A COMPREHENSIVE REVIEW OF PHOTOVOLTAIC DEVICES BASED ON PEROVSKITES

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INTRODUCTION
World Energy resources 2016 reported that 1.3 billion people globally and 93 million in Nigeria have no access to electricity (world energy resources, 2016). Yet, every hour the sun beams more energy onto the Earth than it requires to satisfy the global energy needs for an entire year (National Geographic Report, 2017). Thereby offering a solution to the increasing concern of energy shortage, global warming, and greenhouse gases by fossil fuels (oil, natural gas and coal). Solar energy is the most abundant and cleanest form of energy for our energy-starved planet (Mohammed et al., 2015). It remains a foremost energy resource with unlimited capacity to solve man's numerous energy needs (Abodunrin et al., 2015). Surprisingly, most major solar installation has been in regions with relatively less solar resources (Europe and China) while potential in high resources regions (Africa and Middle East) remain untapped (World Energy resources, 2016). However, in regions other than Africa (like south-western United States, Central and South America, Middle East, the desert plains of India, Pakistan, Australia, etc.), such potential is only limited to generate 125 Gigawatt hours (GWh) from a 1 km 2 land area (Adaramola, 2014). A deliberate transition from conventional sources of electricity energy to renewable and environmentally friendly sources is critical for national development, though recent developments show the Nigerian government backsliding in adopting renewable energy technologies (Akuru, et al., 2017).
The Energy Information Administration (EIA) of the United States government has quoted that, about 10% of the worlds marketed energy consumption is from renewable energy. This 10% comprises of solar, geothermal, hydropower, wind, nuclear, biomass, and biofuel with the remaining 90% from fossil fuels (Chang et al., 2010). With the world energy consumption expected to increase by 71% from 2003 to 2030, it becomes a fact that for the next two to four decades, fossil fuels are likely to remain the primary sources of energy in the world.
Currently, the world's fossil fuels are being consumed more rapidly than they are being created, thus, there is a pressing need for alternative energy sources that are both renewable and environmentally benign (Mao et al., 2007, Chang et al., 2010Elano et al., 2009). Projected increase in global energy demand, predicted to be as high as 1Gw/day, will place significant strains on current energy infrastructure (Espinosa et al., 2012). This looming challenge, coupled with depleting traditional fossil fuel-based energy sources with the threat of climate change, require the development of renewable energy technologies (Wang et al., 2016). Among the renewable energy approaches, photovoltaic (PV) presents a promising route. The current photovoltaics landscape is dominated by silicon solar cells, though these cells are constrained by fundamental cost barriers, such as high temperature processing. However, Green et al., 2012 reported that silicon solar cells have advanced tremendously both in terms of cost of production and efficiency over the past four decades. An alternative to these cells is the third-generation photovoltaic devices (a technology that promises a combination of lower cost and ease of synthesis with a better energy payback matrix) developed from various dye sensitizers, organic and hybrid (organic and inorganic) materials (Seelam and Lingamaller, 2016). Ever since the discovery of the photovoltaic effect by the French Physicist Edmond Bec-querel in 1839, a myriad of emerging solar technologies has been developed, with three of the most highly researched being organic photovoltaics (OPVs), dye-sensitized solar cells (DSSCs), and recently, Perovskite solar cells (Ugwoke, 2014). Among these materials, organometallic halide Perovskites offer captivating prospects owing to their solution processability, broad solar absorption spectrum, low non-radiative recombination losses, etc. Due to the substantial improvement of Power Conversion Efficiency (PCE) of these materials, photovoltaic efficiency has reached prestigious position (approx. 20.1 %) within 5 years.
Though in its development stage, Perovskite solar cells (PSC) has been of much interest to Scientists. The "Science" magazine touted it as one of the top scientific breakthroughs of 2013 (Dawn, 2016).
However, there are issues which need to be resolved in the commercialization of Perovskites and limits is application. (Seelam and Lingamellm, 2016). Hence, this comprehensive review presents the current state of the Art of photovoltaic devices based on Perovskites, highlighting the underlying phenomenon, synthesis, challenges, comparison to other technologies and future outlook.

PHENOMENA
Though a detailed overview of the historical evolution of PSC performance, occurring over a short time period, can be found in several review articles (Snaith 2013;Leijtens et al.,2015;Sun, 2015, andRong et al.,2015), Perovskites have being known over a century ago (Green et al., 2014), they received attention only when Miyasaka et. al. used methylammonium lead halide (Perovskite) as a light harvesting material in excitonic solar cells (Kojima et al., 2009). This group utilized Perovskite as sensitizers in dye sensitized solar cells (DSSC) and achieved the solar-to-power conversion efficiency of 3.2% for (CH3NH3)PbBr3 and 3.8% (CH3NH3)PbI3 (Kojima et al., 2009). However, instability of these devices due to the degradation of Perovskites in liquid electrolyte containing lithium halide prompted Park et al. in 2011 to develop the quantum-dot sensitized solar cells using Perovskite (CH3NH3)PbI3 sensitizers (Im et al,. 2011). But these devices reduced the performance of solar cell in a short span of time due to the dissolution of halides in liquid electrolyte (Im et al., 2011). In order to avoid corrosive liquid electrolyte in Perovskite DSSC, Kim et al., developed the solid-state electrolyte, spiro-OMeTAD (2,2 1 ,7,7 1 -tetrakis (N,N-di-p-methoxyphenylamine)-9,9 1 -spirobiflourence), which can act as a hole transporting material (HTM) .
The architecture of Perovskite solar cells was derived from the dye sensitized solar cell (DSSC) technology (Wang et al., 2016). The traditional architecture of DSSCs consisted of a porous TiO2 scaffold, sensitized by a dye and infiltrated by a liquid electrolyte. As regards the dye, Kojima et al. (2009) investigated CH3NH3PbBr3 and CH3NH3PbI3 as an alternative to replace it. This however yielded moderate success (Kojima et al., 2009;Kojima et al., 2006). Based on the inherent instability of these devices, Lee et al. in 2012 attempted to replace the TiO2 scaffold, used to transport electrons, with an insulator (Al2O3) . This demonstrated for the very first time that Perovskite material could effectively transport electrons without the underlying TiO2 layer. With this insight, the demonstration of a planar geometry solar cell with a Perovskite thin film as the absorber layer evolved (Liu et al., 2013). A high efficiency with this device structure was however achieved.

