Critical Role of Perovskite Film Stoichiometry in Determining Solar Cell Operational Stability: a Study on the Effects of Volatile A-Cation Additives.

Volatile A-cation halide (AX) additives such as formamidinium chloride and methylammonium chloride have been widely employed for high-efficiency perovskite solar cells (PSCs). However, it remains unstudied how they influence the perovskite film stoichiometry and the solar cell performance and operational stability. Hereby, our work shows that over annealing of formamidinium chloride-containing perovskite films leads to a Pb-rich surface, resulting in a high initial efficiency, which however decays during maximum power point tracking (MPPT). On the contrary, perovskite films obtained by a shorter annealing time at the same temperature provide good stability during MPPT but a lower initial efficiency. Thus, we deduce that an optimal annealing is vital for both high efficiency and operational stability, which is then confirmed in the case where methylammonium chloride additive is used. With optimized perovskite annealing conditions, we demonstrate efficient and stable p-i-n PSCs that show a best power conversion efficiency of 20.7% and remain 90% of the initial performance after a 200 h MPPT at 60 °C under simulated 1 sun illumination with high UV content. Our work presents a comprehensive understanding on how volatile AX impacts perovskite film stoichiometry and its correlation to the device performance and operational stability, providing a new guideline for fabricating high-efficiency and operationally stable PSCs.

Ultrathin NiGe films prepared via catalytic solid-vapor reaction of Ni with GeH(4).

A low-temperature (225-300 °C) solid-vapor reaction process is reported for the synthesis of ultrathin NiGe films (∼6-23 nm) on 300 mm Si wafers covered with thermal oxide. The films were prepared via catalytic chemical vapor reaction of germane (GeH4) gas with physical vapor deposited (PVD) Ni films of different thickness (2-10 nm). The process optimization by investigating GeH4 partial pressure, reaction temperature, and time shows that low resistive, stoichiometric, and phase pure NiGe films can be formed within a broad window. NiGe films crystallized in an orthorhombic structure and were found to exhibit a smooth morphology with homogeneous composition as evidenced by glancing angle X-ray diffraction (GIXRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Rutherford back-scattering (RBS) analysis. Transmission electron microscopy (TEM) analysis shows that the NiGe layers exhibit a good adhesion without voids and a sharp interface on the thermal oxide. The NiGe films were found to be morphologically and structurally stable up to 500 °C and exhibit a resistivity value of 29 µΩ cm for 10 nm NiGe films.