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Perovskite oxides as transparent semiconductors: a review
Nano Convergence volume 7, Article number: 32 (2020)
Traditional transparent conducting oxides (TCOs) have been widely used for various optoelectronic applications, but have the trade-off between conductivity and transmittance. Recently, perovskite oxides, with structural and chemical stability, have exhibited excellent physical properties as new TCOs. We focus on SrVO3-based perovskites with a high carrier concentration and BaSnO3-based perovskites with a high mobility for n-type TCOs. In addition, p-type perovskites are discussed, which can serve as potential future options to couple with n-type perovskites to design full perovskite based devices.
Transparent conducting oxides (TCOs) are a group of unique materials for optoelectronic applications, including displays, solar cells, and light-emitting diodes [1,2,3,4]. High transmittance (≥ 80%) in the visible spectrum, high conductivity, and high carrier mobility are required for transparent electronics. Various metal oxide materials, such as Sn-doped In2O3 (ITO) [5,6,7], F-doped SnO2 (FTO) [8, 9], Sb-doped SnO2 (ATO) [10, 11], and Al-doped ZnO (AZO) [12,13,14,15], have been investigated. Researchers have been developing these oxides to exhibit high performance by focusing on transmittance in the visible range, electrical conductivity at room temperature, stability in the working environment, high carrier mobility, and controlling conductive type.
Electrical conductivity is determined by σ = enμ = e2τ (n/m*), where τ, n, and m* are scattering time, carrier concentration, and electron effective mass, respectively. It is clear that the value of n/m* is the key to achieve high performance of TCOs provided that the scattering time is maximized . Typical post-transition metal cations such as In3+ [7, 17], Zn2+ , and Sn4+  have largely spread vacant s-orbitals to form the conduction band minimum (CBM) of TCOs, along with small effective masses. Among these materials, ITO has been widely used in industry thanks to its highest conductivity (~ 104 S·cm−1), along with an electron carrier concentration of the order of 1021 cm−3 [5, 20]. However, the carrier concentration is limited and the total scattering time of the carriers could be reduced due to the solubility limitation of the dopants as well as self-compensation [21,22,23]. Additionally, it is well-known that the markets for transparent devices are large but the critical raw material, Indium (In), is scarce on earth, which has led to high costs . Substitution with ZnO- and SnO2-based binary compounds has found success with optimized electrical performance, but this strongly depends on approaches of deposition and growing conditions . It should be noted that it is difficult to deposit high performance impurity-doped metal oxide thin films. The key factors are to improve crystallinity by deposition methods and control an oxidizing atmosphere [25,26,27]. It was reported that AZO thin films deposited by pulsed laser deposition (PLD) showed an encouraging conductivity of the order of 104 S·cm−1 .
Carrier mobility (μ), defined as μ = eτ/m*, is another important factor for the high performance of transparent oxides. Conduction bands derived from widely spread metal s orbitals result in n-type conductivity and thus excellent electron carrier mobility, whose values range from 10 to 100 cm2·V−1·s−1. However, the applications of transparent oxides can hardly be further developed due to the lack of p-type counterparts for p-n heterojunctions [20, 28, 29] or other p-type devices like photovoltaics . So far no efficient method has been created to introduce shallow acceptors and small hole effective mass due to strongly localized O 2p orbital derived from valence bands of metal oxides, though huge efforts have been made . The carrier mobility of p-type TCOs cannot compare with those of n-type, even though high mobility values have been achieved from Cu-based materials [31, 32] at the sacrifice of hole carrier concentration.
ABO3-based perovskite oxides are common in inorganic compounds. The ideal cubic perovskite structure (Pm-3 m) is composed of an A-site cation (an alkaline-earth, a rare-earth or an alkali element), B-site cation (a transition or post-transition metal element from periods 4, 5, or 6 in the periodic table), and oxygen anions. B-site cation sits at the center of BO6 octahedron . It should be noted that distortions are caused by cations of different radii, which results in octahedral tilting. The structure of perovskite oxides can remain stable even if A- or B-site cations are replaced by a large amount of other elements [33, 34]. Materials with a perovskite structure have drawn high attention for extensive research on a variety of physical properties, including superconductivity , metal–insulator transition , photovoltaic effect , transparent conductivity [38,39,40,41,42,43], magnetism , and ferroelectricity [37, 45]. In particular, using novel perovskite oxides to replace traditional binary TCO materials has become popular. Recently, Zhang et al.  reported that correlated metal compounds (e.g., SrVO3 and CaVO3), as transparent conductors, had an excellent carrier concentration (> 2.2 × 1022 cm−3), an order of magnitude greater than that of ITO (~ 1021 cm−3). Alkaline earth stannates (ASnO3, A = Ca, Sr, Ba) have a wide bandgap (~ 3.0 eV), exhibiting highly transparent visible photovoltaic effective range and excellent electrical properties [46,47,48,49,50], i.e., a mobility up to 320 cm2·V−1·s−1 at room temperature [51, 52]. In contrary to stoichiometric perovskite oxides that are insulating, oxygen-deficient structures transform to semiconductor or conductor, i.e., oxygen vacancy also plays a role in electrical properties [53, 54]. Early research results have revealed high thermal stability of oxygen in perovskite materials [55, 56], thus avoiding the degradation in the oxide-based electronic devices.
