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Article

Protic Ionic-Liquid Precursor Engineering with Methylammonium Acetate for Efficient and Stable Inverted Triple-Cation Perovskite Solar Cells

by
Hanhong Zhang
1,2,
Jun Song
1 and
Yuanlong Deng
2,3,*
1
State Key Laboratory of Radio Frequency Heterogeneous Integration, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
2
School of Sino-Germany Intelligent Production, Shenzhen City Polytechnic (Shenzhen Institute of Technology), Shenzhen 518116, China
3
College of Urban Transport and Logistics, Shenzhen Technology University, Shenzhen 518118, China
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(1), 19; https://doi.org/10.3390/cryst16010019
Submission received: 10 December 2025 / Revised: 17 December 2025 / Accepted: 25 December 2025 / Published: 26 December 2025
(This article belongs to the Special Issue Advanced Research on Perovskite Solar Cells)
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Abstract

Perovskite solar cells (PSCs) have achieved remarkable efficiencies, yet further progress is limited by defect-induced nonradiative recombination and instability associated with uncontrolled crystallization. Here, we develop a protic ionic-liquid precursor engineering strategy based on methylammonium acetate (MAAc) for high-performance inverted (p–i–n) triple-cation perovskite solar cells. Systematic variation of the MAAc content reveals that a moderate concentration yields perovskite films with enlarged grains, suppressed pinholes, and strongly reduced residual PbI2. Steady-state and time-resolved photoluminescence measurements, together with electrochemical impedance spectroscopy and light-intensity-dependent analysis, demonstrate that MAAc effectively suppresses trap-assisted nonradiative recombination, prolongs carrier lifetime, and increases recombination resistance without introducing additional transport losses. As a result, optimized inverted devices deliver a champion power conversion efficiency of 23.68% with a high open-circuit voltage of 1.21 V, a fill factor of ~0.83, negligible J–V hysteresis, and excellent device-to-device reproducibility. Moreover, the MAAc-2M devices exhibit markedly improved operational and shelf stability, retaining 73.2% of their initial efficiency after 30 days, compared to 53.2% for the control. This work establishes MAAc as an effective ionic-liquid additive that simultaneously governs crystallization and defect chemistry, offering a general route to efficient and stable inverted perovskite solar cells via protic ionic-liquid-assisted precursor engineering.

