.9Cu_0.1O/n-In₂O₃:W (IWO) hetero-junction diode was fabricated exhibiting rectifying current-voltage characteristics.     

Indium tin oxide films deposited on polyethylene naphthalate substrates by radio frequency magnetron sputtering

Indium tin oxide (ITO) thin films were deposited on unheated polyethylene naphthalate substrates by radio-frequency (rf) magnetron sputtering from an In₂O₃ (90 wt.%) containing SnO₂ (10 wt.%) target. We report the structural, electrical and optical properties of the ITO films as a function of rf power and deposition time. Low rf power values, in the range of 100-130 W, were employed in the deposition process to avoid damage to the plastic substrates by heating caused by the plasma. The films were analyzed by X-ray diffraction and optical transmission measurements. A Hall measurement system was used to measure the carrier concentration and electrical resistivity of the films by the Van der Pauw method. The X-ray diffraction measurements analysis showed that the ITO films are polycrystalline with the bixbite cubic crystalline phase. It is observed a change in the preferential crystalline orientation of the films from the (222) to the (400) crystalline orientation with increasing rf power or deposition time in the sputtering process. The optical transmission of the films was around 80% with electrical resistivity and sheet resistance down to 4.9 × 10^- 4 Ωcm and 14 Ω/sq, respectively.     

Aluminium doped zinc oxide sputtered from rotatable dual magnetrons for thin film silicon solar cells

This study addresses the electrical and optical properties as well as the surface structure after wet-chemical etching of mid-frequency magnetron sputtered aluminium doped zinc oxide (ZnO:Al) films on glass substrates from rotatable ceramic targets. Etching of an as-deposited ZnO:Al film in acid leads to rough surfaces with various feature sizes. The influence of working pressure and substrate temperature on the surface topography after etching was investigated. It was found that the growth model which Kluth et al. applied to films sputtered in radio frequency mode from planar ceramic target can be transferred to film growth from tube target. Furthermore, the influence of Ar gas flow and discharge power on the film properties was investigated. We achieved low resistivity of about 5.4 × 10^− 4 Ω·cm at high growth rates of 120 nm·m/min. Finally, surface textured ZnO:Al films were applied as substrates for microcrystalline silicon solar cells and high efficiencies of up to 8.49% were obtained.     

Thermal stability of indium tin oxide thin films co-sputtered with zinc oxide

The thermal stability of indium tin oxide (ITO) films and ITO co-sputtered with zinc oxide (ZnO) films at different zinc atomic ratios in various atmospheres are investigated. The resistivity of the annealed ITO films decreased with increased annealing temperatures. The improved electrical properties were attributed mainly to the increase in carrier concentration originating from the significant formation of oxygen vacancies in the ITO films. In contrast, due to the lower oxidation potential of zinc ions, the resistivity of the annealed co-sputtered films showed no significant reduction and an increase with annealing temperatures. The film decomposition due to the high degree outdiffusion of oxygen atoms and aggregation of In atoms observed from the metal-like In phase in the diffraction patterns was responsible for the drastic thermal degradation in the electrical and optical properties of the samples annealed at elevated temperatures in reducing gas atmosphere. In contrast, the superior thermal stability of the co-sputtered films, at an atomic ratio of 60% annealed in reducing gas atmospheres, was ascribed to the stable Zn₃In₂O₆ crystalline structure that appeared in the diffraction pattern. The absorption edge observed from the optical transmittance of these annealed films also showed evidence of carrier concentration evolution in various annealing atmospheres. The lower oxidation potential of the zinc atoms introduced into the ITO films was concluded to be efficient in compensating for the formation of oxygen vacancies resulting in the alleviated decomposition behavior during thermal annealing.     

Formation of zinc oxide film by boiling metallic zinc film in ultrapure water

A simple method for forming zinc oxide (ZnO) films has been discovered. Radio-frequency (rf) sputtered metallic zinc (Zn) film is boiled in ultrapure water at 368 K. The opaque Zn film changes into a transparent film. It is confirmed by transmission electron microscopy and X-ray diffraction that the transparent film is hexagonal ZnO. Optical and morphological properties of the ZnO film are discussed.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

High rate direct current magnetron sputtered and texture-etched zinc oxide films for silicon thin film solar cells

Aluminum-doped zinc oxide (AZO) films were prepared by in-line direct current (dc) magnetron sputtering on glass substrates. Four types of ceramic targets with 0.5 wt.% or 1 wt.% of aluminum oxide and different preparation methods, namely normal sintered, soft sintered and hot pressed, were employed. The influence of different target manufacturing processes, aluminum concentration and sputtering conditions on AZO films were investigated. Depending on the type of targets and deposition conditions, highly transparent films with low resistivity values in the range of 3.6-11 × 10^− 4 Ω cm were obtained. The etching behaviour in hydrochloric acid and the resulting light scattering properties of the AZO films were strongly influenced by the choice of the target and the deposition conditions. The most favourable films have been successfully applied in thin film solar cells with 1.1-μm microcrystalline silicon absorber layer leading to an initial efficiency of 7.8%.     

Capacitance-voltage characterization of silicon oxide and silicon nitride coatings as passivation layers for crystalline silicon solar cells and investigation of their stability against x-radiation

In this work capacitance-voltage measurements of three different dielectric layers, thermal silicon oxide, plasma enhanced chemical vapor deposited (PECVD) silicon oxide, and PECVD silicon nitride, on p-type silicon have been performed in order to obtain characteristics as the energy distribution of the interface trap density and the density of fixed charges. Spatially resolved capacitance-voltage, ellipsometry and lifetime measurements revealed the homogeneity of layer and passivation properties and their interrelation. Additionally lifetime measurements were used to evaluate x-radiation induced defects emerged during electron beam evaporation for sample metallization.     

