Abstract
The search for renewable and sustainable energy for energy security and better environmental protection against hazardous emissions from petro-based fuels has gained significant momentum in the last decade. Towards this end, energy from the sun has proven to be reliable and inexhaustible. Therefore, better light harvesting technologies have to be sought. Herein, the current trends in the development of perovskite solar cells with a focus on device engineering, band alignment, device fabrication with superior light harvesting properties, and numerical simulation of solar cell architectures are critically reviewed. This work will form the basis for future scientist to have a better scientific background on the design of highly efficient solar cell devices, which are cost-effective to fabricate, highly stable, and eco-friendly. This review presents thorough essential information on perovskite solar cell technology and tracks methodically their technological performance overtime. The photovoltaic (PV) technology can help to reduce pollution related to greenhouse gas emissions, criterion pollutant emissions, and emissions from heavy metals and radioactive species by nearly 90%. Following the introduction of highly efficient perovskite solar cell (PSC) technologies, the problems associated with stability, short life-time and lead-based perovskite solar cell configurations have significantly been minimized. The fabrication and simulation of perovskite solar cells has been made possible with advanced technologies and state-of-the-art computational codes. Furthermore, device simulation strategies have lately been used to understand, select appropriate materials, and gain insights into solar cell devices’ physical behavior in order to improve their performances. Numerical simulation softwares such as the 1-dimenional solar cell capacitance simulator (SCAPS-1D), Silvaco ATLAS, and wx-analysis of microelectronic and photonic structures (wxAMPS) used to understand the device engineering of solar cells are critically discussed. Because of the need to produce charge collection selectivity, hole transport materials (HTMs) as well as electron transport materials (ETMs) constitute essential PSC components. In this work, the synthesis of inorganic HTMs, as well as their characteristics and uses in various PSCs comprising mesoporous and planar designs, are explored in detail. It is anticipated that the performance of inorganic HTLs on PSCs would encourage further research which will have a significant influence on the future designs and fabrication of highly efficient solar cells.
1. Introduction
The consistent rise in the advancement of technology, global warming, and enhanced living standards globally is a precursor in the search for clean, secure, and reliable energy resources. Accordingly, traditional fossil fuel energy sources are insufficient to support the sustainable growth of human society and environmental protection [1]. For many years, humans have been using environmentally degrading energy sources which have led to climate changes, global warming, and public health problems. Therefore, rigorous scientific research has gained immense traction in order to identify a sustainable renewable energy. Of the many alternative sources of energy such as biofuels, geothermal energy, wind energy, and nuclear energy, solar energy is one of the most feasible, reliable, and inexhaustible resource. Generally, a solar cell is a device that uses photovoltaic effects or photochemical reactions to transform light energy directly into electrical energy [2]. In recent years, perovskite solar cells have attracted enormous attention because of their remarkable photovoltaic performance. The general formula of PSCs is ABX3 where A is a monovalent cation such as cesium or potassium, B is a divalent cation, and X is a halogen anion. The perovskite materials are regarded as one of the preeminent materials for the next generation of photovoltaic technology because of their distinctive properties, including high electron mobility (800 cm2/Vs), high carrier diffusion length (greater than 1 m), and electrical properties charge transport behavior [3]. High transport excitons are what distinguish organic absorbers from inorganic photoactive materials [4].
The primary drawback towards the development of large-scale production of solar energy generation is its high cost, although in the recent century, the cost of manufacturing conventional silicon-based solar device has decreased from $76.67/W in 1977 to as low as $0.36/W in 2022 [5]. Nevertheless, the cost of fabricating such a solar device is about 10-20 times higher than that of energy obtained through fossil fuels. Perovskite is the promising photovoltaic technology capable of replacing the conventional silicon solar cells and offers robust competition against other emerging solar technologies such as dye-sensitized solar cells [6]. PSCs are an efficient newcomer solar cell technology with enormous commercial alternatives of fulfilling the global energy demands. Unlike the conventional fossil fuels, PSCs convert inexhaustible solar photons in a pollution-free manner [7, 8]. According to Guo et al. [9], PSCs have made an incredible progress in terms of their power conversion efficiencies (PCE), reaching a practical value of over 31.01% in approximately 10 years [5], whereas the conventional silicon solar cells reached 26% after four decades of active research but with serious problems of short-lifetime, instability, high production costs, recombination, reflection, and absorption losses, as well as efficiency problems [10, 11]. On the other hand, a power conversion efficiency (PCE) of 20% has been attained in solution-processed perovskite-based solar cells [12].
The attractive features of PCS include their brilliant photovoltaic performance and cheap processing techniques. PSC scans can easily be synthesized using simple local techniques such as screen printing, spin coating, dual source evaporation techniques, and dip coating [13]. In order to overcome the Shockley-Quisser limit of 35.5% in PSCs, tandem perovskite solar cells have been advanced [14]. Besides, PSCs are environmentally sustainable when converting incident solar energy into electricity. Perovskite solar cells have the potential to contribute to sustainable development goals such as increasing the production of clean energy (SDG 7) and climate action (SDG 13) [15]. The potential use of perovskite materials in solar cells is contributed by device stability and its remarkable efficiency. A mineral calcium titanium oxide, the very first perovskite crystal to be identified, has the same crystal structure as a material known as a perovskite. The chemical formula for perovskite compounds is typically ABX3, where “A” and “B” stand for cations and “X” is an anion which bonds to both of them [16, 17]. Perovskite structures can be created by combining a variety of various components. This compositional versatility allows researchers to design perovskite crystals with a broad range of physical, optical, and electrical properties. Figure 1 shows a representation of a lead-based perovskite.
Today, perovskite crystals can be found in solar cells, memory chips, and ultrasound devices. The basic structure of perovskite solar cells consists of an electron transport layer (ETL) and a hole transport layer (HTL) where the free electrons and holes get injected into. Usually, the anode and cathode in a perovskite solar cell structure are fabricated using fluorine-doped tin oxide (FTO), indium gallium zinc oxide (IGZO) glass materials, and a metal back contact. Currently, p-type or n-type silicon is utilized most frequently for cell architectures, and efficiency is determined either by busbar configuration, junction, or passivation type [19]. Theoretically, contact grids are placed on the rear of the cell rather than the front to avoid shading losses; interdigitated back contact (IBC) solar cells potentially reach better efficiencies. IBC cells, which can achieve efficiencies of 20–22%, use high-purity n-type silicon. Panels using advanced heterojunction (HJT) cells, n-type tunnel oxide passivated interface (TOPcon), and monocrystalline silicon passivated emitter and rear contact (PERC) cells can attain efficiencies in excess of 21% [20]. Tandem silicon-perovskite solar cells have been reported to reach theoretical limits of 43% and have demonstrated efficiency exceeding 30% in testing, although they are still mostly in the development stages and are not yet widely available in the market [21]. Nonetheless, efficiencies can be affected by a number of other variables such as temperature, shade, panel orientation, and irradiance at the installation site. The largest electricity output can be attained and increased by optimizing the overall efficiency of the solar cell installation [22].
2. Lead-Based Perovskite Solar Cells
As a visible light sensitizer, a perovskite structure with a 3.8% photoelectric conversion efficiency (PCE) based on methylammonium lead bromide (CH3NH3PbBr3) and methylammonium lead iodide (CH3NH3PbI3) have been utilized previously [23, 24]. Researchers were drawn to PSCs mostly in the field of photoelectric conversion because of their low cost and simple production procedure. In 2019, the PCE of PSCs rose from 3.8% to as high as 25.2% in 2020 [25]. Lead-based perovskite solar cells appear to have nearly ideal optical and electrical characteristics. Lead-based perovskite solar cells recently outperformed solar cells based on Cu(In,Ga)(S,Se)2, CdTe, and Si, reaching an efficiency of 23.7%. However, there are two drawbacks with Pb-based perovskite solar cells: poor stability and severe toxicity [26]. For each parameter, optimization is carried out to obtain the highest PCE. Due to its exceptional characteristics, including an optimal band gap, a wide absorption spectrum, a good carrier transport system, ease of fabrication on a flexible substrate, a configurable band gap, and a long diffusion length, CH3NH3PbI3 has become a good light harvester [27]. A typical CH3NH3PbI3-based solar cell comprises a p-type (PEDOT: PSS) electrode at the top and n-type (PCBM) electrode at the bottom, as presented in Figure 2 [27, 28]. It has been previously reported that the best-performing planar cell using SnO2 electron transport layer (ETL) has reached an average efficiency of 16.02% obtained from efficiencies measured from both reverse and forward voltage scans, as shown in Figure 2. The outstanding performance of SnO2 ETLs is attributed to the excellent properties of nanocrystalline SnO2 films, such as good antireflection, suitable band edge positions, and high electron mobility [28].
At first, the highest practical PCE of perovskite solar cells based on CH3NH3PbI3 was 3.8% [29, 30], but more recently, the PCE of perovskite solar cells has reached 22.1% due to innovative fabrication procedures, better band alignment, and robust cell architectures [31, 32]. Materials for methylammonium lead halide perovskite are widely available and may be processed through low-cost production approaches. Because of the growing use of photovoltaics, lead has been employed extensively in the solar cell industry, posing serious environmental and occupational health hazards. When solar cell panels, particularly those made of perovskite solar cells, are broken, lead may leak into the environment and contaminate the air, soil, and groundwater [33]. Leaching and movement of toxic elements such as Pb from lead-based perovskite solar cells (PSCs) through water, air, and soil may cause etiological risks to both animals and plants.
2.1. Lead-Free Perovskites
Despite the fact that lead is permitted in solar modules, it would be ideal to find substitutes that preserve the distinct optoelectronic characteristics of lead halide perovskites. Previous studies has proven that less toxic ions like Sn 2+, Bi3+, Ge2+, Sb3+, Mn2+, and Cu2+ could be used as an alternative replacements to Pb2+ in perovskites to engineer new lead-free perovskite solar cells [34, 35]. The introduction of these metal cations not only increases the diversity of perovskite species but also enhances environmentally friendly features of PSCs [35]. For Sn-based PSCs, this type of material has a relatively high absorption coefficient estimated at cm−1, but the oxidation of Sn2+ to Sn4+ is considered the main challenge limiting the advancement of Sn-based PSCs [35]. Numerous approaches have been advanced to prevent the oxidation of Sn in order to enhance its performance, but its chemical stability has become difficult to manage [35]. A variety of non- or low-toxic perovskite materials have been used for the development of environmentally friendly lead-free perovskite solar cells, some of which show excellent optoelectronic properties and device performances [36]. Tin appears to be an essential replacement to lead in order to minimize lead poisoning. Since lead may enter the human body, bind with enzymes, and get stored in soft tissues such as the spleen, kidney, liver, and the brain through blood circulation, lead poisoning eventually manifests as functional abnormalities in the neurological, digestive, and circulatory systems of the victim [37]. Typically, lead poisoning symptoms appear in most people when exposure reaches 0.5 mg/day [38]. A thorough examination of each of the Sn-based perovskites’ fundamental physical characteristics as well as a comparison to those of the Pb-based perovskites shows a close similarity. Therefore, these materials ought to be able to equal the efficiency of the APbI3 systems [39]. Towards this end, all inorganic, lead-free and organic-inorganic hybrid, and lead-free perovskite solar cells have proven reliable and cost-effective.
