A synopsis of progressive transition in precursor inks development for metal halide perovskites-based photovoltaic technology 2021 Journal of Materials Chemistry A

a Perovskite solar cell (PSC) technology has received considerable attention due to the rapid escalation of their solar-to-electrical energy conversion, which has recently surpassed 25% for lab-sized solar cells. Other bene ﬁ ts such as their fabrication through solution processing enable new opportunities for scaling up and rapid production. These features may play a key role in realizing quick installations worldwide, helping to meet the global energy production and consumption demand with a realistic energy pay-back time. This report provides an overview of the progress in developing liquid precursor inks for producing a variety of organic – inorganic halide perovskite-based light absorbing layers. In recent years, a variety of con ﬁ gurations for PSC technology have been reported, where intelligent inks of perovskite precursors have been formulated to facilitate novel designs with impressive solar-to-electrical energy conversions and promising stability. This report highlights the evolution of these novel perovskite precursor ink formulations, and discusses the emerging trends in developing e ﬃ cient, scalable, and robust PSC technology. Moreover, the classi ﬁ cation, advantages, and limitations of various types of perovskite precursor ink are addressed. Speci ﬁ cally, single- and multi-cation-based ink formulations are discussed in relation to their impact on producing e ﬃ cient solar cells, which provides an overview of the recent progress in the development of this emerging and low-cost solar cell technology. Overall, this synopsis provides the current state of the art in designing novel perovskite precursor inks to be used in producing high performance, e ﬃ cient, scalable, and stable con ﬁ gurations of perovskite solar cell technology.

A synopsis of progressive transition in precursor inks development for metal halide perovskitesbased photovoltaic technology Cuc Mai Thi Kim, a Lahoucine Atourki, b Mouad Ouafi cd and Syed Ghufran Hashmi * a Perovskite solar cell (PSC) technology has received considerable attention due to the rapid escalation of their solar-to-electrical energy conversion, which has recently surpassed 25% for lab-sized solar cells.
Other benefits such as their fabrication through solution processing enable new opportunities for scaling up and rapid production. These features may play a key role in realizing quick installations worldwide, helping to meet the global energy production and consumption demand with a realistic energy pay-back time. This report provides an overview of the progress in developing liquid precursor inks for producing a variety of organic-inorganic halide perovskite-based light absorbing layers. In recent years, a variety of configurations for PSC technology have been reported, where intelligent inks of perovskite precursors have been formulated to facilitate novel designs with impressive solar-to-electrical energy conversions and promising stability. This report highlights the evolution of these novel perovskite precursor ink formulations, and discusses the emerging trends in developing efficient, scalable, and robust PSC technology. Moreover, the classification, advantages, and limitations of various types of perovskite precursor ink are addressed. Specifically, single-and multi-cation-based ink formulations are discussed in relation to their impact on producing efficient solar cells, which provides an overview of the recent progress in the development of this emerging and low-cost solar cell technology. Overall, this synopsis provides the current state of the art in designing novel perovskite precursor inks to be used in producing high performance, efficient, scalable, and stable configurations of perovskite solar cell technology. Lahoucine Atourki is currently working as an Assistant Professor at the Department of Physics, Mohammed V University in Rabat, Morocco. His research is centered on the growth and characterization of nanomaterials for energy conversion. Houcine's current interest is developing and understanding 2D electronic passivation of 3D perovskites for solar cell applications. Houcine has a Ph.D. degree in Physics from Ibn Zohr University, Morocco. He has also worked as a post-doctoral researcher at the School of Materials Science and Engineering at Georgia Institute of Technology (USA).

