Over the past decades, perovskite solar cells (PSCs), which employ organic-inorganic metal halide hybrid perovskites as light-harvesting materials, have received extensive attention due to their low production cost, solution processing, and good photoelectric characteristics ^{[1][2][3][4][5]}[1,2,3,4,5]. Since the perovskite materials as a photosensitizer were first introduced into dye-sensitized solar cells (DSSCs) and achieved a power conversion efficiency (PCE) of 3.8% in 2009 [6], the PSCs have achieved significant breakthroughs and rapid evolution. Recent technologies can lead to a certified PCE of 25.7%, including device structure and perovskite film quality optimization, interface engineering, and additive engineering ^{[7][8][9][10]}[7,8,9,10], making PSCs a leading candidate for next-generation photovoltaic technology.

In 2012, the standard solid-state PSCs were assembled for the first time by introducing a solid hole transport material of spiro-OMeTAD to replace liquid electrolytes ^[11][12][11,12]. Since then, the mesoporous structure soon became the most popular geometry to construct PSCs ^[13][14][13,14]. However, a complicated fabrication process, which involves the deposition of a compact layer followed by a mesoporous TiO₂ layer, is needed for this structure [15]. Meanwhile, a sintering process at a high temperature (usually over 500 °C) is required to remove the organic material in the TiO₂ paste and enhance the crystallinity of the resulting TiO₂ film [16]. These tedious procedures increase the cost of device fabrication and are incompatible with the production of flexible PSCs. In order to overcome the above issues, the planar PSCs were then developed. The long carrier diffusion length and carrier lifetime of commonly used perovskite materials guarantee the effective transport of carriers in this type of solar cell ^[17][18][17,18]. The typical planar PSCs have two different structures, including the regular n-i-p and inverted p-i-n structures. As for planar PSCs, developing high-quality electron transport layers (ETLs) is crucial to realize high device performance. The ETLs not only can promote the extraction and transport of photogenerated electrons but also block holes to prevent unfavorable charge recombination ^[4][16][4,16]. Currently, many inorganic semiconductors, especially inorganic metal oxides (MOs) such as TiO₂, ZnO, and SnO₂, are widely adopted as ETLs in PSCs with planar structure, which is attributed to their low-cost, superb versatility, low-temperature processability, excellent electronic properties, and superior device performance ^{[19][20][21][22]}[19,20,21,22]. Among them, ZnO is one of the most promising choices owing to its high transparency, high electron mobility, and suitable energy band structure, which can potentially facilitate electron transfer and reduce undesired recombination loss ^[23][24][23,24]. Moreover, ZnO is easy to crystallize, and its intrinsic properties of thin layers can be adjusted simply by doping and manipulating structural composition ^[25][26][27][25,26,27]. The compact ZnO used as ETL for planar PSCs cannot only simplify the device fabrication process ^[28][29][28,29] but is also easier to prepare by diverse deposition technologies ^{[27][29][30][31][32][33]}[27,29,30,31,32,33]. On the other hand, a lower annealing temperature is required for most of the ZnO preparation processes in planar PSCs (usually ≤200 °C), suggesting that it can be produced at a low cost and is suitable for flexible devices ^{[29][30][31][32][33][34]}[29,30,31,32,33,34]. However, the high quality of ZnO films should be ensured to achieve more efficient charge transport in planar PSCs.

The solution process is the most reported deposition technology of ZnO ETL in planar PSCs, among which the sol-gel method has been widely concerned. In 2014, Lee et al., demonstrated efficient planar PSCs with a sol-gel processed ZnO as ETL, achieving a device efficiency of 8.37% ^[35][60]. In their study, a thin layer of organic molecules, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) was further introduced on the surface of ZnO to reduce the nonradiative recombination induced by traps at the interface of ZnO as well as in the bulk of perovskite layer, thus improving the PCE to 12.2%. Manspeaker et al., obtained MAPbI₃ perovskite on a sol-gel processed ZnO ETL by utilizing a sequential deposition method ^[36][61]. They studied the decomposition mechanism of perovskite and revealed the effect of solvent in the perovskite films during annealing. A restricted volume solvent annealing (RVSA) process has been developed to deposit perovskite films on ZnO, resulting in the generation of reproducible PSCs with an efficiency of 13.7%. Zhao et al., also reported the ZnO films achieved by the sol-gel method that was applied as ETLs in planar PSCs, and the device performance of PSCs based on ZnO ETLs made through sol-gel (SG) and hydrolysis-condensation (HC) manners were compared systematically ^[37][53]. They found that the HC-ZnO film exhibited a relatively flat surface and higher conductivity; thereby, the PSCs could yield a higher PCE of 12.9%, while a PCE of 10.9% was delivered for the PSCs based on SG-ZnO ETL. In 2017, Zhou et al., reported an aqueous solution-processed route to produce the ZnO ETLs for planar PSCs at low temperatures. An ammine-hydroxo zinc complex solution ^[38][62], [Zn(NH₃)_x](OH)₂, was spin-coated on the substrate as the precursor. By utilizing this method, the thermal annealing temperature of ZnO could be reduced to 150 °C, and the prepared ZnO thin films have high transparency and uniformity. Consequently, the PSC with a conventional n-i-p structure showed an efficiency of 10.6% with a high open-circuit voltage of 1.07 V. In addition, by changing the traditional sol-gel method, a simple, effective, scalable approach of combustion synthesis was developed to prepare ZnO ETLs at low temperature for planar PSCs ^[39][63], which was comprised of the fuel of acetylacetone and the oxidizer source of Zn(NO₃)₂, respectively. As a comparison, two traditional sol-gel processed ZnO films were also prepared in parallel.

