The Progress of Additive Engineering for CH3NH3PbI3 Photo-Active Layer in the Context of Perovskite Solar Cells

Methylammonium lead triiodide (CH3NH3PbI3/MAPbI3) is the most intensively explored perovskite light-absorbing material for hybrid organic–inorganic perovskite photovoltaics due to its unique optoelectronic properties and advantages. This includes tunable bandgap, a higher absorption coefficient than conventional materials used in photovoltaics, ease of manufacturing due to solution processability, and low fabrication costs. In addition, the MAPbI3 absorber layer provides one of the highest open-circuit voltages (Voc), low Voc loss/deficit, and low exciton binding energy, resulting in better charge transport with decent charge carrier mobilities and long diffusion lengths of charge carriers, making it a suitable candidate for photovoltaic applications. Unfortunately, MAPbI3 suffers from poor photochemical stability, which is the main problem to commercialize MAPbI3-based perovskite solar cells (PSCs). However, researchers frequently adopt additive engineering to overcome the issue of poor stability. Therefore, in this review, we have classified additives as organic and inorganic additives. Organic additives are subclassified based on functional groups associated with N/O/S donor atoms; whereas, inorganic additives are subcategorized as metals and non-metal halide salts. Further, we discussed their role and mechanism in terms of improving the performance and stability of MAPbI3-based PSCs. In addition, we scrutinized the additive influence on the morphology and optoelectronic properties to gain a deeper understanding of the crosslinking mechanism into the MAPbI3 framework. Our review aims to help the research community, by providing a glance of the advancement in additive engineering for the MAPbI3 light-absorbing layer, so that new additives can be designed and experimented with to overcome stability challenges. This, in turn, might pave the way for wide scale commercial use.


Introduction
Alternative renewable energy sources are considered reliable options for long-term usage due to the limited availability of traditional energy resources (i.e., coal, oil, and gas). Since solar energy is abundant and free (i.e., it comes from a natural source), it is referred to as clean and green energy. Also, solar energy can help reduce air pollution and global warming caused by greenhouse gas emissions if low-cost manufacturing is achieved [1]. This notion of utilizing solar energy further led to the development of photovoltaic technology, resulting in three generations. The first generation of photovoltaic technology was silicon wafer-based solar cells. However, they were not commercially successful due to their high cost. In contrast, second generation solar cells offered a low cost, but compromised in efficiency. Hence, the third generation was developed to be cost-effective and efficient. In this context, perovskite solar cells are considered advanced level third generation photovoltaic technology that could offer low-cost processing with ease of manufacturing and equally high efficiency when compared to their traditional counterparts [2]. This is why the research community has paid attention to perovskite solar cells in the last decade.
When the temperature exceeds 85 • C in the dark, MAPbI 3 forms ammonia or some other volatile species, and PbI 2 (Equation (7)) [12,16]. CH 3 NH 3 PbI 3 → PbI 2 + CH 3 I + NH 3 (7) Whereas, in the presence of light and temperature, PbI 2 intermediate forms and further decomposes, resulting in the formation of metallic lead and molecular iodine. At the same time, volatile species/by-product may rise from CH 3 NH 3 I. In the presence of light and heat, the identified degradation pathways are presented in Equations (8) and (9) [2,[17][18][19][20][21][22][23]: CH 3 NH 3 PbI 3 → [PbI 2 ] + volatile species (rising from CH 3 NH 3 I) (8) PbI 2 → Pb 2+ + 2I − → Pb + + I 0 + I − → (1/2)Pb 0 + (1/2)PbI 2 + (1/2)I 2 (9) Thus, these Pb and iodine sites create defect sites/trap states, resulting in instability of the CH 3 NH 3 PbI 3 active layer and CH 3 NH 3 PbI 3 -based perovskite solar cell devices overall. These defect sites are often reported to be within the grains, at grain boundaries or at the surface of the CH 3 NH 3 PbI 3 perovskite layer. Moreover, these trap states can also alter the energy level alignment and destroy charge transfer properties in CH 3 NH 3 PbI 3 solar cell devices. Additionally, non-radiative recombination becomes more prominent in such a scenario, a clear indication of deterioration of device performance. Therefore, it is crucial/important to chemically passivate these defect sites to achieve longer stability of the CH 3 NH 3 PbI 3 layer [14]. To achieve the aforementioned, additive engineering has been widely applied. Although, the passivation can also be achieved by physically passivating the defects with the help of encapsulating layer [24][25][26][27].
In this review, however, we will focus only on the additive approach. To accomplish this, we have analyzed/scrutinized additives used in the CH 3 NH 3 PbI 3 photoactive layer. Further, the role of the additive and mechanism of the additive interaction with CH 3 NH 3 PbI 3 has also been discussed. Their impact on film morphology, charge transport properties, and impact on stability has been intensively described. It should be noted that, here, we discussed only those additives that are added in the CH 3 NH 3 PbI 3 layer. In addition, scrutiny of the physical passivation layers/encapsulation approach that may result in the same is beyond the scope of this review, as we mainly focused only on the aspect of additive engineering. Further, we aim to discuss those additives that do not alter/destroy the perovskite structure of CH 3 NH 3 PbI 3 , i.e., do not result in mixed cation or mixed halide perovskite. The additives are mainly classified as organic additives and inorganic salt additives. Furthermore, organic additives are analyzed based on the functional group associated with the N/O/S donor atom. At the same time, the inorganic salt additives are further classified as per the group name in the periodic table, i.e., alkali metals, transition metals, other metals, and non-metals. This review aims to analyze the research progress/advancement in the context of additives employed to improve the performance and stability of the CH 3 NH 3 PbI 3 photoactive light-absorbing layer. This can further help design additives, ultimately resolving stability challenges to commercialize the CH 3 NH 3 PbI 3 -based perovskite solar cell. ratios are of the same kind, because of which, the solar cell performance and stability do not depend on the NMP ratio in the precursor [51]. Following the idea of the Lewis acid-base interaction, KIM and co-workers explored pyrrolidone-based solvent additives and compared with commonly used DMF-based precursor [52]. For pyrrolidone-based solvent additives, different N-substituents: N-methyl-2-pyrrolidone (NMP), N-ethyl-2pyrrolidone (NEP), N-cyclohexyl-2-pyrrolidone (CHP), and N-octyl-2-pyrrolidone (NOP) was employed. During the in situ crystal growth, it was observed that solvent additive containing pyrrolidone structure with higher boiling point results in lower vapor pressure over pristine DMF-based precursor. Thus, it resulted in a further improvement in the morphology of perovskite film since the N substituent becomes bulkier with a higher boiling point (the boiling points of NMP, NEP, CHP, and NOP are 154 • C, 202 • C, 204 • C, 286 • C, 303 • C). Furthermore, it was discovered that the strength of coordination of solvent component influenced intermediate formation with PbI 2 , which, in turn, was influenced by the boiling point of the solvent. This was based on the evidence when the intermediate phase vanished with the CHP solvent additive, whereas pure DMF solvent without pyrrolidone-based solvent additives showed DMF: PbI 2 solvate formation. Therefore, it was suggested that the bulkier additive solvent suppress DMF: PbI 2 solvate formation, which shows lack of stability when used without any cosolvent [52]. Some other derivatives, such as DMI, 1-(4-ethenylbenzyl)-3- (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylimidazolium iodide (ETI), 1-methyl-3-propylimidazolium bromide (MPIB), 1-butyl-3-tetrafluoroborate (BMIMBF4) are other examples that result in improved PCE and ambient stability due to hydrophobicity of N atoms with a ring-like structure [53][54][55]. Figure 1 below shows the chemical structures of additives based on amine derivatives.

