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Herein, we will present recent progress in the compact layer CL or hole blocking layer HBL which is known as an important layer and not as an essential layer for perovskite solar cells PSCs. Thus, any change, modification, and replacement in this layer will have a profound effect on the performance and improvement of some characteristics such as photo-stability, durability and hysteresis effect. These changes can improve the applications of PSCs in the flexible cell, industrial mass production, high-scale manufacturing. In this review, we will present recent studies on CLs.

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Herein, we will present recent progress in the compact layer CL or hole blocking layer HBL which is known as an important layer and not as an essential layer for perovskite solar cells PSCs.

Thus, any change, modification, and replacement in this layer will have a profound effect on the performance and improvement of some characteristics such as photo-stability, durability and hysteresis effect.

These changes can improve the applications of PSCs in the flexible cell, industrial mass production, high-scale manufacturing. In this review, we will present recent studies on CLs. Energy consumption is rising and energy sources, especially fossil fuels, are finishing. Therefore, the demand for renewable, accessible energies has increased significantly.

After discovering the photovoltaic effect in [1] , solar cells appeared and attracted a lot of attention to convert solar energy into electricity.

Among the various types of solar cells, PSCs were more important and impressive because perovskite materials, used as a light absorber in PSCs, have excellent advantages such as a great light absorbing capability, small exciton binding energy, direct band gap, and outstanding charge carrier mobility [ 2 , 3 , 4 , 5 ].

PSCs structures include a conductive substrate such as fluorine doped tin oxide FTO , an electron transport layer ETL which involves a compact layer CL and a mesoporous scaffold layer for mesoporous PSCs , perovskite layer as an absorber, hole-transporting material HTM , and a metal electrode.

There are two structures for PSCs. In usual MS-PSCs, mesoscopic scaffold layer formed by small size TiO 2 NPs creates a structure with little spaces which restricts perovskite grains growth inside TiO 2 scaffold and produces many voids, results to augment carrier recombination and leakage currents in PSCs.

They can operate well due to the ambipolar nature of perovskite [8]. P-PSCs devices involve the simplest structure and fabrication processing due to removing high temperature sintered mesoscopic layer.

Also, P-PSCs can be fabricated with a low-temperature process, which is suitable for making flexible solar cells [ 9 , 10 , 11 , 12 ]. Many efforts have been made to improve the efficiency and stability of solar cells, which contain many studies on various layers in PSCs structures.

In this review, we are focused on articles presented for CLs. Therefore, it will diminish charge recombination and augment the open-circuit voltage V OC [ 13 , 14 , 15 ]. The CL is an electron selective layer which can collect the photo-generated electrons from the perovskite layer and transport electrons toward FTO [ 17 , 18 ].

Therefore, it is essential that CLs involve compact uniform films without pinholes and cracks to obtain high efficient PSCs [19]. Furthermore, CLs with optimized thickness are very essential [20]. A thick CL will increase the distance of electrons to transport from the perovskite layer to the FTO, leads to decrease the charge transport. Thus the FTO layer contacts with the perovskite layer directly, leads to the charge recombination with formed holes in the perovskite layer.

The optimized thickness of TiO 2 as a CL reported via traditional methods, the spin-coating method, is 50— nm [20]. In fact, CL thickness is related to the balance between hole blocking capability and carrier transport loss. Full coverage and adequate thickness increase hole blocking ability. Suitable film thickness decreases the carrier transport loss [21]. An appropriate CL should include some properties, such as reduced transport resistance which enhances electron extraction, good transparent in the visible light region, appropriate conduction band CB level which matches CL energy levels with the perovskite layer well to decrease energy loss.

In addition to, superior electron mobility, low-temperature preparation process are important [ 22 , 23 , 24 ]. However, in inverted PSCs which ETL placed promptly on top of the perovskite layer instead of HTL, the ETL should fully cover the perovskite layer to protect the perovskite layer from deleterious oxygen and moisture effects.

Also, the used solvents in the deposition process of the ETL should be considered [ 25 , 26 , 27 , 28 ]. The TiO 2 as an intrinsic n-type semiconductor along with a wide band gap exhibits a substantial role in augmenting electrons transportation and preventing recombination phenomenon between the FTO and HTM layer.

