In perovskite/thin-film tandem solar cells, the top electrode must be composed of transparent conductive oxides (TCOs) to ensure sufficient light transmission to the underlying thin-film subcell. However, the conventional sputtering processes typically used for TCO deposition are accompanied by harsh conditions, such as surface damage by plasma interaction and high substrate temperatures, which can cause severe degradation of the perovskite top cell. These challenges underscore the necessity of introducing a protective interfacial layer that can be deposited under mild conditions to preserve the integrity of the perovskite absorber while maintaining the optical and electrical requirements for high-efficiency tandem integration.

In this work, we explored the use of atomic layer deposition (ALD), a technique well known for its precise thickness control, conformal coverage, and low-temperature processability, to deposit tin oxide (SnO) thin films as a protective layer. The ALD process was systematically optimized under low-temperature conditions in order to suppress thermal degradation of the perovskite absorber. Specifically, the substrate temperature was progressively reduced below 100 °C, and subsequent optimization of the purge cycle time was carried out by gradually shortening the duration from 15 s at the optimized substrate temperature. Furthermore, thickness optimization was conducted by reducing the SnO film thickness starting from 20 nm. This reduction not only improved optical transmittance, thereby facilitating higher photon flux into the bottom thin-film subcell, but also minimized thermal stress and degradation of the perovskite layer by shortening the overall deposition time.

The effect of these parameters on the perovskite absorber was carefully examined, with particular attention paid to the evolution of secondary phases that could deteriorate device performance. Through these investigations, we successfully established an ALD-SnO deposition process suitable for use as a protective layer in perovskite solar cells. Finally, the optimized SnO protective films were integrated into both single-junction perovskite solar cells and perovskite/thin-film tandem solar cells. The results clearly demonstrate the feasibility of ALD-based materials as effective protective layers, highlighting their potential to enable reliable fabrication of high-performance tandem devices without plasma-induced damage or thermal degradation. This study provides important insights into interface engineering strategies for the development of next-generation tandem photovoltaics.