Advances of Electron Transport Materials in Perovskite Solar Cells: Synthesis and Application

Ying Yang; Yuan Luo; Shupeng Ma; Congtan Zhu; Liu Zhu; Xueyi Guo

doi:10.7536/PC200515

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (2) : 281-302 DOI: 10.7536/PC200515

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Advances of Electron Transport Materials in Perovskite Solar Cells: Synthesis and Application

Ying Yang ¹^,²^,³
, Yuan Luo ¹^,²^,³
, Shupeng Ma ¹^,²^,³
, Congtan Zhu ¹^,²^,³
, Liu Zhu ⁴
, Xueyi Guo ¹^,²^,³^,^*

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Abstract

Organic-inorganic hybrid perovskite solar cell(PSC) is a photovoltaic device with great potential for development. In the past decade, many studies have been devoted to the preparation of high-performance PSC, and have made amazing progress. Device efficiency has now exceeded 25%. The electron transport layer plays a vital role in extracting and transporting photogenerated electrons, blocking holes, modifying interfaces, adjusting interface energy levels, and reducing charge recombination. Inorganic n-type materials, such as TiO₂, ZnO, SnO₂ and other metal oxide materials have the advantages of low cost and good stability, which are often used as ETLs in traditional PSC. Organic n-type materials, such as fullerenes and their derivatives, naphthalene diimide polymers and small molecules, have good film-forming properties and strong electron transport capabilities, which are often used as ETLs in inverted PSCs. This review will systematically classify the electron transport materials for perovskite solar cells, outline their preparation methods, introduce their charge transport mechanism and effect in perovskite solar cells. The latest research progress of metal oxide materials, organic molecular materials, composite materials, multilayer electron transport layer materials and their modification methods are systematically discussed. Finally, the practical application and development prospects of the electron transport layer materials towards high-performance PSC are prospected. In summary, this review helps to better understand the preparation and mechanism of various electron transport layer materials related to perovskite solar cells, and provides strategies for further understanding and preparing high-performance PSCs.

Contents

1 Introduction

2 Charge transport mechanism of perovskite solar cells

2.1 Charge transport mechanism of positive perovskite solar cells

2.2 Charge transport mechanism of inverted perovskite solar cells

2.3 The role of electron transport layer in perovskite solar cells

3 Preparation methods of electron transport layer in perovskite solar cells

3.1 Spin coating

3.2 Chemical bath deposition

3.3 Atomic layer deposition

3.4 Other deposition methods

4 Electron transport materials in positive perovskite solar cells

4.1 TiO₂

4.2 ZnO

4.3 SnO₂

4.4 Other metal oxides(WO_X,Nb₂O₅,CeO_X )

5 Electron transport materials in inverted perovskite solar cells

5.1 Fullerene and its derivatives

5.2 Non-fullerene small organic molecules

5.3 Non-fullerene polymer molecules

6 Conclusion and outlook

Graphical abstract

Keywords

perovskite solar cell / electron transport layer / material type / preparation method

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Ying Yang, Yuan Luo, Shupeng Ma, Congtan Zhu, Liu Zhu, Xueyi Guo. Advances of Electron Transport Materials in Perovskite Solar Cells: Synthesis and Application. Progress in Chemistry, 2021, 33(2): 281-302 DOI:10.7536/PC200515

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1 Introduction

With the shortage of traditional energy and the increasingly prominent environmental problems, the sustainable development of human beings is facing serious challenges^[1]. As a new generation of clean energy, solar energy has a very broad development prospects^[2]. Perovskite solar cells (PSCs) have attracted worldwide research interest due to their high efficiency, low cost, and potential for mass production^[3]. In 2009, Kojima et al. First used organometallic lead halide CH₃NH₃PbI₃ as sensitizer in dye-sensitized solar cells, and its photoelectric conversion efficiency was only 3.8%^[4]. And is not stable in liquid electrolyte. In 2012, Kim et al replaced liquid electrolyte with solid hole transport material (HTM) spiro-MeOTAD, and the stability and photoelectric conversion efficiency of PSC were significantly improved, and the PCE of the all-solid-state solar cell was 9.7%^[5]. In 2016, Gr Grätzel et al. In Switzerland used anti-solvent one-step method to prepare highly coordinated mixed cation perovskite thin films, which increased the photoelectric conversion efficiency (PCE) of PSC to 20.8%^[6]. You Jingbi et al. Used phenethylammonium iodide (PEAI) to passivate the surface defects of FA-MA mixed perovskite to reduce surface defects and inhibit recombination, and obtained 23.32% PCE^[7]. In April 2019, the Korea Research Institute of Chemical Technology (KRICT) generated 24.2% of PCE. Recently, it is reported that the highest PCE of the single-junction perovskite solar cell created by Korea Institute of Chemical Technology can reach 25.2%, but the stability of the device is still a problem because the perovskite material is unstable to water and oxygen^[8]. In our research group, MAPbI₃/ agarose composite photosensitive material was used in perovskite solar cells, and the interaction between polymer and perovskite can improve the film formation and crystallinity of perovskite thin films.The water and oxygen stability of the device is further improved, and it is found that a proper amount of agarose doping can passivate the surface defects of the perovskite, thereby improving the electron injection capability from the perovskite to the electron transport layer; The Li⁺- agarose complex formed at the perovskite layer/hole transport layer interface can effectively reduce the water absorption of Li-TFSI and thus prevent the degradation of the MAPbI₃ layer. Under the condition of no glove box, the photoelectric conversion efficiency of the unpackaged device in humid environment is 14. 66%, and 90% of the initial PCE is retained after 1392 H (RH > 70%), which realizes the stability of perovskite solar cells in air humidity conditions under the condition of no glove box^[9].

The structure of perovskite solar cells can be divided into mesoscopic structure and planar heterojunction structure. There are two types of planar heterojunction structures: the formal structure (n-i-p type) and the trans structure (p-i-n type), as shown in figs. 1 and 2^[10]. In PSC, both organic and inorganic n-type materials can be used as the electron transport layer (ETL). Organic n-type materials, such as fullerenes and their derivatives, naphthalimide polymers and small molecules, have been widely used in trans-PSC due to their good film-forming and excellent electron transport properties. Inorganic n-type materials, especially inorganic metal oxides (MO_x), such as TiO₂, ZnO and SnO₂, have the advantages of low cost and good stability. The photoelectric conversion efficiency of PSC using metal oxides as ETL has reached more than 24%^{[11⇓⇓~14]}. Doping and surface modification of the electron transport layer can effectively improve the performance of the device, which needs further design and development. Multilayer electron transport layer materials are considered to be an important research direction for efficient and stable perovskite solar cells due to their synergistic interaction^[3,15⇓~17].

Optimal selection and proper design of the ETL plays a very important role in enhancing the device performance of the PSC^[18,19]. The following conditions should be satisfied for ETL in high performance solar cells: (a) good light transmittance in the visible range to reduce the loss of light energy; (B) the energy level of the ETL should match the energy level of the perovskite material, thereby improving electron extraction efficiency and effectively blocking holes; (C) good electron mobility; (D) having a suitable morphology to form a good interface with the perovskite layer to facilitate crystallization of the perovskite and reduce interfacial recombination; (E) has excellent chemical and thermal stability to prevent it from gradually reacting with the adjacent perovskite layer or metal electrode^[20]. Therefore, an in-depth understanding of the structure of electron transport layer materials and their optimization strategies is essential to improve the performance and long-term stability of devices^[3,21,22]. In this paper, the ETL materials used in perovskite solar cells are systematically classified, and the charge transport mechanism, function and deposition methods of ETL materials are introduced. The latest research progress of ETL materials and their modification methods, such as TiO₂, ZnO, SnO₂, fullerene and its derivatives and non-fullerene organic molecules, are emphatically summarized, which provides a reference for the preparation and design of high-performance PSC^[11].

