Ink engineering for slot-die coated perovskite solar cells and minimodules

The power conversion efficiencies of perovskite solar cells (PSCs) have approached 26% for single-junction and 33% for multi-junction cells. Thus, various scalable depositions are studied to improve the manufacturability of PSCs for market entry. Of all types, slot-die coating is a promising technique thanks to its excellent compatibility with versatile systems. However, the complicated ink chemistry and film formation are major obstacles to scaling up devices. In this review, we systematically discuss ink engineering in the fabrication of slot-die-coated PSCs and perovskite minimodules, covering all functional layers that are processed using solution-based means. We then summarize a range of strategies to improve ink compatibility with slot-die coating, focusing on how to optimize the ink formulation to achieve high-quality films. Finally, we highlight the existing challenges and potential avenues for further development of slot-die-coated devices.


Introduction
Slot-die coating is a conventional technique that has been invented for over 70 years [1], in particular for non-contact manufacturing the large-area high-uniformity/-compactness thin film. Compared to other upscaling techniques, the slot-die coating has many advantages: (i) excellent system compatibility. The slot-die coating is compatible with both sheet-to-sheet and roll-to-roll (R2R) systems [2][3][4]. For R2R, the slot-die coating can achieve high-throughput products (over 600 m min −1 ) [5]. (ii) Broad ink suitability. The slot-die coating can conduct different types of inks with a wide range of viscosities (1-10 000 mPa s) [6]. (iii) Low material wastage. The slot-die coating does not lead to a huge chemical consumption throughout the fabrication process (<1% material) [7]. (iv) High operation accuracy. As a pre-metered technique, slot die coating can control the film thickness (<5 µm wet film or <20 nm dry film) [8], with a continuous and accurate supply of ink fluid. If readers are interested in more fundamentals of slot-die coating, we recommend referencing some noteworthy literature that has been published [9][10][11][12][13].
As a common scalable deposition technique, slot-die coating is a useful fabrication tool for academic study from lab-scale to pilot-level production. For example, perovskite solar cells (PSCs, with an aperture area of normally ⩽1 cm 2 ) and minimodules (PMs, 10-200 cm 2 ) are ideal candidates for slot-die coating thanks to low-temperature solution processing. The slot-die-coated PSCs were first reported in 2015, with a power conversion efficiency (PCE) of 12% on the rigid substrate (0.1 cm 2 ) [14] and 4.9% on the flexible substrate (∼0.2 cm 2 ) [15]. Encouragingly, the recent slot-die-coated PSCs achieved high PCEs on both rigid (23.4%) and flexible (21.3%) devices (0.09 cm 2 ) [16]. For rigid minimodules, the slot-die-coated devices yielded a PCE of 12.9% for triple-mesoscopic structure (60.1 cm 2 ) [17]. The regular structure reached the PCEs of 19.3% (certified) on 17.1 cm 2 and 19.5% on 65 cm 2 [18]. A maximum active area of 174 cm 2 was reported on the inverted structure, with over 18% stabilized PCEs [16]. In the preparation of these high-performance PSCs and PMs, the ink used for coating each functional layer plays a pivotal role that the non-ideal ink formulation inhibits the film quality and its scalability, impairing the device performance and stability. Therefore, in light of the rapid advancements of slot-die-coated PMs, it is imperative to provide a timely review of the ink engineering involved in their development.
In this minireview, we provide an up-to-date overview of the recent progress in ink engineering, highlighting key findings in the one-step single-layer coating on the rigid substrate, and providing future directions on how to develop the slot-die-coated PSCs and PMs. We first introduce the working mechanism of slot-die coating, systematically summarizing the important parameters for each functional layer, such as operational condition, distance, pumping rate, coating speed and drying process. We then discuss a series of ink engineering exploited at the slot-die-coated film, including the perovskite layer, charge transport layer and electrode layer. We believe that an increased understanding of these ink properties is crucial to address the current challenges of achieving consistent and high-quality films. In the final section, we share our concerns and perspectives regarding the development of high-performance slot-die-coated devices.