On the other hand, it remains a challenge to achieve p-type conductivity by binary post-transition metal oxide because of strongly localized O 2p orbitals, resulting in high formation energy of acceptor and large hole carrier masses. Fortunately, A or B sites can easily be replaced by other elements in perovskite oxides, thanks to the high chemical and structural stability. Stoichiometric strontium titanate (SrTiO3, STO) is a group of highly transparent insulators (Eg ~ 3.2 eV) that can be easily doped by Nb , La , or Sb  for n-type STO, or doped by Sc , and In  for p-type ones. Like “chemical modulation of the valence band” (CMVB), introduced by Hosono et al., partial replacement of Al for Sn in cubic perovskite-structures, SrSnO3, can decrease the localization and generate holes in the valence . Perovskite compounds such as La1-xAlSnO3 , La1-xSrxCrO3 [42, 63], La2/3Sr1/3VO3 , SrTiO3:Rh , etc., can be p-type TCO candidates, which demonstrate excellent electrical and optical properties. The new perovskite oxides not only balance the trade-off between conductivity and transmittance, but also show p-type characteristics. Development of p-type perovskite oxides are critical, along with n-type perovskites, to make full perovskite devices.
In this article, we review the novel TCO materials based on perovskite oxides. Section 2 discusses the crystal structure, electronic properties, fabrication methods of the SrVO3- and BaSnO3-based perovskites. SrVO3 films inherit a high conductivity thanks to the high carrier concentration, even though they have a high effective mass. BaSnO3-based perovskites have a high mobility. Section 3 describes the properties and impurity doping of p-type perovskite oxides. Due to their high chemical stability, perovskite oxides can be doped with A- or B-site.
Alkaline earth metal vanadium oxides with the AVO3 (A = Ca, Sr, Ba) composition have a V4+ 3d1 electronic configuration with a single electron in the partially filled t2g band. The first-principle calculations showed three t2g bands at the Fermi energy between -1.5 eV and 1.5 eV . In this article, strontium vanadium oxide (SrVO3, SVO), which has aroused widespread interest, is selected to illustrate that strongly correlated metal oxides are promising transparent conductive materials. SVO possesses cubic symmetry (a = 3.842 Å) with one V atom surrounded by six O atoms to form an octahedral configuration (Fig. 1a). The valence band maximum (VBM) is composed of O 2p orbitals, and the conduction band minimum (CBM) is composed of V 3d orbitals that can be split into e.g. and t2g states (Fig. 1b). The width of 3d bands is narrow enough (W ≈ 2-3 eV) so that the local Coulomb interaction between the electrons is particularly strong, causing electronic correlations .
SVO is an important strongly correlated metal oxide that has been extensively investigated in both theoretical [68,69,70] and experimental researches [71,72,73]. Recently, SVO has been reported as a transparent conductor because of its high electrical properties. Contrary to heavy doping to optimize physical properties in wide-bandgap semiconductors, Zhang et al.  demonstrated that novel materials with strong electron–electron interactions had a higher carrier concentration in comparison with that of ITO. This kind of materials inherit a high conductivity (~ 104 S·cm−1) due to the higher carrier concentration (~ 1022 cm−3) although they have a high effective mass. In addition, SVO films show optical transmittance in the visible range due to lower plasma frequency (ωp* = 1.33 eV), allowing them to be thinner, to increase optical transmittance in the visible spectrum range without suffering from surface scattering because of the much smaller electron mean free path (EMFP = ħ(3π2)1/3(τ/n2/3) (n/m*)) in correlated metal oxides (SrVO3, EMFP= 5.6 nm and CaVO3, EMFP= 3.9 nm)  than other conductors, like Ag (EMFP= 52 nm) and Au (EMFP= 50 nm) [74, 75].