1. Introduction

Perovskite solar cells (PSCs) have quickly moved to the forefront of next-generation photovoltaic research, with certified power conversion efficiencies (PCEs) in inverted architectures now surpassing 26% and rivaling those of crystalline silicon modules [1,2,3,4,5]. Despite these impressive figures, the transition from laboratory devices to practical technologies is still constrained by issues of long-term stability, limited scalability of the fabrication processes, and the environmental and health risks associated with conventional solution processing [6,7,8,9,10]. In particular, solution-processed halide perovskites are extremely sensitive to their crystallization environment and to external perturbations such as electric bias and trace amounts of water; these factors can trigger ion migration, generate trap states and promote phase segregation, all of which accelerate performance loss during operation [6,11,12,13,14]. Taken together, these problems make it clear that rational precursor and process engineering is required if one aims to obtain high-quality perovskite films while simultaneously improving device stability and moving toward greener, more controllable manufacturing routes.
A large body of work has now established that the microstructure and defect landscape of the perovskite absorber are key determinants of both PCE and operational robustness [11,15,16]. When nucleation is poorly regulated and crystallization proceeds too rapidly or non-uniformly, the resulting films tend to contain pinholes, grain boundaries with high defect densities and regions of incomplete pore filling or surface coverage; such features act as centres of non-radiative recombination and inevitably limit the open-circuit voltage and fill factor [17,18,19]. These issues are exacerbated in structurally complex or relatively thick device architectures—for example, carbon-based mesoscopic cells and fully screen-printed devices, where the perovskite must infiltrate multilayer mesoporous scaffolds—as well as in scalable doctor-blade or slot-die coating, where uncontrolled solvent evaporation often leads to rough, discontinuous films [3,5,20,21,22]. Recent studies on two-dimensional Dion–Jacobson perovskites and on blade-coated MAPbI3 further demonstrate that even subtle variations in solvent coordination and additive chemistry can profoundly influence nucleation density, preferred orientation, trap distribution and interfacial recombination, underscoring the central importance of precursor chemistry in dictating film quality and device performance [13,23,24]. Beyond photovoltaic energy conversion, metal-halide perovskites have also become an increasingly important materials platform for emerging electronics, where their coupled ionic–electronic transport can be leveraged to realize resistive switching and in-memory computing functions. For example, Shooshtari et al. reported solution-processed halide-perovskite memristors and demonstrated logic-circuit concepts supported by compact modeling, highlighting the rapid progress of perovskite-based memristive devices toward next-generation digital computing. This broader perspective reinforces that regulating defect formation and ion/charge transport is not only essential for suppressing non-radiative losses and improving operational stability in PSCs, but also represents a generalizable design principle across perovskite device technologies [25].
In this framework, ionic liquids (ILs) have attracted considerable interest as a class of “functional” liquid media for perovskite precursor design. Owing to their negligible vapour pressure, high thermal and electrochemical stability, tunable viscosity and rich coordination chemistry, ILs can not only dissolve metal-halide and organic salts but also take part directly in the film-formation process [26,27]. Systematic solvent-engineering studies show that appropriately tailored ILs stabilize metal-halide colloids, widen the processing window and lessen the reliance on antisolvent steps, while at the same time suppressing defect-mediated recombination and improving operational stability [19,24,28]. For example, protic ILs have been used to form stable metal-halide perovskite colloids that are resistant to aggregation and hydrolysis [27], whereas in-depth mechanistic studies have revealed that the size of the IL cation and the nature of the anion jointly govern coordination strength, viscosity, nucleation kinetics and ultimately the grain-growth pathway [28]. In parallel, the role of water in IL-based systems has been clarified: trace amounts of water can assist in regulating intermediate phases and interfacial chemistry, whereas excess moisture accelerates IL degradation and interface deterioration, underscoring the need to carefully balance water–IL–perovskite interactions.
Among the large library of ILs, methylammonium acetate (MAAc) is particularly attractive because it combines the methylammonium (MA+) cation and acetate (Ac−) anion in a single protic IL. Early work demonstrated that MAAc can act as a green solvent or co-solvent for MAPbI3, affording dense, highly oriented perovskite films through strong N–H···X hydrogen bonding and Pb–O coordination, and enabling efficient PSCs without the need for an antisolvent step [29,30,31]. When used as an additive in carbon-based mesoscopic MAPbI3 devices, MAAc promotes the formation of a MAPbI3−x(Ac)x intermediate, slows down crystallization, favours (110)-oriented grain growth and improves pore filling in the mesoporous scaffold, leading to markedly enhanced PCE and stability. In all-inorganic CsPbIBr2 PSCs, a thin MAAc IL layer at the TiO2 interface creates MA+-rich nucleation sites and strong Pb–O coordination, which yields more uniform crystallization, reduced interfacial trap density and improved operational stability [32]. Furthermore, dynamic spin-coating studies on MAPb(I1−xBrx)3 have revealed that introducing MAAc into DMF-based inks accelerates initial nucleation while moderating the growth of intermediate phases, enabling the formation of compact, large-grain films and PCEs approaching 20% with simplified processing [30]. The advantages of MAAc become even more pronounced in more complex compositions such as Sn–Pb mixed perovskites and air-processed inorganic absorbers. In Sn–Pb systems, MAAc-based ILs coordinate strongly with metal-halide precursors via abundant N–H···X hydrogen bonds and carboxylate–Pb interactions, stabilizing colloidal species, suppressing Sn2+ oxidation and dramatically improving the air resistance of precursor solutions [33]. MAAc-containing inks have enabled anti-solvent-free deposition of wide-bandgap MAPbxSn1−x(I0.75Br0.25)3 films with tunable bandgaps, smooth morphology and competitive efficiency, as well as air-coated Sn–Pb perovskite films with extended shelf life and reduced Pb leakage [34]. Related studies using ammonium-carboxylate ILs have further shown that appropriate IL design can stabilize the perovskite lattice, suppress Sn2+/Sn4+ redox cycling and enable efficient, low-toxicity tin-based perovskites [17,35]. In inorganic CsPbI3 PSCs, in situ generation of MAAC during air fabrication of inverted devices has been shown to convert detrimental water erosion into a stabilizing factor: the hydrolysed MAAC regulates grain growth and binds strongly to CsPbI3 grains, expanding the humidity window for device fabrication while improving efficiency and stability [36]. Collectively, these studies demonstrate that MAAc and related protic ILs can simultaneously broaden the processing window, finely tune crystallization kinetics and passivate deep defects across a wide range of perovskite chemistries and device architectures.
Despite this encouraging progress, most MAAc-related studies have focused on relatively simple MAPbI3-type compositions, all-inorganic CsPbIBr2 or Sn–Pb mixed absorbers, and are often implemented in n–i–p or carbon-based mesoscopic architectures, or in specialised printing/coating schemes [1,2,12,13,23,35,37]. By contrast, there is still a lack of systematic understanding of how moderate amounts of MAAc, introduced directly into the precursor solution of state-of-the-art triple-cation mixed-halide perovskites, regulate the coordination environment, nucleation and growth dynamics and defect chemistry in the dense films required for high-efficiency inverted (p–i–n) devices.
In this work, we address this gap by incorporating MAAc into the FA0.80Cs0.07MA0.13PbI2.64Br0.39 precursor solution as a simple yet effective precursor-engineering strategy for high-performance PSCs. By rationally tuning the MAAc content, we aim to tailor precursor coordination and crystallization pathways, suppress structural and electronic defects in the resulting perovskite films and thereby enhance both photovoltaic performance and environmental stability, while moving toward greener and more scalable solution processing.

