In the 21st century, we are living in an energy-driven world. Sustainability in energy generation is one of the most challenging issues to satisfy the ever-increasing energy demands. Solar energy is readily available in nature, which exhibits a great potential to address the global energy challenge. However, the widespread use of solar energy requires the development of cost-effective and high-performance photovoltaics (PVs) for efficient energy conversion from solar to electric energy. The emergence of organometallic mixed halide perovskite solar cells (PSCs) resulted in an important breakthrough in PV technology. The recently announced record power conversion efficiency (PCE) values of single-junction PSCs and perovskite-based monolithic tandem devices are 25.5 and 29.5%, respectively (The National Renewable Energy Laboratory [NREL], 2021), which outperform other thin-film PV technologies (e.g., CIGS and CdTe) and are comparable to well-established solar technologies such as crystalline silicon photovoltaics (PCE ∼26.1%). Such impressive device efficiency of PSCs is owing to inherent perovskite properties such as large absorption coefficients, adjustable bandgaps, low exciton binding energy, large carrier diffusion lengths, and high charge mobilities (Xing et al., 2013; Wang et al., 2015). However, a number of challenges such as device long-term stability, hysteresis effect, toxicity of lead, and scalable difficulties still remain in the community (Djurišić et al., 2016; Djurišić et al., 2017; Li et al., 2018; Li et al., 2020). In general, five major areas are being investigated to come up with possible solutions for the problems mentioned above. They include (i) perovskite composition and crystal structure (Luo and Daoud, 2016; Mitchell et al., 2017), (ii) engineering of perovskite growth methods (Ng et al., 2015, 2018; Babayigit et al., 2018; Shen et al., 2018; Hu et al., 2020a), (iii) optimization of carrier transport materials and interfaces (Liu et al., 2017; Hu et al., 2020b,c; Wang et al., 2021), (iv) device architectures (Hanmandlu et al., 2018; Ren et al., 2018; Maxim et al., 2020; Shalenov et al., 2020), and (v) encapsulation (Dong et al., 2016; Fu et al., 2019; Emami et al., 2020). Some comprehensive review papers on those topics have also been published (Jiang et al., 2018; Sajid et al., 2018; Ouyang et al., 2019; Hanmandlu et al., 2020; Hu et al., 2020a; Zhou et al., 2020).

This article aims to highlight the development of the predominant metal oxides—TiO₂, SnO₂, and NiO_x—using the charge transporting layers (CTLs) in PSCs. Special attentions are placed on these three types of metal oxides as the state-of-the-art PSCs with record-breaking efficiencies are usually associated with these metal oxide CTLs. A brief review on representative research works on PSCs has been done, indicating the highest achievable PCE for different CTL-based PSCs at different stages of time. The important criteria in selecting metal oxides as CTLs are emphasized. Varieties of effective preparation methods for metal oxide CTLs have been summarized, and their advantages and shortcomings in terms of processing conditions and possibility of future large-scale manufacturing are discussed. This work will provide a direct insight into the PV community for further optimizing of CTLs, which will be one of the critical steps for approaching practical PSCs in the commercial market.

