Stoichiometric MACl/PbCl₂ Additive Engineering for Improved Reproducibility and Carrier Lifetime in FA/Cs...

Chloride-based additive engineering has become nearly indispensable in the development of high-efficiency perovskite solar cells (PSCs), with the majority of record-setting devices achieving power conversion efficiencies (PCE) exceeding 25% using methylammonium chloride (MACl) as a processing additive. However, the extreme process sensitivity and consequent irreproducibility associated with MACl incorporation are rarely addressed. Here, we systematically investigate MACl-processed FA_0.9Cs_0.1Pb-(I_0.9Br_0.1)₃ films and show that MACl-assisted processing can act as a double-edged strategy. Under nominally identical processing conditions, a minority of samples exhibit strong photoluminescence (PL) enhancement, whereas the majority display PL quenching despite their larger grain sizes. Using complementary techniques such as X-ray diffraction (XRD), line-profile analysis, atomic force microscopy (AFM), and time-resolved PL (TRPL), we attribute this bimodal response to compositional and structural heterogeneity, including halide-segregated domains and nonuniform microstrain that increase trap-assisted nonradiative recombination. To mitigate this instability, we replace volatile MACl with a reduced-volatility chloride additive derived from a stoichiometric MACl/PbCl₂ precursor (1:1 molar ratio, hereafter MCP chloride additive). At optimal additive concentration (5 mol %), the MCP-5 yields single-phase diffraction signatures, homogenized PL, and substantially extended carrier lifetimes, increasing the average lifetime by a factor of ∼7-9 (from ∼180 ns to ∼1.3-1.6 μs). In n-i-p PSCs, MCP-5-based devices yield modest but representative efficiencies, with a best PCE of 12.03%, a ∼20% relative efficiency gain, and a 30 mV increase in open-circuit voltage compared to MACl-20-treated devices, together with reduced device-to-device variability. Short-term humidity tests on unencapsulated cells (70-75% relative humidity, 24 h) further show improved PCE retention for MCP-5 compared to MACl-20 and control devices. Overall, these results identify MACl-assisted processing-induced heterogeneity as a key source of irreproducibility and establish MCP chloride additive processing routes as a promising approach to more reproducible mixed-halide PSCs, while highlighting the need for future long-term stability studies.