Introduction

With the emergence of the Internet of Things (IoT) and Big Data, indoor perovskite solar cells (IPSCs) earned notable consideration in response to the global gap in implementing self-power-driven devices. This is due to the rapid modernisation and advancement of lightweight electronic devices worldwide[1]. The energy consumption, size, weight, and cost requirements for devices have changed in the IoT era [2]. Several studies have been conducted over time to develop PSCs due to their high PCE, showing promising commercialisation characteristics and vast area utilisation [3]. These off-the-grid applications replace non-rechargeable batteries with power supplements provided by indoor solar cells [4]. Currently, a single junction PSC's best PCE is measured at 25.8% [5], and the highest FPSC stands at 24.7% efficiency [6]. PSCs are a potential new generation of solar cells due to their high PCE, low price, and simplicity of manufacture. Because it can be produced utilising the roll-to-roll (R2R) method, which is effective for high-volume manufacturing, FPSC is considered a significant commercialisation option for PSC. As a result, the development of FPSC is crucial for PSC applications. If used effectively, FPSCs made on light, and thin substrates can fill the gap between the growing market for flexible electronics and uses for solar energy that rigid solar cells cannot. Yang et al. reported FPSC and found that the PCE was over 21% [7]. The superior optoelectronic qualities, low-temperature solution production processes of perovskite thin-film, and their inherent mechanical flexibility make it possible to build FPSCs [8]. FPSCs are anticipated to play a part in smart, integrated buildings [9], unmanned systems [10], self-powered bioelectronics [11], peak DC link voltage in a quasi-Z-source inverter system[12], voltage control [13] and portable electronics [14]. The first FPSC was fabrication by Kumar et al. using a polyethylene terephthalate (PET) flexible substrate and Zinc oxide (ZnO) nanorods as ETLs to obtain a PCE of 2.62% [15]. There have been numerous attempts to improve the efficiency of FPSCs by changing various elements, for instance, the HTL [16], the ETL [17], perovskite films [18], and the annealing temperature [19].

Among these initiatives, important changes in the design of materials and structures were important in optimising FPSC efficiency [20], such as mechanical flexibility[21] and stability [22]. The PCE of FPSCs is greatly decreased by the challenge of fabricating conventional in-organic transport layers (TLs) using a low-temperature method; for instance, an inverted FPSC device was fabricated using poly(3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT: PSS) as an organic HTL and the PCE was increased to 6.4% [23]. Kim et al. produced FPSCs with an efficiency of 12.2% with a thin Titanium oxide (TiOx) nanolayer of 20 nm fabricated using plasma-enhanced atomic layer deposition at 80 °C [24]. The durability bending test can determine the FPSC material's ductility, fracture strength, bend strength, and resistance to fracture. Shin et al. performed this test for the first time on the devices using ETL from Zinc Stannate (Zn2SnO4) nanoparticles to increase the efficiency of an FPSC to 15.3% while maintaining a temperature below 100 °C [25]. Because of its excellent conductivity and transmittance of graphene electrodes, Yoon et al. fabricated a super-flexible solar cell with a 16.8% optimised efficiency. The performance of FPSCs depends on the crystallinity of perovskite films and the calibre of the transport layers [26]. Jiang et al. fabricated large-grain, low-defect density perovskite films using dimethyl sulphide (DS) as an additive [27]. The efficiency was reported to be 18.41%, where n-methyl-2-pyrrolidone (NMP) and methylammonium chloride (MACl) were used as synergistic additives by Zhang et al. to optimise formamidinium lead iodide (FAPbI3) on FPSC, and the PCE was increased to 19.36% [28]. Furthermore, the efficiency of using artemisinin to passivate defects in perovskite films achieved 21.10% [7]. This research will discuss the FPSC structures, fabrication methods, and indoor FPSC. In addition, the prospects for FPSCs in indoor and outdoor applications and the challenges facing and opportunities for FPSC research will be discussed. Additionally, the performance of several FPSC types will be compared. The authors expect this review to shed some light on the potential of FPSCs and a path ahead for the fabrication of optimised FPSCs after addressing the core challenges of bendability, toxicity and stability, and adaptability Fig. 1.

Fig. 1
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The schematic summary of the structure of the review

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Structure

A perovskite absorber layer, an ETL, an HTL, an anode, and a cathode are the essential components of a PSC. PSCs can be categorised as traditional Electron-Absorber-Hole (n-i-p), inverted Hole-Absorber-Electron (p-i-n), and, in some situations, mesoporous structures (Fig. 2c). In the fabrication of n-i-p architecture design, the ETL is deposited first. In contrast, the HTL is first deposited in the p-i-n. In a typical PSC, an intrinsic perovskite absorber is positioned between the ETL and the HTL. The ETL of the perovskite layer's conductive band receives electrons generated in the perovskite absorber, while the HTL of the layer gets holes. In a PSC device, the ETL and HTL transport electrons and holes in the direction of the matching electrodes and stop them from moving in the opposite direction. To create highly effective FPSCs, it is important to carefully evaluate the components of ETL/perovskite/HTL and their interface. The material and interface are very important in the fabrication of the PSC; the perovskite device usually contains the conducting substrate, charge carriers, absorber layer, and electrode, as shown in Fig. 2d.

Fig. 2
figure 2

a Structure of the flexible perovskite solar cell (FPSC), b Band energy diagram of an FPSC, c Different design architecture of the FPSC, d Image of an FPSC. Reprinted with permission from [29] Copyright 2021 Elsevier B.V Ltd

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In the planar structure, the low-temperature process is often used to fabricate FPSCs, both n-i-p and p-i-n. High-temperature processing materials are not compatible with FPSCs due to polymer substrates, but another new flexible substrate, the flexible glass substrates, has recently used in the fabrication of the FPSC, making it easier to fabricate.

Substrate

The FPSCs substrate is important since it affects the device's final photovoltaic performance and mechanical stability [30]. The three (3) different types of FPSC substrates include composite substrates, flexible glass substrates, and polymer substrates [8]. Titanium foil, copper foil, and plastic substrates are a few common flexible substrates [31]. Two examples of polymer substrates are PEN and PET. Some requirements for flexible substrates in FPSCs include the following; To maintain long-term stable performance, be a barrier layer that prevents moisture and oxygen from contaminating the perovskite, have strong optical properties, and light should be able to penetrate through it. They should also have strong conductivity, mechanical flexibility, and thermal stability. They should also possess good mechanical properties that enable them to withstand deformable transformations when subjected to strain and stress, effectively release stress, and withhold their original qualities. This is because they are exposed to various chemicals during fabrication, such as gases and solvents [32]. However, brittleness and inconsistent crystallisation on multiple substrates are two factors that contribute to perovskite degradation. Most requirements for a good flexible substrate are obtainable from flexible glass (FG) substrates with a hundred micrometres thickness. FG has excellent barrier properties, conductivity, chemical stability, and high tolerance to high temperatures (up to 600 °C) [33]. The increased weight, fragility, and expense may raise some questions compared to other options. The metallic substrate offers great thermal stability, charge conductivity, and corrosion resistance, but because it is naturally opaque, optical absorption is compromised, and efficiency decreases [20].

