Determination of Mobile Ion Densities in Halide Perovskites via Low-Frequency Capacitance and Charge Extraction Techniques.

Mobile ions in perovskite photovoltaic devices can hinder performance and cause degradation by impeding charge extraction and screening the internal field. Accurately quantifying mobile ion densities remains a challenge and is a highly debated topic. We assess the suitability of several experimental methodologies for determining mobile ion densities by using drift-diffusion simulations. We found that charge extraction by linearly increasing voltage (CELIV) underestimates ion density, but bias-assisted charge extraction (BACE) can accurately reproduce ionic lower than the electrode charge. A modified Mott-Schottky (MS) analysis at low frequencies can provide ion density values for high excess ionic densities, typical for perovskites. The most significant contribution to capacitance originates from the ionic depletion layer rather than the accumulation layer. Using low-frequency MS analysis, we also demonstrate light-induced generation of mobile ions. These methods enable accurate tracking of ionic densities during device aging and a deeper understanding of ionic losses.

Open-circuit and short-circuit loss management in wide-gap perovskite p-i-n solar cells.

In this work, we couple theoretical and experimental approaches to understand and reduce the losses of wide bandgap Br-rich perovskite pin devices at open-circuit voltage (VOC) and short-circuit current (JSC) conditions. A mismatch between the internal quasi-Fermi level splitting (QFLS) and the external VOC is detrimental for these devices. We demonstrate that modifying the perovskite top-surface with guanidinium-Br and imidazolium-Br forms a low-dimensional perovskite phase at the n-interface, suppressing the QFLS-VOC mismatch, and boosting the VOC. Concurrently, the use of an ionic interlayer or a self-assembled monolayer at the p-interface reduces the inferred field screening induced by mobile ions at JSC, promoting charge extraction and raising the JSC. The combination of the n- and p-type optimizations allows us to approach the thermodynamic potential of the perovskite absorber layer, resulting in 1 cm2 devices with performance parameters of VOCs up to 1.29 V, fill factors above 80% and JSCs up to 17 mA/cm2, in addition to a thermal stability T80 lifetime of more than 3500 h at 85 °C.