이종접합 CdTe/CdS 박막의 광학 및 전기적 특성 최적화

Abstract

Thin-film cadmium telluride (CdTe) solar cell technology can theoretically improve the conversion efficiency and production costs of conventional silicon solar cell technology. Due to the optimal band gap energy (about 1.4 eV) for solar energy absorption, high light absorption capability and lower cost requirements for solar cell production, CdTe have been widely researched as being suitable for commercial cell production. The effects of the processing conditions on the thickness-uniformity and surface-morphology of the CdTe thin-films were investigated using atomic force microscope (AFM) and ellipsometry for large-area solar cells. The photovoltaic properties of CdTe thin films were analyzed in relation to the different thickness-uniformity and surface morphology caused by the various process conditions. The thickness-uniformity, which was controlled by the process conditions in the sputtering process, affected the photovoltaic properties of the sputtering deposited CdTe thin films. Higher carrier concentration and better optical absorbance were obtained in CdTe thin films with a good thickness-uniformity. As the effects of thickness-uniformity on the photovoltaic characteristics for the large-area CdTe thin film solar cell, chemical mechanical polishing (CMP) process was investigated for an enhancement of thickness-uniformity. Removal rate of CdTe thin film was more affected by the down force than the table speed. RMS roughness and peak-to-valley roughness of CdTe thin film after CMP process were improved to 96.68% and 85.55%, respectively. The optimum process condition was estimated by 100 gf/㎠ of down force and 60 rpm of table speed with the consideration of good removal uniformity about 5.0% as well as excellent surface roughness for the large-area CdTe solar cell. Cadmium sulfide (CdS) is widely used material as a window layer with either CdTe or copper indium selenide (CuInSe2) in the heterojunction thin film solar cells. The phase structure and grain size of CdS thin films were changed as changing the annealing temperature, and then the electrical properties including resistivity and the optical properties were influenced by them. The improved optical transmittance of 72.25% (at average thickness of 843.93 nm and the optical band gap energy of 2.43 eV) was obtained compared to the as-deposited CdS thin films at the annealing temperature of 400℃ because of the decrease in the defect density caused by the annealing process. This temperature was selected for the optimized condition considering to both the electrical properties (resistivity not exceeding 103–order Ωcm) and optical characteristics including transmittance. In this thesis, the thickness uniformity of CdTe thin films was the main point for the energy conversion efficiency and the optimum annealing temperature of CdS thin films was 400℃ for the excellent thin film characteristics. For the application to the large-area thin film solar cells, the improvement of the thickness uniformity in the preparation of CdTe thin films was strongly necessary; therefore, CMP process was firstly employed and the enhanced characteristics were successfully obtained. The optimized optical and electrical characteristics of CdTe/CdS heterostructured thin film solar cells could be achieved considering the optical and electrical properties through these results.