Introduction
The growing population and resulting human activity continue to increase energy consumption, imposing new challenges for energy supply and production. Solutions must meet sustainable production criteria, especially under the current climate change scenarios. 1,2 These stringent conditions have become a source of intense motivation for researchers worldwide to develop clean and alternative energy production schemes focused on reducing greenhouse gas effects and environmental pollution. 3,4 Among the various renewable energy sources, photovoltaics has been considered as a potential source for providing clean, silent, and affordable energy with high solar-to-electrical energy conversion. [5][6][7] Despite the increase in installations of dominant Si-based photovoltaic systems, these have various limitations such as saturated performance for bulk outdoor electricity production, limited aesthetics for modern architecture-based building integrated photovoltaic (BIPV) applications, and noneco-friendly and high energy consumption-based production methods. Moreover, they display limited performance under low light intensity conditions; efficient conversion of electrical energy under simulated light is needed to energize maintenance-free internet of things (IoT) devices and portable electronics. [8][9][10][11] In this regard, third generation-based photovoltaic (PV) technologies such as dye-sensitized solar cells (DSSCs), organic solar cells (OSCs) and perovskite solar cells (PSCs) offer numerous advantages over Si-based PV systems. [9][10][11] For example, they provide exibility in designing solar cells and modules in a variety of sizes, their fundamental materials are abundantly available, and they are fabricated with non-vacuumbased scalable production methods such as established slot-die coating, screen printing or inkjet printing-based material deposition technologies. 12,13 Such unique features make these next generation-based PV technologies a potential source for energizing modern electronics and utilizing their integration in the next generation of BIPV products. They also provide the possibility to produce bulk electricity with much higher solar-toelectrical energy conversion efficiencies compared to standalone Si-based PV systems, for example when smartly used in tandem congurations or installed outdoors under natural climatic conditions. [14][15][16] Among these next generation-based PV technologies, PSCs have received considerable attention due to the rapid escalation of their solar-to-electrical energy conversion, which has recently surpassed 25%. 17 In addition to these reported improvements in conversion efficiencies, other benets such as their fabrication through solution processing enable new opportunities for scaling up and rapid production. 18 Importantly, this may play a key role in realizing quick installations worldwide, thus providing a realistic way of meeting the global energy production and consumption demand. 19,20 This report provides an overview of the progress in developing liquid precursor inks for producing a variety of organicinorganic halide perovskite-based light absorbing layers in emerging PSC technology. formulated to facilitate novel designs with impressive solar-toelectrical energy conversions and promising stability. [21][22][23][24] This report highlights the evolution of these novel perovskite precursor ink formulations and discusses the emerging trends towards developing efficient and robust PSC technology. In addition, the classication, advantages, and limitations of various types of perovskite precursor ink are addressed. Singleand multi-cation-based ink formulations are discussed in light of their impact on producing efficient solar cells. Progress in developing lead (Pb)-free precursor inks is also reviewed, as well as the scalable precursors that have been identied for producing large-area solar modules in a variety of congurations with popular scalable fabrication methods. Overall, this review provides the most up-to-date advancements being made in designing novel perovskite precursor inks for producing high performance, efficient, scalable, and stable congurations of perovskite solar cell technology.

Role of precursor inks in establishing the current state of the art for conversion efficiencies of PSC technology
The progressive advances in PSC solar-to-electrical conversion efficiencies ( Fig. 1) may be attributed to many factors, for example, the known knowledge of design principles for producing solid state DSSCs (ssDSSCs), 25,26 optimizing the active layer thicknesses for improving the diffusion lengths of carriers, [27][28][29] or even proposing intelligent strategies for minimizing both the radiative and non-radiative losses observed in device architectures. [30][31][32][33][34][35] The advances in designing novel perovskite precursor inks [36][37][38][39] have played a key role in addressing the aforementioned key issues, thus leading to extraordinary improvements in solar-to-electrical energy conversions in recent years (Fig. 1).
The striking discovery of potential applications of perovskite-based light harvesters in photovoltaics was preliminarily reported by formulating two types of precursor ink. 40 Briey, CH 3 NH 3 Br and PbBr 2 were dissolved in N,N-dimethylformamide (DMF) solvent, while CH 3 NH 3 I and PbI 2 were mixed in g-butyrolactone (GBL) as a high viscosity solvent, cast on a TiO 2 nanocrystalline lm via the spin coating method (also known as the single-step deposition method). [41][42][43] The solar cells produced in a liquid junction DSSC assembly fashion resulted in 3.81% solar-to-electrical energy conversion when tested under full sunlight illumination. 40 Following these ndings, several additional reports have assessed other novel precursor inks including lead-based compounds. For example, lead iodide (PbI 2 ) or lead chloride (PbCl 2 ) have been combined with other organic-inorganic salts such as methylammonium iodide (CH 3 NH 3 I) or cesium iodide (CsI) and a variety of solvents to further improve the solar-toelectrical conversion efficiencies of the fabricated solar cells. [44][45][46] Among these investigations, the study by Kim et al. mimics the solid-state dye-sensitized solar cell (ssDSSC) device design, in which the TiO 2 -based electron transport layer (ETL) was rst inltrated with MAI and PbI 2 containing a perovskite precursor ink with a GBL solvent kept overnight while stirring at 60 C. 47 The solid-state cell architecture was further processed via casting a hole transporting layer (HTL), i.e. Spiro-OMeTAD, followed by thermal evaporation of gold to produce the nal contact layer. Interestingly, the fabricated solar cells not only exhibited far higher conversion efficiencies (>9%) compared to previous reports, but they also exhibited striking photovoltaic performance stability during periodic measurements when tested without encapsulation for a period of 500 hours in air combined with room temperature (RT)-based ecological conditions.