The direct synthesis of ZnO NPs is the most extensively used solution process. The ZnO NP films as the ETL of PSCs were originally studied and realized in 2014 by the Kelly group [30]. They synthesized ZnO NPs by the hydrolysis of Zn(C₂H₃O₂)·2H₂O dissolved in methanol, and the NPs could be well dispersed in the mixed solvents of butanol and chloroform without extra surfactants or binders. After that, the compact ZnO NP layers were obtained by spin-coating and used as the ETLs for planar PSCs. The ZnO film was obviously thinner and required no calcination or sintering step. By optimizing the ZnO thickness and perovskite crystal growth, the low-temperature processed PSC devices yielded a promising efficiency of 15.7% and 10.2% on glass and plastic substrates, respectively. Then, they further investigated the effect of CH₃NH₃PbI₃ film thickness and morphology on device efficiency of planar ZnO PSCs ^[40][64] and demonstrated that the thermal evaporation of PbI₂ films was a highly reproducible method to fabricate planar PSCs with very precise control over the perovskite thickness. Based on the ZnO NP ETLs, Hwang et al., fabricated the fully slot-die-coated PSCs using a 3D printed slot-die coater ^[41][65]. Consequently, the optimal PCE of 11.96% was produced for the planar PSCs processed at a low temperature, and the results demonstrated the possibility of roll-to-roll mass production of PSCs with ZnO ETL. Zhou et al., developed the ZnO(NP)/CH₃NH₃PbI₃/C planar PSCs without organic hole-transporting layers (HTLs) and metal electrodes at low temperatures ^[42][66]. The device architecture and procedure were simple, and the flexible PSCs based on ITO/PEN substrate performed well after 1000 times of bending. In 2016, Song et al., employed commercial ZnO NPs to prepare ZnO thin films used as ETLs for low-temperature planar PSCs ^[43][67]. To address the interfacial thermally instability problem between ZnO and perovskite, and further boost the photovoltaic performance of the device, they developed FAPbI₃ as the ZnO-based light absorber and a modified two-step deposition technique to grow the perovskite layer. By optimizing the preparation process of FAPbI₃, the fabricated PSCs yielded the best-performing PCE of up to 16.1%, and the heat resistance of the perovskite layer on ZnO was greatly promoted compared to MA-based perovskite. Subsequently, by utilizing the same ZnO NP ETL, Song et al., confirmed that the triple cation perovskite prepared by a one-step anti-solvent method could be a stable active-layer material for efficient PSCs ^[44][68]. An optimum PCE of 18.9% was achieved for the PSC devices with excellent aging resistance and light stability. In 2018, an ultrasonic-assisted method was shown to obtain a ZnO NP solution with high transparency, and the more compact and pinhole-free ZnO NP films were successfully prepared ^[45][69].