It contains advantages such as chemical stability, inexpensive and suitable CB. But, TiO 2 has disadvantages such as low electron conductivity due to low carrier density and low electron mobility which can produce unsuitable charge transport in the perovskite [36].

Also, the photoactive nature of TiO 2 CL indicates a detrimental effect on perovskite layer stability [37]. Therefore other materials were proposed to replace TiO 2 to improve its properties. It's important to be mention that conventional methods for fabrication TiO 2 CLs such as spin-coating and spray pyrolysis need a high-temperature sintering procedure which increases the costs and stops the fabrication of flexible devices. Other methods such as atomic layer deposition ALD [16] , electrochemical deposition [ 40 , 41 ], high pressure pressing [42] , and chemical bath deposition CBD [43] could not utilize low temperature processing.

The presented articles can provide appropriate solutions to solve disadvantages and problems in the CLs. As previously mentioned, the TiO 2 CL indicates low electron extraction owing to its relatively poor carrier mobility. So, semiconductors with higher carrier mobility are used to replace them. However, both devices showed the obvious hysteresis.

The existence of a hysteresis effect, lack of similarity of J-V curve which observes by changing the direction and scan rate, will reduce the accuracy of measured performance.

The PCE Also, the hysteresis effect this device improved compared with that based on TiO 2 CL. Because TiO 2 can excite by UV light easier than SnO 2 which causes TiO 2 operates as a photocatalyst, as a result, accelerates the decomposition of perovskite.

Another kind of metal oxides is zinc oxide. The ZnO CL with low temperature solution-process has good electrical property and high charge mobility. But, ZnO CL showed poor thermal stability. Deprotonation of the methylammonium cation by the ZnO surface produces methylamine and PbI 2.

Deprotonation of the methylammonium cation available in perovskite structure occurs more easily at ZnO surfaces while deprotonation is difficult in AZO, owing to this fact that the acidic property of ZnO has increased by doping of aluminum.

The more acidic metal oxide surfaces provide more thermal stability [51]. In fact, the enhancement of the thermal stability was related to a decrease in the Lewis acid-base reaction between perovskite and CL.

Also, the work function of AZO was 4. The studies have indicated that ZnO as a CL shows some advantages, high electron-transport property, relatively wide bandgap, and electron mobility better than that of TiO 2 [ 21 , 55 , 56 , 57 , 58 , 59 , 60 ]. The high electronic mobility enhances the photo-generated electron transport and therefore provides the fabrication of PSCs devices without hysteresis.

As a result, the passivation of ZnO surface as CL can improve the stability and efficiency of PSCs, as well as remove the hysteresis effect. During EA coordination with Mg, the proton on the hydroxyl end group of EA is quickly eliminated [64]. Mg onto ZnO surface will convert the neutral chelation structure in EA into a charged monodentate structure. ZnO involves an excellent electronic mobility — cm 2 V.

Also, ZSO compound is a transport-conducting oxide with high electron mobility and stability [67]. J SC improves from The highest R sh value shows reduction in short circuits or current leakages [71]. Lower R s and higher R sh create a higher FF and a large electron mobility [72]. The rough surface of the QD-modified TiO 2 nanorods, in comparison to the smooth surface of the unmodified TiO 2 nanorods, can create a bigger surface area and superior contact with the perovskite film.

This QD-modification promotes the light absorption and improves the perovskite crystallization. CQD is a kind of carbon nanomaterials composed of separate, quasi-spherical nanoparticles with sizes lower than 10 nm, which presents good light harvesting property and promising optical, electrical features [78].

The CQDs enhance the energy levels matching between the perovskite and TiO 2 CL that improves electron mobility and electron extraction between the TiO 2 and perovskite layers.

Thus enhance J SC Furthermore, PC 61 BM is expensive and shows variable performances due to the lack of morphological controllability under sintering conditions [83]. In comparison with PC 61 BM and metal oxides, n-type organic small molecules [84] have gotten much attention as ETL, because they can be synthesized easily and modified for matching their energy levels with the perovskite energy levels.

