Passivation of the back contact with Al2O3

Glossary:
Passivation When surfaces are covered with a protective layer to avoid reaction and improve optical and/or electrical properties.
Recombination When the electron emits its energy and returns to the valence band.

There is a lot of research aiming to improve the CZTS-solar cells since these still have much lower efficiency than the corresponding CIGS-solar cells. For CIGS, the quality of the material is already much higher than for CZTS. Therefore, the research on CIGS has lately been focusing on passivation of the surfaces, since the electrons that have been excited in the absorber layer sometimes recombine in the interfaces. For CIGS, several ways to passivate the surface on both the front and back side of the absorber layer have been investigated, for example at the back contact by depositing a layer of Al2O3 with nanosized point openings to provide contact between the absorber layer and the back contact.

In this study, the possibility to deposit a similar layer of Al2O3 with nanosized openings on CZTS-solar cells was investigated. Since the CZTS-material does not have as high quality yet not many electrons will be able to reach the back contact if it is too thick, and therefore ultrathin layers of CZTS was used to see how the passivation layer affect the efficiency. As a reference, CZTS-solar cells were also manufactured in the same way as the passivated but without the passivation layer.

The ultrathin solar cells without the passivation layer were seen to have lower voltage and current, and therefore efficiency, than standard solar cells of CZTS-type with thicker layers. The biggest losses in these reference cells are due to high recombination at the back contact and lower material quality, for example because other combinations of the elements are formed. The ultrathin solar cells with a passivation layer had higher current, voltage and total efficiency than the reference solar cells, and the passivation layer therefore seems to be a good way to decrease the recombination at the back contact. The passivation layer probably also decreased the impact of the other combinations of elements that were leading to lower material quality and therefore losses.

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