Metal-Doped TiO 2 Thin Film as an Electron Transfer Layer for Perovskite Solar Cells: A Review

: The electron transfer layer (ETL) plays a vital role in achieving high-performance perovskite solar cells (PSCs). Titanium dioxide (TiO 2 ) is primarily utilised as the ETL since it is low-cost, chemically stable, and has the simplest thin-ﬁlm preparation methods. However, TiO 2 is not an ideal ETL because it leads to low conductivity, conduction band mismatch, and unfavourable electron mobility. In addition, the exposure of TiO 2 to ultraviolet light induces the formation of oxygen vacancies at the surface. To overcome these issues, doping TiO 2 with various metal ions is favourable to improve the surface structure properties and electronic properties. This review focuses on the bulk modiﬁcation of TiO 2 via doping with various metal ions concentrations to improve electrical and optical properties, charge carrier density, and interfacial electron–hole recombination, thus contributing to enhancing the power conversion efﬁciency (PCE) of the PSCs.


Introduction
Perovskite solar cells (PSCs) have been identified as one of the candidates for nextgeneration solar cell materials since their first debut in 2009 [1].In the past decades, the world has seen tremendous technological advancement through interfacial [2][3][4], composition [5,6], and charge collection efficiency optimization [7], which has led to a significant increase in its efficiency.PSCs mainly consist of a perovskite layer sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL) [8].
The ETL, also known as the electron extraction layer or electron collection layer, has recently piqued the interest of researchers working on PSCs.Although some studies have claimed that an ETL-free PSCs is possible in obtaining cell efficiency of up to 20% [9,10], most researchers still believe that the ETL is vital in facilitating an efficient electron collection and transportation between the perovskite layer and the electrodes [11][12][13].The ETL promotes the transportation of photogenerated electrons that are extracted from the perovskite layer before being collected through the ohmic contact by the electrodes [14].Furthermore, it is also important for eliminating electrical shunts between the transparent electrode and the perovskite layer [15].Best research cell efficiencies for various emerging solar photovoltaic materials.This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO [28].

Best Research-Cell Efficiencies
A standard PSC structure typically consists of multiple layers of thin films that include the conductive contact layer, hole transporting layer, perovskite as an absorber layer, an electron transport layer, and the substrate [32,33].Figure 2 depicts the most common deposition sequence of a standard PSC.A fluorine-or indium-doped tin oxide is usually used as the substrate layer for the PSCs.The thin layer of ETL in this case, TiO 2 , is first deposited on the cleaned substrate, followed by the deposition of the perovskite absorber layer.Another thin layer of hole-transporting material, for example, spiro-OMeTAD, is then deposited, and to complete the whole structure, a final layer of conductive contact is deposited.The layers are stacked together while ensuring that the band alignment of the heterojunction is in accordance with its distinct device configurations.Despite the device configurations, the incorporation of ETL seems to be the most effective way of improving the overall performance of PSCs [34,35].A good structural and photoelectrochemical properties of an ETL can effectively expedite the whole electron collection and transfer process [36,37].The manufacturing of a thin and dense ETL while ensuring a matching conduction band and good electron transfer properties results in a high-performance device.These characteristics reduce interfacial recombination while facilitating electron movement and charge accumulation [38][39][40].

Characteristics of ETL
Since PSCs were originally fabricated on solid-state dye-sensitised solar cells (DSSCs), a mesoporous TiO2 scaffold structure was used (Figure 3c).However, due to the high-temperature process required for fabrication, a planar architecture (Figure 3a,b) similar to other thin-film solar cells is now common [41].The characteristics of the ETL, including its energy level alignment, trap states, charge mobility and morphology-dependent parameters, material, and related interface properties, are vital for PSC performance [25,42].When the energy level of the ETL/perovskite layer is matched, the electron extraction and transport properties are accelerated.This eventually increases the short-circuit current density (Jsc) and the fill factor (FF).The energy level is one of the important properties to ensure the improvement of the open-circuit voltage (Voc) of a photovoltaic device.It can be determined by the energy level differences between the Fermi levels (EF) of the ETL and the EF of the hole-transporting layer (HTL).Figure 4 depicts the energy levels of various inorganic ETL materials.It is important to further investigate TiO2 to develop a pathway to boost the PCE of perovskite solar cells via optimization, doping, and surface modification [23].A good structural and photoelectrochemical properties of an ETL can effectively expedite the whole electron collection and transfer process [36,37].The manufacturing of a thin and dense ETL while ensuring a matching conduction band and good electron transfer properties results in a high-performance device.These characteristics reduce interfacial recombination while facilitating electron movement and charge accumulation [38][39][40].