Besides, introducing sulfur species into their molecular structure is very easy, which leads to enhance the interfacial reaction between the perovskite and the ETL via S-Pb or S-I bonding [ 85 , 86 ]. Devices with molecules PC 61 BM or HATNT show slight hysteresis because the perovskite and these molecule clusters cannot quickly penetrate into each other [89] , thus decrease ions motion in the perovskite and hysteresis effect.

An important topic about the TiO 2 CL is the presence of deep trap states on its surface which creates a high leakage current and charge recombination. A predominant strategy to decrease these trap states in order to produce highly efficient PSCs is interface engineering. Modification, improved perovskite crystallization, and enhanced charge mobility and electrical properties for TiO 2. Also, charge separation improved from perovskite layer to the ITO electrode, because of perfect alignment of PNP energy levels with perovskite energy levels.

Thus, moisture entrance into the perovskite layer strongly was prevented in PNP devices. In fact, the PNP interfacial layer between TiO 2 and perovskite layers augmented the value of photo-generated charge carrier sites and decreased the trapping of charges and accumulation at the TiO 2 interface.

J SC enhanced from Thus, charge recombination blocking effect for any passivation layer depends on CB level of utilized material. This was ascribed to high recombination blocking effect by Nb 2 O 5 CL, which was proven from the dark smaller current.

This suggests that quality and interfacial charge transfer are not the major reasons for improving performance by TiO 2 -Cl. The charge recombination lifetime t r in a TiO 2 -Cl device was predominantly higher than that of the TiO 2 device. The developing of low-cost, easy and low-temperature processes for fabrication of TiO 2 CLs with high electron extraction ability to create PSCs with high PCE and without hysteresis effect, have been attracted much attention.

CuI is an inexpensive and stable hole conductor [97]. However, they can be predominantly extracted to the TiO 2 layer at interface formed between perovskite and TiO 2 layer.

In addition, the higher conduction and valence band energy levels of CuI in comparison with those of TiO 2 can induce a shift of TiO 2 band edge and decrease trap states density. Since the recombination rate hundreds nanosecond scale is much slower than the extraction rate tenths nanosecond scale [98] even if a part of the generated holes moves to CuI, positive CuI can pull the electron to the interface between TiO 2 and perovskite layer which enhances electron extraction and decreases nonradiative recombination Fig.

The optimized device indicates negligible hysteresis, which is attributed to removing trap sites and high electron extraction by CuI TiO 2 as a CL. Reprinted with permission from Ref.

The figure of interface with various amount of CuI islands. Without CuI, electron—hole recombination occurs easily, and with CuI, electrons are pulled to the interface between the perovskite and TiO 2 and are extracted owing to the formed dipole moment. However, too many CuI islands on TiO 2 block the electrons passway. It should be mentioned that the decomposition and deterioration of the perovskites via moisture can be affected by CL.

This improves perovskite crystallization, leading to efficient charge transport and high J SC The increment of V OC from 1. It should be noted that cell stability extremely depends on the surface wetting features. As a result, the C-PCBSD film can form an adhesive film with suitable moisture resistance, which can prevent the decomposition of the perovskites by moisture. In addition to moisture stability, UV stability is also important.

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Recent advancements in compact layer development for perovskite solar cells

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The Impact of Hybrid Compositional Film/Structure on Organic⁻Inorganic Perovskite Solar Cells.

Continue to access RSC content when you are not at your institution. Follow our step-by-step guide. Organometal halide perovskites are promising photo-absorption materials in solar cells due to their high extinction coefficient, broad light absorption range and excellent semiconducting properties. However, a high-temperature processed mesoscopic metal oxide e. Although the planar heterojunction PHJ structure can be considered as the most appropriate structure for flexible PrSCs, they have shown lower PCEs than those with a mesoscopic metal oxide layer. We conclude with our technical suggestion and outlook for further research direction.

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Either your web browser doesn't support Javascript or it is currently turned off. In the latter case, please turn on Javascript support in your web browser and reload this page. Perovskite solar cells PSCs have been intensively investigated over the last several years. Unprecedented progress has been made in improving their power conversion efficiency; however, the stability of perovskite materials and devices remains a major obstacle for the future commercialization of PSCs. In this review, recent progress in PSCs is summarized in terms of the hybridization of compositions and device architectures for PSCs, with special attention paid to device stability.

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