However, an inevitable problem is that the carrier mobility and transmittance of SVO is lower than ITO. The factors those affect the physical properties of the SVO thin films, including: (1) Film crystallinity. The SVO thin films with high performance are all crystal phase deposited on lattice-matching substrates, such as (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), SrTiO3 (STO), NdGaO3 (NGO), etc., as shown in Table 1, instead of on an amorphous substrate (glass or PET) or other substrates with a different crystal structure (Si, GaN, or Al2O3). Note that the properties of the films are not directly affected by the lattice mismatch, but mainly determined by lattice misfit between the SVO film (a(SVO)) and the substrate (as), f = [as-a(SVO)]/as. The f values between SVO bulk and LSAT (3.868 Å), STO (3.905 Å), and NGO (3.863 Å) substrates are +0.67%, +1.61%, and +0.54%, respectively. Under the same growth conditions, the carrier concentration (or mobility) of the SVO films on LSAT and NGO are close, with values of 2.14 × 1022 cm−3 (2.9 cm2·V−1·s−1) and 2.2 × 1022 cm−3 (3.3 cm2·V−1·s−1), respectively . However, the resistivity of the SVO film grown on an LSAT substrate (70 μΩ·cm) is lower than that on a STO substrate (90 μΩ·cm) . (2) Growing methods. To grow high quality thin films, various growing methods have been developed to deposit SVO films to achieve high performance, including pulsed laser deposition (PLD) [76,77,78,79,80], molecular beam epitaxy (MBE) , hybrid MBE [43, 82, 83], and radio frequency sputtering deposition (RF sputtering) . These results show that preparation method has a great influence on the properties of the prepared films. This might be caused by the defects generated in the preparation process. For example, good stoichiometric SrVO3 can be controlled by hybrid MBE, which can reduce the number of defects of perovskite-based oxide films [82, 83]. The resistivity (28 μΩ·cm ) of the SVO films grown by MBE measured at room temperature is lower than those grown by other deposition approaches like PLD (38  and 120 μΩ·cm ).
An open question still facing SVO materials is if it will be one of the candidates to replace ITO in many optoelectronic devices. Except for high optical transmittance and high electrical properties, large area deposition, uniformity, and integration of oxide materials are essential. Amorphous materials have obvious advantages in the deposition of large area homogeneous films thanks to their anisotropy properties. Boileau et al.  investigated the optical and electrical properties of SVO films without long-range crystalline order. The resistivity and transmittance were found to depend on crystalline state. In the crystalline state, the transmittance reached ca. 80% for a 520 nm film and the resistivity was measured to be 1.2 ~ 1.9 × 10−4 Ω·cm, while the electrical properties deceased sharply with a decreasing crystallinity and the resistivity increased by orders of magnitude. The crystallinity of SVO significantly affects the properties of the films, limiting their potential as an alternative to ITO. Besides, the mobility of carriers is low due to the high effective mass induced by strong electron–electron interaction.
Furthermore, the performance of SVO thin films is sensitive to oxygen pressure [77, 79, 85]. In general, SVO is grown under low oxygen pressure, which is different from other strongly correlated perovskite materials, such as LaNiO3 and SrRuO3 grown under high oxygen pressure conditions [86,87,88]. At a high oxygen pressure, V4+ can be easily oxidized into V5+, indicating that optimization of oxygen pressure has a positive effecnt on film properties. It should be noted that the properties of the films prepared in vacuum are not optimal [77, 79]. Wang et al. achieved the highest conductivity at room temperature with an oxygen pressure of 1 × 10−5 mbar . The effect of oxygen pressure on the physical properties of thin films is still not completely clear and requires further investigation.