Characteristics of ETL
Since PSCs were originally fabricated on solid-state dye-sensitised solar cells (DSSCs), a mesoporous TiO 2 scaffold structure was used (Figure 3c).However, due to the hightemperature process required for fabrication, a planar architecture (Figure 3a,b) similar to other thin-film solar cells is now common [41].The characteristics of the ETL, including its energy level alignment, trap states, charge mobility and morphology-dependent parameters, material, and related interface properties, are vital for PSC performance [25,42].When the energy level of the ETL/perovskite layer is matched, the electron extraction and transport properties are accelerated.This eventually increases the short-circuit current density (Jsc) and the fill factor (FF).The energy level is one of the important properties to ensure the improvement of the open-circuit voltage (V oc ) of a photovoltaic device.It can be determined by the energy level differences between the Fermi levels (EF) of the ETL and the EF of the hole-transporting layer (HTL).Figure 4 depicts the energy levels of various inorganic ETL materials.It is important to further investigate TiO 2 to develop a pathway to boost the PCE of perovskite solar cells via optimization, doping, and surface modification [23].
Various oxide metals have been widely used as ETLs, but most studies have shown that TiO 2 is the best one that can serve in both planar and mesoporous forms.Other oxide metals also have advantages but lack certain properties and need surface modification.Various oxide metals have been widely used as ETLs, but most studies have shown that TiO2 is the best one that can serve in both planar and mesoporous forms.Other oxide metals also have advantages but lack certain properties and need surface modification.Table 1 lists a comparison of TiO2 with other oxide metals as an ETL.

Oxide Metal Comparison References
TiO 2

•
It has better hole-blocking properties, a suitable energy level match with the perovskite absorbing layer, and higher optical transparency.

•
A simple method for fabrication at a low cost.

•
The ability of electron transfer and long-time stability of PSCs.

•
A good photocatalyst material, and it is also widely used as an ETL for perovskite solar cells.

•
Has poor ZnO/perovskite interface chemical stability. [49] • The large surface recombination, attributed to large surface defect concentrations, produces low fill factors and, consequently, lower efficiency.

•
The decomposition of perovskite deposited on ZnO during the process of annealing is attributed to the deprotonation of methyl ammonium in contact with ZnO, facilitated by the chemisorbed species on the surface.

•
The device produced is more sensitive to ambient moisture than TiO 2 under illumination.

•
Not commonly used as an ETL. [51] Zn 2 Ti 3 O 8 , BaSnO 3 , α-Fe 2 O 3 /PCBM • These materials have not been extensively adopted, although in individual studies, these materials have exhibited improved performance over more conventional material choices.

Titanium Dioxide (TiO 2 ) as an Electron Transfer Layer (ETL)
TiO 2 is an n-type semiconductor that can be formed into different polymorphs: anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal) [55,56].The structural properties of different TiO 2 polymorphs are depicted in Table 2. TiO 2 in its pure form is not an ideal ETL due to drawbacks in several important criteria, including (a) low electron mobility (0.1-4 cm 2 v −1 s −1 ), which limits the effective electron transport [57]; (b) conduction band mismatch with compositional-tailored perovskite; (c) a high processing temperature of up to 450 • C [38]; (d) induction of the formation of oxygen vacancies at the surface when exposed to ultraviolet light [58]; and (e) high photocatalytic activity at the grain boundaries, which act as charge traps, resulting in the severe loss of photogenerated carriers through recombination and decomposition [59].Thus, the interface between TiO 2 and perovskite retards the photoresponse of the resultant devices and leads to strong hysteresis [60].Extensive research has been conducted to improve the bulk properties of TiO 2 ETLs, thus producing long-term stability of PSCs that include elemental doping [61,62], morphological control [63][64][65], and surface modification [66][67][68][69].Surface modification is one way to produce a good-quality ETL by modulating the interface energetics and improving the physical contact between the ETL and the perovskite layer [70].Both the layer thickness and morphology of the ETL can be controlled to ease electron extraction and transport, thus improving the optical properties of PSCs by inhibiting carrier recombination and promoting charge extraction [71].
The mesoporous morphology of the ETL surface can lead the perovskite material to enter the pores and cause a larger surface area of the perovskite layer [25].In addition, direct contact between the perovskite absorbing layer and the contact layer can increase the recombination sites, causing a substantial loss of the photogenerated electron.In this case, morphological control is implemented by inserting photoanode materials between the perovskite material and the conductive contacts.By ensuring an appropriate band gap for the photoanode, it facilitates the electron transfer and hole-blocking of the contact simultaneously, suppressing the electron-hole recombination rate and enhancing the PCE of PSCs.[72] Elemental doping involves the introduction of either a metal or non-metal dopant to improve the intrinsic properties of the TiO 2 layer.A wide range of elements has been used to dope TiO 2 to modify the light absorption, photocarrier bulk transportation, and surface transfer properties [73][74][75].The incorporation of metal ions may have very little effect on the electron density; however, it helps improve the overall properties of the ETL by modulating the energy level, improving the electron dynamics, and decreasing the defect density [23].

