Although perovskite solar cells (PSCs) have made great achievements during the past few years, the efficiency of PSCs is only up to 25.5%, which is comparable to silicon-based solar cells. However, long-term stability is still an important problem for future commercialization. Enormous efforts have been made to prolong the lifetime of PSCs. The novel passivation strategy and advanced encapsulation are investigated, and great achievements are acquired. However, research on the basic understanding of the perovskite structure and the fabrication process of PSCs is rare, which stints the initial research for the abecedarian. At the same time, the defects among the perovskite film caused by the uncontrollable crystallization process and the fragile ionic nature also deteriorate the efficiency and stability of the perovskite devices. Herein, we summarized the investigations of the mechanism for perovskite materials and the manufacturing process of PSCs. The composition of perovskite materials, the orientation of perovskite grain, and various fabrication processes are explained. Simultaneously, the novel passivation strategy and technology are also discussed. We believe that a deeper understanding of the perovskite mechanism is beneficial to render more facilities for further development of perovskite application.

The world will be a huge village as the world’s population needs more resources regularly, ignoring the reality that Earth’s composition cannot be modified. The energy consumption and associated services that meet human needs for socio-economic development, well-being, and health are growing. All social institutions need energy providers to satisfy people’s needs such as health, space comfort, flexibility, and knowledge exchange and to provide work and income. The energy sector’s two main dilemmas on the road to a sustainable future are ensuring energy availability and decreasing energy’s impact on climate change. One will not be able to live if one does not have enough resources. It is surprising to learn that 1.4 billion people worldwide lack complete access to electricity, with 85% living in rural areas. As a result, the number of rural communities that rely on biomass is projected to grow from 2.7 billion currently to 2.8 billion by 2030.1 

One of the world’s most serious issues is meeting the world’s ever-increasing electricity consumption. It is predicted that global energy consumption will increase by 28% by 2040. This requirement arises directly from the developing and underdeveloped countries, where rapid urbanization, infrastructure development, and economic expansion can occur. This is associated with the fact that significant changes in product quality and complexity would in the immediate future lead to high average power consumption per device. Furthermore, energy sources such as petroleum products and coal affect today’s power generation. These reserves are expected to last until 2040 and 2042, respectively.

Consequently, the emerging threat to global climate change, driven by contaminating CO2 emissions from fuel combustion, including coal and natural gas, also leads to an urgent need for renewable energy. Petroleum products, which emit carbon dioxide, provide more than 80% of the world’s electricity. According to Smalley, 60 TW of power will be expected to provide the developed world’s standard of living for a potential population of 10 billion people.2 The influence of humans on Earth’s environment is undeniable; according to the Intergovernmental Panel on Climate Change (IPCC), CO2 concentrations in the atmosphere have risen to levels not seen in at least 800 000 years, and the increase in CO2 concentration is the source of the greatest contribution to overall radiative forcing. The study discovered a strong connection between global mean temperature and carbon dioxide levels in the atmosphere and ways to reduce possible global temperature rises by limiting carbon dioxide emissions. Regrettably, progress in reducing CO2 emissions seems to have been slow so far.3 Therefore, clean energy output by renewable energy sources including wind, hydroelectric, geothermal, and solar power must be exploited and maximized. Renewable energy sources make up approximately just 13.7% of global energy generation, and biomass fuels contribute significantly.4 Enhancing the production potential of renewable energies would not only improve electricity production efficiency by eliminating inefficient power generation by burning fossil fuels but also reduce the release of CO2 gases into the environment. In general, the simultaneous usage of a number of fuel perspectives would be necessary to satisfy the ridiculously high energy demand soon. This leads to a decarbonized electricity system based on a massive reservoir of renewables generated mainly from wind and solar photovoltaic sources. Photovoltaic (PV) systems are appealing because they produce electricity without polluting the environment by directly converting a free and infinite energy source, solar power, into electric power. Photovoltaic modules are becoming more affordable, and their performance is increasing, indicating that photovoltaic power generation will continue to play a significant and critical part in the future. Solar photovoltaic is a type of renewable energy that can transform future energy infrastructure into clean, secure, scalable, and inexpensive.

Governments and corporations are promoting the implementation and installation of solar PV technology in recognition of this reality. PV materials are available in a wide variety all over the world. Dozens and dozens of companies worldwide develop PV modules, each with different efficiencies and limitations.5 It is the world’s fastest-growing energy technology, with cumulative installations exceeding 50 GW and expected to exceed 200 GW in 2040.6Figure 1 illustrates the various forms of solar cell materials. Due to their superior efficiency and ease of use over other technologies, crystalline silicon (c-Si) solar cells have dominated the market. PV module demand is steadily growing across the world. Because of the high cost of c-Si, researchers worldwide are working to develop new low-cost PV technologies. While thin-film PV technology has shown promise in engaging with the well-established c-Si technology, its low conversion efficiency remains a cause of worry.

FIG. 1.

Different solar photovoltaic technology.

FIG. 1.

Different solar photovoltaic technology.

Close modal

The research community has always struggled to develop solar cells that are affordable, easy to process, effective, and scalable.7,8 The potential difference between the two ends of the p–n junction is determined by light absorption, separation, and charge accumulation on each electrode, which is how the solar cell functions. The voltage difference will produce electrical energy. Solar cells of the first and second generations are made of crystalline silicon and thin-film, respectively. The drawbacks include silicon’s limited availability and high cost. As an alternative, evolving third-generation photovoltaics have been produced. DSSCs, organic photovoltaics, quantum dots, and perovskite solar cells are among them. Third-generation solar cells are made up of fullerenes, hybrid polymers, and perovskite solar cells. The existence of acceptor phases in the first two solar cells will help us compare and assess solar cell types. The sensitized solar cell design is an interesting choice for meeting these requirements. Since the dye-sensitized solar cell (DSSC) was invented, many methods for improving the efficiency of solid-state DSSCs have been investigated. This incorporates dye design to increase electron injection and hole regeneration and to expand the absorption range into the near-infrared regime. Despite this, a thicker mesoporous film still needs to absorb light due to the extremely poor extinction coefficient, increasing hole transport resistance and recombination.9,10 As light absorbers, quantum dot solar cells (QDSCs) use semiconductor nanocrystals, whose big intrinsic dipole moments and bandgap tunability through shape and size control make them an excellent tool for nanoscale light absorber material design for sensitized solar cells. Groundbreaking device structures, where a PbS absorber functions as an HTM, have recorded efficiencies of more than 8% in solid-state configurations.11 Perovskite solar cells are derived from DSSCs, which came out on top in terms of conversion efficiency. Since perovskite-type solar cells have a higher PCE and can be integrated with scalable processes, they are likely to play an important role in massive solar production.12 This was the first observation of perovskite’s ability to serve as light-harvesting materials inside solar cells in DSSCs, and it was a crucial step in the growth of interest in perovskite-based solar cells. Perovskite has the general formula ABX3, with A and B being monovalent and divalent ions, respectively. O, C, N, or halogen can all be used as X.13 They are named after Russian mineralogist Perovskite and have a cubic structure. The first perovskite discovered was CaTiO3. The most common perovskites are CH3NH3PbI3, CH3NH3PbBr3, and the mixed halide system CH3NH3PbI3−xClx, which are often used in solar cell applications. Perovskites’ popularity is likely to have contributed to their power to transform high-quality crystals quickly, employing solution processing techniques and moderate temperatures.14 

Perovskite is a mineral that was first found in the Ural Mountains and was named after Lev Perovskite (the Russian Geographical Society founder). A perovskite definition is any material that has the same chemical composition as that of the perovskite mineral. The actual perovskite (mineral) is a calcium, titanium, and oxygen compound called CaTiO3. Furthermore, a perovskite structure is described as something with the standard form of ABX3 and the same crystallinity arrangement as the perovskite (mineral). Most people only work in solar cells’ science because they do not engage in mineral and geological science. In ABX3, the A+ monovalent cation is normally methylammonium (MA+: CH3NH3+), formamidine [FA+: HC(NH2)2+], Cs+, rubidium (Rb+), or their mixture. B2+ divalent cations are typically divalent metals, such as Pb2+, Sn2+, Ge2+, or a mixture of these metals; X monovalent anions are normally halides, such as I, Br, Cl, or a combination of these halides.15–18 The cavity of the BX64− octahedron comprises 12-fold coordination pores of B metal cation, X anion, and A cation. For various perovskites, the thermodynamically stable arrangement depends on the composition. MAPbI3, for example, is stabilized mostly as a tetragonal structure, while tri-cation perovskite based on FA/MA/Cs is typically stabilized at ambient temperature only as a cubic structure.16 Those halide perovskite materials based on Pb and Sn were stabilized at ambient temperature mainly as cubic perovskite structures. For CH3NH3PbX3, as the halide size increases from X = Cl to Br and I, the unit cell parameter rises from 5.68 to 5.92 and 6.27 Å, respectively. By combining halides, lattice parameters in the cubic process can be tuned, as shown in Fig. 2. The most appropriate perovskites are related to group IV metal halides (especially lead), and it is difficult to outperform them. More in-depth experiments may be more critical than commonly possible techniques for extensively exploring the spectra of possible perovskite structures. Lead-based perovskite-based solar cells have a high defect tolerance and processing capacity for many reasons, particularly high absorption throughout the visible light region, long carrier diffusion distance, adjustable energy bandgap, and easiness of growth when the temperature is too low.