2. Materials and Methods

2.1. Materials

Formamidine hydroiodide (FAI, 99.99%), Cesium iodide (CsI, 99.99%), Methylammonium bromine (MABr, 99.99%), Lead (II) bromide (PbBr2, 99.99%), Lead (II) iodide (PbI2, 99.99%) were all obtained from Advanced Electron Technology (Dongguan, China). Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)]amine (PTAA), [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), (2,3,5,6-tetrafluoro−2,5-cyclohexadiene-1,4-diylidene) dimalononitrile (F4-TCNQ), 4,7-diphenyl-1,10-phenanthroline (Bphen) were provided by Nichem Fine Technology Co., Ltd. (Taiwan, China). Liquid MAAc (99.5%) was provided by Xi’an Bailete. The solvents used in this work were all purchased from Sigma Aldrich (Shanghai, China).
The FA0.83Cs0.07MA0.13PbI2.64Br0.39 precursor solution was prepared by dissolving FAI (285.5 mg), CsI (36.4 mg), MABr (29.1 mg), PbI2 (802.1 mg), and PbBr2 (95.42) in DMF (0.87 mL) and NMP (0.13 mL). MAAc was introduced as a protic ionic-liquid additive to modulate precursor coordination and crystallization kinetics. The nominal FA/Cs/MA ratio of the targeted perovskite phase is defined by the stoichiometric amounts of FA-, Cs- and MA-halide salts in the precursor formulation; MAAc is not used as a stoichiometry-setting A-site source.

2.2. Device Fabrication

After being cleaned ultrasonically with deionized water, acetone, and isopropanol, respectively, the glass/ITO was put under UV-Ozone treatment for 15 min. PTAA: F4-TCNQ layer and PMMA layer were successively coated on the ITO substrate. The perovskite solution was spun onto the ITO/PTAA:F4-TCNQ/PMMA surface at a rate of 6000 rpm for 20 s. During the spin-coating process, 300 μL 2-butanol was dropped on the wet film at the 5–6th second. The film was then annealed at 100 °C for 30 min for crystal growth. In the next step, PC61BM (20 mg mL−1 in chlorobenzene, 2500 rpm for 20 s), and phen (0.7 mg mL−1 in ethanol, 6000 rpm for 20 s) were successively coated on the perovskite surface. The construction of the devices was completed by evaporating a 100 nm Ag layer as the electrode.

2.3. Characterization

A sun simulator (Zolix Sirius-SS, AAA, Zolix Instrument Co., Ltd., Beijing, China) was used to simulate solar irradiation (AM 1.5 G, 100 mW cm−2). The J−V curves were measured using a Keithley 2400 source meter (Tektronix Inc. Shanghai, China), where the light source output was adjusted by employing a calibrated silicon photodiode (ABET technology, Baltimore, MD, USA). EQE curves were obtained using a homemade system consisting of a power source (Zolix Sirius-SS, Zolix Instrument Co., Ltd., Beijing, China), a monochromator (Zolix Omni-λ-300, Zolix Instrument Co., Ltd., Beijing, China), and a source meter (Keithley 2400, Tektronix Inc. Shanghai, China). Scanning electron microscopy (SEM) images were captured using a cold field emission scanning electron microscope (FEI-Quanta FEG 25, Thermo Fisher Scientific, Shanghai, China). The XRD patterns were recorded using an X-ray diffractometer (Ultima IV, RIGAKU, Tokyo, Japan). The steady PL spectra were collected on a fluorescence spectrometer (OmniPL, Zolix Instrument Co., Ltd., Beijing, China) with a 600 nm incident laser beam from the air side. The TRPL was measured using a C12132 HAMATSU Photonics (Arzbergerstr, Germany).