In addition to the solution process, other deposition methods, such as ALD, magnetron sputtering, ED, and electrostatic spraying, have also received extensive attention in ZnO preparation. Lee et al., prepared compact ZnO films as the ETLs of planar PSCs using the ALD technique at a low temperature of 80 °C ^[46][73]. Comprehensive studies were performed to understand the effect of the thickness of the ZnO layer on the PSC performance, and the highest PCE of the device could be obtained at a ZnO film thickness of 30 nm. Using this same method, Dong et al., fabricated the planar PSCs by depositing the ZnO films at 70 °C, producing the best device efficiency of 13.1% ^[47][74]. It was observed that the compact ZnO film prepared by ALD could promote the formation of CH₃NH₃PbI₃ at room temperature when the perovskite precursor contained chloridion, which was attributed to the reaction between ALD-ZnO and CH₃NH₃Cl. The ALD deposition is also suitable for PSCs with the inverted p-i-n structure. In 2015, Chang et al., adopted a low-temperature ALD technology to grow high-quality ZnO film applied as ETL for inverted planar PSCs [32]. The resulting PSC device revealed a remarkable PCE reaching 16.5% with high reproducibility, which is superior to that of the PSC with ZnO NP ETL (10.8%). Furthermore, the applicability of ALD-ZnO ETL in semitransparent PSCs was also demonstrated by employing Ag nanowires as the top electrode, and a record-high PCE of 10.8% was achieved. Meanwhile, the Al₂O₃ films prepared by the ALD process were incorporated to serve as the encapsulation layer, and thus the ambient stability of the device was significantly improved.

Magnetron sputtering is a simple and reliable technique. In 2014, Liang et al., reported the magnetron sputtered ZnO film used as the ETL in planar PSCs ^[48][75]. The device performance was observed to be insensitive to the thickness of ZnO ETL, which was ascribed to the high electric conductivity of ZnO. As a result, the sputtered ZnO gave a PCE of 13.4% for PSC on a rigid substrate, and the flexible PSC on PET substrates showed a PCE of 8.03%. Tseng et al., investigated the effect of the atmosphere in a sputtering chamber on the formation quality of ZnO [33]. The results demonstrated that the properties, such as the conductivity and band structure of ZnO films, could be tuned by optimizing the ratio of working gases in the process of magnetron sputtering. Finally, an efficiency of up to 15.9% was realized for the regular PSCs when the ZnO ETL was produced under the working gas of pure Ar, indicating that magnetron sputtering was a splendid technique to fabricate a ZnO layer with controllable properties in planar PSCs. The sputtered ZnO is also a feasible choice in the inverted PSCs. Lai et al., have demonstrated the performance of inverted structured PSCs with a sputtered ZnO ETL ^[49][76]. In their research, in order to prevent sputtering damage on perovskite, a C₆₀ interlayer was introduced between perovskite and ZnO for protection. The resulting optimized PSC exhibited better performance than that based on C₆₀/BCP.

In 2013, the ED method was employed by Kumar et al., to form a compact ZnO film as ETL [29], and the low-temperature, solution-processed, and flexible PSCs were successfully fabricated by growing ZnO nanorods by chemical bath deposition on electrodeposited ZnO. In addition, Zhang et al., utilized the ED technology for the deposition of the ZnO layer at low temperatures and successfully applied it as ETL in planar PSCs ^[50][59]. The effect of the chemical nature and structure of ZnO and TiO₂ ETLs on the formation of CH₃NH₃PbI₃ stemming from two different techniques was investigated. The optimum PCE of 15% was yielded for the PSC with an electrodeposited ZnO ETL, a planar architecture, and a one-step method prepared perovskite. Using the electrospraying method, Mahmood et al., first deposited ZnO and Al-doped ZnO films and studied their application as ETLs for PSCs in 2014 [27].

Elemental doping has been widely used to improve the quality of ZnO ETL. In 2016, Tseng et al., reported the preparation of high-quality, full-coverage Al-doped ZnO (AZO) films (~20 nm) on transparent conductive substrates by magnetron sputtering, and used it as an ETL for the regular planar PSCs ^[51][77]. Compared with ZnO films, the AZO showed higher conductivity, better acid resistance, and a more aligned energy band with MAPbI₃, resulting in improved photovoltaic performance. The higher V_oc and FF were achieved in the best AZO-based PSC with an efficiency of 17.6%, and the MAPbI₃ films formed onto the AZO had higher thermostability compared with those formed onto the ZnO. Song et al., developed a sol-gel method prepared Mg-doped ZnO (ZMO) used as ETL in planar PSCs ^[52][78]. It was found that the photovoltaic performance of the device was strongly dependent on the Mg doping amount, and the champion PCE of 16.5% was reached for the device with 10% Mg doping. The enhanced PCE originated from the improved optical properties, favorable energy band, efficient electron extraction, and inhibited nonradiative carrier recombination at the ZMO/perovskite interface. More importantly, the CH₃NH₃PbI₃ deposited on 10% ZMO thin films demonstrated better heat resistance, and the fabricated PSC device showed improved stability stored in an N₂ atmosphere and under illumination. Then, the lithium (Li) doping low-temperature processed ZnO (L-ZnO) ETL was reported by Mahmud et al., ^[53][79]. After Li doping, the inherent defects in the ZnO films were effectively passivated, and the Fermi energy position of L-ZnO was downshifted by 30 meV. The shifted energy level helped to reduce the electron injection barrier from perovskite to ETL. Consequently, the triple cation (Rb, MA, FA) PSCs incorporating L-ZnO achieved an increased PCE from 14.1% to 16.1% compared to the pure ZnO, which benefited from the superior charge transfer, lowered leakage current, and suppressed nonradiative charge recombination. Azmi et al., prepared the alkali-metal-doped ZnO ETL by dipping ZnO films into various solutions of alkali-metal hydroxide (LiOH, NaOH, and KOH) ^[54][80]. The metal doping not only significantly enhanced the electron mobility but also induced a more favorable energy band. Additionally, the surface defects of ZnO films were effectively passivated. Particularly, the deprotonation reaction between perovskite and ZnO was weakened, and the durability of PSCs under ambient air conditions was dramatically raised. A champion device efficiency of 19.9% was yielded for the planar PSC fabricated with K-doped ZnO (ZnO-K), whereas the control device only exhibited a PCE of 16.10%. In addition, similar results could be obtained by Li or Cs doping that was performed by adding either caesium carbonate or lithium acetate into the sol-gel ZnO ^[55][81].

