**4. Summary**

In this study, it is proved that suitable amounts of cesium can improve the film morphology and crystallinity of Pb(Ac)2-based perovskite films and adjust the electrical properties of the photoactive layer of perovskite for extracting more charge. PSCs based on these Pb(Ac)2-based perovskite films were demonstrated with a PCE, Voc, Jsc, and FF of 15.57%, 0.98 V, 21.08 mA·cm<sup>−</sup>2, and 0.75, respectively. Additionally, the optimized devices showed negligible hysteresis in the forward and reverse J–V scans. This research shows that a perovskite precursor based on lead acetate is a promising source to achieve highly efficient PSCs, and further improvements will be possible through subtle tuning of the chemical composition.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1996-194 4/14/2/363/s1, Figure S1: Reverse scan and forward scan J–V curve of 5% Cs-doping device; Figure S2: Tapping-mode AFM height images of (a) the pristine and (b) Cs-doped perovskite films.; Table S1: Fitted values of the equivalent circuit parameters from dark Nyquist plots of devices without and with 5% Cs.

**Author Contributions:** Conceptualization, Z.L.; Formal analysis, X.L.; Investigation, M.-S.H.; Supervision, E.-C.L.; Visualization, J.Y.; Writing—original draft, M.-S.H., Z.L., X.L. and J.Y.; Writing—review & editing, E.-C.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. NRF-2016R1A2B2015389) and the Gachon University research fund of 2019 (Grant No. GCU-2019-0350).

**Data Availability Statement:** Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.
