Possibilities for large scale production of CZTS-solar cells

Sputtering Method for depositing films from an original material source.
Annealing Heat treatment of films to get a certain crystal structure.

Sputtering has already been proved as a successful method for producing CIGS-solar cells, and it has the advantage that it can be used also for larger solar cell sizes. For large scale methods, the processes need to be uniform to result in high quality materials, which is therefore an important factor in the development of production methods for CZTS. In contrast to CIGS, deposition of the absorber layer in the CZTS-solar cells is divided into two steps where the sputtering is followed by an annealing in high temperature. In this study, CZTS-solar cells were characterized before and after the annealing to investigate possible improvements towards a large scale production.

In the study, CZTS-solar cells were fabricated on a substrate of 6-inch stainless steel plates with the layers MoNa/Mo/CZTS/In2S3/ZnO and ITO with equipment from the solar cell company Midsummer. The CZTS layer was deposited through sputtering followed by annealing in an atmosphere of Ar and H2S. The composition and morphology of the samples were measured along the diagonal of the sample, and the performance of each part of the sample was evaluated by dividing the sample into 184 small cells of 1x1cm2.

The composition of the absorber layer directly after the sputtering was found to be rather close to the target composition. In the middle of the sample, the tin level was somewhat higher whereas the copper level was higher at the edges, and the morphology of the sample was relatively even. For the annealed sample, the composition of the sample was significantly more uneven. The Sn level had decreased, especially in the middle, and the zinc level had decreased especially at the edge.

This could be explained with a varying temperature distribution in the annealing chamber which gives rise to an uneven annealing for the different parts of the sample. Simulation and measurements with a pyrometer confirmed the uneven heat distribution. The morphology showed a more developed recrystallization for the top part of the sample where the small solar cells also had the highest performance, while the lower part of the sample did not reach the temperature needed for recrystallization. A possible improvement is therefore to develop and stabilize the annealing process to get an even temperature, so that solar cells could be fabricated in a larger scale with as even layers as before the annealing.

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