Polyethersulfone (PES), Polyethersulfone (PES), and Polycarbonate (PC) are optically transparent and with higher glass transmission temperature (Tg) when compared to PEN and PET. Table 1 compares the characteristics of various FPSC plastic substrates. While PEN has the highest melting temperature (Tm), its main limitations are inadequate gas barrier characteristics and a lack of process chemical resistance. Despite its recent discovery, colourless polyimide (PI) has excellent thermal properties but lacks optical transparency, which is necessary for a substrate of light-absorbing devices.

Table 1 Properties of the flexible substrate made of polymer. Printed permission [30] with permission from Elsevier
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With PET and PEN as the dominant candidates, Table 2 summarises the various substrates employed in the fabrication of FPSC. Due to its lower annealing temperature of about 100 °C compared to TiO2, which is usually used to fabricate PSC with good efficiency, PCMB is the primary ETL; however, its main challenge of TiO2 is the annealing temperature of 450 °C, where the flexible substrate will melt. Recently, flexible willow flexible glass (FWGS) was used to fabricate FPSC with the same annealing temperature as a glass substrate [20] which is a breakthrough in the fabrication of the FPSC. The substrate (FWGS) was intensively discussed in the next chapter because of its unique advantages in fabricating the FPSC.

Table 2 Summary of the substrate of FPSCs fabricated
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Flexible Willow Glass Substrate (FWGS)

Another recent development in the fabrication of the FPSC is the introduction of the FWGS. It is among the several thin film materials emerging as promising candidates for highly efficient FPSCs. However, there is a tremendous opportunity for improvement regarding the performance, scalability, and reliability of flexible perovskite solar cells. Here we look at FWGS because of its strong, transparent, low thermal expansion coefficient and ability to withstand high temperatures. In their research, devices made on FWGS were put through bending and fatigue tests to see whether they were suitable for use as flexible solar cell substrates. After 500 cycles of bending, the device's performance gradually decreased (by around 15%). The findings support FWGS viability as a flexible substrate for perovskite solar cells. Their excellent barrier properties strengthen their suitability as a substrate and promote in-depth research [41]. Table 3 below summarises the properties of the FWGS, which can be seen as compatible with the fabrication of the FPSC.

Table 3 Properties of the flexible willow glass substrate [42]
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The R2R method was employed to fabricate high-efficiency IPSC on ITO-coated FWGS, surpassing the 1600 bending procedure at 20.5 mm curvature. The optimised cell delivered PCE of 22.6% under 400 lx LED illumination, and the lightweight cell's specific power (Watt) is 55% higher than that of PET film [43]. Figure 3 shows the optical transmission and bending stress of the FWGS. Table 3 is the summary of the FPSC fabricated on the FWGS, putting their efficiency into consideration. As seen in Table 3 above, FWGS is thin enough to be flexible and maintain its outstanding glass properties, about the same thickness as a copy paper. It offers the natural advantages of glass in a mechanically bendable form factor, making device processing more affordable. Its surface is flawless, exceedingly smooth and flat, and almost defect-free because it was formed through fusion. Its unique edge tabs make roll-to-roll processing fabrication possible.

Fig. 3
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a The optical transmission of the flexible willow glass substrate (FWGS), b The bending stress of the FWGS

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Electron transport layer

Good ETL and deposition techniques must be considered to fabricate high-performance and efficient FPSC devices. The ETL must integrate with the deposition procedure (i.e., it should not cause degradation of the absorber layer). Wang et al. modified the [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) ETL by adding 3-aminopropyl tetraethoxysilane (APTS) and replaced the organic buffer layer with a tin oxide that was ALD-deposited to create high-performance inverted FPSCs (SnOx). High-quality PCBM ETLs that become smooth surfaces were made via the steric-hindrance effect Table 4. The enhanced PCBM layer improved the device's photovoltaic performance by assisting with interface charge transfer and perovskite film passivation [35].

Table 4 Summary of the FWGS
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Been shown that TiO2, due to its favourable energy band and relatively high stability, seems to be a more popular ETL in PSC. However, a compact TiO2 or meso-TiO2 layer requires high-temperature treatment (500 °C), which is impossible on flexible plastic substrates. Consequently, several researchers are developing new methods to enhance the fabrication of TiO2 layers at low temperatures (150 °C), such as the sol–gel method [46], radio-frequency sputtering [47], and electro-deposition [48], so that they can be used on polymer substrates. Unfortunately, TiO2 treated at low temperatures cannot ensure great efficiency (still < 17%). Liu et al. used a thin layer of ZnO nanoparticles to enhance ETL efficiency to 10.2% [49]. Deposited SnO2 ETL at 100 oC with PCE of 13.88% [50] and niobium oxide (Nb2O5) [51]. ZnO is a very common ETL in the FPSC structure as a substitute for TiO2, not only because of its compatible energy level but also because of its low-temperature manufacturing and the adaptability of several fabrication methods such as R2R, spin-coating, thermal evaporation and sputtering among others [52]. The atomic layer deposition and low-pressure aided solution technique were used, and the PCE of the FPSCs determined using the methodologies is over 17% [53]. Therefore, SnO2 will play a crucial role in the fabrication of cheap, highly effective FPSCs; with the substantial PCE gap between rigid PSCs and FPSCs, we are now looking at the SnO2-based FPSCs to investigate the charge recombination process between SnO2/perovskite layers fabricate through low-temperature, for enhancement and to reduce energy losses at the SnO2 layer and perovskite layer interface even more Table 5.

Table 5 Summary of the ETL on the FPSCs
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Because electron transport at the ETL/perovskite interface is slower than hole transfer at the perovskite/HTL interface, hysteresis behaviour in efficiency measurement, recombination in the device, and device stability, such as ion migration, are all affected [32]. Thus, to achieve reliable FPSCs with high PCE, ETL material selection and engineering are critical. SnO2 and TiO2 are currently used as n-type materials in high PCE devices, and no ETL material can match their stability and efficiency. Finally, counter-electrode optimisation and interface recombination systematisation could be the next trend of investigation of FPSCs to accelerate the development of ETL- or HTL-free device Table 6.

Table 6 Summary of the HTL on the FPSC
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Absorber layer

MAPbI3 is a commonly organic–inorganic hybrid perovskite material used in the fabrication of FPSC as an absorber layer because of its high efficiency and low cost. The film layer is made from a solution at a temperature of 100 OC and has lower thermal stability than its perovskite absorber counterpart. The porous structure of the light absorber/wide-bandgap cathodic semiconductor material heterostructure must allow light to be absorbed. It should be chemically stable and very hydrophobic and be able to make good contact with the charge collector material (e.g., Pt, Au, C) [60]. The absorber’s bandgap function is linked to solar cell efficiency [61] example, a PCE of 24.2% was achieved with a bandgap of ~ 1.5 eV [62]. Because of its remarkable semiconducting characteristics and inexpensive cost, it has received interest as an appropriate material for absorber layers in PSCs [63]. MAPbI3 is a good absorber layer because it has a bandgap of 1.50 eV, with a wide absorption spectrum throughout the UV–visible region up to approximately 825 nm, a low trap state density of 1010 /cm3, coupled with a low exciton binding energy of 5–10 meV, long electron and hole diffusion lengths of up to 1 m, a high absorption coefficient (1.5 × 104 /cm) at 550 nm, a tunable bandgap, and increased mobility up to 66 cm2/V/s [18].

