Droplet Manipulation and Crystallization Regulation in Inkjet-Printed Perovskite Film Formation

Open AccessCCS ChemistryMINI REVIEW1 May 2022Droplet Manipulation and Crystallization Regulation in Inkjet-Printed Perovskite Film Formation Yajie Cheng†, Hangjuan Wu†, Junjie Ma, Pengwei Li, Zhenkun Gu, Shuangquan Zang, Liyuan Han, Yiqiang Zhang and Yanlin Song Yajie Cheng† School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 †Y. Cheng and H. Wu contributed equally to this work.Google Scholar More articles by this author , Hangjuan Wu† School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 †Y. Cheng and H. Wu contributed equally to this work.Google Scholar More articles by this author , Junjie Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Pengwei Li School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Zhenkun Gu School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Shuangquan Zang College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author , Liyuan Han State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 Google Scholar More articles by this author , Yiqiang Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Materials Science and Engineering, Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou 450001 College of Chemistry, Zhengzhou University, Zhengzhou 450001 Google Scholar More articles by this author and Yanlin Song *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing Engineering Research Center of Nanomaterials for Green Printing Technology, National Laboratory for Molecular Sciences (BNLMS), Beijing 100190 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101583 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Emerging organic–inorganic metal halide perovskite materials have become the focus of the optoelectronics research community owing to their excellent photoelectric properties. Nevertheless, challenges still exist for transferring the lab-made devices to large-area industrial modules. Inkjet printing (IJP) technology provides a promising way to fill the gap because of its precise droplet control and uniform large-scale deposition functions. Hence, an in-depth understanding of inkjet-printed perovskite films in terms of droplet manipulation and crystallization regulation is critical for upscaling the perovskite devices to commercial usage. In this review, we give an overview of inkjet-printed high-quality perovskite films and provide guidelines on inkjet-printing large-scale high-performance perovskite devices. First, we analyze theories of droplet formation and perovskite nucleation/crystallization dynamics and then focus on summarizing the perovskite film-formation strategies via IJP, in the aspects of ink engineering, the printing process, and posttreatment. Furthermore, we review the recent advances of inkjet-printed perovskite films on optoelectronic devices, such as perovskite solar cells, perovskite light-emitting diodes, and perovskite photodetectors. Finally, we highlight the “Trilogy Strategies,” including ink engineering, printing process, and posttreatment for printing high-quality perovskite films. Download figure Download PowerPoint Introduction In recent years, organic–inorganic metal halide perovskite materials have received tremendous attention due to their exceptional optical and electronic properties in terms of tunable band gaps,1 large defect tolerance,2 high absorption coefficient,3 and ultralong carrier diffusion lengths.4 Intensive research efforts have been devoted to developing the perovskite-based functional optoelectronic devices, such as solar cells,5–7 light-emitting diodes (LEDs),8,9 photodetectors (PDs),10,11 and so on. Despite the stellar rise of laboratory-scale perovskite optoelectronic devices based on the nonscalable spin-coating method,12,13 challenges still exist for upscaling the devices to the commercial scale. The centripetal force during the spinning process deteriorates the uniformity of the solute distribution, leading to poor control of the perovskite nucleation and crystallization for large-area film (≥1 cm2).14,15 To achieve homogeneous large-area perovskite films, a diversity of solution-processed techniques, such as blade-coating,16 slot-die coating,17 screen printing,18 and inkjet printing (IJP),19 have been investigated. Among these technologies, IJP is viewed as a reliable, versatile, and digitized direct writing technique. It provides a promising way to fabricate high-quality perovskite film by accurately regulating the small droplets and enabling