WHAT ARE PEROVSKITES?
Perovskite is a type of mineral that is chemically found on the earth crust. It was first discovered in the Ural Mountains and was named after a Russian noble man and mineralogist, Lev Perovski (founder of the Russian Geographical Society) (Dawn, 2016). The Perovskite solar cells have the same structure of the Perovskite mineral, hence named Perovskite solar cells. A Perovskite structure is anything that has the generic form ABX3 and the crystallographic structure as Perovskite (the mineral).
Depending on the atoms or molecules used in the structure, Perovskites may obtain a set of interesting properties like superconductivity, spintronics and catalytic properties. Hence, scientists and researchers find Perovskites as exciting playground for physicists, chemists and material scientists.

CRYSTAL STRUCTURE
Perovskite are a family of materials with the crystal structure of calcium titanate, that is, ABX3 (Muhammad et al., 2015). There are numerous materials which adopt this structure with exciting applications based on their thermoelectric, insulating, semi conducting, piezoelectric, conducting, antiferromagnetic and superconducting properties (Service, 2014). ABX3 describes the crystal structure of Perovskite class of materials, where A and B are cations and X is an anion of different dimensions with A being larger than X. Crystal structure of Perovskites is illustrated in figure 2.1 below: The crystal structure of Perovskite can be alternatively viewed as corner-linked BX6 octahedral with interstitial A cation. Its crystallographic stability and apparent structure can be deduced by considering a Goldschmidt tolerance factor t and an octahedral factor N. The tolerance factor t is defined as the ratio of the distance A -X to the distance B -X in an idealized solid sphere model as shown in equation (1) below: Where RA, RB and RX are the ionic radii of the corresponding ions.
The octahedral factor µ is defined as the ratio RB / RX. For halide Perovskites (X = F, Cl, Br and I), 0.81< t <1.11 and 0.44 < µ < 0.90 are the typical values. Narrower range of t values from 0.89 to 1.0 dictates cubic structure, while lower values of t stabilizes as a less symmetric tetragonal and orthorhombic structure.
In case of ABX3, the larger cation A is considered as an organic cation typically methylammonium (CH3NH3 + ) with RA = 0.18nm (Li et al., 2008), though ethylammonium (CH3CH2NH3+, Ra =0.23nm) and formamidinium (NH2CH =NH2+, RA=0.19-0.22nm) also provide excellent results. Anion X is an halogen such as iodine with Rx =0.220nm, Bromine (Br) and chlorine (Cl) used in Perovskites with Rx =0.196nm and 0.181nm respectively though in a mixed halide configuration. For cation B, Lead (Pb) with RB = 0.119nm and Tin (Sn) with RB = 0.110nm have been used for high efficiency in PSCs because of lower theoretical ideal band gaps (Pang et al., 2014). Although Sn has similar band gap with Pb and in the same group, due to the ease of oxidation and lack of stability, it performs poorly compared to Pb in efficiency. was measured for solid-state and liquid electrolyte cells, respectively (Kojima et al., 2006).

PEROVSKITE SENSITIZED SOLAR CELLS
Owing to the instability and degradation within minutes of these cells, due to the liquid electrolyte, the idea of adopting a solid-state hole transport medium was born. Kojima et al. in  This cell recorded maximum full sun PCE of between 8 and 10% employing mixed halide Perovskites of iodine and chlorine (Kojima et al., 2009).

MESOPOROUS TIO2 STRUCTURES
The first use of hybrid Perovskite absorbers in photovoltaic cells is based on the typical structure of a dyesensitized solar cell, where the Perovskite absorber is self-assembled within the gaps of a porous TiO2 layer formed by sintering nanoparticles (Song et al., 2015). The typical configuration of this type of Perovskite based solar cells FTO / Mesoporous TiO2 /Perovskite / (spiro-OMeTAD) / electrode is as shown below in figure 2 In this structure, Perovskite materials are deposited onto Mesoporous TiO2, which is used to facilitate electron transport between the Perovskite absorber and the FTO (fluorine-doped Tin oxide) electrode. A subsequent work demonstrated the replacement of the relatively conductive porous TiO2 with an insulating porous Al2O3 layer (Song et al., 2015). It is however important to note that successful pore-filling in these structures is necessary in order to prevent leakage via the device, which has been an issue for thick Mesoporous structures.
The use of Mesoporous structures as a scaffold to fabricate Perovskite solar cells has led to an increase in device performance from 3.8% to over 17% PCE within a few years (Kojima et al., 2009). Just as these structures do not rely on long carrier diffusion length; it is also able to provide a compensation platform for the investigation of new Perovskite materials (Song et al., 2015). While the use of a Mesoporous scaffold requires a comparatively complex device architecture and fabrication process in which many problems could arise, it has consistently delivered high efficiencies that made its use fully worthwhile for laboratory scale investigations (Song et al., 2015).

PLANAR STRUCTURE
In a planar junction Perovskite solar cell, a several hundred nanometer thick absorber layer, is sandwiched between the electron transport layer (ETL) and hole transport layer (HTL) without a Mesoporous scaffold (Song et al., 2015). These cells can deliver efficiency values of over 15% despite being under developed for an even shorter period than their Mesoporous counterparts. This architecture offers the advantages of a simplified device configuration and fabrication procedure, and thus, rapidly acquired the interest of the thin film research community. Planar structures are most commonly illuminated from the n-type side, resulting in the structure glass/TCO/ETL/Perovskite/HTL/metal or p-type side, resulting in the inverted structure glass/TCO/HTL/Perovskite/ETL/metal which functions in a superstrate configuration. Owing to its simplified fabrication and ease of deposition, the planar architecture provides a great promise in future applications, including high performance flexible and portable devices (Song et al., 2015).