3. Results and Discussion

3.1. Analysis of Crystallization Performance

This study focuses on perovskite solar cells with a planar inverted (p–i–n) architecture, a widely adopted configuration owing to its reliable device performance and ease of fabrication. In our experiments, perovskite thin films were prepared using a conventional antisolvent method. As schematically illustrated in Figure 1a, methylammonium acetate (MAAc) ionic liquid (molecular structure shown in Figure 1b) with different molar contents was first incorporated into the perovskite precursor solution. After thorough mixing to obtain a homogeneous ink, an appropriate volume of the solution was dispensed onto the conductive glass substrate. The film was then formed by spin-coating, during which an antisolvent was dripped onto the rotating substrate to induce rapid supersaturation, perovskite precipitation, and film formation. Finally, thermal annealing on a hot plate yielded compact perovskite thin films.
MAAc is a protic ionic liquid composed of CH3NH3+ (MA+) and CH3COO− (Ac−). Compared with the conventional DMF/DMSO solvent system, the Ac− anion in MAAc exhibits strong Lewis basicity, and the lone-pair electrons on the carboxylate O atoms can coordinate with Pb2+ via Pb–O bonds. Although MAAc introduces additional MA+, it is unlikely to function as a bulk compositional dopant. Because the counter anion (Ac−) is not a lattice-compatible halide, MAAc is expected to primarily form transient coordination/hydrogen-bonded complexes in the precursor and intermediate states. During crystallization and annealing, these species are largely expelled from the bulk lattice and may only remain in trace amounts at grain boundaries/interfaces, where they contribute to defect passivation. The absence of a pronounced shift in the absorption edge and the lack of systematic XRD peak shifts support that the bulk composition is largely preserved, whereas the improvements in recombination-related metrics and reduced hysteresis point to suppressed defect-assisted recombination and ion-migration-related losses. At the same time, the –NH3+ group of MA+ can form an N–H···X (X = I−/Br−) hydrogen-bond network with halide anions. Previous studies have shown that precursor solutions using MAAc as a solvent or additive yield colorless or pale-yellow PbI2/MAI solutions accompanied by pronounced NMR and FTIR peak shifts, indicating that Pb2+–Ac− coordination and N–H···I hydrogen bonding act synergistically to weaken the direct Pb–I interaction and simultaneously improve the solubility and colloidal stability of the precursor. On this basis, we propose that introducing MAAc into the ternary-cation perovskite system plays a dual role. On the one hand, MAAc is introduced here primarily as a coordination/hydrogen-bonding additive to regulate the precursor complexation and crystallization pathway. Ac− can coordinate with Pb2+ in the precursor/intermediate states, while MA+ participates in hydrogen-bonding interactions with halides. During antisolvent-assisted spin coating and annealing, acetate-containing species are expected to be largely expelled; however, a trace amount may remain immobilized at grain boundaries/interfaces and contribute to defect passivation. On the other hand, Ac− preferentially complexes Pb2+ and constructs a local coordination motif of “Pb2+–O(Ac−)–(I−/Br−)” around the halides, while N–H···(I/Br) hydrogen bonds further stabilize the halogen positions. As a result, a more uniform and stable precursor complex is formed, as schematically depicted in Figure 1c.
To verify the above hypothesis, a perovskite film without MAAc was prepared as a control, and three MAAc-doped films with MAAc concentrations of 1 M, 2 M, and 3 M were fabricated as experimental groups, denoted MAAc-1M, MAAc-2M, and MAAc-3M, respectively. The surface morphology of these films was first examined by scanning electron microscopy (SEM), as shown in Figure 2a–d. The control film without MAAc (Figure 2a) exhibits poor crystallinity and pronounced pores, whereas the MAAc-1M film (Figure 2b) shows noticeably improved crystallinity, with most pores nearly eliminated. The MAAc-2M film presents an even more compact morphology with no obvious pinholes, while the MAAc-3M film, although displaying further improved crystallinity, still contains a small number of pinholes.
Grain-size distribution statistics (Figure 3) indicate that the average grain sizes of the control, MAAc-1M, MAAc-2M, and MAAc-3M films are 0.57 μm, 0.71 μm, 0.97 μm, and 1.07 μm, respectively. These results suggest that the MAAc-2M film achieves the most favorable combination of crystallinity and compactness, which is beneficial for efficient transport and collection of photogenerated carriers within the grains and for mitigating the detrimental effects of grain boundaries and charge traps.
The crystallinity of the films was further evaluated by X-ray diffraction (XRD), and the results are shown in Figure 2e. The control sample exhibits a distinct diffraction peak near 13.1°, corresponding to residual crystalline PbI2. To evaluate whether MAAc affects preferential crystal orientation, we further compared the relative intensities of the dominant perovskite reflections and performed a semi-quantitative out-of-plane texture analysis based on the same θ–2θ XRD patterns (Table 1). The (110) peak remains dominant for all films, suggesting that MAAc does not induce a pronounced new preferential orientation. In contrast, the intensity of this peak is significantly reduced or even vanishes in the MAAc-2M and MAAc-3M films. This indicates that the introduction of MAAc effectively suppresses PbI2 residue, consistent with our earlier hypothesis that MAAc weakens the direct interaction between Pb and I, improves precursor solubility, and enhances film formation.