Double perovskites with the formula have been sought after in addition to the directly similar Sn- and Ge-based perovskites. These 3D materials have wider band gaps of about 2 eV and are typically more stable in air, although they have indirect band gaps, parity-forbidden transitions, 0D electronic dimensionality, large hole/electron effective masses, and indirect band gaps, which results in low exciton nobilities as well as poor carrier transport. Bismuth-based double perovskite solar cells share similarities with Cs2SnI6-based solar cells, but they have not yet attained high and PCE [39]. Previously, the efficiency of tin-based perovskite solar cells decreased largely with increased cell area because of the inhomogeneity of the tin perovskite films formed by a one-step deposition method; however, this setback has been solved by the two-step deposition approach using appropriate solvents in order to enhance uniformity and improve the cell performance [40]. At present, more new lead-free perovskite materials with tunable optical and electrical properties are urgently required to design highly efficient and stable lead-free perovskite solar cells.
2.1.1. Lead-Free Double Perovskite Solar Cells
In the recent past, metal halide perovskites have attracted interest as semiconductor devices that achieve desirable properties for optoelectronic application; however, two major challenges—instability and the toxic nature of Pb—remain unaddressed [41]. For this reason, lead-free double perovskites (LFDPs) are emerging as the preferred photoactive absorbers because of their promising PV properties such as intrinsic chemical stability, in addition to being environmentally friendly [42]. Lead-free halide double perovskite nanocrystals are considered as one of the most promising alternatives to the lead halide perovskite nanocrystals because of their exceptional characteristics of nontoxicity, robust intrinsic thermodynamic stability, and rich and tunable optoelectronic properties [43]. Recently, lead-free double perovskites have redefined photovoltaic research, despite the fact that a detailed study of their optical, excitonic, and transport characteristics is yet to be understood [44, 45]. Studies by Jain et al. have shown that, in comparison to a pristine LFDP, alloyed versions of LFDPs have a longer exciton lifetime, suggesting a lower electron–hole recombination, which also leads to a higher quantum yield and better power conversion efficiency in the alloyed compounds [45]. Inorganic halide double perovskites show good stability because of their inorganic counterions [46]. LFDPs are therefore remarkable photoactive layers for optimal solar harvesting because they exhibit an increase in hole and electron mobilities [45, 47]. Double perovskites have gained importance due to their similar characteristics to lead halide perovskites and have been found to exhibit interesting optical and morphological properties [46]. Nonetheless, LFDPs have been found to suffer various drawbacks; low photoluminescence quantum yields, preparation of double perovskites needs high temperature which causes significant challenges in device fabrication, and tuning the morphology of double perovskite is quite difficult. Despite these obstacles, double perovskites have gained considerable research momentum recently and have established themselves as promising superstar alternatives to lead halide perovskites [46].
Currently, the LFDP structure, Cs2AgBiBr6, is receiving a lot of interest as a light-harvesting device in PSCs [48]. Numerical simulation of the device configuration, FTO/SnO2/Cs2AgBiBr6/P3HT/Au, based on SCAPS-1D gave a low PCE of 1.81% [48]. This was attributed to the unsuitable band alignment of P3HTAu with the perovskite, its poor carrier mobility, and extensive charge recombination [49]. By changing the absorber’s defect density value from cm-3 to cm-3 for the most effective device, the impact of the defect was evaluated with CuSbS2 as the HTL [48]. Furthermore, by optimizing the double-perovskite layer thickness, which was found to be 400 nm, the device’s photovoltaic performance was further enhanced, and this resulted to a PCE of 18.18% while operating at peak efficiency [50]. The optimized was 1.39 V, was 16.04 mA/cm2, and the FF was 78.34%, which shows that the double-perovskite absorber layer (Cs2AgBi0.75Sb0.25Br6) is a suitable candidate for the design of a highly effective Pb-free DPSC [50]. Figure 3 presents an enhanced LFDP solar cell architecture.
Tin is a key component of lead-free PSCs because it has a similar diameter and valence as lead. This leads researchers to replace lead with tin in order to create ASnX3 perovskite films [51]. However, compared to a lead-based perovskite solar cell, the tin-based device’s greatest PCE was just 6.4% [52, 53]. More crucially, the material’s unstable Sn2+ ion is easily converted into Sn4+, which diminishes the photovoltaic performance [54]. In tin-based perovskite, a strong correlation exists between the Lewis base molecules’ molecular hardness and how well they passivate [51]. By triggering charge redistribution and saturating the dangling states while at the same time decreasing the quantities of deep band gap states, it is demonstrated how the level of hardness of the Lewis adsorbate controls the stabilization of the PV device [51]. The first effect is to change the tin vacancy’s (VSn) stubborn spatial distribution. New lead-free perovskites with good intrinsic stability for solar applications are still challenging to design [53].
2.2. Organic-Inorganic Hybrid Solar Cell
Organic and inorganic hybrid lead halide perovskites have successfully emerged as revolutionary optoelectronic semiconductors for use in various device applications. The long-term stability and lead toxicity of hybrid lead halide perovskites have gained traction; therefore, all-inorganic lead-free perovskites have become an alternative perovskite for use in solar cell and optoelectronic applications [55]. Halide perovskites are produced using cheap ingredients that work with very effective deposition techniques previously employed for organic electronics [56, 57]. In order to make use of the low-cost cell manufacture of organic photovoltaics (OPV) and to gain additional benefits from the inorganic component such as tunable absorption spectra, hybrid solar cells incorporate organic, and inorganic components. Organic-inorganic hybrid perovskite solar cells (PSCs) have recently attracted a lot of interest in the photovoltaic community, but recent research has indicated that a missing hydrogen occasioned by poor stability can cause massive energy losses and may therefore be unreliable in the long run [44]. High solar efficiency of more than 25% has been demonstrated by hybrid organic-inorganic perovskite-based solar cells [58]. Accordingly, the presence of organic molecules in the material that contain carbon and hydrogen is essential for obtaining remarkable photovoltaic performance since these molecules are thought to suppress energy-draining “nonradiative recombination” occurrences. Even though hybrid solar cells have the capacity to produce high power conversion efficiencies (PCE), the efficiencies currently attained are modest [59]. Although the power conversion efficiencies now attained by hybrid solar cells are rather low, they have the potential to reach higher PCEs. The electrical structure of the inorganic substance utilized as an electron acceptor in hybrid solar cells, in particular, has a critical role in the device’s performance. An inorganic acceptor has an ideal electrical structural design. Four main categories of materials have been studied: silicon, metal oxide nanoparticle, narrow band gap nanoparticle, and cadmium compounds which previously gave a PCE of 4% [60]. In inorganic-organic hybrid heterojunction solar cells, organometallic halide perovskites have the ability to function both as a hole conductor and as a light harvester. A 15% power conversion efficiency is provided by the sequential deposition as observed in Figure 4 [26]. Whereas organic–inorganic tin halide perovskites have shown good semiconducting characteristic, the instability of tin in its 2+ oxidation state has exhibited an overwhelming challenge [34].
Because of their high power conversion efficiency and lack of emissions, photovoltaics are thought to be a possible solution to the problems facing renewable energy advancement and clean environment. However, two significant limitations for high-performance solar cells are the restricted spectral absorption range and strong recombination events at electrode/electrolyte interfaces in addition to nonradiative and charge transport losses [60, 62]. In order to make use of the low-cost cell manufacture of organic photovoltaics (OPV) and to gain additional benefits from the inorganic component, such as tuneable absorption spectra, solar hybrid cells combine organic and inorganic components. Even though hybrid solar cells have the capacity to produce higher power conversion efficiencies (PCE), the efficiencies currently attained are modest. The electrical structure of the inorganic substance utilized as electron acceptor for hybrid solar cells, in particular, has a critical role in the device’s performance. An inorganic acceptor has an ideal electrical structural design [60]. Investigated material types include silicon, metal oxide nanoparticles, narrow band gap nanoparticles, and cadmium compounds. The state-of-the-art at the moment is cadmium sulphide (CdS) quantum dots which provide a practical PCE of more than 4% [60, 63].
2.3. Tandem Perovskite Solar Cells
Perovskite-based tandem solar cells (TSCs) are an emerging PV technology with the potential to surpass the S–Q theoretical limit of efficiency of single-junction silicon solar cells, which have the capacity to achieve efficiencies of approximately 45% through complete optimization of the optical and electrical parameters [64]. Through the use of tandem strategy, high efficiency of up to 29% has been achieved [65]. Silicon solar cells have a theoretical bandgap of 1.2 eV, and this implies that the PCE is 32% [66]. Both physically stacking 4-terminal (4-T) and monolithically series integration 2-terminal (2-T) subcells can be used to create perovskite-perovskite tandems [67]. A 2-T arrangement necessitates accurate bandgap matching due to the requirement for current matching, but a 4-T arrangement is mostly dependent on the effectiveness of individual subcells and somewhat resistant to band gap (Eg) identification. However, the 2-T arrangement is favored over the 4-T because it has less parasitic absorption, more practical, and has superior economic characteristics [68]. With regard to hybrid perovskite, a 2-T structure with the ideal Eg ratio of 1.2 and 1.8 eV can theoretically provide a PCE of about 36% [69]. Therefore, improving 2-T perovskite tandems seems to be a fascinating scientific challenge that will encourage the creation of additional hybrid perovskite and reduce the cost of PV to encourage technology transfer [70]. Examples of perovskite-perovskite tandem device is presented in Figure 5. The power conversion efficiency (PCE) of perovskite/perovskite tandem solar cells has surpassed that of single-junction perovskite solar cells [71].