2 Charge Transport Mechanism of Electron Transport Layer in Perovskite Solar Cell

2.1 Charge transport through the electron transport layer in perovskite solar cells

The perovskite light absorption layer has the characteristics of bipolar charge transport, and can play the role of electron and hole transport in addition to the role of light absorption material in the device. In the conventional perovskite solar cell, after the perovskite absorbs sunlight, electrons in the valence band are excited to jump to the conduction band, leaving holes and forming excitons, and the excitons are separated at the perovskite/electron transport layer interface to form free charges. Under the action of the built-in electric field, electrons are transported from the conduction band of the perovskite layer to the conduction band of the electron transport layer, and finally collected by the ITO electrode. In the process of transport, the free charge always recombines at the interface and the internal defects of the material, so reducing the charge recombination is one of the important means to improve the performance of the device^[23]. The electron transport material in the conventional perovskite solar cell is generally an n-type inorganic semiconductor material. Generally speaking, the inorganic electron transport material has higher electron mobility than the organic electron transport material, and the electrons will undergo directional diffusion and drift movement under the action of concentration gradient and built-in electric field (Figure 3A)^[24].

2.2 Charge transport in the electron transport layer of trans-perovskite solar cell

In the trans-perovskite solar cell, the perovskite absorbs light and generates excitons, which are separated at the perovskite/electron transport layer interface, and the electrons are transferred from the conduction band of the Perovskite layer to the LUMO level of the electron transport material, and finally collected by the metal electrode. The electron transport material in the trans-perovskite solar cell is generally an n-type organic semiconductor material. The electrons in the n-type organic semiconductor need to overcome a large potential barrier, and their transfer mainly depends on the conjugated orbitals between organic molecules, so the electron mobility of the organic semiconductor is generally low (Figure 3 B)^[25].

2.3 Role and Influence of Electron Transport Layer in Perovskite Solar Cell

In the PSC, the electron transport layer forms a contact with the perovskite layer for extracting and transporting photogenerated electrons, blocking holes, and suppressing charge recombination. The efficiency of electron extraction and collection, hysteresis in current-voltage tests, and device instability are all closely related to ETL^[23]. For example, the performance of PSCs based on TiO₂ETL degrades rapidly under UV irradiation, and this instability is mainly attributed to oxygen vacancies (Ti³⁺ defect States) on the surface of TiO₂^[25]. There are many oxygen vacancies (Ti³⁺ defect States) on the surface of the TiO₂. The electrons of these sites are about 1 eV below the edge of the conduction band, which can interact with molecular oxygen in the atmosphere and adsorb to the oxygen vacancies to form charge transfer complexes (O₂-Ti⁴⁺). Under the excitation of ultraviolet light, the electrons in the valence band of the TiO₂ jump to the conduction band, and the holes left behind will recombine with the electrons in the oxygen adsorption site, thus desorbing oxygen. The free electrons in the conduction band and the unfilled oxygen vacancies (deep traps) with positive charges are left. Under the excitation of light, the photogenerated electrons in the perovskite will be captured by the deep traps and easily recombine with the holes in the hole transport material, resulting in a rapid decline in the short-circuit current of the device^[26]. In the absence of oxygen, the performance of the device is attenuated in two stages, as shown in fig. 4. In the first attenuation stage, after being excited by ultraviolet light, electrons in the valence band of the TiO₂ jump to the conduction band, leaving holes; Unpaired electrons in the Ti³⁺ defect (Ti³⁺-V_O) combine with holes in the valence band to form Ti⁴⁺ defects (Ti⁴⁺-V_O) and leave free electrons in the conduction band; The Ti⁴⁺-V_O can actively capture the photogenerated electrons in the perovskite layer, and the reduction of the Ti⁴⁺-V_O to the Ti³⁺-V_O;Ti⁴⁺-V_O induces the accumulation of the photogenerated electrons, which provides an additional electron recombination channel for the holes in the perovskite layer and the HTL layer, thus leading to the performance degradation of the PSC. In the second decay stage, :Ti⁴⁺-V_O traps provide a large number of oxidation sites, and I^- in perovskite will be oxidized to I₂ and PbI₂, while CH₃NH₃⁺ will be released. The resulting I₂ will combine with free I^- to form I^3-,The CH₃NH₃⁺ and I^3- will react at the interface to produce volatile CH₃NH₂ and I^3-, which will cause the rapid decomposition of perovskite and lead to a sharp decline in the performance of PSC^[27]. For the formal PSC, the ETL with appropriate morphology can form a good interface contact with the perovskite layer, which is beneficial to the crystallization of high-quality perovskite and reduces the interfacial recombination. For the trans-PSC, the dense and pinhole-free ETL provides both perfect coverage of the perovskite layer and optimal ohmic contact to the top electrode. Impurities and defects in ETL can also affect the photoelectric conversion efficiency of the device, for example, donor and acceptor impurities can provide carriers and increase conductivity; Defects tend to create recombination centers and shorten the lifetime of non-equilibrium carriers^[24].

3 Preparation of Electron Transport Layer in Perovskite Solar Cell

Low defect, crack-free and pinhole-free dense TiO₂ film is an important part of high efficiency PSC. The main preparation methods of dense TiO₂ film are coating, chemical bath deposition, atomic layer deposition, magnetron sputtering, pulsed laser deposition and so on.

3.1 Coating method

Coating method (SC) is a low-cost and simple solution processing method, which can control the thickness of the film in a wide range (such as several nanometers to tens of microns), but it is difficult to prepare a large area of the film, and the film is not uniform. As shown in fig. 5A, a precursor solution prepared by dissolving a metal salt or a hydrate thereof in a suitable solvent is spin-coated on a substrate, and then the deposited film is thermally annealed to be converted into a metal oxide (MO) film,Or the MO nanocrystal or nano colloid synthesized by the solution method is dispersed in a proper solvent to prepare a dispersible spin-coating solution, and the dispersible spin-coating solution is spin-coated on the substrate and then annealed to obtain the MO film. The solution method generally includes a Sol-Gel method (Sol-Gel), a hydrothermal/solvothermal method, a high-temperature thermal decomposition method, and the like^[28,29]^[30]. The sol-gel method is to hydrolyze and condense the precursor in a solution to form a nano-colloid, slowly polymerize the colloid after aging to form a three-dimensional network nano-gel, and obtain the nano-material after further heat treatment. The hydrothermal/solvothermal method is a method for synthesizing nano materials in a reaction kettle by using water/ethanol solution or organic solution as a solvent. In the laboratory, sol-gel method and hydrothermal/solvothermal method are usually used to prepare nanoparticles, nanowires, nanotubes and other nano-arrays of nano-materials, and then deposited on the substrate by spin coating or spray coating. Screen printing is a conventional deposition method for the preparation of mesoporous ETL films^[31⇓~33]. Slit coating, which can be used to deposit uniform wet films and is suitable for flexible substrates, has great potential for mass production of MO ETL^[34]. The MO ETL in most high performance PSCs is prepared by the coating method and has good crystallinity and few defects^[35]. It is worth noting that the environmental humidity and annealing temperature are closely related to the defects and pinholes in MO films^[36].

The dense TiO₂ films prepared by spin coating usually have cracks, pinholes and agglomerations during heat treatment. Xia et al. Induced hydrolysis of titanium diisopropoxide (acetylacetone) (TiAcAc) at 70 ℃ using a solution containing TiO₂ seeds to prepare anatase TiO₂ precursor, and then spin-coated the TiO₂ precursor on FTO and annealed to form a dense anatase TiO₂ electron transport layer^[37]. The annealing temperature and thickness of the TiO₂ affect the conductivity, density, uniformity and surface roughness of the films. The PCE of 13.66% is obtained for the device with a thickness of 40 nm and an annealing temperature of 150 ℃.

3.2 Chemical Bath Deposition

Chemical bath deposition (CBD) is a method used to fabricate a variety of high-quality, large-area semiconductor films, which can be used to synthesize metal oxide films, such as ZnO and SnO₂, as ETL in PSC. The film is slowly deposited by immersing the substrate in a dilute solution containing metal ions and a source of hydroxide, selenide or sulfide ions (fig. 5B). Has that advantage of high efficiency, low cost, low temperature, strong adaptability, high repeatability, large-scale processing and the like. However, the growth of the thin film is largely dependent on the growth conditions, such as temperature, solution concentration, and pH of the solution. At the same time, the waste of residual precursors after deposition should also be properly dealt with^[11].