MESOSUPERSTRUCTURED PSCS (MSSC)
These are PSC device structures having CH3NH3PbI2Cl mixed Perovskite, coated with alumina layer in a photovoltaic cell. Bi et al., reported that the PCE of this type of structure reached 10.9% in 2013 (Bi et al., 2013).
They were so called because the photogenerated electrons are not transferred to alumina because of the difference in band edges of alumina and Perovskite active layer, which acts only as a scaffold for carrying the photoactive layer (Muhammed et al., 2015). The scaffold layers afford processing at lower temperatures by excluding high temperatures annealing step as neither generated electron are injected into the Mesoporous layer nor transported.
With the advantage of processing at lower temperatures, Lee et al. reported a MSSC/PCE value of 12.3% (Though Al2O3 Mesoporous layers was dried at 150°C) .

HYBRID PEROVSKITE SOLAR CELLS
An example of a hybrid planar heterojunction solar cell is a device structure based on TiO2CH3NH3 PbI3-X / P3HT (poly (3-hexylthiophene)). Such device achieved a better photovoltaic performance when the ITO (indium-doped-Tin-oxide) substrate was treated with C60 self-assembled monolayer having an improved PCE of 6.7%. This was achieved with a significant increase in both Jsc (shunt current density) and Voc (open circuit voltages) (Jeng et al., 2013). The active layer of a hybrid planar heterojunction cell can be sandwich between poly (N,N 1 -bis (4butylphenyl) -N, N 1 -bis (phenyl)-benzidine)(poly-TPD) as Hole transporting material layer and electron accepting PCBM layer (Abrusci et al.,2013). A PCE of 12% was thus reported.

FLEXIBLE PEROVSKITE SOLAR CELLS
If the commercialization goal of PSCs must be achieved, further studies on the possibilities of fabricating cells and flexible substrate must be encouraged. However, several works have been done in this field owing to the low temperature solution processability of PSCs. Ability to conform to the contours of the platform holds obvious promises for incorporation of this technology in diverse application (Muhammad et al., 2015). Docampo et al., in 2013 achieved a higher PCE of 10.2% using device structure ITO / ZnO (25nm) / CH3NH3PbI3 /Spiro-MeoTAD /Ag, fabricated using low temperature solution processing techniques .

HYBRID MULTIJUNCTION SOLAR CELLS
Song et al., concluded in the work saying it is anticipated that the demonstration of new solar technologies based on Perovskites, or the integration of an established manufacturing method that uses both Perovskite and existing technologies are particularly promising for the future photovoltaic market (Song et al., 2015). They envisaged a tandem cell configuration where PSCs can be used effectively as a top cell with existing technologies and at very little optimization in terms of bandgap widening and Fill Factor (FF) enhancement (Mailoa et al., 2015). With a reasonable estimate of achieving 20mAcm -2 and Voc of 1.1V at the top Perovskite cell, a silicon cell generating 0.75V Voc will lead to a FF of 0.8 and efficiency of 29.6% (Snaith, 2013). Tandem solar cells have attracted the attention of researchers around the world as it offers an alternative path towards higher efficiencies when compared to those obtained from single solar cell structures. For a good tandem solar cell structure, an optimized top and bottom cell structure must be used in order to achieve maximum conversion efficiency (Olopade et al., 2015).

SYNTHETIC METHODS
Perovskite solar cells can be manufactured with simpler wet chemistry techniques in a traditional laboratory environment, unlike silicon solar cells that need expensive, multi-step processes requiring an extreme temperature and vacuum controlled system. Perovskites can be created using a variety of solvent techniques and vapour deposition methods (Dawn, 2016). Approaches reported for the synthesis of Perovskite active layers are: one-step precursor solution deposition; two-step sequential deposition; dual-source vapour deposition; vapour assisted solution process; and sequential vapour deposition (Liu et al., 2013;Burschka et al., 2013;Hu et al., 2014).
Categorically, we can put the synthesis methods of Perovskite under these broad heading: solvent techniques (solution processing); vapour assisted solution processing and vacuum deposition.