The highest-quality perovskite solar cell may achieve a PCE of more than 31%. Researchers can produce perovskite with a close band gap to the ideal one by manipulating the chemical composition of the perovskite crystal [70]. Engineering multilayered perovskite solar cells, where the layers should have varied band gaps, is another way to approach the optimum band gap [65]. Due to the presence of many layers, most of the sun’s total power is converted to electricity when low-energy photons stimulate electrons in layers with a smaller band gap and high-energy photons stimulate electrons in layers with a wider band gap [72]. The conversion efficiency of 26% has been attained by using multiple junction perovskite solar cells with different band gap sizes [73]. Moreover, in a “tandem cell” arrangement with silicon cells, a tunable layer of perovskite can be added to collect photons, thus improving the power conversion efficiency [74].
2.4. All-Inorganic Perovskite Solar Cells
Due to their exceptional thermal and environmental stability, all-inorganic perovskite materials have been rapidly made by employing pure inorganic cations that can substitute the A-site organic cations in the ABX3 structure. All-inorganic perovskite solar cells (I-PSCs) have currently reached efficiency levels of 19% and have a wide range of potential applications [75]. Perovskites made entirely of inorganic CsPbI3 have a lot of potential for use in tandem solar cells and other photovoltaic combinations. Nonetheless, CsPbI3 perovskite solar cells (PSCs) continue to face a lot of obstacles which cause them to have a lower PCE as compared to the organic-inorganic PSCs counterparts [76]. Figure 6 shows an example of an all-inorganic PSC architecture.
Due to their exceptional qualities, such as a tunable bandgap, remarkable defect tolerance, prolonged exciton diffusion range, high carrier mobility, and absorption coefficient, organometallic lead halide perovskites are potential material for solar cells [77]. Organometallic lead halide PSCs have so far shown outstanding power conversion efficiency, reaching up to 25.2% [77]. However, their operational lifetimes are constrained as a result of the organic components’ susceptibility to environmental degradation. Consequently, scientists are particularly interested in all-inorganic perovskite, particularly cesium lead triiodide (CsPbI3), which has higher chemical and compositional stability. However, the major drawback to these materials is its phase instability in the black phase and the poisonous nature of lead [77].
As a brief overview, the scientific community has given the hybrid organometallic trihalide perovskite (CH3NH3SnI3) significant attention since it was reported in the literature [78, 79]. It has excellent photoelectric properties and is simple to process in solutions. It has a direct band gap of 1.55 eV, a weak binding energy of approximately 0.03 eV, an absorption coefficient greater than 104 cm-1, and a small difference between the band gap potential and the device’s open-circuit voltage (Voc) [80]. Using the iodide HC(NH2)2- (SnI3:FASnI3)-based PSC, the effect on defect density, layer thickness, and doping concentration has been investigated using the 1-dimensional solar cell capacitance simulator (SCAPS-1D) numerical code and found that the , , and , and an optimal PCE of 14.03% was achieved [81].
Lakhdar and Hima [82] reported the highest PCE of 18.16% based on CH3NH3PbI3 solar cell and 9.56% with CH3NH3SnI3 solar cell using Silvaco ATLAS simulation software [83]. Hima et al. [84] investigated the impact of the absorber layer thickness, charge mobility, and defect density on planar PSC with an efficiency of over 20% using analysis of microelectronic and photonic structures (AMPS-1D) [83]. Perovskite solar cells made of sheets of gallium-doped zinc oxide (GZO) have been designed, and a maximum theoretical PCE of 21.24% has been obtained [85, 86]. This is attributed to better carrier concentration of the photogenerated excitons.
2.5. Quantum Dot Solar Cells
A promising low-cost alternative to the current photovoltaic technologies such as crystalline silicon and thin inorganic films is the quantum-dot-sensitized solar cells (QDSCs). Quantum dots (QDs) could be made using low-cost techniques, and their size can be adjusted to customize their absorption spectrum [87]. To fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance, conventional dye-sensitized solar cells (DSCs) are used as a source of nanostructures exhibiting high microscopic surface area, redox electrolytes, and solid-state hole conductors [88]. A size-dependent absorption spectrum is produced by the quantum confinement of the exciton in the absorber material, which is a frequent characteristic of QD-based solar cells [89]. A wide-bandgap material’s nanostructure that has been sensitized with a QD monolayer serves as the foundation of QDSCs [90, 91]. Theoretically, the QDSC can achieve a theoretical PCE of up to 66% due to the occurrence of a unique phenomenon called multiexciton production [92]. This makes QDSC a potential third-generation solar cell multiexciton generation (MEG) [92]. The experimental values of PCE for QDSCs are quite low compared to what is theoretically predicted. Hybridization of electron-hole pairs in the quasi-neutral zone is one possible cause of the observed low PCE. The proper choice of HTL and a suitable ETL is critical in reducing recombination losses and increasing QDSC efficiency [92, 93]. When, for instance, lead sulfide (PbS-TBAI) coated with tetrabutylammonium iodide is employed as the active layer, with tungsten trioxide (WO3) is used as the ETL, a PCE of 15.51% is achieved [92].
The lowest constrained states of QDs will establish the effective band gap for absorption; for instance, the phenomenon of resonant tunneling can be used to increase the internal quantum efficiency for the gathering of charge carriers photoexcited in the QD [94]. A well-known stacking method in the Stranski-Krastanov growth mode can be used to create high-density QD arrays [95]. The vertical alignment of QDs is brought about by strain fields from the bottom QD layer that have extended into the barrier material. Electronic states can take on a wire-like appearance because of the strong vertical coupling among QDs. As a consequence, channeling the electrons and holes through the coupling between aligned QDs can result in elevated internal quantum efficiency for the collection of carriers photoexcited in the QDs. This phenomenon makes it possible to efficiently separate and inject the generated holes and electrons in QDs into the nearby and regions [96].
One can adjust the size and form of the InAs islands and subsequently the quantized levels of energy in order to control the absorption of the light spectrum by adjusting the deposition mode, the thickness of the intermediate layer, and the number of times the island layer is repeated. Figure 7 is a typical example of a quantum dot solar cell design. An essential approach to enhance the oxidation resistance of PbX QDs is through passivation of their surface via halides, a process which relies on the binding of halide ions to the Pb atoms on the surface and accordingly reduces the number of sites where O2 can adsorb [98].
2.6. Lead Colloidal Quantum Dot Solar Cells
One of the most desirable QDs for creating innovative optoelectronic devices like solar cells, photodetectors, and biological labels is the lead sulphide (PbS) colloidal quantum dot solar cell [99]. PbS colloidal QDs have received significant interest as promising building blocks for optoelectronic devices because of their size-dependent band gap and tunability of electronic properties by means of surface chemistry and solution processability [100]. Finding a suitable approach to obtain high-quality QDs in a broad size range using less expensive, toxic-free, and eco-friendly precursors is challenging. Due to their easy solution processing, low material cost, long-term air stability, and potential for customizing their optoelectronic properties by adjusting size, composition, and surface chemistry, PbS colloidal QDs are appealing materials for the next-generation photovoltaic devices [101]. However, after solidification, the lengthy aliphatic ligands that commonly surround PbS QDs in solution function as barriers to charge transfer and transport between nearby QDs [102]. The ligand-exchange method, which is employed to remove such lengthy ligands, can produce a variety of surface traps including vacancies and dangling bonds. Recombination will significantly lower the performance of the device. Nonetheless, surface passivation techniques have been developed, and PbS quantum dot photovoltaics (QDPVs) have witnessed a considerable increase in PCE of over 10% [103]. However, the achieved PCE is still much lower than what is anticipated, and surface traps continue to be a major limiting factor for PbS QDPVs. The primary goals of the work on Schottky solar cells’ performance improvement are to enhance metal species, create ligand strategies, and expand responses into the infrared spectrum [104]. In Schottky solar cells, ternary PbSxSe1x CQDs were used, and they performed better than PbS or PbSe CQDs. Choi et al. modified the PbS CQD/metal contact by adding an incredibly thin oxidized contact layer in order to enhance the Schottky barrier’s quality and the functionality of the device [104]. A PbS CQD sheet was sandwiched between a material with a low work function and a high work function metal anode in an inverted Schottky CQD solar cell, and the best device was produced with a PCE of 3.8% and a record of 0.75 V [105, 106]. An example of a lead-based colloidal quantum dot is presented in Figure 8.
3. Stability of All-Inorganic Perovskite Solar Cells
Due to their outstanding compatibilities with tandem devices, high carrier mobility, and strong thermal stability, all-inorganic perovskite solar cells (PSCs) have received a lot of interest [108]. The power conversion efficiency with all PSCs has exceeded 19% because of extensive research of these devices and ongoing process development. The manufacture for long-term application of PSCs in the air environment, however, still presents significant difficulties due to the comparatively poor phase stability. Many researchers have suggested numerous approaches such as additive engineering, interface engineering, and the construction of all-inorganic perovskite quantum dot solar cells, to enhance the long-term stability of all-inorganic perovskite solar cells [109].
Inorganic cations like Cs+, Rb+, Sn+, and K+ have thus been suggested to be employed to make an all-inorganic perovskite, CsPbX3 (), in order to address the challenges resulting from environmental deterioration [110]. Unlike organic-inorganic hybrid solar cells, this cation substitution attempts to increase the chemical and thermal stability. Currently, CsPbI3, CsPbI2Br, CsPbIBr2, and CsPbBr3 are the most widely used inorganic perovskite solar cells. A good choice of solar energy harvesting is the CsPbI3 material, which has a significantly small band gap (). Among inorganic solar cells, cesium lead halide perovskite solar cells display the highest efficiency [77]. To increase phase stability, adding some bromide ions to Cs-perovskite instead of iodide ions could result in CsPbBr3. However, this material’s high band gap () restricts light harvesting, which consequently lowers the efficiency of the cells [77].
In comparison to hybrid solar cells, inorganic perovskite solar cells are reported to be more stable [77]. In fact, some researchers have noted the remarkable stability of inorganic solar cells throughout over time with or without encapsulation. Inorganic perovskite solar cells, for instance, were noted to be good substitutes to the stability problem of hybrid PSCs, particularly to the moisture instability caused by the material’s high-hygroscopicity characteristics. However, it has been shown that the most widely used inorganic lead halide perovskite, CsPbI3, suffers from severe phase instability problems in ambient air [77, 111]. Even without encapsulation, the all-inorganic PSCs exhibit minimal performance degradation in humid air (90–95% relative humidity, at 25 °C) for more than 3 months (2640 hours) and can withstand extremely low temperatures of up to –22°C [112].
