Chemical and Electronic Properties of Low‐Temperature ALD‐NiO _x and Its Application in Perovskite and Tandem Solar Cells

Atomic layer deposition (ALD) is a versatile approach for depositing nickel oxide (NiO _x ) hole‐selective layers in perovskite solar cells (PSCs), particularly for integration into multijunction devices where low processing temperatures are required. However, deposition temperature can strongly influence the chemical and electronic properties of NiO _x films and their interfaces, affecting device performance. Here, we investigate ALD‐NiO _x layers deposited at temperatures ranging from 75 to 250 °C and correlate their properties with photovoltaic performance. X‐ray photoelectron spectroscopy reveals an increase in stoichiometric NiO content from 76% to 96% with increasing deposition temperature, while only minor variations in work function are observed. PSCs fabricated using these ALD‐NiO _x layers show only marginal performance improvements above 150 °C, with efficiency gains below 1% absolute, mainly due to changes in fill factor and short‐circuit current. As an application‐oriented demonstration, ALD‐NiO _x deposited at 100 °C was implemented in a monolithic CIGS/perovskite tandem device, yielding a power conversion efficiency above 23%, with an open‐circuit voltage of 1651 mV and a fill factor of 76%. This result demonstrates the compatibility of the investigated low‐temperature ALD process with the fabrication of temperature‐sensitive multijunction device architectures.