27 | Page the moisture and light soaking stability. However, when introducing Cs into. MAPbI3, the yellow orthorhombic phase of CsPbI3 easily segregated from.
1. Introduction 1.1. The perovskites as promising solar cells Projected increases in world energy demand and increasing global concern over the issue of climate change have focused research attention on renewable and clean energy sources of which photovoltaics (PV) is a promising example. Organic-inorganic metal halide Perovskite solar cells (PSCs) became one of the most attractive topic in new-generation solar. The power conversion efficiency (PCE) of PSCs has rapidly increased to the certificated 22.1% [1]. The graph figure 1 shows a meteoric rise compared to most other technologies over a relatively short period of time. In the space of three years, perovskite solar cells have managed to achieve power conversion efficiencies comparable to Cadmium Telluride, which has been around for nearly 40 years. Although it could be argued that more resources and better infrastructure for solar cell research have been available in the last few years, the dramatic rise in perovskite solar cell efficiency is still incredibly significant and impressive [1].
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Fig. 1 the increase of power conserving efficiency of different types of solar cells through past few years (1977-2017) (adopted from reference [1])
1.2. The perovskite material components The True perovskite (the mineral) is composed of calcium, titanium and oxygen in the form CaTiO3. Meanwhile, a perovskite structure is anything that has the generic form ABX3 and the same crystallographic structure as perovskite (the mineral) [2]. Table. 1 Organic-inorganic hybrid perovskite of the composition ABX3 (adopted from reference [2]) A
B
X3
Organo
Metal
Trihalide (or trihalide)
Methylammonium
Lead
Iodide (or triiodide)
Plumbate
Chloride (or trichloride)
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The perovskite name picking table: pick any one item from columns A, B or X3 to come up with a valid name. Examples include: Organo-leadchlorides, Methylammonium-metal-trihalides, organo-plumbate-iodides etc [2].
Fig. 2 The lattice arrangement in the perfect crystal of perovskite (adopted from reference [2])
Fig. 3 Generic structure of a standard (non-inverted) perovskite solar cell (adopted from reference [2])
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1.3. The efficiency of perovskite solar cells
Fig. 4 graph describe the absorption of solar radiation by perovskite solar cell (adopted from reference [2]) Perovskite solar cells have supreme optical properties whereas have capability to absorb different frequencies photons from the solar radiance that means high harvesting to the light in ultra violet, visible and infrared regions as shown in figure 4 [2]. The power conversion efficiency (PCE) of a solar cell is defined as: PCE = (J"sc" × V"oc" × FF)/(P"in" ) where Jsc is short circuit current density, Voc is open circuit voltage, FF is fill factor and Pin is incident input power. The high-efficiency (PSCs) are based on perovskite thin films prepared using the spin-coating method. The device used to apply the spin coating called a spin coater. and this technique can be done by spread the coating
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material on the substrate by centrifugal force. Manufacturing and recycling processes are complicated as we can see in figure 5 [3].
1.4. Manufacturing perovskite solar cells
Fig. 5 (a) Flow chart of the fabrication and recycling processes for PVSCs. (b) Schematic illustration of the detailed process of recycling PVSCs via selective dissolution (adopted from reference [4]) During soaking in a polar aprotic solvent, the deposited-metal electrode peels away from the device, leaving the clear electron-transport-layercoated substrate behind, and the hole-transport layer and perovskite layer dissolve [4]. Recently, inkjet printing has become a promising technique for manufacturing perovskite solar cells. inkjet printing has been used to deposit perovskite thin film using both one-step and two-step processes in either planar or mesoporous PVSCs. A mesoporous material is a material
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containing pores with diameters between 2 and 50 nm, according to IUPAC nomenclature [4]. The deposition of perovskite CH3NH3PbI3 with a one-step process on a mesoscopic TiO2 film using an inkjet printing technique is reported. optimizing the perovskite thin film morphology can be done by changing the table temperature and the ink composition using a mixture of MAI, PbI2, and MACl with a molar ratio of (1–x):1:x (x = 0 ∼ 0.9) dissolved in γbutyrolactone (35 wt%) and obtained a flat and uniform perovskite layer. these devices based on inkjet printed perovskite film fabricated on a substrate to 50o C and with a MACl ratio of x = 0.6 exhibited a PCE of 12.3% with an average value of 11.2%. the fabrication and optimization of multipass inkjet-printed perovskite on a planar TiO2 electron transport layer with a one-step process. Recently most of solar energy researchers seeking to fabricating low-cost and high-performance (PSCs) using achievable techniques and rapid mechanisms to spread this type of photovoltaic cells wider [4-6].