3.3 Atomic layer deposition method

Atomic layer deposition (ALD) is a technique of chemical vapor deposition that has been widely used to fabricate ultra-thin, uniform semiconductor films produced in the semiconductor industry^[38]. As shown in fig. 5C, the overall ALD cycle typically includes four steps: (1) injecting precursor A into the reaction chamber and forming a monolayer on the substrate by chemisorption; (2) the unreacted precursor is purged by inert gas flow; (3) the precursor B is injected into the reaction chamber to react with the precursor A adsorbed on the substrate to form a thin product film. (4) Excess precursors and by-products are removed by the inert gas stream^[11]. Due to the limitation of chemisorption, only one monomolecular film is formed in each cycle, so the ALD process can conveniently and accurately control the thickness of the film, and the coverage rate of the product film is high. So far, the ALD method has been successfully used to prepare high quality dense MO ETLs, including TiO₂, ZnO and SnO₂^[38]^[39]^[40]. However, the ALD method is costly and sensitive to the purity of the substrate, which may not be suitable for large area production^[11]. Shalan et al. Deposited a uniform, pinhole-free and low-defect dense layer of TiO₂ with a thickness of 10-400 nm on FTO glass by ALD method, and deposited a mesoporous TiO₂ layer by screen printing^[41]. It is found that the conductivity, surface roughness and interfacial electron transfer rate of the film change with the thickness of the film, and the best photoelectric conversion efficiency (15%) is obtained at a thickness of 200 nm, and the device has excellent stability and repeatability (Figure 6a).

3.4 Other deposition preparation method

In addition to the above processes, there are some other effective ways to prepare specific ETLs according to their different physicochemical properties. For example, Wei et al. prepared an ultra-thin dense layer of TiO₂ from a TiCl₃ solution using electrodeposition (ED) and obtained a PCE of 13.6%^[42]. The advantages of ED are easy and fast deposition and controllable deposition thickness. High-quality TiO₂ films deposited by RF magnetron sputtering (MS) have the characteristics of uniform thickness, high compactness, high transmittance and strong interaction with the substrate^[23]. Magnetron sputtering can be used to prepare TiO₂ thin films with high transparency, full coverage and homogeneity. Huang et al. Prepared dense TiO₂ thin films with { 001 } crystal plane by magnetron sputtering, which showed low incident light loss, highly smooth surface, excellent wettability and high conductivity. The highest photoelectric conversion efficiency of the device was 17.25%^[43]. In addition, there are other methods of depositing ETLs, such as electrostatic spraying, spray pyrolysis, electron beam evaporation, and pulsed laser deposition (PLD)^[44]^[45]^[46]^[47].

4 Novel perovskite solar cell electron transport layer material

4.1 TiO₂

TiO₂ are commonly used as ETLs in conventional perovskite solar devices due to their chemical stability, high charge transport properties, low cost, and ease of fabrication^[10]. The TiO₂ has a low refractive index (2.4 ~ 2.5) and a large band gap (3.2 eV for anatase, 3.1 eV for brookite, and 3.0 eV for rutile). The conduction band bottom (CBM) is about -4. 1 eV, which is slightly lower than the lowest unoccupied molecular orbital (LUMO) level of perovskite, and electrons can be effectively transported from the perovskite layer to the TiO₂ETL, while the valence band top (VBM) is deep, which has excellent hole blocking ability^[11]. Based on the ETL of TiO₂, it can be divided into TiO₂ mesoporous layer and TiO₂ dense layer^[10]. Mesoporous TiO₂ is beneficial to the growth of perovskite crystals, while dense TiO₂ plays an important role in electron transport^[24]. Generally, the TiO₂ film must be calcined at a temperature higher than 400.d egree. C. to remove the organic binder (e.g., ethyl cellulose) and form a porous structure to improve the transport capability, but this is not conducive to the application of the device in a flexible device^[50]. Therefore, a low-temperature process for the preparation of nanostructured TiO₂ layers is considered to be a technological breakthrough^[10].

4.1.1 TiO₂ with different morphologies

The morphology of TiO₂ depends largely on its preparation method, and the morphology of materials has a great impact on the performance of perovskite solar cells. The morphology and size of nanomaterials are closely related to the specific surface area and the pore size of the membrane, thus affecting the performance of the device. Lee et al. Used TiO₂ nanoparticles with particle sizes of 25 nm and 41 nm as the mesoporous layer by spin coating (mp-TiO₂)^[51]. It was found that the larger size of TiO₂ nanoparticles had larger total pore volume and average pore diameter, which was more conducive to the filling of perovskite and the reduction of electron transport resistance. Small-sized TiO₂ nanoparticles have a large specific surface area, which is conducive to the injection of photoelectrons. By optimizing the thickness, 18.03% (25 nm) and 18.72% (41 nm) PCE were obtained, respectively. In addition to the preparation of the mesoporous layer, Liu et al. Used commercial TiO₂ nanoparticles with a diameter of 6 nm to prepare a dense ETL by spin coating. The small size of the TiO₂ nanoparticles is closely packed, which can prevent the perovskite from filtering into the TiO₂ETL. It can also form a rough surface to enhance wettability and facilitate the nucleation and growth of perovskite. The PCE of 11.0% was obtained by optimizing the thickness^[52].

Well-oriented, highly ordered nanoarray structures (nanorods, nanowires, nanotubes) can provide direct channels for efficient charge transport and facilitate the filling of perovskites compared to nanoparticles. The porous TiO₂ nanorod array prepared by Yang et al. Through pulsed laser deposition was used as the ETL of PSC, and 14.1% PCE was obtained^[47]. The resulting anatase TiO₂ nanostructures effectively increase the contact area between the TiO₂ layer and the perovskite layer, and exhibit high electron-hole pair separation and electron extraction capabilities at the interface (fig. 6B). Wu et al. prepared an amorphous titanium oxide nanowire (Am-TNW) thin film as an ETL for PSC by a low temperature (< 150 ℃) solution process. The Am-TNW thin film does not need annealing treatment and has good transmittance. It can be in close contact with the perovskite layer, which is conducive to the penetration of perovskite and can effectively inhibit charge recombination. The device obtained an average PCE of 18.3%^[53]. Tenkyong et al^[54].

The TiO₂ nanotube arrays were prepared by electrochemical anodization on titanium substrate and used in PSC without hole transport materials.The optical band gap and conductivity of the material were found to change with the structure size (length) of the TiO₂ nanotube, and the optimal PCE of 3. 64% was obtained for the tube length of about 9. 4 μm. In addition, Chen et al. prepared a 3D flower-like TiO₂ nano-array layer composed of one-dimensional (1D) anatase TiO₂ nanorods on the FTO substrate by low temperature (80 ℃) chemical bath deposition method to replace the traditional mesoporous TiO₂ layer.It is found that the 3D flower-like TiO₂ nano-arrays can suppress the charge recombination and improve the light collection efficiency (LHE), and the LHE increases with the increase of the chemical bath deposition reaction time of the 3D flower-like TiO₂ nano-arrays.The device achieves the best PCE of 15. 71% with good repeatability and low hysteresis^[55].