Operational mechanisms of slot-die coating
In slot-die processing (see the schematic in figure 1), the ideal meniscus determines the quality of wet films. The gap between the slot-die head and substrate affects the film thickness and the equilibrium between the pumping rate and coating speed influences the coverage, roughness, and uniformity [5]. These parameters are also affected by environmental conditions, such as temperature and relative humidity (RH). To accelerate the drying process of the wet films, the additional air-knife [18][19][20] or hot-stage accessories [20] are created in the slot-die coating facility to mimic the quenching effect. In addition, the gas [14], anti-solvent [21], and vacuum-assisted [22] quenching methods are useful to achieve a homogeneous, pinhole-free, uniform large-area film. Under nitrogen (N 2 ) quenching, the annealing temperature is also crucial, in which the non-ideal temperature could cause impurities in the final film [23]. Thus, the important parameters of coating each functional layer are summarized in table 1.

Ink engineering
Given that a reproducible and steady slot-die coating, the specific solution for each functional layer is referred to as the 'ink' . In terms of ink used in PSCs and PMs, the physical properties, such as viscosity, wetting ability, and volatility, are more related to the coating processing. Furthermore, a better understanding of ink chemistry is the prerequisite for the formation of a high-quality thin film, which will be discussed in detail in the following sections.

Perovskite layer
For many PMs, slot-die or other upscaling approaches are only used for coating the perovskite layer. Methylammonium lead tri-iodide (MAPbI 3 ) is the conventional light absorber [19,22,25,[46][47][48][49]. However, the issues induced by illumination or heating stressors will hinder the device stability. Due to the volatile and diffusive nature, the MA-based devices showed inferior damp-heating and operational stability than that of formamidinium (FA) [33]. Hence, more attention has been paid to FA-based perovskites in recent years. Compared to MAPbI 3 , the formation of pure α-FAPbI 3 is hard to control as a result of the complicated crystallization dynamics (forming yellow phases at room temperature) [18,50]. To optimize the crystallization, composition tuning is a straightforward method. For example, partial replacement of FA by cesium (Cs), i.e. FA x Cs 1−x (normally, x ranges from 0.85 to 0.95), exhibited improved phase stability [18,50,51]. Thus, we mainly focus on the promising FACs-based perovskite. As shown in table 2, the champion slot-die-coated devices in regular (n-i-p) or inverted (p-i-n) structures are all based on the FA x Cs 1-x PbI 3 ink, indicative of its compositional advantage in both small-area and large-area films.

Solvents
The rational selection of solvents is the crucial step in preparing the FACs-based perovskite ink [52]. The solvent can dissolve the perovskite precursor and determines the colloidal distribution of the solution [53,54]. In slot-die coating, the replication of the conventional N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) mixtures (normally 4:1 volume ratio) is not a straightforward task since the DMSO is harmful to the large-area film under N 2 quenching [23]. Without using anti-solvents, the DMSO in the wet films cannot be rapidly removed. Thus, another solvent system is normally used in slot-die-coated perovskite. Based on pure DMF, the additional N-methylpyrrolidone (NMP) can reduce the colloid size and retard the nucleation process [21]. In the crystallization process, the DMF will form the intermediate phases with the perovskite (also called solvent-coordinated complexes) [18], and thus form the impurities (like δ-FA x Cs 1-x PbI 3 ) in the final perovskite. To solve this issue, NMP is introduced in DMF to construct the solvent system (up to 1:1 volume ratio in some scenarios [21]). Instead of forming DMF-coordinated perovskite intermediate complexes, the PbI 2 •NMP adducts are favorable for the α-FACs perovskite as illustrated in figure 2(a). Thanks to the strong interaction between PbI 2 and NMP, the NMP can accelerate the nucleation rate and decrease the formation energy. It is also found that acetonitrile (ACN) and 2-mercaptoethanol (2-ME) can quickly dry the FACs ink [50] and smooth the FA-based perovskite film [29]. However, the 2-ME has a low solubility of Cs + [23], which will inhibit its application in FACs perovskite. Therefore, DMF-NMP is the common solvent system for perovskite ink, which enable the most high-performance PSCs and PMs (see table 2).