1.5. Some obstacles in the way of progressing PSC The core-target of the current photovoltaic research is to create solar cells with high efficiency, easily processable and low-cost solar cell absorber materials. The most compatible type of solar cells to achieve that at the moment are the perovskite solar cells using light absorbing material of CH3NH3PbI3-xClx Organic-inorganic hybrid perovskites based on methyl ammonium lead tri-halide compounds. their light harvesting properties are tunable (available to change their absorbing wavelengths of incident light) by varying the chemical composition, their processing is simple and power-conversion is efficient. The calculations indicate that (PCE) to approach limit of 31.4%. Despite the remarkable progress in increasing
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cell efficiency, there are three significant hurdles for perovskite photovoltaic technology: 1) Improving the long-term stability of the cells. 2) Defining standard characterization protocols to eliminate hysteresis artifacts. 3) Forming pinhole-free films of the highly crystalline semiconductor. Pinhole free thin films of CH3NH3PbI3-xClx can be coated on high conductivity
poly(3,4-ethylenedioxythiophene)
poly(styrene-sulfonate)
(PEDOT:PSS). the research found that remarkable solution for the previous obstacles by using highly conductive PEDOT:PSS interfacial layer. This process enables the fabrication of perovskite solar cells using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron transport layer with more than 12% power conversion efficiency, low hysteresis and excellent operational stability. Figure 6 shows that The current increases as the number of incident photons increase till the saturation of harvesting capability of the material [7].
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Fig. 6 Electroluminescence versus driving current measured on one of the prepared solar cells (adopted from reference [7]) External Quantum Efficiency (EQE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell from outside (incident photons). (EQE) is used to determine the power conversion efficiency (PCE) of perovskite solar cell. the analysis shows that this type of perovskite solar cells can reach efficiencies of 20% upon reducing optical, electrical, and nonradiative recombination losses [7, 8]. Typically, PSCs can be classified into two main categories according to their structure, which are commonly named as mesostructure and planar device. A mesoporous scaffold fabricated with TiO2 or Al2O3 particles at high temperature is commonly employed in the mesoscopic structure, which is relatively complicated and inhibits the vast applications especially in
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flexible or tandem cells, where the bottom substrate or device is sensitive and hard to endure the high temperature treatment [9]. On the contrary, the planar PSCs have a simple device architecture similar to organic or inorganic thin film solar cells and are possible to be fabricated at a moderate temperature, hence, are more qualified for novel efficient photovoltaic devices. The planar device includes n-i-p and p-i-n architectures depending on the light illumination direction. A PIN diode operates under what is known as high-level injection. In other words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. That makes it act like a normal resistor at high frequencies. Approaches to further upgrade the current silicon heterojunction solar cells and improve the efficiency as high as 25.6% are highly attractive. One of the most possible strategies is fabricating tandem device by integrating a PSC with a suitable bandgap above silicon device, which can absorb higher energy photons and realize a high voltage. The pioneer works have already been carried out in several groups and the tandem devices with an exciting efficiency higher than the top subcell has been achieved recently. This thereby further inspires the coming active studies [9].
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Fig. 7 J-V curves of solar cells with and without the PC61BM buffer layer (adopted from reference [9]) The power of the solar cell = current × voltage that means that The solar cell efficiency in case of using the PC61BM buffer layer is higher than without using it as we see in figure 7 [9]. Perovskites of Three-dimensional (3D) lead halide based are emerging as one of the most promising light absorbers in solar cells in the recent years. Rapid material and device optimization has led to their power conversion efficiency (PCEs) exceeding 20% in just a few years after releasing this type of solar cells in December 2013. Among the perovskite choices, methyl ammonium lead iodide (MAPbI3) has become the archetypal light absorber due to its solution process-able, high coefficient, tunable and medium band gap, small exciton binding energy, and long exciton and charge diffusion lengths [9].
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1.6. Remarkable progress in improving PCEs of PSCs Although remarkable progress has been achieved in improving the PCEs in perovskite solar cells, these cells still suffer from the issue of the moisture instability, which is mainly associated with phase transitions and ambient hydrolysis of MAPbI3. Great efforts, such as improving the crystallinity and quality of perovskite films, partial or fully replacing MA + with formamidinium (FA+), substitution I- with Br-, Cl-, or pseudo-halide SCN-, regulating the morphology and crystallinity of the contacting electron transport layers (ETLs), or adding a buffer layer or an encapsulation layer atop are devoted to enhance the stability as well as the efficiencies of the perovskite solar cells. Nevertheless, the moisture sensitivity of 3D MAPbI3based compounds has not been substantially addressed yet [10]. Therefore, it is a great importance to study and develop alternative classes of moisture resistant perovskite compounds for photovoltaic applications in order to enhance its harvesting ability towards the solar radiance. Recently, attempts to utilize two-dimensional (2D) layered hybrid compounds in perovskite solar cells have been made with pioneering progress. For example, Smith I.C. et al. reported the solar cell application of a layered (PEA)2(CH3NH3)2Pb3I10 (where PEA=phenylethylammonium) perovskite light absorber, demonstrates an open-circuit voltage of 1.18 V and a power conversion efficiency of 4.73%. More importantly, it is relatively stable in air containing 52% relative humidity for up to 46 days [9, 10]. Perovskite solar cells based on CH3NH3PbI3-xClx have differences between two electron transporting materials used as n-type selective contacts are evaluated: the classical titanium dioxide (TiO2) and tungsten oxide (WO3) as a potential alternative. WO3 presents the advantage of
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being printable and does not require high annealing temperature: allowing cheap, expendable and efficient solar cells. we observe a drastic improvement of device stability in the dark under ambient conditions of WO3-based devices compared to TiO2 [11]. Exploiting Kelvin probe force microscopy (KPFM) the lower open-circuit voltage is explained, through transient photoluminescence measurements the crucial role played by oxygen vacancies and moisture is pointed out to rationalize this behavior. Finally, demonstrations of perovskite solar cells containing spin-coated or printed WO3 processed under ambient condition and presenting power conversion efficiency close to 10% are provided [11].
