Perovskite Solar Cell (PSC)

1 Introduction
Ongoing innovation in reliance on renewable energy sources (RESs) is having a crucial effect on synthesis and characterization multi-functional nanomaterials. Renewable technologies consider green energy sources, maximization of resources usage while decreasing the impact on the environment, little secondary waste production, and the energy sources sustainability regarding the social and economic future and current needs. Recent studies have shown that fourteen percent of the world’s total energy is from (RESs). By year 2100 there is a projection of rise in supply by 30-80 percent.
(RESs) Can be classified into four categories: solar cells, wind energy, global thermal energy, and biomass [1A]. A particular emphasis is placed on the solar cell due to its advantages: environmental friendliness, plentiful of solar irradiation, and more applicable in both developing and developed countries, even in rich-fuel nations to mitigate the negative implications of fossil fuel on the health and the environment.
Photovoltaic cells convert direct solar energy to electricity. The process involves similar or more energy compared to the higher gap of the materials. In 2014, 55% of the international PV market had been dominated by crystalline silicon technology and polycrystalline. Whereas monocrystalline-silicon modules share the PV market with a 36% and only 9% has been divided between various PV technologies. Some of the technologies used include dot quantum cells, semi -conductors which are amorphous, thin films poly-crystalline in nature, organics and solar cells that have been sensitized through dye.
On the contrary, the process of manufacturing the crystalline silicon structures not only requires a relatively high amount of heat energy [4] but also utilizes some hazardous and rare components including telluride cadmium telluride, Indium- selenide copper, cadmium indium gallium di-selenide and silicon tetrachloride [5,6]. As a result of this, organic and abundance materials like dye-sensitized, organic, and perovskite solar cells (PSC) can be more environmentally-friendly alternatives.
There has been emphasis placed on Solar cells (perovskite) as a competitive and viable alternative in the PV market due to its advantages. The cells have got a power transforming capacity of twenty percent, minimal costs of manufacturing and perfect stability. Because of this, rapid development in mass production and interface aspects of thin perovskite films and attachments has succeeded excellently in improving the working of gadgets. Through the developed technologies, there has been a remarkable increase in grain size of the perovskite as well as the degree of the solid order of the molecule, increases its coverage on the surface, the morphology of the film and minimize on the majority of errors .Therefore, understanding the mineral materials properties have become a young research area and require more investigation.
1 Perovskite solar cell (PSC)
In 2009, PSC was introduced in the literature although the quality of PSC was insufficient. For instance, there was a minimal ability to convert, around less than four percent, and its stability included since during the cell operation the electrodes and perovskite layer dissolved in the liquid electrolyte [7]. After only three years of progress, the (PCE) and stability of PSC was developed to become 9.7% and 500h respectively by Kim et al. [8].
This type of solar cell depends on photovoltaic effect in which photons of incident light can excite the electrons of semiconductor materials. The electron can absorb the photon of incident light and travel from the outermost electron orbit /HOMO to the delocalized region/LUMO, leaving openings in the outermost band/ HOMO, when the material energy gap is lower than the energy of incident light. Diffusion, the charge carriers between the opposite electrodes through the electrodes area, can hinder recombination of electrons and the openings left behind. [3]. consequently, the current arises from charge carriers movement while the voltage generates from the band gap of the materials. Becquerel in 1839 was discovered this effect and is called “Becquerel effect,” but now it is defined as the photovoltaic effect [3].
1 Perovskite material structure
In comparison to existing technologies in PV, PCES that are solar-based are better. Perovskite is a word that refers to Calcium Titanate in its crystal form. It was found by Gustav Rose, a German mineralogist and named after Lev Perovskite a Russian mineralogist. ABX3 is the perovskite materials general structure, where A and B represents cations varying in electronegativity plus atomic radius while X refers to negatively charged ion which is connected with atoms A and B by chemical bonds. The crystal cubic structure of perovskite characterizes by lower electronegativity and bigger atomic radius of atom A than atom B. the type of cations and anions play an essential role in determination perovskite properties materials such as superconductivity, ionic conductivity, and high thermal power.
Fig (1): The cubical crystal structure of perovskite ABX3 [3].
1.1.2. Organo-Metallic Halide Perovskite materials (OMHPs)
A significant amount findings is based on organo-metallic-halide solar cells made of calcium titanium oxide, and Weber discovered this specific composition for the first time in 1978 [9] and then, it was used in 2009 like dye in Solar cells that are dye- (DSSC). The solar cell based on OMHP comprises a liquefied electrolyte .Inappropriate stability and minimal efficiency has contributed to the minimal attention received. After several years of continuous research effort particularly in the solid-state cell, the effectiveness of OMHP-based PV devices in 2012 became (∼10%) and only after three years increased to >21% [3]. Due to a significant development in the last few years in the ability to convert power, stimulated by other distinctive properties such as strong absorption coefficients, mobilities that are high charged, long carrier lifetimes, including the ability to tolerate errors acquired in the processing part, the Solar OMHP-based cell can be promising in solar technology in future.