large-area automated manufacturing. Typically, the jetted ink droplets of perovskite precursor are deposited onto the substrate under precise digital control, then form a liquid membrane with the desired pattern, and finally solidify by posttreatment. Compared with other scalable deposition techniques, IJP has broader and more powerful printing capabilities due to its precise droplet control20 and patterning functions.21 The near-unity precursor utilization of IJP reduces the raw materials consumption (especially of the toxic ones), featuring cost-effectiveness and environmental benignity. Therefore, IJP is certain to be one of the most competitive approaches to the economical and green development of the perovskite optoelectronic industry. Perovskite solar cells (PSCs) have exhibited rapid development: over the past 10 years, the power conversion efficiency (PCE) of PSCs has increased from 3.8%22 to 25.5%,23 outperforming that of the commercialized multicrystalline Si solar cells (PCE = 22.8%).24 However, the PSCs investigated so far are mainly fabricated through the uncontrollable spin-coating method with limited device areas.25 Given the advantages of IJP technology, the Yang group26 initially inkjet-printed a lab-scale PSC with a PCE of 11.6% in 2014. Since that time, significant progress has been made in inkjet-printing of high-performance PSCs. A fully inkjet-printed PSC (printing all inner layers excluding electrodes) with an efficiency of 10.7% was first reported by Gheno et al. in 2018.27 At present, inkjet-printed PSCs have achieved an unprecedentedly high efficiency of over 21%.28 In 2015, our group29 successfully applied IJP technology to the deposition of high-quality perovskite film on a mesoporous TiO2 electron transport layer (ETL), and obtained an incipient PCE of 12.3%. Since then, we have made continuous efforts, such as vacuum-assisted thermal annealing (VTA),15 surface chemistry engineering,14 and crystallization kinetics regulation,30 to optimize the quality of printed perovskite films. Based on the rapid development of inkjet-printed PSCs, IJP technology has demonstrated remarkable applications in perovskite light-emitting diodes (PeLEDs) and PDs. The metal halide perovskite is a promising material due to its high photoluminescence (PL) efficiency, high color purity, and feasible solution processability.31,32 Similar to the PSC counterparts, the record performance of PeLEDs with a 28% external quantum efficiency (EQE) is achieved via the spin-coating method.33 The patterning feature of IJP makes it highly suitable for fabricating large-scale PeLED arrays. However, little work has been reported about inkjet-printed PeLEDs owing to poor control of the printing process. The highest EQE of inkjet-printed PeLEDs is only 9.0%.34 The perovskite PDs prepared by IJP have demonstrated their potential application in the wearable sensor market, owing to their efficient photo-to-electron conversion. For instance, PDs based on the CsPbBr3 microplate array achieved the high responsivity and decent detectivity of 480 A W−1 (1.8 mW cm−2, 2 V) and 2 × 1011 Jones, respectively.35 Our group36 proved that direct IJP in polydimethylsiloxane can produce wafer-level perovskite fluorescent patterns. The development of inkjet-printed LEDs and PDs lags behind that of PSCs, but the progress on inkjet-printed PSCs promises to guide the further development of inkjet-printed PeLEDs and PDs. High-performance perovskite optoelectronic devices rely on the perovskite film formation. Poor film quality with crystallization defects generally leads to severe nonradiative recombination and deteriorates the perovskite photoelectric properties.37 During the process of IJP, the droplet manipulation and nucleation/crystallization regulation of the printed perovskite film are the key factors to obtain high-quality films.14 Versatile methods, including ink engineering, printing process, and posttreatment, are widely used to improve the film quality of inkjet-printed perovskites. However, to the best of our knowledge, there is still no summary of the strategies for optimizing inkjet-printed perovskite thin films in terms of droplet manipulation and nucleation/crystallization regulation. In this review, relying on the theories of droplet formation and perovskite nucleation/crystallization, we summarize the perovskite film-formation strategies via IJP in the aspects of ink engineering, printing process, and posttreatment. Furthermore, we introduce the recent advances of inkjet-printed