Tunable‐Conduction‐Band In–Zn–O‐Based Transparent Conductive Oxide Deposited at Room Temperature

Aluminum (Al)‐doped indium zinc magnesium oxide (Al:IZMO)‐based transparent conductive oxide (TCO) films with a tunable conduction band structure are developed via deposition at room temperature under radio‐frequency (RF) magnetron co‐sputtering for the application of highly efficient photovoltaic devices. First, at the amorphous phase region with the [In]/([In] + [Zn]) compositional ratio of 0.5–0.8, Al‐doped indium zinc oxide (Al:IZO) films obtained by introducing a small amount of aluminum oxide (Al2O3) exhibit higher Hall mobility and lower carrier density than those of indium zinc oxide (IZO) without Al doping. Second, adding magnesium oxide (MgO) to Al:IZO allows the optical bandgap (Eg) control, with constant ionization energy of −7.31 eV, which can be expressed by Eg = 0.83y + 3.33 (eV) as a function of [Mg]/([Zn] + [Mg]) compositional ratio (y) (in the y value region of 0–0.12). This result suggests that the conduction band minimum can be tuned by 0.067 eV under the 8% Mg substitution into the Zn site in the Al:IZMO films while remaining valence band maximum. Al:IZMO‐based TCO films with conduction band controllability are deposited at room temperature for preventing thermal damage. These findings contribute to the device design and development of emerging photovoltaic applications.

lightweight thin-film solar cells has remarkably progressed. Flexible perovskite solar cells fabricated on heat-sensitive substrates with high flexibility, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), have been reported. [20,21] Moreover, despite the high growth temperature of 500-600°C for Cu(In, Ga)Se 2 , by applying the devicepeeling technique using MoSe 2 atomic layers, flexible and bifacial Cu(In,Ga)Se 2 solar cells have been developed, which were transferred to ethylene tetrafluoroethylene (ETFE) or fluorinated ethylene propylene (FEP) films with low heat resistance temperature of 150-200°C as the alternative flexible substrates. [22][23][24][25][26] Al-doped zinc oxide (AZO) [23] and indium tin oxide (ITO) [22,[24][25][26] were deposited at room temperature after device-peeling for the back contact electrode of their flexible and bifacial Cu(In, Ga)Se 2 solar cells. According to this recent research progress on photovoltaic devices, the requirement for low-temperature deposition techniques for TCO layers is considerably increasing. Hence, in this study, the TCO films with controllable CBM are developed for applying high-efficient photovoltaics, which can be deposited at room temperature for preventing thermal damages to the devices.
A transparent amorphous oxide is free from grain boundaries and can accommodate large bond angle distributions. Therefore, the amorphous/polycrystalline semiconductor interface had a lower defect density than that in the conventional polycrystalline/polycrystalline semiconductor interfaces [27] ; thus, the transparent amorphous oxide materials are useful for developing TCO layers in the solar cells. Amorphous indium zinc oxide (a-IZO) [28][29][30][31] is a promising TCO layer with the highest carrier mobility of %60 cm 2 V À1 s À1 [30,31] deposited evenly at room temperature. In IZO, the vacant 5s orbitals in indium cations, which is a posttransition heavy metal element, form the large band dispersion owing to their large spatial overlap in the neighboring atoms; therefore, affording an electron conduction path and the high mobility despite the amorphous phase deposited at low temperature. [32,33] Via the elemental substitution, the electrical properties and energy band structure in IZO can be controlled so that they can be applied as the TCO layers for developing high-performance photovoltaic devices. In the Al-doped a-IZO matrix, [34][35][36] an increase in the Hall mobility caused by co-sputtering Al 2 O 3 and IZO was reported, which can be attributed to extra conduction paths formed by Al cations in the network of In cations. [34] To deposit an electrically stable IZO, Al atoms for the stabilizer ions were doped as a carrier suppressor, thereby reducing oxygen deficiencies. [37] Furthermore, MgO has a wide optical bandgap (E g ) of %7.9 eV, [38] thereby facilitating band structure controllability by mixing in the oxide-based compound semiconductor. Optical E g can be controlled from 3.6 to 3.8 eV by varying the Mg content x of 0-0.5 in In 2 O 3 (Zn 1Àx Mg x O) 3 deposited at a temperature of 300°C, [39] where In 2 O 3 (ZnO) k ¼ 3 is an In 2 O 3 (ZnO) k (k ¼ 3-9, 11, 13, and 15) family with a homologous structure and comprises cubic-bixbyite-structured InO 2 À1 and wurtzite-structured 4In 1/4 Zn 3/4 O 1/4þ layers repeatedly stacked along the c-axis.
The TCO films with continuously tunable CBM deposited at room temperature developed for the solar cell application is the only Al:(Zn,Mg)O films. [3,4,6,40]  ) compositional ratio of 0.55-0.65, the energy band structure obtained by Mg addition is systematically investigated based on the optical E g and ionization energy. Finally, room-temperature deposition of developed Al-doped indium zinc magnesium oxide (Al:IZMO) films with tunable CBM on a heat-sensitive flexible FEP substrate is demonstrated. These findings contribute to the realization of high-performance solar cell devices and the development of emerging photovoltaic applications.

