Understanding the role of oxygen in the segregation of sodium at the surface of molybdenum coated soda‐lime glass

Abstract

Molybdenum (Mo) coated soda‐lime glass is a commonly used substrate for Cu(InGa)Se2 solar cells as it also acts as the sodium (Na) source, which improves the efficiency of these devices. In this work, we investigate how oxygen controls the segregation and accumulation of Na on the Mo surface. A direct relationship between the concentration of surface oxygen and the amount of Na accumulation is showed. Values for the surface segregation ratio and grain boundary diffusion coefficient for Na in Mo are obtained by fitting diffusion data at several temperatures to a model for grain boundary diffusion. The results of this model reveal that surface oxygen controls the Na saturation level through its effect on the surface segregation of Na. An activation energy for grain boundary diffusion of Na is estimated and is similar to that of MoO bond dissociation in MoO3 suggesting the involvement of this bond during Na transport. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2365–2372, 2014

Incorporation of alkali metals in chalcogenide solar cells

“…In two publications, Blosch et al [106,107] have stated that Na easily diffuses from internal interfaces of two MoNa layers possibly because Na moves through grain boundaries. In addition they also found that the microstructure of the MoNa layers strongly influence this diffusion, in accordance with several findings [57,108]. Therefore, Blosch et al introduced multi-stacked layers of MoNa to allow more Na to be released and to arrive to the CIGSe layer.…”

“…However, also here contradictory results have been presented in the literature, stating that SLG provides an optimal amount of Na [16], that it provides too much Na [97], and that it provides too little Na [34]. The reason for this inconsistency is likely associated with the complexity of the Na out-diffusion process, originating from the glass and going through the Mo layer [57]. A schematic representation of the diffusion process based on the H þ /Na þ ion exchange model is shown in Fig.…”

“…It has been theoretically proposed [91] and experimentally verified [92] that the mechanism responsible for Na diffusion is a H þ /Na þ ion exchange process and that the H þ is being stored in the Mo layer during air exposure whereas the source of Na is Na 2 O present in the glass [93]. The diffusion of Na from the SLG through the Mo proceeds via oxygenated Mo grain boundaries [92,94] and it has also been shown that oxygenated Mo layers provide CIGSe with higher Na concentrations [57,95]. It is worth mentioning that the amount of Na that diffuses is of Table 2 Typical composition of SLG (only the main constituents are given) [88].…”

“…There are other disputed topics regarding the influence of Na in CIGSe: the promotion of MoSe 2 by Na [38,[49][50][51][52]; the exact location and effect of Na in the grain boundaries or in the bulk [43,53]; and the relation between oxidized compounds with sodium [5,[54][55][56][57]. The reason for these numerous contradictory results is likely connected with the different growth processes used for the deposition of CIGSe.…”

Reversibility of (Ag,Cu)(In,Ga)Se2 electrical properties with the addition and removal of Na: Role of grain boundaries

“…Heating in air drives Na out of the film where it accumulates on the surface. The presence of oxygen has shown to greatly increase the rate of Na diffusion, 25,26 so the presence of air during the heat treatment facilitates the accumulation of Na on the surface. The newly accumulated Na is once again rinsed away so that the combination of surface accumulation followed by dissolution lowers the Na concentration in the film, making it possible to control Na content by increasing the number of heat/rinse cycles.…”