Starting from 2009, the group of Miyasaka demonstrated the photovoltaic properties of using perovskite in the dye-sensitized solar cell by employing mesoporous TiO₂, exhibiting a PCE of 3.8% (Kojima et al., 2009). In early 2012, the group of Park reported a PCE of 9.7% of PSCs by introducing a sub-micrometer-thick layer of TiO₂ and solid-state hole transporting materials (HTMs) (Kim et al., 2012). In the same year, a PCE more than 10% was reported by the group of Snaith (Lee et al., 2012). In 2013, the group of Grätzel demonstrated high-performance PSCs fabricated by the sequential deposition method with a certified PCE of 14.1% (Burschka et al., 2013). In the following years, the group of Seok reported a dramatical improvement in the device performance of PSCs by using different engineering approaches such as solvent engineering (certified as 16.20%) (Jeon et al., 2014), compositional engineering (certified as 17.9%) (Jeon et al., 2015), and intramolecular exchange (20.1% certified PCE) (Yang et al., 2015c). In 2016, the group of Grätzel reported the use of triple cations in perovskite materials, yielding good device reproducibility and improved device stability, leading to a high PCE of 21.1% (Saliba et al., 2016). Nowadays, the highest certified PCE as reported in literature for PSCs based on TiO₂ electron-transporting layer (ETL) is 24.64% (Jeong et al., 2020). The TiO₂-based ETLs are popular to be used since the early stage of PSCs owing to its high versatility of preparation techniques as well as attributing to the evolution of the architecture from the dye-sensitized solar cells. The representative research works using TiO₂ as the ETL in PSCs with their achieved PCEs are summarized in Figure 1 as indicated in solid circles. Along with such a tremendous improvement in device performance, a compact layer of tin oxide (SnO₂) has been demonstrated as an alternative for ETL. First, Ma et al., 2017 reported a high-temperature processing compact SnO₂ ETL prepared by the conversion of SnCl₂⋅2H₂O precursor to SnO₂ using a sol-gel method for CH₃NH₃PbI₃-based PSCs, showing a PCE of 7.43% (Dong et al., 2015). The group of Dai and Kuang also reported a high-temperature processing SnO₂ mesoporous ETL and as-synthesized colloidal solution, respectively, followed by TiCl₄ surface treatment for CH₃NH₃PbI₃-based PSCs, exhibiting an improvement in their champion device efficiency of 10.18 (Li et al., 2015) and 14.69% (Rao et al., 2015), respectively. However, at that time, the PCEs of SnO₂-based PSCs are still lower than those of TiO₂-based PSCs due to the introduction of a large amount of trap states during the high-temperature processing (Ke et al., 2015). The important breakthrough in achieving high device efficiency for SnO₂-based PSCs was from 2015. The group of Fang firstly introduced low-temperature processing for preparing SnO₂ compact layer based on the sol-gel method using SnCl₂⋅2H₂O as the precursor to achieve a high PCE of 17.21% for CH₃NH₃PbI₃-based PSCs (Ke et al., 2015). Then, the group of You reported a PCE of 20.54% (certified as 19.9%), which was a record-high value in 2016 for (FAPbI₃)_X (MAPbBr₃)_1–X-based PSCs with a low-temperature processing SnO₂ ETL prepared from commercially available SnO₂ colloid solution (Jiang et al., 2016). In the same year, the group of Hagfeldt demonstrated a low-temperature chemical bath deposition for preparing SnO₂ compact layer as ETL used in Cs-containing mixed halide-based PSCs, yielding a stabilized PCE of 20.7% (Anaraki et al., 2016). In 2017, the group of You updated the champion PCE to 21.6% (certified as 20.9%) using low-temperature processing SnO₂ in (FAPbI₃)_1–X (MAPbBr₃)-based PSCs (Jiang et al., 2017). The representative research works using SnO₂ as the ETL in PSCs with their achieved PCEs are summarized in Figure 1 as indicated in solid squares. On the other hand, metal oxides based on NiO_x have been commonly used as HTMs for PSCs. In 2014, Tian et al. (2014) firstly introduced the p-type NiO mesoporous for perovskite-sensitized solar cells, achieving a PCE of 1.5%. The group of Ahmadi demonstrated NiO_x/Ni double layer as HTL deposited by the sputtering process in a planar n-i-p structure for PSCs, yielding a PCE of 7.28% (Abdollahi Nejand et al., 2015). In 2015, the group of Han employed a 10–20 nm NiO compact layer along with mesoporous Al₂O₃ scaffold as HTM in p-i-n-based PSCs, obtaining a promising efficiency of 13.5% (Chen et al., 2015a). The group of Wang reported using NiO_x HTM in mesoporous n-i-p-based PSCs with a further enhanced PCE to 15.03% (Cao et al., 2015). Li et al. (2017) applied sputtered NiO_x for planar PSCs, which exhibit a champion PCE of 18.5%. Furthermore, introduction of dopants, commonly Li (Qiu et al., 2017), Co (Natu et al., 2012), Mg (Chen et al., 2015b), Cu (Chen et al., 2018), Cs (Singh et al., 2020), etc., in NiO_x can further enhance the charge transport properties due to improved carrier conductivity as well as better energy level alignment across the interface of perovskite and HTL. The group of He and Lei doped NiO_x with Cs and Cu, yielding a PCE of 19.35 (Chen et al., 2017) and 20.5% (Yue et al., 2017) for planar p-i-n-based PSCs, respectively. Recently, a latest high PCE of 21.6% was achieved for NiO_x-based PSCs in a p-i-n structure after passivating the perovskite/ETL by using Cd_xZn_1–xSe_yS_1–y quantum dots (Chen et al., 2020). The representative works for NiO_x-based PSCs are summarized in Figure 1 and indicated in solid triangles.