perovskite films in PSCs, LEDs, and PDs. Finally, we propose meeting the challenges of obtaining high-quality inkjet-printed perovskite film and highlight the “Trilogy Strategies” of printing high-quality films designed for high-performance perovskite optoelectronic devices. Perovskite Film Formation during the IJP Process To construct commercialized inkjet-printed perovskite optoelectronics, the formation of a high-quality perovskite active layer on a large scale is critical to fill the performance gap between laboratory-scale devices and scalable devices for commercialization.38 This is the prerequisite and core of the future perovskite optoelectronic industry. Recently, Ge et al.39 summarized the kinetics of perovskite nucleation and crystal growth for different scalable preparation processes and provided guidance for the formation of high-quality perovskite films. It is essential to comprehensively understand the inkjet-printed perovskite nucleation and crystallization mechanism, which can in turn benefit the exploration of perovskite inkjet-printing dynamics. Mechanism of nucleation and crystallization of perovskite The perovskite nucleation/crystallization processes and inkjet-printing procedures (ink engineering, the printing process, and posttreatment) can mutually influence each other.30,40 The nucleation and crystal growth processes can be divided into three stages according to the classical LaMer theory.41,42 As shown in Figure 1a, in stage I, the monomers accumulate in the solution. Although the monomer concentration exceeds the solid solubility (Csol), no obvious nucleation occurs. In stage II, the monomer concentration reaches the critical supersaturation (Cmin), where it reaches the critical level of nucleation, and then nucleation occurs. When the monomer concentration exceeds the maximum supersaturation (Cmax), the nucleation speed is significantly accelerated. In stage III, the nucleation is terminated when the concentration of monomers is lower than the Cmin. The crystal continues to grow via the monomer diffusion mechanism. Eventually, the crystal growth is finalized when the concentration of monomers decreases below the Csol. From the point of view of thermodynamics, a nucleus could be regarded as an ideal sphere of the condensed phase. The total free energy (ΔG) is composed of the surface free energy (ΔGs) and the bulk free energy (ΔGv), where ΔGs refers to the free energy between the particle surface and the particle, and ΔGv refers to the free energy between the large particle and the solute in the solution. As shown in Figure 1b, ΔGs is a positive value proportional to r2, and ΔGv is a negative value proportional to r3 in a supersaturated solution. Therefore, the process of particle nucleation in the solution strongly depends on its critical radius (r*). In other words, particles with a radius smaller than r* dissolve back into the solution, whereas nuclei with radius larger than r* are thermodynamically stable and will further grow.41 Figure 1 | Schematic diagrams of (a) the nucleation growth mechanism of perovskite and (b) the classical free energy of uniform nucleation. (c) Schematic diagram of inkjet-printed film-formation processes. Download figure Download PowerPoint Based on the above theoretical analysis, we can conclude that both the number of nucleation sites and the crystallization speed have an important influence on the quality of the perovskite film. In the case of the IJP process, it is critical to homogeneously achieve fast nucleation and slow crystal growth. In this way, dense, large-area perovskite films with uniform coverage can be inkjet-printed out. However, there exist several hurdles to overcome to achieve this goal: (1) suitable ink composition, (2) printing resolution control, (3) suitable surface properties of printing substrates, (4) annihilation of the coffee-ring effect, and (5) optimization of post annealing. Therefore, we divide the series of IJP regulatory approaches into three phases: ink engineering, the printing process, and posttreatment (Figure 1c). We hope that this systematic summary can shed light on the design and fabrication of large-scale inkjet-printed perovskite optoelectronic devices. Ink engineering Ink properties, such as viscosity, volatility, and surface tension, have an important impact on the perovskite droplet spreading/coalescence and film formation process.43–45 Optimizing the composition of the ink and introducing appropriate additives are effective strategies to improve the morphology of