Cyano/Nitrile Additives
Subsequently, it was observed that the cyano group-containing N atom could be equally effective to passivate grain boundaries [56]. Triazine-graphdiyne (Tra-GD) is a graphene-like material that consists of pyridine-like nitrogen in the highly conjugated framework. It was suggested that balanced charge distribution in the C=N bond in the triazine ring promotes stronger interaction with Pb 2+ , resulting in very tight contact between these two materials. This, in turn, did not allow the ion migration on the surface and successfully passivated grain boundaries [56]. Similarly, graphitic carbon nitride addition also resulted in good morphology due to highly compact bonding between Pb 2+ and N of C=N that further endorsed to achieve power conversion efficiencies (PCEs) up to 21.1% (aperture 0.16 cm 2 ) and 19.5% (aperture 1.0 cm 2 ), featuring an open-circuit voltage of 1.16 V (corresponding to a slight voltage loss of 0.39 V), and improved operational device stability (~90% initial efficiency retained after constant one sun illumination for 500 h) [57,58]. Further, Zhou and co-workers demonstrated that, by employing 1,8-Diazabicyclo [5.4.0]undec-7-ene (DBU), the formation of I − can also be suppressed due to adduct formation with PbI 2 [59]. Moreover, another solvent additive, acetonitrile (ACN), also showed improvement in morphology due to adduct formation with PbI 2 when added in a DMF-based precursor. This further proved to increase PCE from 15.04% to 19.7%, which remained stable without encapsulation for 160 h, with the PCE remaining~60% of the initial value when exposed to the white light [60]. The chemical structures of the mentioned nitrile-based additives are illustrated in Figure 2.  Further, at a glance, the summary of additives that improve stability is presented in Figure 3. In Figure 3, "Extrinsic" represents results reported by conducting stability tests in air, moisture, or in ambient conditions, whereas "Intrinsic" represents results reported by conducting stability tests in inert, inside glovebox (nitrogen or argon filled), or vacuum. All subsequent figures depicting stability time versus additive will use these terms and represent the aforementioned. Here, the stability time refers to the time for which the active layer with additives was reported to be operational/functional in corresponding references. Additional details of additive influence on power conversion efficiency (PCE) and stability are described in Table 1, displaying the additives based on the N donor atom for the MAPbI 3 active layer. The data are taken from references [33][34][35][39][40][41][47][48][49][50][53][54][55][56][57][59][60][61][62] respectively.

O Donor Atom-Based Additives
Similar to N donor, O donor and S donor can also act as Lewis bases due to lone pair of electrons available to co-ordinate with Pb defect site. Additives containing O atom as the Lewis base may consist of various functional groups, such as carbonyls, carboxyl, carboxylates, ester, ethers, and alcohols [28].

Carbonyl and Amide Additives
One such systematically investigated additive is urea. Urea is an eco-friendly compound consisting of a carbonyl group known to interact with PbI 2 [65]. It has been illustrated that PbI 2 and urea forms an intermediate PbI 2 .O=C(NH 2 ) 2 when added to the PbI 2 precursor solution in the double-step spin coating process. The oxygen atoms act as a Lewis base. Whereas Pb 2+ acts as Lewis acid, resulting in the formation of Lewis acid-base adduct. This adduct formation further results in large, flat grains [66,67]. Similar results were observed for the single-step spin coating process, resulting in grain boundary passivation, improved crystalline film, and suppressed non-radiative recombination losses, further improving the efficiency of solar cells [67][68][69]. Moreover, it was shown that incorporating amides in MAPbI 3 reduces the Fermi level by interacting I − defect vacancy, reducing the trap sites, and increasing the work function of MAPbI 3 perovskite. At the same time, the carbonyl group transfers electrons to perovskite, reducing the Fermi level, which in turn helps to achieve better charge transport properties [70]. Recently, a variant of urea, biuret incorporation in MAPbI 3 precursor attributed the intermediate formation with PbI 2 to the electron delocalization in the N-C=O-N system in the presence of carbonyl group and explained this is why the peak shift for C=O vibration is noticed in FTIR spectra (from 1722 to 1713 cm −1 in case of biuret-modified MAPbI 3 film) [71], resulting in higher PCE over urea incorporated devices [72]. Some other additives, such as benzoquinone (BQ), triazine perylene diimide (TPDI), 3,4-dihydroxybenzhydrazide (DOBD), containing the carbonyl group have shown similar properties [73][74][75][76]. Similarly, it was also shown that, if a hydrophobic ring is attached within the additive framework in combination with the carbonyl group (Isatin-Cl additive), it also acts as a shield against humidity in ambient conditions, helping to maintain the PCE nearly to 95% of the initial state [77]. Moreover, the usage of the carbonyl group can also be expanded to improve the flexibility of flexible, printable solar cells (FPSC). Polycaprolactone (PCL) additive in MAPbI 3 has shown desirable improvement in PCE as well as mechanical strength. This improvement in mechanical stability was ascribed to a two-fold increase in grain size (from 200 to 400 nm) and homogenous grain distribution, leading to improved mechanical stability that maintained PCE more than 90% of the initial state after 300 bending cycles for the radius from 20 to 4 mm and stability under illumination for 350 h [78].