Building on the outcomes of the aforementioned research, >10% conversion efficiencies were promptly demonstrated for lab-sized PSC devices, also by incorporating a mixed cationbased perovskite precursor solution ink that was used to achieve the perovskite light absorbing layer through a single-step spin coating method. 48 Unlike the single-step processing-based precursor inks, the pioneering work reported by Burschka et al. used a two-step processing scheme. In this approach, the perovskite-based light absorbing layer in the PSC device structure was achieved by fabricating two individual solutions. 49 The rst precursor ink was obtained by dissolving PbI 2 in DMF and was kept at 70 C before the inltration in step 1 through spin coating. Next, glass electrodes containing the PbI 2 -inltrated electron transport layer (ETL, TiO 2 ) were dipped into a solution of MAI containing 2-propanol solvent for 20 seconds to obtain the desired perovskite crystal-based light harvesting layer. Striking solar-toelectrical conversion efficiencies reached 15% due to achieving a controlled perovskite crystal morphology in response to the previously reported single-step mixed halide perovskite inks. 49 Since the description of these initial techniques, progressive enhancements in conversion efficiencies have continued. The solar-to-electrical energy conversion milestone of 25% was  Bulk perovskite layer is passivated with a 2D perovskite recently surpassed for lab-sized PSCs 17 due to the tremendous efforts being made in developing individual components of this solution-processed PV technology. Designing and developing novel precursor inks for producing high quality perovskitebased light absorbing active layers has remained the focus of reaching high performance and striking stability under several simulated and natural climatic conditions. Fig. 1 summarizes the solar-to-electrical energy conversion evolution. Table 1 provides a list of various perovskite precursor inks used to contribute towards enhancing the conversion efficiencies of labsized PSCs in recent years.

Emerging research trends in perovskite precursor ink development
There are a variety of precursor inks that have been developed by incorporating various novel materials to produce perovskitebased light absorbing active layers. These precursor inks (Fig. 2) can be categorized as classical single-cation based, 60-62 multication based, 63-65 additive based, 53,66-68 or lead-free. [69][70][71] Traditional PSC devices have been initially reported with a classical ABX 3 -based crystalline perovskite active layer. Here, A generally represents methyl-ammonium CH 3 NH 3 + , formamidinium NH 2 CH ¼ NH 2 + or cesium (Cs)-based monovalent organic or inorganic cations. 40,46 B typically represents Pb 2+ , Sn 2+ or even Ge 2+ -based divalent metal cations, and X 3 shares I À , Cl À , Br À or Fl À as monovalent anions during the crystal formation of the traditional perovskite-based light absorbing layer. 72,73 Due to the inherited characteristics such as stable crystal formations, 74-78 good chemical compatibility with other active layers, high efficiency and robust stability under initially simulated environmental conditions, 79,80 single cation-based perovskite precursor inks have been the focus of developing the rst generation of lab-sized PSC devices. [81][82][83] Interestingly, these rst-generation precursor inks exhibited strong processing compatibility when incorporated utilizing both the singlestep and two-step fabrication schemes to produce lab-sized PSC devices. [84][85][86][87][88] Nevertheless, single-step processable precursor inks have remained the preferred choice for scaling up various device designs of PSC technology. 89,90 Recently, trends of multi-cation-based ink formulations have also emerged, not only for suppressing non-radiative losses but also for achieving high performance and better stability. This has mainly been achieved through large grain boundary-based crystal formulations, along with physical and chemical stabilities of perovskites as light harvesters in PSCs. 63,[91][92][93][94] Bi et al. demonstrated a unique multi-cation-based halide precursor ink by incorporating multi-cations such as formamidinium iodide (FAI) and methylammonium bromide (MABr) salts with lead bromide (PbBr 2 ) and PbI 2 . These were dissolved in a mixed solvent system containing dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) solvents. This recipe of perovskite precursor ink resulted in PSC devices with >20% conversion efficiency. 