PEROVSKITE FILM FORMATION
Various processing techniques have been documented to fabricate hybrid Perovskite films. The major methods of fabricating Perovskite solar cells as suggested by Ezike et al. (2017) are Spincoating, Vapour deposition and thermal evaporation methods. Spin-coating methods include onestep, twostep/ sequential deposition and vapour deposition method which include vapour -assisted deposition, and dualsource vapour deposition, and thermal evaporation technique (dual source approach) have been used to prepare CH3NH3PbX3 materials (Ezike et al., 2017).
It has also been suggested that the optoelectronic properties of Perovskite films are closely related to the processing conditions, such as the starting material ratio and the atmospheric conditions during film growth, which lead to a substantial difference in the film quality and device performance . Hybrid Perovskite materials form with crystallinity, even when processed at low temperatures, and the formation of the final Perovskite phase benefits from the relatively high reaction rates between the organic and inorganic species. These advantages substantially expand the choices of available processing methods such as thermal evaporation and solution processing, and facilitate the adoption of new and varied architecture (Song et al., 2015).
In solution processing of Perovskite film, a mixture of MX2 (M=Pb, Sn; X=Cl, Br, I) and AX (A= methylammonium, MA; Formamidinium, FA) is dissolved in an organic solvent and deposited directly to form a film and followed by thermal annealing to produce the final Perovskite phase (You et al.,2014).
In thermal evaporation synthesis of Perovskite film, a dual source is employed for MX2 and AX with different heat temperatures to form the Perovskite film (Liu et al.,2013). Both solution processing and thermal evaporation methods described above are one-step processing methods. In one -step method, both the organic and inorganic halides are stoichiometrically prepared in a common solution and are then spin coated into a thin film.
In sequential deposition synthesis of Perovskite films, MX2 layer such as PbI2 and an AX such as Methylammonium iodide (MAI) are deposited sequentially followed by heat treatment to form the completed Perovskite film (Xiao et al., 2014). The deposition of the MX2 is done by spin-coating while AX can be introduced by spin-coating the AX solution on top of the MX2 layer or the AX solution can be immersed in MX2 layer to induce a solid-liquid reaction or by exposing the MX2 layer to AX vapour at elevated temperatures (Pang et al., 2014). Spin-coating deposition processes allow metal halide and organic halide to be dissolved in organic solvents which is followed by deposition on a substrate from which the formation of the Perovskite is achieved through annealing around 100℃. This method is a low-cost approach but it wastes a lot of precursors.
Two-step sequentially deposition can be carried out in thermal evaporation, by sequential deposition the inorganic and organic components. Here, the PbI2 is first spin-casted followed by solution processing or vacuum assisted deposition of MAI . It is a heterophase reaction resulting in conversion to MAPbI3. A modification of two-step deposition method is the vapour assisted growth of MAI on the PbI2 film.
Compact and uniform PbI2 film obtained by solution processing is exposed to MAI vapour under ambient conditions. In contrast to vapour depositing, this method does not require expensive vacuum equipment and environmental controls. Combining the advantages of solution processing and low temperature vapour deposition, the films grown are pinhole-free offering higher efficiencies Xiao et al., 2014). In vapour deposition method, the substrate is exposed to one or more volatile precursors, which react with the substrate and/or decompose to produce the wanted deposit. It can be when the metal halide is deposited by spin -coating or other methods and volatile organic halide is deposited by ejecting it to give out vapour in the reaction chamber (Ezike et al., 2017). The vapour process is argued to be better than the solution process in planar heterojunction layout because the former produces a flat and even surface (Kesinro et al., 2017).
Dual source vapour deposition method involves simultaneous evaporation of organic and inorganic salts from respective sources at high vacuum. PCE of ~12% was achieved using this method (Liu et al., 2013). Dual source of organic and inorganic halide ejects the vapours to the substrate exposed in the chamber. It is a low-cost method, uniform step coverage, fast deposition, low processing temperature and high throughput.
Thermal evaporation is widely used as a technique for the preparation of thin films for deposition of metals, alloys and many compounds. The requirement is to create vacuum environment where enough heat is given to the evaporants to attain the vapour pressure required for evaporation (Abbas et al., 2015).

SYNTHESIS OF PEROVSKITE-SENSITIZED SOLAR CELLS
A typical DSSC is a Mesoporous n-type Titania sensitized with a light absorbing dye in a redox active electrolyte.
It was in the process of finding a more efficient light sensitizer for DSSCs that Miyasaka et al. reported the first Perovskite-sensitized solar cells, which they formed employing CH3NH3PbI3 and CH3NH3PbBr3 absorbers with an iodide triiodide redox couple (Kojima et al., 2006).  Figure 3 below shows the above detailed process.

SYNTHESIS OF PLANAR STRUCTURE
As earlier stated, planar junction PSCs have a several hundred nanometer thick absorber layer which is sandwiched between ETL and HTL without a Mesoporous scaffold. This architecture offers the advantages of a simplified device configuration and fabrication procedure. These structures are mostly commonly illuminated from the ntype side, resulting in the structure glass/TCO/ETL/Perovskite/ETL/metal (Jeng et al., 2013;Chiang et al., 2014).
The earliest attempts to fabricate planar Perovskite solar cells used single step deposition to deposit the Perovskite absorber layer. Compared to the Mesoporous scaffold, thermal evaporation can be more efficiently applied in a planar configuration, without worrying about the difficulty of Perovskite precursors penetrating into the Nanoporous scaffold. Bi et al., 2013 attempted by thermal co-evaporation of CH3NH3I and PbCl2 and deposited CH3NH3PbI3-xCl3 onto FTO with a thin TiO2 layer resulting in a PCE of 15.4% (Bi et al., 2013).

SYNTHESIS OF MESO PSCS (MSSC) SUPERS STRUCTURED
These devices are fabricated by spin-coating CH3NH3PbI2Cl mixed Perovskite with alumina layer in a photovoltaic cell. A Mesosuperstructure concept was evaluated using ZrO2 Mesoporous scaffold with CH3NH3PbI3 light harvester which exhibited significant photovoltaic activity Voc of approximately 900mV through lower than titania (Kim et al., 2013).
The charge collecting layers were solution-processed in spin-coated chlorobenzene while the active layer was vacuum-deposited by heating reagents CH3NH3I to 70°C and PbI3 to 250°C.

SYNTHESIS OF FLEXIBLE PEROVSKITE SOLAR CELLS
These PSCs consist of fabricating the cells on flexible substrates. Malinkiewicz et al., 2014 investigated this using both regular and inverted device architecture on an ITO coated PET substrate. A CH3NH3PbI3-xClx active layer with PEDOT: PSS and PCBM as hole-transporting and electronic selective contacts were used. A PCE of 6.4% was achieved (Malinkiewicz et al., 2014). A higher PCE of 10.2% was achieved using device structure of ITO/ZnO (25nm)/CH3NH3PbI3/ Spiro-OMeTAD/AG, fabricated using low temperature solution processing methods .