Sulfonyl Additives
Furthermore, carbonyl-based additives are often compared with the sulfonyl-based group, where the O atom is connected to different species, (i.e., S atom instead of C atom). Fang and co-workers have done one such study in which the strength of Pb coordination of the sulfonyl group containing additive was compared with the carbonyl-based additive. For this purpose, two novel fused ring non-fullerene acceptor materials IDIS-Th and IDIC-Th were introduced into MAPbI 3 . IDIC-Th had the Lewis base functional group carbonyl (C=O), and IDIS-Th had a sulfonyl group (O=S=O). Since the sulfonyl group (O=S=O) is a stronger electron-withdrawing group compared with carbonyl (C=O); the sulfonyl group was predicted to have a stronger interaction with Pb ions than carbonyl (C=O) because the two sulfur-oxygen double bonds can chelate Pb 2+ with stronger interaction. Moreover, it was noticed that replacing carbonyl with the sulfonyl group could effectively downshift the lowest unoccupied molecular orbital (LUMO) level of IDIS-Th, which indicated the sulfonyl group is a stronger electron-withdrawing group. Thus, the interaction with Pb would reduce the defect traps density. Further, to confirm the defect passivation effect of IDIC-Th and IDIS-Th, the trap densities (n trap ) in bulk MAPbI 3 films were calculated and found to be 8.85 × 10 15 cm −3 and 4.20 × 10 15 cm −3 , respectively. Undoubtedly, the sulfonyl group reduced the defect states. This further reflected improvements in the contact angles of the film (the hydrophobicity of the films). The improvement in hydrophobicity indicated that the additive molecules at the GBs and surface could inhibit the raid of moisture to MAPbI 3 films. However, both IDIC-Th and IDIS-Th are known for their hydrophobic nature. Nevertheless, these changes reflect improvement in hydrophobic nature. Moreover, the influence of the two molecules on photovoltaic performance was also examined. The PSC with IDIS-Th molecule showed significant improvement in PCE and reached up to 20.01%. Additionally, due to the strong passivation effect of IDIS-Th, PSC with IDIS-Th demonstrated superior stability over PSC with IDIC-Th in ambient conditions, under solar radiation and in the dark at high temperature [79]. Likewise, a comparison of solvent additives hexamethylphosphoric triamide (HMPA) and dimethyl sulfoxide (DMSO) was made, which was further compared to solvent additive NMethyl-2-pyrrolidone (NMP). Here, Lewis basicity, donor number (D N ), and the boiling point of solvents were utilized as comparison parameters to correlate the role of solvents in regulating morphology. This investigation, conducted by Cao and Wei, and co-workers, highlighted that the larger the donor number, the stronger is the Lewis basicity of the solvent additive. The donor number of DMF, NMethyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and hexamethylphosphoric triamide (HMPA) are 26.6, 27.3, 29.8, and 38.8, respectively. Therefore, it was suggested that Pb-O bond strength would increase with an increase in the donor number of the additive [80,81]. Hence, NMP, DMSO, and HMPA were expected to form stronger intermediates over pure DMF. In addition, it is also observed that the strong Lewis base additives exhibit higher boiling points. (The boiling points of the DMF, NMP, DMSO, and HMPA are 152, 202, 189, 235 • C, respectively) [82,83]. This, in turn, reduces the evaporation rate of the solvent during the spin-coating process, resulting in a good film morphology. As a result, DMSO followed by NMP cosolvents displayed excellent PCE of 14.66% and 16.17% over pristine (DMF) (12.25%). No residue of PbI 2 remained and complete conversion into perovskite resulted in less resistance [84]. Further, other solvent additives based on O donor atoms, such as tetramethylene sulfone (TMS) [85] and tetrahydrothiophene oxide (THTO), also utilized a similar mechanism of forming the Pb-O bond and improved performance and stability [86]. The chemical structures and influence of amides/carbonyl/sulfonyl group-based additives onto the stability of the MAPbI 3 layer are presented in Figures 4 and 5, respectively, with a glance summary shown in Table 2.