4.1.2 Interface Modification and Element Doping of TiO₂;

The intrinsic defects of TiO₂, such as oxygen vacancies and low valence Ti³⁺ ions, affect the performance of devices. Surface modification and element doping can improve the defects of materials and reduce the recombination of charges in devices. Surface modification is an effective method to adjust the carrier dynamics at the interface, and its ultimate goal is to reasonably construct an interface structure that is conducive to rapid carrier transfer and inhibition of recombination. Passivation of the surface trap States of the TiO₂ layer and the perovskite layer is an effective modification method to improve the photovoltaic performance and suppress the hysteresis of PSC. For example, Tan et al. Obtained chlorine-modified colloidal nanocrystalline films of TiO₂ by TiCl₄ treatment^[56]. Interfacial Cl atoms can suppress the formation of surface deep trap States, which can improve the interface binding and reduce the recombination of charges at the interface. The PCE of the small area device (0.049 cm²) based on the TiO₂-Cl is 20.1%, and the PCE of the large area device (1.1 cm²) is 19.5%, and the stability is good. Han et al. treated the TiO₂ with bifunctional 4-picolinic acid (4-PA) self-assembled monolayer (SAM), and the modified device obtained 18.9% PCE^[57]. They found that 4-PA treatment of TiO₂ can increase the grain size of MAPbI₃, effectively passivate the interface of UK3 perovskite, and optimize the transfer of electrons and holes, with negligible hysteresis. Shen et al. introduced an ultra-thin graphene quantum dot (QGD) modified layer on the surface of mesoporous TiO₂ film. The GQD embedded in the surface of mesoporous TiO₂ film can promote the extraction and transmission of electrons and reduce the contact resistance of perovskite/mesoporous TiO₂ interface. The photoelectric conversion efficiency of the device was increased from 18.57% to 20.45%^[58]. In addition, Xu et al. fabricated TiO₂/ZnO/C₆₀ three-layer films on transparent glass substrates for the electron transport layer of planar perovskite solar cells^[59]. Among them, the TiO₂ is beneficial to eliminate the charge recombination on the surface of FTO and realize the effective electron collection. ZnO is beneficial to the growth of the CH₃NH₃PbI_3-_xCl_x(x≈0), and ensures proper energy level matching of the TiO₂ layer and the C₆₀ layer to improve the open-circuit voltage; The C₆₀ facilitates enhanced extraction of carriers from the perovskite layer. The PSC based on the TiO₂/ZnO/C₆₀ three-layer electron-transporting material had a PCE of 18.63% (fig. 7A).

Element doping is a method to change the band gap, energy level arrangement, carrier and (surface) trap state density and distribution by introducing dopant ions into the lattice sites of Ti⁴⁺, which plays an important role in improving photovoltaic performance, reducing hysteresis and enhancing the stability of PSC. The doping of high-valence metal elements into the TiO₂ layer (n-type doping) can introduce additional electrons, shift the conduction band of the TiO₂ upward, improve the injection process of photoelectrons, and increase the conductivity. The Nd-doped TiO₂ prepared by chemical bath deposition by Yin et al improves the injection and extraction efficiency of photoelectrons, shows higher conductivity and electron mobility, and lower trap state density. The PSC with 1% Nb-doped TiO₂ obtains more than 19% PCE, and has better device stability^[60]. The doping of equivalent metal elements into the TiO₂ layer can change the band gap, energy level arrangement, carrier mobility and trap state density, but because there are no excess electrons in the dopant, it will not cause the change of carrier density. For example, Sn-doped TiO₂ nanorod arrays can improve electron mobility and increase band gap, resulting in 67% higher device efficiency than undoped ones^[61]. The doping of low-valent metal elements into the TiO₂ layer (p-type doping) usually leads to the decrease of conductivity due to the decrease of electron density, but the doping of appropriate low concentration can reduce the defects of the TiO₂ (oxygen vacancies, Ti³⁺ gaps, etc.), thereby improving the conductivity. The TiO₂ layer doped with low concentration of Y and Mg can reduce the trap state density, move the Fermi level up and reduce the resistivity, thereby improving the ability to transport electrons and block holes. The device shows higher open circuit voltage and fill factor, and obtains PCE of 19.3% and 19.08% (Figure 7B), respectively^[62,63]. The metal ions with smaller radius are doped into the TiO₂ layer, which is usually located in the lattice gap to form doping. For example, Li-doped TiO₂ can reduce the electron trap density and improve the conductivity. The photoelectric conversion efficiency of PSC based on Li-doped TiO₂ is improved from 14.2% to 17.1%, and the hysteresis is negligible^[64].

4.1.3 TiO₂ prepared at low temperatures

The high temperature manufacturing process of TiO₂ is not compatible with low-cost and flexible devices. In recent years, many scientists have extensively studied the low temperature manufacturing process of TiO₂ for flexible devices. Kim et al. Used reactive ion etching (RIE) method instead of conventional high temperature annealing process to prepare mesoporous TiO₂(mp-TiO₂) as the electron transport layer in high-efficiency PSC. The RIE-mp-TiO₂ showed improved electron extraction ability and J-V hysteresis, and the flexible device obtained 17.29% PCE^[65]. You et al. Synthesized TiO₂ nano-sol in acidic aqueous solution by sol-gel method and dispersed the dried TiO₂ nanoparticles in different solvents, then spin-coated and annealed at low temperature to make ETL.It was found that using N, N-dimethylformamide (DMF) as solvent, a uniform and hydrophilic TiO₂ electron transport layer could be formed, and the PCE of the flexible device reached 15.8% and had good mechanical stability^[66]. Xie et al. Used the TiO₂/SnO₂ thin film prepared at a low temperature of 150 ℃ as the electron transport layer in perovskite solar cells, and found that there was a high driving force for electron extraction at the SnO₂/ perovskite interface, and a defect-free physical contact could be formed at the TiO₂/FTO interface. The bilayer thin film showed effective electron extraction and hole blocking capabilities^[67]. The best PCE of the device is 18.85%, which is much higher than the PCE of the device based on TiO₂, which is very attractive for flexible PSC.

4.2 ZnO

ZnO has high electron mobility, good energy level structure (CBM is about -4.17 eV, band gap is 3.37 eV) and high transmittance at visible wavelength. Compared with traditional TiO₂, ZnO can be used to fabricate large area thin films without high temperature sintering. In addition, ZnO is easy to crystallize at low temperature to form various nanostructures^[11]. Nanostructured ZnO can significantly improve the extraction efficiency of carriers due to its large specific surface area, and can also act as a scattering center to enhance the light collection of PSC^[20].

4.2.1 ZnO with different morphologies

The high electron mobility and low temperature processability of ZnO have led to the widespread use of uniform and dense ZnO films with low defects and pinholes in planar devices. Magnetron sputtering, atomic layer deposition and spray pyrolysis are effective methods to prepare dense ZnO films with controllable properties. For example, the dense ZnO thin films prepared by Ngo et al. Through spray pyrolysis under N₂ gas flow have high transmittance, good uniformity and substrate coverage^[68]. The device achieves a PCE of 12.7% and has good stability. For the first time, Zheng et al. Used combustion synthesis to prepare high-quality ZnO thin films, which have the advantages of inherent passivation, high crystallinity, low carbon pollution, appropriate energy level matching, ideal morphology and good chemical compatibility with perovskite layers. Devices based on combustion synthesis ZnO thin films can achieve 17% -20% PCE (Figure 8 a)^[69]. Zhang et al. Synthesized highly crystalline and ultra-uniform ZnO nanoparticles of different sizes by solvothermal method^[70]. It is found that the filling and contact area of perovskite, the electron injection efficiency at the interface between ZnO and perovskite, and the charge recombination ability are affected by the size of the nanoparticles.The device based on 40 nm ZnO nanoparticle mesoporous film achieves the best PCE of 15. 92%.

ZnO thin films based on various nanostructures have also been used in efficient PSCs due to the easy crystallization of ZnO to form various nanostructures. Unlike nanoparticles, one-dimensional ZnO nanostructures are single-crystal and provide a direct electron transport pathway. ZnO nanorods have a vertical arrangement structure, which can increase the contact area with perovskite, but it is difficult to prepare high-quality perovskite films on their surfaces. In order to solve this problem, Li et al. Synthesized ZnO nanorods as ETL by hydrothermal method, and preheated the substrate before spin-coating perovskite, so that the surface coverage, grain size and crystallinity of perovskite were increased^[71]. After optimizing the thickness, the PCE of the device is 10. 34%. ZnO nanowires have a high aspect ratio, which is also beneficial to the penetration of perovskite and the transport of electrons. Etgar et al. Hydrothermally prepared ZnO nanowires for use in PSC, and by optimizing the length and diameter, achieved PCE of 9.06% and 6.39% on rigid and flexible devices, respectively, with excellent stability (Figure 8 B)^[72]. In addition, Islavath et Al. prepared vertically aligned three-dimensional (3D) ZnO nanowall arrays on transparent conductive substrates by using a seed layer-assisted low-temperature solution growth method for a hole-free PSC, and by changing the Al content in the seed layer, the morphology of the ZnO nanostructure can be gradually changed from 1D nanowires to 3D nanowall arrays,The well-aligned 3D ZnO nanowires have a high surface area and a unique cage-like structure, which is beneficial to the transport of electrons, and the PCE of the device is 2. 56%.After that, 1 D ZnO nanowires are grown on the surface of the 3D ZnO nanowire, which can improve the ZnO/perovskite interface, and the PCE of the device is increased to 3. 28%^[73].

