CuInSe₂ Thin Film Solar Cell-Related Materials

Copper indium diselenide (CIS) or copper indium gallium diselenide (CIGS) and cadmium sulfide (CdS) are both promising semiconductor compounds for fabricating thin film solar cells. CIGS solar cells have reached a very high photo-electric conversion efficiency of 19.9%. In order to developing commercial production, a large-scale and low-cost deposition method is needed. Electrodeposition (ED) and chemical bath deposition (CBD) are attractive methods which are not limited by substrate size, deposition temperture and does not require vacuum. However, the problem that has hindered development of these techniques were the poor crystalline quality and electronic properties obtained by ED and CBD. Therefore, it is necessary to improve the crystalline quality by optimizing the ED and CBD as well as recrystallizaion processes through heat treatments. Among various methods of heat treatments, we focus on CdCl2-annealing of CBD-CdS precursor, selenization of one-step electrodeposited CIS precursor and selenizaiton of evaporated InSe/Cu bilayer precursor.In Chapter I, we first introduced the development of solar cells and the theory of photovoltaic effect. Then we introduced the development of CuInSe2 thin film solar cells as well as the function and deposition method of each layer in CuInSe2 solar cells. Lastly, the physical properties of CuInSe2 and CdS were introduced.In ChapterⅡ, we studied the CdS recrystallization mechanism during CdCl2-annealing. A comparative study was carried out on thin films prepared by CBD: CdCl2-annealing (annealing in air with a CdCl2 coating layer) and air-annealing (annealing in air without a CdCl2 coating layer).The air-annealed CdS films were partially oxidated while the CdCl2-annealed samples were not oxidated since the diffusion of Cl ions protected CdS from oxidation during annealing in air. We study the point defects formed in the CdS and explained the anti-oxidation mechanism. In situ Raman spectra was obtained during the annealing to monitor the structure change especially cubic-hexagonal phase transition. Combined with other characterization techniques, we proposed a CdS recrystallization model.In order to prepare high efficient CdS/CdTe solar cells, we deposited hexagonal phase CdS thin film with (002) preferred orientation through optmizing the CBD method. Based on the understanding of the formation of high quality n-type CdS thin film, the efficiency of CdS/CdTe solar cells fabricated by other students in our lab has reached 12.4%.In Chapter III, p-type CuInSe2 films were prepared by selenization of one-step electrodeposited Cu-In-2Se (atomic ratio) precursors. To obtain high-quality, dense, and homogeneous CuInSe2 films for solar cell application, the effects of substrate temperatures during selenization and precursor compositions on the final microstructures were systematically investigated. The precursor layers evolved in very different ways under different selenization conditions. The final microstructures and phases of the films depended critically on the precursor compositions, selenization temperature, and the selenization thermal process history. Low-melting-temperature CuxSe phase, which tended to segregate at the film surface, acted as an element-transport flux agent at higher temperature under high Se vapor, can efficiently assist the CuInSe2 grain growth. Good crystalline quality chalcopyrite CuInSe2 film was obtained by improving the selenization technology. Based on the ED-CuInSe2 thin films, we made the Glass/Mo/CIS/CdS/ZnO solar cells. In order to study the mixed phases in electrodeposited CuInSe2 film, the temperature-dependent Raman spectroscopy was carried out between 83 and 723 K. CuAu-ordered phases were detected by Raman scattering at low temperature. By considering the phonon dispersion curves and using the Ridley model, it showed that the chalcopyrite A1 mode decayed asymmetrically into phonons with different frequencies. However, the A1 mode of the CuAu-ordered CuInSe2 showed almost no frequency change upon increasing the temperature up to 400 K. In order to optimizing the band gap of CuInSe2, we prepared CuIn(Se,S)2 thin film with good crystalline quality by sulfurizing the one-step-electrodeposited CIS precursor in H2S/Ar flow. We also prepared CdTe films by electrodeposition and made CdS/CdTe solar cells with the Voc above 500 mV.In Chapter IV, we prepared pure chalcopyrite CuInSe2 film with good crystalline quality by selenization evaporated Cu/InSe-bilayer precursor. The phases formed at different temperature during selenizaiton were studied. Then based on the results we proposed a chemical reaction path model about the formation of chalcopyrite CuInSe2. In order to obtain device grade CIS film, Br-MeOH was used to etch our CuInSe2 films and the mechanism was investigated. In addition, we study the deposition of other layers in CuInSe2 solar cells such as Mo and ZnO:Al film by magnetron sputtering. We investigated the influence of argon pressure and power density to the property of Mo layer. Then based on it we prepared Mo bilayers with good electrical and structure quality as well as a good adhesion to glass substrate. We also studied the influence of substrate temperature as well as negative ions bombarding effect to the electrical, optical and structure properties of ZnO:Al layer. Then we fabricated ZnO:Al film with high visible light transmittance (>90%) and low resistivity (1.1×10-3Ωcm).