In 2019, the quintuple-cation composition Rb5xKxCs0.05FA0.83MA0.17PbIxBr3x set a new efficiency record of 19.11% for FPSCs, with a good bendability that retained 93% of its initial PCE after about 2000 bending cycles and the thermal stability was good its maintained 68% PCE at 85 °C in a nitrogen environment after 1000 h [57]. In addition, the current record of Jsc of FPSC is 21 mA/cm2 compared with rigid PSCs, which is 25 mA/cm2, noticeable the major problem of the FPSC is the current Jsc loss, which can lead to the main efficiency loss of FPSCs. Hence, the next stage is to close the efficiency gap between FPSCs and rigid PSCs by focusing on electrons/hole charge transport materials and interlayers, bringing each parameter closer to its theoretical limit Table 7.

Table 7 Summary of the Electrode on the FPSCs
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Hole transport layer

The best HTL material for PSC must have compatible energy levels that match the perovskite layer, intrinsically high hole mobility, photochemical and thermal, and long-term air stabilities. To prepare HTL, it should be solution-processed, especially when utilised in traditional (n-i-p) PSCs. Regarding commercial applicability and scalable manufacturing HTL, low cost and ease of fabrication should be regarded [64]. Some important characteristics must be considered while selecting HTL materials for PSCs, such as preventing electrons from traveling in the wrong direction and transferring holes [15]. HTLs should have a valence band maximum (VBM) to improve efficiency. They should also be well-matched with the perovskite and cathode and have a high hole-injection efficiency [65] HTL material in FPSCs is responsible for transporting holes from the perovskite layer to the cathodes.

Unlike rigid PSCs, flexible substrates have a high-water vapour transmission rate; as a result, HTLs should have more low permeability toward oxidising element stability to prevent corrosion or degradation by oxygen, vapour, metal electrodes, etc. [66]. Other options include using nano-barriers to protect against moisture and isolating the perovskite absorber layer and electrode to reduce recombination at the interface [28]. To improve the transport layer of PSC, because of challenges such as instability and high cost, Syafiq et al. investigated copper zinc tin sulfide (CuZnSnS4) as an HTL for its ease of synthesis, band tuning, abundant, non-toxic and good stability, the study shows an excellent opportunity to improve the performance of PCE by using CuZnSnS4 as HTL [67]. In 2012, organic HTLs were used for the first time in all-solid-state PSCs, and using spiro-OMeTAD, since then, it remained the best HTL candidate for high-performing PSCs but has better performance well when doped with certain additives, such as 4-tert-butyl pyridine (tBP) and bis (trifluoromethane) sulfonimide lithium salt (Li-TFSI) [68]. Dopant -free HTLs like P3HT poly(3-hexylthiophene2,5-diyl (P3HT) and PEDOT: PSS [poly (3,4-ethylene dioxythiophene): polystyrene sulfonate] have also been developed [69]. Zhang et al. studied dopant-free P1-based PSCs they were able to achieve an efficiency of 18.30% [28].

It has been demonstrated that the high-conductivity, high-adhesion PEDOT: GO film significantly improves FPSCs crystallinity and charge transfer. Its interface layer also significantly reduced interlayer separation and grain fragmentation, increasing mechanical stability. The PSC devices (1.01 cm2) based on the substrate mentioned above achieved over 85% retention after 5000 bending cycles, which is crucial for the practical deployment of FPSCs in portable and wearable electronics. Nevertheless, printing large-area homogeneous HTLs on plastic substrates to fabricate FPSCs with high PCE and outstanding stability remains a major issue.

Electrode

Transparent conducting oxides (TCOs), such as ITO, coat flexible substrates in FPSC device fabrications due to their high visual transmittance and low sheet resistance. Therefore, providing the best photovoltaic (PV) performance and mechanical stability for flexible electronics is essential. Transparent electrodes should resist repeated bending, twisting, stretching, folding, and reversible plasticising. ITO-free flexible devices have received much research to minimise performance deterioration caused by fragile ITOs. Investigations were carried out on several electrode materials, including gold (Au), as alternatives to traditional ITO for highly efficient FPSCs [56], silk fibroin [73], silver nanowires (AgNWs) [74] metal mesh[75], InZnO (IZO) [76], Al-doped ZnO (AZO) [77], carbon nanotubes (CNTs) [78], and graphene [79].

Gold is one of the best materials for an electrode in high-performance PSCs electrode materials because it retains high efficiencies, is firm, and has good stability. Nevertheless, it is expensive and accounts for about 71% of the cost of a single PSC material [81]. Materials made up of carbon, such as CNTs, GO, and graphene, have attracted much attention because they are mechanically robust, inexpensive, thermally stable, and highly conductive; these are important in the fabrication of FPSCs. A PCE of 11.9% was attained in fabricating all carbon-electrode-based FPSCs. In addition, 86% of the efficiency was retained when subjected to 2000 bending cycles at a radius of 4 mm [82]. Wang et al. used a transparent electrode 1D-2D hybrid Silver nanowire graphene (Ag-NWs-G) instead of a typical ITO to build an ultra-low-cost HTL-free carbon paste-based perovskite solar cell (C-PSCs) with the architecture of (Ag-NWs-G)/SnO2/perovskite/carbon. All-carbon electrode-based FPSCs' promising flexibility, efficiency, and long-term stability have recently proven the good compatibility of carbon materials with FPSCs and the promise of using other electrode materials to create very durable, massive, and efficient FPSCs [83].

Indoor Perovskite Solar Cell (IPSC)

Based on power-conversion efficiency, a study released in early 2015 by Kawata et al. demonstrates that perovskite solar cells perform better in illumination than single-sun illumination. Although the PCE under solar irradiation was only 12.4%, PSCs yielding 64.9 mW/cm2 or 19.8% PCE at 1000 lx were still attained[85]. Perovskite IPSC cell efficiencies have surpassed 33.9% at 200 lx in just a few years[86]. Some important breakthroughs are the efficiency of 35.9% under 1000 lx illumination [87], 33.4% at 400 lx [88], and the highest stand at 40.2% at 1000 lx [89]. The typical indoor light intensities for the living room, workplace workstation, and supermarket can be compared to these illuminance values of 200, 400, and 1000 lx. A significant finding is that PSCs have outperformed their other emerging PV counterparts, organiOPV (30%) and dye-sensitized solar cells (DSSC) (34%), as measured in 1000 and 1200 lx, respectively [90, 91].