OMHPs have a representation of AMX3, the negatively represented electron is marked A like methylammonium (MA) or formamidinium (FA), a negative metal electron like lead (Pb) is marked M or tin (Sn), whereas negative Halide is marked X. Figure (1) indicates crystal structure and unit cell of MAPbI3. This type of solar cell features mobility in high charge carrier to allow electrons in addition to the holes to move unhindered and high diffusion length to protect electron-hole pairs from recombination whereas, low band gap to absorb lighter because the photons of longer wavelength light have sufficient energy to excite electrons.
Fig (2): The general structure of CH3NH3PbI3 perovskite cell (A) cubic (B) tetragonal, and (C) the unit cell [3].
A special emphasis is placed on the structures used in the development of the semiconductor where the active layer materials being transported, the positive electrons and the gaps provide the surface for transporting the materials. According to the kind of transported materials, the lighter one coming first, there are two major types of these structures: [Fig. 2(b)] representing the conventional type and [Fig. 2(c)] representing the invented forms. In general, all layers in the structure of OMHPs solar cell have been investigated and developed because all of them directly contribute to enhancing the device performance. However, the electron transport layer (ETL) has obtained enormous attention [10-12] because, at this layer, the electrons could be collected more efficiently to resist electron-hole recombination.
Fig (3): Schematic diagrams of perovskite solar cells in the (a) n-I-p microscopic, (b) n-I-p planar, (c) p-I-n planar, and (d) p-I-n microscopic structures [9].
1.1.3. Limitations and challenges facing PSC
Despite the capability in the PSC cells, there is need to look into various defects before producing the cells for commercial purposes.
* Stability of the gadgets: In comparison to the existing solar cells ranging up to thirty years, the PSC solar cells are not yet stable for the commercial purposes. The appropriate composition and covering of the PSC modules will help in their stability. For example, the excellent stability being demonstrated in Jeddah Saudi Arabia, despite the hot outdoor conditions [13].
* The dependence of the amount of voltage presently produced: Several issues have been raised concerning mass production of J-V voltage due to the dependency arising from the scanning process. Three possible reasons may be responsible for this behavior which are ferroelectricity [14,15], ion migration [16,17], and unbalanced charge collection rates [18,19].
* Harmful effects and environmental degradation: There OMHPs may contain some significant amount of lead hence raising issues on its negative impact on the environment. Consequently, recent research has studied less toxic elements instead of lead such as Sn and Ge which is called free-lead perovskite solar cell but, these types of the cell would be poor stability because they are sensitive to moisture and air to form oxide films [20,21].
1.1.4. Principles of PSC operation
Photons from incident light travels through the glass and top electrode (Fig. 3), and then they are taken in by the perovskite bond, and each electron in this layer are excited when the photons energy is higher than energy represented of the PSC film. The gaps and electrons that are excited are separated to neutral charge that can be easily converted carriers through internal potential generated from the working ability difference between the transparent electrode and metallic electrode. After wards, the excited positive ions are moved from PSC layer to the electron transparent layer (ETL), while holes transport to opposite direction towards hole transparent layer (HTL). Both electrons and holes complete their journey to the transparent electrode and the metallic electrode for electrons and holes, respectively. The excited electrons continue the journey by moving through a wire connecting the two electrodes because of this the current can generate. There is a possibility to recombine the electrons-holes pairs in the metallic electrode.
Carefully adjusting the HOMO/ LUMO energy levels for each layer is substantial to improve PSC performance and to keep the electron-hole pairs from recombination to reduce the total energy. In contrast, the charge carriers are also energy conservers, which means they will always take the path to decrease resistance. With the right layer structure, some of the recombination in the cell will be blocked by allowing the charge carriers to take a different way. This can be achieved by having the ETLs LUMO a bit lower than the conductive layers LUMO which creates a more attractive route for the electron to move. The same is true for the HTLs HOMO that requires being a bit higher than the active layers HOMO which generates a more favorable way for holes to move. Therefore, each layer has either higher HOMO or lower LUMO for insurance the charge carriers transportation unhindered.
Fig (4): showing how perovskite solar cells operate the y-axis is the energy vs. vacuum in eV. The blue arrows show how the charge carriers move, arrows with a cross is a blocked path.[21]
1.1.5. Preparation Methods of PSC
A particular emphasis is placed on fabrication perovskite thin films through many processes such as vapor – based methods as well as solution based methods. Their outstanding optoelectronic properties could outweigh PV materials such as amorphous Si, CuGaxIn1−xSySe1−y (CIGS), CdTe, and Si. The materials have minimal initial purchase price and fits mass production due to developed production techniques hence improving the commercialization of the PSC cells [23, 24]. Therefore, OMHP materials can be considered as an ideal choice for PV applications.
There are several quality criteria for perovskite films that are vital for any PV gadget aimed at high performance:
o Morphology within the appropriate means
o Similarity
o Purity in each step
o Definite degree of some solid order
Each PSC film requires correct composition of the definite components and a managed degree of the solid order are required to achieve these criteria [3]. Table (1) provides a brief comparison of preparation approaches.
Preparation method