3.1. Perovskite Efficiencies and Band Gap Characteristics
One or more electrons are always present around the nucleus of every atom, and negatively charged electrons are drawn to positively charged nuclei. The number of electrons that each atom has determines how many atoms can form a molecule. Its shared electrons float about the molecule. The outer electrons of the atom for which they orbit are considered to be in its “valence band” [113]. When photons of light “bump” the outer electron of a semiconductor material to a high energy state, they force electrons away from the valence band to the conduction band of a molecule, thus producing an electric current. The band gap is defined as the smallest amount of energy required to move each electron from the valence band to the conduction band [114]. An electron becomes a charge carrier which flows through the material it is a part of, when it moves to the conduction band and is no longer tethered to the orbit of the molecule. For this reason, it can be employed in photovoltaic cells to transport electrical energy.
Different colors of light photons carry varying amounts of energy, which is measured in electron volts (eV). Visible light photons have energies ranging from 1.75 eV (intense red) to 3.1 eV (violet). At 1.34 eV, the bandgap of an ideal photovoltaic material, maximum visible light should transform electrons to charge carriers [114]. Power conversion efficiency measures how much solar energy can be transformed into electricity by a solar cell (PCE). It has been noted that among layers of positively and negatively modified materials with an optimal bandgap, better efficiencies are achieveable [115]. The S-Q limit is the name given to this optimal efficiency [116]. The implication of the S-Q limit is that there is no material with the ideal bandgap that can approach the S-Q limit. By absorbing light over a wider spectrum of wavelengths, multijunction (tandem) solar cells (TSCs), which are made up of several light absorbers with noticeably different band gaps, have a high potential to surpass the S-Q efficiency limit of a single junction solar cell. Due to their customizable band gaps, high PCE up to 25.2%, and simple manufacture, PSCs make excellent candidates for TSCs. Narrow band gap PSCs, dye-sensitized, organic, and quantum dot solar cells are just a few of the numerous solar cell types that can easily be combined with PSCs, resulting in high PCEs - commonly made using a low-temperature solution approach [117].
Crystalline silicon-based technologies nowadays control the PV market because of their low manufacturing costs and strong material and manufacturing process reliability [118]. A remarkable efficiency of 22.2% has been achieved, with typical module efficiencies of around 17–18% [119]. Research cell efficiency stands at over 25%, whereas the theoretical conversion efficiency limit for a silicon solar cell currently approaches 33% at 25°C [120]. Therefore, given the theoretical performance limit of 29.4% with crystalline silicon-based solar cells, only slight performance enhancements are still feasible [119]. This limit is established by taking into account auger recombination and intrinsic losses, including but not limited to the thermalization losses of high-energy photons and the clarity of the absorber layer for subbandgap photons [121]. Perovskite and silicon-based solar cells can be successfully combined to create tandem solar devices that are promising to exceed the single-junction silicon devices currently dominating the photovoltaic market.
3.2. Recent Progress in Chemical Stability of Perovskite Solar Cells
Perovskite solar cells (PSCs) have recently improved their record efficiency from 9.7% to 20.1% [122]. However, there has not been much research done on the stability problems associated with these solar cells, which has limited their outdoor use. To obtain strong repeatability and extended durations for PSCs with high conversion efficiency, the problems of perovskite degradation as well as the stability of PSC devices need to be rapidly addressed. Exciting developments cannot be translated from the lab to industry and outside applications without studies on stability. Further investigation into the device’s susceptibility to high temperatures has revealed that an increase from 300 to 375 K lowers PCE from 31.01% to 27.84% (for 4-T) and 18.56-16.14% (for 2-T) [5].
Atmospheric oxygen and moisture can directly impact the device stability of the components during assembly and testing. First, because CH3NH3PbI3 is sensitive to moisture, the molecule tends to hydrolyze, which results in perovskite degradation. The following reactions occur during device degradation.
Accordingly, UV light, moisture, and oxygen are very important factors for the degradation of perovskite solar cells [123]. Furthermore, the equilibrium of reaction (1b) causes CH3NH3I, CH3NH2, and HI to coexist in the perovskite thin film. There are two main ways that HI can deteriorate in the next step [124]. One involves the redox reaction with oxygen present (1c). Another involves the photochemical process, which occurs when UV radiation causes HI to break down into H2 and I2 (1d) [125, 126]. The progression of the entire degradation process is fueled by the consumption of HI in accordance with reactions (1c) and (1d). Considering oxygen and moisture can both cause organic-inorganic halide perovskite to degrade, the majority of the fabrication process must be done in a glove box filled with inert gases such as helium or argon [127]. But when the constructed devices are measured in natural lighting, organic-inorganic halide perovskite generally experiences considerable deterioration. Perovskite deterioration would result in an unintended efficiency reduction, which limits the use of PSC outdoors [128]. Numerous studies have explored how oxygen and moisture impact the overall stability of perovskite solar cells.
3.3. Perovskite Solar Cell Configuration
Perovskite solar cells offer a wider range of device structure than any other PV technology community. PSC devices are categorized according to their structure, morphology (mesoporous or meso-superstructure), and conductivity type (n- or p-type) (n-i-p or p-i-n and p-n or n-p devices) [129]. The perovskite structure is divided into two main categories: (i) normal/regular p-n types and (ii) planar, either n-i-p (conventional planar) or p-i-n (inverted planar), based on the electrical characteristics of the device whether electrons or holes have been collected at the bottom transparent conducting oxide (TCO). The two components of the traditional planar-designed perovskite device are (a) mesoporous and (b) meso-superstructure, as can be observed in Figure 9.
By using doped charge transport layers inside this device design, it was possible to fabricate p-i-n and n-i-p PSCs with a higher PCE of 20.3% [131, 132]. In both planar and mesoscopic heterojunction arrangement, HTM-free-based PSCs exhibit enhanced performance from 5.5% to 12.8% because of enhanced charge extraction [133]. Additionally, these developments were made possible by conducting interfacial engineering, optimizing the thickness of the transporting and perovskite layers, and introducing new scaffold layers [134]. The performance of PSC devices has not yet been fully explored with regard to variety of topologies, fabrication techniques, perovskite compositions, and charge-selective layers that have been proposed. The most effective absorbers contain lead, although various PSC architectures still have stability problems (ca. 13 mg m-2). PSCs are also susceptible to elevated temperatures, UV radiation, moisture, and oxygen, which is a barrier to their commercialization [135].
3.4. Solar Device Performance Enhancement
The percent of solar energy which a PV cell can translate into usable power is referred to as power conversion efficiency. To compete against fossil fuel energy sources on price, solar module manufacturers have to increase efficiency rates [136]. The current theoretical maximum efficiency of solar modules is approximately 33% [137]. Plasmonic nanoparticles are one of the best approaches to increase efficiency of carbon-based perovskite cells [138, 139]. Also, metal nanoparticles diffract light when subjected to solar radiation, boosting its photocurrent within the cell and speeding up the production of free carriers [140]. It has been found that nanoparticles having scattering capabilities are equally effective than upconversion materials at increasing perovskite cells’ efficiency by 1%. Changing the nanoparticle’s size, shape, and distribution could result in even greater efficiency rates [141].
To boost the efficiency of PSCs, better light management can be used to reduce light loss from the cell. Utilizing transparent conducting oxide layers to reduce absorption losses and silicon oxide layers to harvest more sunlight is one approach to enhance solar cell efficiency [142, 143]. Surface gratings can improve internal reflection and increase internal reflection on a cell’s surface thus increasing the cell’s electrical and optical properties. The majority of solar cells can provide optimal efficiencies above 20% [140]. Accordingly, the efficiency of solar panels is influenced by cell efficiency, cell arrangement and design, and panel surface area [144].
The primary objective is to improve the device’s performance while using an inorganic material such as KSnBr3 as the photoactive layer. The device’s performance is specifically compared to those of analogous devices that are theoretically and empirically designed. It was found that the device performed worse when the absorber contact had a larger fault density and the cell’s optimal operating conditions were high at low temperatures. The following photovoltaic properties were obtained with the solar cell operating under ideal conditions: power conversion efficiency (PCE) of 19.5%, short circuit current density () of 25.55 mA cm-2, open-circuit voltage () of 5.32 V, and fill factor () of 14.68% [145]. Compared to similar solar cell technologies that have been computationally or practically studied, this PCE value is larger. Although KGeBr3 has appealing qualities, it is not frequently used because of high initial cost, intermittent energy source, use of a lot of space, small amount of pollution during manufacture, transport, and installation [145, 146]. The effects of changing a number of parameters on the general performance of the solar cell were carefully explored in this model. These variables include the operational temperature, back-contact metal work function, and the thickness of hole transport and electron transport layers. In addition to the hole transport and electron transport layers, doping densities and the density of the absorber layer’s defects were considered. The simulation findings demonstrated that by adjusting the thickness of the absorber and its defect density as well as the width and doping densities of the hole transport and electron transport layers, the device’s performance could be improved. It was found that the device performed worse when the absorber contact had a larger fault density. Nickel, platinum, and Pt metallic back contacts produced outcomes comparable to those of gold. As a result of their relative affordability, these metals can be used as substitutes for gold.
Similar to every other semiconductor device, solar cells are temperature-sensitive (Table 1). Temperature increases affect the majority of a semiconductor’s material’s properties since a semiconductor’s band gap narrows [148, 149]. It is possible to interpret a semiconductor’s band gap decreasing with temperature as an elevation of the material’s electrons’ energy. As a consequence, an increase in temperature results in a narrowing of the band gap [150]. The component primarily affected by an increase in temperature inside a solar cell is the open-circuit voltage.
