Improved environmental stability of highly conductive nominally undoped ZnO layers suitable for n-type windows in thin film solar cells

Highly conductive nominally undoped ZnO (b-ZnO), obtained by means of an additional plasma near the substrate during sputter deposition, represent an attractive alternative for ZnO:Al (AZO) commonly employed in transparent windows of thin film solar cells. b-ZnO layers exhibit more than twice higher charge carrier mobility in comparison to AZO layers of the same resistivity (1·10−3Ωcm). In consequence, a better transparency in near infrared region and an enhanced short circuit current can be achieved for low band gap thin film solar cells. Replacement of AZO for b-ZnO thus enhances their energy output. In order to allow assessment of suitability of these b-ZnO films for deployment in photovoltaic industry, we examine their stability in various environments, and show pathways to improve it. We demonstrate that the b-ZnO films can exhibit comparable stability to ZnO:Al films in both ambient and heated air over the period of 24 months. However, the examined b-ZnO films degrade faster in accelerated open damp heat (DH) conditions, which we attribute to the lower compactness of columnar microstructure. In order to circumvent this limitation, we introduce a novel multilayered b-ZnO film with an improved environmental stability, as verified by the enhanced optoelectrical performance of DH-treated Cu(InGa)(SSe)2 solar cells.