Advantages

Disadvantages


Deposition involving one phase [25]

* Simple processing
* Minimal cost of fabrication

Precursor composition, processing temperature and deposition parameters must be controlled


Deposition involving two phases deposition [26]

It can be obtained more uniform and dense film than the single-step solution deposition.

Incomplete perovskite conversion.
An exchange between the surface smoothness and the size of each grain.


Two-step vapor assisted deposition
[27]

It provides better control of morphology and grain size via gas-solid crystallization
It can be avoided film delamination

It requires a long time.
Efficiency (10-12%)


Thermal vapor deposition
[28]

Uniform (pin-hole free) film can be prepared easily via this approach

It requires precise control over temperature during deposition.


1.2. Solar cells characteristics
There are five quantities which are considered essential to describe performance characteristics of solar cells: Jsc the density of the short circuit, the voltage potential Vocp in a circuit that is not closed, factor required to fill (FF), plus its optimum Ƞ [29].
Each optimal form, should follow the following criteria:
* FF: it should be as high as possible, because it indicates if the resistances in the cell are good or bad. Thus, it is affected by resistance within its series (Rs) as well as its parallel resistance within the shunt (Rsh), if Rs ranges low and Rsh high, the FF increase which in turn, would improve the cell performance.
Fig (5): IV-curves for a PSC cell with low resistance within the series and high shunt resistance with the result of an almost rectangular shape as shown by the solid blue mark in (a) also in (b). The thin black line in (a) shows how the IV-curve changes when RS increases for a solar cell. The thin black line in (b) shows how the IV-curve changes when RSH decreases for a solar cell, (c) FF is calculated from the IV-curve by dividing area A with Area B. The cell power arises from J and V with the highest point being the product of Imp plus Vmp [22].
* Vocp: it should be as high as possible to drive the excited electrons and holes into opposite paths.
* Jsc: it should be as low as possible since the semiconductor elements leave some holes.
* Ƞ: external quantum efficiency is a substantial quantity to determine the effectiveness of cell performance. The EQE for a viable cell should be as high as possible because it shows how many of the incoming photons that excite electrons that are extracted to create a current. At low wavelength the EQE is zero due to the photons is absorbed by the glass and transparent electrode, without reaching the active layer. Also, if the wavelength is high, the EQE become zero again because photons minimal energy than the energy gap and cannot excite an exciton. The cell efficiency related to all previous factors through the following equation:
where Ps is incident light power density.
1.3. Grain Boundaries (GBs) Role in Solar Cell Performance
In addition to the four key criteria that determine the cell performance discussed in the previous section, many recent studies have drawn attention to grain boundaries in terms of their effect on device performance. Some have argued that for improving the solar cell performance, GBs should be passivated to minimize error capacities which in turn, would enhance extraction of the electrons from absorber film [30] as shown in fig (6). For instance, Shao et al. [31] boosted the PSCs efficiency and mitigated the photocurrent hysteresis by passivation of GBs in MAPbI3 with fullerene.
Fig (6): schematic of passivation layer role in mitigation electron traps [31].
Despite these drawbacks, however, recent research shed light on GBs benefits. For example, according to Seidel et al. [32], the importance of GBs in CH3NH3PbI3 with photochemically active additives was investigated through the Kelvin probe force microscopy. Based on research findings, it can be concluded that these additives decrease the difference within the contact change occurring in the grain boundaries ΔCPDGB which leads to generating photovoltaic carrier as shown in fig (7) leading to improvement in cell workability.
Fig (7): Diagram plotting the solid electronic structure surrounding grain boundaries (GBs) (a) sampled in the dark, (b) sampled with an addition to photochemically active additives [32].
Furthermore, Yun et al. [33] modified MAPbI3 structures by doping Cl ions which lead to enhanced charge separation and collecting carriers effectively in GBs more than grain interiors (GIs) because the CPD at GBs was higher than at GIs under illumination due to fast ion migration [34]. Therefore, as the movement of electrons and holes can be unhindered along GBs, photocurrent can be generated efficiently.
Moreover, the grain size is not a critical factor to improve ion diffusion or to reduce hysteresis of I-V curve because the rating in the nucleation plus the growth in the grain rate affect the grain size attributed to numerous thermodynamic factors [35].