On the other hand, there were also some functional molecules that were adopted as dopants of ZnO ETL. Qin et al., reported a new recipe to prepare ZnO by replacing the generally used ethanolamine with polyethylenimine (PEI) in the precursor solution to provide an alkaline environment ^[56][82], which could reduce the number of hydroxyl groups on the ZnO surface. With this approach, the thermal decomposition reaction of MA-based perovskite on the ZnO was considerably relieved. Additionally, the ZnO prepared from the precursors containing PEI (P-ZnO) was beneficial in inducing the uniform and dense deposition of PCBM on its surface, and block the direct contact between perovskite and P-ZnO layers, hence further improving the thermal stability of perovskite. At last, the PSC with the new ETL combination displayed the best device efficiency of 15.38%. Recently, Wang et al., employed a strong chelating agent of ethylene diamine tetraacetic acid (EDTA) to develop the EDTA-complexed ZnO (E-ZnO) as the ETL ^[57][83]. Compared to pure ZnO, the E-ZnO exhibited more suitable energy levels with perovskite and improved electron extraction and transport characteristics. Additionally, the E-ZnO chelated with organic ligands of EDTA could effectively mitigate the gradual decomposition of perovskite. Combing the E-ZnO with a new preparation process of perovskite film requiring neither annealing nor antisolvent, the fabricated PSC achieved an impressive PCE of 20.39%, and the long-term stability was significantly improved with retaining 95% of its initial efficiency after 3604 h of exposure in air environment.

Surface modification is another reliable alternative to the doping strategy, mainly serving the purpose of regulating the surface and interface properties of ZnO. So far, non-assembled organic molecules, self-assembled monolayers (SAMs), and inorganic coatings have been broadly employed to modify ZnO ETL for the achievement of high-performance PSCs.

Many non-assembled organic molecules have demonstrated the potential to treat ZnO films for better performance. Cheng et al., found that the thermal treatment could lead to the decomposition of perovskite films deposited on the bare ZnO NP ETL ^[58][84]. Therefore, a buffer layer was introduced at the interface of the perovskite and ZnO layers. Notably, the small molecule PCBM can slow down but cannot completely avoid the interfacial reaction of perovskite on ZnO, whereas the polymeric molecular layer of PEI can efficiently avoid direct interaction between ZnO and perovskite. There was no obvious decomposition in perovskite even after an hour of heat treatment at 100 °C, allowing the formation of larger perovskite crystals upon thermal annealing. The PCE of planar PSCs was dramatically raised from 2.9% to 10.2% after the surface modification with PEI. Recently, based on the consideration of eliminating the deprotonation ability of ZnO, Liu et al., introduced methyl ammonium chloride (MACl) on the surface of ZnO NP ETL to improve the surface properties ^[59][85]. After the MACl modification and annealing treatment, ZnO could extract H⁺ from MA⁺ and release the CH₃NH₂ gas, thus avoiding the further protonation reaction between ZnO and perovskite. At the same time, Cl would leave on the surface of ZnO and passivate the interfacial defect states. Consequently, the improved efficiency and strengthened durability of planar PSC devices were achieved simultaneously. Additionally, Azmi et al., performed the sulfur passivation on sol-gel ZnO (ZnO-S) ETL by using a simple chemical modification of 1,2-ethanedithiol (EDT) ^[60][86]. With this surface modification, the proton-transfer reaction at the interface of ZnO/perovskite was efficiently prevented, the perovskite growth with larger grain size and higher crystallinity was facilitated, and the surface defects leading to carrier recombination loss were well passivated. Compared to the pristine ZnO, the PCE significantly increased from 16.51% to 19.65% for the low-temperature planar PSCs based on the ZnO-S ETL, and the unencapsulated device exhibited remarkably improved long-term stability after 40 days of air storage.

