Stabilizing the Chemistry of NiO<sub>x</sub> in Perovskite Solar Cells to Pass the Damp Heat Test

Article Stabilizing the Chemistry of NiOx in Perovskite Solar Cells to Pass the Damp Heat Test Marion Dussouillez 1,2,*,†, Mounir Mensi 3, Ivan Marozau 1, Quentin Jeangros 1, Sylvain Nicolay 1,‡, Christophe Ballif 1,2 and Adriana Paracchino 1,* 1 CSEM Sustainable Energy Center, Rue Jaquet-Droz 1, 2002 Neuchâtel, Switzerland 2 Laboratory of Photovoltaics and Thin Film Electronics, Institute of Electrical and Micro-Engineering (IEM), Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71b, 2000 Neuchâtel, Switzerland 3 X-Ray Diffraction and Surface Analytics Platform, Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue de l’Industrie 17, 1951 Sion, Switzerland * Correspondence: marion.dussouillez@gmail.com (M.D.); adriana.paracchino@csem.ch (A.P.) † Current address: Solarlab Aiko Europe GmbH, Berliner Allee 29, 79110 Freiburg im Breisgau, Germany ‡ Current address: Rolex S.A., David-Moning-Strasse 9, 2504 Biel, Switzerland Received: 7 January 2025; Revised: 10 March 2025; Accepted: 13 March 2025; Published: 18 March 2025 Abstract: NiOx is widely used as a hole transport material in perovskite solar cells (PSCs). This wide band gap p-type material is conveniently deposited via high throughput RF-sputtering, making it suitable for the industrialization of PSCs. Nonetheless, for the cells to pass accelerated degradation tests such as the IEC 61215 damp heat (DH) test, the chemistry of the NiOx film should remain constant at elevated temperaturs to preserve its optoelectronic properties. This study emphasizes that structural defects resulting from Ni vacancies in NiOx lead to significant degradation of the PSCs after just a few hours of exposure to elevated temperatures (85 °C). We introduce here an approach to fine-tune the chemistry of the NiOx film by adjusting the gas flow during sputtering deposition and by incorporating Cs. Through this control on the chemistry of the layer, the optimized NiOx-based PSCs exhibit remarkable stability, with devices passing 5 times the IEC 61215 norm (<5% rel after 5000 h of DH testing) and also showing better stability under light soaking. XPS analysis reveals that the concentration of Ni3+ in the bulk of the standard NiOx film is twice that in the optimized NiOx. This suggests that the Ni3+ concentration, typically equal to the Ni vacancy concentration and beneficial for charge transport in NiOx, may actually compromise the stability of the PSCs. Additionally, the film density of the optimized NiOx film was significantly higher than that of the standard film.