52 In addition to multi-cations, other novel additives have also been introduced in precursor inks to overcome/achieve some of the key bottlenecks and milestones related to technological a Two-step deposition: step 1 ¼ spin coating, step 2 ¼ dipping. b Two-step deposition: step 1 ¼ spin coating, step 2 ¼ spin coating. PVK ¼ perovskite. challenges and long-term performance stabilities necessary for the successful commercialization of PSC technology. [95][96][97][98][99] In this regard, one ground-breaking research study in terms of designing a novel perovskite precursor ink was reported by Mei and co-workers. The striking mesoscopic carbon-based printable perovskite solar cell (CPSC) device design featured the introduction of an additive, i.e. 5-ammonium valeric acid iodide (5-AVAI), in the solution recipe of the traditional perovskite precursor ink. 21 The additive (5-AVAI) induces control over the crystallization process during the inltration of the formulated perovskite ink in a much thicker and more porous structure compared to the traditional PSC device congurations. [100][101][102] As a result, the formulated ink contributed to surpassing the rst >1000 hours stability test of these carbon-based printable perovskite solar cells when exposed to ambient air under full sunlight intensity conditions. 21 The promising characteristics of this device design have yielded benecial features, including successful scaling up and promising conversion efficiencies demonstrated in PSCs ranging from lab-sized to various module sizes, along with robust stability when deployed under various natural and simulated climatic conditions. 103-105 Interestingly, 5-AVAI containing a novel precursor ink formulation has also been incorporated with scalable material deposition methods such as inkjet printing or slot-die coating-based technologies. [106][107][108] In contrast to CPSCs, Bi and co-workers used polymer poly(methyl methacrylate) (PMMA) as the templating agent-based additive in the multi-cation-based precursor ink. They demonstrated perovskite lms of high electronic quality for producing very high conversion efficiency (>21%) in one of the traditional mesoporous n-i-p PSC devices. 53 With the impressive solar-toelectrical energy conversion, the photovoltaic parameters were shown to have an inuence upon adjusting the concentration of PMMA, where the champion efficiencies (21.6%) were recorded with a controlled nucleation and crystal growth based light absorbing layer when a specic concentration (0.6 mg mL À1 ) was used in the fabricated perovskite precursor ink. 53 Moreover, the concept of introducing ionic liquids (ILs) in perovskite precursor inks as one of the other promising additive components has also received attention for achieving high performance and robust stability when tested in several congurations. 99,[109][110][111][112][113] For example, Bai and co-workers introduced 1-butyl-3-methylimidazolium tetrauoroborate (BMIMBF 4 ) ionic liquid in the multi-cation perovskite precursor ink. They observed striking efficiencies of 20% at the steady state when fabricated as p-i-n conguration-based PSC devices containing this IL-based perovskite light absorbing layer. 109 The inclusion of BMIMBF 4 IL as an additive shows promise for inuencing the crystallinity and reducing the defects in the incorporated perovskite-based light absorbing layer in the fabricated PSCs. This also contributed to the stability results when exposed to full-spectrum sunlight in air (RH 40-50%) at 60-65 C, or full-spectrum sunlight at 70-75 C based on the environmental testing conditions. 109 Importantly, incorporating ILs in perovskite precursor inks not only offers control over crystallization dynamics and improved charge transport in the perovskite light absorbing layers, but it also provides a possibility for using them as green solvents for realizing the steps towards making PSCs an ecofriendly technology. [114][115][116][117] For example, Chao and co-workers demonstrated a novel precursor ink utilizing an eco-friendly IL, i.e. methylammonium acetate (MAAc), as one of the possible green solvents for producing an efficient and pinholefree perovskite light absorbing layer. This method revealed promising environmental stability for a period of more than 3000 hours under high humidity (60-80%) conditions. 115 Motivated by such promising ndings, both n-i-p and p-i-n conguration-based PSCs were fabricated by incorporating this novel precursor ink that showed promising solar-to-electrical energy conversions ($16-20%). Notably, 20.05% is the champion conversion efficiency marked for the n-i-p based planar PSC device design under full sunlight illumination. More impressively, the non-sealed fabricated PSCs also exhibited striking long-term photovoltaic performance stability by retaining 93% of the initial device efficiency under air for more than 1000 hours. A minor deviation (<20%) in the initially achieved efficiency was observed aer 700 hours of testing under light stress under the conditions maintained in the glovebox. 