Modification of TiO 2 with Metal Doping
Among the various methods used to enhance the bulk properties of TiO 2 ETLs, doping with metals ions has received much attention, as it is a simple process that can lead to improved charge transportation ability, elimination of hysteresis, modification of the band structure, modulated electron mobility, and improved thin film conductivity of the ETL with a high charge extraction capacity [76,77].The numerous metal dopants that have been reported include tin [42,78,79], iron [80], ruthenium [81,82], sulphur [83,84], niobium [85], zinc [86], tantalum [87], and magnesium [77,88].
For instance, Su et al. ( 2019) reported that Sn-doped TiO 2 prepared by the coelectrodeposition method considerably improved the photovoltaic performance of PSCs by increasing the carrier concentration and enhancing the electron extraction capability (Figure 5a,b).This was shown by the increase in the efficiency from 15.3% using pure TiO 2 to 16.8% when doped with Sn [78].Another study by Wu et al. (2019) showed that gradient doping of Sn 4+ on a compact TiO 2 ETL resulted in a high efficiency of 17.2% owing to the formation of a built-in electric field perpendicular to the substrate, which accelerated electron migration and reduced the charge recombination of the photocarriers and the transfer barrier (Figure 5c) [79].Gradient doping is a rational way to maximize the photon-to-current efficiency of a photoelectrode.The gradient doping forms a homojunction from the top to the bottom of the perovskite film with a built-in field that facilitates the extraction of photogenerated carriers, resulting in an increased carrier extraction length.In particular, Deng et al. ( 2020) successfully attained a slightly better performance (17.77%) in the efficiency of the PSCs by passivation of hydriodic acid (HI) in the preparation of Sn-doped TiO 2 .The results showed perfect alignment of the conduction band with the adjacent perovskite layer (Figure 5d) and the reduction of the defect density at the interface and in the entire film [89].Koech et al. (2021) also reported that an increase of 7.16% PCE was attained in an SnO 2 -doped TiO 2 ETL structure, resulting in an average PCE of 17.35% ± 1.39% as the result of improved electron extraction and transport ability [90].
For instance, Su et al. (2019) reported that Sn-doped TiO2 prepared by the co-electrodeposition method considerably improved the photovoltaic performance of PSCs by increasing the carrier concentration and enhancing the electron extraction capability (Figure 5a,b).This was shown by the increase in the efficiency from 15.3% using pure TiO2 to 16.8% when doped with Sn [78].Another study by Wu et al. (2019) showed that gradient doping of Sn 4+ on a compact TiO2 ETL resulted in a high efficiency of 17.2% owing to the formation of a built-in electric field perpendicular to the substrate, which accelerated electron migration and reduced the charge recombination of the photocarriers and the transfer barrier (Figure 5c) [79].Gradient doping is a rational way to maximize the photon-to-current efficiency of a photoelectrode.The gradient doping forms a homojunction from the top to the bottom of the perovskite film with a built-in field that facilitates the extraction of photogenerated carriers, resulting in an increased carrier extraction length.In particular, Deng et al. (2020) successfully attained a slightly better performance (17.77%) in the efficiency of the PSCs by passivation of hydriodic acid (HI) in the preparation of Sn-doped TiO2.The results showed perfect alignment of the conduction band with the adjacent perovskite layer (Figure 5d) and the reduction of the defect density at the interface and in the entire film [89].Koech et al. (2021) also reported that an increase of 7.16% PCE was attained in an SnO2-doped TiO2 ETL structure, resulting in an average PCE of 17.35% ± 1.39% as the result of improved electron extraction and transport ability [90].A previous study by Gu et al. (2017) reported the doping of Fe 3+ into TiO2 to improve the conductivity of TiO2 compact layers and finally boost the performance of PSCs to attain an efficiency of 18.6%.The Fe 3+ -doped TiO2 solution was synthesized using a sol-gel method by mixing Fe(NO3)3 at different concentrations The results demonstrated that the conductivity was increased significantly due to the reduction in the electron trap density A previous study by Gu et al. (2017) reported the doping of Fe 3+ into TiO 2 to improve the conductivity of TiO 2 compact layers and finally boost the performance of PSCs to attain an efficiency of 18.6%.The Fe 3+ -doped TiO 2 solution was synthesized using a sol-gel method by mixing Fe(NO 3 ) 3 at different concentrations The results demonstrated that the conductivity was increased significantly due to the reduction in the electron trap density by the substitution of Fe 3+ at the Ti 3+ sites, which effectively passivated the oxygen vacancy defects in the TiO 2 compact layer [80].A simulation study was conducted by Mulyanti et al. (2022), which showed that the absorption spectra of the Fe-doped TiO 2 increased to up to Coatings 2023, 13, 4 8 of 13 81.7% by the manipulation of the ETL thickness.It was revealed that the absorption spectra fluctuated more for thicker ETLs, while thinner ETLs resulted in the spectra dropping at short wavelengths [91].