Additives 3.1.2.1. Crystallization regulation
For neat FACs precursor, the diameter of colloids ranges from 1 to 10 nm [20], without the large colloids as found in the MA-based perovskite solution [55]. Based on the DMF-NMP solvent system, a small amount of DPSO could further decrease the colloid size and retard the grain growth, leading to enhanced stability of the perovskite wet films as shown in figure 2(b) [21]. The slot-die-coated PMs achieved a certified PCE of 16.6% on 20.8 cm 2 . In contrast, the alkali salt, like KPb 2 Br 5 , increases the colloid size, resulting in a higher supersaturation state and a large number of nucleation sites [20]. With the aid of KPb 2 Br 5 , the as-crystalized perovskite showed an improved film quality, though it formed the new intermediate phases. A-cation chlorides, such as MACl or FACl, also induced the intermediate phases of MAI·PbI 2 ·MACl or FAI·PbI 2 ·FACl. In combination with the vacuum treatment, the FACl additive could increase the grain size and crystallinity of the FACs perovskite and then evaporate as a volatile compound in the annealing process [22]. Thanks to the high dipole moment than FA [56], the MACl additive can effectively stabilize the pure FAPbI 3 α-phase, which is also widely adopted in slot-die-coated FACs [35]. In addition, the different chloride sources can affect the current density-voltage (J-V) characterization of the devices. Compared to MACl, lead chloride (PbCl 2 ) contributes to the stabilized performance, although both additives improve the initial efficiency under an equivalent amount of chloride (e.g. 2.5 mol% PbCl 2 or 5 mol% MACl) [30]. Further study confirmed that the PbCl 2 could completely inhibit the DMF-perovskite intermediate phases in FACs-based perovskite [18]. Also, other small molecules, such as hydroxyethyl acrylate (HEA) [57], diglycolic acid (DA) [31] and 3-allyloxy-1, 2-propanediol (AP) [58] are found to control the perovskite crystallization and increase the grain size, which are beneficial for the film quality.

Passivation
In addition to acting as nucleation sites to regulate the crystallization process, the additives are able to passivate the perovskite defects. For example, the excess PbCl 2 additive in slot-die-coated FACs also causes the in-situ passivation at grain boundaries due to the formation of PbI 2 via halide substitution. The PbI 2 is well known to have a 'self-passivation effect' on perovskite [59]. Based on FACs with excess PbCl 2 , further addition of KPF 6 could passivate the perovskite film (hydrogen bonding between perovskite and PF 6 -) to   reduce the trap density, removing the hysteresis in J-V curves (regular devices) [18]. In addition, the KPF 6 can shift up the valence band maximum of perovskite film approaching the highest occupied molecular orbital (HOMO) level of the spiro-OMeTAD, in which the reduced interfacial barrier contributed to the open-circuit voltage (V OC ) improvement. The PF 6 of KPF 6 could form the complex on the perovskite surface and block the diffusion of cobalt ions from spiro-OMeTAD, leading to improved device stability under thermal (85 • C) or illumination (at open-circuit condition under 1 Sun) stressors. Similarly, organic halide salts, like 2-(2,3,4,5,6-pentafluorophenyl)ethylammonium iodide (FEAI), did not affect the morphologies of the slot-die coated FACs but would passivate the perovskite film with enhanced photoluminescence lifetime [36]. For polymer additive, the non-electroactive PEO (see table 3) could coordinate with the positive ions in perovskite and maintain the large grain size even under high humidity (55 ± 5% RH), conferring the whole slot-die coating process in ambient conditions [30].