Fig. 8 Post-annealing UV–Visible transmission and reflection spectra for compact and mesoporous TiO2 and WO3 on glass, absorption of CH3NH3IxCl3-x on glass (adopted from reference [11])
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The investigation of the stability of tungsten oxide (WOx or WO3) is done. since this oxide is already commercially available in a printable nanoparticle alcoholic solutions, allowing the production of a mesoporous and rough layer, and has been Already used with success as an interface layer in organic solar cell. But, WO3 devices are more sensitive than TiO2 to ambient moisture under illumination conditions, leading to a rapid decrease of photocurrent during operation. And The presence of water molecules is indeed revealed in the perovskite active layer through photoluminescence measurements, which highlights the detrimental influence of the photocatalytic properties of WO3 on device lifetime compared to TiO2 [11]. The behavior comparison between TiO2 and WO3 is made figure 8 Timeresolved photoluminescence measurements performed at the perovskite / electron selective interface show that the charge transfer efficiency from the perovskite to WO3 is comparable to that of TiO2, and is not negatively affected by a storage period of several days in the dark in ambient conditions [11].
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2. Organic-inorganic hybrid perovskite solar cells 2.1. Synthesis of organic-inorganic hybrid materials The organic-inorganic hybrid perovskites have been known for decades. initiated the study of optoelectronic applications of these materials for devices in the 1990’s. This class of materials is advantageous for solar cell applications particularly with respect to their relatively lowcost and to their superb photovoltaic characteristics. In addition to the tunable bandgap by changing chemical compositions of organic moieties or metal and halide elements can diversify the optical properties and indicate a promise of engineered material properties achievable by employing a variety of preparation methods [12]. preparation of perovskite is very interesting objective but initially we have to determine the film thickness, film quality, film coverage and transparent properties. A fine control over the reaction between the inorganic and the organic species is essential to prepare perovskites with desired properties and required photovoltaic performance. The solution processing is suitable for organic-inorganic perovskites because the precursors of components are substantially soluble in conventional organic solvents and can be readily applied for solution crystal growth. (precursors) means the chemical materials that responsible for the initiation of the chemical reaction. Using this technique, two or three-dimensional perovskites could be successfully prepared as we planned. However, a careful tuning of film microstructure and surface morphology is necessary to obtain a high quality film of better crystallinity, good uniformity and coverage, large domain size of crystals, and small surface roughness [12].
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Fig. 9 SEM images of the top surfaces of CH3NH3PbI3-xClx films (A) prepared at 100 °C for different anneal times in a nitrogen atmosphere. (B) perovskite coverage varying with annealing temperature (adopted from reference [12]) By annealing a PbI2 film and CH3NH3I vapor in a protective atmosphere, films with well-defined grain structure can be fabricated as shown in figure 9. Scanning electron microscopy (SEM) reveals that upon increasing annealing time, many additional small pores form rapidly (Figure 9A, 10 min), and then pore size increases along with smaller pores close up until the final crystalline phase formed (Figure 9A, 60 min). The amount of pinholes in the perovskite film is also reduced with increasing annealing temperature (> 100 °C), but the pore size becomes larger and the morphology evolves from continuous layers into discrete islands of perovskite which leads to worse surface coverage and discontinuities as shown in Figure 9B [12].
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Fig. 10 (A) Optical microscope images of perovskite films prepared in different humid atmospheres on compact TiO2-coated FTO substrates. The scale bar is 20µm for all images. For these images, the perovskite material is light in color while the substrate is darker. (B) Photoluminescence quantum efficiency for perovskite films prepared in different humidities. Each data point is the average of at least three measurements, with error bars indicating standard deviation. (C) Time-resolved photoluminescence measured at 785 nm, with excitation at 510 nm, 3 µJ/cm2 per pulse, at 1 MHz, for films prepared in different humidities (adopted from reference [12])
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2.1.1. Gas-assisted preparation of lead iodide perovskite films the perovskite films made by the gas-assisted method had a much smoother surface than that films made by the conventional method. Spincoating was used to prepare the CH3NH3PbI3 films on FTO glass substrates coated with a dense TiO2 blocking layer. Based on the conventional spin-coating method, an additional step was introduced where a dry argon gas flow was blown over the surface of the perovskite solution during the spin-coating process (shown schematically in Figure 11), promoting evaporation of the solvent and accelerating the supersaturation of the solution. The films deposited by the gas-assisted and the conventional spin coating methods both showed a light color [13].