Carboxylic/Acid Additives
Acid additives have also been incorporated in the MAPbI 3 precursor. The most commonly reported acid additive is 5-amino valeric acid (5-AVA). Dauskardt and coworkers showed that the introduction of 5-AVA into MAPbI 3 precursor could be helpful to increase the mechanical robustness of the perovskite film. To test the mechanical reliability, the double cantilever beam method was used, and the cohesion energy (Gc) was calculated from the critical load at which the film created cracks. It was observed that the incorporation of 5% concentration of 5-AVA into MAPbI 3 precursor increased cohesion energy (Gc) from 0.61 ± 0.27 J/m 2 (pristine) to 6.04 ± 2.04 J/m 2 (with AVA additive) almost 12-fold. This improvement in Gc with 5-AVA was attributed to enhanced interaction forces due to the longer alkyl chains present in the 5-AVA [87]. Additionally, a further increase in plasticity and crack deflection around the additive-containing perovskite grain boundaries was observed. Later, the elastic moduli of MAPbI 3 perovskite films with 5-AVA showed a slight decrease in the elastic modulus from 20.5 GPa (pristine) to 18.9 GPa (5-AVA) (using the nanoindentation method). This decrease in stiffness of the planar perovskite structure made it more mechanically robust and less brittle. Moreover, it was confirmed that adding 5 AVA in MAPbI 3 precursor does not change MAPbI 3 lattice parameters [87]. Simultaneously, another study done by Durrant and the group reported enhanced photostability of MAPbI 3 solar cells due to surface defect passivation in screen printed, HTM free, carbon electrodebased PSC consisting of 5 AVA in MAPbI 3 . They found a 40-fold increase in device lifetime measured under full sun illumination in ambient air (RH ± 15%). Further, it was proposed that AVA is located at grain boundaries and, thus, able to passivate surface defect sites, resulting in enhanced resistivity to oxygen-induced degradation [88]. The impressive improvement in the stability due to the AVA additive is attributed to the interaction of halides with -COOH and NH 3 + group, through hydrogen bonding ((O-H--I) and (N-H--I)), and further verified in the large area cell with an area of 0.25 cm 2 , which demonstrated the efficiency of 6.6% with a decent shelf life of 75 days, maintaining more than 90% of initial efficiency [89]. Later, using polyacrylic acid (PAA), Cao and workers demonstrated that acid additives could be useful for large area (1 cm 2 ) films using the doctor blade method. Using the carboxyl functional groups of PAA, iodide ion vacancies at the perovskite crystal surface could be cross-linked. Thus, the interaction of the PAA molecule with MAPbI 3 passivates defects and improves PCE and stability [90]. In addition, it was demonstrated that bifacial passivation could also be obtained by the interaction of a functional group of -COOH, combined with -C, -S (of thioctic acid (TA)). Lewis acid-base reaction between under-coordinated Pb 2+ and S atom in MAPbI 3 can passivate one side, and at the same time, TiO 2 can interact with COOH through hydrogen bonding, resulting in double-sided passivation [91]. Likewise, various studies reported that if multiple -COOH groups are attached with hydrophobic phenyl ring-it can chelate with Pb 2+ atom and act as a shield against moisture and humidity under ambient conditions, maintaining the overall crystallinity of film. Terephthalic acid (TPA) and trimesic acid (TMA) are examples. The proposed mechanism of stability in such a case is given as follows: negatively charged groups (-COO − ) strongly attract Pb 2+ ions in the precursor solution via electrostatic interactions. At the same time, the hydrogen atom from carboxylic acid can bond covalently with halide anion in solution due to the strong electronegativity of halide ions. Thus, it overall suppresses the loss of iodine molecules and, in turn, slows down the decomposition of MAPbI 3 light absorbing layer. Simultaneously, benzene ring with rigidity and π-π bond effect does not allow interacting with water and protects against thermal stress and UV-illumination in ambient conditions [92,93]. Similar effects were observed when 3,3',5,5'-azobenzene-tetracarboxylic acid (H 4 abtc) containing two benzene rings connected via azo bonding is introduced in PbI 2 precursor in optimum amounts (2 mass %). The purpose of using the azo group was to reduce the stiffness of the perovskite film [94]. However, due to the hydrophobic nature of the attached cation in the acid-containing additive, most of them increase ambient stability (see Figure 6 for chemical structures of acid-containing additives and Figure 7 for associated stability). Further details of acid-based additives in MAPbI 3 are provided in Table 3 below.    In a similar fashion, the acetates in the form of additives also show improvement in crystallinity and grain size, passivating the grain boundaries. One of the examples is formamidine acetate salt (FAAc). It was reported that FAAc controlled the film morphology and improved fill-factor over 80%, which in turn improved PCE to 16.59%. The improved photovoltaic parameter was further associated with the incorporation of FAAc that eliminated the defect and trap density in the MAPbI 3 film and, thus, improved the charge transport efficiency and reduced the hysteresis [98]. On the contrary, methyl ammonium acetate (MAAc) resulted in lower crystallinity and a smaller grain size~200 nm [99]. However, it was suggested to use MAAC together with thiosemicarbazide (TSC) salt in the precursor to overcome lower crystallinity. This combination of additives (10-15% MAAc (molar ratio) and a tiny amount of TSC (3-5% molar ratio)) gave a certified PCE of 19.19% for an aperture area of 1.025 cm 2 , and the high-performing devices were able to sustain over 80% of the initial PCE after 500 h of thermal ageing at 85 • C [100]. Moreover, acetate additives were reported to be used as complementary to halogens. For instance, ammonium acetate (NH 4 Ac) was found to produce full film coverage with higher crystallinity. It was speculated that NH 4 Ac resulted in the formation of methylamine acetate salt by forming NH 4 PbI 3 as an intermediate phase with a volatile by-product NH 3 resulting in a higher nucleation density of grains and full coverage of the surface. This, in turn, resulted in a higher efficiency of 13.9% in carbon-based solar cells (FTO/TiO 2 /perovskite/Carbon), and 17.02% in the two-step spin coating in FTO/TiO 2 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au configuration [101,102]. Further, the influence of NH 4 Ac addition was compared with NaAc and ZnAc 2 . Sadly, the incorporation of sodium acetate (NaAc) into MAPbI 3 precursor reported no improvement in photovoltaic performance. However, the PCEs were comparable. While compared to Zinc-acetate (ZnAc 2 ) additive, devices with ZnAc 2 demonstrated improved fill factor and short circuit current density over reference cells, resulting in improved PCE from 11.1% to 12.30%. Furthermore, against the speculation that Zn 2+ might partially replace Pb 2+ , due to the smaller ionic radius, the small quantity of ZnAc 2 (MAI/PbI 2 /ZnAc 2 (1:1−x:x, x = 7% as of molar ratio)) was reported to assimilate well within the MAPbI 3 framework, and maintained~89% of initial PCE after 1900 h when stored in Rh=40%, dark. However, NH 4 Ac incorporated devices exhibited~96% PCE of the initial stage after 1900 h with the same conditions [101,102]. Later, lead acetate (PbAc 2 ) was employed as an additive, demonstrating excellent PCE improvement from 17.25% to 19.07%. It was further demonstrated that incorporation of PbAc 2 in precursor creates an intermediate phase by forming hydrogen bonding due to interaction with MA + and O from acetate. Thus, this hydrogen bonding acts as a cross-linking agent for the intermediate phase, which later improves the intrinsic stability of devices that maintain almost 95% of the initial PCE for 20 days [103]. A similar mechanism was utilized in another study where the addition of barium acetate (BaAc 2 ) was explored in MAPbI 3 perovskite with a lower concentration (0-2 mg/mL), which suppressed ion migration and improved thermal stability. The devices could sustain~90% PCE of the initial stage after exposure to thermal stress at 90 • C in the dark in inert conditions without any encapsulation up to 400 h [104]. Thus, by employing acetate-based additives (Figure 8), improved stability can be obtained both in ambient and inert conditions ( Figure 9). Further, the acetate-based additives are summarized in Table 4.

