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Deposition and characterization of transparent and conductive sprayed ZnO:B thin films 2011/04/25 指導教授：林克默 學 生：邱巧緣 博士 Outline • Introduction • Experimental • Results and discussion Structural properties Sheet resistance and transmission Electrical properties Optical properties • Conclusion PV Materials & Modules Lab 2 Introduction • Zinc oxide (ZnO) have potential in the design of optoelectronic region and photovoltaic devices due to the simultaneous occurrence of high transmittance in the visible and a low resistivity. • ZnO is n-type wide band gap semiconductor＞3 eV, whose electrical conductivity is mainly due to intrinsic defect such as interstitial zinc atom and/or oxygen vacancies. PV Materials & Modules Lab 3 Experimental • The ZnO:B thin films have been deposited using chemical spray pyrolysis (CSP) technique, undoped ZnO films were prepared using 0.7 M solution of zinc acetate, Zn(CH3COOH)2,2H2O dissolved in the mixture of methanol and de-ionized water with volume ratio 3:1. • Doping of boron concentration was achieved by adding 0.1 M solution of Boric acid (H3BO3) to the reducing solution. All films were deposited at constant substrate temperature (Ts = 450 ℃). PV Materials & Modules Lab 4 Results and discussion • This indicates that the most grains have a strong c-axis orientation along (002) plane, normal to the substrate surface. • The average crystallite size of ZnO:B film estimated from the (002) peak width measured at half height by using the classical Scherrer formula was found to be ~20 nm. PV Materials & Modules Lab 5 PV Materials & Modules Lab 6 • When the boron doping concentration is gradually increased from a zero value (undoped ZnO), the sheet resistance decreases and reaches a minimum value of ~61.5Ω/□ at 1 at.% of boron concentration. • Incidentally, the optical transmission is also maximum (＞90%) at 1 at.% doping concentration. It is interesting to note that the optical transmittance does not have any appreciable dependence on boron doping concentration. It is interesting to note that the optical transmittance does not have any appreciable dependence on boron doping concentration. PV Materials & Modules Lab 7 PV Materials & Modules Lab 8 • The plot in Fig. 3(a) indicates a lowest resistivity of 2.54 × 10-3 Ω cm obtained for boron doping concentration around 1 at.%. Further increase in boron concentration the electrical resistivity increases and attains maxima at 5 at.% of boron concentration studied. • The ionic radii of Zn+2 in ZnO is 0.74 Å and that of B+3 in boric acid is 0.20 Å , so it is possible for B to success-fully substitute Zn site and donate one free charge carrier to the conduction mechanism. From Fig. 3(b), the initial increase in mobility with doping concentration may be due to the improvement in the crystallinity. PV Materials & Modules Lab 9 PV Materials & Modules Lab 10 • Fig.(4) shows a typical curve of UV–Vis transmission spectra in the wavelength range 0.2–2.5 μm for intrinsic ZnO and ZnO:B thin films. The average optical transmittance of ZnO:B film combined with glass substrate is higher than 90% at wavelengths above 0.550 μm The doped film is also opaque to NIR and presents a sharp UV cut-off at approximately 0.380 μm in both films. • The optical band gap was found to be 3.24 eV for intrinsic ZnO film and it increases slightly to 3.27 eV for ZnO:B films. The increase in optical band gap may be attributed to Moss–Burstein shift, which occurs owing to filling up of low energy levels by the conduction electron. PV Materials & Modules Lab 11 Conclusion 1. 利用化學噴霧熱解法(CSP)沉積硼摻雜氧化鋅透明 導電薄膜，觀察研究硼摻雜之濃度變異對薄膜特 性之影響。 2. 經研究分析得知薄膜為多晶薄膜之纖鋅礦結構， 且具有C軸 [002]較明顯之優先取向，其次為[110]。 3. 該薄膜得到之最低電阻率為(1 at.%) ρ = 2.54 × 10-3 Ω cm ，最佳載子濃度η = 9.2 × 1019 cm-3，可見光 穿透率最佳T% ＞90%，光學能隙~3.27eV。 其優異的光學與電特性，是具有競爭做為透明窗 層及太陽能電池電極之潛能。 PV Materials & Modules Lab 12 Thanks for your attention!! PV Materials & Modules Lab 13