ICMCTF2011 Session C2/F4-2: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Transparent conducting oxides (TCOs) are playing an important role in today's opto-electronic devices. However in recent years, there has been an increasing demand for high mobility TCO to satisfy the need of next generation technologies such as solar cells and low emissivity windows coating. The carrier mobilities of conventional single material metal oxides are physically limited by various scattering mechanisms. While most of these scattering events can be overcome by careful tailoring of the microstructure, the main limitation in the mobility of TCO is due to ionized impurity scattering. This scattering arises when charge carriers are deflected by the electrostatic field associated with the dopants.

Ga-doped ZnO (GZO) films have attracted attention for use in window layers of solar cells and transparent electrodes of liquid crystal display panels. We investigated the dominant factors determining moisture resistance of GZO films. GZO films with different Ga contents (1.5, 3.0 and 4.0 wt.%) were deposited on glass substrates at 200 ^°C by ion-plating deposition using dc-arc discharge.^1-3 To control the factors determining resistivity of the films; carrier density and Hall mobility, oxygen gas flow rate (fO₂) was changed in the range of 6 to 30 SCCM. The resistivity of 3.0 wt.% GZO films with a carrier density of 5.8 × 10²⁰ cm^-3 and Hall mobility of 31 cm²V^-1s^-1 was 3.4 × 10^-4 Ωcm and changed by less than 3% over a 500 h reliability test at a temperature of 60 ^°C and a relative humidity of 95%. Optical transmittance in the wave length from 450 to 1200 nm of the sample was above 80% . The resistivity of all the samples changed by less than 25% confirmed by the reliability test described above. On the basis of the analysis of the data obtained by Hall effect measurements, we concluded that the dominant factors determining moisture resistance of GZO films is carrier density, The maximum carrier density, n_max, needed for moisture resistant GZO films within the change of less than 10% is 7 × 10²⁰ cm^-3. The resistivity of GZO films with a carrier density of 1.1 × 10²¹ cm^-3 larger than n_max changes by about 20%. We will further discuss the difference in the contribution to the moisture resistant properties between extrinsic defects, Ga at Zn site, and intrinsic defects including Zn interstitial and oxygen vacancy.

This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under the National Project of Rare Metal Indium In Substitute Materials Development.

¹ T. Yamada et al., Appl. Phys. Lett. 91, 051915 (2007)

² T. Yamada et al., J. Appl. Phys. 107, 123534 (2010)

³ N. Yamamoto et al., J. Electrochem. Soc. 157, J13 (2010)

Transparent conductive oxides (TCO) on polymer substrates are prospected as a key material for next-generation devices such as flexible displays and photovoltaics. The advantages of polymer substrates include light-weight, low cost, a multiplicity of materials with tailored properties, shock absorption and highly flexibility. However, polymer substrates also have disadvantages such as low heat resistance and large thermal expansion coefficient compared with glass substrates. A main challenge for an efficient TCO on polymer substrate is not only to choose conductive oxide materials having capability of growing at low substrate temperature, but also to develop a deposition processes in order to obtain good electrical characteristics. Thus, in this study, we developed a multiple-deposition method to achieve deposit TCO on polymer substrates with lower than 100°C. The influences of process temperature on structural and electrical characteristics of 100 nm thickness Ga-doped ZnO (GZO) film on polymer substrates were investigated. In addition, the effects of bending strains on the characteristics were also discussed.

The structural, electrical, optical and bending properties of highly transparent conductive polycrystalline GZO films deposited on polyester substrates at various temperatures of less than 100 °C by ion plating with dc-arc discharge were investigated. We developed a multiple-deposition method to achieve controllable different polyester substrate temperatures. Analysis of data obtained from XRD measurements and cross-sectional SEM images show that polycrystalline GZO films with wurtzite structure exhibit highly (0002) orientation perpendicular to the substrate. The resistivity and average transmittance in the visible wavelength region of GZO films on polyester substrates were 5.0 x 10^-4 ohm-cm and more than 80 %, respectively.

The mechanical bending properties of the GZO films were investigated on the basis of analysis of data obtained by comparing sheet resistance between before and after a bending test. The bending test was carried out for the GZO films deposited at various process temperatures. The GZO films deposited at lower substrate temperatures exhibit improved bending performance.

We have demonstrated that our multiple-deposition method to achieve controllable different polyester substrate temperatures works very well to control the bending properties of GZO films.