3.2. Optical and Electrical Performance Analysis

To further clarify the impact of MAAc on the optoelectronic properties of the perovskite films, UV–Vis absorption and steady-state photoluminescence (PL) spectra were recorded for the control, MAAc-1M, MAAc-2M, and MAAc-3M samples. As shown in Figure 2f, the absorption spectra of the four films largely overlap, indicating that the introduction of MAAc does not significantly alter the optical bandgap or the overall absorption characteristics of the perovskite. Given the strong short-wavelength absorption of perovskite, an excitation wavelength of 600 nm was selected for PL measurements to generate a sufficient carrier concentration near the film surface. Importantly, the UV–vis absorption spectra of the films are highly consistent across all formulations, suggesting that the optical thickness does not vary significantly among the samples. The steady-state PL spectra are presented in Figure 2g. The control sample shows an emission peak at 778 nm, whereas the PL peaks of the MAAc-2M and MAAc-3M films exhibit blue shifts of 5 nm and 2 nm, respectively. We note that the PL peak position in mixed-halide perovskites can be influenced by multiple factors, including subtle variations in local halide distribution, microstrain, carrier-density-related band filling, and optical self-absorption/interference effects, in addition to defect-related band-tail emission. In the present case, the essentially unchanged absorption onset indicates that the bulk bandgap is not significantly altered, whereas the PL evolution upon MAAc addition is accompanied by strengthened radiative emission and, as shown later, prolonged carrier lifetime. Therefore, rather than treating the PL blue shift as a stand-alone proof, we interpret it as being consistent with reduced band-tail/sub-gap emission and lowered energetic disorder, which is in line with the defect-passivation role of MAAc reported previously [37,38]. Notably, the MAAc-2M sample shows the most evident shift, correlating with its overall optimized film quality and recombination characteristics discussed below.
Based on the aforementioned morphology and crystallinity analysis, the MAAc-2M and MAAc-3M films exhibit clearly superior film quality. We therefore further investigated the carrier dynamics of these samples by fluorescence lifetime imaging microscopy. Figure 4a presents the time-resolved PL decay curves of the control, MAAc-2M, and MAAc-3M films, and the corresponding data are provided in Table 2. Bi-exponential fitting reveals that the average PL lifetime of the control sample is ~230 ns, whereas the lifetimes of the MAAc-3M and MAAc-2M films are prolonged to ~310 ns and ~375 ns, respectively. Since defect states in perovskite films usually act as nonradiative recombination centers that markedly shorten the radiative recombination lifetime, the MAAc-2M film, which exhibits both the longest PL lifetime and the highest steady-state PL intensity, can be inferred to possess the most effective defect passivation, thereby efficiently suppressing nonradiative recombination and extending carrier lifetime [38,39,40,41,42].
To further elucidate the influence of MAAc on device performance, we conducted systematic optoelectronic characterization. Figure 4b shows the electrochemical impedance spectroscopy (Nyquist) plots of the control, MAAc-2M, and MAAc-3M devices measured in the dark under a bias of 1 V. The recombination resistance (Rrec) was extracted by fitting the equivalent circuit shown in the inset of Figure 4b. The Rrec values for the control, MAAc-2M, and MAAc-3M devices are 6.1 kΩ, 7.9 kΩ, and 7.1 kΩ, respectively. The MAAc-2M device thus exhibits the highest recombination resistance, indicative of the strongest suppression of defect-induced nonradiative recombination [40]. In addition, the series resistances of the three devices are 29.9 Ω, 31.1 Ω, and 32.0 Ω, respectively (Figure 5), showing no significant increase, which suggests that the introduction of MAAc does not induce additional transport losses. We further evaluated the ideality factor of the devices via light-intensity-dependent open-circuit voltage (VOC) measurements. As shown in Figure 4c, an ideality factor closer to 1 implies weaker trap-assisted recombination. The ideality factor was extracted from the slope of VOC versus n(I) measured under AM 1.5G illumination with the light intensity varied from 0.1%, 1%, 10%, 50%, 80% to 100% suns using neutral density filters. The ideality factor of the control device is 1.364, whereas those of the MAAc-2M and MAAc-3M devices decrease to 1.206 and 1.225, respectively. This result demonstrates that MAAc incorporation effectively reduces VOC losses, with the 2 M MAAc concentration being particularly beneficial for improving device efficiency. Finally, the distribution of trap-state density in energy space was analyzed by transient absorption spectroscopy (TAS). Figure 4c shows the trap-state density spectra of the three devices, which can be divided into three characteristic regions: below 0.40 eV (region I), 0.40–0.50 eV (region II), and above 0.50 eV (region III). Compared with the control, the MAAc-2M device exhibits a pronounced reduction in spectral intensity in regions II and III, indicating that the introduction of 2 M MAAc effectively suppresses deep trap states associated with surface and grain-boundary defects by forming a more uniform and stable precursor complex. This further corroborates the excellent defect-passivation capability of the MAAc-2M formulation. The major improvements are observed in multiple recombination-related metrics (TRPL/EIS/ideality factor), supporting that the performance enhancement mainly originates from reduced trap-assisted recombination and improved interfacial/bulk quality rather than thickness changes. We also note that excessive MAAc (3M) leads to occasional pinholes, which is consistent with an overly increased viscosity and altered antisolvent extraction dynamics, further explaining why an intermediate loading (2M) is optimal.

