Cadmium telluride, a direct gap semiconductor with the nearly optimum band gap (1.4 eV) for photovoltaic solar energy conversion, has long been recognized as a promising photovoltaic materials and cadmium sulfide is the most commonly used heterojunction partner in CdS/CdTe solar cells, presumably because of its similarity to CdTe in electron affinity. It is essential to enlarge the active area of a solar cell for terrestrial application because the large active area increases the total area efficiency. Therefore, CdS/CdTe solar cells with TCO front electrode, TCO/CdS/CdTe solar cells have been studied. Recently there has been an interest in TCO/CdS/CdTe thin film solar cells to reduce the material cost and increase cell efficiency by increase of transmitance of CdS films. In TCO/CdS/CdTe thin film solar cells, the required properties of CdS films are high optical transmittance and no pinholes in films and that of CdTe films are large grain size, compact microstructure and adequate resistivity about $10^4$ Ωcm.
In this thesis, CdS and CdTe films have been deposited and characterized to find optimum deposition condition and using these films, ITO/CdS/CdTe thin film solar cells were fabricated. CdS films were prepared by chemical mist deposition (CMD) and chemical bath deposition (CBD). CdTe films deposited by close spaced sublimation (CSS). Using CdS and CdTe films, ITO/CdS/CdTe thin film solar cells were fabricated and their photovoltaic properties were investigated. The preparation and properties of CdS films by CMD, CdS films by CBD, CdTe films by CSS, and ITO/CdS/CdTe solar cells are described in order.
CdS films were prepared by CMD process from solutions containing equimolar (0.1M) cadmium chloride and thiourea $[(NH_2)_2CS]$ on glass substrate of which temperature was ranged from 250 to 45℃. Pure CdS films with a hexagonal structure were formed above 300℃ and the number of pinholes in the films decreased as the substrate temperature increased. The optical transmittance of the films deposited at 400℃ was about 75% and the optical band gap of the films was 2.43 eV regardless of substrate temperatures. The dark electrical resistivity of the films deposited at 400℃ was of the order of $10^5$Ωcm and all the films showed high photoconductivity. Boron-doped CdS films were prepared by adding boric acid ($H_3BO_3$) to the starting solution. Grain size of the B-doped CdS films decreased as the molar ratio of $H_3BO_3/CdCl_2$ increased. The dark electrical resistivity of the B-doped CdS showed a minimum value of about $10^2$Ωcm at 0.01 molar ratio and then increased with further increase in the molar ratio.
CdS films were prepared by CBD process using cadmium salts and thiouria solution. Nucleation and growth of CdS films varied with pH value in solution. At the condition of pH = 12 and solution temperature of 90℃, the CdS films on ITO substrate showed about 85% transmittance. After $H_2$ annealing, the transmittance of CdS films increased about 90% due to reduction of number of CdS particles on surface. Structural property and atomic concentration of CdS films were not varied by $H_2$ annealing. The optical band gap of as-deposited CdS films was higher than that of annealed films. It appears to be related to three-dimensional quantum size effect resulting from grain size and oxygen ion in grain boundary of as-deposited CdS films. And annealed CdS films showed optical band gap of 2.40 eV, the band gap of single crystalline CdS, due to reduction of oxygen concentration by $H_2$ annealing. With 560℃ $CdCl_2$ treatment, the transmittance of CdS films showed 95% due to increase of grain size.
CdTe films have been prepared by close-spaced sublimation process with screen printed CdTe layers on glass substrate as a new source. The screen printed CdTe layers were fabricated by sintering slurries, containing CdTe powder or mixtures of Cd and Te powder. The CdTe films prepared with screen printed CdTe layers have a zinc blende structure regardless of substrate materials, but grain size of the CdTe films deposited on CBD CdS substrate was smaller about a few μm than that of the films deposited on sintered because large number of grainboundaries in the CBD CdS substrate enhanced nucleation rate of CdTe films. In the CdTe films prepared by using CdTe powder, the resistivity of the films deposited in $O_2$ gas was about one-tenth lower than that of the films deposited in He gas. The electrical resistivity of the CdTe films showed two activation energies of about 0.15 eV below 280 K and 0.63 eV above 280 K, regardless of gas atmosphere. This demonstrated that active acceptors of CdTe films are cadmium vacancy ($VCd^{2-}$) above 280 K and complex defect ($VCd^{2-}Cl^+$) below 280 K. Our composition analysis indicated that Cd concentration in the CdTe films deposited in $O_2$ gas was lower than that in the CdTe films deposited He gas. The resistivity decrease of the CdTe films with $O_2$ gas appears to be related to decrease in the Cd concentration, resulting in the increase of both complex defect ($VCd^{2-}Cl^+$) and the cadmium vacancy($VCd^{2-}$). As the Cd/Te ratio in CdTe source-layer decreased, the resistivity of the CdTe films decreased and reached at a constant value of about $3\times10^4$Ωcm. In addition, the effect of $O_2$ atmosphere on the resistivity decrease was diminished when the Cd/Te ratio was less than 0.7, where the composition of the CdTe film might be limited by Te solid solubility. The above results suggested that the resistivity decrease of CdTe films deposited in $O_2$ was not by due to the effect of previously-known oxygen doping but by due to the effect of Cd/Te compositional change.
Finally, ITO/CdS/CdTe thin film solar cells were prepared by CBD CdS films and CSS CdTe films. Cell efficiency increased 1% to 5% with decreasing of Cd/Te ratio in source CdTe layer due to decrease of series resistance of cells. The cell efficiency of ITO/CdS/CdTe cells fabricted at Cd/Te = 0.67, substrate temperature of 550℃, and He gas atmosphere was about 5\% because the contact resistance of cells was too high about 8 Ω㎠. Contact resistance in ITO/CdS/CdTe cell is a task to be further studied. Space charge in CdTe films increased with increase of depletion width in CdTe films due to Cd diffusion from CdS films to CdTe films.