Surface agents
Apart from the aggregation of additives on the perovskite surface, the agent ink used for surface treatments could also realize the passivation effects. For example, graphitic carbon nitride (g-C 3 N 4 ) is introduced to treat slot-die-coated perovskite films. These two-dimensional nanosheets could form diffusion barriers, passivate the perovskite surface and block the iodide diffusion to the interconnected metal electrodes [33]. The PCEs of the passivated device increased from ∼14% to 15.6% (with a certified value of 14.2% on an AA of 35.8 cm 2 ). More significantly, the damp-heat (85 • C/85% RH) and operational (60 • C in ambient air at the maximum power point under 1 Sun) stability of the PMs showed dramatic improvements, maintained  [35] or large cation salt, like isobutylamine bromide (iBABr) [18], is introduced on the top of slot-die-coated perovskite, yielding a PCE of 18.6% (10.2 cm 2 ) and 19.5% (65 cm 2 ), respectively.

Charge transport layer
Aside from the perovskite ink, the solution for the charge transport layer requires cautiously adjusting in the slot-die coating. For the electron transport layer (ETL) ink, the as-purchased Alfa-tin (IV) oxide (SnO 2 , 15% in H 2 O colloidal dispersion) solution needs to be diluted and quickly dried by blowing the hot air, which can prevent the aggregation of nanocrystals with a much lower surface roughness (0.95 vs 5.3 nm) [42]. To stabilize the SnO 2 colloids, additional potassium hydroxide (KOH) is used to treat the SnO 2 film. It is found that the K + can facilitate the perovskite nucleation and the charge extraction at the SnO 2 /perovskite interface, both of which contribute to the reduced hysteresis and improved efficiency of the final devices. The flexible PMs based on KOH-treated slot-die-coated SnO 2 achieved a PCE of 14.9% on an AA of 16.07 cm 2 .
To remove the sub-gap valence-band states of the commercial SnO 2 layer, the SnO 2 ink can be synthesized by the reaction between SnCl 4 ·5H 2 O and benzyl alcohol under a microwave-assisted hydrothermal route [43]. The device based on the slot-die-coated SnO 2 yielded a PCE of 15.3% on rigid and 10.4% on a flexible substrate (0.16 cm 2 ). Based on the identical device structure, the slot-die coated hole transport layer (HTL) is further investigated [40]. Although a small molecule of 2,2 ′ ,7,7 ′ -tetrakis(N,Ndipmethoxyphenyl)amine-9,9 ′ -bifluorenylidene (Bifluo-OMeTAD) was reported to show a better performance (with the champion PCE increased from 11.7% to 14.8% [41]) than Spiro-OMeTAD thanks to the lower HOMO and amorphous phase after slot-die coating, the Spiro-OMeTAD is still the most popular HTL for n-i-p devices. The recent study indicated that the solvent system could determine the Spiro-OMeTAD quality in the slot-die coating [40]. Chlorobenzene (C 6 H 5 Cl, with a high-boiling point of 132 • C) and ethyl acetate (C 4 H 8 O 2 , with a low-boiling point of 77 • C) mixed by volume ratio of 6:4 realized the optimal HTL morphology, with the lowest surface roughness of 4.8 nm. The slot-die Spiro-OMeTAD devices achieved a champion PCE of 20.4% on rigid PMs with an AA of 10 cm 2 .
Similar solvent strategies (high-boiling point mixed with low-boiling point solvents) were also used in carbon-based mesoscopic devices [6]. To slot-die coat the hole-blocking layer, like compact TiO 2 , the additional tetralin (C 10 H 12 , with a high-boiling point of 207 • C), isopropanol (IPA, with a low-boiling point of 82.5 • C) and ethanol (EtOH, with a low-boiling point of 78.4 • C) were used to dilute the titanium diisopropoxide bis(acetylacetate) precursor [6]. These solvents enable to form the homogenous film without de-wetting issues. To further reduce the thickness difference between the center and edge, a small amount of surfactant is essential in the solvent mixture. For titania paste of mesoporous TiO 2 layer, IPA mixed with the terpineol (C 10 H 18 O, with a high-boiling point of 219 • C) is an ideal solvent system for slot-die coating. While for zirconia paste, the exclusive use of alcohol-based mixtures, like IPA and EtOH, is sufficient to achieve faster-drying films. This could be due to the higher thickness of mesoporous ZrO 2 than the TiO 2 layer (1000 nm vs 550 nm).
In inverted PMs, most solution-processed functional layers are compatible with slot-die coating. For HTL ink, the IPA dilution is also generally accepted in PEDOT:PSS (Clevios P VP AI 4083, Heraeus), aiming to smooth the slot-die-coated film [8,15]. However, due to its acidic and hygroscopic nature [60], PEDOT:PSS is seldom used in high-performance lead-based PSCs. Thus, alternative candidates, such as self-assembled monolayers (SAMs, e.g. 2PACz), inorganic materials (e.g. NiO x ) and polymer (e.g. poly-TPD), are slot-die coated as shown in table 1. Amongst HTLs, the SAMs are promising ink thanks to achieving record efficiency (a certified PCE of 24.7%) in inverted PSCs [61]. Also, their depositions have been testified by many scalable techniques [62]. Further optimization on slot-die-coated SAMs could be performed in two ways, such as molecular design and morphology control.
For ETL ink, the PCBM is still indispensable for state-of-art slot-die coating, which could be ascribed to its better solubility than C 60 . However, the high-cost and inferior thermal stability of PCBM inspires us to invent new inks [63,64]. For buffer-layer ink, BCP is not the only choice in slot-die coating. PEIE [30] and TBAOH [38] were also reported while their long-term stability still needs more evidence compared to the commonly used BCP. Since the inverted structure has many advantages in tandem cells [65] and the small-area single-junction devices, such as negligible hysteresis under different light intensities or scan rates [66] and excellent operational [67], damp-heat [68] or thermal-cycling stability [69], the slot-die-coated PMs based on the inverted structure is worth of more efforts in the future.