SAM modification was considered a facile and efficient strategy to passivate surface defects and adjust the charging behavior. As a pioneer, Zou et al., developed 3-aminopropanioc acid as the self-assembled molecule (C3-SAM) for the modification of sol-gel ZnO [29]. The deposition of C3-SAM contributed to the improved morphology of CH₃NH₃PbI₃ with reduced pinholes and high crystallinity, thus reducing the defect state density of perovskite. Moreover, the interfacial energy level was better aligned because of the formation of a permanent dipole moment. Therefore, a surged PCE from 11.96% to 15.67% was acquired for the planar PSCs. Subsequently, the highly polar molecules of T2CA and JTCA were synthesized by Azmi et al., for the SAM modification on sol-gel ZnO ^[61][87]. These SAM molecules enhanced the hydrophobicity of ZnO, resulting in the effective improvement of the formation quality of PbI₂ and final perovskite layers. Meanwhile, the increased electric dipole effect of SAMs enhanced the charge extraction property of PSC devices. A decent PCE of 18.82% was reached for the low-temperature ZnO-based PSC, whereas the pristine device delivered only 15.41%. Recently, Song et al., adopted two thiophene acetic acid-based organic molecules, 2-TA and 3-TA, as SAMs on the ZnO surface for interface modification ^[62][88]. The TA-based molecules would interact with ZnO and passivate the trap states on its surface, while the sulfur atom from the thiophene ring could passivate the Pb²⁺ defect of perovskite. additionally, this TA modification promoted perovskite growth with improved crystallinity and induced a more favorable interfacial energy level alignment. Therefore, the carrier recombination loss caused by defects was reduced, and the interface carrier transport dynamics were improved, contributing to a significantly elevated efficiency from 18.1% to 20.6%. Noteworthily, the perovskite film based on the modified underlayer showed an alleviated thermal decomposition reaction.

In addition to the functional organic molecules, in recent years, several inorganic compounds also have been successfully utilized to modify the ZnO ETL, demonstrating the ability to improve device performance. In 2018, Zheng et al., adopted a thin layer of MgO and a sub-monolayer of protonated ethanolamine (EA) to modify ZnO ^[63][35]. The charge recombination at the interface of ZnO/perovskite was inhibited by introducing the modification layer of MgO. Additionally, the contact barrier was reduced profited from the protonated EA and hence promoted charge extraction and transport. This modification also nicely resolved the instability issue at the interface. Based on the MgO-EA⁺ modification, the planar PSCs achieved the optimal PCE of 18.3% with improved long-term stability and fully eliminated hysteresis. Later, Chen et al., constructed a cascade ZnO-ZnS ETL by sulfurizing the ZnO surface to convert it into ZnS ^[64][89]. The sulfide on the surface of ZnO-ZnS could coordinate with Pb²⁺ and generate an electron transport pathway that accelerated electron transfer and reduced charge recombination. Moreover, the ZnS acted as a passivation interlayer to passivate the basic surface of ZnO and avoid the possible proton transfer of perovskite. All these results contributed to enhancing the overall stability of the PSC device and producing a champion PCE of up to 20.7% without appreciable hysteresis. Recently, Pang et al., introduced the PbS quantum dots (QDs) onto ZnO and further deposited the tetrabutylammonium iodide (TBAI) to obtain a new ETL of ZnO/PbS-TBAI for planar PSCs ^[65][90]. The non-wetting surface of modified ZnO improved the crystal quality of perovskite, and the more favorable energy level alignment arising from the tunable surface dipole moment of TBAI accelerated electron transfer and transport. Moreover, the decomposition problem of perovskite was completely solved. Based on the PbS QDs modification and optimized TBAI treatment, the ZnO-based PSC achieved an increased PCE from 14.65% to 20.53% with negligible hysteresis, as well as improved stability. In addition, Tavakoli et al., transferred monolayer graphene (MLG) on the surface of ZnO to restrain the possible deprotonation reaction at the interface ^[66][91], thereby protecting the perovskite film from decomposition at elevated temperatures. The introduction of MLG also enhanced the carrier extraction property of ZnO. With the help of MLG modification, a high PCE of 19.81% and excellent operational stability were achieved for the planar PSC device.