Ester-and Ether-Based Additives
Interestingly, phenyl-C61-butyric acid methyl ester (PCBM), which is commonly employed as a charge transport layer in organic-inorganic hybrid solar cells, was also examined as an additive in the perovskite layer to achieve hole transport material (HTM) free fully printable MPSC (mesoporous perovskite solar cell). It was shown that, when the concentration of PCBM increased from 0 to 0.25 mg/mL, the PCE significantly improved from 8.58% to 12.36%, achieving a PCE enhancement of more than 44%. This enhancement of PCE was further ascribed to increased J sc from 14.56 mA cm −2 to 20.26 mA cm −2 due to higher photogenerated charge separation, and suppressed the charge recombination process. Later, it was illustrated that the PCBM-perovskite intermediate phase was facilitated due to the formation of an intermediate adduct with PbI 2 and carbonyl groups in PCBM [105]. Afterwards, Hu et al. compared PCBM incorporation with only the C 60 unit and C 60 -taurine unit in a single-step spinning coating method in p-i-n configuration. It was reported that C 60 (16.59%), PCBM (15.94%), C 60 -Ta (16.59%) all improved PCE over pristine (14.87%), which was consistent with the previously published report. Further, it was found that the addition of C 60 and its derivatives decreased the trap densities and exhibited higher stability when exposed to ambient conditions RH = 25-50% without encapsulation [106].
Moreover, investigations have shown that ethers can help passivate grain boundaries on the perovskite surface. Poly (propylene glycol) bis (2-aminopropyl ether) (PEA) and Jeffamine are some of the examples that showed interaction with Pb ions due to the lone pair of the electron on the oxygen-containing ether part of the additives. Simultaneously, the MA + cation from perovskite forms hydrogen bonding with the counter ion of the additive. Thus, the polymer works as a cross-linking agent that decreases trap densities and hysteresis, improving the device performance and stability. Moreover, the crosslinking properties can also improve the ductility of the perovskite film while stretching when fabricated on a flexible substrate (such as D2000, a Jeffamine variation) [107,108]. Furthermore, using the mechanism above, ethyl cellulose (EC), a low cost, environmentally friendly, thermally stable, and water-insoluble compound, also demonstrated chemical passivation of defect traps. Thus, because of the passivation effect of EC additive, the MAPbI 3 crystal structure remained stable against moisture and air, maintaining 80% of PCE when exposed unencapsulated to ambient air at RH = 45% in the dark for 30 days [109]. Later on, tetra-orthosilicate (TEOS) additive in MAPbI 3 was also reported to lower the cost of processing by making the fabrication process feasible in the air instead of the glove box, which was found valid for both single and double step spin-coating methods. This achievement is due to the fact that, when TEOS is introduced to MAPbI 3 in air, SiO 2 precipitates, acting as a passivation for the perovskite surface and shielding it from moisture and air. This improved the overall stability in ambient conditions [110][111][112]. Further, cyclic ether compound THF (tetrahydrofuran) also reported improving stability in ambient air, which the pioneer of the PSCs, the Miyasaka group, investigated. The study demonstrated that THF and PbI 2 in DMF interact and form a complex, resulting in dense, homogenous, and pinhole-free film due to complex formation between the Pb and O donor from THF, because of the Lewis acid nature of Pb and Lewis basicity of THF. This, in turn, avoids a direct interaction of PbI 2 with water, improving T80 (T80 is the time at which PCE is degraded to 80% of initial PCE) for 20 days in the ambient environment, with 50% RH, when devices were exposed unencapsulated [113]. Similarly, the liquid additive diethyl ether was also found to influence the crystallization process, resulting in large grains, leading to improved photocurrent properties and performance [114]. The chemical structures ( Figure 10) and an overview illustrating the influence of ester and ether-based additives on the stability of MAPbI 3 -based solar cells is shown in Figure 11, respectively. Further, a table describing ester and ether containing additives and their role in the MAPbI 3 layer is presented in Table 5.

Hydroxyl/Alcohol
The Hydroxyl group can also play a vital part in interacting with perovskite through hydrogen bonding, further resulting in improved crystallinity and performance. For example, enhanced PCE and stability was observed when dibutyl hydroxyl toluene (BHT) additive containing a phenol group was mixed in MAPbI 3 precursor [115]. Furthermore, methanol and isopropanol showed larger grain size, reduced grain boundaries, lessened defect density, and demonstrated efficient charge carrier extraction at the interfaces, leading to improved PCE stability in ambient conditions without encapsulation. Similarly, improvement in PCE was also noticed due to other hydroxyl-based additives, such as 2-methoxy ethanol (ME), 2-ethoxyethanol (EE), and 2-propoxy ethanol (PE) and n butanol [116][117][118]. The chemical structures of the discussed additives are shown in Figure 12. The influence of some of the alcohol additives on the stability of MAPbI 3 perovskite is displayed in Figure 13. Further, a summary of alcohol-based additives, their influence on the PCE, and their role with the perovskite framework is described in Table 6.

Oxygen-Based Multifunctional Group Containing Additives
Other than these additives, other oxygen atom-based additives with multiple functional groups, consisting of C=O, COOH, and OH, were also employed in the MAPbI 3 precursor. One of the examples is reduced graphene oxide (rGO), which was used to achieve fast electron transport rates toward anode, including the growth of large, uniform, smooth, and crystalline perovskite film, resulting in a massive improvement in device performance from 13.6% to 20% [120]. Another example includes poly (amic acid) (PAA) and polyimide (PI) additive. However, the PAA and PI additives also consisted of the N atom, other than C=O, COOH and the OH functional group. The O atoms in PAA and PI form hydrogen bonds with H atoms in CH 3 NH 3 + . The lone pair of electrons of N atoms interact with Pb ions, which stabilized the PVSK framework. Furthermore, these interactions improved the optoelectronic properties of the perovskite layer. As a result, an increment in the grain size was observed when 0.0497 mg/mL PAA-derived perovskite was employed, illustrating the Lewis acid−base interaction between C=O and Pb 2+ that controls the crystallization process and defect passivation of PVSK. Moreover, both PAA and PI are hydrophobic and highly heat-resistant polymers and further contribute to the stability of PSC when operated in a humid and high-temperature environment [121]. The chemical structure of these additives and their influence on stability are shown in Figures 14 and 15, respectively. Table 7 presents the detailed description of these additives.

Sulfide and Organosulfur Additives
Further, sulfur-based donor atoms have also improved crystallinity and device performance [28]. For instance, Yang and co-workers used dimethyl sulfide (DS) additive in MAPbI 3 precursor solution (based on DMSO precursor) and demonstrated a record high PCE of~18.4% for a flexible perovskite solar cell. In addition, the formation of a stable intermediate complex between PbI 2 and DS molecule was observed, which further enhanced stability. The enhancement of stability due to DS solvent additive was ascribed to the smaller electronegativity of the sulfur atom (2.5) in the DS molecule compared to an oxygen atom (3.5) in the DMSO molecule, which allowed sulfur molecules to provide electrons to empty 5d Pb orbitals, ultimately resulting in a hexa-coordinated complex formation. Therefore, with better crystallization and stable intermediate formation, 86% of initial PCE remained for 60 days when unencapsulated devices were exposed to moisture RH~35% in the dark at room temperature [122]. Thiourea is another example of such an organosulfur compound, the addition of which, into the photoactive layer, results in improved performance and stability due to intermediate formation because of the Lewis acid-base reaction. However, the addition of thiourea in the active layer is often compared to the similar compound urea, where the O atom from the carbonyl group (in urea) is replaced with the S atom for thiourea. (i.e., the carbon atom is connected to different species-C=O for urea and C=S for thiourea). Both thiourea and urea were found to show similar effects of increasing grain size, crystallinity, grain boundary passivation, and coordination with PbI 2 in the MAPbI 3 framework [123]. Moreover, many other organosulfur derivatives were reported to form an adduct with PbI 2 as the Pb atom acts as the Lewis acid. One such example is a volatile Lewis base, thioacetamide (CH 3 CSNH 2 ), abbreviated as TAA [124]. Another example is 2-pyridylthiourea, which has an S donor from thiourea, with a better ability to coordinate with PbI 2 , and an N donor from pyridine, which can coordinate with PbI 2 as a Lewis acid base and regulate the co-ordination strength with PbI 2 . This further resulted in an improvement in crystallinity, which in turn resulted in better light-harvesting ability, charge transport, the reduction of defect and recombination loss, leading to PCE of 18.2%. In addition, this allowed retaining the stability of PSC more than 90% when exposed to ambient conditions, at room temperature or at higher temperature in the dark [125]. Recently, Zhang and his co-workers combined thiourea with water and iodine (ITU) ions to reduce iodine defect vacancy produced during the film fabrication process. Afterwards, the grain size was found increased with ITU (3.4 um) compared to when thiourea was added (800-2000 nm), reducing the defects at grain boundaries. Moreover, PSCs with ITU additives displayed a higher V oc , longer decay time, and longer carrier lifetime, with a lower charge recombination rate. Thus, overall, the addition of ITU resulted in improvement in PCE from 17.75% to 20.3%. In addition, the trap densities of the perovskite film significantly reduced as the ITU additive was incorporated into PSC, allowing PSC to stabilize and maintain~80 of initial PCE after 100 h of exposure to 1.5 AMG solar irradiation and 30 days in the ambient atmosphere [126].