The limitation of SVO films for wide applications is their low carrier mobility, which is just on the order of 10 cm2·V−1·s−1. It is difficulty to enhance the mobility because of high effective mass and anisotropic electron carrier transport path. In contrast to strongly correlated SVO, alkaline earth stannates (ASnO3, A = Ba, Sr, and Ca) have been reported to have a high mobility, particularly barium stannate (BaSnO3). Unlike V 3d, Sn 5 s orbitals are isotropic and can still overlap to a large extent even in the amorphous state, without affecting the conductivity. Metal ions with (n−1)d10ns0 electronic configuration play a dominant role in high conductivity and high mobility of amorphous semiconductor materials [89, 90]. According to the literature, BaSnO3 (belonging to the perovskite family), shows a wide band gap (Eg > 3.1 eV) , a high thermal stability , and a high carrier mobility up to 320 cm2·V−1·s−1 in single crystals at room temperature  and 70 cm2·V−1·s−1 in epitaxial films , respectively. Caused by the high mobility, BaSnO3-based materials have been used as a semiconductor layer in oxide thin film transistors (TFTs) [92,93,94,95] and other photoelectronic devices [96,97,98]. It should be mentioned that the mobility of BaSnO3 is much higher than that of SrVO3. In contrast to CBM derived mainly from the localized V 3d states, the first-principle calculations showed that CBM is dominated by dispersive Sn 5 s states (Fig. 2), leading to a smaller carrier effective mass [48, 99]. This is similar to other cations with a (n−1)d10ns0 electron configuration, such as In3+, Zn2+, and Ga3+. Most effective masses of BaSnO3 from the first-principle calculations are between 0.2 and 0.5m0 [38, 48, 96, 100, 101]. They are apparently smaller than titanium- or vanadium-based perovskites, e.g., BaTiO3 (5.3m0) , and SrVO3 (3.0m0) , but close to traditional semiconductors, such as Si (0.27m0) , In2O3 (0.35m0) , and SnO2 (0.25m0) . The smallest value below 0.03m0 was reported by Moreira et al. with first-principle local density approximation (LDA) and generalized gradient approximations (GGA) calculations . The small effective mass has been found in tin-based perovskites , including SrSnO3 (0.23m0), ZnSnO3 (0.17m0), and CdSnO3 (0.23m0). This indicates that cation Sn on B-site is critical to high mobility.
High mobility has not been measured for stoichiometric BaSnO3 and it is known that it is not caused by intrinsic defects, but obtained from La-doped BaSnO3. Negative defect formation energy indicates a high concentration of substituting La for Ba (LaBa) in BaSnO3 , while the charge-state transition energy level, E[LaBa (+/0)], with respect to CBM is just 0.046 eV, indicating LaBa to be a shallow donor, which contributes one electron easily by the ionization of La atoms . To be a good candidate for TCOs, the solubility of dopants should be high and the donor level should be shallow. High carrier mobility of Ba1-xLaxSnO3 (x = 0 to 0.04) crystals were reported by Luo et al., when x = 0.02 the highest mobility was about 103 cm2·V−1·s−1 corresponding to a carrier concentration of ~ 8-10 × 1019 cm−3 . Kim et al.  enhanced the carrier mobility of (Ba, La)SnO3 single crystals to the highest value so far of 320 cm2·V−1·s−1. In the crystal structure of BaSnO3, tin atom is at the center of the octahedra (SnO6) and the band angle of Sn–O-Sn is ~ 180o. When Ba is partially replaced by La, the defects (LaBa) are located away from SnO6 octahedra, causing a low disorder effect. In addition, at a doping concentration of 8 × 1019 cm−3, the mobility showed the highest value . For single crystals, the mobility decreases with an increasing carrier concentration due to scattering. Therefore, a suitable doping level can reduce the dopant scattering in comparison with high carrier concentrations (> 1020 cm−3). Phonon scattering also has an effect on carrier mobility, but such effect is less significant than that of dopant scattering.
Epitaxy-grown BaSnO3 films
Compared to single crystals, the mobility value of La-doped BaSnO3 epitaxial films is much lower due to grain boundaries and dislocations caused by lattice mismatch between (Ba, La)SnO3 and the substrate . To enhance the film mobility, researchers managed to grow La-doped BaSnO3 epitaxial films on other cubic perovskites, such as SrTiO3(001) [51, 52, 110], SmScO3(110) , PrScO3(110) , TbScO3(110) , and DyScO3(001) . The electron mobility was enhanced from 5.8 to 183 cm2·V−1·s−1 at room temperature, as shown in Fig. 3. Furthermore, BaSnO3 epitaxial films were grown on BaSnO3 single crystal substrate with a mobility of 102 cm2·V−1·s−1 . The mobility can be increased from ~ 37 cm2·V−1·s−1 (without buffer) to ~ 56 cm2·V−1·s−1 at room temperature when a bilayer of BaSnO3/(Sr,Ba)SnO3 was inserted as a buffer layer to reduce dislocation scattering . Wang et al.  reported that La-doped BaSnO3 films using SrTiO3 buffer on Si(001) substrate showed a mobility of 128 cm2·V−1·s−1 at room temperature. This result is remarkable, but the fabrication cost has been significantly increased. To address this issue, Sanchela et al.  developed to grow La-doped BaSnO3 films under a highly oxidative ozone (O3) atmosphere, achieving a balance between mobility (115 cm2·V−1·s−1) and processing cost.