printed perovskite films.30 Solute engineering Solute is an important part of perovskite ink. During the film formation process, the solute initially accumulates in the solution. When the solute concentration reaches the critical supersaturation, nucleation occurs. As the solute concentration further increases to exceed the maximum supersaturation and reaches its maximum value, the nucleation speed increases significantly, and the crystal grows. Therefore, solute engineering can manipulate the nucleation and crystallization process of perovskite, improving the morphology of the film. A lot of recent work in solute engineering has been reported exploring the solute effects on film morphology and is summarized below. In 2018, Gheno et al.27 used the Owens–Wendt–Rabel–Kaelble model to study the effects of chloride, bromide, and diiodooctane on the wetting property of perovskite inks (Figure 2a). The low surface energy of the substrate triggered dewetting behavior during the printing process of perovskite. The use of chlorine or bromine tended to increase the wettability of perovskite inks, leading to better impregnation of the ink into the porous material. In contrast, the diiodooctane did not seem to affect the wettability. By optimizing the solute composition of the perovskite ink, they successfully prepared inkjet-printed CH3NH3PbI3-xClx PSCs with an efficiency of 10.7%. Zhang et al.48 manipulated the spreading and crystallization behaviors of perovskite droplets employing lead acetate (PbAc2), lead chloride (PbCl2), and methylammonium iodide (MAI) in printing ink formulation. The perovskite droplets containing pure PbAc2 exhibited fast crystallization speed, resulting in the formation of plate-like crystals. The introduction of chlorine in the perovskite precursor ink significantly alleviated this phenomenon, and inkjet-printed perovskite film with appropriate addition of PbCl2 demonstrated dense and homogeneous crystals. Consequently, a PCE of 16.6% with MAPbIxCl3-x precursors was shown for inkjet-printed PSC in air. Figure 2 | Ink engineering of perovskite in IJP process. (a) Wetting envelopes of the ETL and wettabilities of various perovskite inks. Reprinted with permission from ref 27. Copyright 2018 Wiley-VCH. (b) Top-view scanning electron microscopy (SEM) images of mesoporous TiO2 films coated with the inkjet-printed perovskite from the precursor solution with a molar ratio of 1 − x:1:x for PbI2, MAI, and MACl, respectively, at x = 0, 0.3, 0.6, and 0.9. Reprinted with permission from ref 29. Copyright 2015 Royal Society of Chemistry. (c) Green solvent-based perovskite precursor development for inkjet-printed flexible solar cells. Reprinted with permission from ref 46. Copyright 2021 American Chemical Society. (d) Photographs of binary perovskite ink-systems (PIL/co-solvent) with increasing cosolvent volume. Reprinted with permission from ref 47. Copyright 2018 Elsevier. (e–i) The printed points and films under different mixed solutions of DMF and DMSO. Reprinted with permission from ref 14. Copyright 2018 Elsevier. (j) Images of optical microscopy for the dynamic crystallization process of printed ink. Time here indicated how long the printed liquid films were exposed to the air right after printing. PbX2 (X = Br, I) was used as an ink precursor for all samples. Reprinted with permission from ref 30. Copyright 2020 American Chemical Society. Download figure Download PowerPoint In addition to tuning solute composition, special chemical additives can influence the ink properties and assist and/or delay perovskite crystallization. In 2015, our group29 studied the effect of MACl as an additive to assist the crystalline process. As shown in Figure 2b, uniform perovskite film with complete surface coverage was obtained upon the addition of MACl, while the control sample exhibited isolated disk-like crystal plates. The corresponding PSCs with MACl additive exhibited a high PCE of 12.3%. Subsequently, Hashmi et al.49 used 5-ammonium valeric acid iodide (5-AVAI) as an ink additive to improve the CH3NH3PbI3 crystal quality and increase charge carrier lifetime, resulting in a device efficiency of 8.47%. They found that 5-AVAI could significantly slow down the growth of perovskite crystals, prolonging the crystallization time. Wilk et al.46 developed a new ink formulation containing Lewis base-type coordination additive, thiosemicarbazide (TSC) (Figure 2c). The addition of TSC produced a stable intermediate phase of Pb complexes to facilitate the pure-phase perovskite