Abstract
In this work, the diffusion process of sodium (Na) in molybdenum (Mo) thin films while it was deposited on soda lime glass (SLG) was studied. A small amount of oxygen was present in the chamber while the direct-current (DC) magnetron sputtering was used for the deposition. The substrate temperatures were varied to observe its effect. Such molybdenum films, with or without oxidations, are often used in thin film solar cells, either as back contact or as hole transport layers. Secondary ion mass spectrometry (SIMS) was used to quantify the concentration of the species. A grain diffusion mechanistic model incorporating the effect of grain and grain boundary geometrical shape and size was developed. The model was used to provide an in-depth theoretical analysis of the sodium diffusion in molybdenum thin films that lead to the measured SIMS data. It was observed that not only diffusion coefficients should be considered when analyzing diffusion processes in thin films but also the ratio of grain boundary size to grain size. Both depend on substrate temperature and directly affect the amount of diffused species in the film. The data were analyzed under the light of the film growth speed versus diffusion front speed, the effect of oxygen content, and the effect of substrate temperature on the overall diffusion process. The temperature inversely affects the ratio of grain boundary size and grain size and directly affects the diffusion coefficient, which leads to a preferable temperature at which the highest amount of alkali can be found in the film.
Highlights
High efficiency Cu(In,Ga)Se2 (CIGS) solar cells are always achieved by incorporating small amount of sodium (Na) into the CIGS film to improve open-circuit voltage and fill factor [1]. molybdenum (Mo) coated soda-lime glass (SLG) is a common substrate used for CIGS solar cells
There are several reasons for that: (1) Both sodium and potassium originate from the SLG; (2) when the films are beginning to coalesce into the columnar structure, it is possible for more oxidation to occur because of the higher effective surface area of the molybdenum during the preliminary film coalescence; (3) because the matrix of material is transitioning from an oxidized molybdenum to soda lime glass, it is fairly common to see the intensity of atomic species of small concentration to build-up at such interfaces due to atomic mixing
Computational diffusion modeling was performed for all the molybdenum thin films following the trial-and-error scheme to obtain the corresponding diffusion coefficients
Summary
High efficiency Cu(In,Ga)Se2 (CIGS) solar cells are always achieved by incorporating small amount of sodium (Na) into the CIGS film to improve open-circuit voltage and fill factor [1]. molybdenum (Mo) coated soda-lime glass (SLG) is a common substrate used for CIGS solar cells. On top of that, none of them have considered directly the grain size effect on the SIMS intensity data, which, as it will be shown later, it likely biases the measured diffusion coefficient. Another key element to take into account for the study of alkali diffusion in Mo is oxygen. The analysis of the data examines the rate at which the molybdenum is deposited and its relation to the extent of the oxidation This culminates in analyzing how the diffusion front for the sodium moves with respect to the film growth.
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