FIG. 2.

The energy levels of MAPbBr3, MAPbI3, FAPbI3, MAPb1−x SnxI3, and MASnI3 are depicted in a simplified diagram. Reproduced with permission from H. S. Jung and N. G. Park, Small 11, 10–25 (2015). Copyright 2015 John Wiley and Sons.

FIG. 2.

The energy levels of MAPbBr3, MAPbI3, FAPbI3, MAPb1−x SnxI3, and MASnI3 are depicted in a simplified diagram. Reproduced with permission from H. S. Jung and N. G. Park, Small 11, 10–25 (2015). Copyright 2015 John Wiley and Sons.

Close modal

Perovskite has the general formula of ABX3, where A is the organic or inorganic cation, B is the metal cation, and X is the halide anion. The perovskite structure is shown in Fig. 3. To have access to the tunable bandgaps, their high structure tolerance is another benefit of perovskites, decreasing the strict lattice match requirements among sub-cells.15,19 The tolerance factor of the Goldschmidt t is defined as follows:

t=rA+rB2(rA+rx).
(1)
FIG. 3.

(a) 3D schematic diagram. (b) 2D schematic diagram. (c) Chemical schematic diagram.

FIG. 3.

(a) 3D schematic diagram. (b) 2D schematic diagram. (c) Chemical schematic diagram.

Close modal

The ionic radii of the A, B site cations and X site anions are rA, rB, and rX, respectively. The t value should be 0.81–1.00 to safeguard the three-dimensional metal halide perovskite structure. Hexagonal structures can be shaped at t > 1.00. An example is the BaNiO3 style structure. Since the tightly packed layers of [NiO6] are stacked in a hexagonal fashion rather than the cubic fashion of SrTiO3, the octahedral face sharing occurs. BaNiO3 has a t value of 1.13 (rA = 1.61 Å and rB = 0.48 Å). Non-perovskite structures could be established at t < 0.81. The symmetry of the crystal structure will decrease as t decreases. GdFeO3 at t = 0.81, for example, is orthorhombic (rA = 1.107 Å and rB = 0.78 Å).20 Since perovskite is not a true ionic compound, the tolerance factor is just an estimate. The ion radius strongly influences the value of t. The range A–X ratio to range B–X in the definitive version of the solid sphere model is 0.81–1.11. The μ octahedral element (rB/rX ratio) is 0.44–0.90.15,21–23 A cubic structure may occur at finite temperature when it is between 0.89 and 1. In general, a small t can lead to a tetragonal (β-phase) or orthogonal (γ-phase) configuration, which decreases symmetry. A large t (t > 1), on the other hand, may cause the three-dimensional (3D) B–X network to weaken, culminating in a two-dimensional (2D) structure. The α-phase is normally more stable in a measurement where the DFT is zero, and the phase has always been the most unpredictable, so t = 1 is difficult to please. Transitions between specific configurations occur at specific temperatures in most perovskites, which is interesting.24,25 Different ion sizes and structural stability affect the transition temperature of different perovskites. The phase transition from α to β to γ emerges at 330 and 160 K, respectively, for the archetypal CH3NH3PbI3 halide perovskite.

According to the calculation of the tolerance factor, the composition of perovskite could be easily changed. SrFeOx (2.5 ≤ x ≤ 3) is an excellent example of a family of such compounds. The Fe ions’ valency could be altered by heating a sample in either an oxidizing or a reducing environment. As a consequence, the oxygen level of air can differ from 2.5 to 3. Some Fe ions in SrFeO2.875, for instance, could be allocated to the oxidation state +3, while others can be assigned to +4. SrFeOx is an example of defect perovskites. The homologous sequence AnBnO3n−1, with n = 2–∞, can be used to explain their chemistry. Some other forms of vacancy orderings are identified; for example, Ca2Mn2O5 and La2Ni2O5 have n = 2 structures. The structure distortion in certain perovskites may indeed be attributed to the active ions of Jahn–Teller at the B position. When Mn3+ ions are present in LnMnO3 (Ln = La, Pr, or Nb), the 3d4 electrons divide into 3 tg and 1 eg, electrons. The eg orbital of the [MnO6] octahedron is elongated due to the odd number of electrons in it.

DFT (density functional theory) analyses had already figured out the perovskites’ electronic structure, particularly the typical MAPbI3. Even after accounting for spin–orbit coupling and other impacts such as the van der Walls interaction, the measured bandgap was reasonably agreed with the absorption spectrum’s observed bandgap. In addition, the unexpected (density of state) positions of Pb2+ and I revealed a p–p optical transition, which would have been closely related to the ionic material’s charge transition.26,27 The valence band maximum tends to disperse at the valence band maximum because of the s–p antibonding coupling, resulting in a smaller effective mass (mo). According to several measurements, MAPbI3 had such an effective mass comparable to Si and GaAs, which are extensively used semiconductors. As a consequence, one should still assume a high degree of carrier versatility.28–31 While subsequent research did not support this approximation to the same degree, the fact that MAPbI3 has a low radiation recombination coefficient implies that carrier mobility may be sufficient to withstand radiation recombination.

Furthermore, it is rational to assume that the p–p transition in MAPbI3 is greater than the usual p–s transition in GaAs by comparing the two materials’ state density and absorption spectra.32 The apparent difference in density of states near the conduction band’s minimum value results in a different combined density of states, which results in higher light absorption. As a result of the efficient charge generation and transition, the device structure results in a high photo-current and voltage.

The electronic structure of materials influences the luminescent properties; in addition, the crystal structure and chemical compositions dictate the electronic format. To completely comprehend perovskite luminescence, it is important to research the electronic structure of perovskite materials. The arrangement of perovskite is strongly linked to its band configuration. The inorganic layer influences the luminescent characteristics of organic–inorganic hybrid perovskite. Only by manipulating the bending and stretching among Pb and the halogen atom in the [PbX6] octahedra will the cation change the band structure.33 For instance, the electronic configurations of MAPbX3 (X = I, Br) and CsPbX3 are similar. CH3NH3SnI3 and NH2CHNH2SnI3 are hybrid organic–inorganic compounds with bandgaps identical to that of a possible CsSnI3 cubic perovskite of the same cell dimension.34 Radiative processes such as electron–hole recombination, band-to-band transformations, and transitions between emissive subband stages trigger luminescence in perovskites. Becker and his team prove that the lowest excitonic degree in cesium lead halide perovskites (CsPbX3, with X = Cl, Br, or I) is a strongly emissive triplet condition. Their electronic configuration strongly influences perovskite’s luminescent properties. Both crystal structures and chemical components greatly affect the forbidden bandgap energy.

Ferroelectrics are part of a class of materials known as noncentrosymmetric materials because they lack inversion symmetry. Ferroelectrics are exceptional to this material class. They have at least two stable states, which do not vanish in the unavailability of even an electric field. They have finite electric polarization. The most popular way of measuring electric polarization is polarization reversal, which involves applying an electric field to the transition between two stable non-vanishing states of polarization. The outcome is an electrical hysteresis loop in a ferroelectric material.35–37 The typical research approach for ferroelectric crystalline solids is assisted by the inclusion of cations that are likely to stabilize ferroelectric distortions—concerted displacement in the unit cell of cations and anions leading to a non-fading moment of a dipole. The hybridization of oxygen 2p states and metal states of the A and B cations stabilizes such ferroelectric distortions. Cations with a stereo active lone pair (Pb2+, Bi3+) or a d0 electronic configuration (Ti4+) are prototypical for encouraging these ferroelectric distortions. The Ti 3 d–O 2p hybridization influences the stability of the tetragonal ferroelectric structure with space group symmetry P4mm over the non-ferroelectric cubic structure with Pm-3m space group, according to DFT studies on bulk ferroelectrics such as PbTiO3 and BaTiO3.38 The stereo active lone pair of the Pb 6s highly hybridizes to the O 2p states in PbTiO3, resulting in a non-vanishing dipole and off-centering of the Pb cation.