PEROVSKITE PHASE FORMATION
Depending on the relative sizes of the cation and the octahedron in a Perovskite structure, the Perovskite phase can be three dimensional (3D), two-dimensional (2D) or even one-dimensional (1D) in crystal structure (Stoumpos et al., 2013).
Focusing on 3D organo-metal halide Perovskite phases, it is worth noting that the formation of Perovskite structures usually follows the overall reaction formula AX + BX2 → ABX3. Using this reaction as a typical example, it has been observed that the reaction kinetics of Perovskite phase formation are very fast (Liu and Kelly, 2013 Besides the kinetics of phase formation, another issue is the formation of mixed-cation, mixed group IV metal, and mixed-halide Perovskite phases, allowing for the fine tuning of the optical and electronic properties of the final material (Song et al., 2015). A Perovskite phase with mixed-group IV metals has recently been synthesized using MAI and a mixture of PbI2 and SnI2 (Hao et al., 2014).
MAPbX3 -based Perovskite have been found to exhibit multiple phases as a function of temperature and composition. These different phases possess dramatically different electrical/optical properties as well as stability.
Stoumpos et al., showed that MAPbI3 exhibited an X-phase, δ-phase and γ-phase with transition temperature of 400 0 K, 333 0 K, and 180 0 K respectively (Stoumpos et al., 2013). Phase transformation can also occur in mixed halide systems. A mixed halide MAPbI3-xBrx (0 ≤ X ≤ 3) was used for band-gap tuning and it was observed that the crystal structure transformed from the tetragonal phase to a cubic phase when the percentage of Br present passed a threshold of approximately X approx. 0.5 (Noh et al.,2013). This phase transition has been presumed to explain the improved stability of MAPbI3-xBrx materials in the air and humidity test, making it an interesting addition to our understanding of the specifics of the Perovskite lattice.

PEROVSKITE FILM QUALITY
Based on various processing approaches, Perovskite materials exhibit a wide range of film properties like grain size, morphology, crystallinity, surface coverage, etc. Several works have also shown that Perovskite films exhibit composition/structure dependent properties (Song et al., 2015). Therefore, since it is essential to achieve fine control over the reaction between the inorganic and organic species so as to produce Perovskites within the required properties, various process parameters have to be incorporated. Paramount among these are: stoichiometry, thermal treatment, solvent engineering, additives and environmental control.
The stoichiometry, particularly the ratio of the organic to inorganic component, largely affects the resulting MAPbI3-xClx film quality in terms of film conformity and carrier behaviour. Generally, a solution of PbX2 and MAX with a stoichiometry of 1:1 is used as the precursor to form a pure Perovskite phase (Song et al., 2015). Further studies on film formation based on stoichiometry effects have also been conducted, in terms of the phase, the underlying reaction, and the possible byproducts. Details about this can be found in the works of Lee et al. .
Thermal annealing is an essential step to initiate or accelerate the reaction between the molecules, as well as the film formation. A delicate control of heat treatment is needed due to the fast reaction rate between the organic-inorganic component and their various phase in the low temperature of hybrid Perovskite range materials. Generally, Perovskite films are deposited and annealed in nitrogen or dry air glove boxes with H2O levels less than 5ppm, as the presence of moisture was deduced to deteriorate the Perovskite film. However, Zhou et al.
found that Perovskite films annealed in a mild moisture environment of approximately 30% humidity. And could improve film properties significantly . The speculation is that the moisture could enhance film formation by partially dissolving the reaction species and accelerating mass transport within the film. It could also possibly promote the movement of organic species and accelerate the grain growth resulting in less pinholes in the films. This result indicates that a controlled atmosphere during the film formation will result in high performance Perovskite devices. The quality of hybrid Perovskite films can be determined by the critical role solvents play in all kind of solution processes. The selection of solvents with sufficient solubility for organic and inorganic precursor components is limited due to their distinct nature (Song et al., 2015). have improved the morphology of the Perovskite film . Therefore, the solvents either from the precursor solution or induced during the processing, substantially influencing the molecule/species interaction within the system, and the subsequent film quality.

Stability and efficiency are two factors important for the commercial application of Perovskite solar cells.
For their commercial viability, it is imperative that studies be undertaken on issues of stability and reproducibity to enhance the lifetime of these devices (Muhammad et. al., 2015). Degradation in Perovskite solar cells is a synergetic effect of moisture, ultraviolet light, temperature, and the effects of hysteresis and ion migration.

MOISTURE
Multiple reports have suggested that water is the catalyst required for the irreversible degradation of the Perovskite material (Wang et. al., 2016).  The irreversible degradation of the Perovskite layer is a challenge facing the lifetime of a photovoltaic cell.
However, the challenge is compounded by the nature of the by-products. One of the by-products of this reaction is PbI2, (Eqn 1a) which itself is soluble in water . The decomposition of PbI2 in installed modules could cause significant eco-toxicological problems in the field.
In contrary to the findings of Niu et al. In Situ grazing incidence x-ray diffraction (GiXRD) measurements provided another insight into the decomposition reaction occurring in the CH3NH3PbI3 film .
The authors discovered a new crystalline phase which they speculated as a hydrated compound [(CH3NH3)4 PbI6 · H2O] when they exposed the Perovskite film to 80% relative humidity (RH) for 2.5 hours. The formation of this hydrated compound caused a significant reduction in the film absorption. However, CH3NH3Pb(I1-xBrx)3 based absorbers retained good PCE on exposure to humidity of 55% for 20 days (Noh et al., 2013).