Fig. 11 Schematic procedure for the gas-assisted spin-coating method progressing from left to right (adopted from reference [13])
A TiO2 dense blocking layer deposited on the clean FTO glass by pyrolysis spray of a bis(isopropoxide)-bis(acetylacetonate)titanium(IV) solution at 450 ºC. After cooled to room temperature, the substrate was cut into around
1
cm2.
A
25
microliters 45
wt%
CH3NH3PbI3
(DMF)
dimethylformamide solution, prepared from PbI2 and CH3NH3 in a molar
17 | P a g e
ratio of 1:1, was spread on it, on a spincoater. For the conventional spincoating method, the solution was spun at 6500 rpm for 30 s, while for the gas-assisted method, a 60 psi dry Argon gas stream was blown over the film during spinning at 6500 rpm in 2 s after the spin-coating commenced. The films were then annealed at different temperatures (35-100 °C) on a hotplate for 10 min, and then cooled to room 5 temperature on a steel substrate. A 25 microliters (spiro-OMeTAD) solution (prepared by dissolving 41.6 mg spiro-OMeTAD, 7.5 microliters of a stock solution of 520 mgmL-1 lithium bis(trifluoromethylsulphonyl)imide in acetonitrile and 14.4 microliters 4-tert-butylpyridine in 0.5 mL chlorobenzene) was coated on the perovskite film by spin-coating at 3000 rpm for 30 s. A 70 nm silver layer was deposited by thermal evaporation to form the complete device [13].
2.2. fabrication of high-performance PSCs The power conversion efficiency (PCE) of perovksite-based solar cells has been rapidly increased from 3.8% to more than 19% within the last few years. Due to the singular characteristics of perovskite material such as excellent and tunable optical properties, ambipolar charge transport, and very long electron-hole diffusion lengths [14]. The dense TiO2 compact layers were deposited onto a F-doped SnO2 (FTO, Pilkington) by either dip-coating or spin-coating an ethanolic solution of titanium diisopropoxide bis(acetylacetonate). For the dipcoating method, two critical variables such as a withdrawal rate of coater and a concentration of the Ti precursor solution play a significant role in controlling the thickness and morphology of the resultant thin film. Thus, while we set the withdrawal rate of 1mm/s, we varied the concentration of Ti precursor solution from 1:5 to 1:13 (volume ratio of Ti precursor to
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ethanol). For the spin-coating method, the same Ti precursor solutions to those used in the dip-coating route were spin-coated on FTO substrate at 5000 rpm for 20s. In both cases, the electrodes were aged at 35 ℃ for 1 hour and heated in air at 450℃ for 30min. The surface morphology of the compact
layer
was
investigated
using
a
scanning
electron
microscope(SEM) [14].
2.2.1. Synthesis of ZnGa2O4:Eu3+ ZnGa2O4:Eu3+ nanophosphor has very important property of acting as a light down-shifting/converting material to absorb the high energy photons and emit lower energy photons that match the absorption of perovskite layer well, which can excite CH3NH3PbI3 to generate more photo generated electron-hole pairs, and thus promote the incident light use ratio and increase the power conversion efficiency (PCE) of PSCs [15]. ZnGa2O4:Eu3+ was synthesized through a hydrothermal method. 0.05 mmol Zn(acac)2 zinc acetylacetonate, 0.1 mmol Ga(acac)3, 0.01 mmol Eu(acac)3 and 20 ml oleylamine (is the organic compound with the formula C18H35NH2. It is an unsaturated fatty amine related to the fatty acid oleic acid) were added into 50 ml flask. The solution was heated at 120 °C under vacuum for 30 minutes and at 300 °C under N2 atmosphere for 6 hours. The product was isolated by centrifugation, washed using chloroform and methanol, and dried under vacuum overnight as shown in Scheme. 1. The color of obtained ZnGa2O4:Eu3+ nanophosphor is tan color [15].
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Scheme. 1 preparation of ZnGa2O4:Eu3+ nanophosphor using hydrothermal method
(50ml flask)
0.05 mmol Zn(acac)2 +0.1 mmol Ga(acac)3 +0.01 mmol Eu(acac)3 + 20 ml oleylamine
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drying the product
washing the product
under vacuum
using chloroform
overnight
+ methanol
Fig. 12 (a) TEM image of ZnGa2O4:Eu3+ nanophosphor; FESEM images of mesoporous TiO2 layer (b) without and (c) with ZnGa2O4:Eu3+ (8 mg ml); (d) FESEM image of cuboid perovskite layer on (c) (adopted from reference [15]) ZnGa2O4:Eu3+ nanophosphor figure 12 was successfully synthesized and introduced into mesoporous TiO2 layer of PSCs. A highest PCE of 14.34% and a typical PCE of 13.80% under one sun illumination is achieved, much higher than that of the cell without ZnGa2O4:Eu3+. The large improvement of PCE is attributed to the enhanced light harvesting mainly via the light down-shifting/converting offered by the ZnGa2O4:Eu3+ nanophosphor and improved exciton generation rate [15].