Effect of Al-Doping on In 2 O 3 -ZnO Systems
Two IZO films were co-sputtered using In 2 O 3 and ZnO or AZO (Al 2 O 3 : 2 wt%) targets for discussing the correlation between Aldoping and electrical properties. Figure 1 depicts the XRD spectrum associated with the TCO films of: a) In 2 O 3 -ZnO and b) In 2 O 3 -AZO systems deposited at room temperature via  The overlap between neighboring In 5s orbitals in IZO affords high carrier mobility despite an amorphous phase. [32,33] Furthermore, by co-sputtering Al 2 O 3 and IZO targets, Hall mobility was improved because of the extra electron conduction pathways formed by Al 3s orbitals in the network of In cations, affording higher mobility. [34] Second, oxygen vacancies were reduced by Al-doping, suppressing the scattering of carriers by ionized impurities. Higher chemical bonding energy of Al with O atoms, rather than In and Zn atoms, affords reduced oxygen vacancies. [37,42] In particular, in this study, oxygen gas was not introduced during the IZO sputtering process.  The average T at the wavelength region of 700-1300 nm in Figure 3c was estimated based on data depicted in Figure 3a, b. The optical E g in Figure 3d was extracted by the (αhν) 2 plot as a function of hν, as shown in the supporting information ( Figure S2, Supporting Information  Figure 3c) and optical E g was high (Figure 3d) in the In 2 O 3 -AZO system compared to the In 2 O 3 -ZnO system. Al atom act as a donor in the ZnO. [40,43,44] Hence, at the [In]/([In] þ [Zn]) compositional ratio of 0, Al-doping affords a higher carrier density of 4.50 Â 10 20 cm À3 (AZO) in the In 2 O 3 -AZO system than that of 6.55 Â 10 19 cm À3 (ZnO) in the In 2 O 3 -ZnO system (Figure 2c). Therefore, free-carrier absorption afforded reduced average T   (Figure 3a-c). A metal-like dark film is formed under the zero or low oxygen partial pressure during the sputtering process caused by the oxygen-deficient in In 2 O 3 -ZnO-SnO 2 . [46,47] (Figure 3d) can be expressed as the following equations Finally, relatively high carrier mobility and transparency in the In 2 O 3 -AZO TCO films deposited at room temperature compared with the conventional AZO TCO were obtained at the amorphous phase region with an optimum [In]/([In] þ [Zn]) compositional ratio of 0.55-0.65. The next section discussed the influence of Mg addition to Al:a-IZO films on controlling the CBM band structure.

Conduction Band Tuning of Al-Doped IZO Films via Mg Addition
To realize the CBM controllability for the TCO films, MgO was added to the Al:a-IZO films by co-sputtering using In 2  ) compositional ratio of 0-0.10 should be stable when used as a TCO layer in the photovoltaic devices because the sheet resistance is below 10 Ω sq. À1 In the XRD spectrum, the Al:IZMO films crystallized and became the homologous phase by Mg addition (Figure 4d). This crystallization mechanism should be investigated in more detail. However, one of ) compositional ratio value (y) from 0 to 0.12, the variation in E g and IE for Al:IZMO films can be expressed using the following equations IE ¼ À0.05y À 7.31 (4) The optical E g is widening because of the Mg addition rather than Burstein-Moss effect owing to decreasing trend in the carrier density with the [Mg]/([Zn] þ [Mg]) ratio depicted in Figure 4b. Moreover, because the IE was estimated to be an almost constant value of À7.31 eV, the valence band maximum (VBM) was fixed. By contrast, CBM was shifted by 0.067 eV toward the vacuum level under the Mg substitution into the Zn site by 8%. This finding helps the design of the highly efficient solar cells considering the energy band structure. This E g widening affords enhanced activation energy for donor-related shallow defect states, hindering the climbing of the carriers to the conduction band. [48,49] Furthermore, MgO has high formation energy of oxygen vacancies of 9.8 eV, [48]

Experimental Section
First, the influence of Al-doping on the IZO films is discussed. 1000 nm thick TCO films of In 2 O 3 -ZnO (without Al-doping) and In 2 O 3 -AZO (with Al-doping) systems were deposited on glass substrates at room temperature via a radio-frequency (RF) magnetron co-sputtering system. In 2 O 3 and ZnO targets in the In 2 O 3 -ZnO system or In 2 O 3 and AZO (Al 2 O 3 : 2 wt% doped) targets in the In 2 O 3 -AZO system were used. The [Mg]/([Zn] þ [Mg]) compositional ratio was controlled from 0 to 0.12 by changing the sputtering power from 0 to 140 W. All applied targets had a diameter of 76.2 mm and 99.99% (4 N) purity. The background pressure before the deposition, working pressure, and pure Ar gas flow rate were 2.0 Â 10 À4 Pa, 0.25 Pa, and 10 sccm, respectively. The elemental composition was estimated using energy-dispersive X-ray spectrometry (EDX) (EMAX x-act, HORIBA) under an acceleration voltage of 4 kV via scanning electron microscopy. The X-ray diffraction (XRD) θ-2θ scan was conducted using Cu Kα radiation with a wavelength of 1.5405 Å using a measurement system (X'Pert PRO, PANalytical). Optical E g was estimated from the plot of the (αhν) 2 as a function of photon energy (hν), where the absorption coefficient of the films (α) was calculated from transmittance and reflectance spectra measured using an ultraviolet/ visible near-infrared spectrophotometer (UV-3600, Shimazu). The electrical properties and the ionization energy were investigated using a Hall effect measurement system (ResiTest 8400, TOYO) and by the Y 1/3 plot as a function of hν using the photoelectron yield spectroscopy (PYS) system (BIP-KV221K, Bunkoukeiki), where the Y is photoemission yield intensity, respectively.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.