The device structure and, most importantly, the absorber layer plays are crucial to having a high-efficiency solar cell; here, we discussed the device structures and the perovskite absorber layer used in the fabrication of indoor solar cells, with special consideration on the low light harvesting of the PSC, also emphasizing emphasis on power for the indoor energy harvesting and efficiency obtained. In their study of the effect of the crystal structure and the way to achieve high open circuit voltage under a low-intensity light source of 1000 lx (0.371 mWm−2), absorber layer energy band gap (Eg) of 1.59 eV as shown in Table 8 below and the absorbance wavelength ranges from 450–800 nm [92] Dagar et al., able to fabricate highly efficient PCS using SnO2/MgO ETL with the illumination of 400 lx (in Table 8) and realised the PCE of 26.9% using the spin coating technique. They deposited all the layers at low temperatures except for the electrode, where the thermal evaporation method was employed [93].

Table 8 Summary of the IPSC architecture
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In another analysis and utilisation of low light harvesting, an indoor p-i-n indoor PSC using NiO as the HTL, a light intensity of 2.2 mW/cm2 was used to obtain the PCE of 23%. Another important feature of the NiO on the device is that it acts as a barrier to moisture; hence it enhances the stability of the fabricated device [95]. In their work, they showed an inverted PSC that works efficiently using low light with the use of compact NiO (c-NiO) and nanoparticle nickel oxide (np-NiO) as the HTL, achieving a PCE of 30.12% and 23.9%, respectively, using low light at 1000 lx, also noted that c-NiO has higher PCE due to lower interface recombination at the c-NiO/perovskite junction, having demonstrated that both the two configurations can scale up to 1 cm2 [96] For the stability and efficiency of an IPSC, it's important to consider the interface between the absorber layer and the charge transport layer. Therefore, in the study led by Jie Xu, they inserted ultrathin alkali fluoride (AF) between the perovskite layer and charge transport layers, thus helping reduce interface recombination. As a result, a PCE of 35.7% was achieved under a low light intensity of 1000 lx [87].

This achievement is one of the best records in IPSC, hence offering an updated solution to challenges confronting the fabrication of PSCs. In their effort to fast-track the commercialisation of PSC, they developed an annealing-free rigid and flexible perovskite for indoor and outdoor use through a 3,4-Ethylenedioxythiophene (EDOT)-assisted anti-solvent strategy. The fabricated FPSC delivered a very good indoor PCE of 22.79% at 1000 lx. The developed anti-solvent engineering offers a simple and efficient method for producing indoor FPSC with good performance and low cost (annealing-free) [97]. The solvent engineering was deployed in this research to synthesise high-quality perovskite thin film layers using non-toxic butyl acetate (BA) instead of chlorobenzene, which resulted in an enlarged grain and improved carrier mobility, with an enhanced PCE of 25.74% at 1000 lx [98] The IPSC has so many advantages compared with the outdoors. These include good stability because it is installed indoors and many negative environmental factors. In the case of Kim et al., the fabricated indoor FPSC retained 97.4% of its efficiency after 1000 h [8]. It also has better PCE compared to outdoors; in the case of; at 400 lx, it has a PCE of 23.33%, while outdoors, it is 19.78% [98], 26.9% indoor, and 19.0% outdoor [95], 23.0% indoors, 13.3% outdoors 30.12% indoor, 16.4% outdoor [34] and 35.90% indoor, 22.02% outdoor [87]. So far, the highest reported indoor. It was observed from the review that the good efficiency of the IPSC was usually obtained when the indoor light is at 1000 lx, as shown in Table 8. In addition, the table summarises the design architecture of the IPSC.

Techniques

This section examines FPSCs based on various functional layers created using various fabrication processes. In general, FPSC fabrication techniques can be classified as large-scale and lab-scale.

Spin coating

Spin-coating is a typical method for depositing thin films. The advantages of spin coating are that it is a simple and effective process, is ideal for small and flat substrates, dries quickly, is cost-efficient, and the layer thickness can be carefully regulated. This technique is commonly used in the lab because it can build a thin homogenous coating across a small substrate. It is a popular approach for fabricating small-scale PSC on a laboratory scale with a homogenous thin film and simple film thickness control [99]. However, other strategies must be used to convert the fabrication process to mass production of large-area FPSC and PSC modules [100]. Many studies have been undertaken on printing techniques for PSC fabrication that enable high-volume and large-area PSC production, such as slot die coating [101], blade coating [102] and spray coating [103]. Even though these techniques have demonstrated their inherent benefits for high throughput and low-cost mass manufacturing of high cell efficiency devices equivalent to spin-coating manufactured devices, there are still certain challenges, such as vacuum warping, incomplete coating and pinholes and comet streak [104].

The most significant drawback is that the spin coating method is not ideal for batch or large-scale production. Most perovskites require a high post-heating temperature above 150 OC; hence, they are unsuitable for most plastic substrates [106]. Fortunately, perovskites like Cs0.05(MAFA)Pb (IBr)3 require a lower post-treatment temperature at 100 °C [107], which enables them to be fabricated and thermally treated on temperature-sensitive plastic substrate Spin-coating techniques have been used to achieve the high record efficiency FPSCs so far [99]. However, some known issues with the two-step technique, such as the partial conversion of lead iodide (PbI2) into perovskite, severely limit its use on a wide scale for high-efficiency, repeatable PSCs. Even though this procedure is free from anti-solvent, the edge effect may arise, implying that the greater the area, the more severe the influence on the PSC.

Slot die technique

Another technique for depositing layers in the PSC device is slot die coating. As a pre-metered coating technique, it is extremely effective regarding materials required and minimal ink waste levels compared to other deposition processes. For a normal slot-die coating process, a coating head is positioned close to and across a substrate or web, and ink is pumped into the head using a syringe pump. The ink is then driven out through a narrow slit along the length of the coating head. This technology was originally used alongside spin-coating to fabricate FPSCs. Sears et al. utilised the slot-die technique to deposit HTL while the perovskite layer with spin-coating; they achieved a PCE of 11% [108].

The film thickness of the substrate, ink and blade interaction is affected by the parameters, such as the substrate's surface energy, the ink's surface tension, and the ink's viscosity. The thickness of the dry film can be determined using the following equation for a given substrate. (Krebs, 2009).

$$d= \frac{f}{Sw} x \frac{c}{\rho }$$
(1)

From Eq. (1) above, thickness (d) is in cm, flow rate ( f) in cm3 min‒1, web speed (S) in cm min‒1, coated width (w) in cm, solid content in ink (c) in g cm‒3 and ρ is the density in g cm‒3.