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2.2.2. PSCs based on ZnO nanostructures Its known that TiO2 is the most commonly used material for the electron transport layer in PSCs, but ZnO is an excellent inorganic semiconducting material owing to its large surface area, easy synthesis, large variety of synthesizable nanostructures and low cost of fabrication. It can potentially replace TiO2 as the electron transport layer material in high efficiency perovskite solar cells based on flexible substrates [16]. Table. 2 comparison of the physical properties of TiO2 and ZnO (adopted from reference [16])
Physical property Crystal structure
TiO2
ZnO
Rutile, anatase, and Rocksalt, zinc blende, brookite
and wurtzite
Energy band gap [eV]
3.0–3.2
3.2–3.3
Electron mobility
0.1–4.0 (bulk TiO2)
205–300 (bulk ZnO)
[cm2·V·s−1] Additives
Li, Mg, Ca, B, Ge, Mn, Al, Ge, Sn, Ga, In, Mn, Ni, Cu, Zn, Sn, Al, In, Ti, Zr, N, F, Cl N, F, I
there are Some issues related to ZnO-based PSC thoughThe large enhancements achieved in the device performance suggest that PSCs have great potential and can surpass traditional silicon solar cells. However, despite their high efficiency and relatively low cost, perovskite materials show unstable properties. Nominal operating cell temperatures of 50 °C are common for commercially available photovoltaic modules in 22 | P a g e
extreme environments (areas with high air temperatures and above average insulation), the temperature of an operating cell can easily exceed 85 °C. Taking into consideration the actual environmental conditions and the practical application of these devices, it is imperative that the long-term stability of ZnO-based PSCs be investigated and addressed [16]. The currently available hybrid organic-inorganic perovskite materials are sensitive to moisture and oxygen leading to a decrease in the device efficiency, it is commonplace to fabricate devices in a controlled humidity environment, such as a glove box, to reduce exposure to moisture. This drawback causes problems in large-scale production and affects the longterm stability of PSCs. However, two main components in ambient air, H2O and O2, will affect the chemical stability of PSCs. From a long-term perspective, the air stability of devices is an important issue for the commercial development of PSCs. Currently, there is a lot of work going on to enhance the stability of PSCs. it was studied that the basic mechanism for the degradation of PSCs in ambient air. They proposed some degradation process reactions and confirmed that air and sunlight are necessary for the degradation process [17].
2.2.3. PSCs based on Al2O3 nanorods The researchers are always seeking to develop and experiment a variety of new architectures. One-dimensional nanostructures are naturally introduced in solar cells, because of their excellent charge transport properties and open-pore structure. However, the performances of these solar cells are inferior to their mesoporous counterparts, suggesting that some unique mechanisms maybe held behind devices operation. Here, a three-dimensional optical model combined with a two-dimensional 23 | P a g e
axisymmetric semiconductor model is applied to investigate the influence of the architectural design of scaffolds on the properties of perovskite solar cells based on Al2O3 nanorod arrays. Simulation results show a great dependence of device performance on the density, length and porosity of Al O nanorods, which decided the electron field distribution and carrier 2
3
recombination loss inside the cells. Strikingly, an optimal length of 450 nm for Al2O3 nanorods is obtained for the perovskite solar cells with efficiency over 20% at porosity of 0.7. The results obtained have some guidance function on the fabrication of high efficiency PSCs based on nanorods [18].
Fig. 13 The schematic diagram for the unit PSCs based on Al2O3 nanorods: the 3D structure for optical model, where scell is the width of the unit cell (adopted from reference [18]) 24 | P a g e
2.2.4. The impact of Pd on the light harvesting in PSCs the incorporation of Pd in organic-inorganic hybrid perovskite, CH3NH3PbI3 has a significant effect in a decrease in the band gap energy of over 20% was observed in the samples with Pd, due to the presence of energy levels related with the Pd-I interaction. This feature can improve the light harvesting of this kind of material, which could have significant good consequences in the efficiency performance of photovoltaic devices [19]. Scheme. 2 The schematic of types of devices contained in the architecture (adopted from reference [19])
the distribution of perovskite structures formed with Pd was observed to be uniform with regard to the distribution of Pb, which is evidence of the formation of hybrid Pb-Pd tetragonal perovskite. Also, the reflections
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assigned to tetragonal perovskite in XRD x-ray diffraction patterns show a shift in the samples with Pd, which is evidence of the deformation of the perovskite due to the presence of Pd in Pb sites. Furthermore, XPS revealed that the Pd found in these situations is Pd2+. This leads to small modifications in the interactions between the I- ion and the corresponding B ion, which is responsible for the distortions in the structure. On the other hand, the band gap values obtained theoretically were concordant with those obtained using UV–Vis spectroscopy. DFT calculations showed that new peaks appear in the valence-band-maximum (VBM) when the concentration of Pd increases. The analysis of the PDOS indicates that the contribution of the d-Pd and p-I states is predominant in these peaks, which lead to think in the great impact of the presence of Pd in band structure in the perovskite. This result suggests that, in general, the Pd-I interaction must be strengthened when the concentration of Pd increases [19].