Thiocyanates
Thiocyanates (SCN − containing anions) additives are another subcategory for S atombased additives. It should be noted that thiocyanates are also considered pseudo halides. However, the role of thiocyanates in the perovskite film is very controversial. For example, Yanfa Yan et al. demonstrated that a small amount (5%) of Pb(SCN) 2 addition in the precursor can significantly increase the grain size and crystalline quality of perovskite thin films using a one-step solution process method. This increased the average PCE from 15.57 to 17.80%. It was proposed that SCN − anions do not incorporate in the perovskite film. SCN − react with CH 3 NH 3 + and form volatile products HSCN and CH 3 NH 2 . This allows excess Pb 2+ and I − to form PbI 2 at grain boundaries that sources passivation effect [127]. On the contrary, Kim et al. demonstrated that Pb(SCN) 2 in CH 3 NH 3 PbI 3 partially substitutes I anions and forms CH 3 NH 3 Pb(SCN) x I 3−x at lower temperatures. This phenomena was observed as not dependent on the concentration of Pb(SCN) 2 . However, similar reports of enhancement of crystal size were found. Thus, it was concluded that the influence of processing temperature could affect the purity of the final perovskite film when Pb(SCN) 2 additive is used with MAPbI 3 [128]. Whereas, the addition of potassium thiocyanate (KSCN) also reported to form volatile products HSCN and CH 3 NH 2 , consistent with earlier mentioned results, with no traces of SCN − found in the final perovskite film [127]. In the case of the KSCN additive, however, the residue was KI instead of PbI 2 . Nevertheless, the authors illustrated that the final product upon KSCN incorporation was MAPbI 3 , not KPbI 3 , due to the smaller radius of the K cation. In addition, KI residue improves the crystal quality of perovskite film, resulting in enhancement of charge transport, reduced carrier recombination, and eliminating hysteresis [129].
Conversely, when NH 4 SCN was incorporated in MAPbI 3 , it was reported to form an unstable intermediate phase NH 4 PbI 3−x SCN x due to lower formation enthalpy ( Hf) of the NH 4 PbI 3 over MAPbI 3 , which later resulted in CH 3 NH 3 PbI 3−x SCN x a mixed halide perovskite after annealing of the films [130]. Therefore, when methylammonium thiocyanate (MASCN) was introduced as an additive to the precursor, a rapid vacuum-based drying approach was used to extract volatile intermediate product, so that MAPbI 3 was formed as the final product with no traces of SCN − . The film fabricated, using this fabrication technique, was reported to gain grain size of more than one micrometer with uniform surface morphology, resulting in high crystallinity and significantly large carrier lifetimes (τ1 = 931.94 ± 89.43 ns; τ2 = 320.41 ± 43.69 ns), improving the ambient stability up to 1000 h [131]. However, when a larger cation guanidinium (Gu) was directly added in MAPbI 3 as GuSCN, GuPbI 3 was formed as an intermediate phase present with the MAPbI 3 phase. It was contradictory to the earlier belief and reports, which showed that Gu single-handedly could not form three-dimensional perovskite materials due to a larger ionic size (278 pm) over MA cation (217 pm). As a result, improved crystallinity, grain size, and reduced trap density were noticed further, resulting in improved PCE (from 15.57% to 16.70%) that maintained stability~90% of the initial value after being stored for 15 days without encapsulation [132]. On the other hand, when guanidinium isothiocyanate was added in the PbI 2 precursor, with the dual step fabrication method; mixed cation perovskite was formed (GU) x (MA) (1−x) PbI 3 [133]. Nevertheless, most S donor atom-based additives ( Figure 16) were found to influence ambient stability, as displayed in Figure 17. At a glance, a summary of additives containing S atom and their role in the MAPbI 3 framework are presented in Table 8.

Alkane Additives
Alkane-based additives are another class of additives that was briefly investigated within the MAPbI 3 framework.
Additives such as poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP) have shown interaction with MA cation via hydrogen bonding with fluorine atoms in the additive. Such an additive is quite useful for indoor photovoltaic applications, as the additive is reported to control the nucleation and growth rate on a very thin (~150 nm) active layer of MAPbI 3 through a one-step solution processing method. Thinner light absorber perovskite is usually full of voids, results in a low fill factor, and lower internal quantum efficiency due to poor perovskite film quality. However, increasing the thickness is not suitable to obtain semi-transparent films [134]. As a solution to this problem, a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) additive illustrated less rough morphology when a~150 nm thick perovskite layer was obtained. In addition, it was found that perovskite solar cells with PVDF-HFP in the active layer have a higher charge transfer rate and lower carrier recombination rate [135]. Similar observations were seen when another derivative of PVDF polyvinylidene fluoride-trifluoroethylene polymer P(VDF-TrFE) was employed in a two-step fabrication process instead [136]. Other than these additives, some other haloalkane additives, such as diiodomethane (CH 2 I 2 ) and diiodooctane (C 8 H 16 I 2 ), were also reported to reduce the trap states, resulting in improved morphology and enhanced PCE [137,138]. The chemical structures of alkane additives are shown in Figure 18. Further, Table 9 summarizes their role in the MAPbI 3 framework.

Quantum Dot-Based Additives
Recently, a new area of additive engineering has emerged, based on quantum dots containing different functional groups introduced in the precursor. This allows the functional group to chelate under coordinated Pb vacancy or iodine defect sites at the surface of the perovskite layer (Figure 19). At the same time, a quantum dot improves the crystallinity of film due to the quantum confinement effect and reduces non-radiative recombination centers; thus, passivating defects at grain boundaries [139][140][141][142][143][144]. In some cases, quantum dots can even promote charge transfer from the oxide surface to the perovskite layer, further reducing the charge trap density, boosting fill factor as high as 84% and efficiency as 21.04% [143,145]. The influence of incorporation of QDs onto the stability is shown in Figure 20. A summary of such additives based on quantum dots (QDs) is presented in Table 10.   [139,141,142,145] respectively.