Even though the ABX3 configuration has especially stringent structural constraints, a wider variety of low-dimensional perovskite aggregates can be studied. This achieves substantial structural tunability by switching from the parent 3D structure based on corner-sharing BX6 octahedrons to isolated zero-dimensional (0-D) BX6 octahedral clusters. The size limitation caused by the tolerance factor of the 3D structure is gradually eliminated as the perovskite structure is cut into thin slices. There is no known limit on the length of the interlayer “A” cation in the two-dimensional (2D) layered derivatives of the perovskite structure. Since the MX6 octahedron is isolated and its relative location can be easily modified, the size restriction does not fully apply to 0D derivatives. This size’s structural stability and tuning ability create a rich and fruitful atmosphere for developing unique crystal structures with various physical properties. Although the length of the interlayer organic cations in the different groups of the 2D perovskite system has no absolute limit, it appears that the thickness limit applies to the area octahedron described by the terminal halides shared from four adjacent corners. This is also important in terms of electron count and hydrogen bonds. Because of the need for charge balance, only a certain number of cations are present. As a result, too-wide organic molecules can close the gap between neighboring organic molecules, preventing this number of cations from being appropriate for the ideal perovskite structure. However, if they are too narrow, the frame can be accommodated by allowing the organic molecules to cut apart far smaller than the inorganic substructure’s region. The bandgap of a compound usually increases as the size of the structure is decreased. The composition (C4H9NH3)2−(CH3NH3)n−1SnnI3n+1, whose parental 3D perovskite (n → ∞)CH3NH3SnI3 is a doped semiconductor with a narrow bandgap, is an excellent example. The compound with n = 1 (C4H9NH3)2SnI4 is a semiconductor with a wide bandgap. This can be explained in a generic context by making the following assumption: By removing the metal “B,” the inorganic structure becomes smaller, increasing the ionic contact between “A” and “X.” Since the electronegativity of the two components differs significantly, the energy bandgap between them is greater. Layered and two-dimensional perovskites are an extensive complex material group, so this is probably one of the primary reasons where more and more people would be interested in them because of their characteristics for various applications. The configuration could be interpreted as a three-dimensional anionic framework with BX6 octahedra connecting corners and the A cation filling the anionic framework’s voids. 2D perovskites, on the other hand, are a massive group of materials with a wide range of compositions and properties. The most widely known categorization for 2D perovskites is dependent on their crystal structure, the inorganic layers’ composition, and, more precisely, their composition structural resemblance to the traditional 3D perovskite form (AIBIIX3). As a consequence, cutting along the crystalline plane provides the best results. 〈100〉, 〈110〉, or 〈111〉 of the 3D perovskite structure yields 2D halide perovskite layers, which leads to the 〈100〉, 〈110〉, and 〈111〉 oriented perovskites being three different 2D perovskite families and three different kinds of 2D layers. The low-dimensional perovskite structure is shown in Fig. 4.

FIG. 4.

The low-dimensional perovskite structure.

FIG. 4.

The low-dimensional perovskite structure.

Close modal

〈100〉 perovskite can be made by systematically extracting the B portion from the inorganic structure in the ABX3 structure. Focused layered perovskite (RNH3) 2An−1BnX3n+1 was originally a flat inorganic layer formed by removing layers along with the matrix structure in the 〈100〉 direction. The 〈100〉-oriented perovskites are generally the most abundant particular group of the perovskite derivative family, owing to the ease with which the interlayer RNH3+ cations can be coordinated in such structures. 〈100〉 perovskites are physical characteristics to both inorganic and organic components, allowing for greater compositional diversity. The interaction among organic cations and inorganic substructures, especially the hydrogen bonding strategy, is particularly important in perovskite derivatives’ configuration. The most common type of 2D halide perovskites is 〈100〉 perovskites, which are also the most extensively utilized solar cells.

The 〈110〉 family A2A mBmX3m+2 m > 1 was generated by cutting the layer all along the 〈110〉 orientation of the 3D perovskite structure. This group is much smaller than the 〈100〉 oriented perovskite, which produces corrugated inorganic sheets using the cubic 3D lattice’s diagonal plane. The iodoformamidinium cation plays a special and significant role in the formation of hydrogen bonds through the 〈110〉-oriented perovskite layer channel in the [NH2C(I)=NH2] 2−(CH3NH3)mSnmI3m+2 sequence.39 When EDBE, or 2,2′- (ethylenedioxy)bis(ethylamine), is incorporated into an EDBEPbBr4 perovskite, for example, hydrogen bond interaction is permitted between the two ether classes in the EDBE cation and the marginal ammonium of another EDBE cation, resulting in a staggered structure in the organic layer and the formation of a 〈110〉-sheet template. The iodine atom of the iodoformamidinium cation completes the 12-fold alignment frame that surrounds most methylammonium cations. The perovskite layer’s thickness increases as m increases, where m–∞ refers to the 3D parental perovskite’s configuration. According to this 〈110〉 orientation family, the perovskite structure’s size can be changed from the 3D matrix structure to the thicker 2D layer in m = 2–4 members and the 1D chain in the entire m = 1 member.

Next, there is the 〈111〉-oriented family. According to the formula A′2Aq−1 BqX3q+3, the members with q > 1 are 2D perovskites, in which A′ and A are interlayer and intralayer organic cations, respectively. (H23-AMP)2PbBr6, which originally belonged to the cubic K2PtCl6-type configuration with such a significant number of recognized members, is the most common q = 1 member of the 〈111〉-oriented perovskites. Since this structure lacks the expanded inorganic framework, there are no strong organic cation size limits for the q = 1 member. The structure would be sufficient to facilitate a wide usage of components as long as the charge balance is maintained. The proportion of the acknowledged q = 1 members, on the other hand, has just a few minor cations. Small organic cations are also contained in the recognized q = 2 groups, such as (NH4)3Sb2I9, (CH3NH3)3Bi2Br9, [NH2(CH3)2]3Sb2Cl9, [NH(CH3)3]3−Sb2Cl9, and (CH3NH3)3Sb2I9.40–43 Cs3Sb2I9 has recently been investigated as a potential Pb-free perovskite for use in photovoltaic applications. The energy bands in this material are relatively distributed about the perovskite layer plane, parallel and perpendicular, which means that layering does not limit their use in 3D photovoltaic devices.44 Despite the device’s initial results of Cs3Sb2I9 lacking, these initial device findings indicated that these layered perovskites with a 〈111〉 orientation could be used in electronic devices.

Many researchers have been drawn to renewable energy in recent decades, which has opened up with the help of scientists who do not neglect the strength of the sun’s rays to be the highest abundant source of energy that can meet consumer’s demand. In engineering sciences, developing an environmentally sustainable power source is still a grueling mission.45–48 On the other hand, photovoltaics (PV) tend to have developed steadily in the last ten years, and a large number of attempts to develop organic photovoltaic have been steadfast (OPV). As one of the world’s clean energy sources, solar energy is commonly recognized as one of the world’s most objectionable solutions, unlike liquid fuels, which have culminated in substantial poisoning and environmental damage. Solar power could be used from multiple perspectives, including the legitimate transfer of sunlight photovoltaic energy production (PV). Photovoltaic power production is expected to account for one-third of the global power generation by 2030. Researchers still intend to launch photovoltaic devices with high performance, relatively cheap cost, and large-scale manufacturing since the last decade, but inevitably they have not yet been accomplished. The sustainable development of photovoltaic devices worldwide needs a good advancement in materials and structures to minimize production costs and enhance energy conversion.48,49 Photovoltaic systems are classified into crystalline silicon first-generation, crystalline silicon second generation, and crystalline silicon third generation. Cadmium indium gallium selenide, amorphous silicon dye-sensitized solar cells, perovskite-type solar cells (PSCs), and organic PV are all used in second-generation PV applications. The technology of the third century has been investigated. Importantly, perovskite solar cells (PSCs) have indeed been unanimously marketed by conventional renewable energy technologies as an economical and ecologically achievable renewable energy technology choice to resolve environmental complexities of electricity production and environmental issues.50–53 PSCs have been extensively explored even among the PV approaches over the last period.