2.3. Controlled orientation of perovskite films The perovskites, as light‒absorber component, exhibit broad absorption spectra, large extinction coefficient, high carrier mobility, as well as ambipolar charge transport. Despite the amazing performance, there is still a huge gap between academic studies and industrial applications on account of the poor stability. CH3NH3PbI3 (MAPbI3), the most commonly used perovskite, is vulnerable to the humidity, heat, and UV light conditions. For this reason, doping has been demonstrated as an effective method to improve the stability and photovoltaic performance. For example, due to a more compact and stable crystal structure compared to MAPbI3, mixed halide perovskite MAPb(I1‒xBrx)3 is resistant to the moisture. Cs‒doped [HC(NH2)2]3PbI3 (FAPbI3) film exhibits merits on both 26 | P a g e
the moisture and light soaking stability. However, when introducing Cs into MAPbI3, the yellow orthorhombic phase of CsPbI3 easily segregated from the film, especially for higher doping content, restricting the performance [20]. The doping content of Cs in the perovskite film, could be linearly tuned through altering the composition of precursor solutions. As shown in the X‒ray diffraction (XRD) spectra (Figure 1a), for (MAPbI3)1‒x(CsPbBr3)x, there was no additional CsPbBr3 or CsPbI3 orthorhombic phase observed in the film. For tetragonal phase of perovskite, due to the similar interplanar spacing, the peaks for (110) and (002) facets are highly close to each other, and so for (112) and (200) facets, thereby the two facets for corresponding peak were both labelled in the XRD spectra. As shown in the schematics (Figure 1c), in the tetragonal phase, one Pb atom is coordinated with six I atoms, while one MA group is located in the center of a distorted cubooctahedral pocket with 12 I atoms at the vertices, enclosed by eight PbI6 octahedra [20].
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Fig. 14 (a) X‒ray diffraction (XRD) patterns of fabricated perovskite films composed of (MAPbI3)1‒x(CsPbBr3)x with x ranging from 0 to 0.2. (b) Rocking curve measurement of (224)/ (400) diffraction peaks for x=0 and 0.1. (c) Schematic illustrations of (110), (002), (112), and (200) crystal planes from a perpendicular view (adopted from reference [20])
2.4. Pursuing of efficiency and stability of PSCs Increasing energy demand, environmental issues and limited availability of fossil fuels are demanding the research on sustainable and renewable energy resources. One of the most important and available ways to get energy is the sun light so we can get energy through solar cell and we should to develop our solar cell devices to reach higher efficiency as we 28 | P a g e
can. In the third generation solar cells, the dye-sensitized solar cells (DSSC) provided cost-effective and simple fabrication. the highest reported PCE of a PSC is reached >22.1 % in march 2017 [21].
Fig.15 Chemical vapour deposition (CVD) technique (adopted from reference [21]) Chemical vapour deposition (CVD) technique (figure 15) is reported as a simple onestep technique to fabricate CH3NH3PbI3 and CH3NH3PbI3-xClx based PSC with a PCE of 11%. In this method two temperature zones are present in which the first zone is high-temperature zone and the second is low-temperature zone. the substrate is kept in the low-temperature zone while lead chloride lead iodide and methylamine iodide (MAI) are kept in the high-temperature zone. In the second zone and the precursors are kept according to their vaporization temperature. Perovskite layer is deposited on the substrate when the vaporization temperature reaches. It is made to vaporize the precursors and it is transported to a low temperature zone using Argon as a carrier gas. In this technique high quality films can be obtained by optimizing parameters such as deposition
29 | P a g e
time and annealing temperature according to universal recipes. Schematic diagram of this method is given in figure 16. The reported PCE for CH3NH3PbI3 and CH3NH3PbI3-xClx PSCs are 9.2 and 11.1 % respectively [21].