Inorganic Additives
Inorganic salts can be further explained as per the category in the periodic table, i.e., based on alkali metals, alkaline earth metals, transition metals, other metals, and other non-metals. These additives may exist in the form of inorganic halide salts or inorganic acids [146].

Alkali Metals Additives
Hydro halides are at the top, as hydrogen is the first element in group one of the periodic table. Soe et al. studied the incorporation of hydrohalic acids (HX, X=I, Br, Cl) in MAPbI 3 precursor prepared using DMF. It was found that HX incorporation in the perovskite layer alters the bandgap and unit cell parameters. The HI addition compressed bandgap, whereas HBr widens the bandgap at high concentrations (20-25 vol%). Further, these changes were found correlated with the types of defects present in polycrystalline perovskite thin films combined with the structural strain induced in very small crystallites. Thus, it was concluded that these acids could influence crystallization rate, surface coverage, and improve morphology. However, HCl incorporation showed no influence on bandgap [147,148]. The same was verified by Yan and co-workers who investigated HI (hydroiodic acid) as an iodine quencher in MAPbI 3 precursor solution (prepared with isopropyl alcohol (IPA)), that further lead to chemically passivate grain boundaries and increase in PCE from 16.13% (pristine) to 18.21% (with HI additive) [149]. Likewise, alkali metals, such as Li, Na, and K, combined with halogen counterparts, have been inspected as additives for the MAPbI 3 light absorber layer. Grätzel and Friend co-workers demonstrated the influence of NaI (sodium iodide) on the optical, excitonic, and electrical properties of CH 3 NH 3 PbI 3 perovskite prepared by a two-step sequential deposition technique. It was revealed that the NaI additive helps in the complete conversion of PbI 2 into CH 3 NH 3 PbI 3 , leaving no unreacted PbI 2 and enhancing the crystallinity. Furthermore, KPFM measurements showed a reduction in work function (towards Au metal) for contact-potential difference (CPD) and proved the chemical passivation of perovskite surface due to additive incorporation in the precursor. Additionally, the hypothesis of partial substitution of Pb 2+ with monovalent cations Na was discarded, because replacing Pb 2+ with the Na cations required high energy, which could not be obtained at room temperature, since additives and films were processed at room temperature [150]. Similar reports were published from Chu and co-workers who investigated alkali metal chloride additives (LiCl, NaCl, and KCl) in MAPbI 3 and suggested that mixed halide formation does not occur as Cl atoms easily evaporate due to the annealing process, improving the crystallinity and carrier charge transport [151]. However, as the size of the nuclei increases, the perovskite shows partial replacement and forms a perovskite with a double cation (i.e., in the case of RbX and CsX, where X represents halide) [152]. The partial substitution of cation was also observed when alkaline earth metals were employed as additives in the precursor. Hence, they are not the center of attraction here and it is suggested that alkaline earth metals are best suited to replace Pb due to their divalent nature [153,154]. The chemical structures of alkali metal additives are presented in Figure 21. The example of the influence of alkali metal additives on the stability of MAPbI 3 PSC is shown in Figure 22. Further, the role of alkali metal additives in the MAPbI 3 framework is summarized in Table 11.

Transition Metals Additives
Additionally, some transition metals were also reported as additives in MAPbI 3 precursor. One such example is n-type goethite (FeOOH) quantum dots that act as multifunctional additives. The addition of FeOOH QDs into precursor solution improves not only the performance, but also stability. The investigation by Wang and et al. showed that FeOOH QDs could produce heterogeneous nucleation, passivate the trap states, and obstruct the ion migration. The investigation demonstrated that the oxygen (in FeOOH) as a Lewis base could coordinate with Lewis acid Pb 2+ . The -OH group (in FeOOH) can coordinate with hydrogen atoms of MA + . This interaction delays crystal growth kinetics, resulting in good quality of perovskite films that result in high PCE. Further, the iron in FeOOH QDs as a Lewis acid interact with Lewis base I − . Thus, ion migration of I − and MA + is controlled, resulting in enhanced intrinsic stability of solar cells [157]. Further, the transitional metal halides, either with monovalent cations (i.e., AgI, CuI, CuBr), divalent cation (NiCl 2 , CuCl 2 , ZnCl 2 , CdCl 2 ) [158][159][160][161], or trivalent cation (RhI 3 ) [162] frequently pointed out the improvement in crystallinity and grain boundary passivation.
Interestingly, in all these reports, chlorine-based additive salts were reported to form the MAPbI 3 phase without mixed iodide and chloride as it was proposed that Cl atoms evaporate while annealing the films. Similarly, trivalent cation, such as Rh 3+ , did not replace lead atoms when used in small quantities (e.g., 1 mol%). However, the addition of 5% could result in partial replacement of the Pb atom. Figures 23 and 24 demonstrate the chemical structures and the effect of transition metal additives on the stability of MAPbI 3 PSC, respectively. Further, Table 12 summarizes the details associated with transitional metal additives.

Other Metals Additives
However, the most widely reported metal additive is PbI 2 . Several groups have suggested using excess PbI 2 as an additive and studied the effect of the stoichiometric and non-stoichiometric composition of PbI 2 in MAPbI 3 perovskite [163,164]. Reports suggest that the addition of excess PbI 2 in perovskite increases power conversion efficiency [163,165]. However, the influence of excess PbI 2 on photochemical stability is debatable [166]. Some research groups claim that excess PbI 2 passivates grain boundary [167]; thus, creates extra passivation [168]; enhancing performance and stability overall [164,169]. On the contrary, some reports showed that excess PbI 2 could increase PCE, but decreased PSC stability [170,171]. To resolve the issue, Stevenson et al. demonstrated that the stability associated with excess PbI 2 is affected by solvent coordination capacity with Pb atom, and suggested that excess PbI 2 can work as a stabilizer to improve intrinsic stability if the correct solvent (NMP) is chosen over commonly-used solvent DMF [172]. Other than PbI 2 , adding PbCl 2 also resulted in an equally good perovskite film quality with improved performance, but the outcome was reported as a mixed halide [173,174]. Hence, it is not focused on here. Interestingly, Ngo and the group demonstrated a different approach by using PbS quantum dots as capping ligands in the precursor solution to control nucleation and morphology. This approach allowed improving crystallinity and enhanced grain size, resulting in improved solar cell performance [175]. See Figure 25 for the chemical structures of PbI 2 and PbS. The influence of excess PbI 2 additive on the stability of MAPbI 3 is presented in Figure 26. Further, a relevant summary is presented in Table 13.