Authorized reported efficiencies of over 25.2% were achieved by PSCs, outpacing early findings. Polysilicon is used in inorganic photovoltaic cells such as cadmium gallium selenide54 and cadmium telluride.55 The unprecedented cumulative efficiency is due to adequate accuracy, bandgaps (almost 1.5 eV),51 higher carrier mobility,29 lower exciton binding energy (35–75 meV),56 large absorption coefficient of hybrid perovskite materials,57 and long charge diffusion lengths.58 Organic–inorganic perovskite with such an ABX3 configuration, in which a is cesium, B is Pb or Sn, and X is Cl, Br, or I, have recently been released as an enigmatic class of semiconductors. (Cs), methylammonium (MA) or different formamidinium (FA) techniques exist. It is possible to process perovskites by spray coating, where a nozzle is used to spray little liquid droplets on substrates, and ultrasonic spraying can also deposit the perovskite layer.59 Dip coating is a unique way of building the meniscus’s edge. The cover plate may be turned over on the deposited substrate after the substrate has been separated from the precursor ink,60 two-step deposition.61 

Organic solar cells, hybrid materials and inorganic semiconductors, and nanostructured solar cells are all examples of solution-processed solar cells, making up the third generation of solar cells. Solar cells all receive sunlight and transform it into electricity. The photovoltaic effect is used by solar cells to convert energy. Photons with higher energy than the bandgap energy (Eg) are consumed in the latter, and electrons are excited from their valence state to the conduction band as a result. Asymmetry is built into solar cells, allowing electrons to enter the external circuit when an electrical potential is applied. During the device being irradiated by light, the current density, which is biased with the variable load with the voltage across the device, is being used to describe solar cells’ performance.62,63 The open-circuit voltage (Voc) and short-circuit current density are proportional to PCE (Jsc),

PCE=JscVocFFPsolar.
(2)

The fill factor and solar incident capability are expressed by FF and Psolar, respectively. The importance of Eg has a major impact on the solar cell’s Jsc. Jsc gradually increases with the increase in all visible and infrared spectral areas, since E is significantly higher because photon energy (E) is inversely proportional to wavelength [E(eV) = 1240 λ/(nm)]. Even if PCE rises with Jsc, it must be balanced against Voc, implying that PCE has the highest Eg value. Specifically, as Eg declines, Voc decreases as well. To achieve the best PCE value for c-Si solar cells, the Eg value should be between 1.0 and 1.6 eV. Shockley and Queisser estimated that a solar cell with an Eg of 1.1 eV has a maximum efficiency of 30%.64 With increasing film thickness, the amount of light absorbed in solar cells decreases exponentially.

As a result, the photoactive layer thickness is proportional to the absorption length (1/α, where α is the absorption coefficient in cm−1) and is an essential factor for regulating the solar cell’s PCE. The latter is described as the distance at which 63% of the incident light (non-reflected light) is absorbed. The photoactive layers of Si solar cells must be hundreds of micrometers to millimeters thick due to their low values. The latter raises the content and manufacturing costs of c-Si solar cells substantially.

Perovskite solar cells have been considered a unique form of the solar cell with a unique working concept. Due to its low exciton binding energy, the light-excited charge acts as Wannier-type excitons. The hole was preventing, and hole transport layers direct electrons’ movement and holes when the electrode produces a built-in low voltage. Selective charge transfer is allowed by the energy shift between the perovskite valence band and the hole transport valence band. Selective charge transfer is allowed by the energy offset between the perovskite valence band and the hole-transporter valence band. Exciton diffusion has also been evaluated. The results are obtained both within the bulk and at interfaces.28,65 A research study using a scanning electron microscope showed the first direct proof that perovskite solar cells act as a p–i–n device. Due to the low exciton binding energies, perovskite solar cells do not need bulk heterojunctions, which is an additional benefit.66 As a result, there are no energy losses relative to the energy level offsets needed for bulk heterojunction solar cells, and the Voc to Eg ratio is extremely high, raising the PCE. The equilibrium between series and shunt resistance is thought to be the key factor limiting perovskite solar cells’ efficiency. A dense HTM layer is necessary to prevent pinhole leakage, but it also raises series resistance.67 As a consequence, increasing the thickness of the HTM is necessary. Voc’s significance in perovskite-type solar cells is affected by deeper defects that act as non-radiative recombination centers. During the testing, it was discovered that perovskite defects have low formation energy and a low trap energy level. This phenomenon can be induced by the heavy pairing of the Pb lone pair of the s orbital and the I p orbital and the high ionicity of CH3NH3PbI3, resulting in a low recombination rate and a higher Voc value.68 As a result, Pb is necessary for Pb-containing perovskite solar cells to perform well. As a result, likely, perovskite solar cells will not achieve high PCE values without Pb.

In the commercial production of photovoltaic technology, powerful PSC may be the best option. PSC products are also divided into two sections for an excellent purpose: regular and inverted (Fig. 5). Mesoscopic and planar structures are the two basic types; mesoscopic structures usually have an n–i–p arrangement: compact ETL/mesoporous, ETL/perovskite/HTL/electrode. In the n–i–p configuration, the perovskite material is usually deposited on opaque substrates filled with a lightweight TiO2 layer and an optional mesoporous TiO2 scaffold layer. The standard configuration includes n-type semiconductors along with TiO2, and the inverted structure incorporates p-type semiconducting polymers including poly (3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (styrene sulfonate) (PEDOT: PSS).69 Perovskite solar cells (PSCs) can be made using two different methods depending on whether the electron transport layer (ETL) or the hole transport layer (HTL) is applied first. The standard (negative-intrinsic-positive, NIP-type) or “inverted” (positive-intrinsic-negative, PIN-type) structure is referred to here. In 2009, the former structure was first developed as an ETL employing titanium oxide (TiO2). A PIN-type product using poly(3,4-ethylene dioxythiophene) mixed with polystyrene sulfonate (PEDOT: PSS) as an organic HTL was launched four years later. Currently, these two structures can achieve high power conversion efficiency (PCE) of more than 22%; however, the NIP-type PSC has been proved in single-junction cell recording efficiency and has been published.70–73 One may also split these two structures into two categories: mesoscopic and planar structures. A mesoporous layer is integrated into the mesoscopic framework, while the planar structure is made up entirely of planar layers. The electron transporting layer and the hole-free transporting layer solar cells made of perovskite were also examined. Researchers have looked into six different perovskite solar cell architectures: the mesoscopic configuration of the NIP, the planar configuration of the NIP, the planar configuration pin, the mesoscopic configuration of the pin, the ETL-free configuration, and the HTL Free configuration.

FIG. 5.

Two general device structures of perovskite solar cells.

FIG. 5.

Two general device structures of perovskite solar cells.

Close modal

Since the first PSCs were made from dye-sensitive solar cells, the mesoporous TIO2 film was used as a scaffold (DSCs). The self-assembled perovskite absorber is used to fill a blank in a porous TiO2 layer made by sintering nanoparticles. Figure 6(a) depicts a typical configuration of this type of perovskite solar cell. Perovskite film was deposited onto mesoporous TiO2, which aids in electron transfer between the perovskite absorber and the FTO electrode in this structure.74,75 Its objective is to facilitate perovskite nanoparticles in gradually removing electrons from the photoexcited pigment, which has taken the place of the molecular sensitizer, primarily as a light harvester. However, the finding is that organic–inorganic halide perovskite would conduct electrons and holes on its own spurred subsequent device evolution. The mesoscopic version uses a semiconductor oxide scaffold with completely infiltrated pores to enable the perovskite to protrude well above the nanostructure and develop a capping layer.76,77 However, the mesoporous thin film needs a high temperature annealing process, which is not compatible with the flexible substrate. To simplify the fabrication process, the planar structure was investigated. The temperature to fabricate the planar metal oxide ETL is usually below 200 °C and do not deteriorate the performance of the perovskite devices. Thus, the planar structure gains enormous attention for further study.

FIG. 6.

(a) Schematic illustration of mesoporous PSCs. (b) Schematic illustration of planar PSCs.

FIG. 6.

(a) Schematic illustration of mesoporous PSCs. (b) Schematic illustration of planar PSCs.

