Stabilizing the CZTS-surface by controlling the partial pressure

Irreversible A reaction that can only go in one direction.
Lattice vibrations Vibrations of the atoms in the lattice, i.e. crystal structure.
Partial pressure The pressure from one of the compounds in a gas phase.
Annealing Heat treatment of films to get a certain crystal structure.

An important difference between CZTS- and CIGS-solar cells is the materials’ stability, especially at a high temperature and vacuum. For CZTS, high temperatures will induce losses of tin (Sn) which has been attributed to evaporation of SnS. This is a problem especially at the surface of the material where the products can be transported away from the reaction site, which makes the reaction irreversible. In this study, possible models to explain the decomposition and loss of SnS were investigated. The models were then compared with experimental data and the driving force behind the process was analyzed.

The first model to describe the decomposition is a reversible single step reaction where CZTS is decomposed and SnS and S2 are evolved directly into the gas phase. For this model, the reaction is initiated due to random fluctuations, for example lattice vibrations, which depend on the temperature. The reversibility depends on the partial pressure of the products in the gas phase, here SnS and S2, since these are needed as reactants for the backwards reaction. For every temperature, there is therefore a critical limit for the partial pressures when the reaction rate is zero and the decomposition of the CZTS is equal to the re-formation. To get a stable surface of CZTS in this model, both SnS and S2 have to be present in the surrounding gas phase. For the other model, the decomposition is divided into two steps. In the first reaction, CZTS is decomposed, but SnS remains at the surface while S2 is evolved into the gas phase. The second reaction is then the evaporation of SnS from the surface to the gas phase.  These two reactions can be combined to a complete expression for the reaction rate, which for a certain temperature also is seen to depend on the partial pressures of SnS and S2. There are therefore threshold values for every temperature also in this model where both SnS and S2 are needed for stability. In a gas phase with only S2, however, the first reaction is still reversible since SnS does not occur as a gas, and only the second reaction where SnS is evolved into the gas phase will be irreversible. For model II, the partial pressure of S2 will therefore have an effect on the reaction rate also without the presence of SnS, which is not the case in the first model.

In the experimental part, CZTS films were deposited with sputtering and annealed at 550°C, where the decomposition might take place. During the annealing, Ar was flowing with a controlled speed over one source of sulfur and one of SnS which were then mixed and brought into the annealing chamber, and due to the constant flow the two partial pressures could be calculated from the mass changes in the source materials.

Both models had predicted a constant reaction rate, which up to a certain point was also seen experimentally. The most important difference between the models was that model I predicted that the partial pressure of S2 would not change the reaction rate without the presence of SnS, while model II predicted that the reaction rate would decrease with an increased partial pressure of S2. The experiments supported model II, which was therefore determined to be the more realistic model. In the experiment, the critical pressures needed to stabilize the CZTS surface were also determined.

It has earlier been implied that the driving force for the instability is the high vapor pressure of SnS. However, since SnS does not occur in the gas phase for the decomposition reaction in model II this explanation cannot be valid. The results from this study demonstrated that it is more probably that the process is driven by a redox potential where tin is reduced from Sn(IV) in CZTS to Sn(II) in SnS. Since S2 works as an oxidizing agent, its partial pressure will determine the redox environment and thereby the stability for Sn in CZTS.

The described reactions offer an explanation to why CZTS should not be deposited in only one step with sputtering at high temperatures, as for example CIGS, but in two steps where the material is deposited at a low temperature and then annealed. Understanding the process and the necessary partial pressure could be used to further develop and control the conditions of the annealing, which would enable production of CZTS films with increasing quality.

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