4. Hole Transport and Electron Transport Materials
A critical component of perovskite solar cells is the hole transport material (HTM). Inverted (p-i-n) PSCs frequently use PSS, an organic HTM. Because of their inherent chemical stability and high cost, inorganic hole transport materials (HTMs) are the preferred options for selective contact materials. Inorganic HTMs with the appropriate characteristics such as the right energy level and high carrier mobility can enhance charge transport as well as increase PSC stability at ambient temperatures, and are cheap to fabricate [70]. While being unstable, costly, and acidic, PEDOT: PSS has the potential to harm the absorber [151]. Due to its qualities including low cost, simple synthesis, and high hole mobility, copper zinc tin sulphide (CZTS), an inorganic semiconductor, can be used as a HTM [152]. There are three major layers in a planar heterojunction perovskite-based solar cell which are sandwiched between the two conducting electrodes. Planar heterojunction perovskite-based solar cells typically have the architecture—hole transport material (HTM), back electrode, and electron transport material (ETM). Recent research has shown that improving the conductivity of hole transport materials through doping and increasing charge collection by altering the absorber thickness could have a positive influence on the efficiency of planar heterojunction-based solar cells [153]. Perovskite-based solar cells also require materials that can transport electrons, which is a critical component [154]. It is also possible to quantitatively simulate how alternative electron-carrying materials would affect the final performance of the PV device. Several PV factors, including absorber thickness and numerous PV characteristics such as the thicknesses of the absorber, HTM, and ETM, can be improved using simulation techniques and then put into practice by experimenters [153, 155]. The acceptor concentration and hole mobility of HTM, trap density, and back contact metal’s work function have demonstrated a considerable impact on the device’s performance [70]. Even with these significant advantages, it is still essential to improve the hole mobility as well as the conductivity of HTM, the stability of perovskite as well as the ETM, and the displacement of poisonous lead [156]. By appropriately synthesizing the perovskite absorbers, optimizing selective contact design, and increasing HTM and ETM conductivity, the device’s performance and stability will be enhanced [153, 155]
From Table 2, highly efficient PSCs are obtained by using hole transporting materials (HTMs) which are necessary for removing and moving holes from the perovskite layer towards the electrodes. An effective HTM for a device with long-term stability is being developed using dopant-free HTMs in PSC devices with promising robust operational features [157]. Particularly, the various perovskite materials that distinguish HTMs based on their molecular or device architectures are used to classify perovskite solar cells. Figure 10 shows the various inorganic HTM materials usually applied in PSCs.
The primary purpose of HTMs in solar cell device is to gather and transport excitons after the light harvester injects holes, effectively separating the electrons and holes, which is a significant and critical component of the PSCs [159]. An excellent HTM material has superior photochemical and thermal stability, long-term stability in air, acceptable energy levels that correspond with the perovskite layer, and intrinsically high hole mobility [160].
There are two types of HTMs for PSCs—organic and inorganic semiconductors. Inorganic HTMs such as CuSCN, CuS, CuInS2, and CuI can have simpler, cheaper ingredients than organic HTMs, for instance, 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61(PCBM) and Spiro-OMeTAD, although they often need to be deposited using more expensive non–solution-based methods such as sputtering, atomic layer, or pulsed laser deposition methods [161]. Since they do not need any dopants or additives and generally have greater intrinsic hole mobility than inorganic HTMs, they have better long-term stability. The main disadvantage, however, is that there are not as many options available as there are for inorganic HTMs. Most inorganic HTM devices have lower efficiency than organic-based HTMs [161]. Small molecules, polymers, and organometallic compounds are the three primary categories of organic semiconducting materials used for hole transport layers of PSCs [161].
The adjustment of the essential device design parameters is done to increase cell power efficiency. In perovskite solar cells, HTMs are used to collect and transport holes that have formed within the perovskite absorber layer to the perovskite/HTM interface. Charge extraction through ETMs is very essential. For high-performance PSC devices, HTMs are necessary. Despite being efficient, these materials need a significant amount of doping with additives (for instance, nanoparticle, graphene, and ZnO, polyaniline) to increase charge mobility and substrate compatibility, which causes stability problems with HTMs as well as increased costs and experimental complexity [161]. Due to its excellent thermal stability, low cost, and appropriate energy level, TiO2 has been widely employed as the best ETM for PSC [160]. Most recently, however, the use of zinc oxysulphide (ZnOS) has been proposed as a better electron transport material because of its stability against sulphurization, low toxicity, and robust tunable band gap [162]. A band gap is the distance between the valence band of electrons and the conduction band [163]. There are two main configurations of perovskite solar cells which include n-i-p and p-i-n configurations each of which is inverted relative to the other resulting in planar solar cells with minimal hysteresis and high efficiencies of 16.5% and 20%, respectively [164]. Examples of n-i-p and p-i-n solar cell architectures are presented in Figure 11.
(a)
(b)
To achieve excellent performance, planar perovskite solar cells with a p-i-n structure employ both hole-transport layers and electron-transport layers to enhance the collection of the photo-generated holes and electrons [166]. On the other hand, mesoscopic solar cell is third-generation solar technology. The ability to fabricate the absorber layer using a solution-based method is a noteworthy feature of mesoscopic solar cells. An example of a mesoscopic-based perovskite solar cell is presented in Figure 12.
(a)
(b)
4.1. Effects of Doping on ETM and HTM
Hole and electron transport materials are essential in the transport of excitons. Organic and inorganic p-type semiconductor materials have been frequently used for perovskite solar cell architectures [133]. The electrical conductivity of the layers within the solar cell design is determined by the doping of the photoactive material which will have an impact on the device’s functionality. The overall performance of a device is enhanced by proper doping of HTM and ETM layers, which increases the interface electric field [168, 169]. There are two ways to dope HTM and ETM. The first is by using minority carriers that will significantly lower fill factor and efficiency and change the - curve’s shape to an [170]. Another relates to the majority of carriers which results in an increase in efficiency and fill factor. A better carrier transport and appropriate energy position can indeed be attained with mild doping doses [171]. Deep faults can be avoided by doping optimization and self-doping processes. To understand the doping effect of HTM and ETM on the device performance, the doping levels are varied from 1014 cm-3 to 1019 cm-3 [172]. Figure 13 represents the changes in the PV parameters with the defect density of HTM and ETM [27].
4.2. HTM and ETM Free Perovskite Solar Cells
HTM and ETM are necessary to obtain efficient photovoltaic performance; however, their effect in obtaining efficient solar cell performance is not equal [163]. This means that, upon the removal of HTM, there is direct contact between the photoactive layer and the glass such as indium tin oxide (ITO), which inhibits hole injection and consequently reduces cell performance. An excellent HTM should have high hole mobility, low electron affinity, compatible HOMO and LUMO energy levels with perovskite, high thermal stability and should involve low costs [65]. Technology computer-aided design (TCAD) Atlas has been used to design a number of ETM-free PSCs in order to improve device performance and lower fabrication costs [173]. According to simulations of PEDOT: PSS-CH3NH3PbI3-PCBM as well as CuSCN-CH3NH3PbI3-PCBM p-i-n PSCs has demonstrated a strong agreement with experimental results. In the ETM-free PSCs, various HTMs were chosen and mixed directly with n-CH3NH3PbI3, with CuSCN-CH3NH3PbI3 being the best HTM. The effects of the back electrode material, gradient band gap, thickness, doping concentration, and bulk defect density on the performance are examined in order to better understand the CuSCN-CH3NH3PbI3-based PSC [174]. The design is optimized by using the energy band and the distribution of the electric field. The CuSCN-CH3NH3PbI3-based PSC efficiency is thus increased [175]. HTL-free perovskite solar cells have attracted interest because of the numerous benefits including cost savings, a simpler manufacturing method, and the prevention of oxidation while enhancing stability and a longer lifetime [176]. Table 3 gives a preview of the performance of full perovskite, HTM-free, and ETM-free solar cell.
Currently, attention is shifting towards the minimalist device structure architecture of PSCs for rapid advances of electron transport layer-free PSCs through composition and solvent engineering in order to achieve higher device performance and effective interface energy level alignment [163, 177]. In general, ETM and HTM are responsible for charge extraction, but when these materials are removed, the excitons cannot be extracted, efficiently resulting in hysteresis and poor cell performance [163, 177].
4.3. Hole Conductor Free Tin-Lead Halide Based All-Perovskite Heterojunction Solar Cells
In today’s world, HTM-free PSCs with their straightforward construction and processing, low manufacturing cost, and excellent stability are very important [178]. An effective and novel HTM-free PSC structure with carbon serving as the back electrode, WS2 serving as the ETM, and CH3NH3Pb(I1xClx)3/FA0.75Cs0.25Pb0.5Sn0.5I3 acting as a light the photoactive layer has been found to exhibit an optimal PV performance [179]. This structure was numerically investigated using the SCAPS-1D solar cell simulator. Also, a number of ETMs and solar cell layer characteristics, including absorber thickness, defect density, and doping concentration, were adjusted to find the best possible optimal values [180]. The device’s best performance gave of 0.82 V, of 31.94 mA/cm2, a fill factor of 77.95%, and PCE of 20.53% when carbon is used as the back contact [179]. When gold was used as the back contact, the cell parameters of 0.84 V, Jsc of 34.34 mA/cm2, a fill factor of 78.54%, and PCE of 22.72% were attained [179]. A thin interfacial layer was inserted between the absorber and ETL to adjust the conduction band alignment also lead to increased PCE [181]. The architecture of the suggested structure comprised of Spiro-OMeTAD as the HTL, an absorber layer (MAPbI3), and an ETL layer, CdS [182]. The doping densities of the ETL and HTL as well as the thickness of the perovskite solar layers were adjusted in order to achieve the highest PSC performance prior to the insertion of the interfacial layer. The highest power conversion efficiency of 18% [181] at a thickness of 250 nm for ETL, 400 nm for absorber, and 200 nm for HTL, respectively, with doping densities of 1022 cm3 for ETL and 1019 cm3 for HTL [181].
Due to its application as a light harvester in photovoltaic cells, the inorganic-organic perovskite is currently receiving a lot of interest. Tin–lead halide perovskites have a considerable chance of being used as energy harvesters in mesoscopic solar cells with heterojunction due to their high optical absorption, high carrier mobility, and good stability. The tin–lead halide perovskite’s ability to serve as both a light harvester and a hole conductor in the photovoltaic panel is one of its special qualities in device application [183]. The p-type behavior as well as band gap of the various perovskites are revealed by surface photovoltage as well as optoelectronic properties [184]. In order to achieve both low toxicity and excellent performance of perovskite solar cells, the combination of tin- (Sn-) lead iodide perovskite is thought to be the most attractive low-bandgap photovoltaic material (PSCs) [185]. Their performance, however, still falls short of that of the complete Pb-based equivalent. One of the key elements that significantly influences how well a PSC performs is the HTL [186]. For instance, the band gap and also the resilience of the layers may be affected by changes in the ratio of the methylammonium (MA) to formamidinium (FA) cations during the annealing procedure. The pure MAPbI3 as well as the FAPbI3 solar cells are more stable than their mixture when it comes to the PV characteristics at various temperatures [187].