A perovskite layer was created using the slot-die approach in an ambient environment, where PCE of 3.6% was attained using PET/ITO/ZnO/MAPbI3/P3HT/Au configuration by an active area of 1 cm2 [111]. Therefore, the slot-die coating can be used to create all additional layers except for the Au electrode, which requires a thermal evaporation method. A fully printable FPSC was fabricated using the slot die technique and the electrode by thermally evaporating carbon (C) instead of the Au electrode [65]. High homogeneity, low material waste, scalable manufacturing, customisable film thickness, and high-speed coating are all advantages of slot die coating[111]. Nonetheless, some of its significant drawbacks include a complicated and costly fabrication process, as well as numerous sources of flaws that make diagnosing problems harder. When the manufacturing process has matured, this approach is an excellent choice for the industrial production of FPSC.

Thermal Evaporation technique

Thermal evaporation provides a one-of-a-kind opportunity to deposit multilayers of perovskite materials [112]. This process is performed by heating perovskite precursor into vapour phases in a vacuum chamber, and the technique usually produces a smooth and homogeneous perovskite layer. Furthermore, because the FPSC fabrication and the deposition of the absorber layers need low annealing temperatures, thermal evaporation is particularly well suited to FPSC and flexible optoelectronics applications requiring low processing temperatures; another unique advantage is that toxic solvents are not used in this technique. Hence, it allows perovskite films to be deposited without endangering the underlying layers of tandem devices [113]. This technique enables a high rate of film deposition, minimal damage to the substrate, lower processing temperature and high film purity, the major setback is that film deposition and the substrate temperature is difficult to regulate. According to a study group led by Liang et al. [114], a PSC demonstrated a steady efficiency of 7.73% with nearly no hysteresis, indicating single-source thermal evaporation that is acceptable for a large area of PSC. In addition, Li et al. used single-source thermal evaporation to fabricate the perovskite absorber layer on a flexible PEN/ITO substrate, achieving an efficiency of 13.8% [115]. Thermal evaporation was used to deposit a smooth PbI2 layer, and then MAI was spin-coated to have MAPbI3. As previously stated, thermal evaporation can produce thin films with tiny grains and is not harmful to the FPSC's thin film layers Fig. 4.

Fig. 4
figure 4

The pictures are from our lab set-up a A thermal evaporator for fabricating electrodes, b A schematic diagram of a thermal evaporator FPSC

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A few studies have been published on the heat evaporation technique for perovskite deposition. Regardless, devices made using this technology have shown the ability to achieve exceptional results, with reported PCEs exceeding 20%. While there are still numerous obstacles to overcome, when it comes to commercialisation, the thermal evaporation approach has a distinct edge over solution processing. Nevertheless, the manufacturing procedures are more challenging to regulate the temperature of substrates during the evaporation process than solution-processing approaches. As a result, becoming a general method for commercialising FPSCs is currently difficult [65]. On the other hand, solution processing is a serious challenge with what could be a leading method in the FPSCs fabrication technique. It should be channelled toward collaboration of the thermal evaporation and solution methods, combining the benefits of the two methods. Perovskite solar cells are coming up, and their scalability is now achieved in major labs across the globe Fig. 5.

Fig. 5
figure 5

a The power conversion efficiency of halide perovskite indoor photovoltaic, d maximum theoretical efficiency of IPSCs as a function of bandgap [94]

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Spray coating

Spray coating is popularly used for the noncontact and rapid deposition of perovskite films, resulting in a superior option for large-scale production technology. Small ink droplets are formed from a solution and then sprayed onto a surface using an inert carrier gas. Reported the first use of this method fabricating FPSCs using PET substrate, with an efficiency of 8.1% subjected to a strong bending test [103]. Spray coating was used to fabricate FPSCs with an efficiency of 11%, with the device structure of PET/ITO/TiO2/MAPbIxCl3x/Spiro-OMeTAD/Au [116] TiO2 coated on PET substrate on the device MAPbI3/Spiro-OMeTAD/Ag and was able to achieve an efficiency of 8.5%; they used a one-step spray coating of ETL on various substrates for PSC, demonstrating outstanding performance, well crystallised, and exhibiting mass production potential [117].

In their study, different substrates were ultrasonically sprayed with a solution mixture of MAI and PbCl2 precursors, which was subsequently annealed for one hour in the air at 100 °C to promote the chemical reaction and crystallisation. It was established that the procedure might produce large area FPSCs when done in ambient air at a low temperature of 150 °C. [116]. Spray-coating was used to deposit ETL/HTL and perovskite layers, resulting in pinhole-free, smooth, and uniform perovskite, HTL, and ETL layers with a PCE of 15% [118]. Taheri et al. recently published a paper that combined the optimisation of laser scribing and automated spray-coating of SnO2 thin film layers. The method was used to deposit uniform SnO2 film on a plastic substrate area of 120 cm2, achieving a PCE of 14.3% and demonstrating an efficient approach for the area fabrication of FPSCs [31]. More importantly, because of the flexible substrate, each layer's annealing temperature was less than 150 °C. Spray coating can be considered an effective strategy for producing FPSCs with minimal waste of precursor inks Fig. 6.

Fig. 6
figure 6

Schematic representation of the multistep spin coating process used to create perovskite films [105]

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Blade coating

The blade-coating process is commonly utilised in PSCs, DSSCs, and organic solar cells (OSCs) [119]. The hot plate, blade, and control system are frequently used tools in blade coating [120]. The blade is utilised for thin film deposition, while the hot plate is used for solvent evaporation. The control system regulates the distance between the substrate and the blade, the speed of the blade, and the temperature. To deposit perovskite films, the blade moves from edge to edge of the substrate at the required speed. The annealing temperature for this process is normally between 80 and 160 °C. First reported in 2015, Deng et al., applied this approach to construct PSCs, with a PCE of 15.1% [121]. The major problem of deposition of the perovskite layer is the surface morphology control; according to Dai et al., a technique to overcome this challenge is to improve the process by using hot air or nitrogen-assisted, two-step blade-coating technology and a meniscus blade [20]. However, in their research, Guo et al. fabricate a perovskite with silver nanowires acting as the top electrodes using the blade coating technique: In semi-transparent PSS serves as the HTL [122].

Various procedures have been used to improve blade-coated PSCs' stability and efficiencies, including techniques such as solvent, doping engineering and additive, and interface. The grain size of perovskite prepared by this method is usually bigger than that of perovskite prepared by spin-coating. Blade coating has several advantages, including temperature control during deposition, remarkable compatibility with various substrates, and reduced contamination. More importantly, it can be done at a very low temperature, making it ideal for producing FPSCs. This technique may be a future strategy for commercialising large-area FPSCs in this context Fig. 7.

Fig. 7 
figure 7

a The perovskite film deposition using a slot die setup; b Deposition of pinhole-free PbI2 layer by slot-die coating with gas-quenching technique; c SEM picture of PbI2 film/Perovskite film slot-die coated; d PbI2 films coated in a slot die under varied coating conditions [109]

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Roll-to-roll coating

A flexible substrate, such as flexible plastic or metal foil, is used in the roll-to-roll (R2R) coating technique for FPSCs deposition layers. An unwinder roll feeds the flexible substrate mounted on a roll during the R2R operation, while a rewinder roll collects the flexible substrate on another roll. One or more printing or coating machines and extra process equipment like UV-curing, heating, and drying can be installed between the unwinder and rewinder rolls Fig. 8. The substrate enters at one end of the R2R system, and the finished deposited substrate emerges at the other. In this method of manufacturing FPSCs, the output is increased while damage is reduced. The R2R method has been successfully applied to the manufacturing of solar cells, especially organic solar cells [123], DSSCs [124] and solid-state DSSCs [125].

