[Objective] Perovskite solar cells have drawn considerable attention in recent years. The addition of additives to perovskite precursor solutions is an important method to improve the quality of perovskite films for enhancing the performance of perovskite solar cells. In the past, alkali metal ions were extensively used as additives. Rubidium ions (Rb+) were generally added into perovskite films alongside other kinds of cations, following which the photovoltaic performance of the solar cells was clearly improved. However, few researchers studied the effects of only adding various proportions of Rb+ on perovskite films. In this study, rubidium chloride (RbCl) was used as an additive in perovskite precursor solutions and the morphology and structure of perovskite films were analyzed. [Methods] Perovskite films were fabricated using a two-step method. RbCl was used as an additive into lead (ID iodide (Pbl2) precursor solutions with the RbCl proportions 2%, 4%, 6%, 8%, 10%, and 12% versus Pbl2, and a Pbl2 precursor solution with no RbCl added was used as the control. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis were employed to analyze the difference in surface morphology and structure of the perovskite films. Steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) spectra were recorded using devices of fluorine-doped tin oxide (FTO)/SnOz/perovskite films to study the carrier-transporting properties. The photovoltaic performances of the perovskite solar cells were studied through a solar simulator and external quantum efficiency testing. UV-visible (UV-vis) absorption spectra were recorded to explore the change in light absorption. [Results] The crystalline grain size is clearly enhanced upon adding 4% RbCl. The grain size is 1.61 fim in the control and 2. 14 pm upon adding 4% RbCl. However, a high addition proportion O 8%) damages and distorts the crystal structure, decreasing the film quality. Adding RbCl at a low proportion is beneficial for guiding the growth of perovskite grains, increasing grain size, and forming a dense film with fewer holes. The XRD patterns reveal that the peak at 12. 6° corresponding to Pbl2 is suppressed upon adding RbCl, whereas a new peak appears at 11. 3°. The suppression of the Pbl2 peak and the appearance of the new peak can be attributed to the formation of the RbCl complex and excessive Pbl2, and the complex can be observed in the SEM images, which is confirmed by EDS results. The TRPL results reveal that adding RbCl at a low proportion enhances the transport and extraction of charge carriers, which is consistent with the SSPL results. Furthermore, the photovoltaic performance results reveal that with RbCl as an additive, the photoelectric conversion efficiency of the perovskite solar cells increases from 18.88% to 20.06%, and photoelectric properties such as open-circuit voltage, short-circuit current density, and filling factor are considerably improved. However, the UV-vis absorption spectra show that the absorption is not improved upon adding RbCl and even decreases with a high addition proportion, which is due to the increasing roughness of the perovskite films with increasing RbCl proportion. The enhancement of the photoelectric properties is due to the increase in transport and extraction of charge carriers caused by the improvement in film quality. [Conclusions] This research demonstrates that adding RbCl at low proportions can enhance the grain size and transport of the carriers, improving the photovoltaic performance. The optimal RbCl addition proportion is ~ 4%. This study has considerable potential for improving the performance of perovskite solar cells. © 2024 Tsinghua University. All rights reserved.
