Impact of Back-Contact Passivation

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There is extensive research aimed at improving CZTS solar cells, since their efficiency is still much lower than that of corresponding CIGS solar cells. In CIGS, the material quality is significantly higher than in CZTS. Therefore, recent research on CIGS has focused more on surface passivation, where electrons excited in the absorber layer can recombine at the interface to another layer. Several methods have been studied for passivating both the front and back surfaces of the absorber layer, for example at the back contact by depositing a layer of Al₂O₃ containing nanoscale openings that allow electrical contact between the absorber layer and the contact.

In this study, the possibility of depositing a similar Al₂O₃ layer with nanoscale openings for CZTS solar cells was investigated. Because the material quality is not yet very high, not many electrons would reach the back contact if the absorber layer were too thick; therefore, ultrathin CZTS layers were used to examine how the passivation layer affects efficiency. As a reference, CZTS solar cells were produced in the same way as the passivated ones but without the passivation layer.

The ultrathin solar cells without passivation showed lower voltage and current — and thus lower efficiency — than standard CZTS solar cells with thicker layers. The largest losses in these reference cells are due to high recombination at the back contact and degraded material quality, for example because alternative combinations of the constituent elements are formed. The ultrathin solar cells with a passivation layer showed higher current, voltage, and overall efficiency than the reference cells, indicating that the passivation layer is an effective way to reduce recombination at the back contact. The passivation layer likely also reduces the effect of unwanted secondary phases that degrade the material quality.

Glossary

Passivation: When surfaces are coated to prevent reactions and improve optical and/or electrical properties.

Recombination: When an electron releases its energy and returns to the valence band.