Fig. 8
figure 8

Roll-to-Roll (R2R)-coating process, a perovskite Layer coated on a flexible substrate. Reprinted with permission from [30]

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As a result, the R2R technique is used to make the bulk of reported FPSC devices employing a PET or PEN substrate. The ETL, absorber layer, and HTL can all be deposited using R2R coating methods [126]. To develop high-quality thin films for highly efficient FPSCs, aspects like deposition speed, solvent selection, flow, film thickness, and annealing temperature must be optimised during the R2R process [127]. The metal deposition process via R2R vacuum techniques and encapsulating operation for increased stability can be used to fabricate the electrode. OSCs, DSSCs, and other thin-film solar cells can benefit from the vast amount of work done in printable electrodes, encapsulation, and charge-carrying layer processing [128].

Gu et al. employed R2R and slot die techniques to fabricate PSCs with an area of 0.2 cm2 – 0.5 cm2; the inverted device has a structure of PET/ITO/ZnO/PCBM/perovskite/P3HT/PEDOT: PSS/Ag yielded a PCE of 4.9%. [129]. Hu et al. used R2R, micro-gravure printing and doctor blading to create a highly aligned and ultra-long CH3NH3PbI3 nanowire array of thin films with nanowire lengths of 15 mm with a humidity of 45% and temperature of 28 °C [130]. They demonstrated moderate performance in nanowire photodetectors placed on SiO and PET substrates. R2R processing, according to this study, has a lot of potential for use with FPSCs.

Spray pyrolysis

The spray pyrolysis method is capable of fabricating PSC [131]. The method consists of two interlinked systems, atomisation and deposition, connected through a quartz nozzle that introduces the perovskite. In this process, perovskite ink is converted into droplets by the atomizer. a low-pressure gas stream directs these droplets onto the substrate. This process produces perovskite film deposition as the solvent evaporates [132]. It has been employed to fabricate perovskite andETL [133]. Barrow et al. pioneered its use in 2014, achieving an 11% power conversion efficiency (PCE) in a single-step deposition of a planar perovskite solar cell [103]. The study, utilizing Spray Pyrolysis-produced TiO2, investigates the optical and electrical impacts on PSCs through a 3D finite-difference time-domain (FDTD), resulting in an optimized single-junction PSC design with over 30% efficiency compared to planar reference PSCs [131]. The control of the perovskite layer's quality is achievable by analysing the attributes of liquid precursor droplets during the spray coating process. Surface tension emerges as a critical factor influencing film quality, as elevated surface tension results in inadequate wettability. This is attributed to the accumulation of the perovskite precursor solution in a spherical cap formation over the substrate.

Sputtering

The sputtering method has emerged as a promising technique for fabricating FPSC, showcasing both advantages and drawbacks. One significant advantage of sputtering is its ability to deposit thin films with excellent uniformity, promoting precise control over the perovskite layer'sthickness [134]. This uniformity contributes to enhanced device performance and stability. Additionally, sputtering allows for better scalability in production, making it a viable choice for large-scale manufacturing of FPSCs. Fully inorganic perovskites, with a focus on CsPbBr3 due to its favourable band gap and stability, hold great promise for solar cells and LEDs, with ultrathin films successfully produced through magnetron sputtering, exhibiting highly textured submicrometric crystal structures that vary in texture with deposit thickness[135]. A systematic investigation was conducted to optimize sputtering parameters, resulting in highly efficient perovskite solar cells with a power conversion efficiency of 18.35% and excellent electrical characteristics [136].

On the downside, the sputtering method requires specialized equipment and controlled environments, which can add to the overall fabrication costs. The process also involves high temperatures, posing challenges for certain substrate materials and potentially limiting the choice of flexible substrates [137]. The complexity of sputtering setups poses challenges for smaller laboratories or facilities with limited resources; however, ongoing research and technological advancements are addressing these limitations, with the potential for fine-tuning deposition parameters to enhance sputtering as a competitive method for FPSC fabrication, offering a promising avenue for achieving high-performance FPSCs if associated challenges are effectively managed.

Inject printing

Inkjet printing technology, derived from newspaper and graphics printing, represents a digitalized version of traditional printing. Controlled by computers, it allows for printing desired patterns without needing a specific material or substrate shape, making it a non-contact fabrication process[138, 139]. This method surpasses other deposition techniques due to its capability for on-demand patterning without needing masks. It is cost-effective, utilizes materials efficiently, can be scaled up, enables high-resolution printing, and allows for freedom in design [140]. In 2014, Wei et al. pioneered inkjet printing to fabricate the first planar PSC with a nanocarbon HTL, achieving precise control over interfaces and patterns. Their comparative analysis revealed that preparing a carbon and MAI mixed precursor, then printing it on the PbI2 layer and thermal treat for an hour, resulted in superior performance, transforming the PbI2 layer into the desired MAPbI3 layer [141]. Hashmi et al. investigated the durability of inkjet printing on PSC in ambient conditions exposed to intense UV radiation. They achieved remarkable stability, and the PSCs' good performance under 1.5 Sun light for 1002 h outdoors was made possible using epoxy glue sealing [142]. Hence, this technology is well-suited for FPSC, capable of generating stable and long-lasting PSCs while demonstrating favourable production scalability.

Table 9, a detailed comparison of several FPSC fabrication techniques, reveals distinct advantages, disadvantages and applicability. Certain techniques, such as Roll-to-Roll and Slot Die, are precise and scalable, whereas Thermal Evaporation and Sputtering have special advantages for particular uses. By carefully weighing these methods, researchers can customize their decisions for the best FPSC manufacturing depending on cost, scalability, and substrate compatibility.

Table 9 Comparison of Techniques for Fabricating FPSC
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Applications of FPSCs

Indoor application

FIPSCs can be integrated into IoT systems, mobile phones, watches, buildings, backpacks, electric cars, sensors and portable power supplies. Indium tin oxide (ITO) film is widely used in the PV industry. However, ITO films' brittleness, high processing temperature, and high manufacturing cost limit their use in wear-resistant, durable, and flexible IPVs. As a result, the development of flexible substrates or electrode materials with high conductivity, transmittance, durability, and lightweight significantly impacts the industrialisation of IPSC. Furthermore, because IPSCs typically have curved shapes or flexible characteristics, the mechanical flexibility of electrodes must be considered. Because of the reduced light intensity of the indoor light source, the substrate or transparent conducting electrode (TCE) must be highly transparent and smooth, allowing the maximum amount of photons to flow through while minimising the recombination centre to prevent current leakage. Flexible electronics, such as graphene, conductive polymers, and silver nanowires, have been widely used in IPSCs in recentyears [144]. Until now, most flexible PVs have been fabricated on this substrate. Lucarelli et al. reported in 2016 that flexible PIPVs based on CH3NH3PbI3xClx had efficiencies of 10.8% and 12.1%, respectively, under 200 lx and 400 lx lighting conditions [145]. In addition, Park et al. demonstrated that during spin coating, butyl acetate (BA) substitutes the CB-treated perovskite films, which improves grain nucleation and growth. Under 400 lx, the best device has a great maximum power density of 0.063 mWcm2 and an efficiency of 23.33%. Castro-Hermosa et al. developed flexible perovskite modules and cells using a mixture of SnO2 and mesoporous-TiO2.