Humid environment stability of low pressure chemical vapor deposited boron doped zinc oxide used as transparent electrodes in thin film silicon solar cells

The stability in humid environment of low pressure chemical vapor deposited boron doped zinc oxide (LPCVD ZnO:B) used as transparent conductive oxide in thin film silicon solar cells is investigated. Damp heat treatment (exposure to humid and hot atmosphere) induces a degradation of the electrical properties of unprotected LPCVD ZnO:B layers. By combining analyses of the electrical and optical properties of the films, we are able to attribute this behavior to an increase of electron grain boundary scattering. This is in contrast to the intragrain scattering mechanisms, which are not affected by damp heat exposure. The ZnO stability is enhanced for heavily doped films due to easier tunneling through potential barrier at grain boundaries.     

Properties of plasma enhanced chemical vapor deposited silicon nitride for the application in multicrystalline silicon solar cells

Hydrogenated films of silicon nitride (SiNx:H) were investigated by varying the deposition condition in plasma enhanced chemical vapor deposition (PECVD) reactor and annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in multicrystalline silicon solar cells. By varying the gas ratio (ammonia to silane), the silicon nitride films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, silicon wafer with the film deposited at 450 °C showed the best minority carrier lifetime. The film deposited with the gases ratio of 0.57 showed the best peak of carrier lifetime at the annealing temperature of 800 °C. The performance parameters of cells fabricated by varying co-firing peak temperature also showed the best values at 800 °C. The multicrystalline silicon (mc-Si) solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrate (125 mm × 125 mm) was found to have the conversion efficiency of 15%.     

High deposition rate aluminium-doped zinc oxide films with highly efficient light trapping for silicon thin film solar cells

Aluminium-doped zinc oxide films were deposited on glass substrates at high rates by reactive mid-frequency sputtering. The in-line sputter system allows oxygen influx along the middle and sides of a dual-cathode system. The effect of varying the oxygen flow from the sides on the electrical and optical properties together with the surface morphology after wet chemical etching was investigated. Increasing the amount of oxygen flow from the sides improved the resistivity profile of static prints and gave highly conductive and transparent films in dynamic deposition mode. The etched films developed rough surface textures with effective light scattering that could be controlled by the oxygen balance between the middle and sides. Optimally textured films were used as front contacts in 1 cm² single junction microcrystalline solar cells yielding an initial efficiency of 8.4%. The improvement in light trapping led to short circuit densities higher than that of the reference solar cells.     

Resistive switching behavior of (Zn1−xMgx)O films prepared by sol-gel processes

Highly c-axis oriented sol-gel (Zn_1 − xMg_x)O films were deposited on Pt/Ti/SiO2/Si substrates. Resistive switching behaviors with stable switching and high resistance ratio were demonstrated for the Pt/(Zn_0.9Mg_0.1)O/Pt stacks. The effect of the film thickness and the annealing temperature on resistive switching was discussed. Higher substitution of Mg for Zn results in higher resistance of (Zn_1 − xMg_x)O films, which is beneficial for resistive switching to occur at thinner film thickness. The mechanisms dominating the low and the high resistance states are Ohmic conduction and Poole-Frenkel emission, respectively. The resistance ratio varies from 140 to 1000, which is much higher than the value 25 reported recently for sol-gel (Zn_0.8Mg_0.2)O films. Films annealed at higher annealing temperatures possess higher resistance ratio.     

Surface textured molybdenum doped zinc oxide thin films prepared for thin film solar cells using pulsed direct current magnetron sputtering

In this study, we examined the effect of etching on the electrical properties, transmittance, and scattering of visible light in molybdenum doped zinc oxide, ZnO:Mo (MZO) thin films prepared by pulsed direct current magnetron sputtering. We used two different etching solutions - KOH and HCl - to alter the surface texture of the MZO thin film so that it could trap light. The experimental results showed that an MZO film with a minimum resistivity of about 8.9 × 10^− 4 Ω cm and visible light transitivity of greater than 80% can be obtained without heating at a Mo content of 1.77 wt.%, sputtering power of 100 W, working pressure of 0.4 Pa, pulsed frequency of 10 kHz, and film thickness of 500 nm. To consider the effect of resistivity and optical diffuse transmittance, we performed etching of an 800 nm thick MZO thin film with 0.5 wt.% HCl for 3-6 s at 300 K. Consequently, we obtained a resistivity of 1.74-2.75 × 10^− 3 Ω cm, total transmittance at visible light of 67%-73%, diffuse transmittance at visible light of 25.1%-28.4%, haze value of 0.34-0.42, and thin film surface crater diameters of 220-350 nm.     

Influence of dopant and polymeric matrix on Indium tin oxide/p-zinc phtalocyanine/n-Si hybrid solar cells

The present paper reports on the photovoltaic properties of zinc phthalocyanine (ZnPc) films with various dopants sandwiched between indium tin oxide (ITO) and n-type silicon substrate. The ZnPc films were realized by entrapping the molecules in a poly(methyl methacrylate) matrix upon stirring and heating the components in solution. For the ITO/p-ZnPc/n-Si sandwich structure solar cells with ZnPc doped with I₂ and without post-annealing, the measured short-circuit current density, open-circuit voltage, maximum output power, fill factor and efficiency were 28.8 mA/cm², 0.46 V, 5.55 mW/cm², 0.42 and 5.55%, respectively, under AM 1.5 illumination.