Fig. 16 Chemical vapour deposition technique to fabricate CH3NH3PbI3 (adopted from reference [21]) Table. 3 Summary of evolution in PSCs through the last few years (adopted from reference [21])
year
Configuration
PCE %
2009
CH3NH3PbI3/ TiO2
3.81
2012
TiO2/CH3NH3PbI3/ Spiro-OMeTAD
9.7
2012
Al2O3 /CH3NH3PbI3−x Clx/ SpiroOMeTAD
10.9
2014
TiO2 /HC(NH2)2PbI3/CH3NH3PbI3/ Spiro-OMeTAD
16.01
2014
bl-Y-TiO2/CH3NH3PbI3-xClx
19.3
2015
TiO2/(FAPbI3)1-x(MAPbBr3)x/ PTAA
15.4
(x=0-0.30) 2015
PCBM/C60/BCP/ CH3NH3PbI3/ PEDOT:PSS
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15.1
2.4.1. Improving the electron transportation in the hybrid PSCs CaMnO3 presents a better candidate for the electron transport material in thin film hole transporting material free hybrid perovskite solar cells with the planar architecture than the most common anatase TiO2. the faster charge carrier mobility improved charge transfer and reduced exciton recombination achieved because of the more appropriate band gap of CaMnO3 and better band alignment with the hybrid perovskite [22].
Fig. 17 Schematic representation of a HTM free thin-film perovskite solar cell layers and optical processes during the device operation (adopted from reference [22])
There is a reported model suggests an unoptimized device with a photoconversion efficiency of almost 10% for the low defect concentrations under 1015. Among the various reported inorganic perovskite materials 31 | P a g e
CaMnO3 (CMO) is a material of great interest because of its beneficial band gap of 1.7 eV which is just somewhat higher than in commonly used perovskite material, CH3NH3PbI3 (MAPI) with 1.5 eV, chemical and thermal stability, low cost, non-toxicity and resistance to oxidation. The two most important optical characteristics of the buffer layers in the studied solar cells are their reflectance and refraction figure 17 the Decrease of the light intensity (dark blue photon ray) as it traverses through the device because of the absorption process is illustrated by the light blue photon ray. Some absorption besides transmission is expected in the buffer layer. Insets show the charge carrier transport which occurs within the perovskite layers as an ambipolar diffusion of electrons and holes. Perovskite buffer/absorber layers here replace a layer with TiO2 coated in the absorber material in the with HTM mesoporous solar cells Both characteristics indirectly influence the total cell efficiency by providing a denser flux of photons to the absorber [22].
Fig. 18 Recombination processes in semiconductor materials (adopted from reference [22])
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When the excited electrons come back from the conduction to the valence band due to the recombination of photo-generated electrons and holes, they will release their extra energy as photons. The excited electrons follow the transition selection rule of momentum conservation figure 18 Direct semiconductors exhibit shorter exciton lifetimes while indirect semiconductors typically exhibit longer exciton lifetimes due to the finite wavelength phonon process required prior to exciton recombination. After the investigation of The limitations of TiO2/perovskite interface have been identified and a new alternative buffer layer as an electron transport material of CaMnO3 is introduced which admits a significant scope for the performance enhancement [22].
2.4.2. Flexible PSCs with transparent CNT electrode
Fig. 19 cross-sectional morphology of perovskite/TiO2 nanotubes/Ti electrode (adopted from reference [23]) The cross-sectional morphology of perovskite loaded TiO2 nanotubes is presented in Figure 19. The tube arrays formed on Ti foil are 300 nm in
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length and 60 nm in diameter. A dense layer of perovskite nanocrystals with a size of 100-400 nm completely covers the nanotubes. The flexible CNT network is transferred on top of the perovskite layer as the counter electrode [23]. Flexible solar cell based on titanium (Ti) foil/TiO2 nanotubes (TNTs) with organic–inorganic halide perovskite absorber and transparent carbon nanotube electrode are one of perfect examples of recently solar cells. Where the power conversion efficiency of 8.31% has been achieved, which is among the highest for TiO2 nanotube based flexible solar cells. In addition to the device performance demonstrate a good flexibility of the Ti foil based perovskite solar cells. The Ti foil based solid-state flexible perovskite solar cells have great potential for applications in building [23].
2.5. Enhancement of Structural and Electrical Properties of PSCs Hybrid
organic-inorganic
perovskites
nanoparticles
(NPs)
have
remarkable and promising photophysical properties. New and interesting properties emerge after combining perovskite NPs with semiconducting materials. the UV-Vis spectra recorded for pristine P3HT and P3HT/PNP film are shown in Figure 20a. In Figure 20b, we present the difference between the spectrum of P3HT with and without perovskite nanoparticles (PNPs) to magnify the differences. the Changes in the vibronic structure of P3HT account for different aggregation, while appearance of novel absorption peaks on the blue absorption tail are ascribed to perovskite nanoplatelets [24].