The financial support from the Japan Science and Technology Agency is gratefully acknowledged

Abstract:

Four novel metallo-polymers (P1-P4 as proton donors) were synthesized to study for the H-bonded effects on their nanocomposites to blend with pyridyl ZnO nanoparticles (ZnOpy as proton acceptors). In order to investigate the nanocomposites containing metallo-polymers P1-P4 and surface-modified ZnOpy nanoparticles, nanocomposites P3/ZnOpy and P4/ZnOpy were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and Transmission electron microscopy (TEM) analyses. In the XRD measurements, the introduction of surface-modified ZnOpy nanoparticles to metallo-polymers P3-P4 has induced higher crystallinities in their nanocomposites P3/ZnOpy and P4/ZnOpy. In the TEM studies, due to H-bonds formed between metallo-polymer P3 and ZnOpy nanoparticles, ZnOpy nanoparticles were more homogeneously dispersed in metallo-polymer P3, which was not well dispersed in nanocomposite P1/ZnOpy (without H-bonds). Finally, it suggested that these nanocomposites might become useful materials for future photovoltaic applications.

Transparent conductive oxide (TCO) materials have attracted much attention for use in liquid crystal displays and photovoltaic devices. In particular, Sn-doped In₂O₃ (ITO) is known as a good transparent conductive oxide material. Recently, ZnO has also been studied as a TCO material because material (Zn) cost is very low in comparison to that of ITO (In). ZnO has shown promise for many applications including gas sensors, transport electrodes, piezoelectric devices, varistors and surface acoustic wave devices. Its direct optical band gap of 3.4 eV at room temperature is wide enough to transmit most of the useful solar radiation in ZnO/CuInSe₂ based solar cells. Furthermore, ZnO is a good candidate to substitute for ITO (In-doped In₂O₃) and FTO (F-doped SnO₂) in transparent conductive electrodes. Many techniques have been employed to produce the ZnO thin films including molecular beam epitaxy, metal organic chemical vapour deposition, radio frequency magnetron sputtering, spray pyrolysis and sol-gel methods.

Non-vacuum processes such as spray and sol-gel method are effective for thin film growth because damage to the surface due to plasma is avoided, high vacuum is not required, and equipment costs are low. Furthermore, low temperature growth of ZnO is important for compatibility with photovoltaic device fabrication processes.

In this work, undoped ZnO films on glass substrates were grown by a spray pyrolysis method at room temperature (RT, ~ 300˚C). Polycrystalline ZnO thin films were successfully grown at RT under an air atmosphere. Diethylzinc (DEZ) was used as the Zn source material. The DEZ solution was diluted by some solvent in order to use safely under an air atmosphere. X-ray diffraction indicates that (10-10) and (10-11) peaks are dominant. The lattice constants of the a and c axes are larger than that of ICDD data. The samples develop a c axis (0002) orientation with increasing substrate temperature. Furthermore, the lattice constants of the a and c axes become closer to those of ICDD data with increasing substrate temperatures.

Metal oxide films are utilized as the gate dielectrics for ZnO-based thin film transistors (TFTs) can improve the characteristics of those devices. In this study, transparent dielectric films of yttrium oxide (Y₂O₃) were deposited onto alkali-free glass substrates (NEG OA-10) by a sol-gel spin coating process . The authors report on the effect of annealing temperature on the microstructural, optical and electrical properties of sol-gel derived Y₂O₃ dielectric films. Each as-coated film was preheated at 300^oC for 10 min, and then annealed at a temperature rage from 400 to 550^oC for 1 h under air ambiance. XRD results revealed that annealed Y₂O₃ films were polycrystalline with a cubic structure, exhibited preferential orientation along the (222) plane, and the crystallinity level of Y₂O₃ films enhanced with annealing temperature increased. Those as-coated films annealed at temperature above 450^oC had high optical transparency with average transmittance over 88.0% in the wavelength region of 400-1000 nm. Moreover, chemical compositions of sol-gel derived Y₂O₃ dielectric films were examined. The metal-insulator-metal (MIM) capacitor structure, Al/Y₂O₃/ITO/glass, were fabricated for examining film’s electrical properties, namely dielectric constant, leakage current, and breakdown field, that were evaluated by capacitance-voltage (C-V) and current-voltage (I-V) measurements.