3.3. Photovoltaic Performance Analysis

Based on the above analysis, to systematically evaluate the impact of MAAc on the photovoltaic performance of perovskite solar cells, we fabricated inverted planar devices with the configuration ITO/PTAA:F4-TCNQ/perovskite/PCBM/Bphen/Ag (Figure 6a). In this architecture, F4-TCNQ-doped PTAA was employed as the hole-transport layer (HTL), FA0.80Cs0.07MA0.13PbI2.64Br0.39 served as the perovskite absorber, Bphen-modified PCBM acted as the electron-transport layer (ETL), and Ag was used as the top electrode. Two sets of devices were prepared: a control group and a MAAc-2M group, and their photovoltaic properties were systematically characterized. Figure 6b shows the current density–voltage (J–V) curves of the two devices under optimized conditions, and the corresponding parameters are summarized in Table 3.
The control device exhibits a short-circuit current density (JSC) of 23.29 mA cm−2 (23.37 mA cm−2) under forward (reverse) scan, an open-circuit voltage (VOC) of 1.081 V (1.092 V), a fill factor (FF) of 0.79 (0.80), and a power conversion efficiency (PCE) of 19.88% (20.41%). Pronounced J–V hysteresis is observed for the control device, which can be attributed to the higher defect density in the perovskite film. In contrast, the MAAc-2M device exhibits significantly improved photovoltaic performance: the forward (reverse) JSC is 23.51 mA cm−2 (23.45 mA cm−2), the VOC is enhanced to 1.206 V (1.208 V), the FF increases to 0.826 (0.829), and the PCE reaches 23.50% (23.68%), while the J–V hysteresis is effectively suppressed. This performance enhancement is mainly ascribed to the formation of a more stable coordination environment between MAAc and the perovskite, which improves both bulk film quality and interfacial properties. The external quantum efficiency (EQE) spectra of the two devices are shown in Figure 6c. The integrated JSC values derived from the EQE curves are 22.69 mA cm−2 for the control device and 21.87 mA cm−2 for the MAAc-2M device, which are in reasonable agreement with the J–V measurements. The MAAc-2M device shows an enhanced EQE response in the 300–800 nm range, which is primarily attributed to effective passivation of vacancy-type defects at the perovskite interfaces by MAAc. This passivation suppresses nonradiative recombination and mitigates interfacial degradation during annealing through the formation of a stable coordination network.
To verify the reproducibility of device performance, 24 control and 24 MAAc-2M devices were fabricated. The statistical results (Figure 7, Table 4) show that the average PCE of the MAAc-2M devices is 23.42 ± 0.26%, significantly higher than the 20.51 ± 0.37% obtained for the control group, confirming the robustness of the performance enhancement. Furthermore, maximum power point tracking (Figure 8) reveals that the PCE of the control device decreases from 18.6% to 17.9% under continuous illumination, whereas the MAAc-2M device remains essentially stable, further evidencing its superior operational stability under light soaking.
Finally, the long-term environmental stability of the two devices was evaluated in a drying chamber with a relative humidity of ~5% (Figure 6d). After 30 days of storage, the PCE of the control device decays to 53.2% of its initial value, while the MAAc-2M device still maintains 73.2% of its initial efficiency, demonstrating substantially improved environmental stability. This improvement can be attributed to the synergistic “crystallization regulation–defect passivation” mechanism enabled by MAAc: on the one hand, coordination between Ac− and Pb2+ together with the hydrogen-bond network between MA+ and halides promotes uniform nucleation and large-grain growth, reduces grain boundaries and metastable phases, and thereby lowers the risk of structural degradation; on the other hand, trace acetate-containing species derived from MAAc may remain at grain boundaries and interfaces and potentially passivate under-coordinated Pb2+ and halide vacancies through Pb–O interactions and N–H···X hydrogen bonding, thereby suppressing ion migration and nonradiative recombination. Although acetate-based salts can be hygroscopic, MAAc in this work is introduced as a protic ionic liquid and is expected to interact strongly with the perovskite through Pb–O coordination and MA+–halide hydrogen bonding. Therefore, residual acetate-containing species are unlikely to behave as free hygroscopic salts in the solid film; rather, they may be immobilized at grain boundaries/interfaces, where they passivate under-coordinated Pb2+ and halide vacancies and mitigate defect-assisted degradation. Nevertheless, moisture remains a critical stressor for halide perovskites, and we avoid overgeneralizing the dry-storage results to high-humidity environments without dedicated testing.

