Optimal fabrication processes for high-efficiency chalcopyrite thin-film solar-cells

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The purpose of the present research work is to improve the performance of a chalcopyrite thin-film solar cell and the adhesion of the film to a Mo-coated soda lime glass substrate. The results of our investigation are summarized as follows.In the first step of sulfurization, we prepared a CuGaS_2 thin film on the substrate, where the Cu to Ga ratio of a vacuum-evaporated metallic precursor was equal to 1.0. In the second step, an optical absorber layer of CulnS_2 was prepared on it by sulfurization of a stacked precursor layer consisting of Cu/In. The Cu to In ratio was varied from 1.3 to 2.3. When the ratio is larger than 1.7, the cell exhibited a large short-circuit current density. This is considered to be due to the presence of Cu_xS quantum dots formed in the absorber layer. Solar cells with efficiency up to 11% was obtained using a Cu to In ratio between 1.7 and 2.1.Using a precursor consisting of vacuum-evaporated GaS, In and Cu, we prepared a Cu(In,Ga)S_2 thin film. A high efficiency solar cell was obtained with the GaS thickness of 40nm and the Cu/In ratio of 1.2. When the GaS is made thicker, inhomogeneous distribution of In prevails, eventually leading to an inefficient solar cell. The performance of the best cell was as follows : open circuit voltage is 673mV, short circuit current density 22.7mA/cm2, fill factor 0.61 and efficiency 10.8%.Adhesion of the chalcopyrite film to the substrate was improved by incorporation of Al into the absorber layer. Even if the Cu/(In+Al) ratio of a sputtered metallic precursor is increased to 2.7, peeling of the film was not observed. A CuAlS_2 thin layer was obtained by sulfurization in a closed quartz ampoule as a first chalcopyrite, The layer exhibited p-type conductivity, resistivity higher than 60 Ωcm and energy bandgap of 3.5eV.