**1. Introduction**

Organometallic halide perovskites (APbX3, in which A = methylammonium (MA+) or formamidinium (FA+), and X = Cl<sup>−</sup>, Br<sup>−</sup>, or I−) have attracted considerable attention because of their tunable bandgap, high light absorption coefficient, and long exciton diffusion length over one micrometer [1]. Since being first reported by Kojima et al. in 2009 [2], perovskite solar cells (PSCs) have been intensively investigated, with rapid improvement in power conversion efficiency (PCE) to above 25.5% [3–8]. As a result, PSCs are considered one of the most promising candidates for next-generation solar energy devices.

To prepare a CH3NH3PbI3 perovskite, typically lead iodide (PbI2) is used as the lead source, which chemically reacts with methylammonium iodide (MAI) at a molar ratio of 1:1 [9]. However, in order to obtain a high-quality perovskite film with a uniform morphology, an anti-solvent treatment is needed for the one-step spin-coating process [10]. This anti-solvent treatment requires expensive technology for the stable crystallization of the perovskite grains, which is detrimental to commercialization [11]. To overcome this problem, lead chloride (PbCl2) can be employed to replace PbI2 as the lead source, which has resulted in PSCs with similar performance to those prepared from PbI2 [12]. However, to fully remove the residual MACl from the perovskite film, a lengthy thermal annealing process (of about two hours) is required, which consumes a large amount of energy [13]. Besides lead halides, lead acetate (Pb(Ac)2) is another important lead source, which can avoid the need for the inconvenient anti-solvent treatment and lengthy thermal annealing processes [13]. Zhang and coworkers have shown that the Pb(Ac)2-processed CH3NH3PbI3 perovskite shows a more uniform and compact morphology with increased crystallinity, compared with PbCl2- or PbI2-processed perovskites. As a result, the PSCs based on Pb(Ac)2-processed perovskite films were shown to exhibit a high PCE of 14.7%, which is

higher than that of either PbCl2- or PbI2-based PSCs [13]. To improve the performance of PSCs, morphology control of the perovskite film is crucial, because it is strongly related to the charge generation and dissociation properties of the PSCs [14]. Solvent engineering is a widely used technique for controlling the morphology of perovskite films. For example, the use of additional dimethyl-sulfoxide has resulted in an improved film morphology of Pb(Ac)2-based perovskite [15]. Doping of the perovskite crystal is another effective way to improve the morphology of the perovskite films. For example, Br− and FA+ have been used to partially replace I− and MA+ in the perovskite structure for preparing mix-cation perovskite (FAxMA1-xPbIyBr3−<sup>y</sup>), which resulted in a significantly improved PCE and enhanced stability of the PSCs [16]. For PbI2-processed perovskite, cesium doping into the MA site has been demonstrated to be an efficient way to improve the performance of the associated PSCs [16]. M. Saliba and coworkers have demonstrated that doping with 5 mol% cesium resulted in a uniform and compact perovskite film morphology with fewer pinholes, which significantly improved the PCE of the PSCs from 16.37 to 19.20% [16]. However, there has not been any investigation into the cesium doping effect on the film morphology of Pb(Ac)2-based perovskite and the performance of Pb(Ac)2-based PSCs. Considering the importance and convenience of using Pb(Ac)2 for perovskite preparation, it is crucial to dope Pb(Ac)2-based perovskite with cesium for PSC fabrication.

In this work, we doped the Pb(Ac)2-based perovskite with cesium by adding a small amount of cesium iodide (CsI) into the perovskite precursor. After doping, the perovskite film showed a uniform morphology with enhanced crystallinity and reduced pinholes, which is beneficial for charge transportation. Consequently, the champion device PCE raised from 15.22 to 18.02% with negligible hysteresis and a stable output which is a significant improvement in open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) via cesium doping. Additionally, the average PCE of the Pb(Ac)2-based PSCs was significantly improved from 14.1 to 15.57%. Our results demonstrate the superior effect of cesium doping on the performance improvement of Pb(Ac)2-based PSCs.