115 Despite the numerous striking features acknowledged for this emerging low-cost PV technology, one of the key concerns raised relates to incorporating hazardous and toxic compounds such as lead (Pb)-based compounds, which have thus far remained an essential element in producing high performance PSCs. [118][119][120] Use of Pb in a PSC, however, restricts its deployments in many portable and consumer electronics-based applications since it may leak from such devices and cause bodily harm. 121 Similarly, deploying PSCs outdoors for bulk electricity generation also requires robust encapsulation procedures due to the solubility of Pb as a heavy metal in water. This may generate a negative environmental impact as indicated in several life cycle assessment reports for PSC technology. [122][123][124] Hence, efforts are being made to either reduce or completely replace Pb-based compounds from the next generation of PSC devices. 125,126 In this regard, the emerging trends for demonstrating leadfree precursor formulations include tin (Sn)-127-129 or germanium (Ge)-based halide perovskite precursor inks. 130,131 Similarly, trivalent antimony (Sb 3+ )- [132][133][134] or bismuth (Bi)-135-137 based perovskite precursor formulations have also been explored as possible alternatives for replacing hazardous and toxic leadbased halide perovskite precursor inks. 125,126,138 Among these formulations, Sn-based formulations have thus far contributed to a gradual increase in the solar-to-electrical conversion efficiencies, 128,129,139-141 which has recently exceeded above 13% when tested in inverted p-i-n device design-based PSCs under full sun illumination. 127 Nevertheless, the longterm photovoltaic performance stability was not demonstrated, hence further investigations are needed for producing robust lead-free PSCs in the near future. Table 2 summarizes versatile ink formulations that have been incorporated in many device designs in recent years to produce high performance perovskite solar cell devices.

Research and development trends for advanced scalable precursor inks
Motivated by the successes achieved from solution processed lab-sized PSCs with traditional perovskite precursor inks, efforts have also been made towards scaling up PSC technology on a variety of substrates by designing novel precursor inks with key characteristics as shown in Fig. 3. [153][154][155][156][157][158] In this regard, spin coating has remained the foremost choice for perovskite precursor ink deposition, as well as for the initial scaling up demonstrations of PSC technology (Table 3). [159][160][161] Among these initial pioneering efforts, Matteocci et al. demonstrated the rst n-i-p structure-based solid-state monolithic module 162 with organometal halide perovskite CH 3 NH 3 -PbI 3Àx Cl x ink, produced by dissolving methylammonium iodide CH 3 NH 3 I and lead chloride PbCl 2 in a N,N-dimethylformamide (DMF)-based solvent. The ink was spin-coated at 2000 rpm for 40 s over the nanocrystalline TiO 2 -based electron transport layer (ETL) in air and successively heated at 120 C for 45 minutes to obtain the nal crystalline structure. The fabricated modules exhibited similar ($5.1%) conversion efficiencies when tested with two types of hole transporting layer (Spiro-OMeTAD and poly(3hexylthiophene-2,5-diyl) (P 3 HT) polymer) and thermally evaporated gold (Au)-based contact layers over an active area of 16.8 cm 2 .
As a further advancement, Seo et al. reported an inverted (pi-n) structure-based module design with a perovskite ink composed of CH 3 NH 3 I and PbI 2 , which were stirred in a mixture of dimethyl sulfoxide (DMSO) and g-butyrolactone (GBL)-based solvents at 60 C for 12 h. 163 The ink was deposited onto a PEDOT:PSS/ITO substrate by a consecutive two-step spincoating process at 1000 and 4000 rpm for 20 and 60 s, respectively. The toluene in the nal spin-stage was dripped onto the substrate during spin coating. The perovskite precursor-coated substrate was then dried on a hot plate at 100 C. The fabricated module with the phenyl-C61-butyric acid methyl ester (PCBM)based ETL and Al coated contact layer showed an impressive conversion efficiency (8.7%) over a far larger active area (60 cm 2 ) when tested under full sunlight illumination.