Also, D. Cahen et al. [36] indicated the viable role of GBs in mitigation electron-hole pairs recombination rate because the GBs provide an attractive conductive route for electrons and they improved the electrical properties of GBs in CdTe film by CdCl2 treatment.
Impurities and various types of vacancies would lead to electrically charged GBs [30] in CZTS, CZTSSe, CIGS, and CdTe films. Consequently, the build-in field at the GBs and GB band bending contributes to separate photo-generated electron-hole pairs effectively [36], but in some cases, GBs might lead to cell shunting when GBs cross the junction. Therefore, high density of GBs in OMHPs would boost the cell effectiveness if films preparation approaches were prices controlled.
2. Experimental Techniques
2.1- Microscopy involving Physical Probe
This type of experiment is vital in measuring some local functional properties such as carrier lifetimes, work function, conductivity, and the response of piezoelectrics. Therefore, measuring the electrical, optical properties and studying the cross-sectional morphology of materials by utilizing (SPM) has contributed to the knowledge of each processes available in devices [37]. For example, investigation of charge generation and separation would base surface set out measurements photo-voltage and the current of the photo [39, 40]. Due to scanning probe measurements, it could be described and explained how variance in the conditions of processing affect morphology as well the output of the gadget. Despite lack of one item to thoroughly look into the PSC cell, methods of scanning each probe give substantial data hence suitable for research on photovoltaic.
Fig (8): The block diagram of the feedback system in a probe microscope [41].
2.1.1. Microscopy imaging at the atomic level
It is considered as the first member of the physical microscopes group; Gerd Binnig and Heinrich Rohrer invented in 1981 [42,43]. STM characterizes as efficient method to study pixel resolution on a surface to the atomic resolution and visualize the materials atomic structure. Furthermore, exciton binding energy and the electrostatic potential distribution of semiconductors can be measured by STM [44]. The atomic force microscope is created within a short span of time after the tunneling microscope creation.
2.1.2. Kelvin Probe Force Microscopy
A significant factor that is measured here is the variance in contact between the sample and the AFM tip [45]. Thus, it can be considered a surface potential detection approach. Furthermore, it allows researchers to identify the local CPD in the sample area below the apex of the tip because a control loop fed with the deflection signal automatically minimize the electric force in between the AFM and the CPD by implementing a counter resolution, as depicted in fig (9).
Fig (9): KPFM a control loop continuously mitigates the force of electricity electric between AFM and CPD [45].
A recent study has measured the CPD of perovskite film with the added pbI2 by employing KPFM under illumination and dark conditions. According to findings, the CPD of the film under illumination has become stable and higher than CPD under dark condition [30], see fig (10).
Fig (10): CPD standing voltage application result as administered to the tip below 500 nm with an illumination of 0.3 Wm-2 [30].
Also, using the spatial resolution in studying the electrical properties inside the cell as the best method has become applicable and it provides valid correlation between properties in electrical form and physical features on the sample because both images are acquired from similar location [41].
Another crucial study of halide perovskite films conducted by Seidel et al. [32] indicated that KPFM can be an effective technique to investigate the change in the CPD images caused by the change of the functioning work in the measured area. For example, a higher work function shows a brighter area in the CPD image.
Employing KPFM under illumination and dark conditions would be viable to point out the difference of surface ability in the boundaries of each grain. For instance, see fig (11) B CPD of perovskite film under light appears more flat across GBs than the same film under the dark condition, fig (11) A, because KPFM measurements, under illumination would reduce the band bending due to production of photovoltic transporters. Consequently, each fabricated film seems uniform in the potential landscape which in turn enhances photovoltic performance.
Fig (11): CPD images of CH3N3PbI3 thin films with benzoquinone as photochemically active additive (A) in dark, (B) illuminated by laser and (C) distributions of Histogram with and without the provision of light [32].
In the general, this technique is a substantial technique to measure various significant parameters such as:
o Work function distribution in active perovskite layer [36].
o Contact potential difference at each grain surroundings in carbon containing cells [32].
O possible characters through the structure of gadgets in PSC cells with multiple junctions [37].
* Potential profiles across the device under applied voltage and illumination by obtaining cross sectional electrostatic potential imaging of the cell such as GaInP2 [46], silicon [47] semiconductors two and six [48],semiconductors three and five [49] plus chalcopyrite [50,51] cells.
* Charge carrier dynamic in organic field-effect transistor (OFET), as exemplified in fig (12), [45].
Fig (12): Charge carrier dynamics in a transistor with an organic field-effect
(OFET) by KPFM [45].
However, it cannot provide information on thickness, but it can be overcome this point by utilizing transmission electron microscopy (TEM). Another limitation of KPFM is that the measured properties of this technique represent the characteristic of the surface but not belonging to the bulk material [52].
2.1.3-High resolution microscopy
Mainly helps in the study of morphology and the sub-layer morphology of some devices such as donor-acceptor networks of the solar cell [53, 54]. Compared with the electronic microscopes, the atomic force microscopy has two significant advantages which are high resolution (up to 1000000X) and three-dimensional topography [41]. Based on the tip-surface distance and application, there are several AFM operation modes, and it can be divided into:
• Static mode or contact form: In static form, the AFM is scanning in a regular force between the AFM and CPD as indicated in fig A (13). The force generates from the interactions in the exchanges arising from the electrical orbits overlapping at the atomic distance. It is widely used for hard surfaces.
• Non-contact form : Here scanning occurs as the edge is at a constant height where it contacts the sample surface, see fig B(13). The frequency and amplitude of the driving signal are maintained constant. This mode of AFM is mostly used in ambient conditions or liquids.
388620321310AB 00AB 1031875131254400
Fig (13): AFM operation contact form (A), (B) non-contact form [41].
In addition to previous AFM modes, it can be categorized into both conductive and photoconductive AFM (pc-AFM). cAFM and pcAFM have been employed to map the conduction route and photocurrent of microcrystalline materials with a relatively high resolution, respectively [55,56]. Also, the dependence of Vocp on light intensity can be studied by pcAFM [57,58].
Fig (14): Diagram of pc-AFM setup [59].
2.1.4. Time-resolved Electric Force Microscopy
Electrostatic force microscopy can be used in finding out varying occurrences such as such labeling internal errors defects [60], taking the local dielectric constants measurements [61-63], and investigating the conduction of ionic elements [64]. However, this technique could not be practical to obtain varying information on the time scales in line with enormous physical properties. Because of this, electrostatic force microscopy that is time resolved [65] has been used to study many phenomena particularly dynamic information such as photocurrent generation on varying scales of time from the order of seconds [66] to nanoseconds [67]. Although there are some other techniques that have been used for the same purpose like the optical microscopy in near- field plus with pulsed laser optics scanning [68], the different approaches demand expensive hardware and complicated optics.
Several studies have relied on EFM and KPFM to investigate some electrical dynamic features such as charge injection, transport and trapping processes. For example, recent groups have utilized KPFM to acquire physical possible maps of photoexcited organic semiconductor blends under photoexcitation as it is discussed in section 2.1.2. Nevertheless, there is unclear association between KPFM visuals and changing processes of producing charge. This means that tr-EFM would be an effective alternative approach to gauge the accumulation of photogenerated carrier plus the potential within surfaces, See fig (15) [69].
Fig (15): Polymer blend film charging rate obtained by (A) KPFM, (B) tr-EFM [69].
A recent study carried out by Karatay et al. [67] has shown that tr-EFM figures can be utilized to acquire the physical maps indicating photovoltaic quantum efficiency maps as a basis of several parameters including , processing, composition of materials, photodegradation, and excitation wavelength. Also, Ginger et al. [70] compared between tr-EFM and SKPM in terms of identification properties of polymer solar cell and they concluded that trEFM is more effective than SKPM to investigating local trap formation because the images of SKPM are minimally sensitized to formation of trap and exhibit a response that is complex. On the other hand, employing SKPM combined with trEFM could be valid met...