| Transparent conductive oxide (TCO) thin film is indispensable in flat panel displays, solar cells and transparent optoelectronic devices due to its low resistivity and good optical transmittance. The most widely used TCO in present market is mainly tin-doped In2O3 (ITO). However, the large consumption of indium in increasingly expanding market, especially in liquid crystal displays (LCD) and solar cells, makes it high price and difficult to a stable supply. Simultaneously, one consideration of improving the energy conversion efficiency of solar energy in photovoltaic cells is how to effectively utilize the sun energy in the near infrared (NIR) region because the NIR accounts on 52% of the energy in the air-mass 1.5 global solar irradiance spectrum (300-2500 nm). Moreover, novel TCO and transparent oxide semiconductor (TOS) thin films with high optical and electrical properties are of very important in new transparent electronics.In this dissertation, novel amorphous and polycrystalline tungsten-doped tin oxide (SnO2:W) TCO thin films are developed in terms of above mentioned questions and requirements. Pulsed plasma deposition (PPD) method are used to prepare the SnO2:W thin films, the properties of the films and the dependence on various preparation parameters are systematically investigated. The preparation conditions of SnO2:W thin films with good optical and electrical properties are established. Also, SnO2:W thin films with smooth surface and much better transparency are successfully synthesized by sol-gel method, a much simple and much convenient technique apt to mass production and large area uniform coating. This implies the promising application of the films in solar cells. The possible reactive mechanisms in sol-gel process are proposed. The calculated results show that the bottom of conduction band of SnO2:W is mainly ascribed to W 5d states, which results in high conductive SnO2:W films. To study the transparent diode based on SnO2:W films, novel p-type lithium-doped nickel oxide (NiO:Li) thin films are developed, and NiO:Li/SnO2:W p-n junction is fabricated. It reveals the possibility of SnO2:W thin films in application of transparent electronics. The various measurements for all prepared samples, such as XRD, AFM, XPS, SEM, EDS, IR, Raman, stylus profilometer, four-point probe and Hall system, were applied to analyze and characterize the properties of the films. The main results are as follows:Amorphous SnO2:W thin films were deposited by PPD at (Ar+O2) mixed atmosphere, and the structural, electrical and optical properties have been investigated as functions of tungsten doping content and oxygen partial pressure. The lowest resistivity of 2.1×10-3 ohm·cm was reproducibly obtained, with carrier mobility of 30 cm2V-1s-1 and carrier concentration of 9.6×1019 cm-3 at the oxygen partial pressure of 1.8 Pa. The average optical transmission was above 80% in the visible region from 400 to 700 nm, with the optical band gap ranging from 3.91 to 4.02 eV. The root mean square (RMS) and average roughness were measured to be 15.7 nm and 11.9 nm respectively.Polycrystalline SnO2:W thin films were fabricated on quartz glass substrates by improved method:at pure Ar working gas using PPD with post-annealing. Doping of tungsten can effectively enhance the conductivity of the films while maintaining high transparency. The structural, electrical and optical properties of the films have been investigated with different annealing temperatures and tungsten-doped contents. The lowest resistivity of 6.7×10-4 ohm·cm was obtained, with carrier mobility of 65 cm2V-1s-1 and carrier concentration of 1.44×1020 cm-3 in 3 wt.% tungsten-doped films annealed at 800℃in air. The average optical transmittance is about 87% in the visible region from 400 to 700 nm, and about 90% in the near-infrared region from 700 to 2500 nm.XPS analysis shows that In SnO2:W thin film, the Sn 3d spectrum consists of a doublet with binding energies of 486.4 eV for Sn 3d5/2 and 494.9 eV for Sn 3d3/2, corresponding to the binding energies of the Sn4+ ion in SnO2. For the spectrum of W 4f doublet, the binding energies of 4f7/2 and 4f5/2 are 35.2 eV and 37.4 eV, respectively, which implies that tungsten atoms were fully oxidized to W6+ ions. From the XRD results, no additional peaks due to impurities phase were observed and this shows W substituted Sn lattice site and provide more electrons. This indicates that W doping can contribute more free carrier concentration than other dopant such as Sb when the doping concentration are equal. In other words, for the same carrier concentration there needs fewer dopant of W in SnO2:W than in other common doped oxides such as SnO2:Sb, In2O3:Sn or ZnO:Al, whose valence differences between the dopant and substituted ion are only 1. This may also be an influence factor in the process of obtaining high mobility because fewer dopant can largely reduce the ionized impurity scattering centers.Transparent conductive SnO2:W films were also synthesized on quartz glass substrates by sol-gel dip-coating method with the precursors SnCl2·2H2O and WCl6. It was found that the films were highly transparent and the average optical transmission was about 90% in the visible and near infrared region from 400 to 2500 nm. The lowest resistivity of 5.8×10-3 ohm·cm was obtained, with the carrier mobility of 14.2 cm2V-1s-1, and carrier concentration of 7.6×1019 cm-3. The structural properties, surface morphology and chemical states for the films were investigated. Although the electrical properties of films need further improve, sol-gel method is relatively cheap and easy to operate, and the prepared films possess excellently transparency and low average roughness of about 1.8 nm.XRD analysis reveals that the peak intensity of sol-gel prepared SnO2:W films became weak and even wide, makes the crystallinity of the films worse, as tungsten doping content increased. When tungsten content is large, amorphous particles appeare in the films, makes the roughness decrease. The average roughness of the SnO2:W films is 1.92 nm,1.87 nm,1.82 nm,1.61 nm respectively, corresponding to the tungsten doping content of 0,1,3,5 at.%.The calculated results showed that the valence bands of tin oxide were composed mainly of O 2p states and the conduction bands consisted mainly of Sn 5 s states for undoped films. However, the bottom of conduction band is changed with W doping. The bottom of conduction band of SnO2:W is due to W 5d states. W is potential doping elements which result in high conductive SnO2 films.The p-type conductive NiO:Li targets were prepared by solid reaction method. By using these targets, p-type conductive NiO:Li transparent thin films were developed on the glass substrates by PPD. The electrical, optical and structural properties were characterized by four-probe method, Hall system, spectrophotometer and AFM. The conductivity of 15 S·cm-1, with carrier mobility ranging from 0.30 cm2V-1s-1 to 0.42 cm2V-1s-1, carrier concentration from 2.98×1020 cm-3 to 2.1×1020 cm-3 film was obtained, while some literatures report the carrier concentration is about 1018,1019 orders of magnitude or lower.The p-n heterojunction diodes of NiO:Li/SnO2:W were fabricated, based on the experimental results of n-type SnO2:W and p-type NiO:Li thin films. The I-V characteristic curves of p-n junctions were analyzed through investigating on the conductivity of each single layer. The rectifying current-voltage characteristics were obtained. The whole diode shows an average visible transmission of about 40%.
|
PDF Full Text Request