4.2.2 Surface Modification and Element Doping of ZnO

Although the performance of devices based on ZnO electron transport layer has made great progress, the serious interface recombination and instability of ZnO thin films limit its further development. For example, the intrinsic defects of ZnO (such as zinc vacancies, oxygen vacancies, dangling bonds and defects formed by the sudden rupture of ZnO crystal structure, etc.) And the new defects introduced during the solution manufacturing process can provide additional recombination paths and reduce device performance. The surface of ZnO is alkaline, and it will deprotonate with CH₃NH₃⁺ when it contacts with perovskite, which makes the perovskite deposited on the surface of ZnO easy to decompose during heat treatment^[20,74]. This can be solved by surface modification or element doping. Surface modification is an effective method to suppress charge recombination. Coating ZnO electron transport layer with wide band gap oxide can passivate the interface and enhance the device performance. Li et al. Deposited Al₂O₃ monolayer on ZnO nanorods by ALD method, which can passivate interface defects and accelerate the separation of carriers, thus increasing the photoelectric conversion efficiency of the device from 11.72% to 16.08%, and avoiding the chemical instability caused by chemical residues^[75]. In addition, block copolymers such as PCBM and polyethyleneimine (PEI), which have a larger negative conduction band edge than ZnO, are also used to modify the interface between ZnO and perovskite, inhibiting interfacial recombination (fig. 9a)^[76]. For example, Zhang et al. Used PCBM as an interface modification layer to prepare a PSC based on ZnO-PCBM bilayer electron transport material^[77]. Wherein the ZnO layer can improve the energy level arrangement and enhance the extraction efficiency of carriers; The PCBM layer may reduce hysteresis and enhance thermal stability of the perovskite. This PSC has more efficient device performance compared to the PSC of a single layer ETL.

In addition, element doping is also an effective way to improve charge recombination and improve the quality of ZnO thin films. Metal dopants such as Li, Na, K, Mg, Al, Sn, In and Ni can passivate the surface defects of ZnO and improve the electron mobility and Fermi level^[78]^[79]^[79]^[80]^[81]^[82]^[83]^[84]. The PSC of the ZnO ETL with K doping achieved a PCE of 19.9% with excellent air stability and no significant hysteresis (Fig. 9B)^[79]. At present, significant achievements have been made in the modification of ZnO thin films, but problems such as long-term stability still need to be further optimized and studied.

4.2.3 ZnO prepared at low temperatures

In recent years, more and more attention has been paid to the flexible devices manufactured by low-temperature process. The low temperature processability of ZnO thin films makes it have great potential in the development of flexible devices. Liu et al. Prepared ZnO nanoparticle films by spin-coating at temperatures below 100 ℃ for flexible devices, and obtained 10.2% PCE, which promoted the development of low-temperature ZnO films in flexible devices^[85]. Jung et al. prepared the ZnO electron transport layer at a temperature below 100 ℃ for use in flexible devices. By optimizing the thickness of the ZnO layer, the size/morphology of the MAPbI₃, and the residual PbI₂ phase, the device achieved the best PCE of 12.34% and had good mechanical stability^[86]. Lim et al prepared ZnO thin films at low temperature for flexible devices, and inserted PEIE modification layer on the surface of ZnO, which could avoid the decomposition of perovskite layer, improve the crystallinity of perovskite thin films and reduce the electron transport barrier, and the device obtained 11.9% PCE^[87].

4.3 SnO₂

SnO₂ has been considered as a promising ETL material due to its wide band gap (CBM: -4.2 ~ -4.5 eV, band gap: 3.5 ~ 4.0 eV), low refractive index (< 2), high electron mobility (≈250 cm²·V^-1·s^-1) and suitable energy band structure, which can efficiently extract and transport electrons, reduce charge accumulation and recombination losses, and eliminate J-V hysteresis in planar PSC^[11].

4.3.1 SnO₂ with different morphologies

Baena et al. used ALD to prepare a 15 nm thick pinhole-free dense layer of SnO₂ for use in a planar PSC^[88]. The film has high coverage and excellent electron extraction ability, and the device exhibits high and stable current density with a PCE of more than 18%. Hagfeldt et al. prepared a high quality SnO₂ electron transport layer by spin coating and chemical bath deposition (SC-CBD) for use in a planar PSC, which can achieve a high fill factor, high hole blocking ability, good stability and improved hysteresis, with a PCE of nearly 21% (Figure 10 a)^[49]. Ke et al. prepared the dense thin film of SnO₂ by spin-coating the precursor solution of SnCl₂·2H₂O at room temperature. The thin film has a smooth surface, high transmittance, electron extraction and hole blocking ability, and the device obtained the best PCE of 17.21% (Figure 10 B)^[89]. Jiang et al. Spin-coated a solution of small SnO₂ nanoparticles on an ITO substrate to form a pinhole-free dense film of SnO₂ for use in a planar PSC. The high crystallinity of the SnO₂ nanoparticles is conducive to reducing the density of defect States in the SnO₂, and the device obtained a PCE of 19.9% without hysteresis^[90]. Li et al prepared SnO₂ mesoporous film with large particles of SnO₂ nanoparticles for mesoporous PSC, and obtained PCE of 6.5% by optimizing the thickness of the SnO₂ film. After the surface of the SnO₂ film was treated with TiCl₄ aqueous solution, its PCE increased to 10.18%^[91].

One-dimensional SnO₂ nanomaterials, such as SnO₂ nanorods, SnO₂ nanowires, and SnO₂ nanotubes, are used in PSC because they have fewer grain boundaries to facilitate the rapid transport of electrons. For example, Zhang et al. prepared a vertically-aligned SnO₂ nanorod array in an acid solution by a hydrothermal method as an ETL, which has good anti-reflection capability and can enhance light collection, and its one-dimensional vertically-aligned structure can provide an efficient electron transport channel. The device shows a high electron transport rate and a low charge recombination rate, and obtains the best PCE of 15.46%^[92]. Gao et al. prepared a novel hollow domed SnO₂ nanotube array (SnO₂-DNT) using ZnO nanorods as templates by in-situ template self-etching (iTSE) method for PSC. The SnO₂-DNT as ETL has a large contact area and effective carrier collection capability, and the device obtained the best PCE of 12.26% and showed good long-term stability and UV stability^[93]. In addition, two-dimensional SnO₂ nanosheets are also used in PSC because they can form a good porous structure, which is conducive to the penetration of perovskite. Mahmood et al prepared vertically aligned two-dimensional SnO₂ nanosheets as ETLs by one-step electrospray method, and found that the SnO₂ nanosheets had high transmittance and good anti-reflection ability, which was conducive to light collection^[94]. The well-aligned porous structure is beneficial to the permeation of perovskite and the transport of electrons, and the device has low hysteresis and improved environmental stability and achieves an average PCE of 14.27%.

4.3.2 Surface Modification and Element Doping of SnO₂;

SnO₂ has been proved to be an ideal electron transport layer material with high performance. However, due to the inherent defects of SnO₂ film (tin gap, oxygen vacancy, etc.), the interface recombination of SnO₂/ perovskite is serious, resulting in its low open circuit voltage and fill factor^[95]. The performance of SnO₂ devices can be improved by interface modification. Hao et al. Used KCl to passivate the defects of the SnO₂/ perovskite interface, which can improve the efficiency and stability of the device and eliminate the hysteresis phenomenon. A planar PSC with PCE of 20.5% was obtained^[96]. Yang et al. Introduced self-assembled monolayers (SAMs) with different functional groups into the surface of SnO₂ to form various chemical reactions with the perovskite layer, thereby reducing the density of trap States and promoting interfacial charge transfer^[97]. The device surface modified with 4-picolinic acid (PA) SAM achieved a PCE of 18.8%. Xie et al. Modified SnO₂ films with a small amount of graphene quantum dots (GQDs) to improve the electronic properties of SnO₂^[98]. It is found that the photogenerated electrons in the GQDs are transferred to the conduction band of the SnO₂ to fill the electron traps, increase the electron density and improve the conductivity of the SnO₂, which is beneficial to improving the electron extraction rate and reducing the interface charge recombination. The device obtains the best PCE of 20.23% (Figure 11A).