Close modal

In the mesoporous coating, the compact layer is normally added to a fluorine-doped tin oxide layer (FTO), removing electrons and blocking holes. One of three methods is used to apply the TiO2 layer: first, the colloidal dispersion of TiO2 nanoparticles is spin-coated before being thermally treated. Second, titanium precursor solutions are spin-coated and then thermally treated (titanium source: TiCl4, titanium isopropoxide). Third, titanium is deposited through spray pyrolysis [titanium source: titanium diisopropoxide bis(acetylacetonate)18].45 Spin coating, screen printing, magnetron sputtering, or electro-spinning are used to deposit a mesoporous film on the compact layer’s surface. The mesoporous layer’s composition and thickness influence the overall performance. The pore-filling process can be aided by reducing the thickness of the mp-TiO2 layer, resulting in increased efficiency. The mesoporous layer also suppresses hysteresis.45 A subsequent study found that a relatively conductive porous TiO2 layer could be replaced with such an insulating porous Al2O3 layer. Perovskites have more potential than only being used as sensitizers. They can move both electrons and holes between cell terminals, as shown by the efficient use of an insulating Al2O3 scaffold. The following is a standard fabrication technique for mesoporous scaffold-based perovskite solar cells: FTO substrates with a light-blocking TiO2 layer and a mesoporous oxide layer that had gone through the different heat treatment stages were used. After that, a perovskite absorbent coating is added. In N,N-dimethylformamide, MAPbI3 based on pure iodine is normally precipitated from a mixture of PbI2 and MAI, γ-butyrolactone (GBL), or a related solvent system in a one-step procedure. When combined PbCl2 and MAI in a molar ratio of 1:3 in DMF were applied to an insulating mesostructured Al2O3 system, one-step processing was effective in mixed halide composites.78 

It is important to mention that good pore-filling in mesoporous structures is critical for preventing leakage through the device, which has been an issue with dense mesoporous structures in the past. As a result, a thin layer of perovskite is commonly used to enhance light absorption and prevent shunting pathways over mesoporous structures. This deposition method necessitates several important parameters that affect grain size, crystallinity, and subsequent film surface coverage. Solvent development methods, for example, use a limited amount of usable droplets during spin coating. This method was previously used to extract solvent from a perovskite solution while improving grain size, covering mesoporous structure, and protective layer uniformity.79 The use of mesoporous architectures as the basis for the manufacture of perovskite solar cells has improved the device’s performance from 3.8% to over 17% in just a few years.80 Since the mesoporous structure does not concentrate on long carrier diffusion lengths, it can explore new perovskite materials in a forgiving environment. A mesoporous system necessitates a more complex device design and fabrication process, leading to several issues. It has continuously obtained tremendous efficiencies, making its use for laboratory-scale investigations entirely worthwhile. It is important to remember that defining the difference between planar and mesoscopic cell structures is not always simple. Nanocrystalline oxide films are often used as an electron trap coating in “planar” embodiments, and the fluoride tin oxide (FTO) coated glass substrates that protect the perovskite are heavily corrugated themselves.81 In contrast, adding a lightweight perovskite capping layer to PSCs with a mesoscopic scaffold boosts their performance even further.82 

Since such a low-temperature solution method could create the ETL, the p–i–n type arrangement has become even more prominent. Perovskites were suitable for transporting holes when the hole transporting material (HTM) layer was deposited alongside the ETM layer. This has been a significant factor in the production of p–i–n structured solar cells. The fullerene derivative and PEDOT: PSS, respectively, were electron- and hole-selective surfaces. The fabrication process of HTL usually does not need a high-temperature annealing process, thus making them suitable for flexible perovskite optoelectrical devices.83–85 On PET-based conductive substrates, Bolink and colleagues reported 7% efficiency. After 50 bending cycles, this photovoltaic solar design became more robust, with a 0.1% reduction in electrical conversion output. It suggests that PSCs may be well suited to roll-to-roll processing.71,86–88 The standard form PSCs comprise n-type metal oxide preventing layers (BL), both with and without (planar or hetero-structured/mesoporous) metal oxide scaffold (mp-TiO2, Zn2SnO4, ZnO) sensitized with perovskite material, both with and without organometallic halide perovskite over-layer, HTMs such as (2,2,7,7-tetrakis(N,N p-dimethoxyphenylamino)-9,9- spirobifluorene (spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6- trimethylphenyl)amine] (PTAA).89–95 Nevertheless, polymer HTMs such as spiro-OMeTAD and PTAA are endured from lower cost and higher stability. The development of this HTM remedy is the toughest process and high-purity required. Besides that, the organic HTMs restrict the commercial production of PSCs.

HTM free perovskite solar cell is one of the options for air and moisture stability. The task of developing planar p–i–n on a uniform TCO electrode is to produce a pinhole-free perovskite film through the one-step spin-coating process to prevent leakage. Samples from this PbCl2/CH3NH3I solution with a 1:3 molar ratio yield superior results than cells obtained in yet another 1:1 molar formulation of PbI2/CH3NH3I. The latter film of perovskite showed a porous structure and low crystallinity; even after annealing, no scaffold was used at ∼100 °C.96,97 One of the major advantages of mesoporous scaffolds may promote a continuous conformal covering of the absorber’s pores.80 Adding CH3NH3Cl to the PbI2/CH3NH3I solution or NH4Cl enhanced the film morphology and boosted the crystallinity, resulting in improved device performance.98 When using the NH4Cl additive, good morphology and crystallinity were acquired, and PCE was elevated. About <0.1% to 9.9%, the FF has exceeded 80%. These findings indicate that the perovskite layer’s morphology and crystallinity are essential to the efficiency of the device.

Since the exploration of PSCs, a huge effort has been made to manufacture slightly elevated perovskite films with extensive coverage, no pinholes, and high crystallinity using efficient and simple manufacturing technologies. Adjustments to device design, the introduction of new charge carrier materials (ETLs and HTLs), and enhancements to the absorber layer were all part of the roadmap to increased performance. Different approaches for fabricating perovskite solar cell devices have been developed. Manufacturing processes are classified into (A) wet chemistry and (B) vapor processing. Two wet processing methods are the one-step deposition (OSD) and sequential deposition system (SDM). At just about the same time, the vapor-assisted techniques include the VASP, chemical vapor deposition (CVD), thermal vapor deposition (TVD), and microwave irradiation (MIP).

The reliability of the perovskite film seems to be the most important element in determining PSCs’ efficiency. Different techniques for fabricating a higher-quality perovskite film have been discovered to date. The manufacturing methods significantly affect PSC’s efficiency, as do the film’s properties such as morphology, consistency, crystallinity, and purity of the process. According to x-ray diffraction tests, perovskites’ trigger energy (56.6–97.3 kJ mol−1) is much smaller than that of silicone (280–470 kJ mol−1).99 The perovskite’s low activation energy means that many low-temperature methods can be used to create them. Since insufficient film coverage can lead to shunting paths by establishing direct interactions between ETLs and HTLs, complete film coverage of the substrate must be given to the manufacturing technique. In addition, if the active layer is not filled, the incident light will pass without being absorbed.

Spin coating is considered a widely known process for obtaining large areas, high uniformity, and laboratory-scale perovskite films. It paves the way for continued use to achieve robust, high-performance, and reliable perovskite solar modules, with a wide range of applications. This type of deposition technology benefit includes achieving different uniform layers with high layer thickness and strong reproducibility in film formation, which could be achieved by controlling various parameters. The single-step deposition was widely used in perovskite solar cell manufacturing being simple to use and relatively inexpensive. A pinhole-free and good stoichiometry could be achieved in the perovskite film by careful monitoring of the perovskite precursors. The stoichiometric composition of organic halides (CH3NH3X) or lead halides (PbX2) is dissolved in GBL, dimethylformamide (DMF), or DMSO in the nitrogen glovebox to prepare a solution of methylammonium lead (CH3NH3PbX3), as shown in Eq. (3) for the preparation of the solution methylamine lead halide (CH3NH3PbX3),

PbX2+CH3NH3XCH3NH3PbX3.
(3)

The CH3NH3PbX3 film has been deposited on the surface of the TiO2 layer by spin coating the CH3NH3PbX3 precursor.100 First, the precursor was spin-coated and eventually increased to form a dense layer of CH3NH3PbX3 after a few seconds. The perovskite films were rinsed after finishing the coating process. Figure 7 shows a schematic of the one-step spin coating and two-step spin-coating strategy for the perovskite absorber layer. The one-step deposition approach came to prominence due to its one-step easy fabrication and gradual crystallization. The resulting CH3NH3PbX3 films have high-density defects, which limits PSC efficiency. The consistency of perovskite films is determined primarily by deposition conditions, including the temperature of gluing, atmosphere, materials, and thickness of the film, as described in that deposition phase. A one-step deposition method relies on the uniformity of film thickness and morphology control to achieve the necessary film quality. The approach to one-stage deposition also faces the challenge of reducing pinholes in the perovskite film. To overcome all of the above challenges in a one-step deposition, the ASD method is used to track crystal growth and film consistency.79 

FIG. 7.