In addition to these earlier reports of precursor ink development with the spin coating-based material deposition method, advanced single-and two-step processable precursor inks have also been developed and tested with more established scalable approaches (as depicted in Fig. 4), such as blade coating and slot-die coating to fabricate large area perovskitebased solar modules. [164][165][166][167] For example, Razza et al. used a two-step approach where the pre-heated PbI 2 precursor ink was rst coated on ETL layers using a blade coating method followed by rapid evaporation of its solvent (DMF) through an airow. 164 Next, the perovskite crystalline layer was achieved by directly dipping the substrates containing PbI 2 -coated ETL layers into the MAI solution with different dipping times. The perovskite modules fabricated with this approach exhibited 4.3% conversion efficiency under full sunlight illumination over an active area of 100 cm 2 .
Deng et al. introduced the novel concept of adding a surfactant to the perovskite precursor ink and demonstrated impressive control over the crystallization morphology of the large-area perovskite-based light absorbing layer when deposited with a blade coating scheme. 168 The fabricated modules showed very high ($15%) conversion efficiency when tested under full sunlight illumination with a p-i-n device design. Similarly, Giacomo et al. demonstrated slot-die coating of a precursor solution for a perovskite layer, which was prepared under room temperature conditions in an inert N 2 environment. 169 The solution was prepared by dissolving 1 mM of lead compounds and 3 mM of CH 3 NH 3 I in 1 mL of DMF solvent. The solution was stirred for at least 10 minutes before nal use. Slot-die coating of this perovskite precursor ink was performed inside a glovebox, which allowed deposition of a wide range of thicknesses. The wet layer thickness of the perovskite was in the range of 3-4 mm to get a dry layer thickness of 350 nm. With these optimizations, a 12.5 Â 13.5 cm 2 module with power conversion efficiencies above 10% over an active area of approximately 152 cm 2 was achieved in the planar n-i-p conguration. 169 Recently, slot-die coating of a novel perovskite precursor ink containing MAI, PbI 2 and 5-ammonium valeric acid (5-AVAI) in GBL (tested with the drop-casting 170 and inkjet printing methods 107,108 ) has also been demonstrated. 106 The fabricated triple mesoscopic printable perovskite solar modules showed an impressive conversion efficiency of 12.87%, which was attained over an active area of 60.08 cm 2 under full sunlight illumination. 106 Similar scalable demonstrations of organic-inorganic lead halide perovskite-based novel inks with the techniques discussed above have also been utilized to scale up PSC technology on exible substrates. Flexible modules in various device designs have been developed and reported both by research labs and commercial players in recent years. 171-174 Therefore, there is promising evidence of successful fabrication of the novel organic-inorganic lead halide-based precursor ink formulations for achieving module production, where several device  designs can be potentially achieved on both rigid and exiblebased substrates according to the targeted applications. Table  3 provides a summary of the recently reported scalable device congurations of perovskite solar modules designed using scalable perovskite precursor ink formulations on rigid glass and exible substrates.

Forthcoming directions in precursor inks, technological research, and development of next-generation PSC technology
The progressive research and development of PSCs over the past decade provide future directions and guidelines to overcome the remaining key challenges necessary for reaching commercial breakthroughs imagined for this efficient PV technology as envisioned in Fig. 5. Among them, the toxicity and environmental impact of traditional lead-based precursor inks pose a challenge for the safe deployment of PSCs. In this regard, not only should the safety protocols but also the effective recycling procedures be chosen for the safe removal of hazardous and toxic materials such as PbI 2 with high yield from the installed panels of PSC technology aer their end of life. Interestingly, notable progress has been reported in many scientic reports where effective recycling of PbI 2 , methylammonium iodide (MAI) and expensive contact layers has been successfully demonstrated for the labsized PSCs. [190][191][192] These proof-of-concept based striking demonstrations provide pathways in designing effective recycling protocols for large area perovskite solar modules, which may signicantly suppress the environmental concerns, which have been heavily debated during the emergence of this low-cost PV technology. [193][194][195] Moreover, the abundance, availability, cost and sustainability of potential lead compound substitutes and their respective precursor inks must be further investigated to justify their bulk-scale usage to produce the socalled large area eco-friendly perovskite-based PV panels. 196 On the other hand, despite the growing number of reports on achieving stable photovoltaic performance under various environmental conditions while incorporating advanced perovskite precursor inks, 103,104,197 realistic assessments of PSC device stability based on recently suggested stability testing protocols 197 have yet to be established. Thus far, there are several reports of either stable performances or degradation of initial efficiencies in various device designs of PSCs containing novel precursor inks, using the suggested stability testing protocols for PSC technology. This again raises concerns due to the uncertainty in deploying this emerging PV technology in many suitable applications.