The intrinsic point defect, oxygen vacancy (VO), plays a crucial role in various oxide semiconductors, including ZnO, SnO2, and In2O3. The first-principle calculated results show that the formation energy of VO has a negative value in perovskite stannates , indicating defects will form spontaneously along with grown films. BaSnO3 films with a high oxygen vacancy concentration were fabricated at low oxygen pressures [54, 119] and reducing conditions (Ar containing 5% H2 gas) . Oxygen vacancies are donor defects which could contribute twice as many electron carriers per single-unit defect. Experimentally, high n-type carrier concentrations have been reported under O-poor growth conditions [54, 121, 122]. Son and coworker  reported that the mobility of La-doped BaSnO3 film was 78 cm2·V−1·s−1 at room temperature by high-temperature annealing under a N2 environment to generate VO. Later, the same group enhanced the mobility up to 122 cm2·V−1·s−1 by controlling the oxygen pressure using H2 environment annealing . In addition, it is known that oxygen vacancies could be generated by a vacuum annealing process in undoped BaSnO3 films [124, 125]. Vacuum annealing methods have been reported to treat La-doped BaSnO3 films on MgO substrates with a carrier mobility of 101.6 cm2·V−1·s−1. The value was larger than that of the film deposited without annealing (~ 74 cm2·V−1·s−1) . Therefore, it is important to investigate the effect of oxygen vacancies on physical properties of BaSnO3 films.
Conventional TCOs, including ZnO, SnO2, and In2O3, typically exhibit n-type electronic conductivity, and are widely used in various fields. However, the lack of high performance p-type TCOs has limited their applications [127, 128]. In general, p-type TCOs were synthesized by acceptor doping to create holes in the valence band, but the hole concentration is usually not high enough due to recombination and strong localization [129, 130]. The conductivity and hole concentration suffer from doping bottlenecks, intrinsic defects, and the solubility limit of the acceptors. Although previous studies explored some approaches to change n-type TCOs to p-type ones, the hole carrier mobility is usually an order of magnitude lower than that of electron carrier. Moreover, Cu+-based oxides (CnMO2, M = B, Al, Ga, In, Sc) with d10 configuration were used for intrinsically p-type doped materials . Layered oxychalcogenides  and d6 spinel oxides  were reported to achieve p-type conductivity.
Due to their high chemical stability, ABO3-based perovskite transition metal oxides are of great interest in p-type materials. Even with A- or B-site doping, it is still stable for perovskite structures. Lanthanide-based perovskite LaRhO3 (LRO) with A-site doping (substituting La for Sr or Ca) was reported as a candidate for p-type semiconductors . Nakamura et al.  reported a p-type single-crystalline LRO semiconductor and investigated the physical properties of a heterojunction, which was comprised of n-type Nb:SrTiO3. The bandgap of p-type LRO was just 1.3 eV, as shown Fig. 4.
In comparison with Cu+-based p-type oxides, the localization of O 2p orbitals is reduced from hybridization with metal d orbitals using a method called “chemical modulation of the valence band” (CMVB) . The correlated metal oxides, Cr2O3, V2O3, LaCrO3, LaVO3, and LaCoO3, were identified to be p-type TCOs [33, 42, 137,138,139]. The VB consists of a hybridization between O 2p orbitals and transition metal 3d orbitals, leading to a reduction of the hole localization at the top of VB. In A-site doped perovskites LaBO3 (B = V, Cr, Co), the holes at the top of valence band (VB) could be introduced by replacing Sr2+ for La3+. Zhang et al.  grew a new p-type Sr-doped LaCrO3 on SrTiO3 substrate by molecular beam epitaxy. The conductivity was up to ~ 50 S cm−1 due to a high hole carrier concentration of 7.5 × 1021 cm−3. In the periodic table, V element is located to the left side of Cr. Therefore, the size of the 3d wavefunction of V3+ is expanded, resulting in a smaller U (d–d Coulomb repulsion energy) in LaVO3 than that of LaCrO3 . The conductivity of p-type La2/3Sr1/3VO3 (LSVO) films was enhanced to nearly ~ 900 S·cm−1 at room temperature . Meanwhile, the films exhibited a high transmittance up to 70%, indicating that they possess the potential to be a high performance p-type TCO. Takashima et al.  grew p-type La0.67Sr0.33CoO3 films on a SrTiO3 substrate by controlling the oxygen atmosphere and prepared p-n heterostructure. Furthermore, p-BaSnO3:K/n-BaSnO3:La whole perovskite p-n junctions were reported, and results show that they are stable at elevated temperatures (up to 300 °C) .