Here, the (FASnI3)0.6(MAPbI3)0.4 PSCs with such an inverted structure have been successfully created using a CuI/PEDOT:PSS bilayer structure as the HTL [185]. The CuI/PEDOT:PSS film is seen to have a smooth shape and good interfacial contact, and the CuI injected can partially impact the crystallization behavior of (FASnI3)0.6(MAPbI3)0.4and can promote the growth of high-quality perovskite films [188]. In addition, the high mobility of CuI and the cascading energy alignments in the device enables effective hole extraction as well as transport from the perovskite to the anode [188]. Moreover, the electrochemical impedance spectroscopy measurement showed that the photovoltaic performance is observed to be highly influenced by the concentration of CuI, which controls its thickness. Therefore, by employing a double-layer CuI/PEDOT:PSS as HTL with a CuI concentration of 10 mg/mL, the produced PSCs based on 60% Sn perovskite results in an increased high power conversion efficiency of 15.75% with negligible J-V hysteresis [185]. The PSCs exhibit enhanced stability, which is an essential quality for the development of high efficient PSCs which is attributed to the double HTL of CuI/PEDOT:PSS [185].
4.4. Effect of HTMs on the Efficiency PSCs
Hole transport materials are essential for extracting holes from a perovskite layer by conveying the holes to the counter electrode, and preventing the flow of electrons back to the metal electrode [189]. Furthermore, HTMs placed on the perovskite layer could shield the delicate layer from disintegration brought on by exposure to ambient air [128]. Accordingly, improving the effectiveness and lifetime of PSCs requires careful consideration in HTM design. Although many organic HTMs have been fabricated with satisfactory efficiency, it is still unclear how molecule shape affects how well HTMs work. Isomers, which are available to study the structure-property relationship of HTMs, have distinct electronic properties but the same chemical content as well as highly similar molecular structures [190]. The effects of the heteroatom and the addition of double bonds to the conjugation on the molecular optical, electrochemical, hydrophobic, film-forming, and photovoltaic performance in PSCs have been explored. Higher hole mobility and enhanced hole extraction ability are produced by the molecules with double bonds due to their significantly improved planarity and intermolecular charge transfer [191].
4.5. The Metal Back Contact in PSCs
Metal back contacts in photovoltaic cells should result in the design of low-cost, chemically resistant, and easily processed and fabricated photovoltaic cells. The back contact of a PV cell is essential in ensuring effective charge transport to the cell’s external circuit. Since the onset of PV cells, gold has been the default back contact for PSCs, with the function of carrying charges from the HTM to the external circuit. However, the high cost of Au has hindered the fabrication of PSCs in large scale. For this reason, it is critical to investigate different metal back contacts that can be used in place of Au. Platinum is a noble metal that is frequently employed in dye-sensitized solar cells (DSSC) and could as well be used in perovskite solar cells. Although platinum is significantly less expensive than gold, the problems associated with cost and abundance remain; hence it has not been given much consideration in the PSC devices. The key criteria for selecting alternative metals are strongly tied to ensuring a high PCE and strong stability under external aging circumstances, beside the cost-related considerations. Table 4 discusses some of the efforts made to replace gold with alternative metal back contacts.
It is observed from Table 4 that the metal work function values increase from 4.26 to 5.1 eV while power conversion efficiency increases from 11.29 to 25%. A work function of 5.1 eV [193] for gold gives an optimum cell operation. Nonetheless, other metals can also be used in place of gold which is not only expensive but can also migrate across the HTL into perovskite films, resulting in the degradation of the devices [194]. Therefore, alternative metal back contacts such as Pd and Ni that are cheap and readily available, with similar operational performance as gold have been tested. Moreover, nickel, platinum, and Pd metallic back contacts also produced results comparable to those of gold. Due to their relative affordability, several metals can be utilized as gold substitutes. Also, due to the synergistic effect of perovskite decomposition and metal migration, both Au and Ag can react with the halide ions in the hybrid perovskite solar cell [195]. But since the perovskite layer is directly lit when the devices are working, the back contact architecture employed in p-i-n design can reduce transmission losses.
Perovskite solar cells have shown to be particularly useful due to their widespread availability, low cost, and improved efficiency. PSCs are the fourth-generation solar cells with back contacts and a light-absorber layer made of either a hybrid organometallic halide or an inorganic perovskite material. The other components are the electron transport layer and the hole transport layer which enable charge extraction. This implies that there are different forms of perovskite which include organic-inorganic hybrid perovskite, lead-based perovskite and all-inorganic perovskite solar cells. As a result, perovskite solar cells have gained popularity and are believed to revolutionize the photovoltaic industry. For photovoltaic technology to be commercially viable, it typically needs to have at least a 20-year operating lifespan with less than a 10% efficiency decline in performance [196]. Compositional engineering, interface engineering, as well as the development of all-inorganic perovskite and encapsulation techniques, have all been heavily focused on in order to improve the intrinsic stability and extrinsic stability, and as a result, significant success has been achieved [33].
5. Solar Cell Fabrication Approaches
There are various methods which are employed in solar cell fabrication. The major methods are herein discussed. The fabrication methods which comprise vacuum and non-vacuum techniques, are touted to significantly improve the efficiencies of PSCs because PSCs, dye-sensitized solar cells (DSSCs), and thin-film PVs share similar structural architectures [197]. However, the actual study has revealed something different [198], despite having a reasonably simple process and high output efficiency. Several alternative non–vacuum-based strategies have been proposed. Some of the processes like screen printing and doctor blading have been effectively used for the production of large-scale perovskite films [199]. The only vacuum-based technique that has ever been proven to produce good cell performance is thermal evaporation. The sputtering approach has rarely been used, either because there is no good target for it or because high-energy species might harm perovskite materials resulting to poor stability. The fabrication methods of PSCs can be divided into various categories which include one-step process, two-step process, vapor-assisted process, and thermal evaporation processes among others [200].
The term “solar manufacturing” refers to the production and assembly of components for the entire solar value chain with solar photovoltaic (PV) panels being the most dominant examples. PV panels are made up of numerous subcomponents, including wafers, cells, glass, back sheets, and frames [201, 202]. With silicon accounting for over 95% of the modules supplied today, silicon is by far the most prevalent semiconductor material for use in solar cell technology. An example of a silicon-based solar cell preparation method is illustrated in Figure 14.
Pure silicon, a substance that is not pure in its natural state, is the primary component of silicon-based solar cells. In order to create solar cells, silicon dioxide from either crushed quartz or quartzite pebbles is first added to the electric arc furnace, in which a carbon arc is used to liberate the oxygen. Molten silicon and carbon dioxide are the end products, as can be noted in equation (2).
At this point, silicon still needs to be purified before it can be utilized for solar cell manufacture [205, 206]. The key to the only vacuum-based technique that has ever been proven to produce good cell performance is thermal evaporation, although other techniques can not be ruled out. To the best of our knowledge, sputtering has never been used, either because there is not a good target for it or because high-energy species might harm these unstable perovskite materials. The fabrication methods of PSCs can be divided into four categories: one-step process, two-step process, vapor-assisted process, and thermal evaporation process, which is to liquefy it, clean it with distillation, and then deposit it into a silicon seed sample, among other methods such as spin coating, and thermal evaporation. High towers are where chemical cleaning is typically used before proceeding into the reaction chamber. Here, ultrapure silicon is created from filtered gas [207]. Boron is added to silicon during the manufacturing process to allow for the flow of electrons, and phosphorous is then diffused into silicon during the processing of cells. In order to create a semiconductor that can conduct electricity, the resulting pure silicon is doped using phosphorous or boron to create an excess of electrons that can reach the conduction band [208]. The bright silicon disks need an anti-reflective coating, often made of titanium dioxide.
Typically, two layers are formed, and the space between them acts as a wall for the electrons, preventing them from getting beyond the barrier on their own but allowing them to do so with the aid of photons [209]. More electrons usually collect on the cell’s upper side, where they push one another aside. Electrons may rotate through wires that are connected to the top of the cell [210]. Boron is used for doping, which introduces impurities, and crystallization, which melts the mixture. Unwanted contaminants are removed through regulated cooling. After the silicon brix has hardened, it is withdrawn and then sliced into films that are 0.2 mm thick. These slices are then utilized to make solar cells [211]. Wafering is done in two steps: first, the brix is cut into slices, then the slices are pushed into the wire sole, and finally, the long wire holding the slurry is inserted through the brix. It has a number of silicon carbide, which removes the wafer from the brix, after which they are cleansed and brought to the neighboring cell plate [212].
5.1. Spin Coating
Spin coating is a useful technique for applying homogeneous thin layers of exceptionally sticky or hydrophobic polymers to planar oraxi-symmetrical substrates. Utilizing centrifugal force, spin coating uses a liquid-vapor interface to deposit a homogeneous film on a solid surface. A liquid is typically placed in the middle of a circular surface and quickly spun to create uniform sheets that range in thickness from 1 to 10 microns [213]. In this method, a tiny drop of coating material is put into the substrate’s center before the substrate is rotated at a regulated high speeds. The substrate spins during spin coating process around an axis that must be parallel to the area to be coated [214]. As a result, a thin coating film forms on the surface as the coating material extends toward and eventually moves away from the substrate’s edge. The type of coating (viscosity, drying rate, % particles, surface tension, etc.) and the spin process parameters, such as rotation speed, determine the final film thickness and other attributes of the device [215]. The spin coating technique for the preparation of solar cell device is presented in Figure 15. Two-dimensional lead halide perovskites (2D perovskite) have emerged as ambient stable photovoltaic materials owing to their unique layered structure.
(a)
(b)
(c)
5.2. Thermal Evaporation
A well-established technique for coating a thin layer is thermal evaporation, whereby the material evaporates in a vacuum as a result of high-temperature heating, thus allowing the vapor particles to move and immediately contact a substrate where they again transform into a solid material [217, 218]. However, it is challenging to precisely regulate the thickness and create a uniform surface when using the spin-coating method to deposit perovskite films. To address these bottlenecks, the thermal evaporation method was recommended. Compared to the spin-coating approach, this technique offers superior reproducibility and film quality [219]. Thermal evaporation phenomenon illustration is presented in Figure 16.