Non-Metal Inorganic Salts
Furthermore, the controlled nucleation, improved morphology, enhanced crystalline size, and grain size with lower trap density were also noticed with additives based on other non-metals salts. During the two-step method, which is one of the commonly used fabrication approaches to fabricate planar heterojunction complex lead halide perovskite solar cells, the complete conversion of PbI 2 into CH 3 NH 3 PbI 3 is often observed restricted by the inadequate diffusion of CH 3 NH 3 I into PbI 2 film, which affects the short circuit current density. To overcome this problem, Pathipati and co-workers used ammonium iodide (NH 4 I) in a small quantity (5 wt%) in the PbI 2 solution and found this approach increases the porosity of the PbI 2 film, allowing to improve film morphology and increase grain size up to 500 nm. This, in turn, improves the fill factor and short circuit current density, improving the PCE compared to the pristine solar cell [177]. Similarly, a drastic improvement in fill factor (from 27.53 without additive to 80.11 with NH 4 Cl additive) was noticed when NH 4 Cl was added in the MAPbI 3 precursor. Moreover, as reported earlier, due to the quick evaporation of chlorine atoms, mixed halide formation did not take place [178]. Furthermore, improved performance and higher stability were observed with different ammonium halide salts (NH 4 X, where X can be I, Cl, Br) even when using the single-step solution process method. Although, NH 4 I showed the highest performance amongst all [177][178][179]. Another ammonium-based additive, NH 4 H 2 PO 2 (ammonium hypophosphite-AHP), was reported to show similar behavior. Surprisingly, one of the by-products (H 3 PO 2 ) from CH 3 NH 3 I synthesis was found suppressing the formation of molecular iodine (i.e., the oxidation of I − to I 2 ) [180,181]. This led to the idea of using NH 4 H 2 PO 2 (ammonium hypophosphite-AHP) as an additive in the PbI 2 /CH 3 NH 3 I precursor. Huang and co-workers found that AHP forms a complex with PbI 2 , which leads to improvement in crystallinity and grain size. In turn, this improves PV performance due to crystalline grain boundaries [182]. Moreover, in a recent report, it was illustrated that hydrazinium iodide (N 2 H 5 I), an iodine quencher in the MAPbI 3 active layer, can form an intermediate complex with PbI 2 , and enhance crystallinity and grain size~1100 nm, causing grain boundary passivation. This, in turn, reduces hysteresis in solar cells, improves efficiency, and intrinsic stability of solar cells up to 4400 h [183]. There were similar reports by Huang et al., who reported hydrazinium chloride (N 2 H 5 Cl) incorporation in MAPbI 3 [184]. Whereas hypophosphorous acid (HPA) was used to assist crystallization dynamics for large area substrates [185]. The chemical structures and influence of non-metal inorganic additives on the stability of MAPbI 3 are presented in Figures 27 and 28, respectively. A summary of these additives is presented in Table 14.

Conclusions and Outlook
To conclude, we covered the progress and updates in the area of additive engineering for the MAPbI 3 photoactive layer in perovskite solar cells. As known, the poor intrinsic and extrinsic stability causes a restriction in the substantial commercialization of the PSC. Intrinsic instability is considered due to the presence of under-coordinated Pb sites or escape of a volatile product or formation of molecular iodine; either of these creates a defect in the perovskite surface. Therefore, in this review, we summarized various additives that can coordinate with the mentioned defect sites and improve stability. Higher efficiency (more than 25%) was achieved, but achieving higher stability is still far away. Further, the role of additives into MAPbI 3 precursor was presented, which influences nucleation growth, film morphology, crystallinity, grain size, and grain boundary passivation via complex/intermediate formation were conferred. To understand the aforementioned, we classified additives based on organic and inorganic compounds. Furthermore, organic additives were scrutinized as per their functional group, and a relevant coordination mechanism that impacted stability was described. Similarly, inorganic additives were classified based on metal and non-metal halides. Moreover, we focused on additive influence on stability concerning different conditions (intrinsic/ambient/mechanical robustness). Our analysis suggests that amines, heterocyclic amines, and their derivatives, followed by carbonyl and acetate functional groups, can improve intrinsic stability. On the other hand, acid, esters, ether, and alcohol, and S atom-based additives can be mainly valuable to boost ambient stability. This summary of additives further aims to boost designing new additives that can finally help achieve stability challenges.
Yet, further developments are required while adopting the additive approach in the fabrication of MAPbI 3 -based PSCs. So far, additives employed in the MAPbI 3 lightabsorbing layer do not have any rule of selection. The majority of additives have opted from existing literature available for DSSCs or organic solar cells, utilizing the N/O/S donor atoms to combine with under-coordinated Pb 2+ sites to improve stability because of the Lewis acid-base reaction. Hence, the first and the topmost prospect is to design an additive that can help sustain MAPbI 3 under operational conditions, such as illumination, heat, moisture, oxygen, water. To do so, applying machine learning is highly recommended. While additives are primarily being experimented on (on a trial and error basis), the number of published reports are growing exponentially, with time, without any outcome to overcome stability issues. Therefore, a systematic empirical investigation should be conducted using analytical, statistical tools to understand the correlation of the additive, its properties, and its influence on morphology, optoelectronic properties, and stability with various conditions concerning numerous characterization techniques. This might help in the suitable material selections to further experiment by establishing the relationship between the chemical structure of additives and photovoltaic characteristics and lifetime. Thus, it would reduce the time and cost required for the experiment to improve stability, eventually driving industrial interest in product development.
The second prospect includes improved operational stability, which is reported to gain only a few thousand hours. While at least 10,000 h of operational stability is required to commercialize as the indoor application, with respect to thin-film photovoltaics. On the other hand, traditional photovoltaic has a lifetime of~25 years. Thus, the second vision is to overcome short-term stability issues to commercialize the MAPbI 3 -based PSC. Moreover, the duration of operational stability should also be reproducible. Henceforth, fabricating reproducible stability is the third prospect to work on, as testing short-or long-term reproducibility is a time-consuming process. The next challenge is upscaling. Currently, the majority of research reports are based on small area devices. Nevertheless, large-area devices will be required for commercial purpose. However, morphology, performance, distribution of surface defects, grain size, and trap states will be changed when the large area will be used. Therefore, the fabrication of large-area devices is another prospect that requires high PCE and stability. Another future scope of development is in the area of toxicity of PSC. Since some precursors used to prepare MAPbI 3 ink are not environment friendly. Hence, alternative, environment-friendly green solvent/precursors should be further explored to give equally good efficiency and stability.
In the end, we hope this review will help scientists and researchers to design and synthesize new additives that can provide the solution for the stability challenge of MAPbI 3 . Thus, ultimately achieving stability by applying additive engineering, commercializing MAPbI 3 -based PSC can be achieved.

Conflicts of Interest:
The authors declare no conflict of interests.