Electrode layer
To achieve the large-area electrode, conventional thermal evaporation is not suitable as requiring a high vacuum. Hence, choosing the appropriate electrode ink based on low-vacuum or vacuum-free conditions is crucial in the fabrication of PSCs and PMs since the properties of electrode ink can significantly affect the efficiency and stability of the device, as well as its scalability. Although Ag [15] or Cu-based ink [70] were reported or proposed to have good compatibility with scalable printing techniques, the non-metal material, like carbon [71], is the promising candidate considering both the economic and environmental aspects. The carbon electrode has low cost and high stability, which has been successfully used for scalable devices [72][73][74]. To fabricate the large-area carbon electrode, the carbon paste was normally deposited by screen printing, in particular in triple-mesoscopic PMs [75]. In this structure, the carbon electrode also serves as the hole collector. With high pore size and porosity, the fabricated carbon electrode enables the infiltration of the perovskite. For slot-die coating, the carbon paste (containing carbon nanoparticles and graphite flakes) only requires the low-boiling point solvents of IPA and EtOH to dilute thanks to the high thickness (over 10 µm) [6]. These additional solvents could improve the flowability of the carbon paste and the homogeneity of the carbon film. The slot-die-coated carbon electrode is shown in figure 3(a). Based on this technique, the carbon paste could also be coated on HTL or buffer layer to fabricate the regular or inverted devices.
Nonetheless, the alcohol solvents in carbon paste would be detrimental to the perovskite film when directly coated. Hence, indirect approaches were investigated. For example, the carbon paste could be slot-die-coated on a substrate to form a wet film first [76]. Then, this wet film was soaked in ethanol for a solvent-exchange process. This process can be conducted at room temperature, leading to improved conductivity and flexibility. The final carbon film (in figure 3(b)) can be peeled off from the substrate and directly attach (under 0.7 MPa) to the surface acting as the rear electrode, in which the interface adhesion is even much stronger than the normal Au electrode. A similar dry-press method was further used for the slot-die-coated PSC stack. As illustrated in figure 3(c), the carbon film could combine with the Ag layer, both of which can be performed by slot-die coating the corresponding inks (pastes), to form a bilayer electrode [32]. Then, the slot-die-coated PSC stack could combine with the carbon/Ag bilayer by the calendar press to finish the entire device. This process is not only practical but also holds great promise for the mass production of PSCs and PMs.