Element doping is another important way to improve the performance of SnO₂ devices. The doped SnO₂ can improve the conductivity and cause the Fermi level of the SnO₂ to move up, which is conducive to the transfer of interface charges and reduce the recombination of interface charges. At present, the effects of doping metals such as Li, Mg (fig. 11b), Zn, Al, Y, Ga, La, Mo, Nb, Sb into the SnO₂ as dopants on the device performance have been studied^[99]^[100]^[101]^[102]^[103]^[104]^[105]^[106]^[107]^[108]. For example, Anaraki et al. prepared a 5% Nb-doped SnO₂ electron transport layer by chemical bath deposition (CBD) for use in a planar PSC. The device has a lower series resistance and a higher short-circuit current and fill factor, achieving an optimal PCE of 20.5% with higher repeatability^[107]. Xu et al added La to the SnO₂ precursor solution to reduce the agglomeration of the SnO₂ crystal and form a uniform surface without pinholes. The SnO₂ doped with La effectively reduced the energy band offset, improved the conductivity and transparency, and promoted the charge injection and transfer. The open-circuit voltage of 1. 11 V and the PCE of 17. 08% were obtained^[105]. Song et al. Synthesized Y-doped SnO₂ by solvothermal method, which can form a more suitable energy level structure with the perovskite layer, better promote the transport of carriers, and obtain the best PCE of 20.71%, with a slight hysteresis^[103]. In addition, Hui et al. Doped the SnO₂ film prepared at low temperature with red carbon quantum dots (RCQ) rich in carboxylic acid and hydroxyl groups, and found that the SnO₂ film doped with RCQ had excellent electron mobility and good hydrophilicity, which was conducive to passivating the traps and defects at the TiO2 perovskite interface and promoting the formation of highly crystalline perovskite films^[109]. The device based on RCQ doped SnO₂ achieves the best PCE of 22.77% and has excellent humidity stability. Through the continuous efforts of scientists, the photoelectric conversion efficiency based on SnO₂ planar PSC has been improved to more than 23%, which has great prospects for development^[7].

4.3.3 SnO₂ prepared at low temperatures

SnO₂ is also widely used in flexible PSC due to its low crystallization temperature. Liu et al. fabricated a highly crystalline SnO₂ electron transport layer without ligands at low temperature (100 ℃) by spin coating and hydrothermal treatment for flexible PSCs, and found that hydrothermal treatment could not only promote the complete removal of organic active agents coated on the surface of SnO₂ nanoparticles by hot water vapor, but also promote the crystallization through the high vapor pressure of water to form high-quality SnO₂^[110]. The device achieves the best PCE of 18.09% and good environmental stability, and still maintains more than 90% of the initial PCE after 1000 bends. Chen et al. Synthesized high-quality SnO₂ electron transport layer for flexible devices by low temperature (120 ℃) preparation process, and studied the effects of different solvents and deionized water on the crystallization process of SnO₂.It is found that the continuous, pinhole-free, compact and highly crystalline SnO₂ film can be prepared by using 5% water and butanol as the solvent of the SnO₂. The optimal PCE of the device is 18% and the device has good mechanical stability^[111]. Zhong et al. Used the SnO₂ film prepared at low temperature (140 ℃) in the flexible PSC, and inserted the fullerene derivative with hydroxyl (CPTA) modification layer at the SnO₂/MAPbI₃ interface, which can increase the conductivity and reduce the interface barrier to improve the device performance. The device obtained the best PCE of 18.36% and had good environmental stability and mechanical bending durability^[112].

4.4 Other metal oxides (WO_x, Nb₂O₅, CeO_x)

Tungsten oxide (WO_x) is an n-type semiconductor with good stability and acid corrosion resistance, wide band gap (2.0 to 3.0 eV), and high electron mobility (10~20 cm²/V·s). These excellent properties make the WO_x suitable for use as an ETL for perovskite solar cells. Compared with the conventional TiO₂, the amorphous WO_x electron transport layer prepared by the low-temperature process shows higher conductivity and light transmittance, but the WO_xETL PSC shows higher J_SC and lower V_OC and FF due to easy charge recombination^[113]. Wang et al. Developed non-annealed, undoped amorphous tungsten oxide as an electron transport layer for flexible perovskite solar cells using vacuum evaporation technology^[114]. It is found that a uniform and dense α-WO_x layer with a thickness of 30 nm can not only reduce the pinholes in the perovskite layer, but also enhance the charge transport. The fabricated perovskite solar cells still showed stable PCE after more than 1000 bending cycles. The optimum PCE of the solar cell with a 30 nm thick α-WO_x is increased to 15.85%.

Niobium oxide (Nb₂O₅) has good photoelectric properties and chemical stability. Therefore, Nb₂O₅ is also considered as a candidate material for ETL. Ling et al. Prepared amorphous Nb₂O₅ by RF magnetron sputtering in argon atmosphere at room temperature. Amorphous Nb₂O₅ films have high transparency in the visible light region and have high mobility and conductivity^[115]. The PCE value of the PSC with the amorphous Nb₂O₅ electron transport layer can reach 17.1%. In addition, Liu et al. Used the large area Nb₂O₅ prepared by electron beam evaporation as the ETL of planar PSC, and studied the effect of the thickness of the Nb₂O₅ layer on the efficiency of the device^[116]. The optimal PCE of the 60 nm thickness non-heat-treated Nb₂O₅ in the rigid and flexible PSC are 18. 59 and 15. 56%, respectively, and it also shows great potential for large area PSC. After that, they modified the perovskite film by adding dimethyl sulfide (DS) additive to increase the PCE of flexible PSC based on Nb₂O₅ to 18.40%^[117].

Cerium oxide (CeO_x) is also considered as a potential ETL due to its wide band gap, good transparency, large dielectric constant, and good chemical and thermal stability. Wang et al prepared CeO_x thin films at low temperature (150 ℃) by sol-gel method and spin-coating method^[118]. The PSC with the CeO_x film showed high performance and high stability, and the PCE obtained was 14.32%. In addition, Chen et al. Have used solution-treated CeO_x as the effective ETL for trans-PSC and obtained an optimal PCE of 17.1%^[119]. They found that the CeO_x layer not only protects perovskite from water attack, but also acts as a diffusion barrier to prevent corrosion of metal electrodes, thus improving the long-term stability of PSC.

The performance parameters of the metal oxide electron transport layer materials in formal perovskite solar cells are summarized in Table 1. To sum up, in the formal inorganic electron transport materials, uniform, pinhole-free and agglomeration-free dense planar thin films are an important part of efficient PSC, and metal oxide nanomaterials with low cost, simple process and adjustable nanostructure also show great advantages. In the aspect of preparation method, the spin-coating method has the advantages of simple process and low cost, but is difficult to prepare uniform, pinhole-free and large-area films; Sputtering technology and atomic layer deposition can produce uniform, dense and high quality films, but the equipment is expensive. Chemical bath deposition is simple and has good reproducibility but consumes a large amount of solution. In terms of material morphology, spherical nanoparticles can form dense and uniform films, and have large specific surface area, which is conducive to electron injection. Well-oriented and highly ordered nano-array structures (such as nanorods, nanowires, nanotubes, nanosheets, etc.) Have fewer grain boundaries, which can provide direct channels for charge transport and facilitate the filling of perovskites. Interface modification and element doping are two main optimization methods for electron transport materials.Interface modification (such as chloride passivation treatment, self-assembled monolayer modification, insertion of modification layer, etc.) Can construct an interface structure which is conducive to rapid carrier transfer and inhibits charge recombination to reduce the density of interface trap States, improve the energy level structure and improve the performance of devices. Element doping can improve the device performance by changing the energy level arrangement, carrier and trap state density and distribution of ETL. For inorganic ETL, the low temperature process with low cost and low energy consumption is still a major breakthrough direction, and has great development potential in flexible devices.

