고려대학교

초록/요약

Transparent conducting oxide (TCO) films, which are degenerate wide band-gap semiconductors with low resistance and high transparency in the visible wavelength range, have been used extensively in optoelectronic devices such as flat panel displays and solar cells. The majority of TCO films are n-type conductors such as indium tin oxide (ITO), tin dioxide (SnO2), and zinc oxide (ZnO). The conductivity of the ZnO films can be increased by several orders of magnitude by doping with Al, Ga or F or by creating oxygen vacancies. These films are typically prepared by magnetron sputtering, chemical vapor deposition, and a sol-gel process. Among these methods, sputtering is widely used in industrial products because films of high quality, i.e., high density, strong adhesion, high hardness, etc., can be achieved at low substrate temperatures with a relatively uniform film thickness over a large area. The microstructure and the properties of the sputtered films strongly depend on process parameters such as the partial pressure of oxygen, the sputtering power, and the substrate temperature. Therefore, precise control of each parameter is essential for obtaining high-quality sputtered films. Highly transparent ZnO films with low resistivity for solar cell applications were fabricated at a low temperature by RF magnetron sputtering. Al-doped ZnO (ZnO:Al) films were deposited on glass substrates at a substrate temperature of 200 ℃. Structural, electrical and optical properties of the ZnO:Al films were investigated in terms of the reparation conditions. The ZnO:Al films show a hexagonal wurtzite structure preferentially oriented in the (002) crystallographic direction. The transmittance of the ZnO:Al films in the visible range is 90 %. The lowest resistivity of the ZnO:Al films is about 5.7 10-4 Ω•cm at the Al content of 2.5 wt %. After deposition, the smooth surface of as-deposited ZnO:Al films were etched in diluted HCl (0.5%) to investigate the variation of electrical properties and surface morphology due to a textured surface. An etching time plays an important role on the formation of surface-textured ZnO:Al films using wet chemical etching. In Chapter 4, It has been reported the noticeable effect on the electrical properties of ZnO:Al films which is obtained with the addition of hydrogen. Also, I had carried out research to know how the different Ar+H2 ambient affects the structural, electrical and optical properties of AZO films deposited on glass substrates. The ZnO:Al films obtained with the addition of hydrogen to the sputtering gas were characterized. The improvement in conductivity produced by the action of H2 and the structural modifications of the deposited films were examined. The HAZO films showed a lower resistivity and a higher carrier concentration and mobility than the AZO films. In addition, the effect of H2 flow ratio on the structure and composition of hydrogenated AZO thin films have also been studied. The Other study examined the effect of deposition temperature on the electrical and the optical properties of thin-film HAZO fabricated by radio frequency magnetron sputtering using a ceramic target. Various AZO:H films were prepared on glass over a substrate temperature range from room temperature to 250℃. The intentional incorporation of hydrogen was shown to play an important role in the electrical properties of the AZO:H films by increasing the free carrier concentration. The addition of 2% H2 in Ar at a growth temperature of 150℃ produced an AZO:H film with excellent electrical properties and a resistivity of 3.21ｘ10−4 Ω•cm. The UV-measurements showed that the optical transmission of the AZO:H films was above 86% in the visible range with a wide optical band gap. In chapter 5, this study addresses the optimization of RF magnetron-sputtered hydrogenated ZnO:Al (HAZO) films which is used as front contacts in microcrystalline silicon solar cells. The front contact of a solar cell has to be highly conductive and highly transparent to visible and infrared radiation. Depending on their structural properties, these films developed different surface textures upon post-deposition etching using diluted hydrochloric acid. The light-scattering properties of these films could be controlled simply by varying the etching time. Moreover, the electrical properties of the films were not affected by the etching process. Therefore, within certain limits, it is possible to optimize the electro-optical and light-scattering properties separately. The microcrystalline silicon (μc-Si:H)-based p-i-n solar cells prepared using these new texture-etched AZO:H substrates showed high quantum efficiencies in the long wavelength range, thereby demonstrating effective light trapping. Using the optimum AZO:H thin-film textured surface, we achieved a p-i-n μc-Si solar cell efficiency of 7.78%. Also, we investigated the effect of the multi-step texturing process on the electrical and optical properties of hydrogenated Al-doped zinc oxide (HAZO) thin films deposited by rf magnetron sputtering. After deposition, the surface of HAZO films was multi-step textured in dilute HCl (0.5%) for the investigation of the change in the optical property and the surface morphology due to etching. The microcrystalline silicon (μc-Si:H)-based p–i–n solar cells prepared using these new texture-etched HAZO substrates showed high quantum efficiencies in the long wavelength range, thereby demonstrating effective light trapping. Using the optimum AZO:H thin-film textured surface, we achieved a p–i–n μc-Si solar cell efficiency of 7.08%. In chapter 6, we used RF-magneton sputtering method to fabricate Al doped AZO films as well as hydrogenated AZO(HAZO) films, analyzed electro-optical properties of TCO along with interface properties of TCO/p type a-Si:H layer in order to apply to silicon hetero-junction solar cells. Inserting buffer layer to increase interface properties between TCO/ p type a-Si:H layers, we measured the contact resistance using the CTLM (circular transmission line model) pattern, analyzed the depth profile through AES (auger electron spectroscopy) analysis and observed the changes in properties of TCO thin film through heat treatment. In chapter 7, there are technologies to fabricate silicon hetero-junction. After cleaning and texturing n-type (100), we deposited i-a-Si on the both sides of substrate using HW-CVD, and deposited n type a-Si:H as BSF layer at the back in the PE-CVD method. After depositing p type a-Si:H at the front and then depositing Transparent Conductive Oxide using rf magnetron sputtering, we used the thermal evaporator to form the top electrode and back electrode to finish the silicon heterojunction solar cell solar cell. When we fabricated the small area silicon heterojunction solar cell of 1×1㎝2 in size, using the developed technology, the open circuit voltage (Voc) was 675mV, the short circuit current (Jsc) was 35.84mA/cm2, and the Fill factor was 71.46%. As for the solar cell efficiency, it was 17.3%.