crystallization. Finally, an encouraging PCE of 11.4% under a large device area (1 cm2) was achieved. Solvent engineering During the process of perovskite film formation, the nucleation and crystallization of the perovskite are accompanied by the evaporation of the solvent. Solvent properties, such as viscosity, surface tension, and evaporation rate, have a significant impact on the printing behavior of the droplets and the dynamics of nucleation/crystallization, which ultimately influences the film quality.50–53 Therefore, the solvent composition has been extensively investigated. In 2018, Öz et al.47 developed a new green solvent system, which consists of protic ionic liquids (PILs) with water, alcohol, and acetonitrile (Figure 2d). Three methylammonium-based PILs were prepared with different anions including formate, acetate, and propionate. This PILs-based ink system for perovskite precursors can successfully replace toxic solvents such as N,N-dimethylformamide (DMF). Perovskite inks containing propionate-based PILs demonstrated the best compatibility with lead halide precursors, resulting in ultrasmooth and crystallized perovskite thin film. Consequently, the PCE of the multication mixed halide PSC prepared with the PIL/acetonitrile solvent system exceeded 15%. The rheological behavior and the wetting capability of a potential solvent system have a close relationship with the perovskite grains. We systematically investigated the physical properties of mixed solutions [DMF/dimethyl sulfoxide (DMSO)] toward the inkjet-printed growth of large-size perovskite grains.14 Owing to the difference in viscosity and boiling point, the high proportion of DMSO solvents spread slowly leading to the formation of smaller printing dots, which results in high surface roughness of PbI2 film (Figures 2e–2i). In contrast, the high proportions of DMF solvents were inclined to form “islands” that reduce the grain growth space (Figures 2e–2i). The 1:1 solvent ratio resulted in a uniform PbI2 film with few pinholes in favor of large crystal grain growth, which enabled PSCs with high PCEs of 18.64% for a small area (0.04 cm2) and 17.74% for a large area (2.02 cm2). To solve the problem of rapid crystallization of inks during the printing process, our group30 further designed a new mixed cationic perovskite ink system which can controllably delay the crystallization rate of the perovskite (Figure 2j). In this new ink system, the printing solvent consisted of N-methyl-2-pyrrolidone (NMP) and DMF, and PbX2 was replaced by PbX2-DMSO (X = Br, I) complex as a printing precursor. The introduction of NMP can prolong the retention time of the liquid film due to its ability to adjust the viscosity and surface energy of the solvent, while the PbX2-DMSO complex can prevent the rapid reaction among the precursor ions and thus retard the nucleation and growth rate of perovskite. Based on the synergistic effect, the crystallization rate of the printed perovskite can be controlled and slowed down, and the printed Cs0.05MA0.14FA0.81PbI2.55Br0.45 perovskite film exhibited high uniformity and large grain size (over 500 nm). In addition, the printed perovskite film had lower defect density and improved carrier lifetime compared with the control sample. Combining these advantages, printed PSCs achieved high PCEs of 19.6% (0.04 cm2) and 17.9% (1.01 cm2). This research has fully demonstrated the influence of ink properties on the quality of printed films and device performance, which can promote the development of printing inks in the IJP process. The effect of the interaction between solute and solvent on the nucleation and crystallization of perovskite should be further explored to maximize the role of ink engineering in the formation of high-quality perovskite films. During the nucleation and crystallization process, the solute agglomeration leads to the formation of crystal nuclei, and the evaporation of the solvent results in the formation of the supersaturated state. The interaction between the solute and solvent can induce an intermediate phase, such as the complex of PbX2-DMSO, which can delay the nucleation and crystallization of perovskite.54–57 Therefore, precise design of the ink properties can optimize the nucleation and crystallization dynamics to achieve high-quality perovskite films. Printing process In the IJP process, a nozzle is used to jet tiny ink droplets onto the substrate at a certain speed. By adjusting the spacing of the ink droplets, a uniform dot matrix can be printed. The dots