One-step and two-step spin-coating procedures for CH3NH3PbI3 formation. PbI2 was mixed with CH3NH3I in N,N-dimethylacetamide (DMA), which was spin-coated and heated for one-step coating. For a two-step coating, a PbI2-dissolved N,N-dimethylformamide (DMF) solution was first spin-coated on the substrate and dried, and then a CH3NH3I-dissolved isopropyl alcohol (IPA) solution was spin-coated on the PbI2 coated substrate.

FIG. 7.

One-step and two-step spin-coating procedures for CH3NH3PbI3 formation. PbI2 was mixed with CH3NH3I in N,N-dimethylacetamide (DMA), which was spin-coated and heated for one-step coating. For a two-step coating, a PbI2-dissolved N,N-dimethylformamide (DMF) solution was first spin-coated on the substrate and dried, and then a CH3NH3I-dissolved isopropyl alcohol (IPA) solution was spin-coated on the PbI2 coated substrate.

Close modal

The perovskite solution’s precursor is reduced to a liquid phase in the ASD process in a DMSO and GBL mixed solution. The perovskite was spin-coated on the substratum then. During spin-coating, the low anti-solvent concentration is dropped, and the substratum is cured at 100 °C and culminates in a uniform and smooth, perovskite wide grain film. The enhanced surface morphology of the perovskite film leads to a significant increase in PCE.101 For the preparation of high-quality Pb-based perovskite crystalline films, anti-solvents such as CHCl3, C6H5 (CN), (CH2)3CH2O, C6H5Cl, C6H6, C6H5 (CH3)2, CH3OH, C2H5OH, (CH3)2CHOH, C6H5CH3, CH3CN, and C2H6O2 have been used.102,103 The ASD method greatly modifies the surface morphology of the MAPbI3 film.

During the one-step deposition process of perovskite layer deposition, several voids and pinholes are found. On the other hand, the anti-solvent technique reveals pinholes with huge, tightly packed MAPbI3 crystals due to their smooth and homogeneous surface morphology. PSC’s performance and reliability are significantly improved when the MAPbI3 film seems to be of higher quality and covers the entire surface. The primary objective behind using antisolvent was to minimize the solubility of CH3NH3PbI3 in the DMSO and DMF combined solvent rapidly, allowing for faster nucleation and crystal formation in the film. Yang’s team used a 1:3 molar ratio of PbCl2 and CH3NH3I to make the perovskite layer using a one-step spin-coating process at room temperature. The perovskite film morphology is influenced by all spin-coating parameters, including speed, time, annealing temperature, and solvents that dissolve the precursor.92 The incorporation of good additives has encouraged perovskite crystal growth to improve the morphology, stability, and excitonic and optoelectronic properties of hybrid inorganic–organic perovskite films. Polymers, metal half salts, organic halide salts, polymers, complement, nanoparticles, and inorganic acids are the most commonly used compounds to reinforce thin-film perovskites’ morphology. Su et al. first solved this issue with the introduction of CH3NH3PbI3 polyethylene glycol (PEG) in the manufacturing phase, successfully increasing PCE by 25% with 1 wt. % PEG. Crystals in pristine perovskite film grow and accumulate in the large domain with low coverage (86.44%). In contrast, 1 wt. % PEG for perovskite films decreases the domain’s void size, increasing the coverage range (98.13%).104 Nevertheless, an intriguing aspect is that even if PEG volume is raised, the size and amount of the crystal and void increase.

The one-step deposition method using a PbX2 and MAX mix in a typical solvent triggers uncontrolled perovskite precipitation, resulting in major morphological variations and a diverse variety of photovoltaic output in the resulting devices, limiting their real-world applications. The perovskite film developed with a one-step deposition process has poor surface coverage, showing inevitable uniformity. The two-step solution deposition approach was presented to mitigate this. First, a film was deposited over a nanoporous TiO2 layer by spinning a DMF solution at 70 °C. Second, after reacting with the MAI solution, the layer was transformed into a perovskite layer. In this process, the MAI layer is applied by spinning the solution MAI over the layer PbI2 and dipping the film PbI2 over the MAI solution. The spinning speed and time used to spin-coat the MAI solution over the PbI2 film affect the consistency of the perovskite layer deposited. The dipping time and concentration are important when dipping the MAI layer.105 When the composite film TiO2/PbI2 is plunged into a 2-propanol CH3NH3I solution, the substratum changes the color almost instantly from yellow to dark brown in the formation of CH3NH3PbI3. This process can confine the PbI2 crystallites to a small size, increasing the conversion rate into CH3NH3PbI3.60 The material and optoelectronic properties of the MAPbI3 are affected by the reaction time and solution concentration during the dipping phase. A specific experiment has been done with mp-TiO2 substrates; nevertheless, through mild modification of this technique, the PbI2 film was pre-wetted and dipped in iso-propanol 1 s until it was immersed in the MAI solution. Since two-stage processing is based on the MAI immersion layer’s second creation, perovskite formation is not as complete as a precursor solution. Due to the low temperature (less than 1 min) and the fast mixing time during spin-coating, it was assumed that this was the case that MAI diffusion into the grid was too slow to form perovskite crystals or maybe just enough to form perovskite on the PbI2 surface. Non-stoichiometry would have a detrimental effect on the overall system performance.106 

The two-step spin-coating method makes the crystal size of the perovskite simple to control. The concentration of MAI in the second stage of the spinning cover was observed in this phase to significantly change the size of MAPbI3.107 The size of the MAPbI3 crystals increased as the MAI concentration decreased; for example, a 38 mM solution produced 700 nm-sized crystals, while a 63 mM solution produced 90 nm-sized crystals. Regardless of the MAI concentration, the structure of the MAPbI3 crystal was cuboid. However, apart from morphological control via two-step spin coating with a change in MAI concentration, it is also worth noting that the MAPbI3 size has a major effect on photovoltaic efficiency. Photocurrent density increased as MAPbI3 size increased, and the fill factor typically improved as MAPbI3 size increased. The enhanced photocurrent with increasing size was discovered to be due to better light-harvesting efficiency.

Im et al. investigated the effect of the MAI concentration on the surface morphology and discovered that the grain size decreases as the MAI concentration rises.107 Although this process produces a film better than the one-step deposition method, there are some shortcomings. The correlation between surface ruggedness and grain size is a major downside. The surface ruggedness increases as the grain size increases. A rough perovskite film of greater grain size causes high leakage current and surface recombination loss. A smooth little grain film, on the other hand, has a low carrier lifetime and diffusion length. As a result, a suitable composition must be developed. Another drawback is that the perovskite partially dissolves during the second step. A rough surface with pinholes and voids may be the most direct effect, which could be quickly created during the two-step procedure. This deficiency may be overcome by adding an appropriate chemical or using a low-concentration MAI/FAI mixture to improve perovskite crystal growth conditions.

The vapor-assisted solution (VASP) method can be thought of as a two-step procedure with a change. VASP streamlines the preparation of perovskite films, allowing for high reproducibility and mass manufacturing of devices. Solution processes are an innovative approach for making hybrid perovskite films, and they are based on the basic concept that the substance can form quickly from the interaction between metal halides and organic halide salts in solution. VASP takes full advantage of perovskite materials’ hybrid nature, particularly the low temperature of sublimation of organic halides and high reaction rate between inorganic and organic species. VASP provides a lot of versatility for delicate film growth control with delayed nucleation and rapid reorganizational progress. After further film annealing, vaporized MAI/FAI reacted to PbI2 to formulate the second process’s perovskite phase. In general, this method could ensure greater interaction with the two precursors than the solution.