The appropriate energy band alignment between CTLs and the perovskite absorbing layer is one of the fundamental criteria to ensure efficient charge extraction in PSCs. The conduction band minimum (CBM) of ETL is slightly lower than the conduction band of the perovskite, while the valence band maximum (VBM) of HTL is slighly higher than the valence band of the pervoskite so that the CTLs can extract the corresponding charge carriers readily. Meanwhile, the energy barrier between the VBM of ETL and perovskite (and barrier between CBM of HTL and perovskite) is sufficiently large to block the counter carriers generated from the perovskite layer, suppressing the opportunity of charge recombination within the devices. The built-in potential (V_bi) can be influecend by the types of CTMs and the interfacial quality between the CTL and perovskite (Lin et al., 2017; Shin et al., 2019). The larger V_bi of devices enables PSCs to have a stronger ability to separate charges and transport and collect photo-generated carriers, yielding a higher V_OC. A variety of research works have demonstrated that minimizing the interfacial charge recombination between CTL and perovskite layer can effectively enhance the V_OC of PSCs (Jung et al., 2019; Kaneko et al., 2019; Aidarkhanov et al., 2020). Carrier mobility is another critical factor affecting the charge transport properties. Carrier mobility should be high to avoid charge accumulation at the interfaces of CTL/perovskite. The bandgap of CTMs should be wide and possess small refractive indexes so that the amount of visible light penetrating through the CTL to the perovskite absorber layer in the PSCs can be maximized.

The morphology of CTLs should be controlled well in PSCs. The underlying ETL or HTL will determine the morphology of the perovskite film deposited on the top of CTLs for n-i-p or p-i-n device structures, respectively. It is known that the optimized morphology of the CTL-coated substrate can assist in the crystallization of the subsequent depositing perovskite layer and improve the perovskite film coverage on the CTL, inhibiting the formation of pinholes to prevent the introduction of undesired shunt paths in PSCs (Ng et al., 2016; Wang et al., 2019). Furthermore, the interfacial quality between the CTL and perovskite, which is of significant importance to determine the PCE and stability of PSCs, is strongly influenced by the surface morphology of CTLs. Intensive research works have exhibited different strategies to passivate the defects, which likely concentrate at material interfaces to suppress charge recombination and hysteresis effect (Shao and Loi, 2020). Meanwhile, the interface between the CTL and conductive electrode should also be optimized for forming of a good ohmic contact for PSCs (Babaei et al., 2020; Tseng et al., 2020).

It is noteworthy that metal oxide materials used as CTLs should be insensitive to ultraviolet light and possess very low photocatalytic effect in order to maintain a long-term stability of PSCs operating under strong sunlight. Metal oxide CTLs, unlike organic materials such as the commonly used PEDOT:PSS HTL, are usually not hygroscopic, which are more robust to moisture, allowing them to be the protection layers and preventing the perovskite active layer from degradation in the humid ambient air. Nowadays, metal oxide CTLs have shown their outstanding chemical stability, which is one of the essential factors for achieving long-lifetime PSCs for future commercialization (You et al., 2016; Lei et al., 2019; Singh et al., 2019; Thambidurai et al., 2020). Figure 2 indicates the general properties of three different CTMs as discussed in this work.

The stability of PSCs can be enhanced by incorporating metal oxide CTLs for both ETL and HTL in the same device. However, nowadays, device stability remains a challenge since the majority of PSCs employ metal oxide CTL as either ETL or HTL while using organic or fullerene-based materials as the counter-CTL. For PSCs with all-inorganic CTLs, the processing temperatures of the metal oxide layer, which is located above the perovskite, usually should be below 100°C to prevent the thermal decomposition of the perovskite absorber. Spin-coating of a dispersion solution containing pre-synthesized metal oxide nanostructures on top of the perovskite is one of the possible techniques to yield metal oxide CTL at a low temperature. Considering the commercial standards for practical PSCs, the preparation methods for metal oxide CTLs should be cost-effective and compatible with low-processing temperature for flexible substrates as well as scalable for large solar modules. The mechanical flexibility of metal oxide CTLs should be also carefully investigated together with the perovskite absorber and flexible substrates during the blending process. Meanwhile, the interfacial engineering between the perovskite absorber and the CTL should be considered as well to modify the interface properties and passivate the defects, which is an important strategy to enhance device efficiency and stability.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

MS conceived the idea. MS and AN wrote the manuscript. AN and CC were involved in the manuscript discussion and correction. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.