combine with each other to form a line array, and then the lines further merge to form a complete liquid film.58 The droplets production of drop-on-demand (DOD) printing is discontinuous and controlled by digital signals from an imaging computer. The schematic diagram of the working principle of DOD printing is shown in Figure 3a. For DOD printing, the droplet formation is mainly based on piezoelectric transducers,61 as shown in Figure 3b. Piezoelectric IJP has outstanding advantages, making it the mainstay of industrial printers. First, piezoelectric IJP with an actuation pulse feature has more precise control over the volume and shape of ink droplets, producing high patterning resolution up to 5000 dots per inch (dpi). Second, piezoelectric IJP has a wider choice of inks. Due to its special working principle, piezoelectric IJP does not require instantaneous high-temperature heating (200–300 °C) of the ink. This feature can prevent inks from decomposition, especially for inks containing organic and biological materials.62,63 Therefore, piezoelectric IJP is the choice of priority for fabricating various functional perovskite devices, such as solar cells,5,6,64 LEDs,8,9 and PDs.10,11 In the process of IJP, the printing process is an important factor affecting the quality of the film.65–67 The optimization of the printing process can be achieved via droplet manipulating, including droplet formation and printing parameters control. Figure 3 | Schematic diagrams of the working principle of (a) DOD printing and (b) piezoelectric IJP. (c) A schematic diagram showing the operating regime for stable operation of DOD printing. Reprinted with permission from ref 59. Copyright 2011 American Institute of Physics. (d) Photo sequence of drop formation. Reprinted with permission from ref 60. Copyright 2009 American Chemical Society. (e) SEM images of cross sections of PSCs with IJP perovskite absorber layers printed with different resolutions. A spin-coated reference is shown as well. Reprinted with permission from ref 28. Copyright 2019 Wiley-VCH. (f) Cross-sectional and top-view SEM images of the printed films with different thicknesses by adjusting the printing parameter. Heterogeneous nucleation dominated films: aligned crystal grains; homogeneous nucleation dominated films: stacked crystal grains. Reprinted with permission from ref 48. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint Understanding the theory of droplet formation is essential to accurately control the droplets and obtain high-performance printed devices. In the printing process, the initial step is to select the appropriate ink according to the specification window of the printhead, as shown in Figure 3c.68 The printability of an ink formulation is quantified by the Ohnesorge number (Oh), which accounts for rheological ink characteristics including density ρ, viscosity η, surface tension σ, and a given nozzle diameter d.42 These parameters are usually expressed as dimensionless Reynolds number (Re) and Weber number (We) derived from the Navier–Stokes flow equation: O h = We Re = η ( ρ σ d ) 1 / 2 .(1) Ohnesorge et al.59 showed that the inverse Ohnesorge number Z = Oh−1 should be between 1 and 10 to obtain the best printing results. At low Z values, viscous dissipation prevents ink from ejecting; while at high values, the main droplet is accompanied by a large amount of satellite droplets.68 In the case of a typical nozzle with 20-μm diameter, the ink viscosity should be between 1 and 25 mPa s, and the surface tension should be between 25 and 50 mN m−1.60,69 To prevent nozzle clogging, the tolerable particle size is about 1% of the orifice diameter. And high boiling point solvents (Tb over 150 °C) and their mixtures are often used to avoid clogging, premature drying, and uncontrollable film formation.42 The droplet formation sequence observed on the DOD printer is shown in Figure 3d.59 The long-extension fluid tail is a characteristic of the DOD process.70 The droplet derives from the initial liquid column, thinning to form a leading droplet with an elongated tail or ligament. The final ligament rupture will lead to the formation of satellite droplets. If these droplets are still present at the time of impact, they will result in a noncircular impact footprint. This behavior has a detrimental effect on deposition precision, resolution, and accuracy. To promote droplet merging, it is critical to select the appropriate printing distance between the nozzle and the substrate. The separation distance use