In addition, new consensus statements need to be developed to address advanced stability testing protocols from the perspective of precisely controlled indoor ecological conditions and some severely cold outdoor climatic conditions, which have not been specically covered in the current stability testing protocols. 8 The possible upcoming deployments of PSC technology in batteryless and maintenance-free IoT devices [198][199][200] inside modern buildings with precisely controlled ecological conditions or stand-alone communication gadgets in severely cold outdoor climatic conditions call for new testing standards. Notably, degradation rates could vary under such environmental conditions, but these are not known or have rarely been reported to date.
Interesting research questions may provide future research and development directives towards the forthcoming generation of PSC devices over the next decade. Considering the progress that has already been made, it is expected that future technological developments would resolve key research problems of achieving eco-friendly and robust PSC technology with certied integration in vast applications.

Summary and conclusions
The development of progressive precursor inks reviewed in this work provides evidence for their contributions towards achieving striking milestones such as high-performance or versatile conguration designs and highlights unique integration possibilities of emerging PSC technology. Moreover, the intelligent precursor inks designed with novel materials reported in the recent past have offered pathways for scalability over both rigid glass and exible polymer foil-based substrates. As a result, activities related to commercializing PSC technology have greatly accelerated as its integration in smart sensors, charging stations and BIPV based applications continues to be demonstrated. 201 Nevertheless, additional materials screening is needed for the development of the next generation of novel precursor inks of the perovskite light absorbing layer in order to meet the growing demand, especially from the construction industry where there is keen interest in aesthetically appealing solar panels for smart buildings and nature-friendly ecological architecture. [202][203][204][205][206][207][208][209] This demand may potentially be met by creating precise and exible patterning that allows individual PSCs to be assembled into aesthetic structures based on solar modules for energy generation.  The current methods such as slot-die coating, spraying or blade coating techniques offer limited exibility, and have mostly been used to produce serially connected pattern-based modules in traditional n-i-p or p-i-n based device congurations. 188,210,211 Notably, alternative schemes such as drop on demand inkjet printing may provide more freedom of design, since advanced precursor inks of both perovskite-based materials and other active materials can be fabricated to create individual PSCs. These PSCs can be designed with aesthetically attractive arbitrary patterns, which may be intelligently connected to create electricity at targeted watt scales.
Surprisingly, except for the few demonstrations of printing individual active layers [212][213][214][215] (including the perovskite-based light absorbing layer 107,216 ), inkjet printing has been rarely reported in fabricating complete device designs of PSC technology, despite knowing its potential for fabricating large areabased active layers at high speed. In this regard, one of the most critical limitations realized in producing active layers through inkjet printing is when the nozzles choke the cartridges, which severely limits drop volumes (1-10 picolitres). This in turn makes it difficult to print nanoparticle-based solution processable inks with the necessary precision and resolution. Moreover, the delicate cartridges frequently exhibit chemical incompatibility and oen react with the fabricated inks that are composed of novel materials and solvents, thus making it difficult to achieve the desired patterns with ease and accurate precision. [217][218][219][220] These key challenges call for the development of advanced precursor inks to be used with high chemical compatibility, not only to achieve precisely patterned perovskite-based light absorbing layers but also for other active layers to produce fully printed PSC technology with pattern exibility. One of the potential precursor inks to achieve a perovskite light absorbing layer with high precision has been demonstrated utilizing an inkjet printing technique to produce carbon-based printable perovskite solar cells. 107 The precursor ink showed great compatibility during inkjet printing when fabricated with a novel additive (5-ammonium valeric acid iodide; 5-AVAI), which signicantly slows the perovskite crystallization formation and allows the fabricated ink to pass through the inkjet nozzles without reacting with them. Thus far, this stable device conguration 103,105 of PSCs has been demonstrated with schemes such as inkjet printing and screen-printing methods. There is immense potential to achieve further module fabrication with customized designs to generate appealing patterns based on individual cells as discussed earlier in this section. 221 Hence, a transformation from the rst generation of serially connected carbon-based printable perovskite solar modules 106,178 to aesthetically appealing solar panels can be anticipated, which could provide guidelines to produce the next generation of traditional n-i-p and p-i-n conguration-based PSCs, especially when considering evolved design principles. This could assuredly enable realistic outcomes such as the rapid deployment of PSCs worldwide and the resulting positive inuence on climate change expected to arise from this promising photovoltaic technology.

Conflicts of interest
The authors have no conicts to declare.