Moreover, this method effectively prevents partial perovskite dissolution, particularly during the dipping phase. As a result, the stoichiometry of perovskite films may be enhanced. To prevent pinhole forming problems within perovskite films throughout perovskite layer deposition, the Snaith group suggested the VASP (Fig. 8).108 The PbI2 coating is spin-coated over the substrate’s surface instead of using a solvent dipping process. The MAI is then applied via a vapor deposition technique. The VASP depends on MAI’s kinetic reactivity and the thermodynamic stability of perovskite during the growth process. It makes PV applications with a specific grain structure, wide crystal sizes, maximum surface coverage, and restricted surface ruggedness. This approach uses gas–solid crystallization to regulate morphology and grain size. It would be concluded that an innovative two-step approach, vapor-assisted PSC approaches challenging PSC devices and, maybe, a major advancement in the field of heat treatment, where the time needed could be reduced to the same number of single steps/two-step methods.

FIG. 8.

Schematic illustration for obtaining the perovskite layer by the vapor-assisted solution process.

FIG. 8.

Schematic illustration for obtaining the perovskite layer by the vapor-assisted solution process.

Close modal

Inside the perovskite absorber and the corresponding interfaces, perovskite solar cells contain numerous defects that affect device efficiency and stability. Interestingly, comprehensive research has been conducted to establish passive strategies that increase stability for high-efficiency perovskite solar cells. Important research areas in solar cells include state-of-the-art passivation techniques within every perovskite cell layer, which primarily improve carrier extraction, reduce recombination of the carrier, and improve cell stability. The International Union of Pure and Applied Chemistry (IUPAC) says that passivity in architecture and physical chemistry is due to a less weakened or rusted substance by the environment it can use. The application of passivation includes the micro-coating of the outermost surface of a protective material formed by a chemical process with the base material.109 The transformation process from the “active state” to “passive state” creates a passive movie. Passivation explicitly refers to perovskite solar cells’ chemical passivation, reducing the defect trap status to maximize the charge transfer between various interfaces, or physical passivation, which isolates certain functional layers from the outside atmosphere to prevent the system from degrading. PSC stability and efficiency are among the most successful and widespread strategies for surface-passivation with organic materials.110 The basic defect schematic diagram is shown in Fig. 9. The secondary bond among organic molecules and perovskite layers is not good enough to protect the perovskite absorber from degradation caused by defect-attacking oxygen and water. Because of the feasibility of chemical and mechanical passivation, passivation of inorganic materials has increasingly been preferred by investigators. Substances containing lead, alkali metal halides, transition elements, oxides, hydrophobic compounds, etc., have already been extended to the surface and interfacial passivation of PSCs. The nucleation and crystallization mechanism of perovskite absorbers is predominantly stimulated by these inorganic substances by chemical passivation of defects throughout grain boundaries and surface or creating a mechanically protective coating to inhibit humidity oxygen penetration, improving the efficiency and stability of PSCs.

FIG. 9.

The basic understanding of defects and passivators among the perovskite film.

FIG. 9.

The basic understanding of defects and passivators among the perovskite film.

Close modal

In particular, moisture, oxygen, light, and high temperatures have been shown to cause rapid deterioration of perovskite solar cells. Damage of PSCs caused by moisture, oxygen, and light under illumination has all been intensified by the existence of trapped charges. Passivation of extended and point imperfections was also shown to significantly improve the stability of organic–inorganic halide perovskite materials and devices. The humidity response of CH3NH3PbI3 has also been demonstrated to produce a reversibly hydrated product. Any extra moisture could contribute to an irreversible deterioration if the whole film is converted to the hydrated product. In lighting, trapped surface charges and grain limits have been observed to facilitate the growing once evaporated of highly volatile CH3NH2, leading to irreversible humidity degradation of perovskite materials.111–114 To block holes and transport electrons, plenty of perovskite solar cells (PSCs) utilize titanium dioxide (TiO2).

Nevertheless, due to its photocatalytic properties through sunlight, TiO2 has been believed to trigger the perovskite layer’s degradation. Electrons could be produced under UV illumination in the TiO2 conduction band, which contributes to highly oxidative O2, which tends to cause perovskite decomposition.115 Although other electron transport materials (ZnO, SnO2) are undertaken to replace TiO2, several research programs have investigated the passivation of TiO2 by (I) eliminating trap conditions or (II) physically separating TiO2 from the committed perovskite layer to boost oxygen tolerance.116 By plunging the mesoporous TiO2 substrate into a Sr(NO3)2 solution, Lee et al. have shown enhanced CH3NH3PbI3 (MAPbI3) solar cell UV stability.117 An ultrasonic SrO layer in mp-TiO2 is inhibited by a photoactive reaction involving perovskite and TiO2. However, the SrO integrated perovskite solar cells showed a lower efficiency and improved UV stability, with 60% of the original PCE after 100 h of UV exposure. Those with no SrO interlayer preserved just 34% of the initial PCE. In I–V computation, the SrO/TiO2-based solar cells displayed greater hysteresis, perhaps due to the lower conductivity or lower extraction rates of the SrO. A suitable set of passive molecules and improved PCE can also boost moisture stability. An example of this is the benzylamine alteration of the films of formamidinium lead iodide (FAPbI3). In this scenario, the amino groups help link such molecules with two potential possibilities to the Pb–I framework: (i) by coordinating with the ions of Pb+2 or (ii) establishing H bonding with the iodide ions. The benzene rings’ hydrophobic nature contributes to the perovskite layer’s moisture stabilization, while the π-conjugation arrangement facilitates the strengthened charge transfer. Integrating into this treatment significantly improves VOC and JSC, leading to improved PCE from 14.2% to 17.2%, and enhanced the devices’ stability in humid air for more than four months.118,119 

Another unique interface engineering technique that has been illustrated is Lewis base passivation. It is very well recognized that Lewis bases could donate a pair of non-bonding electrons to coordinate with and passivate the under-coordinated vacancies of Pb2+ or I (iodine), creating a Lewis adduct. A Lewis base surface procedure has been confirmed by depositing on top of the perovskite surface a thin film of thiophene and pyridine.120 Pb2+ point defects could be passivated effectively by intense coordinate bonding between the sulfur atom in thiophene and the nitrogen atom in pyridine with under-coordinated Pb2+. A substantial rise in the time-resolved photoluminescence (TRPL) life of the perovskite films and enhanced stability was reported during this Lewis base procedure. The general passivation mechanism is shown in Fig. 10(c).

FIG. 10.

The basic mechanism of passivation of Lewis acid and base. Reproduced with permission from Wang et al., Adv. Mater. Interfaces 8, 2002078 (2021). Copyright 2021 John Wiley and Sons.

FIG. 10.

The basic mechanism of passivation of Lewis acid and base. Reproduced with permission from Wang et al., Adv. Mater. Interfaces 8, 2002078 (2021). Copyright 2021 John Wiley and Sons.

Close modal

Ionic bonding implies a huge transformation between one or more atoms of valence electrons to have an electrostatic attraction between a positively charged cation and a negatively charged anion. Due to the charged existence of perovskite defects, ionic bonding may be used as another special passivation technique. The specific injection of ions in addition to a certain defect with a charge binds to a defect and effectively removes the resulting trap conditions. It is proven potential to boost PSCs’ stability and performance; ionic bonding passivation has recently attracted tremendous interest.

In halide perovskites, the passivating effects of anionic defects in halide perovskites, such as coordinated I, anti-site PbI3, and MA+ vacancy, were studied as cationic substances consisting of metal ions and organic molecules. By ionic bonding and other electrostatic interactions with perovskite materials, metal ions are negatively charged. It proved to be effective passivating agents for the chemical cations and organic cations. When a research group discovered that sodium ions Na+, present on substrates, the first potential to have a passivation effect on metal ions could diffuse over time through grain boundaries. These Na+ enriched grain frontiers resulted in lower defect densities and higher PL lives, which indicates the good clearance of faulty grain boundaries.121 

Zwitterions have both positive and negative charged compounds, a remarkable feature that has demonstrated value in enhancing PSC photovoltaic performance. The ability to passivate negatively and positively charged ionic defects at the very same time is among the most potential technologies of zwitterions.122,123 It was confirmed that choline zwitterions, such as L-α-phosphatidylcholine, choline chloride, In perovskites, L-α-phosphatidylcholines with positive charged ammonium quaternary [−N(CH3)3+] and with negatively charged phosphate (−PO4), choline chloride, and choline iodide, contain both quaternary ammonium positively charged and negative halides. A thin layer of these choline zwitterions on perovskite films completes passivation by spinning. A theoretical study has shown that two normal, deep surface defects, anion Pb–I anti-sites and cationic Pb clusters, would passivate these zwitterions. It was found that choline could transfer around 0.8 electrons in Pb–I anti-site defects to the perovskite layer, thus minimizing the electric trap status due to the defect. The addition of excess halide ions by choline salt can also prevent trapping loads by creating new hybridized states that cross the traps with conducting band frontier states.