5.3. Inkjet Printing
One of the most efficient methods for producing large perovskite solar cells is inkjet printing. However, the produced perovskite film appears discontinuous with escalating flaws because ink crystallizes immediately after printing. It significantly limits the use of inkjet printing technology for producing perovskite photovoltaic systems [222]. Using printed perovskite solar cells, noncontact inkjet printing allows quick and digital deposition together with robust control over layer formation - properties of the 0.04 cm2 [223].
Since it provides new opportunities in a range of applications, such as tandem cell design and building-integrated photovoltaics, semi-transparency is a desirable and significant characteristic in solar cells. Metal halide perovskite can be produced as a thin film and possesses the ideal characteristics to serve as a photoactive layer in solar cells, although its chemical makeup can alter its band gap [224]. The solar cell’s efficiency is typically compromised when great transparency is achieved. Semi-transparent perovskite solar cells can be created via ink-jet printing without relying on their composition or thickness [225]. The method is based on a technology that may be scaled up. Inkjet printing of arrays of transparent pillars that are made of inactive photopolymerizable liquid compositions and partially covered by the perovskite are possible to design. The transparency and efficiency of the solar cells can be digitally controlled by printing this material at specified locations and array densities [226]. Without an upper metal contact, this new semitransparent architecture exhibits 11.2% efficiency and 24% average transparency [226]. The curves generated from inkjet-printed photovoltaic device of the configuration OMeTAD/Cs0.05MA0.14FA0.81PbI2.55Br0.45/C60/TiO2/FTO in various solvents is presented in Figure 17.
It is evident from Figure 17 that solvent effects play a critical role in the cell performance. For instance, printing the cell device architecture in a binary mixture of N-Methyl-2-Pyrrolidone (NMP) and dimethyl formamide (DMF) gave a cell with good operational I-V characteristics compared to one printed in a binary mixture of dimethyl sulfoxide (DMSO) and dimethyl formamide or in dimethyl formamide (DMF).
5.4. One-Step Method
Due to its simpler operation and lower cost, the one-step deposition approach has frequently been used in the manufacture of perovskite solar cells. With careful management of the perovskite precursors, the perovskite film may be produced with an appropriate stoichiometry and without any pinholes [227]. Typically, gamma-butyrolactone (GBL), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a combination of two or all three solvents has been used to dissolve organic halides methylammonium/formamidinium iodide (MAI/FAI) and inorganic halides such as PbI2. To create a dense, phase-pure, and pinhole-free perovskite layers, the precursors are mixed and subsequently spin-coated and annealed at temperatures between 100 and 150°C [228]. As-produced MAI in commercially available PbCl2 were dissolved in DMF in a 3:1 molar ratio in order to alter the halide anion ratio [229] which establishes a remarkable starting point for the one-step method with a 10.9% power conversion efficiency. After 30 s of spin coating and 100°C of postannealing, the perovskite layer is formed [230].
5.5. Two–Step method
The two-step perovskite deposition process does not require complete precursor preparation; instead, PbX2 (X=Cl, Br, or I) and MAI/FAI layers must be separately spin coated [231, 232]. On a substrate, a PbX2 seed layer is first created by spin-coating or doctor-blading. The inclusion of the MAI/FAI would then be completed by either spin-coating with the MAI/FAI solution or dipping the PbX2-coated substrate into the solution, often isopropanol. After proper baking, the finished perovskite films would have been created [231, 233]. The appearance and quality of perovskite films may be better controlled by changing parameters in either phase thus making it more process-tunable than the one-step technique, despite the steps becoming more complex [234]. Figure 18 represents an example of a two-step DMSO-aided procedure.
5.6. Pulsed Laser Deposition
A high-intensity pulsed laser beam is used in the thin film deposition process known as pulsed laser deposition (PLD) to vaporize a target material before depositing it as a thin film on a substrate. It is a method for creating thin films where physical vapor deposition using PLD approach is applied [236]. The pulsed laser deposition approach has benefits over other methods, including stoichiometry, adaptability, versatility, reduced deposition temperature, and ability to develop metastable substances. Pulsed laser deposition is utilized more often in materials research because of these benefits [237]. For the purposes of solid oxide fuel cells, light-emitting diodes, and solar cells, thin films have been produced using the pulsed laser deposition technique [220].
The layer of material to be deposited is focused by a pulsed laser beam with a pulse duration of 10 to 50 ns [238]. Each laser pulse causes a little amount of the substance to be vaporized with the appropriate energy fluency (1–5 J/cm2). The substrate is generally placed opposite the target where it receives a plume of the ablated material that is expelled from the target during ablation. Figure 19(a) illustrates a pulsed-laser deposition system, whereas Figure 19(b) presents the cell architecture from the PLD technique.
(a)
(b)
5.7. Quantum Dots
Due to its remarkable optoelectronic features and straight-forward fabrication methods, lead halide perovskite quantum dots (PQDs), sometimes known as perovskite nanocrystals, are regarded as one of the most promising groups of photovoltaic materials for solar cells [240]. Because of their distinctive optical characteristics such as variable wavelength, restricted emission, and high photoluminescence quantum yield (PLQY), perovskite quantum dots (PQDs) have recently attracted a lot of attention [241]. The stability of PQDs needs to be further enhanced for industrial applications such as lighting and backlight devices in order to stop their degradation due to heat, oxygen, moisture, and light. Unstable PQDs could easily be degraded by oxygen and moisture [242]. Crystal formation from simple ion migration may reduce the PLQY of PQDs. Important techniques for addressing such issues include surface coating and better band alignment [241].
Compared to their bulk and quantum well equivalents, semiconductor quantum dots (QDs) have already been acknowledged as advantageous optical gain materials. Due to the 3D quantum confinement effect, QDs have well-separated delta function-like densities of states, size-tunable emission wavelengths, and strong optical oscillators, which promise low-threshold but also temperature-insensitive optical gains [242]. An illustration of quantum dot preparation device is presented in Figure 20(a), while the cell architecture prepared using this approach is shown in Figure 20(b).
(a)
(b)
6. Numerical Simulation Strategies
Various simulation softwares such as Personal computer one-dimensional (PC 1D), Accelerated mobile pages (AMPs), COMSOL Multiphysics, Technology computer-aided design (TCAD), the general-purpose photovoltaic device model (GPVDM), and Silvaco ATLAS, have been used to optimize the electrical properties of solar cell designs [244]. This review will, herein, consider a few numerical simulators commonly applied in solar cell analysis.
6.1. Solar Cell Capacitance Simulator
The 1-dimensional solar cell capacitance simulator (SCAPS-1D) was developed under the direction of Prof. Burgelman of Ghent University, Belgium [245–247]. The simulation model clarifies the foundation of solar cells and reveals the key variables that affect how well solar cells perform. The three fundamental equations for semiconductors—the Poisson, the continuity, and the equations for holes and electrons—are numerically solved in the simulation application. The Poisson and the continuity equations serve as the foundation for the SCAPS-1D program [248]. Because of its benefits, including ease of use and control, SCAPS-1D has been used in numerical simulation of solar cells. Equation (3) gives the Poisson expression.
Here, is the electrostatic potential, is the acceptor concentration, and is the donor concentration. The explanations for the continuity equations are expressed according to equations (4) and (5) where is the rate of carrier recombination and is the current density of electrons.
Here, is the current density for holes.
Furthermore, equations (6) and (7) define the drift-diffusion current relations.
where represents the electron’s diffusion coefficient.
Here, is the hole diffusion coefficient.
The continuity equations are given by where the recombination and generation rates are U and G.
Transients or switching times are unnecessary since solar cells function is in a constant state. The carrier concentrations for thermal equilibrium, other than steady state-conditions do not change over time. This can be shown by equations (9), (10), and (11).
This reorganizes the equation above such that
SCAPS-1D simulation gives important photovoltaic characteristics such as the Fill factor (FF), short-circuit current (), open circuit voltage (), and power conversion efficiency (PCE) [249]. Furthermore, the program calculates the recombination profiles and energy band diagrams. The equations for calculating FF and are herein presented as (12), (13), (14), and (15).
Empirical Fill factor
and the is determined from equations (13) and (14).
The equation for the short-circuit current density can be approximated by the expressions (16) and (17), respectively
In this case, stands for generation rate, and stand for hole and electron diffusion lengths, whereas represents the area of the cellin cm2, is light generated current, saturation current, is the thermal voltage, is the doping concentration, is the excess carrier concentration and is the intrinsic carrier concentration.
The one-dimensional equations that affect the conduction of the charge carriers in semiconductor materials when they are in a stable condition are numerically solved using SCAPS-1D software. The one-dimensional SCAPS-1D program is used to carry out numerical simulations of p-p-n perovskite solar cells. The single-shot calculation of the SCAPS-1D simulation software is based on solving the Gummel iteration scheme with Newton-Raphson sub-steps wherein the initial step of the calculation starts with a simple guess of assuming null value for the quasi-Fermi levels throughout the solar cell architecture [250]. An illustration of the SCAPS panel for setting simulation devices is presented in Figure 21.
6.2. Silvaco ATLAS
The two- and three-dimensional device simulator ATLAS is based on physical principles. It makes predictions about the electrical characteristics of specific semiconductor architectures and offers information about the internal physical processes involved in the functioning of the device [251]. Moreover, Silvaco provides an extensive tool set for the development, enhancement, and analysis for digital cell libraries, allowing Integrated (IC) design teams to investigate the effects of various device models, design principles, and cell topologies in order to enhance the efficiency of their state of charge sparse optimal control (SoCs).
A technology computer-aided design (TCAD) simulation may not only show you how well the reverse current-voltage graph appears, but it can also explain why the device is failing. TCAD can be used to express device and process modifications effectively, saving manufacturing cycle times while highlighting possible performance enhancements. Test theories in TCAD is excecuted by manipulating model parameters and coefficients to alter how different physics affect the device including wide-bandgap power semiconductors [252]. The researcher may have to find out the source of device failure in order to develop a physical grasp of the device performance [253–255].
6.2.1. TCAD Silvaco Simulation of Epitaxial Structures and Wafer Design
The four polytype hexagonal silicon carbide (4H-SiC) has been selected in designing epitaxial structures. One of the easiest materials to grow or buy in the market is 4H-SiC. The SiC/SiO2 interface can lead to poor-quality SiC/SiC surface being used as a sensor surface rather than SiO2/SiC [256]. Silvaco graphical user interface is illustrated in Figure 22, whereas the ATLAS simulation methodology scheme is presented in Figure 23.