Among others, ruthenium (Ru) can also be used as a doping material, as it has an ionic radius comparable to that of Ti 4+ (0.062 nm and 0.061 nm, respectively).Ti 4+ can be replaced by Ru 4+ without any strain or secondary phases.Wang et al. (2018), reported on the introduction of Ru ions into the compact layer of TiO 2 using the sol-gel spin coating technique to improve the properties of the ETL thin film.The titanium precursor solutions were prepared by mixing titanium diisopropoxide bis (acetylacetonate) solution in ethanol at a 1:10 vol ratio, followed by the addition of RuCl 3 .Different concentrations of Ru 4+ ions (0 mol%, 0.5 mol%, 1 mol%, 2 mol%, and 5 mol%) were used for the doped films.It was found that the density of the electron traps decreased dramatically without showing a negative effect on the optical properties.The results showed that at a concentration of 1 mol% Ru 4+ , Ru-TiO 2 -based solar cells revealed the highest efficiency of 18.35%, with improvement in carrier density and conductivity compared with pure TiO 2 owing to the matching band gap with a low resistivity, which affected the fill factor (FF) and open-circuit voltage (V oc ) of the device.The reduction in the excessive charge accumulation eventually resulted in increased FF and V oc while promoting effective electron injection between the TiO 2 compact layer and the perovskite interface, thus increasing the overall performance of the PSCs [82].In another study by Xu et al. (2018), a one-step spray pyrolysis technique was implemented in the doping of TiO 2 with Ru.It was found that the conductivity of Ru-doped TiO 2 was improved compared with using pure TiO 2 , as shown by the enhanced photovoltaic performance of the PSCs [81].
Sulphur (S)-doped TiO 2 has been demonstrated to enhance photocurrent, light absorbance, and photocatalytic activity as well as modify the band gap energy in other applications, such as coatings and photocatalysis [83,92].However, very few works have reported on using S as a dopant for TiO 2 .Abd Mutalib et al. (2022) reported on the use of an S-doped TiO 2 ETL in the planar PSCs by using a modified sol-gel method.The S-doped TiO 2 ETL was prepared by mixing of 1.2 mL of acetyl acetone with 7 mL of titanium isopropoxide (TTIP), 35 mL absolute ethanol, and varied concentrations of thiourea (5 mol%, 10 mol%, and 15 mol%.) as the sulphur source.The final solution was then stirred for 180 min to produce a transparent and homogeneous solution.The produced S-doped TiO 2 showed high absorbance, photocatalytic activity, and photo-current density.This proved that the increase in the electron collection capacity of the carrier charge and photocurrent density enhanced the PCE by 6.0% and showed a better efficiency of 18%, as shown in Table 3 [84].It showed that the charge transfer resistance (RCT) in the S-doped TiO 2 perovskite device was lower than that of the un-doped TiO 2 device.The lower resistance could facilitate higher electron transfer in the perovskite device, resulting in a higher overall PCE and finally enhancing the performance of the PSCs.
Table 3.The photocurrent-voltage (J-V) measurements for perovskite solar cell device using S-doped TiO 2 and un-doped TiO 2 with a 0.07 cm 2 active area [84].ETLs.A simple sol-gel spin coating process was used to deposit the parent solution of Nb-doped TiO 2 sol on the FTO/TiO 2 -bl substrate, and NbCl 5 was used as the source of Nb.During the mixing process, the amount of NbCl 5 added into the parent solution varied from 0 to 5 at.% of Nb.The obtained films were characterized, and it was found that at higher visible light transmittances, greater charge transfer conductivities, and higher Coatings 2023, 13, 4 9 of 13 Nb incorporation efficiencies, the compact layer ETL demonstrated superior performance compared with its mesoporous counterpart.The results showed that the doping of 5 at.% Nb into the compact TiO 2 ETL produced higher PCE compared with the mesoporous TiO 2 ETL, with 33.1% and 28.8% for the compact and mesoporous ETLs, respectively.The higher carrier injection with higher Nb incorporation efficiency produced during the addition of Nb to the TiO 2 crystal structure provided a band gap reduction in the ETL, which gave more potential between the conduction band of the perovskite layer and the conduction band of the TiO 2 ETL layer [93].