ULTRAVIOLET RADIATIONS
Illuminations with UV light can cause degradation in Perovskite solar cells. UV sensitivity is attributed to use of TiO2 as photo anode in PSC. Proposed degradation mechanism for CH3NH3PbI3 under UV illumination was given by (Ito et al., 2014) and equations 2a to 2b shows: At the interface between TiO2 and CH3NH3PbI3, we have: Leijtens et al. showed that for Perovskite solar cells, this TiO2 layer is susceptible to UV-induced degradation (Leijtens et al., 2013). This was investigated by measuring a 5h efficiency decay curve, measured under 1 sun AM 1.5G illumination for devices with and without encapsulation and a UV filter. The results showed that, un-intuitively, the encapsulated device degrades more rapidly than the non-encapsulated device. These authors after several measurements to confirm the UV light degradation of PSCs proposed three methods to circumvent the problem viz: pacifying the trap states; avoiding UV light from reaching the TiO2 layer and or; replacing the TiO2 scaffold with another material.
The use of Sb2S3 blocking layer was investigated to reduce UV induced degradation by Ito et al. The technique involves depositing a Sb2S3 layer at the TiO2/CH3NH3PbI3 interface (Ito et. al., 2014).  et al., 2015). Here, the authors heated the films in an analysis chamber under an ultra-high vacuum. Removing the presence of water and air allowed for the isolation of the effect of temperature on the film degradation. The films were characterized using the I/Pb and N/Pb ratios, extracted from hard x-ray photoelectron spectroscopy (PES). A reduction of these ratios indicates the conversion of the Perovskite into PbI2. Heating at 100°c for 20 minutes led to a significant reduction of both ratios. Further heating at 200°c caused both ratios to drop to a minimum, 2 and 0, respectively. This is indicative of a film consisting of 100% PbI2. The author suggested the following reaction for the temperature induced decomposition as shown in equation 3 below: Characterization results clearly shows the instability of the Perovskite material under elevated temperatures.
Thermal decomposition of Perovskite layer has also been reported to depend on the underlying layer . Although initial reports used a TiO2 scaffold, the requirement to reduce the fabrication temperature has been to replace TiO2 with ZnO. These authors also reported that the ZnO/ CH3NH3PbI3 interface appears to accelerate the thermal decomposition of the Perovskite layer. The film is less thermally stable, when compared to TiO2. Yang et al. used in situ absorption measurements to uncover the mechanism causing this instability .

STABILITY OF THE ELECTRON TRANSPORT LAYER (ETL)
Mesoporous Perovskite solar cells require an electron transport layer (ETL) and most commonly used is TiO2.
Pathak et al. reported that non-stoichiometry defects such as oxygen vacancies and titanium interstitials can form in this layer (Pathak et al., 2014).  (Song et al., 2015). Besides TiO2, PCBM has also been used as a transporting layer. Just as TiO2 layer is sensitive to ultraviolet light, PCBM is not stable in air (Song et al;.

STABILITY OF HOLE TRANSPORT LAYER (HTL)
Spirobifluorene (spiro-OMeTAD) and poly (triarlyamine) PTAA are the most studied Hole transporting layer (HTL). The use of spiro-OMeTAD requires an addictive, e.g., 4-tert-butylpyridine (tBP), which can react with the Perovskite materials causing instability. Other HTLs that have been used include PEDOT: PSS and P3HT (poly (3-hexylthiophene)). The acidic nature of PEDOT: PSS also becomes a concern for the long-term stability of solar cells (Song et al., 2015). Yang et al. studied the ability for the hole transport layer to protect the underlying Perovskite film from moisture induced degradation (Yang et. al;. Three HTLs were investigated viz; spiro-OMeTAD, PTAA [poly (bis (4-phenyl) (2, 4, 6-trimethylphenyl)] and P3HT. Both PTAA and P3HT caused a reduction in the degradation rate under the investigated relative humidities. P3HT served to reduce the degradation rate by a factor of 6 while spiro-OMeTAD layer caused an acceleration of the Perovskite decomposition. The difference in their degradation rates was explained by the discrepancies in the mechanical toughness of the layer.
SEM images showed that spiro-OMeTAD layer underwent significant cracking, hence increased decomposition rate, while other HTL layers formed a conformal barrier layer which reduced the ingress of moisture to the Perovskite film.
In 2014, Habisrentingeer et al. investigated the influence of the HTL on the thermal and moisture-induced degradation of Perovskite solar cells (Habisrentinger et al;. Spiro-OMeTAD, P3HT and PTAA HTLs yielded the highest efficiency results. One advantage of using P3HT is that the cost is reduced, due to both reduced material cost, and reduced layer thickness. The authors suggested that P3HT could be 10 times cheaper than spiro-OMeTAD . Yan et al. replaced PEDOT: PSS with a thin polythiophene (PT) film, deposited by electrochemical polymerization (Yan et al., 2015). The optimized efficiency for PT/CH3NH3PbI3 devices was higher than the reference devices using PEDOT: PSS. They proposed that the electrochemical polymerization employed to form the PT film is compatible with large-scale production. HTLs commonly used are organic materials. Kim et al. incorporated an inorganic HTL, NiOx (Kim et al;. The authors observed that such inorganic oxide films display better environmental stability than their organic counterparts. However, organic HTLs gave higher device efficiency.

STABILITY OF PSCS VIA BUFFER LAYER
The addition of buffer layer was shown to improve the device efficiency and stability. (Guarnera et al., 2015).
They did this by incorporating Al2O3 Nano particles between the Perovskite absorber and the HTL. This improvement was due to an improved fill factor (FF). The buffer layer allows for a reduction in the thickness of the HTM layer causing a reduction in series resistance, thus improving fill factor. The author later propose that the degradation was not induced by moisture or UV radiation, but by the migration of metal from the top contact to the Perovskite layer. The buffer layer stopped the movement of metal to the Perovskite absorber which would otherwise cause shunt pathways (Guarnera et al., 2015).

STABILITY OF THE ELECTRODE
High efficiency PSCs make use of Gold (Au) as the electrode material but this material is prohibitively expensive  et al., 2015). They found that PSC, using Ag as the electrode display worse environmental stability than those using Au electrode. Further investigation revealed that exposure to humidity (RH ~ 50%) caused a formation of AgI, making the electrode color to change from a reflective metal to a yellowish color. This however results to reduced efficiency. This environmental instability of the electrode is ultimately caused by the nature of the HTL, indicative of the inter-related nature of the degradation pathways within all layers in a PSC. Mei et al. fabricated a hole-conductor-free PSC with a printed carbon electrode (Mei et al., 2014) and the device displayed excellent stability. This was as a result of the presence of the thick carbon layer providing excellent protection and the stability also enhanced with the elimination of the hole transport medium. They concluded that though the stability is impressive, the device architecture contains layers processed at elevated temperatures (400°c) which does not align with the desire of mass production.
Multiple novel applications of carbon electrodes have recently been reported (Li et al., 2015). These were archived by fabricating a flexible, fibre, supported PSC using nanotubes (CNT) fibres via a solution coating technique.
They laminated a CNT network onto CH3NH3PbI3 removing the need for the HTL and metal electrode .