Outlook
As efficiency continues to rapidly grow, there is an urgent need for a comprehensive understanding of ink engineering in slot-die coating. In order to advance this technique, we wish to express our thoughts and recommendations as follows.
(a) Although the crystallization dynamics might be different, the slot-die coating can learn successful ink strategies from spin-coating or other scalable deposition methods. For example, innovative compositions, solvent systems, and additives can be adopted in slot-die-coated perovskite. In addition, the knowledge acquired from interface engineering has the potential to be leveraged in slot-die coatings, such as the pre-or post-treatment of perovskite interface. For the perovskite surface, various passivation agents, like ammonium halides [77], have been reported to regulate electronic behaviors and defect species, which could be useful for the as-prepared perovskite film by slot-die coating. Furthermore, the formation of high-quality perovskite film is highly dependent on the underlying substrate, and this phenomenon is particularly sensitive in slot-die coating. Consequently, we anticipate that modifying the buried interface [61,78] will be more significant and challenging in slot-die coating. (b) The top electrode is the crucial layer to realizing the all-slot-die-coated devices. At present, many so-called fully slot-die-coated devices are still using the evaporated electrode [14,22,39]. The fully-slot-die-coated devices could be achieved in the carbon-based mesoscopic devices (also called the triple-mesoscopic [75]) with the infiltration of perovskites [6,17]. As a promising electrode for slot-die coating, the carbon can suppress the ion migration and lead-halide reaction at the laser scribing (P1-P2-P3) interconnection. However, with the low conductivity and high thickness (over 100 times thicker than metal electrodes), the carbon-based devices show a sacrificed efficiency due to the high series resistance. Therefore, subsequent investigations should pay attention to enhancing the electrical conductivity of the carbon electrodes, such as in combination with low-cost metal nanoparticles or conductive layers. (c) In slot-die coating, large amounts of solvents are required for mass production, which can pose environmental and safety challenges. In the above discussion, the results showed that many functional layers can be compatible with green or low-toxicity solvents. However, the most common FACs perovskite ink still involves the high toxicity of the DMF and NMP. For perovskite solvents, a recent study found that the DMSO had the least detrimental effects on both human health and the environment [79], and EtOH could also be used for the FA-based perovskite [80]. Thus, a friendly solvent system should be created and optimized for lab-scale slot-die-coated perovskite, which enables practical application for future manufacture. (d) Dry air or nitrogen blowing is widely applied in the slot-die coating process to avoid using anti-solvents.
However, these blowing parameters, such as air-knife position, gas pressure, and flow rate, are difficult to control accurately since slight turbulence in the automated fabrication system will affect the ideal manufacturing. Hence, alternative methods can be introduced to replace the blowing treatment, such as the vacuum flash-assisted process (with low pressure at ∼20 Pa) [81], hot-coating technique [30,82], near-infrared irradiation heating (3-6 kW for 20-250 s) [38] and intense pulsed light [44]. Also, we appeal to be more transparent about these methods, such as providing more descriptions of the corresponding facilities, which will be beneficial for the whole slot-die coating community. (e) For many slot-die-coated PMs, only one functional layer is slot-die-coated while the others are still spin-coated or evaporated. Besides, the common polymer HTL, like poly(bis(4-phenyl)(2,4,6trimethylphenyl)amine) (PTAA), is seldom deposited by the slot-die coating. These phenomena imply the challenges of ink development and the inferior reproducibility in slot-die processing. Thus, a stable and universal ink is highly necessary. We encourage researchers to give more details about ink preparation for the benefit of inter-laboratory comparison. To facilitate the industrial commercialization of PSCs and PMs, the integration of slot-die coating with other sophisticated techniques is promising to manufacture the complete devices.
The slot-die coating process is constrained by the complexity of the ink chemistry and processing parameters. Thus, it is essential to continuously optimize these factors to overcome these limitations. By investing additional efforts in ink engineering, we anticipate that slot-die coating could enable the production of more scalable perovskite modules for future applications.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.