Indoor performance was improved in devices with a mesoporous TiO2 scaffold layer above SnO2. Under 200 and 400 lx illuminations, the cells had maximum power densities of 9.77 and 19.2 mWcm2, respectively [43]. In addition, Brown et al. reported FPSCs based on ultra-thin FG substrates covered with ITO roll-to-roll in 2021. Under 200 and 400 lx LED illumination, the improved devices based on FG integrated with a mesoporous scaffold over SnO2 compact layers achieved a record efficiency of 20.6% (16.7 mWcm2) and 22.6% (35.0 mWcm2), respectively [43]. Furthermore, due to their superior stretchability and flexibility, some publications on ITO-free flexible PVs may encourage the development of indoor FPSCs [146]. Furthermore, given PSCs' exceptional dim-light performance, there is huge potential for self-powered woven e-textiles to be used indoors [43]. Based on the research being done on flexible IPSCs, further research and studies on adaptable indoor FPSCs are expected to be published soon. In terms of ETL's future, much progress has been made recently in the study of PSC. This achievement hinges on the capacity to analyse perovskite materials and electron transport layers (ETL). We have presented an overview of electron transport materials and some of their parameters in this portion of the review paper; morphology control, perovskite structure, and ETL material selection have all been identified as critical aspects that directly impact the performance of the FPSC. Hence, the relationship between device structures and needed ETL and perovskite features was examined, which is critical for achieving the desired efficiency of perovskite solar cells. To achieve good spectral matching, the absorber layer material of IPSCs should have a medium bandgap and a narrow absorption spectrum in the visible light range, considering the diverse emission spectra of the light source [147]. The fabrication methods impact the performance of perovskite cells and the crystal development and shape of perovskite. Furthermore, by selecting an effective ETL, the charges from the perovskite effectively separate, affecting the solar cell's power conversion efficiency. This result demonstrates the relevance of morphological control and material selection in perovskite solar cells. The mesostructured device is currently used in one of the finest cells.

IPSCs have shown improved power conversion efficiency and powerful technology in the IoTs system as a device to convert indoor light to electricity [148]. For Example, DSSCs [91], chalcogenide (i.e., GaSe2 and CuInGaSe2) solar cells [149], OPVs [150] PSCs [151] Because of their advantages for easy manufacturing, good performance, and tunable bandgap, PSC have gotten a lot of interest in the IPSC study. The primary advantages of IPSCs are acceptable absorption spectra, minimal trap-mediated charge recombination and leakage currents, large open-circuit with low energy loss, and superior stability [1]. There have been inconsistencies between the IoT system's compactness and the portable power source's complexity, weight, and unreliability. The inconvenient power supply causes problems with regular use and reduces the life of mobile gadgets [152]. As a result, the IoT system requires the creation of low-cost, lightweight, simple, and self-powered gadgets. Due to the Sun's emission spectrum and the indoor light source, PV is currently incompatible with indoor applications. Indoor lights such as fluorescent lights (FL) and light-emitting diodes (LED) emit spectra in the range of 200 nm –700 nm with low intensity, while the solar AM1.5 spectrum emits spectra in the range of 300 nm–1100 nm [153].

As a result, selecting a light-absorbing material with an appropriate bandgap to absorb the interior light source successfully is critical. According to the theoretical simulations, absorber materials with a bandgap of 1.8 –1.95 eV are optimal for high-performance IPSCs. That is why high-performance solar cells, such as crystalline silicon solar cells with a bandgap of 1.1 eV to 1.6 eV, are unsuitable for indoor applications [154]. Furthermore, due to the low indoor light source intensity that may only create a few carriers, IPSCs have higher requirements for reducing flaws and interface contact. Wang et al. found that doping chlorine (Cl) into perovskite reduced the bulk defects of the material, which suppressed non-radiative recombination and ion migration and increased the power density to 3.525 mWcm2 from 231.78 mWcm2 under 400 lx [155]. The role of interface flaws on indoor flexible PSCs is investigated in another paper using the TiO2 compact and the mesoporous layer; interface recommendations are critical for device indoor performance [95]. To fabricate IPSCs, researchers have tested various PV materials to absorb indoor light sources. DSSCs, for example, have been observed to function effectively in low-light environments. Nonetheless, the expensive cost of this type of equipment prevents its commercialisation from progressing further [156]. On the other hand, PSCs have the advantages of high PCE, cheap cost, and lightweight, making them ideal for use in IPSCs. The perovskite bandgap can easily be optimised without expensive preparatory expenses or difficult synthesis processes [157]. Furthermore, the bandgap of perovskite materials can easily change from 1.18 to 2.6 eV by a simple solution-processed component engineering, and its excellent light absorption ability and long carrier diffusion distance are also more suitable for low light conditions [158].

Outdoor application

Wearable electronic devices in the future are predicted to be self-powered by conformable energy storage systems that can give energy output as needed. Highly efficient FPSCs have been widely researched recently to realise lightweight energy supply systems for novel wearable electronics, such as solar backpacks, electric sunglasses with solar cells for lenses, solar-powered clothing, solar-powered tent, solar-powered boats, electronic skin, and textiles. Apart from wearable electronics have been integrated into vehicles. In addition, fibre-based FPSCs, as an example, can be woven into e-textiles such as clothing, headgear, and bags to create large-scale wearable electronics. [53].