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Fig. 20 (a) UV-Vis absorption spectra of P3HT (black) and P3HT/PPs blend (blue). (b) Difference between UV-Vis spectra of P3HT with and without perovskite NPs (adopted from reference [24])
Generally, to achieve high-performance photovoltaic devices, highabsorption cross sections for efficient light collection, efficient long-range charge separation, and low-loss charge transport and collection are required. The light absorption coefficients of some hybrid organic– inorganic perovskites HOIPs (for example, MAPbI3) are among the highest for materials with solar-cell conversion efficiencies >10%, which significantly reduces the required thickness of the absorber layer to hundreds of nanometers. The HOIPs used in solar cells can display long diffusion lengths, from a hundred nanometres to several micrometres, depending on the form of the thin films or single crystals, for both holes and electrons. The long carrier lifetimes in combination with high efficiencies, necessitate a consideration of photon recycling when discussing carrier transport in thin films under illumination. It has been
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reported that initially photoexcited carriers can recombine away from the excitation spot to regenerate photons, which can then be reabsorbed to form charge carriers at significant distances away from the initial excitation point. The long transport lengths suggest that these materials can function effectively in thin films, because charges can be transported in the perovskite over long distances. The exciton binding energy (EB), which defines the lowest energy required to dissociate an exciton (electron–hole pair), must be small to achieve efficient charge-carrier separation [24, 25].
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Conclusion It is exciting that hybrid perovskites have opened up an entirely new chapter of discovery and developing the solar cells. One of the most important features in the perovskite materials is that it can absorb a wide range of solar radiance through controlling the bang gab of the material by changing the type of the used metal and halide or the percentage of the halide that in the chemical composition of perovskite material (for example CH3NH3PbI3-xClx its PCE 19.3%). there are different techniques to fabricate the perovskite solar cells (PSCs), one of the most efficient and fast techniques is the chemical vaper deposition (CVD) technique. The most common obstacle facing the progress of PSCs is that its vulnerability to the humidity but it has been reported that a mixed halide perovskite MAPb(I1‒xBrx)3 is resistant to the moisture. Using CaMnO3 (CMO) as the buffer layer results more efficient PSCs, and proposed device benefits from the advantageous properties of CMO, like low cost, non-toxicity, stability in chemical and thermal processes and resistivity to oxidation. The Ti foil based solid-state flexible PSCs with transparent CNT electrode have great potential for applications in building. ZnO nanostructures is an excellent inorganic semiconducting material owing to its large surface area, easy synthesis, large variety of synthesizable nanostructures, and low cost of fabrication. It can potentially replace TiO2 as the electron transport layer material in high efficiency PSCs based on flexible substrates. Al2O3 nanorods raised the PSCs efficiency to over 20% at porosity of 0.7.
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natrevmats
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Arabic summery
خِاللِالسنننن ااقِالة ا مِال
ننننِامِ،ظِهرِِ اِهِ ئلِِفيِالِهت مِب ل اادِالعِ نننناامِاالغِارِ
العِ اامِالهِجا مِ"البِارِاﭬ سِكِ اقِ"ِك اادِااعِدةِلتِطاارِالخِالا ِال ش ساِم بسببِخ ص ئ صه ِ البِصننرامِاالكِهرب ئامِال ِت ازةِ،لذلكِكِ نِ نِال ِهمِالتركازِع ىِالخالا ِالش ن سنناِم ال ِعت دةِ ع يِالباراﭬسنننك اقِِفيِهِذاِالبحثِ.اِعتبرِالفصنننِلِاألالِك ةد مِاِاِا ننن ِكالًِ نِاآلتىِ تكاانِ ِ دةِالباراﭬ سك اقِِ،ااِ صفِلِ ذاِالباراﭬ سك اقِِهيِالخِالا ِال ش سامِالااعدةِاالتىِ اسنننعىِالع ءِلتطااره ِبشنننكلِ ِسنننت رِ،ك ِاعرضِل حمِع ِمِعِنِكافامِزا دةِكف ءاقِ ِ الخالا ِالش ساِم ال ِعت دةِع يِالباراﭬ سك اقِِخِاللِالس ااقِالة ا مِال هذةِالخالا ِال ش ساِم ِ.الف صِلِالث يِاِا اادِالع ِ ال
اامِاالغِارِال ِع
ِامِاع امِتص اعِ
ِكالًِ نِاآلتىِكافامِتِركابِالباراﭬ سك اقِِ ِنِ
اامِ،تِ ص اعِخالا ِ ش ساِمِ ِعت دةِع يِالباراﭬ سك اقِ عِ لامِ
األداءِ نِخِاللِإِ شِراكِأ ااعِ ِخت فمِ ِنِالع صِرِفيِالتِ ص اعِفيِ ط قِال اِ،ك ِاِا
ِ
تأثارِهذةِالع صِرِال ِدخِ مِع يِقدرةِ ِ دةِالباراﭬسك اقِِع يِعِ امِإ تص ص ال ِاءِ،ك ِ اعرضِطراةمِل س نِاطرةِع يِاِتج هِأفالم الباراﭬس نك اقِِ،طرقِلتِحسننانِِةلِاإللكترانِفيِ الخالا ِال ش ساِمِال ِعت دةِع يِالباراﭬ سك اقِِ،اإ ت ج خالا ِ ش ساِمِت ت زِب ل ِرا مِاكافامِ تعزازِالخص ئصِالهِاك امِاالكهرب ئامِل خالا ِالش ساِمِال ِعت دةِع يِالباراﭬسك اقِ.
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