5 Trans-perovskite solar cell electron transport layer material

5.1 Fullerenes and their derivatives

Fullerenes and their derivatives are usually used as electron transport layers in trans-perovskite solar cells because of their good electron transport ability and avoiding high temperature sintering^[3]. Its nano-scale spherical shape can effectively connect with perovskite to achieve uniform distribution on the surface of the light-absorbing layer, resulting in highly selective contacts for electron extraction^[131]. In addition, the fullerene material can also passivate the interface traps of the perovskite active layer and reduce the potential barrier between the electrode and the perovskite layer^[3]. Fullerenes (C₆₀ and C₇₀) and their derivatives (methyl [6,6] -phenyl-3-butyrate (PC₆₁BM),6] -phenyl -C₇₁- iso-methyl butyrate (PC₇₁BM)) (fig. 12) are several commonly used organic electron transport layers in high-efficiency p-i-n planar heterojunction perovskite solar cells^[132].

5.1.1 Common fullerenes and their derivatives

Primitive fullerenes such as C₆₀ and C₇₀ have higher electron mobility and lower cost advantages than PCBM. However, they have poor solubility and are prone to crystallization during solution preparation, so it is still a challenge to prepare dense films of native fullerenes by solution deposition. Dai et al. prepared a mixture of C₆₀ and C₇₀ with high performance and enhanced stability as an electron transport layer by solution processing, obtaining 14.04% PCE^[133]. Liu et al. Deposited C₆₀ETL having various thicknesses in an evaporation chamber by vapor deposition^[134]. It is found that the ultra-thin C₆₀ETL is beneficial to the extraction and collection of electrons and the reduction of space charge accumulation at the interface. The PCE of PSC with 1 nm C₆₀ETL has reached 18.0% without J-V hysteresis.

Nie et al. Used PC₆₁BM as an electron transport material for CH₃NH₃PbI₃ perovskite solar cells^[135]. By optimizing the crystal structure of CH₃NH₃PbI₃ to compensate for grain defects, the photoelectric conversion efficiency is as high as 18%. Upzma et al. Prepared a PC₇₁BM fullerene electron transport layer with a broader absorption spectrum and a lower HOMO by solution treatment, and successfully manufactured a high-performance low-temperature perovskite solar cell without hysteresis, with an average PCE of 13.9%^[136]. Today, PCE with PCBM as ETL has increased to 20.9%^[137,138].

In addition, Xing et al. designed a new hydrophilic fullerene derivative C₇₀-DPM-OE (fig. 12) by combining electron-rich functional groups with fullerenes, in which the phenyl groups of the diphenylmethyl fullerene (DPM) moiety were modified by oligomeric ether chains (OE)^[139]. It is found that the electron-rich oligomeric ether (OE) chain can not only reduce the work function of the metal cathode, but also passivate the trap state of the perovskite layer. Compared with the device with the PC₆₁BM, the device with the C₇₀-DPM-OE as the electron transport layer has a significant improvement in the photoelectric conversion efficiency, and achieves a PCE of 16%. In addition, Chen et al. prepared a new fullerene pyrrolidine derivative, N-methyl-2-pentyl- [60] fullerene pyrrolidine (NMPFP, fig. 12) electron transport layer by a simple solution process^[140]. Compared with the PC₆₁BM device, the NMPFP film has higher conductivity, better hydrophobicity and stronger stability. The PCE of the device with NMPFP as the electron transport layer is 13.83%, and the hysteresis is negligible.

5.1.2 Modification of Organic Electron Transport Layer

The surface modification of the organic electron transport layer can also effectively improve the device performance^[141]. In the organic electron transport layer, Chang et al. Found that PCBM treated by fluoride can be uniformly dispersed in the bulk heterojunction (BHJ) perovskite film, filling the pinholes between the perovskite grains and passivating the interface defects, thus improving the current density. The planar heterojunction device with fluorinated PCBM achieved 16.17% PCE and had high stability^[142]. Wang et al. synthesized indenodithienyl-containing ladder conjugated diimide dimer (DTT2FPDI) by a two-step method, and used DTT2FPD/PCBM as a double-layer electron transport material for trans-perovskite devices^[143]. Due to the appropriate energy level, high electron mobility and excellent hydrophobicity of DTT2FPDI, the device has a higher PCE (19. 4%), which is superior to the device using a single PCBM electron transport layer. Ren et al. Used PCBM/N2200 as a double electron transport layer (ETL) to fabricate a high-performance planar heterojunction perovskite solar cell^[144]. The results show that the PCBM/N2200 double ETL increases the built-in potential of the device and reduces the interface energy barrier of the MAPbI₃/PCBM, thus improving the V_OC. In addition, non-fullerene material N2200 can effectively inhibit the agglomeration of PC₆₁BM, promote the growth of perovskite surface grains and passivate the interface defects of perovskite, thus increasing the short-circuit current (JSC) and fill factor (FF) of PSC. In addition, the N2200 improves the air and light stability of the PSC. These results may be due to the interaction between PbI₂ as a Lewis acid and N2200 as a Lewis base.

The organic electron transport materials modified by doping show good device performance. By doping a small amount of methylammonium iodide (MAI) as an n-type dopant into the fullerene (C₆₀-SAM) of a crosslinkable silane molecule with a hydrophobic functional group, the free electron density is increased and the conductivity is improved, and the device achieves a PCE of 19.5% and a fill factor of 80.6%^[145]. Snaith et al. Doped 4- (1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl) -N, N-diphenylaniline (N-DPBI) into the C₆₀, which can improve the conductivity of the C₆₀, reduce the work function of the C₆₀, adjust the surface wettability of the surface film of C₆₀, reduce the trap state density of devices, improve the air stability of devices, and obtain the best PCE of 18.3%^[146]. Jiang et al. incorporated 2,6-dimethoxypyridine (2,6-Py) as a dopant into a PC₆₁BM and fabricated a high-efficiency planar perovskite solar cell with a PCE of 19.41%^[147]. It is found that 2,6-Py not only acts as a Lewis base to passivate the perovskite surface without corroding the perovskite, but also improves the conductivity and mobility to achieve efficient electron extraction and transport. Zhu et al. Used quasi-spherical carbon quantum dots (CQDs) doped PCBM films in trans-PSC^[148]; It is found that the doping of CQDs can improve the Fermi level, conductivity, electron mobility and charge extraction efficiency of PCBM. CQDs have high electron density and can fill the electron traps produced by PCBM dimerization. The groups on the surface of the CQDs can adsorb the I^- and prevent the I^- from diffusing through the PCBM layer to the silver electrode, thereby inhibiting the decomposition of the perovskite and improving the long-term stability of the device. After optimizing the doping amount of CQDs, the PCE of the device is 18. 1%.

5.2 Non-fullerene organic small molecular

In recent years, non-fullerene organic small molecule materials have attracted more and more attention because of their adjustable energy level, easy synthesis, low production cost and good solubility. Some ETL materials based on non-fullerene organic molecules have been used in PSC^[149].

For example, Jiang et al. Successfully used a 3D-based perylene diimide (PDI) molecule (TPE-PDI4, fig. 13A) as an electron transport material in trans-perovskite solar cells^[150]. It is found that TPE-PDI4 has good electron mobility, thin film morphology and good energy level matching with the perovskite layer, and it has better water resistance than the PC₆₁BM, which can more effectively protect the underlying perovskite layer to enhance the environmental stability of the device. The device based on TPE-PDI4 shows a higher photoelectric conversion efficiency (16.29%). In addition, Miao et al. Found that an alcohol-soluble non-fullerene small molecule perylene diimide derivative (PDIN) can be used as an effective electron transport material for p-i-n perovskite solar cells^[151]. The amino group at the terminal of PDIN may reduce the work function and energy barrier of the cathode, and PDIN can also passivate the trap centers on the surface of the perovskite layer and quench the photoluminescence of the perovskite layer. By using a green orthogonal solvent (2,2,2-trifluoroethanol) as the solvent for PDIN, it can easily form high-quality PDIN films on perovskite layers. Compared with the device based on the PC₆₁BM electron transport layer, the device using PDIN as the electron transport layer achieves a higher PCE (15. 28%).