Material
Overall, it has been confirmed that the bulk modification of TiO 2 by doping with different metal ions is an effective way to reduce the band gap and provide control of the electronic properties as well as the trap states of ETL.This results in an increased charge carrier density in the absorbing layer and improved electron transportation from the ETL to the conductive oxide substrate.The ion migration initiated by the illumination-induced electric field can be hindered or even stopped by the passivation of the grain boundaries in the device, which can decrease the packing density of the crystal lattice [94,95].Metal doping can also lower the density of recombination hubs correlated with under-coordinated atoms, and the modification of the ETL/perovskite interface plays a role in facilitating the charge transport and the suppression of hysteresis and interfacial recombination to realize highly efficient and stable PSCs [96].

Conclusions
In the construction of PSCs, it is believed that the ETL is still a key player to ensure the good performance of the PSCs.This review covers the use of TiO 2 as an ETL and the characteristics of TiO 2 ETLs as well as their issues and drawbacks.Several metal-doping materials were discussed, focusing on the improvement of the bulk properties of the doped TiO 2 ETL, which include the modification of the electric structure and intrinsic properties, the improvement of the electrical conductivity by increasing the carrier density and mobility, and the modification of the bandgap alignments and interface trap density and distribution.It is believed that more studies should be conducted focusing on the availability of the materials, the simplicity and flexibility of the method, and the environmental impact of the overall process.

Figure 1 .
Figure 1.Best research cell efficiencies for various emerging solar photovoltaic materials.This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO[28].

Figure 2 .
Figure 2. Sequential deposition of the layers in a typical PSC and standard PSC architecture.

Figure 2 .
Figure 2. Sequential deposition of the layers in a typical PSC and standard PSC architecture.

Table 1
lists a comparison of TiO 2 with other oxide metals as an ETL.

Table 1 .
Comparison between TiO2 and other oxide metals as electron transfer layers (ETLs) for perovskite solar cells (PSCs).
[43]It has better hole-blocking properties, a suitable energy level match with the perovskite absorbing layer, and higher optical transparency.Asimplemethodforfabrication at a low cost.Theability of electron transfer and long-time stability of PSCs.A good photocatalyst material, and it is also widely used as an ETL for[43]

Table 1 .
Comparison between TiO 2 and other oxide metals as electron transfer layers (ETLs) for perovskite solar cells (PSCs).
a Pressure at pO 2 is 101.325KPa.The italic a, b and c are the unit cell parameters.