| Summary: | 碩士 === 龍華科技大學 === 工程技術研究所 === 99 === Zinc oxide is a valuable wide band gap semiconductor. In this study, a new method to prepare the nano scale ZnO electrode for dye sensitized solar cell (DSSC) has been developed. The zinc oxide nano materials, including nanowires and nano dendrites, were first deposited by a low pressure chemical vapor deposition (LPCVD) system, and then being used as the active layer for DSSC. According to literatures, columnar semiconductor electrodes work better than particulate type used in DSSC, due to larger contact area for current flow, and higher voltage output resulted from sharp band edge structures. The electrical experiment data of this research do show convincing results.
The growth of ZnO nanomaterials was performed in a horizontal hot wall LPCVD system. The CVD chamber is made by a 600 mm long, 50 mm ID, quartz tube, operated at 160-200 torr, with sublimation temperature and deposition temperature at 120 oC and 600 oC, respectively. The CVD precursor is zinc acetylacetonate, one gram for each run, while oxygen and water vapor were introduced as the oxidant gas. It shows that the composition of deposits and morphologies are strongly dependent on the temperature, pressure, and substrate being used. To develop a novel nano structured ZnO for DSSC application, doping of iron was used by pre-mix and co-sublimation 0.05-0.09 g iron acetylacetonate together with 1 g of zinc acetylacetonate. Thus, three types of DSSC electrodes have been studied, pure ZnO nano columns, Fe：ZnO nano columns, and Fe：ZnO nano branched dendrites.
SEM images reveal that, for the Fe doped ZnO LPCVD, on the copper net surface, deposits were large area, high density, with remarkable alignment arrays of one dimensional nano columns. Most of them have a diameter ranging from 20 to 150 nm, and length from 2 to 15.4 nm. It was suggested that there are three primary factors influencing the shape of nano ZnO; the difference of surface energy value for respective crystal planes, the mass transfer rate of CVD precursor across the boundary layer, and doping of foreign elements. Since, on the depositing surface, adatoms of dopant such as Fe may stop the lateral moving of ledge, thus create unique crystal growth habit. Doping is assumed to be a very efficient way to form ZnO nanostructures.
XRD patterns confirms that these nano columns were made of ZnO(101) and ? phase Fe2O3. From several systematically experiments, collected data point out that at a condition of O2 flow rate at 30 sccm, 600oC, the Fe:ZnO nano columns can give the best performance for DSSC. The solar cell uses such materials as the active layer has an output voltage of 0.74 V, and current density of 0.14 mA/cm2. The ZnO dendritic electrodes were prepared by a two-step LPCVD process. ZnO nano columns were first prepared as described above, and then these samples were used as the substrates for the secondary LPCVD. SEM studies discovered that, at the second run, ZnO phase will nucleate above the present surface structures of the ZnO nano columns, thereby create a designed dendrite growth of ZnO “nano trees”. From electrical parameters measurement, these ZnO nano branches show better performance. The maximum output voltage for branch electrode DSSC is 0.82 Volt, while it is only 0.76 Volt for DSSC uses pure ZnO nano columns as electrodes. The current density for branch electrode DSSC is 0.12 mA/cm2. That value is 0.05 mA/cm2, for pure ZnO nanocolumn using DSSC. However, the output voltage of branch electrode DSSC drops very fast. It was regarded as the large contact area between electrolytes and electrodes will consume ions so quickly that the electrolyte is difficult to recover. On the other hand, even the Fe：ZnO nano column using DSSC has a output voltage of 0.77 Volt, and a current density of 0.07 mA/cm2; performs only moderately better than pure ZnO nano column using DSSC. But either output voltage or current density can sustain for a longer period. It was suggested that Fe：ZnO nano column using DSSC may be the candidate for following study.
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