Organic and inorganic metal halide perovskite solar cells both have challenges of instability that inhibit their economic potential. There is almost no doubt that somehow the perovskite structure is detrimental to too much moisture and oxygen. However, a significant portion has also been documented to increase photovoltaic efficiency. The position of moisture in or on the perovskite surface is difficult to isolate the possible influencing factors. One aspect that makes it extremely difficult to recognize this phenomenon is a water molecule’s tendency to be an electron acceptor or a chemical donor. Exceptionally high humidity also occurs early in testing; MAPbI3 perovskite films were most vulnerable to perovskite color switches in the atmosphere. Due to exposure to many polar chemical solutions, particularly water, perovskite has proven to decay into PbI2. Hydrophobic large cation molecules are efficient in increasing the moisture stability of perovskites. Still, the insulating nature of these cations with minimum charge transfer leads to comparatively lower performance in some related devices, especially in comparison to 3D perovskites. Significant organic molecules of alky ammonium cations are used as a substrate between the lead halide octahedral planes “slicing” 3D. The perovskite structure has also been found to have an increased tolerance to moisture in layered 2D structures.

Hysteretic photovoltaic characteristics have been seen in PSCs. There is a lot of work involved in overcoming hysteresis by passivating the electron transport layer. Hysteresis can occur due to either (i) macro-interface defects that produce capacitive efficacy over a system; (ii) trapped conditions mainly on the c-TiO2 plate, which can be loaded and emptied at different rates during reverse and forward voltage scans; or (iii) interface band compatibility with scanning directions. Hysteresis of interface defects and ETL trap circumstances can be counterbalanced by defect passivation that prevents charging at the ETL/perovskite interface. The use of chlorine such as CBD or ETL Cl-doping is a standard technique for passivating ETL defects.124,125 Organic materials such as polymers, graphene, or fullerene derivatives were determined for the ETL surface’s passivation. These materials could provide physical barriers that reduce ETL trap states between ETL and perovskite surfaces, improve efficiency, and reduce hysteresis by reducing the charge build-up at perovskite/ETLs. An easy comparison strategy is the addition of phenyl-C61-butyric acid methyl ester (PCBM) or the fullerene/graphene derivative between ETL and perovskite. Substantially, high-performance perovskite devices have strengthened their compositional complexity by adding elements such as Na, K, or Cs of group 1 (alkali metals). Additional layer for trap mitigation on the ETL/perovskite interface. CsBr’s modification of the TiO2 surface strengthens its interaction with the perovskite to rapidly eliminate photo-generated electrons, which results in an exceptionally high cell fill factor of almost 80%. This cell has 1.14 V Voc and 2% PCE with insignificant hysteresis.126 

With their superb optical and electronic properties, such as high optical absorption coefficients, long life of the carrier, high carrier mobility, and low and mid-gap states, PSCs have attracted a huge amount of research in the past decade.15 Due to continuous efforts in material design, process optimization, and technological design, perovskite solar cells (PSCs) have achieved a recognized PCE of over 25% only one decade after their ground-breaking work in 2009.70,127,128 Long-term stability as well as lead toxicity is frequently viewed as two key issues that would need to be resolved before commercial exploitation of PSCs, with impressive PCEs achieved.

Research in the perovskite sector is currently being applied as technological innovations until commercialization is practical; the emphasis is on overcoming many hurdles. Similarly, scientific research’s primary goals are identifying large-scale processing, improving long-term stability, advancement of flexible solar cells, and toxic-free devices. Significant amounts of ionic defects, such as iodide vacancies, uncoordinated lead/halide ions, and clusters that serve as non-radiative recombination centers, would cause uncontrollable decomposition on the surface/interface of the perovskite. The fill factor (FF), open-circuit voltage (Voc), short-circuit current (Jsc), and PCE of PSCs could also be significantly reduced by the low interface trap densities (∼1010 cm−2). Consequently, passivation of the perovskite surface/interface to prohibit defects initiation is crucial to strengthen PSC performance further. Until this point, numerous methodologies have been created to resist PSC deterioration, among which surface passivation may be the best strategy utilized to improve device performance and stability.129,130 

As per the International Union of Pure and Applied Chemistry (IUPAC), passivation, in actual science and designing, alludes to a material turning out to be passive, less influenced, or consumed by the climate wherein it will be utilized.131 Passivation includes using an external layer of a protecting material as a miniature covering, made by the compound response with the base material—the progress cycle from the active state to the passive state by arranging a passivating film. In passive perovskite thin films, an assortment of materials has already been utilized, such as polymers,132,133 small organic molecules,134–136 inorganic nanocrystals,137,138 alkaline halides,126 organic halide salts,139,140 and low-dimensional perovskites.141–143 Among them, polymers with multi-functional groups could render superior passivation effects, and the illustration is shown in Fig. 10(a).

Here, the post-treatment of blended perovskites MAxPbI3 (MA: CH3NH3) is developed to overcome perovskite polycrystalline films’ surface defects for efficient solar cells. Interestingly, instead of the 2D layered PEA2PbI4 perovskite proposed in several initial experiments, the organic halide salt F-PEAI was also used for a 3D perovskite that fills in as a substantially more compelling passivation additive.143–146 In the actual devices, we cautiously control the transformation interaction of F-PEAI and PEA 2PbI4 utilizing direct x-ray diffraction (XRD) confirmation and demonstrate the presence of F-PEAI in the perovskite rather than PEA2PbI4. Interestingly, as a result, a power conversion efficiency (PCE) of 19.48% has been achieved. The basic passivation mechanism is shown in Fig. 10.

In a typical Lewis acid, the fullerene and its derivatives own the property to accept the foreign electron, which was widely used in the electron transport layer (ETL). In the traditional conventional structure, C60 was successfully assembled in the surface of the TiO2 layer to enhance the electron transport and decrease trap states.147,148 Liu and co-workers added C60 into the perovskite precursor to produce a heterojunction that exhibited enhanced efficiency and stability.149 The most representative fullerene derivative PC61BM was also widely investigated. Introducing fullerene derivatives through the anti-solvent process was broadly used.150,151 Shao and co-workers demonstrated that the spin-coated PC61BM on the surface of perovskite could assemble in the grain boundaries to hinder ion migration and enhance the charge extraction efficiency.152 Similar to C60, the concept of heterojunction was realized to passivate the negatively charged (for example, iodide rich) defects, which rendered enlarged grain size and increased stability. Wu et al. demonstrated the graded heterojunction through the antisolvent fabrication process. The significant increase in Voc revealed the reduced recombination loss that attributed to the formation of the graded heterojunction.153 

Superior achievements were conducted by applying fullerene derivatives with various functional groups, such as carboxyl,154 amino,155 or hydroxyl.156 These results show the great potential of fullerene and its derivatives for the application in the field of perovskite photovoltaics. The general passivation mechanism is shown in Fig. 10(b).

In conclusion, the development history of perovskite has been discussed. The tunable perovskite structure renders various multifaction methods to regulate the properties of perovskite. Changing the composition of the concentration could effectively tailor the characteristics of perovskite. The easy tailoring process makes perovskite suitable for various optoelectrical applications. Furthermore, the crystal structure of perovskite is discussed, various orientations of grain are summarized, and different major orientations could render different optoelectrical characters, which could inspire the researcher to control the performance of perovskite solar cells. At the same time, the fabrication process of PSCs is elaborated. Since perovskite devices are compatible with various fabrication processes and the fabrication process could effectively affect the final performance of PSCs, the fabrication process plays an important role for the fabricated perovskite devices. For the actual application of perovskite, figuring out the large-scale fabrication solution and the related high-performance device is a significant challenge for industrialization. In the meantime, a novel and effective passivation strategy was summarized. Although the frangible ionic nature would introduce various defects among the bulk perovskite film and surface, it could render various feasible passivation strategies through the combination of ionic bonds. Perovskite materials show great potential for application in the optoelectrical market; although the efficiency and stability are still behind the commercial products, we believe that through a deeper understanding of the working mechanism and the application of the passivation/encapsulation technique, the optoelectronic devices would finally meet the market requests.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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