6.3. WxAMPS Simulation Method
A 1D solar cell simulation program called widget that provides the analysis of microelectronic and photonic structures (wxAMPS) was developed by the Nankai University of China and the University of Illinois at Urbana-Champaign [257]. This numerical simulation method adheres to the analysis of microelectronic and photonic structures (AMPS) physical concept, adds the share of tunneling currents, enhances convergence and speed, and gives better visualization options. The kernel of wxAMPS is based on the upgraded version of AMPS code, while the user interface was created using the cross-platform library wxWidgets [258]. The performance of solar cells is numerically analyzed using the wxAMPS tool. The numerical operation is to solve the Poisson equation which connects charge and electrostatic potential in addition to the hole and electron continuity equations, which represents the behavior of the device. Figure 24 represents the user graphical interface for the wxAMPS simulation software. In wxAMPS, the simulation procedure is undertaken in three steps: first, the ambient operational environment, including temperature, solar spectrum, and bias voltages is set up in which standard values can be fed or the operator can adjust the data accordingly [259]. Secondly, the material properties of the device are entered for each layer. These are provided through a database, editable using common spreadsheet programs, simplifying entering data for many layers and for quick manipulation of properties such as absorption coefficient. After the simulation is initialized, the output is provided in a variety of forms, including files readable by both spreadsheet programs and directly through the graphical user interface [259].
To better simulate different types of solar cells, wxAMPS integrates two alternative tunneling models—the intraband tunneling framework which supports realistic properties in heterojunction solar cells and the drift-diffusion model. Trap-assisted tunneling current is a key component for tunneling recombination at junctions. Moreover, a new technique that combines the Newton–Raphson approach with the Gummel iteration method has been employed to improve the overall convergence property of the model [260]. Several model outcomes of the simulation are compared in order to obtain the best output parameters to be used in designing high-performance practical cells [261]. Therefore, a solar cell engineer can choose the available model to use for a particular set of materials such as organic or inorganic materials.
7. The Future Outlook of Perovskite Solar Cells
Empowering green energy to reach its full potential is essential in addressing the growing environmental problems the world is facing today as a result of increased pollution occasioned by the use fossil fuels and woody biomass [262]. The utilization of pure renewable sources of energy has gained traction in the advancement of human civilization. Solar energy is perhaps one of the most promising new energy options out of the many available possibilities including nuclear and wind power [263]. Poor stability, despite the fact that power conversion efficiency has so far reached 25.8%, is one of the main obstacles impeding full commercialization of perovskite solar cells (PSCs) [264]. Researchers from all over the world have used a variety of strategies, including structural modification as well as fabrication procedures, in a variety of ways to ensure the needed level of stability and optimal performance are attained. Various factors such as high efficiency, simple manufacture, low cost, and stability are a key component in the commercialization of PSCs. Nevertheless, because different experiments use different testing protocols such as humidity and temperature as well as encapsulation techniques, the stability results offered by diverse researchers cannot be accurately compared [264]. In contrast to stability-related features including lifetime and deterioration rates, PCE is a well-defined metric that can be validated in accordance with set standards. It is also imperative to standardize the necessary test conditions for PSC stability testing, with a focus on elastic modulus, heat resistance, device hysteresis, as well as stability on exposure to light, moisture, and oxygen for each fabrication procedure. It should be understood that although inorganic solar cell test findings have methodologies that have been authorized, they are not frequently used in the analysis of PSC stability. In order to successfully advance PSCs and meet market demands, Research and Development (R&D) goals should be stated [265].
The key challenge in designing very efficient solar cells has been the choice of a robust absorber layer (perovskite), poor device structural engineering, inappropriate band alignment at the absorber interface, as well as carrier recombination at the rear and front contact of the device, which hinders the performance parameters of open circuit voltage , , , and PCE [266]. However, more recently, most of the researchers have shown interest in iron disilicide- (FeSi2‑) based solar cells because it is an excellent and promising light-absorbing material for solar cell applications owing to its remarkable characteristics such as direct band gap energy of 0.80-0.87 eV, optical absorption coefficient (α) greater than cm−1 at photon energies above 1 eV, which is ~200 times larger than that of crystalline silicon (c-Si) [266]. Moreover, it is chemically stable, highly resistant against environmental and chemical degradation, humidity, oxidation, cosmic rays, radioactive exposures, and high-thermoelectric power coefficient of , and has a large diffusion length of about 38 μm [266]. The FeSi2 photoactive layer is considered binary, nontoxic, and abundant semiconducting material since the forming elements, both Fe and Si, are ecofriendly and mostly available on the earth's crust. According to Ali et al. [267], a numerical simulation of a solar cell architecture based on FeSi2 and PEDOT:PSS HTM gave a remarkable power conversion efficiency of 39.44%. Therefore, with the appropriate device engineering, proper band alignment, and proper fabrication approach, a PCE greater than the S-Q limit should be achievable for practical applications.
Three-dimensional perovskites have driven the remarkable development of organic-inorganic halide perovskite solar cells (PSCs) over the past ten years [268]. Nonetheless, the uncertainty surrounding the stability of 3-D PSCs casts some doubt on their practical performances. Some improvement in the stability of 3D perovskite devices has been made by utilizing several technological and scientific approaches [269]. However, enhancing the halide perovskite’s intrinsic chemical stability is the most effective method. On the other hand, 2D perovskites exhibit exceptional stability in ambient circumstances and have been accepted as an alternative to their 3D counterparts [270]. Although the photovoltaic performance of the first generation 2D PSCs has been rather subpar, new findings indicate that they are also capable of generating high PCE levels above 20% [271]. PSCs can be significantly shielded by well-developed encapsulation techniques against exposure and other degrading conditions such as oxygen, moisture, and UV radiation [272]. To achieve the full potential of PSCs, it is essential to have an understanding of the causes of their inherent instability and how to address them. The internal elements that contribute to PSC deterioration such as compositional and phase segregation are linked to transformation in the interfaces of the multiple stacked layers of PSCs and are therefore responsible for their mechanistic degradation. Finally, the momentum to study PSCs should be enhanced, and it would lead to an important breakthrough if flexible tandem solar cell structures are considered more seriously [273]. This could improve device performance while taking into account increased mechanical stability [274]. A study on the robustness of tandem cells in contrast to single junction cells and whether the advantages of flexible photovoltaics are exclusive would be of special interest in the future commercialization of PSCs.
8. Conclusions
The performance of PSC devices has not yet been fully explored with regard to the variety of topologies, fabrication techniques, perovskite compositions, and charge selective layers that have been proposed for better solar cell performance. To boost the efficiency of PSCs, better light management can be used to reduce light loss from the cell by utilizing transparent conducting oxide layers to minimize absorption losses and silicon oxide layers to harvest more photon energy. Despite the toxic nature of lead, tin–lead halide perovskite’s ability to serve as both a light harvester and a hole conductor in the photovoltaic panel is one of its special qualities in device application. Due to their exceptional qualities, such as a tunable bandgap, remarkable defect tolerance, prolonged exciton diffusion range, high carrier mobility, and better absorption coefficient, organometallic lead halides perovskites are potential materials for solar cells. However, their operational lifetimes are constrained as a result of the organic components’ susceptibility to environmental degradation. In the recent past, metal halide perovskites have attracted interest as semiconductor devices that achieve desirable properties for optoelectronic application; however, two major challenges, instability and the toxic nature of Pb, remain unaddressed. For this reason, lead-free double perovskites (LFDPs) are emerging as the preferred photoactive absorbers because of their promising PV properties such as intrinsic chemical stability and environmental friendliness. Accordingly, lead-free double perovskites have redefined photovoltaic research despite the fact that a detailed study of their optical, excitonic, and transport characteristics is yet to be understood. Organic-inorganic hybrid PSCs have recently attracted a lot of interest in the photovoltaic community, but recent research has indicated that a missing hydrogen occasioned by poor stability can cause massive energy losses and may therefore be unreliable in the long term. A promising low-cost alternative to current photovoltaic technologies such as crystalline silicon and thin inorganic films is quantum-dot-sensitized solar cells (QDSCs). Quantum dots (QDs) could be made using low-cost techniques, and their size can be adjusted to customize their absorption spectrum. Theoretically, the quantum dot solar cell (QDSC) has been determined to achieve a theoretical PCE of up to 66% due to the occurrence of a unique phenomenon referred to as multiexciton production. However, the experimental values of PCE for QDSCs are quite low compared to what is theoretically predicted. Nonetheless, one of the most desirable QDs is PbS colloidal quantum dots (QDs), which have received significant interest as promising building blocks for optoelectronic devices because of their size-dependent band gap and tunability of electronic properties by means of surface chemistry and solution processability. Although many organic HTMs have been fabricated with satisfactory efficiency, it is still unclear how molecule shape affects how well HTMs work. Metal back contacts in photovoltaic cells should result in the design of low-cost, chemically resistant, and easily processed and fabricated photovoltaic cells. The back contact of a PV cell is essential to ensuring effective charge transport to the cell’s external circuit. Compositional engineering will be the main strategy because this will considerably increase perovskite lattice entropy. Additionally, the various film deposition procedures, component engineering of an all-inorganic perovskite materials, and energy loss mechanisms are essential. In this regard, researchers have successfully used a variety of approaches, including solution-processing and coevaporation techniques, to improve film quality with optimal grain size and uniform coverage. The deposition parameters, including evaporation rate, solvents, and temperature, have a significant impact on the dynamics of crystallization. To properly adjust the deposition settings and produce pinhole-free, smooth, and big grain-size films, more studies on the kinetics of crystallization are essential. The long-term operational stability of inorganic PSCs is proposed to receive considerable interest in the future because they are cost-effective to fabricate and have the potential to post remarkable power conversation efficiency. The use of numerical analysis of perovskite solar cells to investigate interface engineering and overall device performance has been presented. The simulation features of various computational strategies such as SCAPS-1D, Silvaco ATLAS, and WxAMPS with regard to perovskite solar cells are critical in the design of solar cells with high performance. The simulation tools herein discussed have gained traction in guiding the fabrication of practical solar cells that can be introduced into the production workflow for commercial applications.
Data Availability
The data associated with the findings of this study are available from the corresponding author upon reasonable request.
Consent
This article has the consent of all the authors.
Conflicts of Interest
The authors have no competing interests
Authors’ Contributions
GGN did the analysis, writing, and editing. JKK performed the method development, editing, and supervision. All authors have read and approved the manuscript.
Acknowledgments
The authors are grateful to the Department of Chemistry and Directorate of Research, Egerton University, Njoro Campus, for supporting this study.