HYSTERESIS EFFECTS
The origin of hysteresis and its mechanism is a highly debated topic and has become an area of intense research.
Understanding the ferroelectric behaviour of Perovskite solar cell materials may be critical in increasing its efficiency and stability. Ferro electricity may affect the photo excited electron hole pairing and separation . This behaviour of PSC material came under focus on the report of the hysteresis in current voltage scans, dependent on the scan rate and direction, light soaking history and contact material and interfaces (Unger et al., 2017). Recent reports on hysteresis have attributed the following to its causes: grain boundaries and size; charge trapping at the interface; ion migration within the crystal structure of Perovskites; defects states and surface imperfections of Perovskites (Stephan et al., 2015). Though the frontiers of understanding the intricacies causing hysteresis are evolving, its impact on operating stability of the device remains quite unclear (Wang et al., 2016).
Snaith et al. have proposed capacitive effects, ferroelectric behaviour of absorber, and defect densities to be the sources of hysteresis behaviour .
In organic-inorganic halide Perovskites, grain boundaries and imperfection on its surface may introduce localized states, which will serve as trap centres for photogenerated carriers. Because of its susceptibility to defect formation arising from low thermal stability of these PSC materials, these trap centres could induce a field across that can counteract with the overall photo voltage of the device. Hence, hindering its efficiency (Shao et al., 2014).
Reports have shown C60/fullerene passivation eliminates this kind of photocurrent hysteresis (Xu et al., 2015;Shao et al., 2014). The fullerene is found to interact with halide rich defective regions at the grain boundaries; leading to the passivation of localized trap states. Baena et al. reported a reduction in hysteresis by replacing TiO2 with SnO2 as the electron transport layer (ETL) (Baena et al., 2015). A 15nm SnO2 layer was deposited by low temperature atomic layer deposition (ALD). Planar Perovskite devices with SnO2 ETL thereby achieved efficiencies greater than 18%. This reduction in hysteresis the authors attributed to a more favourable band alignment at the SnO2/Perovskite interface.

ION MIGRATION EFFECTS ON THE STABILITY OF PSCS
The process of ion migration within the crystal structure of Perovskite is another issue that could potentially have an impact on device stability (Eames et al., 2015). Ion migration is sought to be a major reason for hysteresis observed in PSCs (Pellet et al., 2014). The diffusion of intrinsic ionic defects in organic-inorganic halide Perovskites has implications in terms of their long-term stability and performance efficiency. However, recent studies have revealed that the migration iodide ion vacancies under the influence of either electric field or light illumination could alter the collection of efficiency of the photogenerated carriers, causing hysteresis in PSCs (Zhao et al., 2015). Eames et al. also reported that the ionic conductivity in the PSCs depend on the intrinsic iodide ion vacancies, which is found to be dependent on the synthesis conditions and thermal processing techniques (Eames et al., 2015).  et al., 2015).

ATTEMPTS TO IMPROVE INTRINSIC STABILITY OF PSCS AND ITS GROWTH
Variations on the most commonly used Perovskite material (CH3NH3PbI3) structure may lead to improved environmental stability. Noh et al. tuned the stoichiometry of CH3NH3Pb(I1-xBrx)3 of Perovskites by substituting I ions with Br ions (Noh et al., 2013). The author concluded that the stability of PSCs devices incorporating Br was found to be significantly improved. This improvement they attributed to a reduced lattice constant and a transition from tetragonal to cubic phase. The desire for non-toxic, solution processable solar cells has led a few researchers to consider alternatives to lead (Pb) within the Perovskite structure (Wang et al., 2016). The most appropriate element to replace Pb is tin (Sn), as it is also a group 14 metal having four electrons in its outer shell.
Noel et al. formed a device by spin-coating CH3NH3SnI3 on TiO2. The authors observed a decolouration/degradation of both the non-encapsulated and encapsulated devices they tested (Noel et al., 2014).  (Smith et al, 2014). They reported better efficiencies with the use of the 2D layered synthesized Perovskite material as the absorber in PSC devices. The layered Perovskite also did not decompose after samples were exposed to RH of 52% in 46 days.
Ternary halide Cs3Sb2I9 was also investigated (Saparov et al., 2015). The lead-free material does not consist of a hydrophobic layer to protect the film. The authors recorded an enhanced stability which they attributed to the inorganic crystal structure or film grain size. MAPbI3 band gap value is 1.55eV. This reported a higher value than the ideal value for a simple junction solar cell. By replacing the organic cation, it is possible to tune the band gap . Eperon et al. showed that replacing the MAI cation with the larger FAI results in a reduction of the band gap to 1.48eV. The narrower band gap allows FAI-based Perovskites to generate photocurrent over a larger spectral region, resulting in an increase in Jsc (shunt current density). Additionally, the author also showed that FAI Perovskite exhibit improved charge transport characteristics compared to MAI Perovskite, allowing for easy integration into planar geometry devices. They achieved this by incorporating an addictive (butylphosphonic acid 4-ammonium chloride (4-ABPACl)) into a one-step spin coating method to modify the CH3NH3PbI3 surface (Li et al., 2015). XRD     Source: Naveen et al. (2016).

