Studies On Preparation Of ZnO-Based TCO Films And Their Photoelectric Properties

ZnO:Al (AZO) and ZnO:Al:Ti(AZTO) ceramic target materials with high density and high conductivity have been prepared by means of solid-phase method under normal pressure using ZnO, Al2O3and TiO2powder as their raw materials. The influence of sintering temperature on the microstructure, density and electrical properties of AZO target material has been studied. Highly c-axis preferred orientations of AZO films have been prepared by R.F magnetron sputtering on glass substrates. The effects of sputtering pressure, sputtering time and oxygen partial pressure on the photoelectric properties of the films have been studied. In comparison of the photoelectric properties of AZO and AZTO with that of ITO, we try to use the AZO or AZTO film in substitution for ITO film. On the other hand, the AZO film was deposited on the p-type silicon, and the refractive index, the surface reflectivity and lifetime of minority carrier of the film were tested. The experimental results are as follows:(1) The AZO and AZTO ceramic target materials were prepared by means of the solid phase method under normal pressure sintering at deferent temperature using ZnO, Al2O3,TiO2powder. XRD results show that the quality of the the crystallization of the target materials is quite good; FESEM results indicate that for the target material sintering at1400℃, the grain size in the sample is relatively larger, the concentration of defects such as grain boundary and microvoid in the sample is relatively lower; its density and resistivity are94.86%and1.01×10-2Ω·cm, respectively.(2) The mean positron lifetime τm of the AZO decrease with the rising of sintering temperature, and the τm reaches its minimum value when the sintering temperature at1400℃; however τm increases for the sample sintering temperature at1500℃. This means that with the sintering temperature increasing, the grain size of target increases, the concentration of defects reduce, holes close, the electron density increases and the positron lifetime decreases; while for sample sintering at1500℃, the grains grew up abnormaly, the impurity phase is easy to precipitate on the grain boundary, the electron density decreases and positron lifetime increases.(3) The AZO and AZTO transparent conductive thin films were deposited on glass substrates by R. F Sputtering method; the effects of different sputtering pressure, work time and oxygen content on the photoelectric properties of AZO transparent conductive thin film have been studied. The results showed that the AZO film deposited at the sputtering pressure lower than0.5Pa has a good property of crystallization and a low resistivity, and its average transmissivity in visible light area is higher than90%. For the AZO film sputtering for120min., the thickness of the film is about350nm, the resistivity, Hall mobility and carrier concentration of the film are4.038988×10-3Ω·cm,9.969720cm/vs and1.550015×10/cm (minus show the N type semiconductor), respectively; its average transmissivity in visible light area (380～780nm) is91.7%. In comparison of the photoelectric properties of the films sputted at different O: Ar ratio, it can be found that the quality of the crystallization of the AZO film sputted at zero O:Ar ratio is quite good, its resistivity is relatively low, and its transmissivity in visible light area is relatively high, thus it can be inferred that the AZO film is not in a oxygen-deficient state.(4) There is a blue peak at440nm in the PL spectrum of AZO thin film, this is due to the formation of zinc interstitial atoms and oxygen vacancies present in the film induced by Al doping, and the electrons jump from shallow donor energy level formed by oxygen vacancies to the top of the valence band together with the electrons jump from the energy level of the zinc interstitial atoms to the top of the valence band. In comparison of the ZnO, AZO and AZTO films, it can be found that the UV luminescence peaks of both of AZO and AZTO films have a blue-shift.(5) In comparison of the photoelectric properties of AZO with that of ITO, we try to find a possibility to use the AZO film in substitution for ITO film in the future.(6) A new structure of solar cell were designed, where the AZO and AZTO transparent conductive films are used in substitution for the traditional Si3N4reflection film in a crystalline silicon solar cells. The preparation procedure of the solar cell is as follow:the silicon nitride layer was deposited on the p-type polycrystalline silicon by means of the PECVD method, then the AZO and the AZTO transparent conductive film were deposited on the same wafer by means of the R. F Sputtering method. The thickness, the refractive index, the surface reflectivity and the minority carrier lifetime of the film have been measured. Afterthen a feasible solutions for improving the passivation has been put forward. In comparison of the industrial silicon nitride layer, the solar cell prepared in this work has a better reflection effect and a low cost.