전구체 용액을 이용한 비진공 방식의 CIS계 태양전지용 박막 제조 및 특성 평가

Abstract

Carbon-based energy such as coal, oil and natural gas faces difficulties with various aspects caused by increase in energy demands, production cost and decrease in its resources. Furthermore, such energy sources make huge amounts of carbon oxides, sulfur oxides and nitrogen oxides into atmosphere when they burn, which lead to greenhouse effect. According to these situations, a lot of researches for developing renewable energy have been studied until now.Thin film solar cells have the great potential to convert sunlight into electricity at low cost, because of high material use efficiency, high optical absorption property, tunable band-gap energy and so on. Recently, polycrystalline Cu(In,Ga)Se2 (CIGS) absorber layers are used for the most efficient thin film photovoltaic cells. However, previous reports were based on high vacuum deposition systems and high temperature heat treatment process such as thermal evaporation and sputtering technique. During several decades, so, production cost has not reduced under $1/W since CIGS solar cells developed first time, where renewable energy could be expected to substitute for fossil fuels. For achieving ‘grid parity’, non-vacuum and room-temperature deposition methods should be studied.In this thesis, CIS chalcopyrite absorber layers were synthesized on Mo-sputtered soda-lime glass substrate by non-vacuum coating using precursor solution and thermal annealing methods. Precursor solution was prepared by mixture of metal chloride precursors, polypropylenecarbonate(PPC) as a thermal-decomposable organic binder, and non-toxic acetone as a solvent. A low-cost, non-toxic precursor solution was deposited on substrate by simple doctorblade-coating method which can be applied to continuous roll-to-roll production. And then, precursors were converted into CIS crystalline phase in quart tube furnace under a low vacuum environment.The morphology, composition and crystallinity of the synthesized thin films were characterized by SEM, EDX, and XRD analysis. By XRD, TGA measurements, in addition, growth mechanism of CIS grains was analyzed for heat-treated samples at varied temperature. Chloride phase precursors decomposed and perfectly disappeared over 100˚C, but then they formed metal chalcogenide phase such as Cu2-xSe and In2Se3. Between 300˚C and 400˚C, CIS chalcopyrite structure started to evolve, finally completed its crystallization to chalcopyrite structure near 500˚C. Also, composition and concentration of precursors were major factors to forming thin film structure. In most cases, stoichiometric precursor paste could made 500nm thickness of continuous CIS film by 500˚C annealing in a vacuum environment for 30mins. By optical absorption spectrum, furthermore, CIS thin film synthesized on bare substrate showed great light absorbance behavior at visible and near-IR light region. And band-gap energy was calculated to be about 1.15eV by Tauc relation. However, SEM, EDX, XRD, AES results showed phase separation between CIS grains and amorphous Se-rich film. Although that film had very low contact resistance property, amorphous layer can act as electrophilic recombination centers. This phase separation phenomena should be solved for device fabrication and its high performance.