4. Conclusions

In this work, we developed a precursor-engineering approach for high-efficiency inverted perovskite solar cells by introducing the protic ionic liquid methylammonium acetate (MAAc) into the triple-cation mixed-halide composition FA0.80Cs0.07MA0.13PbI2.64Br0.39. By varying the MAAc content, we identified an optimal loading (MAAc-2M) that produces perovskite films with substantially improved morphology and crystallinity, featuring larger grains, a strongly reduced pinhole density and suppressed residual PbI2. Steady-state and time-resolved photoluminescence, together with electrochemical impedance spectroscopy and light-intensity-dependent V_OC measurements, consistently indicate that MAAc incorporation lowers the density of trap-assisted nonradiative pathways, extends the carrier lifetime and increases recombination resistance, while leaving the charge-transport resistance essentially unaffected.
As a consequence, devices based on the MAAc-2M formulation achieve a champion PCE of 23.68% with an open-circuit voltage of 1.208 V, a fill factor of ~0.83 and negligible J–V hysteresis, clearly outperforming the MAAc-free control cells. Transient absorption spectroscopy evidences a pronounced reduction of deep trap states, and both maximum power point tracking and storage tests demonstrate significantly enhanced operational and environmental stability, with 73.2% of the initial efficiency retained after 30 days compared with 53.2% for the control. Taken together, these results support a dual mechanism in which Ac−–Pb2+ coordination and MA+–halide hydrogen bonding jointly regulate nucleation and crystal growth while passivating under-coordinated Pb2+ and halide vacancies at grain boundaries and interfaces. The study thus establishes MAAc as an effective ionic-liquid additive for triple-cation inverted PSCs and outlines a general design principle for exploiting protic ionic liquids to co-optimize film formation, defect chemistry and stability in scalable perovskite photovoltaic technologies.

Author Contributions

Conceptualization, J.S. and Y.D.; methodology, H.Z.; software, H.Z.; validation, H.Z., J.S. and Y.D.; formal analysis, H.Z.; investigation, H.Z.; resources, Y.D.; data curation, H.Z.; writing—original draft preparation, H.Z.; writing—review and editing, H.Z.; visualization, H.Z.; supervision, Y.D.; project administration, J.S.; funding acquisition, Y.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Innovation Teams of Ordinary Universities in Guangdong Province (Natural Sciences) (2024KCXTD065).

Data Availability Statement

The original contributions presented in this study are included in the article.

Acknowledgments

The author sincerely expresses gratitude to all co-authors for their invaluable theoretical contributions and steadfast support throughout this research. We confirm that we used ChatGPT-5 solely for the purpose of language editing and checking gramma to improve readability. We have reviewed the output carefully and take full responsibility for the content of the publication. No AI tools were used to generate data or scientific conclusions.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had a role in the design of the study, writing of the manuscript, and in the decision to publish the results.