COMPARISON TO OTHER TECHNOLOGIES
The advantage of Perovskite solar cells over existing solar technology lies within their excellent optical and electrical properties, low cost raw materials, facial film and device fabrication. It is anticipated that the demonstration of new solar technologies based on Perovskites, or the integration of an established manufacturing method that uses both Perovskite and existing technologies will be particularly promising for the future photovoltaic market (Song et al., 2015).
Perovskite materials have the potential to disrupt the current photovoltaic landscape owing to their unique use as an active layer in PV modules. This is so because Perovskites can deliver high open circuit voltages (VOC) under full sun illumination, leading to light harvesting from a broad spectrum of incident solar radiation. The fundamental loss in a solar cell which is the difference between VOC and the potential of the lowest energy photon generating a charge has been studied (Snaith, 2010). Snaith reported that thermodynamic treatment limits this loss to the tune of 250-300meV, varying the band gap, based on Shockley-Queisser treatment. Onset of the IPCE spectrum determines the lowest energy absorbed photon. For CH3NH3PbI3-xCl2 Perovskite, the onset is 1.55eV (800nm) with the best Voc of 1.1 gives a loss in potential of 450meV which is lower than the reported loss of 0.59eV for best commercially available PV technology CdTe at an efficiency of 19.6% (Green et al., 2013). Thus, Perovskite solar cell, with the current state of the art, is at par with commercial technologies like CIGS, GeAs, and crystalline silicon.
Film formation of the absorber layer is the lay factor that determines the eventual performance of a PSC.
The successful demonstration of high-performance Perovskite solar cells based on Mesoporous oxide scaffolds has proven the importance of the film quality, e.g. surface coverage and roughness, loading percentage, and crystallinity. However, a high temperature annealing process is needed to fabricate the Mesoporous scaffold which may increase the processing complexity and cost, and decrease the compatibility of implementing a high-performance flexible product, as well as integrating tandem cells into existing technologies; e.g. Si and CIGS modules.
Within a short span, Perovskites have demonstrated that they possess the right mix of properties to offer a solution to our energy needs (Muhammad et al., 2015). Though they evolved out of liquid electrolyte DSSCs, they are now established as a class of their own with extensive focused research pushing the efficiency limit beyond 20%. Exploration of tandem cell configuration with Perovskite based cell as the top cell will push further the achievable efficacy limit. When the issues of stability and the use of lead is addressed, it can go a long way in maturing this technology for commercial application, though in the present legal framework, use of lead is not a problem as CdTe based solar cells has received wide acceptance despite Cd content (Muhammad et al., 2015). The use of lead extensively in lead acid batteries and its content at comparable levels in CIGS and silicon modules to Perovskites suggest that in the short term, the concern may not be pressing, but these technologies are increasingly being phased out and alternatives are explored to minimize the environmental impacts of these heavy metals. Noel et al. reported that the replacement of lead with tin in Perovskite solar cell is already under investigation and may offer an environment friendly alternative (Noel et al., 2014).

CONCLUSION AND FUTURE OUTLOOK
Advantages of PSCs which has captured tremendous attention are its; ability to fabricate large-scale transparent or semi-transparent flexible devices, simplicity of processing, easy optimization for structural design and material engineering; projected cost efficiency with superior PCE, longer electron / hole diffusion length, broad spectral absorption and high open circuit voltage, etc. With the amount of research effort underway, guided by the adherence to the issue of best practices, this technology holds great promise to addressing our energy needs in this present-day energy-starved nation, Nigeria. Balancing electron and hole transporting properties of PSCs, engineering its band gap, enhancing its fill factor either by doping or by improving its morphology, replacement of Pb with Sn, use of mixed halogen Perovskites, use of composite of TiO2 and Sb2S3, use of TiO2 free oxides, use of CNT polymer Nanocomposites, use of layered hybrid Perovskites, employing NiO as the p-type semiconductor, processability to improve HTL and ETL layer designs and the use of several different processing techniques such as Atomic layer deposition (ALD), high pressure pressing, chemical sintering, sol-gel and electrodeposition are known alternative optimization methods that can improve the efficiency and stability of PSCs devices. The solar industry should focus more on the quality and development of its technology.
Additionally, researchers should also focus on improving the competitiveness of solar power against both conventional and other renewable energy sources (Ehsanul et al., 2017). Hopefully, more research efforts will be dedicated toward PV technologies in the near future to enhance their efficiency, stability, manufacturability, and availability, to reduce balance-of-system (BOS) costs and reduce the costs of modules.
Just recently, scientists in Hong Kong reported that they have successfully developed Perovskite-silicon tandem solar cells with the world's highest power conversion efficiency of 25.5%.
It is recommended that the poor and incipient status of solar integration in the vastly populated Nigeria should be viewed in a positive light by potential foreign investors as such status is a guarantee that solar power and thermal industry is a firsthand investment opportunity (Ozoegwu et al., 2017). Solar energy resource is available in all parts of the country with an average sunshine hour of 5.535KWh/m 2 /day. Adekunle et al. (2015) carried out an analysis of global solar irradiance over climatic zones in Nigeria for solar energy applications. The authors found that generally, in all the climatic zones, coefficients of variation of solar radiation were high and mean values were low in July and August. Contour maps showed that high and low values of global solar irradiance and clearness index were observed in the Northern and Southern locations of Nigeria, respectively. The vast expanse of Sahel Savanna in the Northern region of Nigeria provides more than enough land space for this kind of project. According to Tunde et al. (2008), a yield of 5 -20W/m 2 is estimated for solar power. Assuming an area of about 300Km 2 and at 10% solar panel efficiency; the power achievable: = (5W/m 2 X 30 X 10 6 m 2 ) to (20W/m 2 X 30 X 10 6 m 2 ) = 150MW -600MW.
This land space accounts for only about 0.03% of Nigeria's total land mass. Multiplying this land space by a factor of 10 gives a geometric feasibility of up to 6000MW of power. In addition, if one million homes in Nigeria own a 1000W solar power system on their rooftops, the cumulative power production will be 7000MW of power which can add notably 45% to the present electricity consumption per capita.
In this review, we discuss the current state of the Art for photovoltaic devices based on Perovskites, highlighting the underlying phenomenon, synthesis, challenges, comparison to other technologies and future outlook. Accordingly, we conclude that despite a few drawbacks, solar energy technology is one of the most promising renewable energy sources to meet the future global energy demand.