The world's first solar-powered aircraft, known as Sunrise (I), was built in 1974 with solar-power subsystems consisting of six (6) spherical sun cells weighing 3.39 kg [159]. For the first time, the successful flight of Sunrise vehicles above regular airways demonstrated the viability of long-distance solar-powered flight at high altitudes. However, traditional power generation systems such as lithium-ion batteries have progressed significantly in recent years, and power densities at a small scale still need to be increased to meet the needs of microscale aerial vehicles that must integrate power supply and onboard electronics within a limited payload capacity. In this regard, FPSCs with excellent power efficiency and good stability, as a promising alternative energy source, will have a huge potential for applications of this sort. As previously said, FPSCs that produce high power-per-weight are typically lab-scale unit cells based on lightweight substrates, active layers, ETL, and HTL materials, according to the literature[160]. The FPSC presented by Kaltenbrunner et al. is regarded as a milestone in high-power-per-weight flexible electronics because they reported an ultrathin (3 μm) FPSC used to power aviation models. It had a 12% efficiency and a power per weight of 23 Wg−1 [161] thickness, including the PU capping, was only 3 m. Aside from the obvious benefit of high power-per-weight, FPSCs can be manufactured in situ or carried in rolls after fabrication. FPSCs are more appealing for outer space and other lightweight applications because of their flexibility in preparation, lightweight, and ease of transportation. While it is critical to understand how resilient they would be in severe otherworldly settings (i.e., high-energy incident radiation) before considering them for practical space applications [162], and extreme temperature cycling [163]. Several groups have already begun studying the impacts of high-energy space particle irradiation on FPSCs, with promising results in terms of the active layer, ETL, and HTL stabilities and performance attenuation. Daniel et al. investigated the net effect of high-energy electron beam irradiation on perovskite films. [162]. The performance stability of quartz substrate-based PSCs exposed to 1 MeV electron irradiation was investigated. Despite the small charge carrier diffusion length drop, the flexible device showed virtually little performance deterioration following e-beam radiation. Miyasaka et al. carried out systematic experiments [164]. A high-energy electron beam and proton irradiations also confirmed the very high tolerance of FAMAPb(IBr)3 and MAPbI3-based PSCs to outer space particle irradiation. They ascribed perovskite's radiation tolerance to the thin absorber layer (500 nm), long carrier diffusion length that exceeds absorber thickness, and free carrier defects tolerance, all of which can reduce the impact of radiation-induced defects on photovoltaic performance.

Prospective and challenges

FPSCs have several advantages, including cheap cost, efficiency, and big easy-to-fabricate areas, and are likely to become the photovoltaic industry's cornerstone in the future. It's worth noting that perovskite uses its superior features by consistently displaying higher PCE under the same bandgap settings. Furthermore, the benefits of the simple big area, solution method, and flexibility are critical in propelling FPSCs to commercialization.

To create highly efficient FPSCs, a high-quality perovskite film with a reasonable bandgap, superb shape, highly oriented crystalline structure, and full coverage is essential. Currently, additive engineering, interface/grain-boundary passivation, and compositional engineering of perovskite approaches have all been used to improve the characteristics of perovskite films [56]. Despite significant improvements in FPSC efficiency and adaptability, manufacturing FPSCs using a mass-fabrication process at a low cost remains a challenge. Traditional FPSC devices, for example, are still produced on ITO/PEN and ITO/PET substrates; however, the rigidity of ITO generates cracks in the layer, which leads to device failure after repetitive bending. Another significant challenge in fabricating FPSC is the sensitivity of perovskite materials to surrounding elements like oxygen and moisture[165]. The sensitive nature of these materials demands careful handling and controlled fabrication conditions to prevent degradation and ensure the stability and effectiveness of the finished solar cell device.

To address these limits, some progress has been achieved using low- cost carbon [82], polymer-based interfacial layers [123], and organic composite materials [166]. In addition, the active layer significantly impacts the overall performance of FPSCs. Due to the inevitable loss of homogeneity in the films, all reported FPSCs with good efficiency are confined to small areas (Lab scale) since the device area is inversely related to the PCE. As a result, it's critical to develop technology for fabricating large-area FPSCs without compromising device efficiency.

Most FPSC devices are made by spin coating, which involves shear forces spreading the perovskite precursor solution across a substrate. Because this method is simple and accessible, it has been employed to build most FPSC devices. However, there is a risk of poor process repeatability and waste of precursors when using this technology between research laboratories. The R2R fabrication technology is one of the most promising approaches to realizing large-area and commercial applications. To speed up this technique, more research should be focused on it. Apart from the efficiency of FPSCs, the mechanical-photovoltaic loss of FPSCs must also be considered. For wearable device applications, for example, more research is needed into how to meet the requirements of flexibility and stretchability for complicated body movements. Many efforts are still being made to make it easier to use FPSCs, such as long-term stability and low-cost mass manufacturing. Considering the accomplishments of FPSCs in recent years, it is realistic to expect a bright future. In addition, the absorber materials (MAPbI3, CsSnI3 MAFAPbI3, CsPbI3, and CsPbBrI2) have a tuneable bandgap (1.18–2.6 eV), allowing them to be made into high-performance FPSCs for a given indoor light source [154]. Undoubtedly, there are still numerous challenges to the commercialization of Indoor FPSCs. In addition, lead leakage can be substantially minimized with physical encapsulation and non-toxic materials. Several PSC-based businesses have recently concentrated on producing comparable goods but have yet to enter the IoT market [9].

Conclusion

In conclusion, significant progress has been made in developing FPSCs for indoor and outdoor applications. This results from a deeper understanding of raw materials' chemical and optoelectronic properties, fabrication techniques, and device working mechanisms. This understanding has resulted in the highest FPCS, which has an efficiency of 24.7%. This paper presents a detailed overview of critical materials for producing FPSCs, including discussions on every layer of the device; substrates, ETL, HTL, electrodes, and active materials. Although advancements in flexible substrates, a new and/or improved substrate is still required to meet the needs for stability, flexibility, low-cost products, and heat tolerance, it was noticed from the review that polymer substrates like PET and PEN have poor thermal tolerance, and a low processing temperature is needed to fabricate the FPSC device. Hence, willow flexible glass will likely be the dominant material for FPSC in the foreseeable future. Nevertheless, it is still expensive and sometimes fragile, which are major reasons hindering its adaptability. The flexible plastic substrates are mostly used in the fabrication of the FPSC. Furthermore, the high water and oxygen permeability affects its long-term stability. Various fabrication methods were critically analysed, with the R2R fabrication technique having been ascertained as a potential method for the fabrication of FPSC for large-scale production and commercialisation. In addition, studies have been conducted on several absorber materials with comparable chemical components but distinct crystal shapes. According to the investigation, various perovskite materials were studied; these materials perform less well in FPSC than CH3NH3PbI3. However, the variety of chemistries examined throughout the few articles suggests that the CH3NH3PbI3 absorber layer is still the best for fabricating FPSCs devices. TiO2 stands out as an electron transport layer, and transparent conducting oxides will continue to dominate the space for the construction of PSCs but are not suitable for FPSC because of their high annealing temperature; some charge carriers such as ZnO, NiO, and SnO are adaptable with SnO mostly used because of its stability and suitable band gap, and NiOx was considered to be the best candidate for the fabrication of HTL. Therefore, It is critical to create a cost-effective technology for applying moisture barrier coating to plastic substrates, producing large-scale substrates at a low cost. We reviewed the FPSC's indoor and outdoor uses for the application materials. FPSC technology is projected to dominate the market, whereas commercial development of perovskite is predicted to increase because of its high efficiency and low-temperature fabrication. The hunt for novel materials for substrate, charge carriers, and perovskite is projected to become more intense. But the FPSC tandem is now considered the way forward to achieving long-time stability and higher efficiency FPSC.