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Crystals 16 00019 g001
Figure 1. (a) The preparation process of perovskite films. (b) The chemical structure formula of MAAc. (c) Schematic diagram of the interaction between MAAc and perovskite.
Figure 1. (a) The preparation process of perovskite films. (b) The chemical structure formula of MAAc. (c) Schematic diagram of the interaction between MAAc and perovskite.
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Crystals 16 00019 g002
Figure 2. SEM images of four types of (a) control (b) MAAc-1M (c) MAAc-2M (d) MAAc-3M. (e) The XRD test results of the four types of films. The photophysical properties of four types of perovskite films (f) UV−vis absorption spectra (g) normalized PL spectra.
Figure 2. SEM images of four types of (a) control (b) MAAc-1M (c) MAAc-2M (d) MAAc-3M. (e) The XRD test results of the four types of films. The photophysical properties of four types of perovskite films (f) UV−vis absorption spectra (g) normalized PL spectra.
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Crystals 16 00019 g003
Figure 3. Statistical analysis of grain size data in SEM images of (a) Control, (b) MAAc−1M, (c) MAAc−2M, (d) MAAc−3M.
Figure 3. Statistical analysis of grain size data in SEM images of (a) Control, (b) MAAc−1M, (c) MAAc−2M, (d) MAAc−3M.
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Crystals 16 00019 g004
Figure 4. (a) Fluorescence lifetime imaging of three types of perovskite films. (b) Nyquist curves of the three devices. (c) Control, MAAc−2M and MAAc−3M perovskite solar cells’ open-circuit voltage response to different light intensities. (d) tDOS of the three types of devices, as measured by the TAS method.
Figure 4. (a) Fluorescence lifetime imaging of three types of perovskite films. (b) Nyquist curves of the three devices. (c) Control, MAAc−2M and MAAc−3M perovskite solar cells’ open-circuit voltage response to different light intensities. (d) tDOS of the three types of devices, as measured by the TAS method.
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Figure 5. The series impedance diagram of the three devices.
Figure 5. The series impedance diagram of the three devices.
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Figure 6. (a) The device structure diagram of perovskite solar cells in this study. (b) The short-circuit current density-voltage curve of the perovskite solar cell with the highest performance after being treated with Control, MAAc-2M. (c) The external quantum efficiency curve of the two devices. (d) Long-term stability tests of the two devices.
Figure 6. (a) The device structure diagram of perovskite solar cells in this study. (b) The short-circuit current density-voltage curve of the perovskite solar cell with the highest performance after being treated with Control, MAAc-2M. (c) The external quantum efficiency curve of the two devices. (d) Long-term stability tests of the two devices.
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Figure 7. The 24 sets of repeated device parameters for the control and MAAc−2M (a) short−circuit current density (JSC). (b) fill factor (FF). (c) open−circuit voltage (VOC). (d) power conversion efficiency (PCE).
Figure 7. The 24 sets of repeated device parameters for the control and MAAc−2M (a) short−circuit current density (JSC). (b) fill factor (FF). (c) open−circuit voltage (VOC). (d) power conversion efficiency (PCE).
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Figure 8. The steady-state output curves of the two devices.
Figure 8. The steady-state output curves of the two devices.
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Table 1. Semi-quantitative preferential orientation analysis derived from θ–2θ XRD.
Table 1. Semi-quantitative preferential orientation analysis derived from θ–2θ XRD.
SamplesI(110)/I(220)I(310)/I(220)I(110)/ΣI(6peaks)I(310)/ΣI(6peaks)
Control1.5730.5150.4550.149
MAAc-1M1.5320.6480.4250.180
MAAc-2M1.4560.6650.4130.189
MAAc-3M1.5470.6550.4250.180
Table 2. The PL lifetime parameters from the PL decay curves in Figure 4a.
Table 2. The PL lifetime parameters from the PL decay curves in Figure 4a.
Samplesτ1 (μs)A1τ1/ΣAxτx (%)τ2 (μs)A2τ2/ΣAxτx (%)τave (μs)
Control137.354.2340.245.8230.2
MAAc-2M225.054.7557.645.7375.3
MAAc-3M199.754.8494.945.2335.4
Table 3. Photovoltaic parameters for the best-performing control, and MAAc-2M PSCs.
Table 3. Photovoltaic parameters for the best-performing control, and MAAc-2M PSCs.
SamplesScan DirectionVOC (V)JSC (mA cm−2)FFPCE (%)
Controlforward1.08123.290.7919.88
reverse1.09223.370.8020.41
MAAc-2Mforward1.20623.510.82623.50
reverse1.20823.450.82923.68
Table 4. Average PV parameters of the control and MAAc-2M PSCs from 24 devices.
Table 4. Average PV parameters of the control and MAAc-2M PSCs from 24 devices.
StylesVOC (V)JSC (mA cm−2)FFPCE (%)
Control1.113 ± 0.00622.84 ± 0.320.813 ± 0.00820.51 ± 0.37
MAAc-2M1.203 ± 0.00623.49 ± 0.320.822 ± 0.00723.42 ± 0.26
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Zhang, H.; Song, J.; Deng, Y. Protic Ionic-Liquid Precursor Engineering with Methylammonium Acetate for Efficient and Stable Inverted Triple-Cation Perovskite Solar Cells. Crystals 2026, 16, 19. https://doi.org/10.3390/cryst16010019

AMA Style

Zhang H, Song J, Deng Y. Protic Ionic-Liquid Precursor Engineering with Methylammonium Acetate for Efficient and Stable Inverted Triple-Cation Perovskite Solar Cells. Crystals. 2026; 16(1):19. https://doi.org/10.3390/cryst16010019

Chicago/Turabian Style

Zhang, Hanhong, Jun Song, and Yuanlong Deng. 2026. "Protic Ionic-Liquid Precursor Engineering with Methylammonium Acetate for Efficient and Stable Inverted Triple-Cation Perovskite Solar Cells" Crystals 16, no. 1: 19. https://doi.org/10.3390/cryst16010019

APA Style

Zhang, H., Song, J., & Deng, Y. (2026). Protic Ionic-Liquid Precursor Engineering with Methylammonium Acetate for Efficient and Stable Inverted Triple-Cation Perovskite Solar Cells. Crystals, 16(1), 19. https://doi.org/10.3390/cryst16010019

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