Jung et al. studied a novel small molecule (N, N '-bis (1-indanyl) naphthalene-1,4,5,8-tetracarboxylic diimide, NDI-ID, fig. 13b) electron transport material based on naphthalimide (NDI)^[152]. NDI-ID is composed of N-substituted indanyl groups, which have both cycloaliphatic and aromatic characteristics and show high electron mobility. The device with NDI-ID as the ETM achieves a high PCE of 20.2% and exhibits high long-term stability, retaining 90% of the initial PCE without packaging after 500 H at 100 ℃.

In addition, Wang et al. synthesized two other small organic molecules, tetraphenylethylene-pyrrolopyrroldione (TPE-DPP4) and tetraphenylethylene-isoindigo (TPE-ISO4), in trans-perovskite solar cells^[153]. TPE-DPP4 and TPE-ISO4 have the energy level matching with the perovskite layer and have strong electron extraction and electron transport capabilities. The devices with TPE-DPP4 and TPE-ISO4 as electron transport materials showed PCE of 18.44% and 18.19%, respectively.

5.3 Non-fullerene polymer molecular

N-type conjugated polymers have also been developed as efficient trans-electron transport materials. For example, naphthalimide (NDI) -based polymer semiconductors have been used as electron transport materials in PSCs due to their high electron mobility, low LUMO level, good solution processability, and stability^[154].

Among them, Kim et al. Developed a new polymer P (NDI2DT-TTCN) (fig. 14) with naphthalimide as the core to replace PCBM in the trans device^[155]. Compared with PCBM, the novel polymer P (NDI2DT-TTCN) shows better π-π stacking and deeper LUMO level, which not only improves the electron extraction ability, but also forms a hydrophobic surface to prevent environmental interference and has excellent mechanical stability. The device has a PCE of 17% with negligible hysteresis.

In addition, other new N-type conjugated polymers have also been synthesized. Tian et al. prepared N-type conjugated polymers PN with varying amounts of fluoro and amino side chains (PN-F25% and PN-F50%, fig. 14)^[156]. The amino side chain in the ETL can effectively improve the interface contact and electron collection of the perovskite layer, and the fluorine side chain can enhance the hydrophobicity of the polymer and improve the stability of the device. The results show that the PCE based on PN-F25% can reach more than 17%.

The performance parameters of the organic electron transport layer materials in the trans-perovskite solar cells are summarized in Table 2. To sum up, trans-organic electron transport materials are expected to become an important part of high-efficiency PSC. Fullerenes and their derivatives have been widely used because of their good electron transport ability, passivation of interface defects and reduction of interface barriers. Non-fullerene electron transport materials show great potential for development due to their high mobility, adjustable energy levels and easy synthesis. Organic electron transport materials have also been doped (such as methyl ammonium iodide (MAI), 2,6-dimethoxypyridine (2,6-Py), carbon quantum dots (CQDs), etc.) And surface modified (such as fluoride treatment, insertion of modified layers, etc.) To improve the photovoltaic performance and stability of devices. In the process of actually producing high-performance trans-PSC, many efforts need to be made, such as developing simple synthesis methods to produce new and efficient organic electron transport materials to reduce costs; Improving the electron mobility of the organic electron transport layer; Nhancing the chemical stability and the thermal stability of the organic electron transport layer and the like.

6 Conclusion and prospect

In this review, we focus on the material properties, morphologies, preparation methods and optimization strategies of various electron transport materials. Each electron transport material has its own advantages and disadvantages. For example, in formal PSC, TiO₂ has good chemical stability and high charge transport performance, but its preparation temperature is high and its UV-induced device performance degradation is serious. ZnO has the advantages of low preparation temperature, high electron mobility and easy preparation of various nanostructures, but it has serious interface recombination and instability. The SnO₂ has the advantages of proper energy band structure, high electron mobility, low preparation temperature and weak photocatalysis, but has more surface defects and serious interface recombination; In trans-PSC, fullerenes and their derivatives have good electron transport ability and can avoid high temperature sintering, but their film forming ability and stability are poor. Non-fullerene polymers and small organic molecules are easy to synthesize, have adjustable energy levels and high electron mobility, but the photoelectric conversion efficiency of devices has not yet made a surprising breakthrough. At present, there is no electron transport material with outstanding photoelectric properties, morphology control, stability and other aspects to replace other electron transport materials, so various electron transport materials are still being explored and used. Scientists hope to find a perfect optimization strategy to improve the performance of materials, which can adjust the energy level, improve the crystal surface structure, improve the electron transport ability, control the film morphology and reduce the surface vacancies and other defects of films.

At present, seeking higher device efficiency and stability is a hot spot in the field of PSC research, but the inherent photoelectric characteristics and inevitable defect States in ETL will hinder the improvement of device efficiency and lead to non-negligible hysteresis. So far, a large number of excellent ETL materials with good stability and various nanostructures have been developed, as well as optimization strategies to improve the photoelectric conversion efficiency and stability of devices. However, for PSC to be commercialized, it must be able to be produced on a large scale, so it is very important to produce dense, uniform, pinhole-free and crack-free ETL with high quality at low cost. In addition, low-temperature process can effectively reduce production costs and energy waste, and is conducive to the manufacture of flexible devices, so the production of high-performance PSC by low-temperature manufacturing process is also a major technology that needs to be broken through. In a word, the development and optimization of electron transport materials and their preparation need to be explored continuously, and in-depth research on ETL in material science and device physics can promote the development of high-performance PSC for commercial application.

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Funding

National Natural Science Foundation of China(61774169)

Qingyuan Innovation and Entrepreneurship Research Team Project(2018001)

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1 Introduction

2 Charge Transport Mechanism of Electron Transport Layer in Perovskite Solar Cell

2.1 Charge transport through the electron transport layer in perovskite solar cells

2.2 Charge transport in the electron transport layer of trans-perovskite solar cell

2.3 Role and Influence of Electron Transport Layer in Perovskite Solar Cell

3 Preparation of Electron Transport Layer in Perovskite Solar Cell

3.1 Coating method

3.2 Chemical Bath Deposition

3.3 Atomic layer deposition method

3.4 Other deposition preparation method

4 Novel perovskite solar cell electron transport layer material

4.1 TiO2

4.1.1 TiO2 with different morphologies

4.1.2 Interface Modification and Element Doping of TiO2;

4.1.3 TiO2 prepared at low temperatures

4.2 ZnO

4.2.1 ZnO with different morphologies

4.2.2 Surface Modification and Element Doping of ZnO

4.2.3 ZnO prepared at low temperatures

4.3 SnO2

4.3.1 SnO2 with different morphologies

4.3.2 Surface Modification and Element Doping of SnO2;

4.3.3 SnO2 prepared at low temperatures

4.4 Other metal oxides (WOx, Nb2O5, CeOx)

5 Trans-perovskite solar cell electron transport layer material

5.1 Fullerenes and their derivatives

5.1.1 Common fullerenes and their derivatives

5.1.2 Modification of Organic Electron Transport Layer

5.2 Non-fullerene organic small molecular

5.3 Non-fullerene polymer molecular

6 Conclusion and prospect

References

Funding

RIGHTS & PERMISSIONS

4.1 TiO₂

4.1.1 TiO₂ with different morphologies

4.1.2 Interface Modification and Element Doping of TiO₂;

4.1.3 TiO₂ prepared at low temperatures

4.3 SnO₂

4.3.1 SnO₂ with different morphologies

4.3.2 Surface Modification and Element Doping of SnO₂;

4.3.3 SnO₂ prepared at low temperatures

4.4 Other metal oxides (WO_x, Nb₂O₅, CeO_x)