硫化鎘薄膜應用於太陽電池之研究

本研究分為兩大方向，第一部份係利用化學水浴法以及真空蒸鍍法成長硫化鎘薄膜於玻璃基板，並分析硫化鎘薄膜之基本特性。像是硫化鎘薄膜之晶相結構、表面型態以及光穿透特性。利用此兩種方法所製備之硫化鎘薄膜均有(111)之結晶方向，經過退火處理後可以產生(002)之結晶方向，硫化鎘薄膜能隙由2.387 eV減小至2.376eV，隨後再上升至2.433 eV。由能隙改變與XRD分析可判斷其內部晶格之轉變是由四方晶體結構轉變為六角晶體結構，在退火條件400℃下也可產生較大粒徑之薄膜結構。而化學水浴法中加入氯化銨緩衝液可得到較平坦之表面，其平均粗糙度可由258nm減小至14nm。另一部份則是太陽電池元件製作，利用真空蒸鍍的方式將硫化鎘結合砷化鎵基板(CdS/GaAs)，針對窗戶層材料的研發對於砷化鎵太陽電池之特性研究，而硫化鎘薄膜利用不同退火溫度，分析其光穿透率與光吸收之能隙變化。CdS/GaAs太陽電池元件利用AM1.5 (97mW/cm2) 之光源照射下，其量測之開路電壓Voc=375mV，短路電流密度Jsc=78.1mA/cm2，填充因子FF=29.6%，光電轉換效率η=1.21%。化學水浴法所製備之硫化鎘薄膜則應用於染料敏化太陽電池上，最主要是利用CdS/TiO2之雙層電極結構改變其基本架構，在硫化鎘薄膜成長中並使用硼掺雜。CdS/TiO2 雙層電極結構之太陽電池元件與TiO2單層電極結構相比，其元件效率有明顯之增加。

關鍵字

太陽電池；硫化鎘

並列摘要

In this thesis, a CdS/GaAs heterostructure was interesting to develop a non epitaxy and low cost process and was expected to have higher absorption efficiency than homo GaAs photovolitaic devices. Cadmium sulphide (CdS) thin films were deposited by vacuum evaporation on GaAs substrate and post annealing was also carried out in an atmosphere of nitrogen. Optical and crystal qualities of CdS window layer has been investigated by x-ray diffraction (XRD) and scanning electron microscopic (SEM) measurement. It was found that either a hexagonal or a cubic structure was existed within the CdS layer. From SEM images, it was also found that grain size of CdS films increased with increasing annealing temperature. Average grain size of CdS film was also increased during the wet-CdCl2 treatment. Transmittance of CdS films was found as high as 80% at different annealing temperature. It can be seen that the blue-shift presented in the optical transmittance spectra. During the AM 1.5 illumination, open-circuit voltage, short-circuit current density, and fill factor of our CdS/GaAs heterostructure PV devices were measured and discussed in detail. The GaAs substrate was used in this study, the front contact of CdS/GaAs was aluminum. The aluminum was deposited on the down side of GaAs substrate by electron beam evaporation, and thermal annealing of GaAs substrate was carried out in nitrogen atmosphere at 450 oC for 30 min, to make the ohmic contact between GaAs substrate and aluminum contact. In order to remove the oxide of the GaAs substrate, the GaAs substrate was put into a non-selective, wet etch solution (H2SO4 : H2O2 : H2O = 5 : 2 : 2) about 10 seconds to obtain the pure surface. After etched, the GaAs substrates were cleaned by acetone, methanol and de-ionized water under ultrasonic stirring for 10 min sequentially. The CdS films were deposited on the etched side of GaAs substrate by vacuum evaporation technique in a turbo-molecular pumped vacuum system operating in the 10-5 ~ 10-6 Torr rage of vacuum pressures. The distance between source and substrate was 7 cm, the source temperature above 500℃. An aluminum mesh electron was used for solar cells back contact, the solar cells with an area of 0.25cm2. In this study, CdS films were also deposited on glass substrate to analysis the quality and property of CdS films. The CdCl2 treatment was used after CdS films deposited, the CdS films were immersed in 0.3 M CdCl2 solution and then annealed at 400 oC. The thicknesses of CdS films were determined in suface profiler system; Veeco Dektak 6M, the optical transmission spectra of CdS films were measured by spectrophotometer; Hitachi U-2800, the surface morphology were observed by field-emission scanning electron microscopy; JEOL JSM-6700F and the crystal structures were determined by the X-ray diffraction measurement; Rikagu D/MAX2500. The research direction in this study was divided into two parts. First, the characterization of cadmium sulfide (CdS) thin films were investigated whose deposited methods were carried out by chemical bath deposition (CBD) and vacuum evaporation (VE). The crystal structure and morphology of CdS thin films were analyzed by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively. For the as-grown samples of the two deposited methods, it was found that the strongest diffraction peak (111) was easily observed. The cubic phase of the as-deposited CdS films will transfer to the hexagonal phase when the annealing process was carried out above 300oC. Also, the x-ray peak corresponding to the hexagonal phase would be significantly seen at the (002) plane. The bang gap of CdS thin films was varied that we can judge that form the inner crystal structure changed. On another hand, the solar cell device was fabricated by using CdS/GaAs structure. The solar cell test were carried out by using illuminated current-voltage characteristics, the open-circuit voltage, short-circuit current density and fill factor of the device were 375 mV, 78.1 mA/cm2 and 29.6% respectively, the conversion efficiency of 1.21% under AM 1.5 conditions. Furthermore, the conversion efficiency of dye-sensitized solar cell increased by using CdS-CBD/TiO2 double electrode structure.