Rf-sputtered aluminium doped zinc oxide films: Enhanced damp heat stability by means of plasma etching treatment

- Sputtered textured ZnO:Al films have been treated by Ar-plasma to improve their electrical stability.
- The effect of Ar-treatment was monitored by using damp heat degradation test.
- Wet and Ar-treated ZnO:Al layers showed improved long-term electrical stability.
- Wet and Ar-treated films showed crystalline rearrangement and lattice defects variation up to 30 nm depth.
- Chemical/structural modification, induced by Ar plasma bombardment, gave electrical stability enhancement.

ZnO:Al films, obtained by RF-sputtering and textured by wet etching, have been subjected to argon plasma etching treatment in order to stabilize their electrical properties over time, being well known the electrical degradation of these transparent conductive films when exposed to air. Specifically focused on photovoltaic application as front electrode, the effect of argon plasma treatment on long-term electrical stability of ZnO:Al thin film was monitored by performing damp heat degradation cycles in environmental test chamber. There was evidence for strong slowdown of electrical degradation when ZnO surface had been subjected to Ar plasma treatment. Different etching times have been used and it has been observed that surface modification induced by plasma treatment gave its more pronounced beneficial effect after the first 10 min of the process. In particular, for no-plasma treated samples the carrier mobility worsening is about three times larger than that related to Ar-treated samples after long-lasting aging test. The stabilization mechanism of the electrical properties was investigated by studying the main modifications of the plasma etched materials in terms of optical, chemical, morphological and structural properties. Structural analysis revealed that crystalline rearrangement occurs down to 30 nm depth below the plasma treated film surface, where a thin nanocrystalline layer forms. PL analysis revealed a different distribution of deep defect centres for plasma treated film surface compared to untreated one. In particular, donor-like lattice defects (Vo and Zni) decreased whereas Vzn defects became predominant. Crystalline rearrangement and change of type and/or amount of deep defect centres inside the plasma modified top layer have been hypothesized as responsible for the enhancement of the electrical stability.