With regard to B-site doped materials, researchers substitute tetravalent ions on B-site by trivalent ions to prepare p-type perovskite oxides. SrTi1-xScxO3 is a p-type semiconductor with Sc3+ replacing Ti4+ in SrTiO3. However, the acceptor levels were so deep that the holes were localized . Guo et al.  reported In-doped SrTiO3 films to create hole carriers and investigated the optical properties. The average transmittance is up to 80% in the visible region. For Al-doped SrSnO3 (Eg > 3.9 eV), the transmittance was increased, too .
Perovskites BaSnO3 and BaZrO3 doped by trivalent elements (Ga, Sc, In, Y) are able to incorporate oxygen at low temperatures. The hole concentration decreases at elevated temperatures . Wang et al. reported that Y-doped BaSnO3 showed p-type behavior under a controlled atmosphere . It is well known that the electronic conductivity and Seebeck Coefficient of semiconducting materials are measured by high-temperature sintering techniques. These procedures may lead to unreliable results such as changing transport properties from p- to n-type . The holes induced by acceptor-doped perovskite oxides are compensated by intrinsic defects (oxygen vacancies) [146,147,148]. Acceptor-doped perovskite, BaSnO3, has also gained attention in various electrochemical devices [149, 150].
Conclusion and outlook
In this article, we comprehensively review the recent developments of perovskites with both high carrier concentration and mobility. SrVO3 with strong electron–electron interactions exhibits a higher carrier concentration of 2.2 × 1022 cm−3 than that of ITO. It also maintains a high conductivity of 104 S·cm−1 and optical transmittance in the visible range by taking advantage of its lower plasma frequency (ωp* = 1.33 eV). The performance of SrVO3 films are affected by crystallinity, mismatched substrate, and growth condition. A low resistivity of 28 μΩ·cm was achieved at room temperature in SVO films, which were grown by hybrid MBE in order to reduce the number of defects. Various lattice matched materials, including (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), SrTiO3 (STO), and NdGaO3 (NGO) have been used as substrates to prepare SVO films. Meanwhile, the performances of SVO thin films are found sensitive to oxygen pressure. Due to the high effective mass, the carrier mobility is just on the order of 10 cm2·V−1·s−1. On the other hand, BaSnO3-based perovskites with dispersive Sn 5 s states on the CBM, leading to a small carrier effective mass, have the highest carrier mobility of 320 cm2·V−1·s−1 in the single crystals and 183 cm2·V−1·s−1 in the epitaxial films at room temperature. It is easy to substitute Ba2+ by La3+ in BaSnO3 to introduce electron carriers due to the low formation energy of LaBa. Researchers have also managed to grow LBSO films on lattice matched substrates or insert a buffer layer between the LBSO and the substrate to enhance the carrier mobility. The performance of LBSO film is also affected by oxygen vacancy which could be controlled by depositional condition. Because of the high performance of SVO and LBSO films, they are expected to be excellent candidates of TCOs to further optimize the physical properties.
Moreover, perovskites with chemical and structural stability could be doped on A- or B-site by low valence metal ions to create stable holes in the valence band. Although various acceptor-doped perovskite oxides have been reported, their physical properties are still inferior to the corresponding n-type materials. It is necessary to further optimize the performance of p-type perovskites. Furthermore, perovskite oxides can be doped as both n- and p-type conductors, so full perovskite devices might be possible eventually.
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Haiying He thanks the support by the National Natural Science Foundation of China (No. 11747090) and the Key Project of Department of Education of Guangdong Province (No. 2016GCZX008). Zhihao Yang thanks the support by the Guangdong Basic and Applied Basic Research Foundation (No. 2019B1515120008).
The authors declare that they have no competing interests.
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He, H., Yang, Z., Xu, Y. et al. Perovskite oxides as transparent semiconductors: a review. Nano Convergence 7, 32 (2020). https://doi.org/10.1186/s40580-020-00242-7
- Perovskite oxide
- Transparent conducting oxide